“behavior of temperature-responsive microgels at...

“Behavior of Temperature-Responsive Microgels at

Interfaces:

Deformability and Interfacial Activity”

Von der Fakultät für Mathematik, Informatik und Naturwissenschaften der RWTH Aachen

University zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften

genehmigte Dissertation

vorgelegt von

Master of Science

Yaodong Wu

aus Anhui, China

Berichter: Universitätsprofessor Dr. Andrij Pich

Universitätsprofessor Dr. Alexander Böker

Tag der mündlichen Prüfung: 26.09.2014

Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek online verfügbar.

To my parents

i

Contents

Kurzfassung .................................................................................................................. I

Abstract ....................................................................................................................... III

List of Abbreviations and Symbols ........................................................................... V

Chapter 1. Introduction .............................................................................................. 1

1.1. Microgels ........................................................................................................ 1

1.1.1. Synthesis of microgels ............................................................................. 1

1.1.2. Internal structure of microgels ................................................................. 6

1.2. Nanoparticles at interfaces ........................................................................... 8

1.2.1. Conventional solid particles ..................................................................... 8

1.2.2. Soft microgel particles ........................................................................... 10

1.3. Challenges in the microgel research .......................................................... 12

1.4. References .................................................................................................... 14

Chapter 2. Motivation and Aims .............................................................................. 21

Chapter 3. Synthesis of Microgels with Controlled Cross-linking Density and

Copolymer Composition ............................................................................................ 23

3.1. Introduction ................................................................................................. 23

3.2.1. Materials ................................................................................................ 25

3.2.2. Synthesis of microgels ........................................................................... 26

3.2.3. Characterization methods....................................................................... 28

3.3. Results and discussion ................................................................................. 29

3.3.1. PVCL/acrylamides-based neutral microgels.......................................... 29

3.3.1.1. PVCL/NIPAm and PVCL/NIPMAm microgels............................. 29

3.3.1.2. PVCL/NIPAm microgel system with different cross-linking

densities…......................................................................................................... 31

3.3.1.3. Microgels synthesized with different amounts of surfactant .......... 36

3.3.2. PVCL-based microgels with functional groups ..................................... 37

3.3.2.1. PVCL/AAEM/VIm microgels ........................................................ 38

3.3.2.2. PVCL/AAEM/AAc microgels ........................................................ 41

3.4. Conclusions .................................................................................................. 42

3.5. References .................................................................................................... 43

Chapter 4. Internal Structure of Microgels by Light Scattering and Microscopy

...................................................................................................................................... 49

ii

4.1. Introduction ................................................................................................. 49

4.2. Experimental section ................................................................................... 51

4.2.1. Sample preparation ................................................................................ 51



4.3.1. Internal structure of microgels by light scattering ................................. 53

4.3.2. Microscopy observation of microgels with different cross-linking

degrees… .............................................................................................................. 55

4.3.3. Polymer network domains in microgels observed by AFM .................. 61

4.3.4. Microscopy observation of PVCL-based microgels with functional

groups… ............................................................................................................... 62

4.4. Conclusions .................................................................................................. 67

4.5. References .................................................................................................... 68

Chapter 5. Deformation and Spreading of Copolymer Microgels at the

Air/Water and Air/Solid Interfaces .......................................................................... 73

5.1. Introduction ................................................................................................. 73

5.2. Experimental section ................................................................................... 76

5.2.1. Sample preparation ................................................................................ 76


5.2.3. Computer simulation of microgels at the interface ................................ 77


5.3.1. Dependence of microgel modulus on the cross-linking density ............ 82

5.3.2. Interfacial activity of microgels at the air/water interface ..................... 84

5.3.3. Conformation of microgels at the air/water interface ............................ 86

5.3.4. Dimensions of microgels varied in the cross-linking degree ................. 87

5.3.5. Deformation of microgels ...................................................................... 90

5.4. Conclusions .................................................................................................. 92

5.5. References .................................................................................................... 93

Chapter 6. Behavior of Temperature-Responsive Copolymer Microgels at the

Oil/Water Interface .................................................................................................... 97

6.1. Introduction ................................................................................................. 97

6.2. Experimental section ................................................................................. 100

6.2.1. Microgel synthesis ............................................................................... 100

6.2.2. Interfacial tension................................................................................. 101

iii

6.2.3. Interfacial dilatational rheology ........................................................... 101

6.2.4. Preparation of microgel-stabilized emulsions ...................................... 102

6.2.5. Preparation of emulsion-templated materials ...................................... 102

6.2.6. Fluorescence microscopy ..................................................................... 103

6.2.7. Scanning electron microscopy ............................................................. 103

6.3. Results and discussion ............................................................................... 103

6.3.1. Dynamic interfacial tension ................................................................. 103

6.3.1.1. Influence of microgel concentration ............................................. 103

6.3.1.2. Influence of temperature ............................................................... 106

6.3.1.3. Influence of cross-link density ..................................................... 108

6.3.1.4. Influence of microgel size ............................................................ 109

6.3.2. Equilibrium state of the interfacial tension .......................................... 110

6.3.2.1. Influence of chemical composition of microgel ........................... 110

6.3.2.2. Influence of temperature ............................................................... 112

6.3.3. Discussion on the temperature dependence of interfacial tension ....... 114

6.3.4. Dynamic interfacial tension of microgel mixtures............................... 119

6.3.5. Interfacial dilatational rheology ........................................................... 121

6.3.6. Microgel-stabilized emulsions ............................................................. 123

6.3.6.1. Emulsions prepared at different oil/water ratios ........................... 123

6.3.6.2. Emulsions prepared at different microgel concentrations ............ 125

6.3.6.3. Summary of emulsions prepared with PVCL/NIPAm (1:1)

microgels… ..................................................................................................... 126

6.3.6.4. Emulsions prepared with microgels of different chemical

compositions ................................................................................................... 128

6.3.7. Materials based on microgel-stabilized emulsions .............................. 129

6.4. Conclusions ................................................................................................ 133

6.5. References .................................................................................................. 135

Chapter 7. Summary ............................................................................................... 141

Chapter 8. Outlook and Future Experiments ....................................................... 145

8.1. Interfacial behavior ................................................................................... 145

8.1.1. Microgels with ionizable groups or specific structures ....................... 145

8.1.2. Other techniques .................................................................................. 146

8.2. Potential applications and improvements ............................................... 146

8.2.1. Asymmetric colloidal particles ............................................................ 146

iv

8.2.2. New microgel-based capsules .............................................................. 147

8.2.3. Designed biodistribution of microgels for therapy .............................. 148

8.3. References .................................................................................................. 149

Acknowledgements .................................................................................................. 151

Curriculum Vitae ..................................................................................................... 153

Kurzfassung

I

Kurzfassung

Mikrogele sind ultraweiche intramolekulare vernetzte Polymernetzwerke. Eine

der wichtigsten Eigenschaften der Mikrogele ist die Grenzfläschenaktivität, die eine

große Rolle in ihre Anwendungen wie Herstellung von Filmen, Emulsionen oder

Kapseln spielt. Um den Einfluss der chemischen Zusammensetzung,

Vernetzungsdichte und Temperatur auf die Grenzflächenverhalten der Mikrogele zu

untersuchen, temperatur-sensitive Copolymermikrogele Poly(N-vinylcaprolactam-co-

N-isopropylacrylamide) (PVCL/NIPAm), Poly(N-vinylcaprolactam-co-N-

isopropylmethacrylamide) (PVCL/NIPMAm) und Homopolymeranaloga mit

unterschiedlicher Zusammensetzung und Vernetzungsgerad wurden synthetisiert und

charakterisiert.

Die Grenzfläschenaktivität der Copolymermikrogele wurden mit einem

Pendant-Drop-Tensiometer gemessen. Die Grenzfläschenspannung IFT der

Copolymermikrogele liegen zwischen den beiden IFT der Homopolymermikrogelen.

Dies bedeutet, dass die chemische Struktur die Grenzfläschenaktivität der

Copolymermikrogele beeinflusst. Die dynamische Grenzflächenspannung von

Mikrogelen mit niedrigerer Vernetzungsdichte verändert sich schneller als die

vonMikrogele mit höherer Vernetzungsdichte. Dies stimmt mit den

Simulationsergebnissen überein, dass die leicht vernetzten Mikrogele schneller an der

Luft/Flüssigkeit und Flüssigkeit/Flüssigkeit Grenzflächen als die dicht vernetzten

Mikrogele verbreiten, da die leicht vernetzten Mikrogele leichter verformbar sind

Weiterhin beweisen die Messungen mit einem Rasterkraftmikroskop, dass die

Verformbarkeit der Mikrogele auch an der Luft/Feststoff Grenzfläche mit dem

abnehmenden Vernetzungsgrad zunimmt. Außerdem ist die Abnahme der

dynamischen Grenzflächenspannung für PVCL/NIPAm und PVCL/NIPMAm

Kurzfassung

II

Mikrogelen bei T<VPTT (Volumenphasenübergangstemperatur) schneller als bei

T>VPTT, da die Verformbarkeit der Mikrogele sich mit steigender Temperatur

verschlechtert. Im weitern zeigen die Abmessungen der Dimensionen der Mikrogele

in zwei Zustände: auf einem festen Substrat und in einer Lösung , dass die Adsorption

der Mikrogele an eine Grenzfläche zu einer stärkeren Verformung als die Schwellung

in einer Lösing führt.

Abstract

III

Abstract

The interfacial activity is an important property of microgels which consist of

intramolecular cross-linked polymer networks, as it is involved in applications like the

fabrication of films, emulsions or capsules. To investigate the influence of the

chemical composition, cross-linking density and temperature on the interfacial

behavior of microgels, thermo-sensitive poly(N-vinylcaprolactam-co-N-

isopropylacrylamide) (PVCL/NIPAm), poly(N-vinylcaprolactam-co-N-isopropyl-

methacrylamide) (PVCL/NIPMAm) copolymer microgels and homopolymer

analogues were synthesized, and their properties were determined.

The interfacial activities of PVCL/NIPAm and PVCL/PNIPMAm copolymer

microgels were measured with a pendant drop tensiometer. The interfacial behaviors

of the copolymer microgels fall between that of homopolymer microgels. It means

that the chemical structure contributes to the variation in the interfacial tension. The

dynamic interfacial tension of microgels with lower cross-linking densities evolves

faster than that of microgels with higher cross-linking densities. This is consistent

with the simulation results that loosely cross-linked microgels spread faster at the

air/liquid and liquid/liquid interfaces than densely cross-linked ones. Meanwhile, the

dimensions of the adsorbed microgels on a solid substrate characterized with atomic

force microscopy indicate that the deformability of the microgels at the air/solid

interface also increases with the decreasing cross-linking degree. Besides, the

evolution of the dynamic interfacial tension for PVCL/NIPAm and PVCL/NIPMAm

microgels at T<VPTT (volume phase transition temperature) is faster than that at

T>VPTT because of the reducing microgel deformability with increasing temperature.

Furthermore, the comparison between the dimensions of microgels on the solid

Abstract

IV

substrate and in solution shows that adsorption of microgels induces higher

deformation than the swelling-induced expansion.

List of Abbreviations and Symbols

V


Chemicals

AAc acrylic acid

AAEM acetoacetoxyethyl methacrylate

ACMA 2, 2’-azobis(N-(2-carboxyethyl)-2-methyl-propionamidine)

AMPA 2, 2’-azobis(2-methylpropionamidine) dihydrochloride

BIS N, N’-methylenebisacrylamide

CTAB cetyltrimethylammonium bromide

NIPAm N-isopropylacrylamide

NIPMAm N-isopropylmethacrylamide

MAA methacrylic acid

PCL polycaprolactone

SDS sodium dodecyl sulfate

VCL N-vinylcaprolactam

VIm vinylimidazole

Instruments and methods

AFM Atomic Force Microscopy

DLS Dynamic Light Scattering

DSC Differential Scanning Calorimetry

FESEM Field Emission Scanning Electron Microscopy

JKR model Johnson-Kendall-Roberts model

NMR Nuclear Magnetic Resonance

PDT Pendent Drop Tensiometry

PSD Power Spectral Density

SANS Small-Angle Neutron Scattering

SLS Static Light Scattering

STEM Scanning Transmission Electron Microscopy

TEM Transmission Electron Microscopy

UV Ultraviolet


VI

General abbreviations and symbols

D Diffusion Coefficient

E' Storage Modulus

E" Loss Modulus

Volume Fraction of Polymer in Microgels

Effective Volume Fraction of Polymer in Microgels

G Shear Modulus

γ Interfacial Tension / Surface Tension

h Height

HIPE High Internal Phase Emulsion

LCST Lower Critical Solution Temperature

Mw Molecular Weight

MWCO Molecular Weight Cut-off

o/w emulsion Oil in Water Type Emulsion

Rcont Contact Radius

Rh Hydrodynamic Radius

Rp Radius of the Particle

Rs Microgel Radius by SLS

σsurf Gaussian Width of Microgel Surface

T Temperature

t Time

V:N Molar Ratio of VCL/NIPAm

V:NM Molar Ratio of VCL/NIPMAm

VPTT Volume Phase Transition Temperature

w/o emulsion Water in Oil Type Emulsion

Chapter 1. Introduction

1


1.1. Microgels

Microgels can be defined as a class of colloidal dispersions of gel particles

composed of cross-linked polymer network, with the particle size range of 10 – 1000

nm.[1]

They have been increasingly attractive in colloidal science and technology. As

an important class of soft matter, microgels find many applications in different fields

such as coating, medicine delivery and emulsion stabilization.[2-4]

Most of the aqueous

microgel systems are based on poly(N-isopropylacrylamide) (PNIPAm), poly(N-

vinylcaprolactam) (PVCL), poly(acrylic acid) (PAA) or poly(2-(diethylamino)ethyl

methacrylate) (PDEA).[1-4]

The special attention is focused on PNIPAm- or PVCL-

based microgels because of their temperature-responsive property which enables the

fabrication of stimuli-responsive materials.[2, 5]

1.1.1. Synthesis of microgels

The microgel synthesis is aimed to control the particle size and size

distribution, the colloidal stability and the selective incorporation of functional groups

in microgels.[1-3]

The possible synthetic routs for microgel preparation can be

summarized as following: (1) polymerization from monomers and cross-linkers in

homogeneous phase or in micro-droplets; (2) physical self-assembly or post cross-

linking of polymers in homogeneous phase or in micro-droplets; and (3) disruption

from macrogel by mechanical grinding or photolithographic techniques.[1, 3]

Figure 1.1 lists the vinyl monomers that are used in the preparation of

microgels investigated in this thesis. Among these monomers, N-vinylcaprolactam

(VCL), N-isopropylacrylamide (NIPAm) and N-isopropylmethacrylamide (NIPMAm)


2

are temperature-sensitive and nonionic. Acetoacetoxyethyl methacrylate (AAEM) is a

nonionic group but can react in a wide range of post modification reactions.[6]

Vinylimidazole (VIm) contains cationic group and acrylic acid (AAc) contains

anionic group. A bifunctional monomer N, N’-methylenebisacrylamide (BIS) with

two double bonds is the most widely used cross-linker for microgel synthesis (Figure

1.2).[1]

NO

N-Vinylcaprolactam (VCL)

O

O

O O

O

Acetoacetoxyethyl methacrylate (AAEM)

NH

O

N-isopropylacrylamide (NIPAm)

NH

O

N-isopropylmethacrylamide (NIPMAm)

N

N

Vinylimidazole (VIm)

OH

O

Acrylic acid (AAc)

Figure 1.1. Chemical structures of monomers used for microgel synthesis.

HN

HN

O O

N, N’-methylenebisacrylamide (BIS)

Figure 1.2. Chemical structure of cross-linker used for microgel synthesis.


3

Generally, the syntheses of microgels from monomers are initiated by ionic

radical initiators. Peroxide- and azo-based compounds are two kinds of initiators

commonly employed in microgel synthesis. For the synthesis of PNIPAm or

PNIPMAm microgels, potassiumperoxodisulfate (KPS) is frequently used.[1, 7-11]

However, the acidic environment caused by the decomposition of KPS can lead to the

hydrolysis of VCL.[12]

Thus, for the synthesis of microgels containing VCL unit, azo-

based initiators are frequently used, and two of them are shown in Figure 1.3. As

reported, the cationic initiator 2, 2’-azobis(2-methylpropionamidine) dihydrochloride

(AMPA) is an effective initiator for the synthesis of nonionic PVCL,[13]

PVCL/NIPAm[14]

and PVCL/AAEM,[6]

and cationic PVCL/AAEM/VIm microgels.[5]

For the incorporation of anionic functional group acrylic acid (AAc) into VCL-based

microgels by copolymerization, the cationic initiator AMPA is not suitable, due to its

using can cause coagulation during the particle formation. Therefore the cationic

initiator 2, 2’-azobis(N-(2-carboxyethyl)-2-methyl-propionamidine) (ACMA) instead

of AMPA is used in the synthesis of PVCL/AAEM/AAc microgels.[15]

N

N

H

H

H

N

N

H

H

H

Cl Cl

2, 2’-azobis(2-methylpropionamidine) dihydrochloride (AMPA)

N

N

NH

OH

ONH

HN

NH

HO

O

2, 2’-azobis(N-(2-carboxyethyl)-2-methyl-propionamidine) (ACMA)

Figure 1.3. Chemical structures of initiators used for microgel synthesis.


4

A powerful method for the synthesis of temperature-responsive microgels is

precipitation polymerization. During the synthesis, the monomer(s), cross-linker, and

initiator are dissolved in water. At the typical polymerization temperature (50 –

80 °C), free radicals are produced by the decomposition of initiator.[11]

Then the

polymerization is started with the attack of radicals on monomers, followed by chain

growth. As the polymerization temperature is much higher than the lower critical

solution temperature (LCST) of temperature-sensitive polymers (32 °C for both

PNIPAm and PVCL, and 43 °C for PNIPMAm), the growing polymer chains collapse

when a critical chain length is achieved and form precursor particles.[1-3, 14]

A

schematic mechanism of precipitation polymerization is presented in Figure 1.4.

Figure 1.4. The formation of microgels through precipitation polymerization.[3]

Reprinted with permission, copyright 2010 Springer.

It is proposed that the precursor particles grow by different ways: (1)

aggregation to form larger colloidally stable particles; (2) deposition onto formed

polymer particles; and/or (3) addition of monomers or macroradicals.[3]

During the

synthesis, the growing particles are shrunk and stabilized by electrostatic charges

originate from the initiator. Though in collapsed state, microgel particles still contain

a lot of water.[16]

Based on this, the precipitation polymerization is different from the

classical emulsion polymerization of water-insoluble monomers, which produces

latex particles with compact structures.[17]

After the polymerization is completed, the


5

reaction mixture is cooled down to room temperature (below LCST of polymer

chains). Then, microgel particles become swollen and develop a “hairy”

morphology,[3, 7]

which will be discussed in detail hereinafter. The stability of

microgel in swollen state relies on the steric mechanism through the formation of

hydrogen bonds between polymer segments and water molecules.[3, 16, 18]

The

precipitation polymerization offers numerous advantages for microgel preparation. It

is able to produce dispersions with low polydispersity and to control parameters such

as particle size,[19]

charge,[5, 15, 20]

and cross-link density.[21, 22]

It should be noted that

there are still some limitations for precipitation polymerization, such as high reaction

temperature, difficulty in synthesis of particles below 50 nm, and formation of sol

fration during the polymerization.[3]

The size of microgels can be controlled by the amount of surfactant added in

the synthesis.[23-25]

In the reaction mixture, the surfactant molecules adsorb on

precursor particles and make them more stable. With more surfactants added in the

synthesis, more precursor particles survive from coagulation.[23]

As a certain amount

of monomers, more growing particles means smaller sizes they are.[25]

The cationic

surfactant cetyltrimethylammonium bromide (CTAB) and anionic surfactant sodium

dodecyl sulfate (SDS) are commonly used in microgel synthesis. Figure 1.5 exhibits

the chemical structures of CTAB and SDS. As expected, the reported results show

that with the increase in the concentration of CTAB or SDS, the hydrodynamic size of

microgels decrease.[11, 14, 24]

Moreover, the presence of surfactant effectively prevents

the coagulation which leads to micro-scale particle and large polydispersity.[14, 26]

After the synthesis, the surfactant can be removed from the microgel solution by

centrifugation or dialysis.[25]

In addition, the size of microgels in surfactant-free


6

polymerization can be regulated by the temperature and concentration of monomers

or cross-linkers.[3]

N Br

Cetyltrimethylammonium bromide (CTAB)

O

S

O

OO

Na

Sodium dodecyl sulfate (SDS)

Figure 1.5. Two common surfactants used for the synthesis of micrgels.

1.1.2. Internal structure of microgels

Microgels consist of chemically intramolecular cross-linked polymer network.

Such kind of cross-linked internal structure makes microgels characteristically

different from other colloidal particles like polymer latex or colloidal supramolecular

aggregates.[17, 27]

The cross-linked polymer network provides microgels both swelling

capability in a good solvent and topological integrity.[28]

So far, the microgel

structures have been thoroughly investigated by scattering methods, differential

scanning calorimetry, rheology, nuclear magnetic resonance spectroscopy (NMR),

and atomic force microscopy (AFM).[13, 22, 28-32]

With higher reactivity, the cross-

linker (BIS) is normally consumed faster than monomers during synthesis. Therefore,

the distribution of cross-linker in microgels is inhomogeneous. Through small-angle

neutron scattering (SANS) as well as static light scattering (SLS) and dynamic light

scattering (DLS), the microgel structure has been proved to be a radial profile, where

the density of cross-linker decreases from the center towards the surface of

microgels.[7, 28, 33]


7

Due to the intrinsic cross-linked polymer network, microgels can undergo

volume phase transitions in response to environmental changes, such as temperature,

pH, solvent composition, ionic strength, and type of surfactant.[34-40]

Microgels

composed of temperature-responsive polymer like PNIPAm, PNIPMAm or PVCL,

experience a volume phase transition upon heating above a critical temperature close

to the LCST of corresponding linear polymers.[41, 42]

This typical temperature is called

volume phase transition temperature (VPTT) of microgels. Below VPTT, microgels

are swollen by solvent which is water in aqueous system. As temperature increases

above VPTT, a part of water is expelled from the inside of microgels due to the break

of hydrogen bond between water and polymer and increase in hydrophobic

interactions, which makes microgels shrink.[18, 41]

If containing charged groups,

microgels are also sensitive to the variation in pH of environment. For example, as pH

decrease from 6 to 4, more VIm groups in PVCL/AAEM/VIm copolymer microgel

are ionized. As a result, PVCL/AAEM/VIm microgels swell to a higher extent, due to

the increasing repulsiveness among more positive charged groups.[5]

It should be

noted that even in collapsed state, microgels still retain considerable amount of water

inside.[16, 41]

Figure 1.6 presents some important features of microgels, including the cross-

linked polymer network and radial profile with decreasing cross-linker density

towards the surface, as well as temperature- and pH-responsiveness. With such a

unique structure, microgels are highly porous, compliant and stimuli-responsive.

Upon these properties, microgels have great potential in applications like surface

coating, drug delivery, and emulsion stabilizer or capsule shells.[2, 4, 29, 43-46]

When

used in coatings and emulsions, the interfacial behavior of microgel is very important.


8

Figure 1.6. Some key features of microgels: internal structure and responsiveness to

environments like changes in temperature or pH.[3, 5-7, 47]

Reprinted with permission,

copyright 2004 American Institute of Physics, 2009 Royal Society of Chemistry, and

2010 Springer.

1.2. Nanoparticles at interfaces

The behavior of nanoparticles adsorbed at interfaces (air/liquid, liquid/liquid

and liquid/solid) has been extensively investigated, because of the wide application of

such particles in colloidal science and technology.[48]

One of the applications

concerning the interfacial behavior of nanoparticles is the famous “Pickering

emulsion”.[49, 50]

1.2.1. Conventional solid particles

For surfactant molecules in oil/water mixture systems, the hydrophilic-

lipophilic balance (HLB) determines the position of aggregated surfactants, whether

in water, oil or a third phase.[49]

In the case of rigid solid colloidal particles, the


9

position mainly depends on the contact angle which is made by the particle with

the interface. Figure 1.7 describes the positions of particles at the oil/water interface

with different . For hydrophilic particles, is normally less than 90o and a

larger part of the particle surface stays in water than in oil. In contrast, for

hydrophobic particles, is greater than 90o and a larger part of the particle surface

resides in oil than in water. The anchoring of particles to the interface is very strong,

which makes the particle-stabilized emulsions stable. The energy required to

remove of particle from the interface can be estimated by Equation 1.1.[49, 51, 52]

(Equation 1.1)

where is the radius of particle, is the oil/water interfacial tension. As the energy

for detachment of a particle from the interface is much higher than the thermal energy

, the particle is usually considered to be irreversibly attached.[52]

However, if

particles are very small (< 1 nm in radius), comparable to small surfactant molecules,

is comparable to thermal energy. Consequently, a constant particle exchange

occurs at the interface due to the displacement for extremely small particles.[49, 51]

In several recent reviews, some general rules have been summarized for

producing particle-stabilized emulsions: (1) particles should be partially wettable by

both oil and water; (2) the continuous phase of the emulsion is generally the one in

which a larger fraction of the particle surface resides; and (3) the interactions between

particles play an important role in emulsion-stabilizing.[52, 53]

The interfacial behavior

of particles can be controlled by size, shape, wettability, roughness, and the

interaction between particles. As a result, the properties of emulsions stabilized by

solid particles can be manipulated, to which the viscoelasticity of the interface is

highly relevant.[48-52, 54-59]


10

Figure 1.7. Solid particles at the oil/water interface. Upper part shows the position of

a spherical particle at a planner oil/water interface for contact angle (left),

(center) and (right). Lower part shows the corresponding

position of particles at a curved oil/water interface.[49]

Reprinted with permission,

copyright 2003 Elsevier.

1.2.2. Soft microgel particles

With an internal soft and porous structure, microgel particles have distinctly

different interfacial properties from classical rigid solid particles. Microgels, spherical

in solution, are strongly deformed and flattened at the oil/water interface.[60, 61]

It has

been revealed that the aqueous-born microgel particles reside mainly in the water

phase for both oil-in-water and water-in-oil emulsions.[52, 61]

Figure 1.8 presents a

schematic of microgel particles at the oil/water interface, which shows the differences

as compared to the interfacial behavior of rigid solid particles in Pickering emulsion

systems exhibited in Figure 1.7. Unlike rigid solid particles, the exact location of

oil/water interface is not clear for microgels. To some extent, the oil/water interface

could slightly penetrate inside the porous microgel.

A remarkable merit for microgel-stabilized emulsions or microgel-based

capsules is the sensitivity to environmental changes brought by stimuli-responsive


11

microgels, such as temperature-responsive PNIPAm or PVCL microgels.[43, 44, 62-66]

Stability of such emulsion systems is strongly reduced as temperature increases above

VPTT of microgels, which might lead to the break of emulsions.[44, 62, 63]

The collapse

of microgels incorporated in capsules causes contraction of the whole capsules, which

is very useful in the application of drug release.[43]

For microgels copolymerized with

ionic monomers like methacrylic acid (MAA), the interfacial property depends on pH,

ionic strength and oil polarity.[44, 64-67]

Figure 1.8. Left part shows the FreSCa cryo-SEM image of microgels at the

water/heptane interface viewed from the oil side, and the schematic representation of

microgel at the interface. Right part schematically describes the microgels at the

interface of oil in water (upper right) and water in oil (lower right) emulsions.[52, 61]

Reprinted with permission, copyright 2012 American Chemical Society.

The structure of latex particles at fluid/fluid interface can be directly observed

in situ through optical microscopy.[68, 69]

It has been also successfully employed for

microgel-stabilized emulsions.[70]

However, the resolving capability of optical

microscopy is limited. To understand the detail structure of microgels at the interface,

freeze-fraction cryo-scanning electron microscopy (cryo-SEM) is employed

usually.[60, 71, 72]

Through the observation on the surface of emulsion droplet stabilized


12

by PNIPAm/MAA microgels, several important features of the assembly structure are

summarized: microgels are highly deformed, interpenetrate, and are linked by

filaments.[52]

It can be easily related to the intrinsic “fluffy” structure of microgels as

shown in Figure 1.6. Recently, a special technique freeze-fracture shadow-casting

cryo-SEM (FreSCa) was successfully used on the study of assembly of

PNIPAm/MAA microgels adsorbed at a flat oil/water interface.[61]

The soft microgel

particles are also proved to be deformed with the surface chains stretching out at the

interface.

Besides, microgel particles attached on a solid surface which can be

considered as the air/solid interface have been widely investigated by AFM.[73-75]

From the information provided by AFM measurement (the morphology and

dimensions like contact radius and height), the conformation of microgels upon

deformation can be obtained. From the theoretical point, the classic model deduced by

Johnson, Kendall and Roberts (JKR model) clearly describes the deformation of rigid

particles on a solid substrate (adhesion between elastic bodies).[76-78]

However, for

small soft particles microgels which highly deform upon particle adsorption, the JKR

model is not applicable. Recent theoretical effort provided a new scaling relationship

for nanoparticle deformation where the surface energy of the particle is considered

besides the other particle parameters which are all included in JKR model.[79]

It is

demonstrated that the JKR theory is suitable at the small deformation, whereas

significant deviation from the JKR theory occurs at the high deformation (comparable

with the particle size).

1.3. Challenges in the microgel research

The microgel research has been growing fast as microgels have specific

structures and wide applications. To increase the potential of microgels in both basic


13

research and practical applications, several key challenges need to be addressed.

Firstly, new controlled synthesis route of monodisperse microgels with specific

functional monomers and structures can expand the classes of microgels with broader

applications.[1, 3]

Secondly, a precise elucidation of the internal particle structure is

necessary for better understanding properties of microgels.[2]

Thirdly, the fabrication

of novel microgel-based hybrids, such as nanoparticle/microgel, drug/microgel and

polymer/microgel complexes, can be used in catalytic and release fields.[3, 80]

Finally,

new applications like microgel-stabilized emulsion, microgel-based capsules and

porous scaffolds, need to be paid more attention.[43, 52, 81, 82]

So far, a lot of work has been done in the microgel research. Apart from the

conventional emulsion- or precipitation-polymerization, microfluidics provides

significant potential in producing monodisperse microgel particles efficiently.[83]

Photo-cross-linking is another novel method to prepare microgels by irradiation of

photo-cross-linkable copolymers.[84]

Moreover, it is also very important to control the

spatial distribution of functional groups inside the microgel particles during

synthesis.[4]

Light and neutron scattering methods are very helpful in determining the

core-corona structure of microgels.[28]

Besides that, NMR characterization has also

been successfully used to detect the heterogeneous morphology of microgels.[13, 26]

Microgels can be applied to catalyst carrier and drug delivery. Hybrids of functional

microgels and metal nanoparticles or proteins like enzymes have been produced.[15, 80]

Furthermore, microgels can also be used to stabilize emulsions which are

environmental responsive.[52]

Other novel applications, such as microgel-based

capsules and three dimensional scaffolds, have been developed as well.[43, 81, 82]

These

new forms of materials greatly extend the applications of microgels.


14

In spite of the big achievement, further investigation and concerted effort are

still required to figure out the complicated relationship between microgel structures

and properties to develop new materials and microgel-based systems.

1.4. References

[1] Pelton R. and Hoare T., Microgels and their synthesis: An introduction In

Microgel Suspensions; Wiley-VCH Verlag GmbH & Co. KGaA: 2011, p 1-32.

[2] Saunders B. R., Laajam N., Daly E., Teow S., Hu X. H. and Stepto R.,

Microgels: From responsive polymer colloids to biomaterials, Advances in Colloid

and Interface Science, 2009, 147-48, 251-262.

[3] Pich A. and Richtering W., Microgels by precipitation polymerization:

Synthesis, characterization, and functionalization In Chemical Design of Responsive

Microgels; Pich A. and Richtering W., Eds. 2010; Vol. 234, p 1-37.

[4] Richtering W. and Pich A., The special behaviours of responsive core-shell

nanogels, Soft Matter, 2012, 8, 11423-11430.

[5] Pich A., Tessier A., Boyko V., Lu Y. and Adler H. J. P., Synthesis and

characterization of poly(vinylcaprolactam)-based microgels exhibiting temperature

and pH-sensitive properties, Macromolecules, 2006, 39, 7701-7707.

[6] Boyko V., Pich A., Lu Y., Richter S., Arndt K. F. and Adler H. J. P., Thermo-

sensitive poly(N-vinylcaprolactam-co-acetoacetoxyethyl methacrylate) microgels: 1 -

synthesis and characterization, Polymer, 2003, 44, 7821-7827.

[7] Stieger M., Richtering W., Pedersen J. S. and Lindner P., Small-angle neutron

scattering study of structural changes in temperature sensitive microgel colloids,

Journal of Chemical Physics, 2004, 120, 6197-6206.

[8] Meyer S. and Richtering W., Influence of polymerization conditions on the

structure of temperature-sensitive poly(N-isopropylacrylamide) microgels,

Macromolecules, 2005, 38, 1517-1519.

[9] Berndt I., Pedersen J. S. and Richtering W., Structure of multiresponsive

"intelligent" core-shell microgels, Journal of the American Chemical Society, 2005,

127, 9372-9373.

[10] Berndt I., Pedersen J. S. and Richtering W., Temperature-sensitive core-shell

microgel particles with dense shell, Angewandte Chemie-International Edition, 2006,

45, 1737-1741.


15

[11] Pelton R. H. and Chibante P., Preparation of aqueous lattices with N-

isopropylacrylamide, Colloids and Surfaces, 1986, 20, 247-256.

[12] Imaz A. and Forcada J., Synthesis strategies to incorporate acrylic acid into N-

vinylcaprolactam-based microgels, Journal of Polymer Science Part A: Polymer

Chemistry, 2011, 49, 3218-3227.

[13] Balaceanu A., Demco D. E., Moeller M. and Pich A., Microgel heterogeneous

morphology reflected in temperature-induced volume transition and (1)H high-

resolution transverse relaxation NMR. The case of poly(N-vinylcaprolactam)

microgel, Macromolecules, 2011, 44, 2161-2169.

[14] Balaceanu A., Mayorga V., Lin W., Schürings M.-P., Demco D., Böker A.,

Winnik M. and Pich A., Copolymer microgels by precipitation polymerisation of N-

vinylcaprolactam and N-isopropylacrylamides in aqueous medium, Colloid &

Polymer Science, 2013, 291, 12-31.

[15] Agrawal G., Schurings M. P., van Rijn P. and Pich A., Formation of

catalytically active gold-polymer microgel hybrids via a controlled in situ reductive

process, Journal of Materials Chemistry A, 2013, 1, 13244-13251.

[16] Pelton R., Temperature-sensitive aqueous microgels, Advances in Colloid and

Interface Science, 2000, 85, 1-33.

[17] Tauer K., Hernandez H., Kozempel S., Lazareva O. and Nazaran P., Towards

a consistent mechanism of emulsion polymerization - new experimental details,

Colloid and Polymer Science, 2008, 286, 499-515.

[18] Sun S. T. and Wu P. Y., Infrared spectroscopic insight into hydration behavior

of poly(N-vinylcaprolactam) in water, Journal of Physical Chemistry B, 2011, 115,

11609-11618.

[19] Blackburn W. H. and Lyon L. A., Size-controlled synthesis of monodisperse

core/shell nanogels, Colloid and Polymer Science, 2008, 286, 563-569.

[20] Ito S., Ogawa K., Suzuki H., Wang B. L., Yoshida R. and Kokufuta E.,

Preparation of thermosensitive submicrometer gel particles with anionic and cationic

charges, Langmuir, 1999, 15, 4289-4294.

[21] Varga I., Gilanyi T., Meszaros R., Filipcsei G. and Zrinyi M., Effect of cross-

link density on the internal structure of poly(N-isopropylacrylamide) microgels,

Journal of Physical Chemistry B, 2001, 105, 9071-9076.

[22] Senff H. and Richtering W., Influence of cross-link density on rheological

properties of temperature-sensitive microgel suspensions, Colloid and Polymer

Science, 2000, 278, 830-840.

[23] Wu X., Pelton R. H., Hamielec A. E., Woods D. R. and McPhee W., The

kinetics of poly(N-isopropylacrylamide) microgel latex formation, Colloid and

Polymer Science, 1994, 272, 467-477.


16

[24] Andersson M. and Maunu S. L., Structural studies of poly(N-

isopropylacrylamide) microgels: Effect of SDS surfactant concentration in the

microgel synthesis, Journal of Polymer Science Part B: Polymer Physics, 2006, 44,

3305-3314.

[25] Kleinen J., Microgels and polyelectrolytes, PhD Thesis, RWTH Aachen

University, Aachen, Germany, 2012.

[26] Balaceanu A., Synthesis, morphology and dynamics of microgels with different

architectures, PhD Thesis, RWTH Aachen University, Aachen, Germany, 2013.

[27] Stuart M. A. C., Supramolecular perspectives in colloid science, Colloid and

Polymer Science, 2008, 286, 855-864.

[28] Richtering W., Berndt I. and Pedersen J. S., Determination of microgel

structure by small-angle neutron scattering In Microgel Suspensions; Wiley-VCH

Verlag GmbH & Co. KGaA: 2011, p 117-132.

[29] Saunders B. R. and Vincent B., Microgel particles as model colloids: theory,

properties and applications, Advances in Colloid and Interface Science, 1999, 80, 1-

25.

[30] Senff H. and Richtering W., Temperature sensitive microgel suspensions:

Colloidal phase behavior and rheology of soft spheres, Journal of Chemical Physics,

1999, 111, 1705-1711.

[31] Hofl S., Zitzler L., Hellweg T., Herminghaus S. and Mugele F., Volume phase

transition of "smart" microgels in bulk solution and adsorbed at an interface: A

combined AFM, dynamic light, and small angle neutron scattering study, Polymer,

2007, 48, 245-254.

[32] Hashmi S. M. and Dufresne E. R., Mechanical properties of individual

microgel particles through the deswelling transition, Soft Matter, 2009, 5, 3682-3688.

[33] Reufer M., Dıaz-Leyva P., Lynch I. and Scheffold F., Temperature-sensitive

poly(N-Isopropyl-Acrylamide) microgel particles: A light scattering study, The

European Physical Journal E, 2009, 28, 165-171.

[34] Flory P. J. and Rehner J., Statistical mechanics of cross-linked polymer

networks II. Swelling, Journal of Chemical Physics, 1943, 11, 521-526.

[35] Saunders B. R., Crowther H. M. and Vincent B., Poly[(methyl methacrylate)-

co-(methacrylic acid)] microgel particles: Swelling control using pH, cononsolvency,

and osmotic deswelling, Macromolecules, 1997, 30, 482-487.

[36] Crowther H. M. and Vincent B., Swelling behavior of poly N-

isopropylacrylamide microgel particles in alcoholic solutions, Colloid and Polymer

Science, 1998, 276, 46-51.


17

[37] Debord J. D. and Lyon L. A., Synthesis and characterization of pH-responsive

copolymer microgels with tunable volume phase transition temperatures, Langmuir,

2003, 19, 7662-7664.

[38] Lietor-Santos J.-J., Sierra-Martin B., Vavrin R., Hu Z., Gasser U. and

Fernandez-Nieves A., Deswelling microgel particles using hydrostatic pressure,

Macromolecules, 2009, 42, 6225-6230.

[39] Oh K. S. and Bae Y. C., Swelling behavior of submicron gel particles, Journal

of Applied Polymer Science, 1998, 69, 109-114.

[40] Sierra-Martin B., Lietor-Santos J. J., Fernandez-Barbero A., Nguyen T. T. and

Fernandez-Nieves A., Swelling thermodynamics of microgel particles In Microgel

Suspensions; Wiley-VCH Verlag GmbH & Co. KGaA: 2011, p 71-116.

[41] Wu C., Zhou S. Q., Auyeung S. C. F. and Jiang S. H., Volume phase transition

of spherical microgel particles, Angewandte Makromolekulare Chemie, 1996, 240,

123-136.

[42] Wu C., A comparison between the 'coil-to-globule' transition of linear chains

and the "volume phase transition" of spherical microgels, Polymer, 1998, 39, 4609-

4619.

[43] Berger S., Zhang H. P. and Pich A., Microgel-based stimuli-responsive

capsules, Advanced Functional Materials, 2009, 19, 554-559.

[44] Brugger B., Rosen B. A. and Richtering W., Microgels as stimuli-responsive

stabilizers for emulsions, Langmuir, 2008, 24, 12202-12208.

[45] Bai Y. Y., Zhang Z., Deng M. X., Chen L., He C. L., Zhuang X. L. and Chen

X. S., Thermo- and pH-responsive microgels for controlled release of insulin,

Polymer International, 2012, 61, 1151-1157.

[46] Bysell H., Mansson R., Hansson P. and Malmsten M., Microgels and

microcapsules in peptide and protein drug delivery, Advanced Drug Delivery Reviews,

2011, 63, 1172-1185.

[47] Heyes D. M. and Branka A. C., Interactions between microgel particles, Soft

Matter, 2009, 5, 2681-2685.

[48] Bresme F. and Oettel M., Nanoparticles at fluid interfaces, Journal of Physics-

Condensed Matter, 2007, 19.

[49] Aveyard R., Binks B. P. and Clint J. H., Emulsions stabilised solely by

colloidal particles, Advances in Colloid and Interface Science, 2003, 100, 503-546.

[50] Vignati E., Piazza R. and Lockhart T. P., Pickering emulsions: Interfacial

tension, colloidal layer morphology, and trapped-particle motion, Langmuir, 2003, 19,

6650-6656.


18

[51] Boker A., He J., Emrick T. and Russell T. P., Self-assembly of nanoparticles at

interfaces, Soft Matter, 2007, 3, 1231-1248.

[52] Richtering W., Responsive emulsions stabilized by stimuli-sensitive microgels:

Emulsions with special non-Pickering properties, Langmuir, 2012, 28, 17218-17229.

[53] Leal-Calderon F. and Schmitt V., Solid-stabilized emulsions, Current Opinion

in Colloid & Interface Science, 2008, 13, 217-227.

[54] Binks B. P. and Clint J. H., Solid wettability from surface energy components:

Relevance to pickering emulsions, Langmuir, 2002, 18, 1270-1273.

[55] Binks B. P. and Lumsdon S. O., Pickering emulsions stabilized by

monodisperse latex particles: Effects of particle size, Langmuir, 2001, 17, 4540-4547.

[56] Georgieva D., Schmitt V., Leal-Calderon F. and Langevin D., On the possible

role of surface elasticity in emulsion stability, Langmuir, 2009, 25, 5565-5573.

[57] Madivala B., Vandebril S., Fransaer J. and Vermant J., Exploiting particle

shape in solid stabilized emulsions, Soft Matter, 2009, 5, 1717-1727.

[58] San-Miguel A. and Behrens S. H., Influence of nanoscale particle roughness

on the stability of Pickering emulsions, Langmuir, 2012, 28, 12038-12043.

[59] Cui M. M., Emrick T. and Russell T. P., Stabilizing liquid drops in

nonequilibrium shapes by the interfacial jamming of nanoparticles, Science, 2013,

342, 460-463.

[60] Destribats M., Lapeyre V., Wolfs M., Sellier E., Leal-Calderon F., Ravaine V.

and Schmitt V., Soft microgels as Pickering emulsion stabilisers: Role of particle

deformability, Soft Matter, 2011, 7, 7689-7698.

[61] Geisel K., Isa L. and Richtering W., Unraveling the 3D localization and

deformation of responsive microgels at oil/water interfaces: A step forward in

understanding soft emulsion stabilizers, Langmuir, 2012, 28, 15770-15776.

[62] Ngai T., Behrens S. H. and Auweter H., Novel emulsions stabilized by pH and

temperature sensitive microgels, Chemical Communications, 2005, 331-333.

[63] Monteux C., Marliere C., Paris P., Pantoustier N., Sanson N. and Perrin P.,

Poly(N-isopropylacrylamide) microgels at the oil-water interface: Interfacial

properties as a function of temperature, Langmuir, 2010, 26, 13839-13846.

[64] Li Z. F. L. Z. F. and Ngai T., Stimuli-responsive gel emulsions stabilized by

microgel particles, Colloid and Polymer Science, 2011, 289, 489-496.

[65] Brugger B. and Richtering W., Emulsions stabilized by stimuli-sensitive

poly(N-isopropylacrylamide)-co-methacrylic acid polymers: Microgels versus low

molecular weight polymers, Langmuir, 2008, 24, 7769-7777.


19

[66] Ngai T., Auweter H. and Behrens S. H., Environmental responsiveness of

microgel particles and particle-stabilized emulsions, Macromolecules, 2006, 39, 8171-

8177.

[67] Sun G. Q., Li Z. F. and Ngai T., Inversion of particle-stabilized emulsions to

form high-internal-phase emulsions, Angewandte Chemie-International Edition, 2010,

49, 2163-2166.

[68] Tarimala S. and Dai L. L., Structure of microparticles in solid-stabilized

emulsions, Langmuir, 2004, 20, 3492-3494.

[69] Tarimala S., Ranabothu S. R., Vernetti J. P. and Dai L. L., Mobility and in situ

aggregation of charged microparticles at oil-water interfaces, Langmuir, 2004, 20,

5171-5173.

[70] Tsuji S. and Kawaguchi H., Thermosensitive pickering emulsion stabilized by

poly(N-isopropylacrylamide)-carrying particles, Langmuir, 2008, 24, 3300-3305.

[71] Brugger B., Rutten S., Phan K. H., Moller M. and Richtering W., The colloidal

suprastructure of smart microgels at oil-water interfaces, Angewandte Chemie-

International Edition, 2009, 48, 3978-3981.

[72] Schmidt S., Liu T. T., Rutten S., Phan K. H., Moller M. and Richtering W.,

Influence of microgel architecture and oil polarity on stabilization of emulsions by

stimuli-sensitive core-shell poly(N-isopropylacrylamide-co-methacrylic acid)

microgels: Mickering versus Pickering behavior?, Langmuir, 2011, 27, 9801-9806.

[73] Lau A. W. C., Portigliatti M., Raphael E. and Leger L., Spreading of latex

particles on a substrate, Europhysics Letters, 2002, 60, 717-723.

[74] Dreyer J. K., Nylander T., Karlsson O. J. and Piculell L., Spreading dynamics

of a functionalized polymer latex, Acs Applied Materials & Interfaces, 2011, 3, 167-

176.

[75] Horecha M., Senkovskyy V., Synytska A., Stamm M., Chervanyov A. I. and

Kiriy A., Ordered surface structures from PNIPAM-based loosely packed microgel

particles, Soft Matter, 2010, 6, 5980-5992.

[76] Johnson K. L., Kendall K. and Roberts A. D., Surface energy and the contact

of elastic solids, Proceedings of the Royal Society of London Series a-Mathematical

and Physical Sciences, 1971, 324, 301-313.

[77] Barthel E., Adhesive elastic contacts: JKR and more, Journal of Physics D-

Applied Physics, 2008, 41, 163001-163020.

[78] Liu K. K., Deformation behaviour of soft particles: a review, Journal of

Physics D-Applied Physics, 2006, 39, R189-R199.

[79] Carrillo J. M. Y., Raphael E. and Dobrynin A. V., Adhesion of nanoparticles,

Langmuir, 2010, 26, 12973-12979.


20

[80] Pich A. and Richtering W., Polymer nanogels and microgels In Polymer

Science: A Comprehensive Reference; Matyjaszewski K. and Möller M., Eds.;

Elsevier: Amsterdam, 2012, p 309-350.

[81] Cho E. C., Kim J.-W., Fernandez-Nieves A. and Weitz D. A., Highly

responsive hydrogel scaffolds formed by three-dimensional organization of microgel

nanoparticles, Nano Letters, 2008, 8, 168-172.

[82] Gan T., Guan Y. and Zhang Y., Thermogelable PNIPAM microgel dispersion

as 3D cell scaffold: effect of syneresis, Journal of Materials Chemistry, 2010, 20,

5937-5944.

[83] Hoare T., Macroscopic microgel networks In Hydrogel Micro and

Nanoparticles; Wiley-VCH Verlag GmbH & Co. KGaA: 2012, p 281-315.

[84] Vo C. D., Kuckling D., Adler H. J. P. and Schohoff M., Preparation of

thermosensitive nanogels by photo-cross-linking, Colloid and Polymer Science, 2002,

280, 400-409.

Chapter 2. Motivation and Aims

21

Chapter 2. Motivation and Aims

The interfacial activity is an important property of microgels for their

applications in fabrication of films, emulsions or capsules. Consisting of

intramolecular cross-linked polymer networks, microgels are highly deformable. Such

a property resembles the deformation of cells during, for example, the translocation

through a small vascular channel or the attachment and adhesion of cells on a solid

substrate.

The aim of the thesis is to investigate the influence of the chemical

composition, cross-linking density and temperature on the interfacial activity of

microgels. For this, thermo-sensitive PVCL/NIPAm and PVCL/NIPMAm copolymer

microgels as well as corresponding homopolymer microgels were synthesized.

Afterwards, their properties and internal structures were determined. Then, interfacial

behaviors of those microgels at different interfaces were investigated. Interfacial

activities of microgels at the air/liquid and liquid/liquid interfaces were measured with

a pendent drop tensiometer. The deformability of microgels upon the cross-linking

density was studied through dimensions of microgels on a solid substrate (detected by

AFM). The final goal is to describe the correlation between the interfacial behavior

and the microgel deformability depending upon the cross-linking density and

temperature.

Chapter 3. Synthesis of Microgels

23

Chapter 3. Synthesis of Microgels with Controlled Cross-

linking Density and Copolymer Composition

3.1. Introduction

The controlled synthesis of microgels is very useful from the point of view of

application as desired structures and properties can be achieved. By optimization of

the synthesis procedure, we can control the particle size and its distribution, the

monomer ratio, the cross-link density and the selective incorporation of functional

groups in microgels.[1-3]

So far, the mostly investigated microgel systems are based on

NIPAm or VCL because of the temperature-responsiveness.

Since 1980s, the polymerization of NIPAm-based microgels has been

extensively developed.[4-9]

Besides, NIPMAm-[10-13]

and VCL-based[14-20]

microgels

are also synthesized and their structures and properties are thoroughly investigated.

These temperature-responsive microgels were successfully copolymerized with

ionizable monomers like acrylic acid (AAc), vinylimidazole (VIm), or 4-

vinylpyridine.[17, 20-24]

In this way, the microgels are combined with pH-sensitivity,

increasing the potential for applications.[22, 25-28]

It is implied that VCL is more

biocompatible than acrylamides, due to the direct connection of amide group to

carbon-carbon backbone chain so that its hydrolysis will not form small amide

compounds.[29-31]

Thus, copolymerization of VCL and acrylamide could be an

interesting and practical method for biomedical application of thermo-sensitive

materials. Balaceanu et. al. have synthesized PVCL/NIPAm and PVCL/NIPMAm

copolymer microgels with a one-pot procedure, and found that the monomer units of

VCL and acrylamides are statistically distributed in the colloidal networks,


24

independent of the copolymer composition.[32, 33]

It is found that PVCL/NIPAm

microgels have VPTT around 35 °C independently from the copolymer composition;

while PVCL/NIPMAm microgels show a nonlinear increase of VPTT from 34 to 45

°C with the increase in NIPMAm fraction in copolymer structure.

Apart from the conventional microgels with statistically distributed monomer

units, novel core-shell microgels consisting two different chemical structures in the

core and shell parts are obtained by using a two-step synthesis procedure. For

example, the core-shell microgel systems of PNIPAm/NIPMAm are synthesized with

doubly temperature-responsiveness because the monomers have different phase

transition temperatures.[34-37]

The volume transition of such core-shell microgel can be

adjusted by the cross-link density and shell thickness. Recently, core-shell microgel

systems of VCL-core/NIPMAm-shell and NIPMAm-core/VCL-shell microgels have

been developed. They were investigated by DLS and a modified Flory-Rehner theory

on the correlated morphological changes during the volume transition temperature.[38]

In this work, we mainly focus on the copolymer microgels with a random

monomer distribution. It has been revealed that the structures and properties of

microgels strongly depend on the chemical composition and cross-linker density.[17, 33,

39] Therefore, we synthesize copolymer microgels based on VCL, NIPAm, and

NIPMAm with different monomer ratios, cross-linker densities and size distributions.

In this Chapter, besides the synthesis procedures, the characterization of microgels

like chemical composition by nuclear magnetic resonance (NMR) spectroscopy and

hydrodynamic size by DLS are presented as well. The NMR results show that the

monomer ratios in the copolymer microgels are consistent with the feeding ratios in

the synthesis. From the DLS results, it is found that the swelling property of microgels

represented by the normalized hydrodynamic radii ( ⁄ ) strongly depend


25

on the monomer ratio and the cross-linker concentration. Besides, the addition of

surfactant during the synthesis reduces the size of microgels as expected.

Furthermore, the viscosities of suspensions of microgels with different cross-linker

densities are checked with a viscosimeter. The critical mass concentration, as the

effective volume fraction is 1, increases with the cross-liker content of microgels.

Besides the neutral PVCL/acrylamide copolymer microgels, ionizable PVCL/AAEM-

based microgels are synthesized by the addition of VIm or AAc monomers. The

hydrodynamic size and zeta potential of PVCL/AAEM/VIm and PVCL/AAEM/AAc

microgels at different pH values are detected by DLS and Zetasizer, respectively. The

pH-responsiveness of these microgels indicates a successful incorporation VIm and

AAc groups into the copolymer microgels.

3.2. Experimental section

3.2.1. Materials

N-vinylcaprolactam (VCL), N-isopropylacrylamide (NIPAm) and N-

isopropylmethacrylamide (NIPMAm) (Sigma-Aldrich) were purified by high-vacuum

distillation at 80 °C before use. Acetoacetoxyethyl methacrylate (AAEM),

vinylimidazole (VIm) and acrylic acid (AAc) were obtained from Sigma-Aldrich and

used as received. The initiators 2, 2’-azobis(2-methylpropionamidine)

dihydrochloride (AMPA) (Sigma-Aldrich) and 2, 2’-azobis(N-(2-carboxyethyl)-2-

methylpropion-amidine) (ACMA) (Wako) were used as received. The cross-linker N,

N’-methylenebisacrylamide (BIS) and surfactant cetyltrimethylammonium bromide

(CTAB) and sodium dodecyl sulfate (SDS) were obtained from Sigma-Aldrich and

used as received. Deuteroxide (D2O) was obtained from KMF GmbH and used as

received.


26

3.2.2. Synthesis of microgels

PVCL/acrylamide copolymer microgels

PVCL/acrylamide copolymer microgels were synthesized according to

previously reported procedures.[32]

For copolymer microgels with different chemical

compositions, appropriate amounts of VCL, NIPAm and NIPMAm (see Tables 3.1

and 3.2) and 0.06 g of BIS (3 mol%) were added to 145 mL deionized water. To

synthesize microgels with constant chemical composition but variable cross-linking

degree, VCL (1.220 g), NIPAm (0.993 g) (VCL:NIPAm = 1:1 mol:mol) were used

and the amount of BIS was varied (0.05, 0.25, 1 and 3 mol% of monomers). To obtain

microgels with the same chemical composition and cross-linking degree but various

sizes, VCL (0.397 g), NIPMAm (1.820 g) (VCL:NIPMAm = 1:5 mol:mol) were used

and the concentration of surfactant CTAB was varied (0.37 mM and 1.10 mM). For

all synthesis, a double-wall glass reactor equipped with stirrer and reflux condenser

was purged with nitrogen. The solution of the monomers was placed into the reactor

and stirred for 1 h at 70 °C with purging with nitrogen. After that the 5 mL water

solution of initiator (5 mg/mL) was added under continuous stirring, the reaction was

carried out for 8 h.

After synthesis, all microgel suspensions were dialyzed by a Millipore

Labscale TFF system with Pellicon XL Filter (PXC030C50) for 4 days. During the

dialysis process the microgel solution was pumped continuously through the

membrane with a pore size much smaller than the size of microgels to remove small

organic molecules and oligomers. The waste-water drained away directly into the

sewer and fresh water was added to the dialyzed solution to keep the total volume

constant.


27

The polymerization technique used is the precipitation polymerization which

involves the formation of microgel particles by a homogeneous nucleation

mechanism. The process is carried out above the volume phase transition of the

microgels, while the particles are in collapsed state. Three sets of microgel samples

were synthesized: a) with variable molar ratios between VCL and NIPAm, and VCL

and NIPMAm respectively; b) with variable cross-linking degree and c) with variable

size.

PVCL/AAEM/VIm copolymer microgels

PVCL/AAEM/VIm copolymer microgels were synthesized with the same

procedure as PVCL/acrylamide microgels, which has been reported elsewhere.[17]

The

amounts of VCL, AAEM and VIm were 1.783, 0.321 and 0.071 g, respectively. The

mole ratio for VCL/AAEM/VIm is 9:1:0.5. Microgels were synthesized with different

amounts of BIS, which were 0.06, 0.02 and 0.007 g (3, 1 and 0.35 mol% of

monomers). The monomers were added in 145 mL water in a double-wall glass

reactor equipped with stirrer and reflux condenser. Then, the mixtures were stirred for

1 h at 70 °C with purging with nitrogen. After that the 5 mL water solution of initiator

(5 g/L) was added under continuous stirring, the reaction was carried out for 8 h. As a

reference, PVCL/AAEM microgels with monomer ratio of 9:1 were synthesized

under the same condition. After synthesis, PVCL/AAEM/VIm and PVCL/AAEM

microgels were dialyzed with a cellulose membrane (MWCO 12 000 – 14 000, Carl

Roth GmbH, Germany) against water for 4 days.

PVCL/AAEM/AAc copolymer microgels

PVCL/AAEM/AAc copolymer microgels were synthesized with a reported

procedure,[22]

which is different from that of VCL/AAEM/VIm. The mole ratio for


28

VCL/AAEM/AAc is 9:1:0.7. Microgels were synthesized with different amounts of

BIS, which were 0.06, 0.02 and 0.007 g (3, 1 and 0.35 mol% of monomers). The

cationic initiator AMPA induces the flocculation during synthesis because of the

existence of anionic AAc groups, and a widely anionic initiator potassium persulfate

(KPS) can lead to hydrolysis of VCL groups.[31]

Thus, another anionic initiator

ACMA was used and proved to be effective.[22]

VCL (1.877 g) and AAEM (0.338 g)

were added in 120 mL water in a double-wall glass reactor equipped with stirrer and

reflux condenser. To prevent the flocculation during synthesis, SDS (0.02 g) was

added to the reaction mixture. Then, the mixtures were stirred for 1 h at 70 °C with

purging with nitrogen. After that the 5 mL water solution of ACMA (0.07 g) was

added under continuous stirring. Precursor particle core rich in AAEM is formed after

the addition of initiator, due to the faster consumption of AAEM.[14]

After 5 minutes,

25 mL water solution of AAc (0.08 g) was added to the mixture. Then the reaction

was carried out for 8 h. After synthesis, VCL/AAEM/AAc microgels were dialyzed

with a cellulose membrane (MWCO 12 000 – 14 000, Carl Roth GmbH, Germany)

against water for 4 days.

3.2.3. Characterization methods

NMR spectroscopy

The composition of copolymer microgel samples were characterized with a

400 MHz Bruker L-S NMR spectrometer at room temperature (around 23 °C). The

calibration of the chemical shift in NMR spectra was made using TMS, based on

proton of D2O (4.8 ppm).[40]

Before NMR measurements, microgel samples were

lyophilized and then dissolved in D2O.


29

Dynamic light scattering (DLS)

The size of the microgel particles was measured with an ALV/LSE-5004 Light

Scattering (DLS) Multiple Tau Digital Correlator and Electronics with the scattering

angle set at 90°. The samples were measured at different temperatures (from 20 to 50

°C) and the temperature fluctuations were below 0.1 °C. Prior to the measurement

microgel samples were diluted with water for chromatography from Merck Millipore,

which is filtered through a 100 nm filter before use.

Zeta potential

Zeta potential measurements were carried out on a Malvern Zetasize NanoZS.

Before the measurements, the pH of microgel solutions were adjusted to different

values using 0.1 M HCl and NaOH aqueous solution. The measurements were

performed at 25 °C after an equilibration of at least 2 min.

Viscometry

The intrinsic viscosity of microgel suspension was determined by an automatic

Schott Micro Ostwald viscosimeter equipped with a water bath. The measurement

was conducted at 25 °C.

3.3. Results and discussion

3.3.1. PVCL/acrylamides-based neutral microgels

3.3.1.1.PVCL/NIPAm and PVCL/NIPMAm microgels

The copolymer microgels of PVCL/NIPAm and PVCL/NIPMAm with

different monomer ratios are obtained from Dr. Andreea Balaceanu. The synthesis

and size information can be found in the published paper and thesis.[32, 33]

These

results show that the monomer ratio in microgels can be controlled by tuning the


30

feeding ratio during synthesis. It was revealed that the monomer units in

PVCL/NIPAm and PVCL/NIPMAm microgels are statistically distributed in the

colloidal networks independent of the copolymer composition. As these microgels are

used in the investigation of interfacial behavior of microgels in other Chapters, here

we exhibit the basic information of the microgels extracted from Dr. Andreea

Balaceanu’s work. Table 3.1 shows the monomer ratios in PVCL/NIPAm and

PVCL/NIPMAm copolymer microgels, and the hydrodynamic radii of the microgels

below and above VPTT (20 and 50 oC). The normalized hydrodynamic radii

⁄ that represent the swelling properties of the microgels are shown as well.

Table 3.1. Monomer amounts and copolymer composition in PVCL/acrylamides

microgels; and the hydrodynamic radii of the microgels at 20 and 50 oC and the

swelling properties represented by ⁄ .[32, 33]

Smaplea VCL

(g)

NIPAm or

NIPMAm

(g)

VCL:acrylamide

(mol:mol),

measb

at 20

oC (nm)

at 50

oC (nm)

V:N (5:1) 1.905 1.905 4.6 366 160 2.29

V:N (1:1) 1.220 1.220 1.01 431 247 1.75

V:N (1:5) 0.437 0.437 0.3 399 190 2.10

V:NM (5:1) 1.872 0.342 5.04 360 183 1.96

V:NM (1:1) 1.16 1.06 1.08 371 209 1.77

V:NM (1:2) 0.78 1.43 0.48 419 204 2.06

V:NM (1:3) 0.592 1.623 0.33 472 241 1.96

V:NM (1:5) 0.397 1.820 0.2 444 216 2.06

a, V:N and V:NM represent PVCL/NIPAm and PVCL/NIPMAm, respectively; and

the ratios in the brackets are the expected monomer ratio of copolymer microgels. b,

measured molar ratio by NMR for PVCL/NIPAm and by Raman for

PVCL/NIPMAm.


31

3.3.1.2. PVCL/NIPAm microgel system with different cross-linking densities

Chemical composition by NMR

Microgels are composed of intramolecular cross-linked polymer networks.

The density of cross-linker plays an imperative role in the properties of either

microgel suspension or individual microgel particle.[39, 41]

We synthesized

PVCL/NIPAm (monomer ratio 1:1) copolymer microgels with different amounts of

cross-linker (BIS), and determined their monomer ratios with NMR. Figure 3.1 shows

the NMR spectra of PVCL/NIPAm (1:1) microgel with 3 mol% BIS content in D2O,

and the chemical structures of the VCL and NIPAm units. According to previous

work, the peak at around 1.2 ppm is assigned to CH3 protons (R1) of NIPAm unit; and

resonances at around 1.6, 2.5 and 3.3 ppm are assigned to CH2 protons unconnected

with N and C=O bond (R2), connected with N (R3) and near the C=O bond (R4) in the

ring of VCL unit, respectively.[18, 32]

Through multi peaks fitting in Origin software,

the slightly overlapped peaks of R1 and R2 are separated (Figure 3.1 b).

6 5 4 3 2 1 0

R4

NO

R4

R2R2

R2

R3

R3

NH

O

R1

R1

ppm

R2 (PVCL)

R1 (PNIPAm)

a

D2O

2.5 2.0 1.5 1.0 0.5 0.0

ppm

b

R2 (PVCL)

R1 (PNIPAm)

Figure 3.1. (a) 1H NMR spectra of PVCL/NIPAm (1:1) microgel with 3 mol% BIS;

(b) the separation of peaks R1 and R2 by multi peaks fitting, determining the monomer

ratios in the copolymer.


32

Based on the areas of R1 and R2 peaks, the monomer ratios of VCL and

NIPAm in the copolymer microgels are calculated, and the results are presented in

Table 3.2. It is found that the measured monomer ratios in copolymer microgels are

consistent with the initial feeding monomer ratio in the reaction mixture. Considering

the high conversion of the polymerization process and the narrow size distribution of

obtained microgels, we believe that the determined values are accurate and depict real

copolymer composition of microgels with a reasonably small error range.[32]

Table 3.2. Monomer ratios of PVCL/NIPAm microgels determined by NMR.

VCL

(g)

NIPAm

(g)

BIS

(g)

BIS

(mol%)

VCL:NIPAm

(mol:mol), theora

VCL:NIPAm

(mol:mol), measb

1.220 0.993 0.060 3 1 1.08

1.220 0.993 0.030 1.5 1 0.96

1.220 0.993 0.020 1 1 1.16

1.220 0.993 0.010 0.5 1 1.01

1.220 0.993 0.005 0.25 1 0.97

1.220 0.993 0.002 0.1 1 1.17

1.220 0.993 0.001 0.05 1 1.07

a, theoretic molar ratio; b, measured molar ratio.

It should be noted that the quantitative determination of the content of cross-

linker in copolymer microgel by NMR or other spectroscopies is very difficult. That is

because the amount of cross-linker in microgel is too small compared to monomers,

and signals of cross-linker are highly overlapped by those of monomers. However, the

qualitative changes in cross-linker density can be confirmed according to the variation

in properties of microgels.

In our synthesis protocols the total concentration of monomers was kept

constant to achieve a precise and reproducible synthesis. However, we changed the

amount of cross-linker BIS in the reaction mixture to obtain microgels with variable


33

cross-linking concentrations without considering the slight change of the total

monomer concentration, since the amount of BIS used here was much smaller

compared to the amounts of co-monomers, VCL and NIPAm. In the one-batch

precipitation polymerization, such a small difference in the monomer concentration

would not bring a big variation in the structure and the property of microgels.

Microgel size and temperature responsiveness

The hydrodynamic sizes of the obtained microgel samples are characterized

with DLS at different temperatures. The results summarized in Figure 3.2 show that

PVCL/NIPAm microgels exhibit temperature sensitivity and the volume phase

transition temperatures (VPTT) are around 35 °C, independent on the cross-linker

content.

20 30 40 50 60100

200

300

400

500a 3%

1.5%

1%

0.5%

0.25%

0.1%

0.05%

Rh (

nm

)

Temperature (oC)

20 30 40 50 60

1.0

1.5

2.0

2.5

3%

1.5%

1%

0.5%

0.25%

0.1%

0.05%

Rh /

Rh

, co

llap

sed

Temperature (oC)

b

Figure 3.2. (a) Hydrodynamic radii ( ) and (b) normalized hydrodynamic radii

( ⁄ ) of the PVCL/NIPAm microgels with different cross-linker contents

versus temperature. The legends show cross-linkers contents in mol%.

The measured hydrodynamic radii ( ) of microgels are normalized by

dividing the hydrodynamic radii in the collapsed state ( ), and plotted

versus temperature (Figure 3.2b). We can see that the swelling capability of microgel


34

decreases as the cross-linker density increase (the trend shown by the arrow). This

indicates that the microgel network becomes more compact and less elastic at higher

cross-linker density, leading to lower swelling capability. It is believed that in

aqueous solution at temperature above VPTT microgel particles in the collapsed state

still contain considerable amount of water[42]

and therefore should have larger size

compared to completely dried particles.

Viscosity of PVCL/NIPAm microgel suspensions

It has been reported that the viscosity of suspensions of hard spheres depends

only on the volume fraction of particles.[43]

Such kind of concept has also been

successfully employed for colloidal suspensions, where the effective volume fraction

( ) needs to be considered.[39, 44]

An expression given by Batchelor for hard-sphere

system, which has been proved to be valid for microgel systems, is used for

describing the relationship between the relative viscosity and the effective volume

fraction.[39, 43, 44]

(Equation 3.1)

Here, the effective volume fraction can be substituted by , is the

mass concentration of the microgel dispersion and is the shift factor for converting

the mass concentration to the effective volume fraction. The relative viscosities of

diluted solutions PVCL/NIPAm (1:1) microgels with different cross-linker densities

are shown in Figure 3.3. The dash lines are fits according to Equation 3.1, and the

values of shift factor are summarized in Figure 3.4. We can see that the value of

increases with the decrease in cross-linker density. The variation tendency of with

cross-linker density here agrees with the reported results.[39]


35

A critical mass concentration can be calculated as ⁄ when

. If we intend to study the behavior of individual particle at the interface, the

microgel solutions should be diluted to certain concentration below . According

to Senff’s work, the effective volume fraction can be compared to the volume fraction

of the “dry” polymer which is calculated from the mass content and the polymer

density.[44]

From Figure 3.4, we can see that decreases with the reduction of

cross-linker content. It means that the microgels with lower cross-linker contents can

reach a certain effective volume fraction at lower concentrations compared to the

microgels with higher cross-linker contents. This might be due to the increasing

swelling capability of microgels with decreasing cross-linking densities, as described

in Figure 3.2b.

0.0 0.2 0.4 0.6 0.8 1.01.0

1.2

1.4

1.6

1.8

3%

1.5%

1%

0.5%

0.25%

0.1%

0.05%

re

l

c (wt%)

Figure 3.3. Relative viscosity of PVCL/NIPAm microgels with different cross-linker

contents at 25 °C versus the mass concentration of microgel dispersion. The cross-

linker content is given in mol% in the legends. The dash lines show fits based on

Equation 3.1.


36

0 1 2 30

10

20

30

40

50

0.00

0.05

0.10

0.15

0.20

0.25

cc

rit (

wt%

)

k

BIS content (mol%)

Figure 3.4. The values of shift factor k and critical mass concentration ( )

versus the cross-linker content for PVCL/NIPAm microgels.

3.3.1.3. Microgels synthesized with different amounts of surfactant

The addition of surfactant during the synthesis can reduce the size of

microgels.[1, 8]

The role of the surfactant in the microgel synthesis is to increase the

colloidal stability of the precursor particles and subsequently lower the size of the

primary particles as the number of the particles is increased. With the increase in the

concentration of surfactant, more precursor particles survive from the aggregation and

grow to a smaller size as the monomer amount is constant.[45]

Here, we add different amounts of CTAB to the reaction mixture during the

synthesis of PVCL/NIPMAm (1:5) copolymer microgel. Through DLS results (Figure

3.5), we can see that with the increase in the concentration of CTAB, the

hydrodynamic radii of microgels decrease. In the swollen state, the hydrodynamic

radius of PVCL/NIPMAm microgel is around 450 nm without CTAB during the

synthesis, while the radii of microgels are around 200 and 60 nm with 0.37 and 1.10

mM CTAB added during the synthesis, respectively. In the collapsed state, Rh of

PVCL/NIPMAm microgels decrease from 200 nm without CTAB to 20 nm with 1.10


37

mM CTAB. Although the microgels size varies with the amount of surfactant added

during synthesis, the swelling ability remains the same for microgels, regardless of

surfactant. It is because the surfactants do not change the chemical composition and

the cross-linker density of the microgels. From Figure 3.5b, it can be observed that

curves of ⁄ versus temperature of microgels synthesized with different

amounts of CTAB highly overlap with each other.

20 30 40 50 600

100

200

300

400

500

Rh (

nm

)

Temperature (oC)

NO CTAB

0.37mM CTAB

1.10mM CTAB

a

20 30 40 50 60

1.0

1.5

2.0

2.5

b

Rh / R

h,

co

lla

ps

ed

Temperature (oC)

NO CTAB

0.37mM CTAB

1.10mM CTAB


( ⁄ ) of the PVCL/NIPMAm (1:5) microgels synthesized with different

amounts of CTAB. The symbols of triangle, circle and square represent 0, 0.37 and

1.10 mM CTAB added during synthesis, respectively.

3.3.2. PVCL-based microgels with functional groups

By incorporation of monomers with ionizable groups, the temperature-

sensitive microgels can be endowed with pH-responsiveness. PVCL/AAEM

copolymer microgels with thermo-sensitivity have been developed by precipitation

polymerization and investigated extensively.[14-16, 46]

Pich and coworkers have

successfully incorporated VIm groups in PVCL/AAEM-based microgels.[17]

Recently,

PVCL/AAEM/AAc copolymer microgels are obtained through the copolymerization

of VCL, AAEM and AAc monomers.[22]

With the units of PVCL and ionizable groups


38

of VIm and AAc, the PVCL/AAEM/VIm and PVCL/AAEM/AAc microgels exhibit

both temperature- and pH-sensitivity. Such temperature- and pH-sensitive microgels

may be used in drug release and catalysis.[22, 25]

3.3.2.1. PVCL/AAEM/VIm microgels

In the synthesis of PVCL/AAEM/VIm copolymer microgels with various

cross-linker contents (3, 1 and 0.35 mol%), the content of VIm in monomers is kept

constant at 5 mol%. After the synthesis and purification, PVCL/AAEM/VIm

microgels with different cross-linker densities are characterized by 1H NMR.

10 8 6 4 2 0

8 7 6

3

2

dc

b

ppm

a

1

N

N

1 2

3

Figure 3.6. 1H NMR spectra of PVCL/AAEM microgels with 1 mol% BIS (curve a),

and PVCL/AAEM/VIm microgels with BIS content of 3 mol% (curve b), 1 mol%

(curve c) and 0.35 mol%. The inset highlights the interesting part (6 – 8 ppm).

From Figure 3.6, we can see that in the 1H NMR spectra of

PVCL/AAEM/VIm microgels, peaks between 7 – 8 ppm belonging to protons in VIm

group appear, contrary to PVCL/AAEM microgel.[47]

The inset in Figure 3.6

highlights the interesting part (6 – 8 ppm), where peak 1, 2 and 3 represent protons of

CH between two nitrogen atoms (7.67 ppm), CH close to C–N (7.22 ppm) and CH


39

close to C=N (7.04 ppm), respectively. It shows that the incorporation of VIm group

in microgels is successful.

The hydrodynamic sizes of PVCL/AAEM/VIm microgels are detected by

DLS, and the results are presented in Figure 3.7. At 20 °C, the hydrodynamic radii of

microgels are around 350 nm for those synthesized with 3 and 1 mol% BIS, and 450

nm for those synthesized with 0.35 mol% BIS. However, microgels in collapsed state

are about 130 nm, regardless of cross-link density. That means the number of polymer

segments in a microgel particle does not change with cross-linker amount for

PVCL/AAEM/VIm microgels. Through the normalized hydrodynamic radii

( ⁄ ) of the PVCL/AAEM/VIm microgels, we can see that the swelling

capability of microgels with 1 mol% is close to those with 3 mol% BIS, but much

lower than those with 0.35 mol%.

20 30 40 50 60100

200

300

400

500

Rh (

nm

)

Temperature (oC)

0.35%

1%

3%

a

20 30 40 50 60

1

2

3

4

b

Rh / R

h,

co

lla

ps

ed

Temperature (oC)

0.35%

1%

3%


( ⁄ ) of the PVCL/AAEM/VIm microgels at pH 7. The symbols of

triangle, circle and square represent 3%, 1% and 0.35% BIS in mol added during the

synthesis, respectively.

As reported, the size of PVCL/AAEM/VIm microgels can be influenced by

pH of the solution.[17]

Figure 3.8 presents the pH-dependence of hydrodynamic radii


40

and zeta potential of PVCL/AAEM/VIm microgels synthesized with 1 mol% BIS.

The variation tendency of microgel size highly agrees the variation of zeta potential

with pH. As pH of the solution decreases from 8, of PVCL/AAEM/VIm microgels

increase gradually due to the ionization of VIm groups, which leads to the

enhancement of electrostatic repulsion in microgel particles (an increase in zeta

potential). When the solution pH is higher than 8, the zeta potential of microgels is

close to zero and the microgel size remains at a constant value as VIm units stay in

neutral state. A maximum of PVCL/AAEM/VIm microgel can be found at pH

around 3.5, which correlates with the maximum ionization of VIm groups. The

decrease of microgel size at pH < 3.5 is attributed to the screening effect of the

electrostatic repulsion between VIm groups by excess of protons. In the contrast, the

change in solution pH has no influence on the hydrodynamic radius of PVCL/AAEM

microgel, as the zeta potential does not change with pH. It has been confirmed that the

VIm content can also influence the microgel dimension.[17]

Here, we mainly focus on

the microgel synthesized with a constant concentration of VIm (5 mol%).

2 3 4 5 6 7 8 9 10 11100

200

300

400

500 VCL/AAEM/VIm

VCL/AAEM

Rh (

nm

)

pH

a

2 4 6 8 10-20

-10

0

10

20

Ze

ta P

ote

nti

al (m

V)

pH

VCL/AAEM/VIm

VCL/AAEMb

Figure 3.8. Influence of pH on (a) hydrodynamic radii and (b) zeta potential of

PVCL/AAEM/VIm and PVCL/AAEM microgels with 1 mol% BIS at 20 °C.


41

3.3.2.2. PVCL/AAEM/AAc microgels

It is difficult to determine the existence of AAc group in PVCL/AAEM/AAc

microgels by spectroscopy because AAEM unit also contain carboxyl groups.

Nevertheless, the influence of pH changes on microgel size suggests that the

incorporation of AAc in the copolymer microgel is successful. The hydrodynamic

radii of PVCL/AAEM/AAc microgels were detected by DLS, as shown in Figure 3.9.

20 30 40 50 60 70 8060

70

80

90

100

Rh (

nm

)

Temperature (oC)

0.35%

1%

3%

a

20 30 40 50 60 70 80

1.0

1.1

1.2

1.3

1.4

1.5

b

Rh / R

h,c

oll

ap

se

d

Temperature (oC)

0.35%

1%

3%


( ⁄ ) of the PVCL/AAEM/AAc microgels at pH 6. The symbols of

triangle, circle and square represent 3%, 1% and 0.35% BIS in mol added during

synthesis, respectively.

At 20 °C, the hydrodynamic radii of PVCL/AAEM/AAc microgels are around

95 nm for those synthesized with 3 and 1 mol% BIS, and 85 nm for those synthesized

with 0.35 mol% BIS. As temperature increases to 40 °C, the hydrodynamic sizes of

microgels with different cross-linker content decrease as microgel particles shrink.

When temperature is above 40 °C, microgels are in collapsed state, and the

hydrodynamic radii remain at a constant value, around 70 nm for microgel with 3

mol% BIS and 65 nm for microgels with 1 and 0.35 mol% BIS. Through the

normalized hydrodynamic radii ( ⁄ ) of the PVCL/AAEM/VIm


42

microgels, we can see that the swelling capability of microgels is independent on the

cross-linker content, unlike PVCL/AAEM/VIm microgels.

5 6 7 8 9 1060

90

120

150

180

-35

-30

-25

-20

-15

-10

Rh (

nm

)

pH

Zeta

Po

ten

tial (m

V)

Figure 3.10. Influence of pH on hydrodynamic radii (solid square) and zeta potential

(open square) of PVCL/AAEM/AAc microgels with 1 mol% BIS at 20 °C.

It should be noted that the PVCL/AAEM/AAc microgels are synthesized in

the presence of SDS. Without the addition of surfactant, coagulation occurs. The sizes

of them are smaller compared to those of PVCL/AAEM/VIm micorgels in either

swollen state or collapsed state.

3.4. Conclusions

We synthesized the copolymer microgels based on VCL, NIPAm, and

NIPMAm with different monomer ratios (the mole ratio of VCL to acrylamides varies

from 0.2 to 5). The chemical composition of copolymer microgels were determined

by NMR spectroscopy. It is found that the monomer ratio in copolymer microgel is

close to the initial feeding ratio during the synthesis.


43

PVCL/NIPAM (1:1) microgels with different cross-link densities (0.05 – 3

mol%) were obtained. The viscosity of microgel solution reflects the variation

tendency of cross-linker content. Microgels with lower cross-linker contents can reach

a certain effective volume fraction at lower concentrations compared to the microgels

with higher cross-linker contents. The hydrodynamic sizes of all microgels are

characterized by DLS at different temperatures. As expected, the swelling property of

microgels strongly depends on the synthesis condition such as monomer feeding ratio,

the concentrations of cross-linker, and the addition of surfactant.

Besides the neutral PVCL/acrylamide copolymer microgels, ionizable

PVCL/AAEM-based microgels are synthesized by the addition of VIm or AAc

monomers. The hydrodynamic size and zeta potential of PVCL/AAEM/VIm and

PVCL/AAEM/AAc microgels at different pH values are detected by DLS and

Zetasizer, respectively. The pH-responsiveness of these microgels indicates a

successful incorporation VIm and AAc groups into the copolymer microgels.

The successful synthesis of microgels with controlled cross-link density and

copolymer composition is achieved in this Chapter. The structures and interfacial

behaviors of these microgels will be in detail discuss in the following Chapters.

3.5. References

[1] Pelton R. and Hoare T., Microgels and their synthesis: An introduction In

Microgel Suspensions; Wiley-VCH Verlag GmbH & Co. KGaA: 2011, p 1-32.








44

[4] Pelton R. H. and Chibante P., Preparation of aqueous lattices with N-

isopropylacrylamide, Colloids and Surfaces, 1986, 20, 247-256.

[5] Acciaro R., Gilanyi T. and Varga I., Preparation of monodisperse poly(N-

isopropylacrylamide) microgel particles with homogenous cross-link density

distribution, Langmuir, 2011, 27, 7917-7925.

[6] Meng Z. Y., Smith M. H. and Lyon L. A., Temperature-programmed synthesis

of micron-sized multi-responsive microgels, Colloid and Polymer Science, 2009, 287,

277-285.

[7] Meyer S. and Richtering W., Influence of polymerization conditions on the

structure of temperature-sensitive poly(N-isopropylacrylamide) microgels,

Macromolecules, 2005, 38, 1517-1519.



Polymer Science, 1994, 272, 467-477.

[9] Zha L., Hu J., Wang C., Fu S., Elaissari A. and Zhang Y., Preparation and

characterization of poly (N-isopropylacrylamide-co-dimethylaminoethyl

methacrylate) microgel latexes, Colloid and Polymer Science, 2002, 280, 1-6.

[10] Duracher D., Elaissari A. and Pichot C., Characterization of cross-linked

poly(N-isopropylmethacrylamide) microgel latexes, Colloid and Polymer Science,

1999, 277, 905-913.

[11] Hu X. B., Tong Z. and Lyon L. A., Synthesis and physicochemical properties

of cationic microgels based on poly(N-isopropylmethacrylamide), Colloid and

Polymer Science, 2011, 289, 333-339.

[12] Tirumala V. R., Ilavsky J. and Ilavsky M., Effect of chemical structure on the

volume-phase transition in neutral and weakly charged poly(N-

alkyl(meth)acrylamide) hydrogels studied by ultrasmall-angle X-ray scattering,

Journal of Chemical Physics, 2006, 124.

[13] von Nessen K., Karg M. and Hellweg T., Thermoresponsive poly-(N-

isopropylmethacrylamide) microgels: Tailoring particle size by interfacial tension

control, Polymer, 2013, 54, 5499-5510.




[15] Pich A., Lu Y., Boyko V., Arndt K. F. and Adler H. J. P., Thermo-sensitive

poly(N-vinylcaprolactam-co-acetoacetoxyethyl methacrylate) microgels: 2.

Incorporation of polypyrrole, Polymer, 2003, 44, 7651-7659.

[16] Pich A., Lu Y., Boyko V., Richter S., Arndt K. F. and Adler H. J. P., Thermo-

sensitive poly(N-vinylcaprolactam-co-acetoacetoxyethyl methacrylate) microgels. 3.


45

Incorporation of polypyrrole by selective microgel swelling in ethanol-water

mixtures, Polymer, 2004, 45, 1079-1087.








[19] Imaz A. and Forcada J., N-vinylcaprolactam-based microgels: Synthesis and

characterization, Journal of Polymer Science Part a-Polymer Chemistry, 2008, 46,

2510-2524.

[20] Imaz A. and Forcada J., Synthesis strategies to incorporate acrylic acid into N-

vinylcaprolactam-based microgels, Journal of Polymer Science Part A: Polymer

Chemistry, 2011, 49, 3218-3227.

[21] Pinkrah V. T., Snowden M. J., Mitchell J. C., Seidel J., Chowdhry B. Z. and

Fern G. R., Physicochemical properties of poly(N-isopropylacrylamide-co-4-

vinylpyridine) cationic polyelectrolyte colloidal microgels, Langmuir, 2003, 19, 585-

590.




[23] Hoare T. and Pelton R., Functional group distributions in carboxylic acid

containing poly(N-isopropylacrylamide) microgels, Langmuir, 2004, 20, 2123-2133.

[24] Kleinen J., Klee A. and Richtering W., Influence of architecture on the

interaction of negatively charged multisensitive poly(N-isopropylacrylamide)-co-

methacrylic acid microgels with oppositely charged polyelectrolyte: Absorption vs

adsorption, Langmuir, 2010, 26, 11258-11265.

[25] Cheng C., Zhu X. M., Pich A. and Moller M., Aqueous microgels modified by

wedge-shaped amphiphilic molecules: Hydrophilic microcontainers with hydrophobic

nanodomains, Langmuir, 2010, 26, 4709-4716.



[27] Schachschal S., Pich A. and Adler H. J., Aqueous microgels for the growth of

hydroxyapatite nanocrystals, Langmuir, 2008, 24, 5129-5134.

[28] Kleinen J. and Richtering W., Rearrangements in and release from responsive

microgel-polyelectrolyte complexes induced by temperature and time, Journal of

Physical Chemistry B, 2011, 115, 3804-3810.


46

[29] Lau A. C. W. and Wu C., Thermally sensitive and biocompatible poly(N-

vinylcaprolactam): Synthesis and characterization of high molar mass linear chains,

Macromolecules, 1999, 32, 581-584.

[30] Dimitrov I., Trzebicka B., Muller A. H. E., Dworak A. and Tsvetanov C. B.,

Thermosensitive water-soluble copolymers with doubly responsive reversibly

interacting entities, Progress in Polymer Science, 2007, 32, 1275-1343.

[31] Ramos J., Imaz A. and Forcada J., Temperature-sensitive nanogels: poly(N-

vinylcaprolactam) versus poly(N-isopropylacrylamide), Polymer Chemistry, 2012, 3,

852-856.







[34] Berndt I. and Richtering W., Doubly temperature sensitive core-shell

microgels, Macromolecules, 2003, 36, 8780-8785.

[35] Berndt I., Pedersen J. S. and Richtering W., Structure of multiresponsive

"intelligent" core-shell microgels, Journal of the American Chemical Society, 2005,

127, 9372-9373.

[36] Berndt I., Pedersen J. S., Lindner P. and Richtering W., Influence of shell

thickness and cross-link density on the structure of temperature-sensitive poly-N-

isopropylacrylamide-poly-N-isopropylmethacrylamide core-shell microgels

investigated by small-angle neutron scattering, Langmuir, 2006, 22, 459-468.

[37] Berndt I., Pedersen J. S. and Richtering W., Temperature-sensitive core-shell

microgel particles with dense shell, Angewandte Chemie-International Edition, 2006,

45, 1737-1741.

[38] Balaceanu A., Verkh Y., Demco D. E., Moller M. and Pich A., Correlated

morphological changes in the volume temperature transition of core-shell microgels,

Macromolecules, 2013, 46, 4882-4891.



Science, 2000, 278, 830-840.

[40] Gottlieb H. E., Kotlyar V. and Nudelman A., NMR chemical shifts of common

laboratory solvents as trace impurities, Journal of Organic Chemistry, 1997, 62,

7512-7515.


47

[41] Schmidt S., Zeiser M., Hellweg T., Duschl C., Fery A. and Mohwald H.,

Adhesion and mechanical properties of PNIPAM microgel films and their potential

use as switchable cell culture substrates, Advanced Functional Materials, 2010, 20,

3235-3243.



[43] Batchelor G. K., Effect of brownian-motion on bulk stress in a suspension of

spherical particles, Journal of Fluid Mechanics, 1977, 83, 97-117.

[44] Senff H. and Richtering W., Temperature sensitive microgel suspensions:

Colloidal phase behavior and rheology of soft spheres, Journal of Chemical Physics,

1999, 111, 1705-1711.




3305-3314.

[46] Boyko V., N-vinylcaprolactam based bulk and microgels: synthesis, structural

formation and characterization by dynamic light scattering, PhD Thesis, Dresden

University of Technology, Dresden, Germany, 2004.

[47] Srivastava A., Waite J. H., Stucky G. D. and Mikhailovsky A., Fluorescence

investigations into complex coacervation between polyvinylimidazole and sodium

alginate, Macromolecules, 2009, 42, 2168-2176.

Chapter 4. Internal Structure of Microgels

49

Chapter 4. Internal Structure of Microgels by Light

Scattering and Microscopy

4.1. Introduction

Microgels are chemically cross-linked polymer networks, which are swollen

by good solvents. The incorporation of cross-linker into the microgel network is often

inhomogeneous, due to its faster consuming rate than monomers during the

copolymerization.[1]

This brings a complex and heterogeneous network structure,

affecting the properties of microgels.

The inhomogeneous structure of microgels has been investigated by proton

NMR transverse relaxation measurements.[2, 3]

As different kinds of proton mobility

are detected, the structure regions in microgel particles can be classified in two parts,

core and corona. In the core-corona model, a higher cross-linking degree in core part

is suggested. The scattering methods including small-angle neutron scattering (SANS)

as well as static light scattering (SLS) and dynamic light scattering (DLS) have also

been widely used to investigate the internal structure of microgels.[4-9]

The results

confirmed the core-corona structure of microgels. Such kind of structure has a radial

profile, where the density of cross-linker decreases from the center towards the

surface of microgels.[5]

Microscopy is a powerful method for direct observation of microgel particles.

The visualization of microgel particles has been successfully achieved by scanning

electron microscopy (SEM), transmission electron microscopy (TEM) and scanning

transmission electron microscopy (STEM).[10-13]

From the microgel morphologies

observed by electron microscopes, the core-corona structure is recognized.[11, 14, 15]


50

Cryo-SEM and Cryo-TEM are also useful approaches to capturing the microstructure

of microgel particles in solution state.[16-18]

Besides the electron microscope,

microgels tagged with fluorescein can also be visualized under confocal

microscopes.[16, 19]

However, with limited resolution, the confocal microscopes hardly

detect the detailed microstructure within the microgels. To determine structures of

soft samples at the nanoscale, atomic force microscopy (AFM) is widely used. The

AFM measurements can be carried on at trivial conditions, such as room ambient and

even under water, unlike SEM or TEM where high vacuum condition is needed. The

observation of PNIPAm-based microgels with AFM gives not only the morphology

but also the dimension information like contact radius and height.[20-22]

The low-

height corona part of microgels with heterogeneous structure can be readily captured

as AFM works well at the nanoscale. Moreover, the AFM measurements can be

implemented under water to detect microgels adsorbed at an interface.[23-26]

Microgel

particles, even in collapsed state, contain a lot of water in a solution.[27, 28]

The

particles are very soft and the fluffy polymer chains at the surface of microgels swing

in water. Therefore the in-situ characterization of microgels under water with AFM

can hardly observe the core-corona structure. However, the AFM observation on

microgel samples on solid substrates at dry state can show us an obvious core-corona

structure of microgel particles.[22]

Here, we investigate the internal structure of PVCL-based copolymer

microgels with SLS and microscopy methods (AFM and FESEM). The SLS results

show that PVCL/NIPAm and PVCL/NIPMAm microgels, rich in either VCL or

acrylamides, have core-corona structures with a radial profile. The observation of

PVCL/NIPAm copolymer and PVCL homopolymer microgels by AFM provides a

visualized evidence of core-corona structure that influenced by cross-linker content.


51

We also deposit PVCL/NIPAm copolymer microgels on silicon wafer at the

temperature above the VPTT of microgels. The microgels are fixed on the solid

substrate in collapsed state due to the fast spin-coating process. The AFM observation

shows that the microgels do not have a core-corona structure as the fuzzy corona part

of the microgel contracts with the core part to form a single monolithic structure.

Additionally, PVCL/AAEM-based microgels with functional groups are characterized

with AFM and FESEM. It is interesting that the core-corona structure is found in

PVCL/AAEM /VIm microgels, but not in PVCL/AAEM/AAc microgels. Moreover,

the AFM observation on microgels of very low cross-linking density provides a

possible method to look into the domains in the cross-linked polymer networks.


4.2.1. Sample preparation

Microgel synthesis

PVCL/NIPAm microgels with different monomer ratios and cross-linker

contents are used in this Chapter. The synthesis and size characterization of microgels

are presented in detail in Chapter 3.

Deposition of microgels on a solid substrate

The deposition of microgels was carried out on a spin-coating machine

(Convac 1001S, Germany) with the speed of 3000 rpm for 1 min. The silicon wafer

used for deposition was cleaned by sonication in isopropanol for 10 min, dried in air

stream, and treated with UV/O2 for 12 min. The concentration of microgel solution

used for spin coating was 0.2 mg/mL. At this microgel concentration, sufficient

amount of particles could be well distributed on the silicon wafer without clustering


52

effect. The quality of the samples was evaluated with a Zeiss Axioplan 2 optical

microscope. For each sample, a drop of 75 μL microgel solution was used. The

deposition process was conducted at room temperature (about 23 °C).

For samples prepared above VPTT, the microgel solution was incubated in a

water bath at 50 °C for around 10 min. The freshly cleaned silicon wafer was fixed on

the spin-coating machine and heated with an infrared lamp. The infrared lamp was

hung around 5 cm on the top of the spin-coating plate, and kept working during the

spin-coating process.


Light scattering


Scattering (DLS) Multiple Tau Digital Correlator and Electronics with the scattering

angle set at 90°. The samples were measured at 20 °C and the temperature fluctuations

were below 0.1 °C. Prior to the measurement microgel samples were diluted with

water for chromatography from Merck Millipore, which is filtered through a 100 nm

filter before use. The SLS measurements were conducted on a Sofica with a

wavelength of 633 nm and a Fica with a wavelength of 495 nm at 20 °C. The

measured data were fitted with a model of polydisperse microgels.[5, 6]

Microscopy characterization

AFM images were recorded on Agilent 5500 and Veeco Nanoscope Atomic

Force Microscopes with a tapping mode. Cantilevers with resonance frequencies of

250 – 300 kHz and spring constants around 42 N/m were used. The data were

analyzed with Agilent Picoimage, Nanoscope Analysis and ImageJ software. FESEM


53

images were obtained on a Hitachi S4800 high resolution scanning electron

microscope with field emission cathode at an accelerating voltage of 1 kV.


4.3.1. Internal structure of microgels by light scattering

For a homogeneous sphere with the radius , the distribution of polymer mass

is uniform inside the particle, where the form factor is

{ [ ]

}

. However, for microgel particles, it has been

proved that the cross-link density is inhomogeneous in the particle, due to a higher

reactivity of cross-linker. It means the cross-link density inside the microgel is higher

than the outside part, which leads to a fuzzy surface of particles as shown in Figure

1.6. Stieger and coworkers[5]

proposed a model of the polymer mass profile of

microgel particle with radius and a Gaussian of width , implying that the

radius measured by scattering method is . In the inhomogeneous

model, the form factor is { [ ]

[

]}

. Such

model has been successfully employed in PNIPAm microgels.[5, 6]

Here we use SLS to

investigate copolymer microgels of PVCL/NIPAm and PVCL/NIPMAm. Each

system includes two copolymer compositions: one is VCL-rich and the other is

acrylamide-rich. The dependence of intensity on of PVCL/NIPAm and

PVCL/NIPMAm microgels is presented in Figure 4.1.


54

1E-3

1E-5

1E-4

1E-3

0.01

0.1

I(q

) (a

.u.)

q (nm-1)

PVCL/NIPAm (5:1)

inhomogeneous microgel fit

homogeneous sphere fit

a

1E-31E-5

1E-4

1E-3

0.01

I(q

) (a

.u.)

q (nm-1)

PVCL/NIPAm (1:5)



b

1E-31E-5

1E-4

1E-3

0.01

0.1

I(q

) (a

.u.)

q (nm-1)

PVCL/NIPMAm (5:1)



c

1E-3

1E-5

1E-4

1E-3

0.01

I(q

) (a

.u.)

q (nm-1)

PVCL/NIPMAm (1:5)



d

Figure 4.1. Static light scattering profiles of (a) PVCL/NIPAm (5:1), (b)

PVCL/NIPAm (1:5), (c) PVCL/NIPMAm (5:1) and (d) PVCL/NIPMAm (1:5) at 20

°C. The dash-dotted lines represent homogeneous sphere fit, and the solid lines

represent inhomogeneous microgel fit.

Both inhomogeneous microgel fit and homogeneous sphere fit are performed

on the static light scattering profiles. We can see that the inhomogeneous microgel

model fits the experimental data very well, while the homogeneous sphere fit deviates

from the experimental result a lot. This means PVCL/NIPAm and PVCL/NIPMAm

copolymer microgels, rich in either VCL or acrylamide, are inhomogeneous in

polymer mass distribution.[5, 6]

Based on the inhomogeneous microgel fit, the values

of and are obtained and summarized in Table 4.1. For PVCL/NIPAm and

PVCL/NIPMAm copolymer microgels, the values of are around 50 nm,


55

indicating the thickness of the fuzzy surface of the particle. The overall particle size

from SLS ( ) is smaller than the hydrodynamic radius from DLS ( ). It is attributed

it to that some dangling polymer chains attached to the microgel surface contribute to

the hydrodynamics.[5]

An interesting observation is that the difference between and

is much larger for VCL-rich copolymer microgels than NIPAm- and NIPMAm-

rich copolymer microgels. It could be originated from the different chemical

structures. However, the intrinsic reason is still unknown and needs further

investigation.

Table 4.1. Summary of parameters obtained by inhomogeneous microgel fit of SLS

data and hydrodynamic radius from DLS.

Samples (nm) (nm) (nm)a (nm)

b

V:N (5:1)c 206 51 308 366

V:N (1:5) 295 47 389 399

V:NM (5:1) 198 45 288 351

V:NM (1:5) 336 47 430 444

a, ; b, is the hydrodynamic radius from DLS; c, V:N and V:NM

represent the monomer ratio of VCL:NIPAm and VCL:NIPMAm in copolymer

microgels.

4.3.2. Microscopy observation of microgels with different cross-linking degrees

Using proton transverse relaxation NMR and Flory temperature-induced

volume transition theory, Balaceanu and coworkers have revealed that PVCL

homopolymer microgels have a core-corona structure, and the polymer segment

distribution is influenced by cross-linker concentration.[3]

However, the direct

microscopy observation on the structure of PVCL homopolymer microgels is seldom

reported. Herein, we used AFM to characterize PVCL homopolymer microgels. From


56

the images in Figure 4.2, we can see that PVCL homopolymer microgels also show

core-corona structures.

Figure 4.2. AFM topography images of PVCL homopolymer microgels with different

BIS contents: 6 mol% (a and d), 4 mol% (b and e), and 1.6 mol% (c and f). Graphs

(d), (e) and (f) are the magnified scanning of square area in (a), (b) and (c),

respectively. The lines in d), e) and f) highlight the height of corona part of microgels.

Through the cross section profile (Figure 4.3), we can obtain the height and

contact angle of microgel particles on silicon wafer, as summarized in Table 4.2. We

can see that as the amount of cross-linker decreases, the height and contact angle of

microgels decreases. For PVCL microgels with 1.6 mol% BIS, the contact angle is

even lower than 10°, and the cross section profile is very flat, indicating a strong

deformation of microgels on the solid surface.


57

0 100 200 300 400 500 600 700

0

20

40

60

80

1.6 mol% BIS

4 mol% BIS

nm

nm

6 mol% BIS

Figure 4.3. The height cross-section profiles of PVCL homopolymer microgels with

different BIS contents at the position indicated by white lines in Figure 4.3 d, e and f.

Table 4.2. Dimensions of PVCL microgels with different cross-linker (BIS) contents

deposited on the silicon wafer detected by AFM.

BIS content

(mol%)

Contact radius

( , nm)

Height

( , nm)

Contact angle

( , °)

6 255 72 70

4 268 44 46

1.6 291 12 8

Besides the PVCL microgels, the PNIPAm-based microgels also have core-

corona structures, detected by cryo-FESEM or AFM.[16, 17, 22, 29]

Figure 4.4 shows the

AFM images of some PVCL/NIPAm (1:1) microgels varied in cross-linker contents.

We can see that copolymer microgels exhibit the representative core-corona structure

in AFM images, especially in phase images. Due to the low density of polymer

network, the corona part of microgel is very thin in thickness, which can be found in

the height cross-section profile (around 2 nm). As cross-linker content in microgels

decreases, the size of corona part becomes larger. The same as PVCL microgels, the

height of PVCL/NIPAm copolymer microgels decreases with the cross-linker content.


58

0 1 2 3 4 5

0

50

100

1.6 1.8 2.00

2

4g

nmm

0 1 2 3 4 5

0

20

40

60

1.4 1.6 1.8-2

0

2

nmm

h

0 1 2 3 4 5

0

10

20

30

0.0 0.5 1.0 1.5

0

1

2

i

nmm

Figure 4.4. AFM results of PVCL/NIPAm microgels with different BIS contents: 3

mol% (a, d and g), 1 mol% (b, e and h), and 0.5 mol% (c, f and i). Graphs (a), (b) and

(c) are topography images; (d), (e) and (f) are phase images; (g), (h) and (i) are the

height cross-section profiles of microgels at the position indicated by white lines in

(a), (b) and (c). The insets in (g), (h) and (i) highlight the height of corona part of

microgels (~ 2 nm).

The dimensions of PVCL/NIPAm copolymer microgels upon cross-linking

density are summarized in Table 4.3. We can see that the height and the contact angle

of microgels on the solid substrate decrease with the reduction of cross-linker content.

For microgels with 3 mol% BIS, the height is around 100 nm and the contact angle is

30 degree, whereas for microgels with 0.05 mol% BIS, the height is less than 10 nm


59

and the contact angel is even less than 1 degree. The contact radius ( ) of

microgels increases with the cross-linker content: from around 500 nm for microgels

with 3 mol% BIS to more than 2200 nm for microgels with 0.05 mol% BIS. The

results show that the microgels, especially with low cross-linker contents, are highly

spread on the solid substrate. The deformation of microgels upon cross-linking

density will be discussed in detail in Chapter 5.

Table 4.3. Dimensions of PVCL/NIPAm (1:1) microgels with different cross-linker

contents deposited on the silicon wafer detected by AFM.

BIS content

(mol%)

Contact radius

( , nm)

Height

( , nm)

Contact angle

( , °)

3 510 97 30.0

1.5 862 53 12.0

1 915 39 9.8

0.5 1204 27 6.5

0.25 1295 20 2.7

0.1 1354 11 1.4

0.05 2241 8 0.8

PVCL/NIPAm (1:1) microgels with 3 mol% BIS are deposited on the silicon

wafer at 50 °C which is above the VPTT of microgels. The sample is characterized

with AFM, and the results are shown in Figure 4.5. Compared with the sample

prepared at room temperature (Figures 4.2a, d and g), the size of microgel deposited

above VPTT is much smaller. Furthermore, the microgels do not show a core-corona

structure, in both topography and phase images. From Figure 4.5c, we can see that the

ends of the height cross-section profile do not have flat part with low height as seen

for samples prepared at room temperature.


60

0.0 0.2 0.4 0.6 0.8 1.0

0

10

20

30

40

0.35 0.40 0.45

0

5

10

mnm

c

Figure 4.5. (a) Topography and (b) phase images of PVCL/NIPAm (1:1) microgels

with 3 mol% BIS deposited on silicon wafer at 50 °C; (c) the cross-section profiles of

microgels at the position indicated by white lines in (a).

As temperature increases from room temperature to higher ones above VPTT,

the polymer chains collapse and the microgels shrink to a smaller size. It is indicated

that the volume of the corona part is reduced more than the core part during the

volume phase transition.[30]

The fuzzy corona part of the microgel contracts with the

core part to form a single monolithic structure.[5, 6]

Some work has been reported on

the in situ observation of microgels under water by AFM.[23-26]

The volume phase

transition of microgels on a substrate was probed through increasing temperature.

However, the microgel particles are restricted due to a strong adhesion between the

polymer chains and the substrate, especially for microgels with low cross-linking

density.[23, 24, 26]

Therefore, the microgel size changes significantly in the vertical

direction (height) but slightly in the horizontal direction (contact radius). For

microgels with high cross-linker contents, the restriction of the adsorption, though


61

still exists, is not very strong and the particles can shrink at both height and contact

radius.[26, 31]

Here, most water is expelled out from the microgel upon the fast spin-

coating process and the whole process for the sample preparation is kept at a

temperature above VPTT. Thus, the microgels are fixed on the solid substrate in

collapsed state. The result should better reflect the morphology and size of microgels.

4.3.3. Polymer network domains in microgels observed by AFM

Polymer gels, including microgel, consist of the three-dimensional network of

flexible polymer chains with chemical or physical cross-linking.[32]

It is assumed that

the cross-linked network structure can be divided into small domains centered with

cross-linking junctions.[32, 33]

Keerl et al. infered internal nanophase separated ‘dirty

snowball’ structure in PNIPAm/NIPMAm copolymer microgels at the phase

transition temperature from SANS results, and the size of collapsed segment is around

10 nm.[34]

A direct observation of PNIPAm and poly (acrylamide) gel surfaces has

been successfully implemented on AFM by Suzuki and coworkers.[35]

Sponge-like

domains of 100 nm size and of 10 nm size were found in PNIPAm and PAAm (poly

acrylamide) gel, respectively. Normally, the observation of microgels by AFM is

executed on the samples with high or medium cross-linking degree. The overlaps of

layers of polymer networks make the surface of microgel smooth under AFM. Here,

we use AFM to study the PVCL/NIPAm microgels with a very low BIS content (0.05

mol%) specifically.

As shown in Figure 4.6, the sponge-like domains is clearly found. In the

topography images, the dark-colored meshes are surrounded by the bright-colored

domains, like holes distributed on a polymer film. The average size of sponge-like

domains, analogous to the center-to-center distance of neighboring holes, can be


62

estimated by power spectral density (PSD) profiles converted from AFM images.[36]

From Figure 4.6c, the peak of PSD profile is located at 23 μm-1

, which means the

average domain size in PVCL/NIPAm (0.05 mol% BIS) microgels is around 43 nm. It

should be noted that even with very low cross-link degree, the overlaps of polymer

network layers could not be avoided, that obstructs the accuracy of estimating the

domain size. Therefore, the domain size obtained here is larger than that gained from

scattering methods.[34]

The average diameter of holes in Figure 4.6a, which can be

considered as polymer network mesh, is around 17 nm. It is bigger than the mesh size

(around 2.5 nm) estimated for PNIPAm microgels with higher cross-linker content (~

1 mol%) by Saunders.[7]

0 20 40 60 80 100 120 140

-4

-3

-2

-1

0

1

Lo

g (

PS

D/n

m2)

d-1 (m

-1)

c

Figure 4.6. (a) Topography and (b) phase images of sponge-like domains detected by

AFM in PVCL/NIPAm microgels with a low BIS content (0.05 mol%); (c) a 2D

power spectral density plot of image (a), d in the horizontal axis is the center-to-center

distance of dark dots.

4.3.4. Microscopy observation of PVCL-based microgels with functional groups

According to the reported procedures,[11, 37]

PVCL/AAEM/VIm and

PVCL/AAEM/AAc microgels with different cross-linking degrees were synthesized.

As shown in Chapter 3, the incorporation of functional groups brings the copolymer

microgels temperature and pH-sensitive properties. The morphologies of


63

PVCL/AAEM/VIm and PVCL/AAEM/AAc microgels were obtained with AFM and

FESEM, shown in Figures 4.7 and 4.8.

The microscopy results show that PVCL/AAEM/VIm microgels exhibit

distinct core-corona structures, regardless of cross-linking degree. The corona-like

shell in dark shadows surrounding the PVCL/AAEM/VIm microgel particles are

found in FESEM images, which has also been reported by Pich et al. before.[11]

As the

AFM is much more sensible at the small scale, the corona part of microgels is more

distinguishable in AFM images than FESEM images. In the height cross-section

profile of PVCL/AAEM/VIm microgels, it is obviously found the corona part with a

very low height outside the core part (Figure 4.9a). From the center toward the edge

of microgel, the height decreases slightly at the beginning, then drops sharply to a low

position, and goes down slowly to the end of microgel particle.

Figure 4.7. AFM topography (a, b and c) and FESEM (d, e and f) images of

PVCL/AAEM/VIm microgels with different BIS contents: 3 mol% (a and d), 1 mol%

(b and e), and 0.35 mol% (c and f).


64

Compared to PVCL/AAEM/VIm microgels, the PVCL/AAEM/AAc microgels

hardly show core-corona structure in both AFM and FESEM morphologies

represented in Figure 4.8. The height cross-section profile decreases sharply from the

flat top to the substrate surface (Figure 4.9b). It should be noted that the synthesis

procedure of PVCL/AAEM/AAc microgels differs from that of PVCL/AAEM/VIm

microgels. A different initiator and an extra surfactant SDS used in

PVCL/AAEM/AAc microgel synthesis might be the reason leading to a structure

without fluffy corona part. However, an additional surfactant CTAB is used in the

synthesis of PVCL microgels, but it does not make the core-corona structure vanish

(Figures 4.3 and 4.4).

Figure 4.8. AFM topography (a, b and c) and FESEM (d, e and f) images of

PVCL/AAEM/AAc microgels with different BIS contents: 3 mol% (a and d), 1 mol%

(b and e), and 0.35 mol% (c and f).


65

0.0 0.2 0.4 0.6 0.8 1.0 1.2

0

40

80

120

160

m

nm

a

0.0 0.2 0.4 0.6 0.8 1.0

0

40

80

120 b

m

nm

Figure 4.9. Cross-section profiles of (a) PVCL/AAEM/VIm and (b) PVCL/AAEM/

AAc microgels with 1 mol% BIS at the position indicated by white lines in Figures

4.7 and 4.8. The arrows show the positions of two ends of the microgel particles.

The dimension of PVCL/AAEM/VIm and PVCL/AAEM/AAc microgels on

the silicon wafer detected by AFM is summarized in Table 4.4. As expected, for both

kinds of microgels, the height and contact angle decrease, whereas the contact radius

increases with the decreasing cross-linking density. Compared with PVCL/NIPAm

(1:1) microgels (Table 4.3), changes in microgel size upon deposition on the solid

substrate are much less for PVCL/AAEM/VIm and PVCL/AAEM/AAc microgels.

Contact angles of particles on the solid substrate are around 50 – 60° for

PVCL/AAEM/VIm microgels and 65 – 70° for PVCL/AAEM/AAc microgels. Both

values are higher than those of PVCL/NIPAm microgels with similar cross-linking

density.

The result indicates that PVCL/AAEM-based microgels are stiffer and spread

less on the silicon wafer compared with PVCL/NIPAm and PVCL microgels with

similar cross-linking densities. This is probably due to the incorporation of AAEM

monomers into the microgel. It has been found that AAEM segments are mostly

accumulated in the core of mcirogels due to their higher reactivity than VCL

monomers.[11, 38]

The accumulation of AAEM segments makes the core more rigid

which contributes to high values of heights and contact angles.


66

Table 4.4. Dimensions of PVCL/AAEM/VIm and PVCL/AAEM/AAc microgels with

different cross-linker (BIS) contents deposited on the silicon wafer detected by AFM.

Samplesa Contact radius

( , nm)

Height

( , nm)

Contact angle

( , °)

PVAV_3%BIS 313 148 62

PVAV_1%BIS 329 144 61

PVAV_0.35%BIS 387 126 53

PVAA_3%BIS 90 113 70

PVAA_1%BIS 94 109 65

PVAA_0.35%BIS 99 105 65

a, PVAV and PVAA represent PVCL/AAEM/VIm and PVCL/AAEM/AAc

microgels, respectively.

As references, the PVCL/AAEM and PVCL/AAEM/VIm (0.9 % VIm)

microgels with 1 mol% BIS are synthesized as well. The obtained microgels are

detected with AFM, as shown in Figure 4.10. It can be found that PVCL/AAEM

microgels do not exhibit apparent corona-like shell in AFM height morphology,

consistent with the reported results.[11]

However, the cross-section profile indicates the

existence of the corona part with fuzzy polymer chains surrounding the core part.

From the cross-section profile, the corona part is very small, around 50 nm from the

end to core part. Even containing very few amount of VIm (0.9 mol%),

PVCL/AAEM/VIm microgels have obvious core-corona structure in AFM

topographic morphology. Compared to PVCL/AAEM/VIm microgels with the same

amount of BIS but a higher content of VIm (5 mol%), as shown in Figure 4.8b, the

corona part of PVCL/AAEM/VIm (0.9 mol% VIm) microgels is smaller. The height

cross-section profiles indicate distances from the core to the surface are about 150 nm

for low VIm content microgels (0.9 mol%, Figure 4.10d), and around 250 nm for high

VIm conten microgels (5 mol%, Figure 4.9a).


67

0.0 0.2 0.4 0.6 0.8 1.0

0

50

100

150

nmm

c

0.0 0.2 0.4 0.6 0.8 1.0

0

50

100

150 d

mnm

Figure 4.10. AFM topography images of (a) PVCL/AAEM, and (b) PVCL/

AAEM/VIm with 0.9 mol% VIm. (c) and (d) show the cross-section profiles of

microgels at the position indicated by white lines in (a) and (b). The arrows show the

positions of two ends of the microgel particles. The BIS content is 1 mol% for all

microgels.

4.4. Conclusions

In this Chapter, we investigate the internal structure of PVCL-based

copolymer microgels with SLS and microscopy methods (AFM and FESEM). The

SLS results show that PVCL/NIPAm and PVCL/NIPMAm microgels, both with

different monomer ratios in copolymer, have core-corona structures with a radial

profile as reported for PNIPAm microgels previously. The characterization of

PVCL/NIPAm copolymer and PVCL homopolymer microgels by AFM gives

morphologies and dimensions (contact radius and height) of microgels on a solid

substrate. It provides visualized evidence of core-corona structure that influenced by

cross-linker content. The AFM characterization of PVCL/NIPAm copolymer

microgels deposited on a solid substrate at the temperature above VPTT reveals the

structure of microgels in the collapsed state, which is highly consistent with the

reported results by SANS and SLS. The fuzzy corona part of the microgel contracts

with the core part to form a single monolithic structure. Besides, PVCL-based


68

microgels with functional groups are characterized with AFM and FESEM. It is

interesting that the core-corona structure is found in PVCL/AAEM /VIm microgels,

but not in PVCL/AAEM/AAc microgels, which might be originated from their

different synthesis procedures. Additionally, the AFM observation on microgels of

very low cross-linking density provides a possible method to look into the domains in

the cross-linked polymer networks.

4.5. References



Polymer Science, 1994, 272, 467-477.

[2] Guillermo A., Addad J. P. C., Bazile J. P., Duracher D., Elaissari A. and

Pichot C., NMR investigations into heterogeneous structures of thermosensitive

microgel particles, Journal of Polymer Science Part B-Polymer Physics, 2000, 38,

889-898.





[4] Richtering W., Berndt I. and Pedersen J. S., Determination of microgel

structure by small-angle neutron scattering In Microgel Suspensions; Wiley-VCH

Verlag GmbH & Co. KGaA: 2011, p 117-132.







[7] Saunders B. R., On the structure of poly(N-isopropylacrylamide) microgel

particles, Langmuir, 2004, 20, 3925-3932.

[8] Mason T. G. and Lin M. Y., Density profiles of temperature-sensitive

microgel particles, Physical Review E, 2005, 71.


69










[12] Pich A., Berger S., Ornatsky O., Baranov V. and Winnik M. A., The influence

of PEG macromonomers on the size and properties of thermosensitive aqueous

microgels, Colloid and Polymer Science, 2009, 287, 269-275.

[13] Cheng C., Zhu X. M., Pich A. and Moller M., Aqueous microgels modified by

wedge-shaped amphiphilic molecules: Hydrophilic microcontainers with hydrophobic

nanodomains, Langmuir, 2010, 26, 4709-4716.

[14] Jones C. D. and Lyon L. A., Synthesis and characterization of multiresponsive

core-shell microgels, Macromolecules, 2000, 33, 8301-8306.

[15] Hoare T. and McLean D., Kinetic prediction of functional group distributions

in thermosensitive microgels, Journal of Physical Chemistry B, 2006, 110, 20327-

20336.







[18] Crassous J. J., Ballauff M., Drechsler M., Schmidt J. and Talmon Y., Imaging

the volume transition in thermosensitive core-shell particles by cryo-transmission

electron microscopy, Langmuir, 2006, 22, 2403-2406.

[19] Li Z. F., Kwok M. H. and Ngai T., Preparation of responsive micrometer-sized

microgel particles with a highly functionalized shell, Macromolecular Rapid

Communications, 2012, 33, 419-425.

[20] Lau A. W. C., Portigliatti M., Raphael E. and Leger L., Spreading of latex

particles on a substrate, Europhysics Letters, 2002, 60, 717-723.

[21] Dreyer J. K., Nylander T., Karlsson O. J. and Piculell L., Spreading dynamics

of a functionalized polymer latex, Acs Applied Materials & Interfaces, 2011, 3, 167-

176.


70

[22] Horecha M., Senkovskyy V., Synytska A., Stamm M., Chervanyov A. I. and

Kiriy A., Ordered surface structures from PNIPAM-based loosely packed microgel

particles, Soft Matter, 2010, 6, 5980-5992.

[23] Hofl S., Zitzler L., Hellweg T., Herminghaus S. and Mugele F., Volume phase

transition of "smart" microgels in bulk solution and adsorbed at an interface: A

combined AFM, dynamic light, and small angle neutron scattering study, Polymer,

2007, 48, 245-254.

[24] Wiedemair J., Serpe M. J., Kim J., Masson J. F., Lyon L. A., Mizaikoff B. and

Kranz C., In-situ AFM studies of the phase-transition behavior of single

thermoresponsive hydrogel particles, Langmuir, 2007, 23, 130-137.

[25] Tagit O., Tomczak N. and Vancso G. J., Probing the morphology and

nanoscale mechanics of single poly(N-isopropylacrylamide) microgels across the

lower-critical-solution temperature by atomic force microscopy, Small, 2008, 4, 119-

126.

[26] Burmistrova A., Richter M., Uzum C. and von Klitzing R., Effect of cross-

linker density of P(NIPAM-co-AAc) microgels at solid surfaces on the

swelling/shrinking behaviour and the Young's modulus, Colloid and Polymer Science,

2011, 289, 613-624.





123-136.









3235-3243.

[32] de Gennes P.-G., Scaling Concepts in Polymer Physics; Cornell University

Press: Ithaca, NY, 1979.

[33] Flory P. J. and Rehner J., Statistical mechanics of cross-linked polymer

networks I. Rubberlike elasticity, Journal of Chemical Physics, 1943, 11, 512-520.


71

[34] Keerl M., Pedersen J. S. and Richtering W., Temperature sensitive copolymer

microgels with nanophase separated structure, Journal of the American Chemical

Society, 2009, 131, 3093-3097.

[35] Suzuki A., Yamazaki M. and Kobiki Y., Direct observation of polymer gel

surfaces by atomic force microscopy, Journal of Chemical Physics, 1996, 104, 1751-

1757.

[36] Kannaiyan D., Cha M.-A., Jang Y. H., Sohn B.-H., Huh J., Park C. and Kim

D. H., Efficient photocatalytic hybrid Ag/TiO2 nanodot arrays integrated into

nanopatterned block copolymer thin films, New Journal of Chemistry, 2009, 33,

2431-2436.







Chapter 5. Deformation and Spreading of Microgels

73

Chapter 5. Deformation and Spreading of Copolymer

Microgels at the Air/Water and Air/Solid Interfaces

5.1. Introduction

Deformability plays an important role in not only synthetic materials, but also

natural ones. A typical example in nature is that the extraordinary flexibility of

mammalian cells enables them to deform in order to adapt the blood circulation

environment.[1, 2]

In a simulation of renal filtration of soft particles, microgels can

translocate through pores that are only one tenth of the particle diameter under

physiologically relevant pressures.[3]

Such a translocation depends on the

deformability of microgels which is controlled by the cross-linking density.[1, 3]

It

gives a hint for the design of soft micro/nano particles with desired in vivo

biodistributions.[1]

Besides the translocation of cells or soft particles, the in vitro cell

culture on a solid substrate also concerns the deformation and adhesion of cells.[4, 5]

A

similar deformation has also been observed for microgels deposited on a solid

substrate.[6]

Microgels are soft particles consisting of an intramolecular cross-linked

network of polymer chains swollen with a solvent. They are currently investigated

due to their role in applications including coating,[7, 8]

drug delivery and release,[9, 10]

sensors,[11, 12]

cell culturing substrates[6, 13]

and emulsion stabilizer.[14, 15]

Although a

completely unanimous definition on microgel size range is absence, it is generally

accepted that the swollen particles are in the range of 10 – 1000 nm.[13, 16, 17]

Macromolecular elasticity confers deformability with enhanced colloidal stability of

suspensions of microgel. These unique features invoke for improved understanding of


74

interfacial behavior of soft colloids and their dynamics at a fluid/fluid or fluid solid

interfaces. In particular microgels exhibit a radial gradient in the cross-linking density

resulting in a core-shell structure.[18]

Adsorption of such tree-dimensional branched

macromolecules enhances steric repulsion of the chains; therefore surface-induced

scission of covalent bonds will need to be considered depending on the cross-linking

density and the strength of adsorption.[19, 20]

The dynamics of interfacial attachment in a fluid medium is of interest and

applicability to several physical and biological systems.[21]

From the theoretical point,

the classic model deduced by Johnson, Kendall and Roberts (JKR model) clearly

describes the adhesion between elastic bodies.[21-23]

According to this model, a contact

radius of an elastic sphere with radius on a solid substrate can be expressed

as ⁄ ⁄ , where is the particle shear modulus and W is the work

of adhesion. This scaling relation indicates that to achieve a large contact area and

consequently enhanced adhesion requires large particles (

) with very low

shear modulus (soft particles, ).[24]

On the other hand, this formula works

very well for strongly cross-linked large particles which are subjected by a small

shape deformation during adhesion.

However, in the case of weakly cross-linked soft small nanoparticles, for

example microgels, which highly deform upon particle adhesion, the JKR model is

not applicable. Recent theoretical effort provided a new scaling relationship for

nanoparticle deformation as ⁄ , where the surface energy of the

particle is considered besides the other particle parameters which are all included in

JKR model.[24]

They demonstrated that the JKR theory is applicable at small values of

the deformation ( ), whereas significant deviation from the JKR theory is

observed at higher deformations (comparable with the particle size).


75

In this Chapter we try to address the issue of the interfacial attachment of

microgels depending on the cross-linking density. For this purpose, we investigated

monodisperse colloidally stable PVCL/NIPAm microgels synthesized with different

amounts of cross-linker. We assessed the interfacial tension of the microgel at the

water/air interface. Particular emphasis is on the adsorption dynamics depending on

the cross-linking density. Adsorbed microgels on the solid surface were investigated

by mapping the profile and measuring the contact radius with scanning force

microscopy.

We demonstrate that the softness of microgel colloids strongly influence the

dynamics of adsorption rather than the equilibrium air/water interfacial tension. It is

remarkable that the time needed for soft particles to adsorb at the air/water interface

can be three orders of magnitude faster than that for stiffer ones. Insight on the

adsorption dynamics could be gained by the computer simulation. The data shows that

two effects concur and lead to enhanced adsorption dynamics: low friction coefficient

for loosely cross-linked microgels combined with the chain flexibility which sustains

deformation and spreading at the interface. These results are complemented by the

detailed analysis of the adsorption of microgels on the solid surface. We note in this

regard that the density of the polymer network limits the spreading of microgels at an

interface, while in solution opposes the expansion by the osmotic pressure from the

solvent. It is also found that a loosely cross-linked microgel can spread and cover a

large surface area with a radius of two- to four-fold greater than the hydrodynamic

radius of the microgel in the swollen state.


76


5.2.1. Sample preparation

Microgel synthesis

PVCL/NIPAm microgels with different monomer ratios and cross-linker

contents are used in this Chapter. The synthesis and size characterization of microgels

are presented in detail in Chapter 3.

Deposition of microgels on a solid substrate

The deposition of microgels was carried out on a spin-coating machine

(Convac 1001S, Germany) with the speed of 3000 rpm for 1 min. The silicon wafer

used for deposition was cleaned by sonication in isopropanol for 10 min, dried in air

stream, and treated with UV/O2 for 12 min. The concentration of microgel solution

used for spin coating was 0.2 mg/mL. At this microgel concentration, sufficient

amount of particles could be well distributed on the silicon wafer without clustering

effect. The quality of the samples was evaluated with a Zeiss Axioplan 2 optical

microscope. For each sample, a drop of 75 μL microgel solution was used.


Dynamic light scattering (DLS)


Scattering Multiple Tau Digital Correlator and Electronics with the scattering angle

set at 90°. The samples were measured at 20 and 50

°C, below and above VPTT of the

microgels, respectively. The temperature fluctuations were below 0.1 °C. Prior to the

measurement microgel samples were diluted with doubly distilled water and filtered

through 2.5 µm filter.


77

Dynamic surface tension

The dynamic surface tension of microgel solutions was measured through the

pendant drop method on a DSA 100 tensiometer (Kruss, Germany). Aqueous

dispersions of microgels were prepared with deionized water (Milli-Q Academic A-10

system, Millipore). The microgel solutions used for surface tension measurement are

of 0.2 mg/mL, the same as used for spin-coating. To reduce the experimental errors

from the evaporation of the droplet, the needle was put in a cuvette with some water

at the bottom sealed with a para film. The setup is shown in Figure 5.1.

Figure 5.1. Schematic illustration of the pendent drop sealed in a cuvette with water

at its bottom in the surface tension measurement.

Atomic force microscopy

Atomic force microscopy (AFM) images were recorded on an Agilent 5500

Atomic Force Microscope in tapping mode. Cantilevers with resonance frequencies of

250 – 300 kHz and spring constants around 42 N/m were used. Picoimage software

provided by Agilent was used to analyze the recorded AFM results.

5.2.3. Computer simulation of microgels at the interface

We consider two microgel particles of nearly equal molecular weight differing

in the cross-linking density, presented in Figure 5.2. The microgels have an ideal

network structure similar to diamond lattice: each cross-link has the functionality four


78

(originates four subchains). To provide a “spherical” shape of the particle, the

network is placed into a sphere with further deleting the outer beads. Weakly cross-

linked microgel consists of 64 subchains, each of 19 beads; 571 beads form dangling

chains so that the total number of the beads in the particle is equal to 1787. The

densely cross-linked particle consists of 1785 beads including 212 subchains, each of

8 beads, and 89 beads in the dangling chains, as shown in Figure 5.2.

Figure 5.2. Initial (before annealing) diamond lattice structures of weakly (a) and

highly (b) cross-linked microgels. The total number of beads in the microgel, the

number of the beads per subchain, and the number of the subchains are 1787, 19, 64

(a) and 1785, 8, 212 (b), respectively.

Explanation of the observed evolution of the surface tension coefficient can be

done based on the mobility of the microgels and their volume in the solution (water).

All the issues are studied with the dissipative particle dynamics (DPD) simulation

technique.[25, 26]

Detailed description of the simulations can be found in literatures.[27,

28]

Each microgel particle, depicted in Figure 5.3, was placed in a cubic box of a

constant volume with periodic boundary conditions in

the x, y and z directions. We set the total number density in the system as

, so that the total number of the microgel and solvent particles (denoted as M and S,


79

respectively) in the box was . Repulsion between the particles of the

system is quantified by interaction parameters and which can be related

to the Flory-Huggins parameters according to the formula:

at .[27, 29]

Annealing of the microgels (transformation of the initial

structure, Figure 5.2, into equilibrium one) was performed in good (

) and poor ( and ) solvents.

Figure 5.3. The absolute value of the total displacement of the microgel as a

function of time steps t for different solvents and cross-linking densities: (a) dense

microgel in poor (red circles) and good (black squares) solvents; (b) dense (red

circles) and loose (black squares) microgels in the poor solvent; (c) dense (red circles)

and loose (black squares) microgels in the good solvent.

After attaining the equilibrium swelling degree, an average velocity of the

center of mass of the microgel was examined. The velocity is defined as a

derivative of the coordinate of the center of mass with respect to time ,

⁄ ⁄ , which can be approximated as a displacement of the center


80

of mass during short time interval : the shorter the interval, the better the

approximation. In the computer simulations, we measured the absolute value of

every simulation steps and plotted the total displacement | |

| | as a function of time . Mobility of the microgel is

different in good and poor solvents, as seen in Figure 5.3. The linear fit of is

quite accurate and average velocity of the microgel is determined by a slope of the

straight. We can see that the dense microgel has higher mobility in the poor solvent.

The ratio of the average velocities (slopes) of the dense microgel in different solvents

is

⁄ .

The same behavior is observed for the loose microgel (not shown). In both

cases, the microgel has a compact form in the poor solvent. The characteristic size of

the microgels, which is defined as a square root of the mean square radius of gyration,

was measured in units of the box cell: ,

,

and

. The swelling coefficient was defined as ⁄ :

and . Therefore, the difference in the mobility can be

explained by a model of impenetrable spheres: the friction coefficient obeys to the

Stokes’ law and proportional to the radius of the microgel. The arguments in favor of

this statement can be extracted from Figures 5.3 b and c: the velocities of both

microgels in the poor solvent are practicaly identical like their sizes,

⁄

, and mobility of the dense microgel in the good solvent (water) is 20% higher:

⁄ . Therefore, if we deal with equal weights of the dense and loose

microgels in the good solvent, the probability to reach the surface for the loose

particles is higher because they have approximately threefold excess in volume,

(

⁄ ) despite of 20% reduction in the mobility.


81

In order to simulate a water/air interface, we considered two incompatible

liquids (denoted by W and A) in the simulation box which were strongly segregated,

( and ). One of the liquids (“air”, A) was considered to be

poor for the microgel (M), , whereas both dense and loose microgels were

in the swollen state in the second liquid, . Such choice of the parameters

provided localization of the microgels at the interface with their further spreading.

Indeed, the high interfacial energy of bar water\air interface, , is

thermodynamically unfavorable if one of the subphases contains the microgels. Some

of them will be localized at the interface reducing the area of water/air contacts and,

as a result, the interfacial energy.

Computer simulations were performed in a box of a constant volume

with periodic boundary conditions in the x and y directions. The

upper part of the box was filled by a poor solvent (A) which occupied 30% of the box.

The bottom part of the box was filled by the good solvent (W). The dense and the

loose microgels were first pre-annealed in one-component good solvent (

and ). Then the corresponding swollen microgel was placed into

the bottom subphase in such a way to make a contact with the interface through one or

two polymer beads. Such contact was enough to keep the particles at the interface. In

the simulation, the lateral size of the microgel is described as (a square root of the

mean square of components of the radius of gyration), the contact diameter of the

microgel at the interface. The vertical size is , the height of the microgel at the

interface.

Based on the computer simulation demonstrated above, we obtain the

spreading dynamics and equilibrium shape of the particles at the interface.


82


5.3.1. Dependence of microgel modulus on the cross-linking density

The shear modulus is a representative parameter to describe the elasticity of

gels. A theory about predicting of the shear modulus of microgels from the

information of size and cross-linking density has been deduced and proved to be valid

on PNIPAm-based microgels,[30]

(

) (

) ⁄

(Equation 5.1)

where is the shear modulus of the microgel particle, is the volume fraction of

polymer within the particle, is the polymer volume fraction in the collapsed

microgel particle, is the average number of monomers between cross-links, and

is the monomer size.

Here the volume fraction of polymer in the particle can be considered as

. Assuming that the polymer mass does change with the

collapse of the particle, we can calculate that (

) ⁄

(

) ⁄

. The value of

is the average volume of the segment between cross-links,

thus

, where is the total number of cross-links in the

particle and is the volume of the microgel in the collapsed state. From the Flory

temperature-induced volume transition theory,[31, 32]

the value of can be obtained.

According to Equation 5.1, the shear modulus of microgels particles is

assessed. As seen from Figure 5.4, the shear modulus of microgels, regardless of the

cross-linking density, increases with temperature. We can also find a transition

temperature for microgel modulus at around 35 °C, equal to the volume phase

transition temperature. Besides, the shear moduli of microgels at a certain temperature


83

increase with the cross-linker content, as seen in Figure 5.5. This explains the

phenomenon that the higher cross-linker content leads to a lower swelling capability.

20 30 40 5010

100

1000

G (

kP

a)

Temperature (oC)

3%

1.5%

1%

0.5%

0.25%

0.1%

0.05%

Figure 5.4. Shear modulus revised according to the Flory shear modulus concerning

the polymer networks in solvent.

0.1 1

100

1000

50 oC

20 oC

G (

kP

a)

BIS content (mol%)

Figure 5.5. Dependence of shear modulus of microgels on cross-linker content.

It has been reported that the modulus of the individual microgel particle can be

obtained from the AFM indentation force measurement,[6, 30, 33, 34]

or calculated from

the plateau shear modulus of concentrated microgel suspensions.[35]

However, both

methods involve theoretical assumptions and inevitable experimental errors. In this


84

work, we mainly estimate the shear modulus of the individual microgel to obtain the

qualitative relation of the mechanical property and cross-linking density of the

microgel. For this reason, we adapt the predicting calculation. As the cross-linker

content increases from 0.05 to 3 mol%, the shear modulus becomes ten times higher

(Figure 5.5). The results firmly depict that the less cross-linking in the particle, the

softer is it.

5.3.2. Interfacial activity of microgels at the air/water interface

To study the effect if the cross-linking concentration on the microgel

deformation (spreading) at the air/solid interface, we first discuss the adsorption of

microgels at the water/air interface. In this regard, drops of aqueous microgel

suspensions are formed in air and the interfacial tension is measured as a function of

time. In a pendent drop tensiometry (PDT) the drop profile is automatically detected

and fitted with the Young-Laplace equation, extracting the interfacial tension value

.[36, 37]

Adsorption of microgels at the air/liquid interface lowers the system free

energy which translates into an effective interfacial tension reduction, therefore by

monitoring as a function of time we can obtain information about the adsorption

kinetics. Figure 5.6 shows that regardless of the cross-liking degree of the microgel

the surface tension decreases and eventually reaches a steady state with an

equilibrium value equal to about 46 mN/m. Since the microgel contains 50 mol%

VCL, we expect the surface tension to be different from previously reported

equilibrium surface tension for PNIPAm microgels (~ 42 mN/m).[38]

Remarkably the time required to achieve a steady state of the surface tension

increases with the particle cross-linking density. To make a more explicit comparison

we plotted in Figure 5.6b the half time , defined by the inflexion point


85

corresponding to the time it takes for the surface tension .[39]

is the surface tension at time , is the surface tension the instant that the drop is

formed and is assumed equal to the surface tension of water (72 mN/m at 23 °C),[40]

and is the surface tension at the steady state. The data suggest that weakly cross-

linked microgel particles exhibit marked adsorption rates compared to highly cross-

linked ones. In principle, adsorption depends on the concentration and requires the

diffusion and spreading of the microgels to the air/water interface. The surface tension

data do not differentiate between the two processes. Nevertheless, it has been

demonstrated that the interfacial activity is related to the microgel deformation

(spreading) at interfaces.[38, 41-44]

In the next section, we propose computer simulations

which explain the data on the surface tension.

1 10 100 100040

45

50

55

60

65

70 a

3%

1.5%

1%

0.5%

0.25%

0.1%

0.05%

(m

N/m

)

Drop age (s)

0.1 1

1

10

100

t* (

s)

BIS content (mol%)

b

Figure 5.6. (a) Dynamic surface tension for microgels with different cross-linker

contents. (b) half time correspond to the inflection point of the dynamic surface

tension curves (a) (see text for detail). The microgel concentration was kept constant

and corresponds to 0.2 mg/mL.


86

5.3.3. Conformation of microgels at the air/water interface

The spreading dynamics of dense and loose microgels is presented in Figure

5.7 as a function of the lateral size of the microgel (a square root of the mean

square of components of the radius of gyration) versus time steps. A slope of the

tangent to each point of the fitting curve characterizes a rate of the spreading of the

polymer microgel in x and y directions. One can see that both microgels increase the

lateral size with time from the size in the bulk solution ( ) up to equilibrium

value at the interface ( ). The loose microgel spreads faster in the beginning of

the process because of the lower elastic moduli. Slowing of the microgel spreading at

the late stage is due to approaching the equilibrium state (plateau value of ).

Therefore, we can conclude that faster spreading of the loose microgel is another

contribution to the faster decay of the surface tension coefficient for the case of

weakly cross-linked microgels.

Figure 5.7. The average lateral diameter of the adsorbed dense and loose microgel

particle as a function of time steps.


87

Comparison of the equilibrium shapes of the loose and dense microgels at the

interfaces is presented in Figure 5.8. One can see that the loose microgel has more flat

shape with characteristic diameter , and height

in

comparison with the denser microgel ,

. Therefore, one

needs a smaller number of the loose microgels in order to cover the water/air interface

and its saturation occurs earlier with the loose particles.

Figure 5.8. Snapshots of equilibrium structures of the dense (upper) and loose

(bottom) microgels at interfaces.

5.3.4. Dimensions of microgels varied in the cross-linking degree

The morphologies of microgel particles with different cross-linker

concentrations captured with AFM and corresponding dimensions (contact radius and

height) have been shown in Chapter 4 (Figure 4.2 and Table 4.3). Different form solid

rigid spheres, the cross-linking density of microgels increases towards the particle

center.[18]

There are some polymer chains dangling on the particle surface, which

results in fuzzy particle surface.[6, 45]

Such an inhomogeneous internal structure,

namely the core-corona structure, can be figured out from the AFM images and

corresponding height profiles. We believe that the exterior part of the microgel on the


88

solid substrate is the corona formed from fluffy polymer chains. The results in

Chapter 4 show that the microgels, especially with low cross-linker contents, are

highly spread on the solid substrate.

Figure 5.9 shows characteristic dimensions of microgels in solution and

adsorbed on the solid surface. When the microgel adsorbs from the bulk solution to a

solid substrate, several areas can be distinguished: i) a thick dense core of the

microgel determines the height h; ii) a thin extended outer layer of the microgel

defines the contact radius ; iii) the most expanded microgel conformation in

solution defined by the hydrodynamic radius ; and iv) the collapsed state of the

microgel in solution above VPTT is represented by .

Figure 5.9. (a) Height image of dehydrated microgel on solid surface and (b) a

scheme of the height profile and (c) is density profile of the swollen and collapsed

microgel according to reference[18]

to highlights the relation between the dense core

and height, as well as hydrodynamic radius and contact area.


89

While the AFM measurements give the size of microgels adsorbed on the soild

substrate (contact radius and height), the DLS results provide the size of microgels in

either swollen or collapsed state in bulk solution. The hydrodynamic radii of

microgels in both swollen and collapsed states measured with DLS at 20 and 50 oC,

respectively are shown in Figure 5.10. The hydrodynamic radii of microgels in

solution state increase if the cross-linker concentration is reduced. This effect is due to

the formation of the loose colloidal networks with larger amount of the dangling

chains on the surface what leads to the increase of hydrodynamic radius.[31, 45, 46]

However, this trend can be observed when the cross-linker content is above 0.25

mol%. As the cross-linker content is lower than 0.1 mol% during synthesis, the

hydrodynamic radii of microgels are only around 400 nm. That might be due to the

cross-linking is not enough to hold polymer chains in the microgel network when

small amount of cross-linker is used in synthesis.

0 1 2 3

0

800

1600

2400

Rh, collapsed

Rh, swollen

Mic

rog

el d

imen

sio

ns (

nm

)

Cross-linker amount (mol%)

acont

height

Figure 5.10. Microgel dimensions in contact with the solid surface and in solution

depending on the cross-linker content. The shape of the microgel on SiO2 is

described by its height and contact radius denoted by . For comparison, the

hydrodynamic radius in swollen ( ) and collapsed state ( ).


90

5.3.5. Deformation of microgels

The contact area is systematically larger that the radius of the particle in the

swollen state, unless the microgel is highly cross-linked. We note in this regard that at

an interface the density of the network limits the spreading of the microgel, while in

solution opposes the expansion by the solvent (osmotic pressure). Curiously, the

loosely cross-linked particle spreads and covers a large surface area with a radius of

two- to four-fold greater than the hydrodynamic radius of the microgel in the swollen

state (Figure 5.11). This indicates that the spreading involves a significant

deformation (stretching) of the microgel relative to that induced by the swelling. In

the swollen state, the particle contains 80 – 90 % water,[31]

which partially evaporates

upon dehydration in ambient conditions and the microgel should in principle shrink at

the surface (air is the bad solvent).

In principle, when a colloidal particle with a radius in contact with a solid

substrate, the ratio of the reduced height in the vertical direction describes

the extent of deformation of the particle (Figure 5.9).[47]

For instance, a hard sphere

does not deform ( ), whereas a highly compliant soft particle flattens and the

deformation may approach its diameter ( ). Microgels exhibit a radial gradient

in the cross-linker concentration resulting in a core-shell structure. As a consequence,

the shell and core respond differently to the environment cue, because of a higher

cross-linking degree at the interior of the particle. Expansion of the shell provides

colloidal stability whereas the densest core delimits their size in the collapsed state. A

balance between the expansion by the solvent and the elasticity of the network defines

the equilibrium size of the microgel particle. However, it is not known that to which

extent the presence of an interface modifies this balance and therefore affects the

shape and size of the microgel particle.


91

Figure 5.11 indicates that the highly cross-linked microgel placed on a surface

assumes a hemispherical shape with a contact radius close to the hydrodynamic radius

of the particle in solution and the height is limited by the compressibility of the dense

core. As the cross-linker content is reduced from 3 to 1 mol%, the height decreases

with the intramolecular cross-linking, whereas the contact radius changes slightly.

The deformation increases and becomes saturated at around 90% relative to the

diameter of the microgel in the collapsed state. Above this limit, deformation of the

loosely cross-linked microgel undergoes a small change, despite an increase an

increase of more than 500% of the contact radius relative to the radius of the

expanded state.

Figure 5.11. The contact radius, normalized to the hydrodynamic radius in the

swollen state, as a function of the cross linker concentration (solid symbol). Reduced

height of the microgel, normalized to the hydrodynamic radius in the collapsed state,

as a function of the cross linker concentration (open symbol).


92

The contact radius and height results from a balance between spreading which

tends to maximize the contact area and the restoring network elasticity which tends to

minimize deformation of the microgel on the solid surface. In contrast, the

hydrodynamic radius of the microgel in solution results from a balance of osmotic

pressure and network elasticity of the microgel. The data suggest that adsorption of

microgels induces deformation exceeding the swelling-induced expansion. In

particular, a loosely cross-linked microgel sustains large deformation due to swelling,

and adsorption further amplifies the strain and eventually may induce uncontrolled

CC-bonds scission leading to fragmentation of the microgel.[19, 20]

5.4. Conclusions

This Chapter studies adsorption and deformation of microgels at the air/water

and air/solid interfaces. Through systematic variation of microgel cross-linking degree

based on PVCL/NIPAm. The dynamic light scattering and tensiometer were used to

determine size and surface activity of the microgels respectively. The conformation of

microgels at the air/water interface was simulated. The results show that faster

spreading happens for the loose microgel which takes more flat shape compared to the

dense microgel. To characterize deformation of the microgel on the solid substrate,

the microgel samples were deposited on the silicon wafer and dried. We used AFM to

investigate the morphology as well as determine the contact radius and height of the

adsorbed microgels. The data indicate that the deformability of the microgels

increases with decrease of the cross-linking degree. The results suggest that

adsorption of microgels induces deformation exceeding the swelling-induced

expansion. In particular, a loosely cross-linked microgel sustains large deformation


93

due to swelling, and adsorption further amplifies the strain and eventually may cause

CC-bonds scission inducing destruction of the microgel.

5.5. References

[1] Merkel T. J., Jones S. W., Herlihy K. P., Kersey F. R., Shields A. R., Napier

M., Luft J. C., Wu H. L., Zamboni W. C., Wang A. Z., Bear J. E. and DeSimone J. M.,

Using mechanobiological mimicry of red blood cells to extend circulation times of

hydrogel microparticles, Proceedings of the National Academy of Sciences of the

United States of America, 2011, 108, 586-591.

[2] Byun S., Son S., Amodei D., Cermak N., Shaw J., Kang J. H., Hecht V. C.,

Winslow M. M., Jacks T., Mallick P. and Manalis S. R., Characterizing deformability

and surface friction of cancer cells, Proceedings of the National Academy of Sciences

of the United States of America, 2013, 110, 7580-7585.

[3] Hendrickson G. R. and Lyon L. A., Microgel translocation through pores

under confinement, Angewandte Chemie International Edition, 2010, 49, 2193-2197.

[4] Davidson P. M., Ozcelik H., Hasirci V., Reiter G. and Anselme K.,

Microstructured surfaces cause severe but non-detrimental deformation of the cell

nucleus, Advanced Materials, 2009, 21, 3586-3590.

[5] Badique F., Stamov D. R., Davidson P. M., Veuillet M., Reiter G., Freund J.

N., Franz C. M. and Anselme K., Directing nuclear deformation on micropillared

surfaces by substrate geometry and cytoskeleton organization, Biomaterials, 2013, 34,

2991-3001.




3235-3243.

[7] Wen Q. and Pelton R., Microgel adhesives for wet cellulose: Measurements

and modeling, Langmuir, 2012, 28, 5450-5457.

[8] Wen Q., Vincelli A. M. and Pelton R., Cationic polyvinylamine binding to

anionic microgels yields kinetically controlled structures, Journal of Colloid and


[9] Bysell H., Mansson R., Hansson P. and Malmsten M., Microgels and

microcapsules in peptide and protein drug delivery, Advanced Drug Delivery Reviews,

2011, 63, 1172-1185.


94

[10] Wanakule P., Liu G. W., Fleury A. T. and Roy K., Nano-inside-micro:

Disease-responsive microgels with encapsulated nanoparticles for intracellular drug

delivery to the deep lung, Journal of Controlled Release, 2012, 162, 429-437.

[11] Ancla C., Lapeyre V., Gosse I., Catargi B. and Ravaine V., Designed glucose-

responsive microgels with selective shrinking behavior, Langmuir, 2011, 27, 12693-

12701.

[12] Wang D., Liu T., Yin J. and Liu S. Y., Stimuli-responsive fluorescent poly(N-

isopropylacrylamide) microgels labeled with phenylboronic acid moieties as

multifunctional ratiometric probes for glucose and temperatures, Macromolecules,

2011, 44, 2282-2290.











Matter, 2009, 5, 2681-2685.




[19] Sheiko S. S., Sun F. C., Randall A., Shirvanyants D., Rubinstein M., Lee H.

and Matyjaszewski K., Adsorption-induced scission of carbon-carbon bonds, Nature,

2006, 440, 191-194.

[20] Lebedeva N. V., Sun F. C., Lee H. I., Matyjaszewski K. and Sheiko S. S.,

"Fatal adsorption" of brushlike macromolecules: High sensitivity of C-C bond

cleavage rates to substrate surface energy, Journal of the American Chemical Society,

2008, 130, 4228-4229.

[21] Barthel E., Adhesive elastic contacts: JKR and more, Journal of Physics D-

Applied Physics, 2008, 41, 163001-163020.

[22] Johnson K. L., Kendall K. and Roberts A. D., Surface energy and the contact

of elastic solids, Proceedings of the Royal Society of London Series a-Mathematical

and Physical Sciences, 1971, 324, 301-313.


95

[23] Liu K. K., Deformation behaviour of soft particles: a review, Journal of

Physics D-Applied Physics, 2006, 39, R189-R199.

[24] Carrillo J. M. Y., Raphael E. and Dobrynin A. V., Adhesion of nanoparticles,

Langmuir, 2010, 26, 12973-12979.

[25] Hoogerbrugge P. J. and Koelman J., Simulating microscopic hydrodynamic

phenomena with dissipative particle dynamics, Europhysics Letters, 1992, 19, 155-

160.

[26] Koelman J. and Hoogerbrugge P. J., Dynamic simulations of hard-sphere

suspensions under steady shear, Europhysics Letters, 1993, 21, 363-368.

[27] Rudov A. A., Khalatur P. G. and Potemkin, II, Perpendicular domain

orientation in dense planar brushes of diblock copolynners, Macromolecules, 2012, 45,

4870-4875.

[28] Rudov A. A., Patyukova E. S., Neratova I. V., Khalatur P. G., Posselt D.,

Papadakis C. M. and Potemkin, II, Structural changes in lamellar diblock copolymer

thin films upon swelling in nonselective solvents, Macromolecules, 2013, 46, 5786-

5795.

[29] Groot R. D. and Warren P. B., Dissipative particle dynamics: Bridging the gap

between atomistic and mesoscopic simulation, Journal of Chemical Physics, 1997,

107, 4423-4435.

[30] Hashmi S. M. and Dufresne E. R., Mechanical properties of individual

microgel particles through the deswelling transition, Soft Matter, 2009, 5, 3682-3688.





[32] Balaceanu A., Demco D. E. and Pich A., Responsive copolymer microgel

morphology, Proceedings of the Romanian Academy Series a-Mathematics Physics

Technical Sciences Information Science, 2011, 12, 296-301.




2011, 289, 613-624.

[34] Evangelopoulos A., Glynos E., Madani-Grasset F. and Koutsos V., Elastic

modulus of a polymer nanodroplet: Theory and experiment, Langmuir, 2012, 28,

4754-4767.

[35] Stokes J. R., Rheology of industrially relevant microgels In Microgel

Suspensions; Wiley-VCH Verlag GmbH & Co. KGaA: 2011, p 327-353.


96

[36] Song B. H. and Springer J., Determination of interfacial tension from the

profile of a pendant drop using computer-aided image processing .1. Theoretical,

Journal of Colloid and Interface Science, 1996, 184, 64-76.

[37] Thiessen D. B., Chione D. J., McCreary C. B. and Krantz W. B., Robust

digital image analysis of pendant drop shapes, Journal of Colloid and Interface

Science, 1996, 177, 658-665.

[38] Zhang J. and Pelton R., Poly(N-isopropylacrylamide) microgels at the air-

water interface, Langmuir, 1999, 15, 8032-8036.

[39] Hua X. Y. and Rosen M. J., Dynamic surface-tension of aqueous surfactant

solution. 1. Basic parameters, Journal of Colloid and Interface Science, 1988, 124,

652-659.

[40] Pallas N. R. and Harrison Y., An automated drop shape apparatus and the

surface-tension of pure water, Colloids and Surfaces, 1990, 43, 169-194.










[44] Richtering W., Responsive emulsions stabilized by stimuli-sensitive microgels:

Emulsions with special non-Pickering properties, Langmuir, 2012, 28, 17218-17229.

[45] Eckert T. and Richtering W., Thermodynamic and hydrodynamic interaction

in concentrated microgel suspensions: Hard or soft sphere behavior?, Journal of

Chemical Physics, 2008, 129.

[46] Berndt I., Pedersen J. S., Lindner P. and Richtering W., Influence of shell

thickness and cross-link density on the structure of temperature-sensitive poly-N-

isopropylacrylamide-poly-N-isopropylmethacrylamide core-shell microgels

investigated by small-angle neutron scattering, Langmuir, 2006, 22, 459-468.

[47] Carrillo J. M. Y. and Dobrynin A. V., Molecular dynamics simulations of

nanoimprinting lithography, Langmuir, 2009, 25, 13244-13249.

Chapter 6. Microgels at the Oil/Water Interface

97

Chapter 6. Behavior of Temperature-Responsive Copolymer

Microgels at the Oil/Water Interface

(The results and descriptions of this Chapter have been published in the paper: Yaodong Wu,

Susanne Wiese, Andreea Balaceanu, Walter Richtering, Andrij Pich. Behaviour of

Temperature-responsive Copolymer Microgels at the Oil/Water Interface, Langmuir, 2014,

30, 7660–7669.)

6.1. Introduction

Microgels have become increasingly important in polymer and colloidal

science due to their attractive properties and potential applications. So far, a lot of

work on synthesis of microgel particles has been done and mostly such colloids are

prepared in an aqueous medium and consist of water-soluble polymers, such as

poly(N-isopropylacrylamide) (PNIPAm), poly(N-isopropylmethacrylamide)

(PNIPMAm), poly(N-vinylcaprolactam) (PVCL), poly(acrylic acid) (PAA) or poly(2-

(diethylamino)ethyl methacrylate) (PDEA).[1]

Among these microgels, special

attention has been focused on PNIPAm-, PNIPMAm- and PVCL-based microgels due

to their temperature-sensitive behavior. These polymers feature the lower critical

solution temperature (LCST) and their corresponding microgels have the volume

phase transition temperature (VPTT), close to the LCST of linear polymers.[2-6]

The

temperature sensitivity of these polymers facilitates the fabrication of “switchable” or

“stimuli-responsive” materials.[7, 8]

Indeed, the responsive microgels are effectively

used as the drug carrier or the emulsion stabilizer.[7-11]

As the drug carrier, microgel

particles can be used in either a dispersed state or an assembly state, e.g. colloidosome

capsules.[8]

The application of microgels in the synthesis of colloidosomes or emulsions is

based on the adsorption of microgel particles at the oil/water interfaces. The oil/water


98

interfacial tension is reduced when microgel particles are adsorbed onto the interface

and this effect leads to the stabilization of emulsions.[7, 10-13]

The microgel-stabilized

emulsion is a new kind of particle-stabilized emulsion, referred by Richtering as

“Mickering emulsion” compared to the traditional “Pickering emulsion”.[13]

Compared to surface-modified latexes or other inorganic/polymer hybrid particles

used in the conventional Pickering emulsions, microgels are easy to synthesize, highly

deformable at the interface and importantly environment-responsive.[7, 10-12]

The

emulsion stability is deeply influenced by the environment because of the

temperature- and pH-sensitivity of microgels.[7, 10-16]

Through the studies of the

interfacial behavior of modified inorganic nanoparticles at the liquid-liquid interfaces,

it was revealed that the assemblies of nanoparticles can be controlled by tuning the

size of the nanoparticle, the chemical characteristics of the ligands, and environment,

such as temperature and pH.[17-20]

Until now, the interfacial tension in the Pickering

emulsion system has been widely investigated, and some theoretical models have

been developed; nevertheless, these theories are normally based on the hard latexes or

inorganic particles.[17-21]

Microgels are colloidal particles whose behavior falls

between that of hard spheres and ultra-soft systems (e.g., polymer coils in solution).[22,

23] The assembly laws deduced from the interfacial behavior of hard particles at the

oil/water interface seem not to be completely suitable for microgels, since soft

microgel particles are highly deformed at the oil/water interface.[16, 24-26]

With the temperature-sensitive unit, NIPAm, and the pH-sensitive unit, MAA,

the interfacial behavior of PNIPAm-co-MAA microgels, strongly depends on the

temperature and the pH, and so does the emulsion stability.[7, 9-11, 14, 24]

The interfacial

properties of PNIPAm-based microgels as a function of temperature were studied by

Monteux and coworkers. A minimum interfacial tension was found around VPTT of


99

the microgel. They assumed that the decrease in the interfacial tension with

temperature below VPTT came from the formation of denser layers of microgels at

the interface; and that the increase in the interfacial tension with temperature above

VPTT arose from loosely packed aggregations of microgels at the interface.[12]

Compared to the microgel with ionizable groups, neutral microgels should be more

suitable for investigating the interfacial behavior because the lack of electrostatic

interaction would make the system much simpler.

It is known that the variation of temperature induces a change in the

hydrophilic/hydrophobic balance of microgels, consequently leading to a size

variation. However, both the size and the hydrophilic/hydrophobic balance have a

great influence on the interfacial behavior of colloidal particles.[18, 19]

Which one of

them is more imperative in the change in the interfacial tension of microgel systems

with temperature is still unclear. A recent paper from Li et. al. reports the adsorption

kinetics of PNIPAm microgels at the oil/water interface. It is demonstrated that the

spreading of microgels at the oil/water interface, governed by microgel deformability,

plays an important role in the decline rate of the interfacial tension.[27]

In this Chapter, we investigate the interfacial behaviors of these microgels. In

our previous work,[28]

we demonstrated that in PVCL/NIPAm and PVCL/NIPMAm

microgels there is a statistical distribution of the monomer units in the colloidal

networks independent of the copolymer composition. This gives a straightforward

possibility to study systematically the influence of the chemical structure on the

interfacial behavior of microgels. So far the influence of the copolymer structure and

the cross-linking degree of microgels on the oil/water interfacial tension has not been

investigated thoroughly. The equilibrium interfacial tension and the time for reaching

the equilibrium state can be considered as the thermodynamic aspect and the kinetic


100

aspect, respectively. Dependences of these two aspects on temperature, chemical

composition and cross-linker degree of microgels are analyzed. The results indicate

that both the chemical composition of copolymer microgels and the cross-linking

degree have a governing influence on the deformation-controlled interfacial behavior

of microgels at the toluene/water interface. The former strongly affects the volume

phase transition temperature which determines the temperature-dependence of the

softness of microgels, and the latter directly influences the softness of mcirogels.

Furthermore, we did a primary study on the application of microgels based on

the interfacial activity. The emulsions stabilized by PVCL/NIPAm and

PVCL/NIPMAm microgels are prepared. It is found that the type of the emulsion

relies on the chemical composition of microgels. PVCL/NIPAm microgels and

PVCL/NIPMAm microgels with low NIPMAm contents tend to stabilize w/o

emulsions, while PVCL/NIPMAm microgels with high NIPMAm contents are likely

to make o/w emulsions. For PVCL/NIPAm microgels, a HIPE is obtained at the

oil/water volume ratio of 3/7. Unlike the interfacial activity, the type of emulsion is

not influenced by the cross-linking density.


6.2.1. Microgel synthesis

PVCL/NIPAm and PVCL/NIPMAm microgels with different monomer ratios

and cross-linker contents are used in this Chapter. The synthesis and characterization

of microgels are presented in detail in Chapter 3.


101

6.2.2. Interfacial tension

The dynamic interfacial tension of microgels at the toluene/water interface was

measured through the pendant drop method on a DSA 100 tensiometer (Kruss,

Germany) equipped with a thermostat. Aqueous dispersions of microgels were

prepared with deionized water (Milli-Q Academic A-10 system, Millipore) previously

saturated with toluene. A drop of toluene (previously saturated with deionized water)

was formed in microgel dispersions, and then dynamic interfacial tension was

determined through rapid acquisition of the drop image, edge detection, and fitting of

the Laplace-Young equation conducted by computer automation. As toluene is lighter

than water, we formed a toluene drop from bottom to up using a curved needle, as

shown in Figure 6.1. For the temperature dependence experiment, fresh interfaces

were produced at each temperature.

Figure 6.1. Schematic illustration of the pendent drop from bottom in the interfacial

tension measurement.

6.2.3. Interfacial dilatational rheology

The interfacial dilatational rheology was recorded by the DSA 100 tesiometer

(Kruss, Germany) on which the interfacial tension measurements were implemented.

For the dilatational rheology measurement, an oscillating drop module (ODM, Kruss,

Germany) was installed on the tesiometer. The drop area of was oscillated at the


102

frequency of 0.2 Hz at different temperatures for homopolymer microgels of PVCL,

PNIPAm and PNIPMAm with 3 mol% BIS. The measurements started 5 min after the

formation of the drop. The Fourier analysis of the raw data was performed with the

software packages supplied by the manufacturer.

6.2.4. Preparation of microgel-stabilized emulsions

Microgel-stabilized emulsions were prepared by homogenizing the microgel

dispersion with proper amounts of chloroform using an Ultra Turrax T25

homogenizer (17 mm head) operating at the speed of 8000 rpm for 3 min. The total

emulsion volume was 10 mL. The o/w ratio (v/v) ranged from 1/9 to 9/1. The

concentration of microgels in the aqueous phase varied from 0.05 – 0.5 wt%.

6.2.5. Preparation of emulsion-templated materials

The emulsion-template materials were prepared with the same procedure for

emulsion preparation. Appropriate amount of polycaprolactone (PCL, Mw ~ 80 k),

purchased from Sigma-Aldrich and used as received, was dissolved in chloroform.

For the porous scaffold-like materials, the microgel solution (0.5 wt%) of

PVCL/NIPAm (1:1) with 3 mol% BIS was used, and the PCL/chloroform solution (4

wt%) was used as the oil phase. The mixture with oil/water ratio of 3/7 was

homogenized with Ultra Turrax T25 homogenizer (17 mm head) operating at 8000

rpm for 3 min. To obtain porous materials, the mixture was freeze-dried.

For the emulsion-template colloidosomes, the microgel solution (0.015 wt%)

of PVCL/NIPMAm (1:5) microgels with 3 mol% BIS, and the PCL/chloroform

solution (2 wt%) was used as the oil phase. The mixture with oil/water ratio of 1/9

was homogenized with Ultra Turrax T25 homogenizer (17 mm head) operating at

8000 rpm for 3 min. Then the mixture was stirred with a magnetic stirrer under

ambient condition to remove chloroform by evaporation overnight.


103

6.2.6. Fluorescence microscopy

Fluorescence microscopy was performed on a Zeiss Axioplan 2 microscope

equipped with XBO 75 illuminating system (Xenon lamp) and using a filter

565 nm and 620 nm. To facilitate the fluorescence microscopy characterization,

the oil phase contains Nile Red with the concentration of 0.1 μg/mL.

6.2.7. Scanning electron microscopy

Field emission scanning electron microscopy (FESEM) was carried out on

Hitachi S-4800 field-emission SEM. The freeze-dried samples were broken with

tweezers and stick to the sample holder before the measurement. The samples of

colloidosomes were prepared by placing a drop of emulsion on silicon wafer. Before

being placed into the SEM, the samples were dried under ambient conditions.


6.3.1. Dynamic interfacial tension

6.3.1.1. Influence of microgel concentration

Due to the fact that no monomers carrying acidic or basic groups or ionic

surfactants were used for microgel synthesis, we can consider that only small amount

of ionizable groups in the microgel structure originates from the initiator residues

incorporated into polymer chains. In this situation we consider the investigated

microgels as purely temperature-sensitive colloids where no pH-sensitive effects are

present. The characterizations of microgels, such as chemical composition by NMR,

size at different temperatures by DLS and morphology by AFM, are presented in

Chapters 3 and 4. Because of the different reactivity ratios of the monomers and the

cross-linker, microgels synthesized have a radial density profile in the internal

structure, where the segment density gradually decays at the surface.[4, 29, 30]


104

Figure 6.2 shows the interfacial tension of toluene against aqueous solutions of

PVCL/NIPMAm microgels at different concentrations. It is obvious that the

interfacial tension decreases with time, and eventually approaches an equilibrium

value. The time for reaching the equilibrium interfacial tension decreases with the

increase in the concentration of microgels. The plots, especially at low

microgel concentrations, interpret the classical kinetics of interfacial tension with

three distinct regimes (Figure 6.2b), which is similar to the dynamic interfacial

behaviors of polymer-grafted inorganic nanoparticles and proteins.[17, 31]

The first

stage is a kind of induction regime, where the interfacial tension decreases slightly. In

the second regime, a sharp decline in the interfacial tension is observed. In the last

regime, the interfacial tension reaches a quasi-equilibrium value.

1 10 100 1000 10000

10

20

30

40

0.0014 mg/mL

0.0046 mg/mL

0.0145 mg/mL

0.046 mg/mL

0.145 mg/mL

0.28 mg/mL

(m

N/m

)

Drop age (s)

a

Figure 6.2. (a) Time dependence of the toluene/water interfacial tension with

PVCL/NIPMAm (5:1) microgels of various concentrations at 24 °C. The red lines

show the fitting results based on Equation 6.1. (b) Schemetic showing of three

regimes of the plots marked on a typical curve (0.046 mg/mL in Graph

a).


105

Generally, the decrease of interfacial tension in the second regime can be

explained by the adsorption of microgel particles to the toluene/water interface. In the

early stage, microgels within the vicinity of toluene drop adsorb onto the interface,

which induces the gentle reduction of interfacial tension. The diffusion of microgel

particles from the bulk solution to the interface increases the interface coverage. After

a certain period of time, the interface coverage reaches a critical value, and then a

steep decline in the interfacial tension is observed and maintains the status for a short

time from several to tens of seconds. When the interface coverage approaches to the

maximum value, the interfacial tension reaches an equilibrium value. For microgels of

lower concentrations, a longer time is needed for the adsorption of sufficient amount

microgel particles from bulk solution to the toluene/water interface. Therefore, the

induction time increases with the decrease of microgel concentration. When the

concentration of microgel is very low (here lower than 0.0014 mg/mL), the induction

time is so long that the toluene droplet becomes unstable before the second regime

starts. The induction period is only found for dilute microgel solutions. In the

measurable time scale, the induction time is difficult to be detected for concentrated

microgel solutions, because the dynamic interfacial tension decreases very fast to a

stable level.[17, 31]

One possible explanation for the acceleration of the interfacial

tension decay is that the capillary force between the particles at the interface that

increases with the particle concentration makes the soft microgels easier deformed.[27]

It should be noted that in the conventional Pickering emulsions, solid particles seldom

lead to appreciable changes in oil/water interfacial tension.[16, 32, 33]

However,

microgels are organic colloidal particles, and their deformation leads to a higher

number of polymer segments adsorbed at the interface.[27]

This leads to a significant

decrease in the interfacial tension.


106

To obtain the equilibrium interfacial tension, an empirical equation (Hua-

Rosen equation) is employed (Equation 6.1).

(Equation 6.1)

where , and are the initial interfacial tension, the equilibrium interfacial

tension and the interfacial tension detected at time , respectively. is a

semiempirical parameter having the unit of time and corresponds to the time when

, and is a dimensionless constant. The Hua-Rosen equation has

been successful for small molecular surfactant systems, macromolecular systems and

globular protein systems.[34-36]

We carried out the fitting of experimental data using

Equation 6.1 for samples with medium microgel concentrations, which had typical

decay. In Figure 6.2, we can see a good agreement between the fitting

results and the experimental data.

6.3.1.2. Influence of temperature

It has been reported that temperature has a considerable influence on the

interfacial/surface tension of small molecular surfactants, polymers, and colloidal

particles.[34, 37-41]

Normally, as temperature increases, the diffusion rate increases.[34]

That means faster adsorptions of solutes and/or dispersed substances occur at higher

temperatures, reflected by the faster interfacial/surface tension decay rate. However,

through the experimental results of interfacial tension of temperature-sensitive

polymers, such as PNIPAm, a different temperature dependence of surface tension

were observed.[39, 40]

Below LCST, the rate of the surface tension reduction increases

with the increase of temperature, which is similar to the behavior of non-ionic

surfactants.[34, 42]

On the contrary, the surface tension decay rate decreases as

temperature increases above LCST. Kawaguchi et al. attributed the slowdown of

surface tension decay to aggregation of PNIPAm chains which retarded the diffusion


107

of polymers from the bulk solution to the interface.[39]

Zhang and Pelton supposed that

the unfolding of PNIPAm at the air/water interface above LCST prolonged the time

for reaching a steady-state surface tension.[40]

They adopted the same explanation to

the similar phenomena observed in the case of PNIPAm microgels.[41]

0 100 200 300 400 500

5

10

15

20

25

30

35

24.0 oC

27.5 oC

31.0 oC

35.5 oC

39.5 oC

43.0 oC

(m

N/m

)

Drop age (s)

Figure 6.3. Time dependence of the toluene/water interfacial tension with

PVCL/NIPMAm (5:1) microgels of 0.15 mg/mL at different temperatures. The red

lines show the fitting results based on Equation 6.1.

The dynamic interfacial tension of toluene against aqueous microgel solutions

was measured at various temperatures. At each temperature, the interfacial tension of

freshly formed interface was measured. As shown in Figure 6.3, the dynamic

interfacial tension represents a similar trend versus time at different temperatures,

which decays with time and finally approaches to an equilibrium value as discussed

before. A good agreement between the fitting through Equation 6.1 and the

experimental data was also found in plots at different temperatures.

However, the equilibrium interfacial tension and the time for reaching the equilibrium

status behave differently at different temperatures. The resulting Hua-Rosen

parameters, describing the dynamic interfacial tension, from the fittings in Figure 6.3


108

are presented in Table 6.1. The lowest value of was found at around 31 °C. A

similar change of with temperature was found in the dynamic surface tension of

linear PNIPAm solution.[39]

It was mentioned that the value of might reflect the

hydrophobic character of the surfactant, but there is no direct evidence to prove this.

However, others’ work show that temperature has no special influence on the value of

.[40, 41]

Table 6.1. Dynamic interfacial tension parameters of Hua-Rosen equation for

PVCL/NIPMAm (5:1) microgels of 0.15 mg/mL at different temperatures (fitting in

Figure 6.3).

(°C) (mN/m) (s)

24.0 8.5 1.2 1.94

27.5 7.1 1.0 2.06

31.0 5.6 0.5 0.81

35.5 5.2 5.4 1.09

39.5 5.6 51.3 2.26

43.0 6.6 124.6 3.32

6.3.1.3. Influence of cross-link density

Cross-link density plays an important role in the microgel structure and

consequently influences the property of either the individual microgel particle or the

entire microgel suspension.[4, 43-45]

Figure 6.4 presents the dynamic interfacial tension

of toluene against the solutions of microgels with different cross-linking densities. We

can obviously see that microgels with lower cross-linker concentrations, which are

softer, have the faster kinetics of reduction of interfacial tension with time. A similar

phenomenon was observed by Pelton’s group on the dynamic surface tensions of

PNIPAm microgels at the air/water interface.[41]

It was proposed that the rate of

surface tension lowering was partially influenced by the rate of particle spreading


109

after the adsorption onto the interface. In Chapter 5, both the experimental data and

the computer simulation results of microgels at the air/water interface have shown that

a faster spreading happens for weekly cross-linked microgels compared to denser

ones.

10 100 1000

10

20

30

40

3%

1%

0.25%

0.05%

(m

N/m

)

Drop age (s)

Figure 6.4. Dynamic interfacial tension of toluene against the solutions of microgels

with different cross-linking densities at 24 °C. The values in the legend represent the

molar ratio of cross-linker, BIS, to monomers added during synthesis. The

concentration of microgel solution is 0.15 mg/mL.

6.3.1.4. Influence of microgel size

It was reported that the size of nanoparticles strongly affects their interfacial

behaviors.[17, 18]

To figure out whether the size of microgel particles can influence the

equilibrium interfacial tension, we synthesized a series of microgels with the same

chemical composition, but of different sizes. By adding different amounts of

surfactant CTAB during the synthesis, the size of microgel can be tuned.[46]

From

Figure 6.5, we can see that the more CTAB is used in synthesis, the smaller are

microgels obtained. However, the changes in the microgel size have no impact on the

interfacial tension of microgel solutions against toluene, both below and above VPTT.


110

Thus, within the range of microgel size investigated here, the effect of size on the

interfacial tension can be ignored. It ensures the comparison of the interfacial

behaviors of microgels with different compositions, which are also varied in size,[28,

47] is reasonable.

0 100 200 300 400 500 60010

15

20

25

30

35

100 1000

NO CTAB

0.37 mM CTAB

1.10 mM CTAB

(m

N/m

)

Drop age (s)

a

Rh (nm)

0 100 200 300 400 5005

15

25

35

10 100

b

NO CTAB

0.37 mM CTAB

1.10 mM CTAB

(m

N/m

)Drop age (s)

Rh (nm)

Figure 6.5. Dynamic interfacial tension of PVCL/NIPMAm (1:5) microgels solution

(0.15 mg/mL), synthesized with different amounts of surfactant CTAB, against

toluene. (a) Below VPTT (24 °C) and (b) above VPTT (49 °C). The inset shows the

size distributions of microgels, which decrease with the increase in CTAB

concentration. The temperature trends of hydrodynamic radii of microgels are shown

in Figure 3.5.

6.3.2. Equilibrium state of the interfacial tension

6.3.2.1. Influence of chemical composition of microgel

The variation of the equilibrium interfacial tension ( ) and the size of

copolymer microgels with the molar ratio of acrylamide at room temperature are

summarized in Figure 6.6. It is found that the size of PVCL/NIPAm microgels has no

significant change with the change in the fraction NIPAm (around 400 nm in ),

while the size of PVCL/NIPMAm microgels increases slightly with the fraction of

NIPMAm (from 350 to 450 nm in ).[28]

As synthesized in the presence of CTAB,


111

the corresponding homopolymer microgels are much smaller than the copolymer

microgels. Consequently, an irregular trend of size variation with the chemical

composition is found. However, the values of for PVCL/NIPAm and

PVCL/NIPMAm systems increase with the fraction of NIPAm and NIPMAm in the

copolymer. It does not follow the changing trend of size at all. For both

PVCL/NIPAm and PVCL/NIPMAm copolymer microgel systems, the change in

is very slight as the fractions of NIPAm and NIPMAm increase from 0 to 0.5. As the

fractions of NIPAm and NIPMAm increase from 0.5 to 1, the values of increase

significantly.

0.0 0.2 0.4 0.6 0.8 1.0

8

9

10

11

12

100

200

300

400

500

eq

(mN

/m)

Fraction of NIPAm in PVCL/NIPAm

a

Rh (

nm

)

0.0 0.2 0.4 0.6 0.8 1.0

8

10

12

14

16

100

200

300

400

500

eq

(mN

/m)

Fraction of NIPMAm in PVCL/NIPMAm

b

Rh (

nm

)

Figure 6.6. Variation of and with the fraction of (a) NIPAm and (b) NIPMAm

in copolymer microgels at 24 °C. The concentration of microgel solution is 0.15

mg/mL.

Most of the microgels investigated here were synthesized without CTAB,

except the homopolymer PVCL, PNIPAm and PNIPMAm microgels and

PVCL/NIPMAm copolymer microgels varied in size. For these microgels, a small

amount of CTAB was used. We believe that no CTAB residues are left in the solution

after the 4-day dialysis with a continuous fresh water exchange. If there were CTAB


112

residues left in the solution, they would lead to the same interfacial behavior of PVCL,

PNIPAm and PNIPMAm microgels. However, they are different from each other

(Figure 6.6). The PVCL/NIPMAm microgels of different sizes (synthesized in the

presence of CTAB) and a control sample prepared without CTAB have the same

interfacial behavior (Figure 6.5). From this we conclude that CTAB has been

successfully removed from microgel samples during the dialysis step.

6.3.2.2. Influence of temperature

The equilibrium interfacial tensions at different temperatures for

PVCL/NIPAm and PVCL/NIPMAm microgels are presented in Figure 6.7. At the

same temperature, of PVCL homopolymer microgels is always lower than those

of PNIPAm and PNIPMAm homopolymer microgels. The value of for PNIPAm

microgels decreases with temperature firstly, and reaches a minimum around VPTT,

then increases with temperature slightly when temperature is higher than VPTT. This

kind of transition temperature for interfacial tension is consistent with the reported

results about the interfacial tension of NIPAm-based microgels.[12]

For PVCL and

PNIPMAm microgels, the values of also decrease with temperature below VPTT,

however, stay at a plateau value above VPTT. The interfacial behaviors of

PVCL/NIPAm or PVCL/NIPMAm copolymer microgels fall between those of PVCL

and PNIPAm or PNIPMAm homopolymer microgels, and depend on the chemical

compositions of microgels.

Figures 6.7c and d show the kinetics of interfacial tension reduction as

reflected by for microgels at different temperatures. We can see that the values of

remain at a constant level below VPTT and increase with temperature above VPTT.

The kinetics of interfacial tension ( ) also depends on the chemical compositions of

copolymer microgels. The increase in for PVCL- and PNIPAm-rich microgels


113

above VPTT is more obvious compared to that for PNIPMAm-rich systems. It means

that at higher temperature (above VPTT), the reduction of interfacial tension for

PVCL- and PNIPAm-rich microgel systems is slowed down to a larger extent

compared to PNIPMAm-rich microgel systems. Though the reason for that is still

unknown, such a dependence of the interfacial behavior on the chemical composition

could be made use of in the application of microgels.

20 25 30 35 40 454

6

8

10

12

14 PVCL

V:N (5:1)

V:N (1:1)

V:N (1:5)

PNIPAm

eq (

mN

/m)

Temperature (oC)

a

20 25 30 35 40 45 504

8

12

16 PVCL

V:NM (5:1)

V:NM (1:1)

V:NM (1:2)

V:NM (1:3)

V:NM (1:5)

PNIPMAm

eq (

mN

/m)

Temperature (oC)

b

20 25 30 35 40 45

0

50

100

150

200

250

300

PVCL

V:N (5:1)

V:N (1:1)

V:N (1:5)

PNIPAm

t* (

s)

Temperature (oC)

c

20 25 30 35 40 45 50

0

50

100

150

PVCL

V:NM (5:1)

V:NM (1:1)

V:NM (1:2)

V:NM (1:3)

V:NM (1:5)

PNIPMAm

t* (

s)

Temperature (oC)

d

Figure 6.7. Dependence of the interfacial tension and time to equilibrium on

temperature for microgels with different ratios of VCL:NIPAm (a and c) and

VCL:NIPMAm (b and d). V:N and V:NM represent PVCL/NIPAm and

PVCL/NIPMAm, respectively; and the ratios in the brackets are the monomer ratio of

copolymer microgels. The concentration of microgel solution is 0.15 mg/mL.


114

The intrinsic reason for the difference between the interfacial behavior of

PVCL, PNIPAm and PNIPMAm microgels observed here is still not understood. It is

revealed in this paper that these copolymer microgels have the similar internal

structure, regardless of the chemical composition (SLS results in Figure 4.1), and that

the size of microgels has no impact on the interfacial behavior (Figure 6.5). The

probable reason for such a difference we can think out is the variation in the chemical

structure. However, it is still an open question how the chemical structure has such an

influence on the interfacial behavior observed here. Though the reason for that is still

unknown, such a dependence of the interfacial behavior on the chemical composition

could be made use of in the application of microgels.

6.3.3. Discussion on the temperature dependence of interfacial tension

We can easily find turning points on the curves (Figure 6.7). Before

such kind of point, the value of decreases monotonously with the increase in

temperature. Here, the turning point can be considered as the transition temperature of

the interfacial behavior, and its exact position was defined as the intersecting point of

two trend lines (approximate slopes). According to our previous work, the phase

transition temperature of microgels can be determined by DSC.[28]

The transition

temperatures determined by DSC and the curves are summarized in Table

6.2. We can see that the interfacial behavior of microgels is highly consistent with the

phase behavior of microgels. For PVCL/NIPAm microgels, both kinds of transition

temperatures are not influenced by the monomer ratio and remain in a certain range,

although the phase transition temperatures (34 – 36 °C) are slightly higher than the

transition temperature derived from the interfacial behavior (31 – 33 °C). For

PVCL/NIPMAm microgels, both transition temperatures depend on the monomer


115

ratio in copolymers. When the VCL:NIPMAm ratio is higher than 1, the transition

temperatures are around 34 – 36 °C. However, if the VCL:NIPMAm ratio decreases

from 1 to 0.2, the transition temperatures increase rapidly to 45 °C. The results

indicate that the behavior of copolymer microgels with VCL:NIPMAm ratios above

and below 1 is dominated by VCL and NIPMAm, respectively. The transition

temperatures of follow exactly the same trends of the VPTT of microgels which

stands for exclusively the chemical structure of microgels. Considering the

independence of on copolymer microgel size (Figure 6.5), it can be inferred that

the chemical constitution of microgels has a dominant influence on the interfacial

tension.

Table 6.2. Comparison of transition temperature of the interfacial behavior and that of

phase behavior of microgels with different monomer ratios.

Samplea Phase transition temperature by

DSC (°C)[28]

Transition temperature of γ

(°C)

V:N (5:1) 34.9 32.7

V:N (1:1) 35.5 31.8

V:N (1:5) 35.0 31.7

V:NM (5:1) 33.8 32.6

V:NM (1:1) 35.9 32.4

V:NM (1:2) 41.3 40.8

V:NM (1:3) 42.3 42.0

V:NM (1:5) 44.6 45.0


the ratios in the brackets are the monomer ratio of copolymer microgels.

It is believed that the reduction of interfacial tension is caused by the

adsorption of microgel particles at the oil/water interface. At the early stage, the

adsorption of nanoparticles is expected to be a diffusion-controlled process. For


116

nonionic nanoparticles, the process can be quantitatively described as the following

equations through the short time ( ) and the long time ( )

approximations,[17, 34, 42]

, or (

⁄)

;

, or (

⁄)

(Equations 6.2)

where is the initial interfacial tension and is the equilibrium interfacial tension,

is the bulk concentration of nanoparticles, is the interfacial density of

nanoparticles, and is the diffusion coefficient. For the adsorption of nonionic

surfactant, a mechanism of mixed diffusion-activation is suggested, where a linear

relationship exists between ⁄ and .[17, 34, 42]

Here, is the diffusion

coefficient of the particles at different temperatures and is the particle diffusion

coefficient at 24 °C. From the interfacial tension, to obtain the values of , the

interfacial density of particles Γ is needed, as seen in (

⁄)

and

(

⁄)

. However, as the microgels are very soft and deformable,

the size of the particles adsorbed at the interface is hardly known. As a consequence

of that, the interfacial density of microgel particle is not possible to obtain. Therefore,

the relative value of ⁄ is calculated.

The plots of ⁄ versus were obtained from DLS and calculated

through the approximations in Equations 6.2 (Figure 6.8). We found that the

⁄ plot according to light scattering data has a good linear dependence

in the small range of investigated here. Compared to that, the calculation based

on Equations 6.2 shows a different result. When T < VPTT, the values of ⁄

depend on linearly, and are also located near the fitted line of light scattering


117

results for both approximations. That means the change in diffusion coefficients

derived from interfacial tension (through Equations 6.2) with temperature is

consistent with the one in the bulk solution (light scattering results). But when T >

VPTT, the values of ⁄ calculated with Equations 6.2 deviate far away

from the fitting line of light scattering results. It indicates that the adsorption of

microgels onto the oil/water interface is not simply controlled by diffusion

mechanism, especially above VPTT.

0.0031 0.0032 0.0033 0.0034-3

-2

-1

0

1

2

T > VPTT T < VPTT

DLS

IFT_short time

IFT_long time

linear fit for DLS

ln(

DT/D

0)

T-1 (K-1)

Figure 6.8. Dependence of apparent diffusion coefficients for PVCL/NIPMAm (5:1)

microgels on temperature. The values were obtained from DLS results (square) and

calculated through short time (circle) and long time (triangle) approximation by

Equations 6.2.

At the equilibrium state a balance exists between the adsorption and

desorption at the oil/water interface for the small molecular surfactant or the solid

inorganic particles in the Pickering emulsion.[17, 34]

The increase in temperature can

bring stronger thermal fluctuations that result in a weak particle attachment at the

fluid interfaces.[17, 48]

In our work, the oil drop was freshly formed in each experiment.

At the temperature above VPTT, the interfacial tension of the freshly formed droplet


118

decays, as usual. That means the adsorption of microgels onto the interface is steady.

According to the literatures about the microgel-stabilized emulsion, the increase of

temperature leads to the breaking of emulsion, where macroscopic floccules are

observed.[7, 12]

However, even in the broken emulsions, microgels are found to be

adsorbed at the oil/water interface.[49]

So far, none has found significant desorption for

microgel particles from the oil/water interface even upon increasing temperature.

Monteux and co-workers reported the same interesting phenomena for

PNIPAm-based microgels, where the variation of the interfacial tension versus

temperature presents a minimum value around VPTT. They correlated the decrease in

the interfacial tension below VPTT to the increasingly compact packing of microgel

particles at the interface, and the increase in interfacial tension above the VPTT to the

aggragates of microgels at the interface at high temperatures.[12]

It has also been

revealed that soft microgels deform at the interface.[13, 14, 24, 26]

At T < VPTT, the

swollen microgels are able to deform easily. They can spread their dangling chains at

the particle surface to cover the interface as much as possible. That is the reason some

chain bridges were found between adjacent microgels in some reports.[13, 14, 24]

The

results of dynamic interfacial tension of microgels with different cross-link densities,

shown in Figure 6.4, strongly support this view. With lower cross-linker

concentrations, microgels are softer, thus have faster kinetics of the decay of

interfacial tension with time. However, at T > VPTT, the microgel particles deform

less. Without good spreading ability, a higher number of particles and a closer

packing density are needed to reach the maximum coverage of the interface. Besides,

the mobility at the high packing density might be much slower due to crowding or

jamming of colloidal particles at the interface.[50-52]

Therefore, a strong increase of


119

is observed at T > VPTT. The adsorption of microgels below and above VPTT is

schematically shown in Figure 6.9.

Figure 6.9. The adsorption of microgels below and above VPTT.

Such a deformation-controlled mechanism could also explain the

independence of the interfacial tension on the microgel size shown in Figure 6.5.

Assuming that the use of surfactant in the synthesis does not change the internal

structure and the cross-linking concentration of microgels, the deformability of the

copolymer microgels should remain the same. Therefore, the interfacial tension

decays for PVCL/NIPMAm microgels synthesized with different amounts of CTAB

highly overlap to each other.

6.3.4. Dynamic interfacial tension of microgel mixtures

The temperature dependence of the interfacial behaviors of PVCL and

PNIPAm microgels are similar to each other in both and , but are different to

that of PNIPMAm microgels, especially in the kinetics represented by (Figure 6.7).

With the increase in temperature, microgel particles shrink because of the change in

the hydrophilic and the hydrophobic interactions.[2-6]

The decrease in the size results

in a denser microgel coverage at the interface, which consequently lower the


120

interfacial tension.[12]

When the temperature is higher than VPTT, the increasing

thermal fluctuation and the loose aggregation lead to a slight increase in the value of

, and a sharp decrease in the kinetics of the interfacial tension decay (increase in

). However, the kinetics of the interfacial tension decay of NIPMAm-rich microgels

seems not to be affected by temperature. It indicates that microgels with chemical

structures rich in NIPMAm are probably easy to adsorb at the oil-water interface.

The interfacial tension of the mixing solution of PVCL and PNIPMAm

homopolymer microgels were investigated at different temperatures, i.e. 24, 37, and

44 °C. They are below, between, and above the VPTT of PVCL and that of

PNIPMAm homopolymer microgels, respectively (Figure 6.10). At 24 °C, of the

PNIPMAm system is much higher than that of the PVCL system, while of them are

very close to each other (Figure 6.7). Therefore, two reduction slopes with a small

time interval in the plot are observed. A similar result was obtained at 37 °C,

but the time interval between two interfacial tension reducing slopes is longer.

Because the temperature is above VPTT of PVCL microgels, the rate of the interfacial

tension reduction for PVCL microgels becomes slower as temperature increases.

However, the temperature has no effect on the kinetics of the dynamic interfacial

tension of the PNIPMAm microgel system. In the mixed solution of PVCL and

PNIPMAm microgels, only one step is observed in the dynamic interfacial tension

curve. With a faster kinetics, PNIPMAm microgels adsorb onto the interface earlier,

which decrease to the first plateau. Due to a slower kinetics, the influence of PVCL

microgels on begins to appear later than the first plateau, inducing a second plateau

in the plot. When temperature is above the VPTT of PNIPMAm microgels,

the difference between of PVCL and PNIPMAm is very slight, although the gap in


121

of them is even larger (Figure 6.7). Therefore, only one plateau without any step is

observed in the plot at 44 °C.

0 100 200 300 400 5005

15

25

35

0 100 200 300 400 500 600 7005

15

25

0 100 200 300 400 500 600 7005

15

25

35

24 oC

(m

N/m

)

Drop age (s)

37 oC

(m

N/m

)

44 oC

(m

N/m

)

Figure 6.10. Dynamic interfacial tension of mixture solution of PVCL and

PNIPMAm at 24, 37 and 44 °C. Arrows show the steps between two kinds of quasi-

equilibrium induced by different microgels. The weight ratio of PVCL and

PNIPMAm in the mixture is 1:1, and the total concentration of microgels is 0.15

mg/mL.

6.3.5. Interfacial dilatational rheology

The interfacial tension results show that microgels are good surface active

materials, with high application potential in the emulsion stabilization field. To

further understand the structure of microgels at the toluene/water interface, the

dilatational rheology of the interface was investigated. Figure 6.11 shows the

temperature trend of the dilatational modulus of the toluene/water interface with

PVCL, PNIPAm and PNIPMAm homopolymer microgels with 3 mol% BIS. We can


122

see that the storage moduli (elastic moduli, ) of the interface are higher than the loss

moduli (viscous moduli, ) for all microgel solutions at the temperatures between 24

to 45 °C. The values of moduli are of the same order as the reported results of

PNIPAm-co-MAA microgels at the heptane/water interface and PNIPAm microgels

at the air/water interface, but two orders lower than the results for hard latex

particles.[15, 53, 54]

20 25 30 35 40 45 501E-4

1E-3

0.01a

E'

E''

Dil

ati

on

al m

od

ulu

s (

N/m

)

Temperature (oC)

20 25 30 35 40 45 501E-4

1E-3

0.01

b

E'

E''

Dil

ati

on

al m

od

ulu

s (

N/m

)

Temperature (oC)

20 25 30 35 40 45 501E-4

1E-3

0.01

c

E'

E''

Dilata

tio

nal m

od

ulu

s (

N/m

)

Temperature (oC)

Figure 6.11. The dilatational modulus of the toluene/water interface with (a) PVCL,

(b) PNIPAm and (c) PNIPMAm microgels with 3 mol% BIS. The concentration of

microgels is 0.15 mg/mL.

The work of Cohin et al. shows that the compression dilatational surface

moduli for PNIPAm microgels at the air/water interface decrease with the increase in

temperature till VPTT and then increase with temperature.[53]

However, the


123

temperature trend of the dilatational surface modulus for linear PNIPAm polymer

solution remains a plateau below LCST, and has a clear increase above LCST.[54]

For

all microgels studied here, the values of of the oil/water interface increase with

temperature. This means the elasticity of the oil/water interface adsorbed with

microgels increase as temperature increase. However, compared to linear polymers,[54]

the increasing rates of the interfacial elasticity for microgel systems are much lower.

The loss modulus of the interface increases slightly with temperature for

PVCL microgels, but remains at a certain level for PNIPAm microgels. For

PNIPMAm microgels, the values of of the interface decrease with temperature.

The results indicate that the viscous property of the oil/water interface with the

adsorption of microgels depends on the chemical structure of microgels. As

temperature increases, a decline is only found in the viscous property of PNIPMAm

microgels-adsorbed interface. That means the interface with the adsorption of

PNIPMAm microgels is more solid-like than those covered by PVCL and PNIPAm

microgels above VPTT. Typical winkles were found on the drop surface during the

volume contraction above VPTT for PNIPMAm microgels, leading to inaccuracy of

the results based on picture analysis of drop shape. Thus, the interfacial dilatational

rheology measurements for the PNIPMAm system were only implemented below

VPTT.

6.3.6. Microgel-stabilized emulsions

6.3.6.1. Emulsions prepared at different oil/water ratios

Figure 6.12 shows the photographs of the emulsions prepared with

PVCL/NIPAm (1:1) microgels at different oil/water volume ratios. Macroscopic

phase separation is found for these mixtures except the one with the oil/water ratio of


124

3/7. For the samples prepared with the oil/water ratios of 1/9 and 2/8, the emulsions

are at the lower part of the mixtures. For the sample prepared with the oil volume

fraction of 30%, the entire mixture is at the emulsion state without macroscopic phase

separation. For samples with the oil volume content more than 40%, the emulsions are

at the upper parts of the mixtures.

Figure 6.12. Photographs of emulsions prepared with PVCL/NIPAm (1:1) microgels

of 3 mol% BIS at the oil/water volume ratios from 1/9 to 9/1. The concentration of

microgels in water phase is 0.5 wt%. The pink color of the samples originates from

Nile Red.

Figure 6.13 shows the fluorescence microscopy results of the emulsions. It can

be clearly found that the continuous phase is the oil stained by Nile Red, which shows

red color under the fluorescence microscope. That means the emulsions are of water-

in-oil (w/o) type, different from the reported results of the oil-in-water (o/w)

emulsions based on PNIPAm/MAA microgels.[7, 10, 11, 55]

Several characteristic

emulsion statuses are found in the fluorescence optical microscopy results. The

emulsions prepared with the oil/water ratios less than 3/7 are fluid, and the aqueous

drops are dispersed very well in the oil phase without collision (Figure 6.13a). By

contrast, the droplets in the emulsion with the oil/water ratio of 3/7 (Figure 6.13b)

have a higher density, similar to the high internal phase emulsions (HIPE) prepared

with PNIPAm/MAA microgels.[9, 56]

For the emulsion prepared with the oil/water

ratio of 4/6, the droplets collide with the neighbor ones and exhibit irregular


125

polygonal shapes rather than spheres (Figure 6.13c). As the oil/water volume ratios

range from 5/5 to 9/1, the emulsions are unstable under the pressure of coverslip. The

coalescence happens quickly at the beginning of optical microscopy measurement.

Small aqueous droplets with collision are fused into bigger continuous phases with

small amount of oil between them (Figure 6.13d). After several minutes, the

coalescence becomes very slow and the system tends to be dynamically stable.

Figure 6.13. Fluorescence microscopy of the emulsions prepared with the oil/water

volume ratio of (a) 1/9, (b) 3/7, (c) 4/6 and (d) 6/4, showing different typical emulsion

statuses. The oil phase stained by Nile Red exhibits red color under the fluorescence

microscope. The concentration of microgels in water phase is 0.5 wt%.

6.3.6.2. Emulsions prepared at different microgel concentrations

The emulsions with the oil/water ratio of 3/7 prepared with different microgel

concentrations are shown in Figure 6.14. For systems with higher concentrations of

microgels (0.5 and 0.25 wt%), HIPE-like emulsions are obtained. When the

concentration of microgels is low (0.1 and 0.05 wt%), macroscopic phase separation


126

occurs and a thin emulsion layer is located at the interface of water and oil phases. As

microgel particles play the role of stabilizer in the system, a sufficient amount of

particles is needed for the formation and subsequent stability of emulsions. We also

prepared emulsions using the microgels with different cross-linker contents, but found

no significant influence on the formation of emulsions.

Figure 6.14. Photographs of emulsions prepared with PVCL/NIPAm (1:1) microgels

of 3 mol% BIS at the oil/water volume ratio of 3/7. The concentration of microgels in

water phase varies from 0.05 to 0.5 wt%.

6.3.6.3. Summary of emulsions prepared with PVCL/NIPAm (1:1) microgels

The statuses of emulsions prepared with PVCL/NIPAm (1:1) microgel

solutions and chloroform are summarized in Table 6.3. It is found that the emulsions

are of w/o type, regardless of the microgel concentration and the oil/water ratio. The

emulsions were prepared with an Ultra Turrax, having a very strong shearing force.

Before the operation of the Ultra Turrax, the aqueous phase is at the upper part and

the oil phase is at the lower part because chloroform has a higher density than water.

For the samples with the oil/water ratio lower than 3/7, some water is brought into the

oil phase during the strong shearing, and a w/o emulsion forms at the lower part. For

the sample with the oil/water ratio of 3/7, the water phase and the oil phase are mixed

perfectly. As the volume fraction of the water phase is close to that of the internal

phase of HIPE (74%), the w/o HIPE in the entire mixture is formed. For samples with

the oil/water ratio above 3/7, an appropriate amount of oil is brought into the upper


127

water phase to form w/o HIPE. It is interesting that the emulsion with the oil/water

ratio of 2.5/7.5 which has a closer ratio to 74% compared to that of 3/7, is not of

HIPE.

Table 6.3. Summary of the emulsions prepared with PVCL/NIPAm (1:1) microgel

solutions and chloroform.

Cross-linker

content

Oil/water

ratio (v/v)

Microgel

concentration

Emulsion status

3 mol% 1/9 0.5 wt% w/o at the lower part

2/8 0.5 wt% w/o at the lower part

2.5/7.5 0.5 wt% w/o at the lower part

3/7 0.5 wt% w/o HIPE in the entire mixture

0.25 wt% w/o HIPE in the entire mixture

0.1 wt% unstable

0.05 wt% unstable

4/6 0.5 wt% w/o HIPE at the upper part






1.5 mol% 1/9 0.5 wt% w/o at the lower part


3/7 5 wt% w/o HIPE in the entire mixture

0.05 mol% 1/9 0.5 wt% w/o at the lower part


3/7 0.5 wt% w/o HIPE in the entire mixture


128

6.3.6.4. Emulsions prepared with microgels of different chemical compositions

The effect of the chemical composition of microgels on the emulsion is studied

as well, and the results are summarized in Table 6.4. At the oil/water ratio of 1/9, w/o

emulsions are obtained for PVCL/NIPAm microgel systems, regardless of the

monomer ratios of VCL/NIPAm. For PVCL/NIPMAm microgels with the

VCL/NIPMAm ratios of 5:1 and 1:1, the emulsions are of w/o type; and for that with

the VCL/NIPMAm ratio of 1:5, o/w emulsions are obtained. It is interesting that

stable o/w emulsions can be gained with PVCL/NIPMAm (1:5) microgels even at a

low microgel concentration (Figure 6.15), unlike PVCL/NIPAm systems (Figure

6.14). In Figure 6.15, we can see that the oil droplets, showing red color under

fluorescence observation, are dispersed in the continuous water phase.

Table 6.4. Summary of the emulsions prepared with PVCL/NIPAm and

PVCL/NIPMAm microgels of different chemical ratios.

Microgelsa Microgel concentration Emulsion status (oil/water = 1/9)

V:N (3:1) 0.5 wt% w/o at the lower part



V:NM (5:1) 0.5 wt% w/o at the lower part

V:NM (1:1) 0.5 wt% w/o at the lower part

V:NM (1:5) 0.5 wt% o/w at the lower part

V:NM (1:5) 0.015 wt% o/w at the lower part


the ratios in the brackets are the monomer ratio of copolymer microgels.


129

Figure 6.15. Optical microscopy of the emulsions (oil/water = 1/9) prepared with

PVCL/NIPMAm (1:5) microgels of 3 mol% BIS at the concentration of 0.015 wt%.

(a) is the bright field observation and (b) is the fluorescence observation. The insets in

(a) is the photograph of the emulsion.

6.3.7. Materials based on microgel-stabilized emulsions

The emulsion-template materials have been developed in different forms, such

as 3D porous scaffold,[57]

2D nanoporous film,[58]

and colloidosomes.[59]

The 3D

porous scaffold is an ideal matrix for the cell cultivation applied in tissue

engineering.[60, 61]

Functional nanoparticles are embedded into the pore walls of

scaffolds to make the materials multi-functional, more suitable for the biomedical

application.[62, 63]

As described above, the w/o emulsion prepared with PVCL/NIPAm (1:1)

microgel at the oil/water ratio of 3/7 resembles HIPE which is usually employed for

the fabrication of scaffold.[57]

The HIPE obtained mentioned above was freeze-dried

and observed with FESEM. As shown in Figure 6.16, the fragments of the emulsion

droplets can be found in the cross section of the freeze-dried sample. With a high

resolution in Figure 6.16b, we can see both close-packed hexagonal arrays and loosely

packed clusters, which have been reported for microgels adsorbed at the oil/water

interface of the emulsions observed by cryo-SEM.[13, 14, 24]

The packed microgels form


130

thin films at the interface. We believe that such a packing layer structure of microgels

at the oil/water interface is essential in the stability of the emulsions.

Figure 6.17. FESEM images of the cross section of freeze-dried sample of the

emulsion prepared with PVCL/NIPAm (1:1) microgel at the oil/water ratio of 3/7.

Figure (b) is the magnified view of the place marked by the rectangle in Figure (a).

The freeze-dried emulsion is macroscopically floccule-like. To make a

stronger supporting frame, the chloroform solution of PCL and aqueous solution of

microgels were homogenized with Ultra Turrax at the same condition for the

preparation of the HIPE described above. The sample was freeze-dried and observed

with FESEM. As shown Figure 6.17, the freeze-dried sample is like a porous material.

The fragments of the emulsion droplets can be also found in the cross section of the

material. Compared with the freeze-dried emulsion samples without PCL, the pores

are smaller in the PCL-containing “porous material”. The high-resolution images

(Figures 6.17b and c) of an open “droplet” show that the microgels are embedded in

the wall of the “porous materials” constructed by polymers (PCL) in the oil phase.

Such kind of materials containing microgels could be used in tissue engineering or

cell cultivation with the advantage of the environmental stimuli controlled release.


131

Figure 6.17. FESEM images of the cross section of the porous materials from the w/o

emulsions (oil/water = 3/7). Figures (b) and (c) are the magnified views of the places

marked by rectangles in (a) and (b), respectively. The aqueous phase contains

PVCL/NIPAm (1:1) microgels at the concentration of 0.015 wt% and the oil phase

contains PCL (Mw ~ 80 k) at the concentration of 4 wt%.

The emulsion-template colloidosomes are a kind of novel capsules with

permeability and have been developed rapidly in the last decade.[8, 59, 64-67]

As

mentioned above, the o/w emulsions can be obtained by mixing PVCL/NIPMAm (1:5)

microgel solutions and chloroform at the oil/water ratio of 1/9 (Table 6.4 and Figure

6.15). It has been reported that microgel-based capsules through an emulsion template

can be successfully obtained by mixing the microgel solution and polymer/chloroform

solution.[8, 66]

The preparation process of such capsules is schematically shown in

Figure 6.18.

At the first stage, the PCL/chloroform solution is prepared and added to the

microgel solution. Then the mixture is homogenized with Ultra Turrax to obtain a

stable o/w emulsion (Figure 6.18a). During the emulsification process, the microgel

particles adsorb to the oil/water interface and stabilize chloroform droplets. At the

second step, the chloroform is removed, and the water-insoluble PCL chains

precipitate on the inner surface and bind the microgel particles together (Figure 6.18b).


132

As a result, capsules are formed with a PCL wall containing integrated microgel

particles.

Figure 6.18. Schematic representation of the capsule preparation: (a) the

emulsification process of the mixture of PCL/chloroform solution and aqueous

microgel solution, where microgels adsorb to the oil/water interface and stabilize

chloroform droplets; (b) after the removal of chloroform, capsules are formed with a

surface that has integrated PCL chains and microgels.

The morphologies of the capsules are characterized with FESEM. Figure 6.19

shows the typical examples of the colloidosome capsules. Due to the inhomogeneous

size of emulsion droplets, the capsule sizes are not uniform. From the FESEM images,

we can see that the capsule diameter ranges from around 5 to 20 μm. On the surface

of the capsules, the microgel particles are hexagonally close-packed, similar to the

reported results of both soft microgels and stiff solid latex at the surface of

colloidosome capsules.[8, 59, 64, 66]

From the high-resolution images, we can see that the

microgel particles are entrapped within the PCL layer, and small holes exist on the

wall. These holes would make the capsules permeable. As PCL is a biodegradable

polymer, the capsule wall can be broken with the degradation of PCL. Both effects

increase the potential of the formed microgel-based capsules in the biomedical

application like drug release. It should be noted that due to the coalescence of


133

neighboring droplets in the emulsion, the coagulation among adjacent capsules is

found (Figure 6.19c).

Figure 6.19. FESEM images of the colloidosomes from the o/w emulsions (oil/water

= 1/9). The aqueous phase contains PVCL/NIPMAm (1:5) microgels at the

concentration of 0.015 wt% and the oil phase contains PCL (Mw ~ 80 k) at the

concentration of 2 wt%. The insets show the magnified views of the places marked by

the rectangles, and the arrows indicate the holes on the wall of capsules.

6.4. Conclusions

In this Chapter, the interfacial behaviors of the temperature-responsive

copolymer microgels based on VCL and two acrylamides, NIPAm and NIPMAm,

with various monomer ratios, are investigated. A classic kinetics of the interfacial

tension decay with three distinct regimes is found in the dynamic interfacial tension

plots of the copolymer microgels. This is similar to the dynamic interfacial behavior

of inorganic nanoparticles and proteins. The equilibrium interfacial tensions are

obtained through the data fitting using the empirical equation (Hua-Rosen equation).

We looked through the influences of monomer ratio and temperature on the

interfacial behavior of microgels. As the copolymer microgels have the similar

internal physical structure, regardless of the chemical composition, and the size of

microgels has no impact on the interfacial behavior, the variation of the interfacial

tension for different microgels should be derived from the chemical structure. The


134

increase in monomer ratio of NIPAm or NIPMAm in microgels leads to an increase in

the interfacial tension. Furthermore, a transition of the interfacial behaviors of

microgels around VPTT is observed, which is highly consistent with the phase

behavior of microgels. Below VPTT, the equilibrium interfacial tensions of all

microgel systems decrease as temperature increases. Above VPTT, the equilibrium

interfacial tensions remain at a certain level for PVCL- and PNIPMAm-rich microgel

systems, and increase slightly for PNIPAm-rich microgel systems. When the

temperature is below VPTT, the interfacial behavior of microgels complies with a

diffusion-controlled process. However, when the temperature is above VPTT, the

diffusion-controlled mechanism seems not to be suitable for the interfacial assembly

of microgels studied here. It is proposed that the deformability (softness) of microgels

plays an imperative role in the interfacial behavior. Because of the reducing

deformability of microgel with increasing temperature, the evolution of the dynamic

interfacial tension for microgel at T < VPTT is faster than that at T > VPTT. The

dynamic interfacial tension results of the microgel systems with different cross-

linking densities strongly support this point.

The emulsions stabilized by PVCL/NIPAm and PVCL/NIPMAm microgels

are prepared. The type of the emulsion depends on the chemical composition of

microgels. It is found that PVCL/NIPAm microgels and PVCL/NIPMAm microgels

with low NIPMAm contents are inclined to make w/o emulsions, while

PVCL/NIPMAm microgels with high NIPMAm contents are prone to o/w emulsions.

For PVCL/NIPAm microgels, a HIPE is obtained at the oil/water volume ratio of 3/7.

The cross-linking density has no significant to the type of emulsions. Based on the

two kinds of emulsions prepared, two different emulsion-template materials can be


135

made. Some preliminary results of the porous scaffold materials based on the w/o

HIPE and the colloidosome capsules based on the fluid o/w emulsion are presented.

The study of interfacial behavior of microgels at the oil/water interface in this

work might provide a new insight for better understanding the properties of microgels

at the fluid interfaces and future applications, related to the stabilization of emulsions

or the fabrication of microgel-based capsules for drug deliveries.

6.5. References









[4] Balaceanu A., Demco D. E., Moller M. and Pich A., Microgel heterogeneous




[5] Wu C., A comparison between the 'coil-to-globule' transition of linear chains

and the "volume phase transition" of spherical microgels, Polymer, 1998, 39, 4609-

4619.



123-136.





[9] Li Z. F., Ming T., Wang J. F. and Ngai T., High internal phase emulsions

etabilized eolely by microgel particles, Angewandte Chemie-International Edition,

2009, 48, 8490-8493.


136

[10] Ngai T., Auweter H. and Behrens S. H., Environmental responsiveness of

microgel particles and particle-stabilized emulsions, Macromolecules, 2006, 39, 8171-

8177.

[11] Ngai T., Behrens S. H. and Auweter H., Novel emulsions stabilized by pH and

temperature sensitive microgels, Chemical Communications, 2005, 331-333.

[12] Monteux C., Marliere C., Paris P., Pantoustier N., Sanson N. and Perrin P.,

Poly(N-isopropylacrylamide) microgels at the oil-water interface: Interfacial

properties as a function of temperature, Langmuir, 2010, 26, 13839-13846.

[13] Schmidt S., Liu T. T., Rutten S., Phan K. H., Moller M. and Richtering W.,

Influence of microgel architecture and oil polarity on stabilization of emulsions by

stimuli-sensitive core-shell poly(N-isopropylacrylamide-co-methacrylic acid)

microgels: Mickering versus Pickering behavior?, Langmuir, 2011, 27, 9801-9806.




[15] Brugger B., Vermant J. and Richtering W., Interfacial layers of stimuli-

responsive poly-(N-isopropylacrylamide-co-methacrylicacid) (PNIPAM-co-MAA)

microgels characterized by interfacial rheology and compression isotherms, Physical

Chemistry Chemical Physics, 2010, 12, 14573-14578.

[16] Li Z. F. and Ngai T., Microgel particles at the fluid-fluid interfaces,

Nanoscale, 2013, 5, 1399-1410.

[17] Kutuzov S., He J., Tangirala R., Emrick T., Russell T. P. and Boker A., On the

kinetics of nanoparticle self-assembly at liquid/liquid interfaces, Physical Chemistry

Chemical Physics, 2007, 9, 6351-6358.

[18] Binks B. P. and Lumsdon S. O., Pickering emulsions stabilized by

monodisperse latex particles: Effects of particle size, Langmuir, 2001, 17, 4540-4547.

[19] Binks B. P. and Clint J. H., Solid wettability from surface energy components:

Relevance to pickering emulsions, Langmuir, 2002, 18, 1270-1273.

[20] Boker A., He J., Emrick T. and Russell T. P., Self-assembly of nanoparticles at

interfaces, Soft Matter, 2007, 3, 1231-1248.

[21] Ranatunga R., Kalescky R. J. B., Chiu C. C. and Nielsen S. O., Molecular

dynamics simulations of surfactant functionalized nanoparticles in the vicinity of an

oil/water interface, Journal of Physical Chemistry C, 2010, 114, 12151-12157.


Matter, 2009, 5, 2681-2685.


137

[23] Eckert T. and Richtering W., Thermodynamic and hydrodynamic interaction

in concentrated microgel suspensions: Hard or soft sphere behavior?, Journal of

Chemical Physics, 2008, 129.


and Schmitt V., Soft microgels as Pickering emulsion stabilisers: role of particle





[26] Richtering W., Responsive emulsions stabilized by stimuli-sensitive

microgels: Emulsions with special non-Pickering properties, Langmuir, 2012, 28,

17218-17229.

[27] Li Z., Geisel K., Richtering W. and Ngai T., Poly(N-isopropylacrylamide)

microgels at the oil-water interface: adsorption kinetics, Soft Matter, 2013, 9, 9939-

9946.











[31] Beverung C. J., Radke C. J. and Blanch H. W., Protein adsorption at the

oil/water interface: Characterization of adsorption kinetics by dynamic interfacial

tension measurements, Biophysical Chemistry, 1999, 81, 59-80.

[32] Aveyard R., Binks B. P. and Clint J. H., Emulsions stabilised solely by

colloidal particles, Advances in Colloid and Interface Science, 2003, 100, 503-546.

[33] Vignati E., Piazza R. and Lockhart T. P., Pickering emulsions: Interfacial

tension, colloidal layer morphology, and trapped-particle motion, Langmuir, 2003, 19,

6650-6656.

[34] Eastoe J. and Dalton J. S., Dynamic surface tension and adsorption

mechanisms of surfactants at the air-water interface, Advances in Colloid and



138

[35] Gilcreest V. P., Dawson K. A. and Gorelov A. V., Adsorption kinetics of

NIPAM-based polymers at the air−water interface as studied by pendant drop and

bubble tensiometry, The Journal of Physical Chemistry B, 2006, 110, 21903-21910.

[36] Tripp B. C., Magda J. J. and Andrade J. D., Adsorption of globular proteins at

the air/water interface as measured via dynamic surface tension: Concentration

dependence, mass-transfer considerations, and adsorption kinetics, Journal of Colloid

and Interface Science, 1995, 173, 16-27.

[37] Rojas Y. V., Phan C. M. and Lou X., Dynamic surface tension studies on

poly(N-vinylcaprolactam/N-vinylpyrrolidone/N,N-dimethylaminoethyl methacrylate)

at the air-liquid interface, Colloids and Surfaces A-Physicochemical and Engineering

Aspects, 2010, 355, 99-103.

[38] Li G., Prasad S. and Dhinojwala A., Dynamic interfacial tension at the

oil/surfactant- water interface, Langmuir, 2007, 23, 9929-9932.

[39] Kawaguchi M., Hirose Y. and Kato T., Effects of temperature and

concentration on surface tension of poly(N-isopropylacrylamide) solutions, Langmuir,

1996, 12, 3523-3526.

[40] Zhang J. and Pelton R., The dynamic behavior of poly(N-isopropylacrylamide)

at the air/water interface, Colloids and Surfaces A-Physicochemical and Engineering

Aspects, 1999, 156, 111-122.

[41] Zhang J. and Pelton R., Poly(N-isopropylacrylamide) microgels at the air-

water interface, Langmuir, 1999, 15, 8032-8036.

[42] Eastoe J., Dalton J. S., Rogueda P. G. A. and Griffiths P. C., Evidence for

activation-diffusion controlled dynamic surface tension with a nonionic surfactant,

Langmuir, 1998, 14, 979-981.







2011, 289, 613-624.



Science, 2000, 278, 830-840.




3305-3314.


139



[48] Bresme F. and Oettel M., Nanoparticles at fluid interfaces, Journal of Physics-

Condensed Matter, 2007, 19.

[49] Liu T. T., Seiffert S., Thiele J., Abate A. R., Weitz D. A. and Richtering W.,

Non-coalescence of oppositely charged droplets in pH-sensitive emulsions,

Proceedings of the National Academy of Sciences of the United States of America,

2012, 109, 384-389.

[50] Stratford K., Adhikari R., Pagonabarraga I., Desplat J.-C. and Cates M. E.,

Colloidal jamming at interfaces: A route to fluid-bicontinuous gels, Science, 2005,

309, 2198-2201.

[51] Subramaniam A. B., Abkarian M. and Stone H. A., Controlled assembly of

jammed colloidal shells on fluid droplets, Nature Materials, 2005, 4, 553-556.

[52] Herzig E. M., White K. A., Schofield A. B., Poon W. C. K. and Clegg P. S.,

Bicontinuous emulsions stabilized solely by colloidal particles, Nature Materials,

2007, 6, 966-971.

[53] Cohin Y., Fisson M., Jourde K., Fuller G. G., Sanson N., Talini L. and

Monteux C., Tracking the interfacial dynamics of PNiPAM soft microgels particles

adsorbed at the air-water interface and in thin liquid films, Rheologica Acta, 2013, 52,

445-454.

[54] Cicuta P., Stancik E. J. and Fuller G. G., Shearing or compressing a soft glass

in 2D: Time-concentration superposition, Physical Review Letters, 2003, 90.

[55] Brugger B. and Richtering W., Emulsions stabilized by stimuli-sensitive

poly(N-isopropylacrylamide)-co-methacrylic acid polymers: Microgels versus low

molecular weight polymers, Langmuir, 2008, 24, 7769-7777.

[56] Sun G. Q., Li Z. F. and Ngai T., Inversion of particle-stabilized emulsions to

form high-internal-phase emulsions, Angewandte Chemie-International Edition, 2010,

49, 2163-2166.

[57] Silverstein M. S., Emulsion-templated porous polymers: A retrospective

perspective, Polymer, 2014, 55, 304-320.

[58] Koh H. D., Kang N. G. and Lee J. S., Fabrication of an open Au/nanoporous

film by water-in-oil emulsion-induced block copolymer micelles, Langmuir, 2007, 23,

12817-12820.

[59] Dinsmore A. D., Hsu M. F., Nikolaides M. G., Marquez M., Bausch A. R. and

Weitz D. A., Colloidosomes: Selectively permeable capsules composed of colloidal

particles, Science, 2002, 298, 1006-1009.


140

[60] Viswanathan P., Chirasatitsin S., Ngamkham K., Engler A. J. and Battaglia G.,

Cell Instructive microporous scaffolds through interface engineering, Journal of the

American Chemical Society, 2012, 134, 20103-20109.

[61] Cao Z. B., Wen J. C., Yao J. R., Chen X., Ni Y. S. and Shao Z. Z., Facile

fabrication of the porous three-dimensional regenerated silk fibroin scaffolds,

Materials Science & Engineering C-Materials for Biological Applications, 2013, 33,

3522-3529.

[62] Nandagiri V. K., Gentile P., Chiono V., Tonda-Turo C., Matsiko A., Ramtoola

Z., Montevecchi F. M. and Ciardelli G., Incorporation of PLGA nanoparticles into

porous chitosan-gelatin scaffolds: Influence on the physical properties and cell

behavior, Journal of the Mechanical Behavior of Biomedical Materials, 2011, 4,

1318-1327.

[63] Li D., Ye C., Zhu Y., Qi Y. Y., Gou Z. R. and Gao C. Y., Fabrication of

poly(lactide-co-glycolide) scaffold embedded spatially with hydroxyapatite particles

on pore walls for bone tissue engineering, Polymers for Advanced Technologies,

2012, 23, 1446-1453.

[64] Thompson K. L., Armes S. P., Howse J. R., Ebbens S., Ahmad I., Zaidi J. H.,

York D. W. and Burdis J. A., Covalently cross-linked colloidosomes,

Macromolecules, 2010, 43, 10466-10474.

[65] Wang H. L., Zhu X. M., Tsarkova L., Pich A. and Moller M., All-silica

colloidosomes with a particle-bilayer shell, Acs Nano, 2011, 5, 3937-3942.

[66] Ao Z., Li Z. F., Zhang G. Z. and Ngai T., Colloidosomes formation by

controlling the solvent extraction from particle-stabilized emulsions, Colloids and

Surfaces A-Physicochemical and Engineering Aspects, 2011, 384, 592-596.

[67] Wei Z. J., Wang C. Y., Zou S. W., Liu H. and Tong Z., Chitosan nanoparticles

as particular emulsifier for preparation of novel pH-responsive Pickering emulsions

and PLGA microcapsules, Polymer, 2012, 53, 1229-1235.

Chapter 7. Summary

141

Chapter 7. Summary

We synthesized PVCL-based thermo-sensitive copolymer microgels and

investigated the influence of the chemical composition, cross-linking density and

temperature on their interfacial behavior.

The polymerization technique used is the precipitation polymerization which

involves the formation of microgel particles by a homogeneous nucleation mechanism.

Three sets of microgel samples were synthesized: a) with variable molar ratios

between VCL and NIPAm, and VCL and NIPMAm respectively; b) with variable

cross-linking degree and c) with variable size. The chemical composition of

copolymer microgels were determined by NMR spectroscopy. The results show that

the monomer ratio in copolymer microgel is close to the initial feeding ratio during

the synthesis. The hydrodynamic dimensions of all microgels are characterized by

DLS at different temperatures. As expected, the swelling property of microgels

strongly depends on the chemical structure, the cross-linking density and the presence

of the surfactant. Besides the neutral PVCL/NIPAm and PVCL/NIPMAm copolymer

microgels, ionizable PVCL/AAEM-based microgels are synthesized by the addition

of VIm or AAc monomers. The hydrodynamic size and zeta potential of

PVCL/AAEM/VIm and PVCL/AAEM/AAc microgels at different pH values are

detected by DLS and Zetasizer, respectively. The pH-responsiveness of these

microgels indicates a successful incorporation VIm and AAc groups into the

copolymer microgels.

The internal structure of PVCL-based copolymer microgels were investigated

with SLS and microscopy methods (AFM and FESEM). The SLS results show that

Chapter 7. Summary

142

PVCL/NIPAm and PVCL/NIPMAm microgels, both with different monomer ratios in

copolymer, have the core-corona structure with a radial profile as reported for

PNIPAm microgels previously. The characterization of PVCL/NIPAm copolymer and

PVCL homopolymer microgels by AFM gives morphologies and dimensions (contact

radius and height) of microgels on a solid substrate. It provides visualized evidence of

a core-corona structure depending on the cross-linking density. The AFM

characterization of PVCL/NIPAm copolymer microgels deposited on a solid substrate

above VPTT reveals the structure of microgels in the collapsed state, where the

corona part is hardly to observe. Besides, PVCL-based microgels with functional

groups are characterized with AFM and FESEM. It is interesting that the core-corona

structure is found in PVCL/AAEM /VIm microgels, but not in PVCL/AAEM/AAc

microgels, which might be originated from their different synthesis procedures. In

addition, the AFM observation on microgels of very low cross-linking density

provides a possible method to look into the domains in the cross-linked polymer

networks.

We further studied adsorption and deformation of microgels at the air/water

and air/solid interfaces. The surface activity of the PVCL/NIPAm microgels varied in

cross-linking degree was detected with a tensiometer and the conformation of

microgels at the air/water interface was simulated. The results show that faster

spreading happens for the loose microgel which takes more flat shape compared to the

dense microgel. The contact radius and height of the adsorbed microgels, extracted

from AFM images, indicate that the deformability of the microgels increases with

decrease of the cross-linking degree. The results suggest that adsorption of microgels

induces deformation exceeding the swelling-induced expansion. In particular, a

loosely cross-linked microgel sustains large deformation due to swelling.

Chapter 7. Summary

143

Furthermore, the interfacial behaviors of PVCL/NIPAm and PVCL/NIPMAm

microgels varied in the monomer ratio, cross-linking degree and size are investigated.

A classic kinetics of interfacial tension with three distinct regimes, similar to the

dynamic interfacial behavior of inorganic nanoparticles and proteins, are found in the

dynamic interfacial tension plots of copolymer microgels. The equilibrium interfacial

tensions are obtained through the data fitting using the empirical equation (Hua-Rosen

equation).

As the copolymer microgels have the similar internal physical structure,

regardless of the chemical composition, and the size of microgels has no impact on

the interfacial behavior, the variation of the interfacial tension for different microgels

should be derived from the chemical structure. A transition of the interfacial

behaviors of microgels around VPTT is observed, which is highly consistent with the

phase behavior of microgels. Below VPTT, the equilibrium interfacial tensions of all

microgel systems decrease as temperature increases. Above VPTT, the equilibrium

interfacial tensions remain at a certain level for PVCL- and PNIPMAm-rich microgel

systems, and increase slightly for PNIPAm-rich microgel systems. When the

temperature is below VPTT, the interfacial behavior of microgels complies with a

diffusion-controlled process. However, when the temperature is above VPTT, the

diffusion-controlled mechanism seems not to be suitable for the interfacial assembly

of microgels. It is proposed that the deformability (softness) of microgels plays an

imperative role in the interfacial behavior. Because of the reducing deformability of

microgel with increasing temperature, the evolution of dynamic interfacial tension for

microgel at T < VPTT is faster than that at T > VPTT. The dynamic interfacial

tension results of microgel systems with different cross-linker densities strongly

support this point.

Chapter 7. Summary

144

Last but not the least, we used PVCL/NIPAm and PVCL/NIPMAm microgels

varied in the monomer ratio and cross-linking degree to produce microgel-stabilzed

emulsions. The type of the emulsion depends on the chemical composition of

microgels. It is found that PVCL/NIPAm microgels and PVCL/NIPMAm microgels

with low NIPMAm contents are inclined to make w/o emulsions, while

PVCL/NIPMAm microgels with high NIPMAm contents are prone to o/w emulsions.

For PVCL/NIPAm microgels, a HIPE is obtained at the oil/water volume ratio of 3/7.

The cross-linker density has no significant to the type of emulsions. Based on the two

kinds of emulsions prepared, two different emulsion-template materials can be made.

They are porous scaffold materials based on the w/o HIPE and colloidosome capsules

from the fluid o/w emulsion.

Chapter 8. Outlook and Future Experiments

145


The interfacial behaviors of PVCL-based microgels on the solid surface or at

the oil/water interface are investigated in this thesis. The results can provide a new

insight for better understanding the properties of microgels at interfaces and future

applications, like the stabilization of emulsions or the fabrication of microgel-based

capsules for drug deliveries. However, the topic of soft microgel particles at the

interface is very complex, and needs further investigation. Here we propose some

concepts for the future experiments.

8.1. Interfacial behavior

8.1.1. Microgels with ionizable groups or specific structures

The ionizable groups in functional microgels play an important role in the

structure and consequently the property of microgels. A lot of work has been done to

investigate the interfacial behavior of PNIPAm-based ionizable microgels. The charge

of the microgel influences the assembly structure of microgel layers at the interfaces.

However, the intrinsic role of charges still needs further investigation. Recently, the

core-shell microgels with different components in core and shell parts are obtained.[1]

This enables the study on the influence of spatial distribution of functional groups on

the interfacial behavior of microgels. Further experiments could be carried out with

the following sets:

1) Polyelectrolytic or ampholytic microgels with acidic or basic groups

located in the core or shell of the particle;


146

2) Core-shell microgels with different VPTTs in core and shell parts, like

PVCL-core/NIPMAm-shell and PNIPMAm-core/PVCL-shell;

3) Core-shell microgels with stiff insensitive core, polystyrene for example,

and soft sensitive shell, like NIPAm or VCL; or reverse ones.

It should be noted that there are many sorts of microgels with unique

structures, and specific properties. They are worth well investigating.

8.1.2. Other techniques

Many other techniques, such as interfacial rheology and cryo-FESEM and

liquid-cell AFM, have also been used to characterize interfacial layers. Interfacial

rheology is very useful in investigating the mechanical property of microgel layers at

the fluid interface and their response to external stimuli. Cryo-FESEM is another

practical tool to detect the assembly microgel suprastructures at the liquid/liquid

interfaces. However, either rheometer or cryo-FESEM cannot take into account both

the bulk state and visualization of microgel structures at fluid/fluid interfaces. Liquid-

cell AFM offers a potential in observing the microgel structure at the fluid interface in

situ. To achieve this aim, we need choose the oil phase carefully, due to a strong

fluctuation of the fluid interfaces. Silicone oil is a possible option as its viscosity is

much higher than water. The interface of water and silicone oil could be relatively

stable for the liquid-cell AFM characterization of adsorbed microgels.

8.2. Potential applications and improvements

8.2.1. Asymmetric colloidal particles

We have shown that the adhesion of microgels on a solid substrate is very

strong. With such a feature, microgels can be applied to microgel-supported adhesives


147

which are useful in papermaking industry. Besides the indeformable silicon wafer

used in this work, microgels can also be deposited on a soft stretchable substrate,

PDMS for example. After the deposition, we can stretch the substrate. Then the

mcirogel particles on the substrate surface would be elongated in the stretching

direction.[2]

With some cross-linking treatments, like UV irradiation of light sensitive

groups, the stretched microgel particles would be fixed in an asymmetric shape. After

removed from the substrate, anisotropic particles can be obtained.

Figure 8.1. Fabrication of asymmetric colloidal particles upon stretching.

Another potential application of microgels on a solid substrate is to form

asymmetric hybrid colloids. Metal nanoparticle precursors can be incorporated inside

the microgels with functional groups, for instance, HAuCl4 in PVCL/AAEM/AAc

microgels.[3]

As demonstrated in Chapters 4 and 5, the polymer density decrease from

the center to the surrounding of the particle profile. If such a structure is exposed in

UV light for an appropriate time, Au nanoparticles would only form in a superficial

layer where the UV light reaches. After eluted from the substrate, asymmetric

colloidal hybrids can be produced.

8.2.2. New microgel-based capsules

The preliminary fabrication of microgel-based emulsions, as well as capsules

and scaffolds is successful in this work. The properties of microgel-stabilized

emulsions, like stability and stimuli responsiveness, need to be studied in detail. For


148

the future application of the capsules and porous scaffolds, several issues should be

addressed. Firstly, the parameters for producing such kinds of materials from

emulsion need to be optimized to obtain monodisperse capsules and interconnected

porous scaffolds. Secondly, more experiments, like drug load and release or

mechanical test, need to be implemented to ensure that microgel-based capsules and

scaffolds are effective materials for biomedical application. Thirdly, polymers with

the glass transition temperature or melting temperature close to the VPTT of

microgels can be used to increase the temperature sensitivity of the formed capsules.

8.2.3. Designed biodistribution of microgels for therapy

In nature cells can deform to translocate through small vascula and recover to

original shape without loss of physiological functions. As microgels are soft porous

particles, they can deform like cells when translocating through a channel much

smaller than the particle size. In a simulation of renal filtration of soft particles,

microgels passed through pores which were ten times smaller than the particle

diameter under physiologically relevant pressures.[4]

Such a property could be applied

to anti-cancer therapy for which the translocation of drug carriers through tumor

vascular is demanded (Figure 8.2).[5, 6]

Additionaly, the deformability (softness) also

governs the biodistribution and circulation times of soft gel particles in blood.[7]

Because of the diverse retention sizes of human organs, the circulation time and

distribution of gel particles in the body differ upon the organ. Therefore we can

design microgels with an appropriate softness for target therapeutic applications due

to their preferential accumulation in a tissue.


149

Figure 8.2. Microgel-based nanocarriers for tumour targeting of long-circulating

therapeutics by the translocation of deformable particles loaded with drugs through

tumor vascula.

8.3. References

[1] Richtering W. and Pich A., The special behaviours of responsive core-shell

nanogels, Soft Matter, 2012, 8, 11423-11430.

[2] Lee K. J., Yoon J. and Lahann J., Recent advances with anisotropic particles,

Current Opinion in Colloid & Interface Science, 2011, 16, 195-202.




[4] Hendrickson G. R. and Lyon L. A., Microgel translocation through pores

under confinement, Angewandte Chemie International Edition, 2010, 49, 2193-2197.

[5] Duncan R., The dawning era of polymer therapeutics, Nature Reviews Drug

Discovery, 2003, 2, 347-360.

[6] Jhaveri A. M. and Torchilin V. P., Multifunctional polymeric micelles for

delivery of drugs and siRNA, Frontiers in Pharmacology, 2014, 5, 77.

[7] Merkel T. J., Jones S. W., Herlihy K. P., Kersey F. R., Shields A. R., Napier

M., Luft J. C., Wu H. L., Zamboni W. C., Wang A. Z., Bear J. E. and DeSimone J. M.,

Using mechanobiological mimicry of red blood cells to extend circulation times of

hydrogel microparticles, Proceedings of the National Academy of Sciences of the

United States of America, 2011, 108, 586-591.

Acknowledgements

151

Acknowledgements

At the end of the thesis, I would like to express my gratitude to many people who

helped me during my four-year doctorate study in Aachen.

Firstly, I would like to thank Prof. Dr. Andrij Pich for giving me a good opportunity of

pursuing my PhD study in the Functional and Interactive Polymers group at DWI. I sincerely

appreciate him for his kind support in the development of my research work and useful

discussions with him.

I am thankful to Dr. Ahmed Mourran for his constructive suggestions and kind helps

on AFM measurements and Prof. Dr. Igor Potemkin for his help on the computer simulation

support. I would also like to thank Prof. Dr. Walter Richtering from the Institute of Physical

Chemistry (IPC) at RWTH Aachen University for his valuable comments and interesting

discussion on my PhD work. Thanks to Dr. Xiaomin Zhu for his kind help in my arriving in

Aachen. In addition, the financial support from the China Scholarship Council (CSC) for my

first three years of PhD study in Germany is highly appreciated.

I am grateful for all my other colleagues at DWI and ITMC who helped me during my

memorable years. Many thanks to Dr. Andreea Balaceanu, Ms. Karla Dörmbach, Ms. Garima

Agrawal, Ms. Ricarda Schröder, Mr. Dominic Kehren and Mr. Christian Willems for the work

side by side since I was in Aachen. Thanks to Ms. Angela Huschens and Mr. Rainer Hass for

their kind help as well. The kind help from Ms. Susanne Weise at IPC/RWTH for light

scattering measurements and Mr. Rustam A. Gumerov at Physics Department in Lomonosov

Moscow State University for computer simulation is also appreciated. To my other colleagues,

I appreciate your kindness and will never forget the happy time with you guys of working

together, funny outing-trips and delicious BBQs. Although I do not list all your names here, I

cherish everything in my heart.

Acknowledgements

152

Thanks to my dear Chinese friends I met in Aachen: Dr. Hailin Wang, Dr. Yingchun

He, Dr. Zhirong Fan and Mrs. Fan – Yunfei Jia, Dr. Qizheng Dou, Mrs. Dou – Dr. Cheng

Cheng and their cute daughter – Tangdou, Dr. Jingbo Wang and Mrs. Wang – Yuting Zhao,

Dr. Lei Li, Dr. Heng Zhang, Ms. Huihui Wang, Mr. Qingxin Zhao, Ms. Yelin Bao, Ms. Helin

Li, Ms. Chi Zhang, Mr. Yongliang Zhao, Ms. Yanqing Li, Ms. Qianjie Zhang, Mr. Baolei Zhu,

Mr. Huan Peng, Mr. Chao Liang, Ms. Yanlan Zheng, Mr. Hang Zhang, Mr. Zhihua Hong, Mr.

Baochun Wang and Mrs. Wang – Congling Wang, Ms. Yumei Wang, Mr. Zhi Chen, Mr.

Kang Han, Ms. Xiaolin Dai and Mr. Lei Wu. I will cherish the happy memory of having lunch

together, hanging out from time to time and sport games every week. Besides, I would also

like to thank my friends who chatted with me by phones or emails. Wherever you guys are, in

China, UK or USA, you always encouraged me to get free from the trapped thoughts. Thank

you!

Lastly but most sincerely, I would like to express my greatest appreciation to my

family for their warm support and selfless love to me. Confucius said: “While your parents

are alive, do not journey afar. If a journey has to be made, your direction must be told.” My

dear mother, father, grandma, uncles and aunts: now I am completing my far journey. I did

not make such a journey alone (and I cannot make it alone), but with all of you backing me

and as well along with so many lovely friends. I love you very much!

Yaodong Wu

08 July, 2014

153

Curriculum Vitae

Personal Details

Family Name: Wu

First Name: Yaodong

Birth Date: 31. December 1986

Birth Place: Dongzhi, Anhui, China

Nationality: Chinese

Education

09/2003 – 07/2007

Bachelor of Engineering, Material Science and Engineering, Tianjin University,

Tianjin, China

09/2007 – 07/2010

Master of Science, Polymer Chemistry and Physics, Fudan University, Shanghai,

China

09/2010 –

Doctorate study in Macromolecular Chemistry, RWTH Aachen University, Aachen,

Germany

154

List of publications

Papers

1. Yaodong Wu, Susanne Wiese, Andreea Balaceanu, Walter Richtering, Andrij

Pich. Behaviour of Temperature-responsive Copolymer Microgels at the

Oil/Water Interface, Langmuir, 2014, 30, 7660–7669.

2. Yaodong Wu, Cheng Cheng, Jinrong Yao, Xin Chen, and Zhengzhong Shao.

Crystallization of Calcium Carbonate on Chitosan Substrates in the Presence of

Regenerated Silk Fibroin, Langmuir, 2011, 27, 2804–2810.

3. Florian Schneider, Andreea Balaceanu, Artem Feoktystov, Vitaliy Pipich,

Yaodong Wu, Jürgen Allgaier, Wim Pyckhout-Hintzen, Andrij Pich, Gerald

Schneider. Monitoring the Internal Structure of Poly(N-vinylcaprolactam)

Microgels with Variable Crosslink Concentration, submitted.

4. Yaodong Wu, Rustam A. Gumerov, Andrey A. Rudov, Igor I. Potemkin, Andrij

Pich, Ahmed Mourran. Statics and Dynamics of Interfacial Attachment of

Microgel Colloids, in progress.

Conference contributions

1. Yaodong Wu, Ahmed Mourran, Andrij Pich. Deformation and Adhesion of

Microgels on a Solid Substrate, Poster Presentation at Bayreuth Polymer

Symposium 2013, Bayreuth, Germany, September 15th

– 17th

, 2013.


Pich. Behaviour of Thermo-sensitive Copolymer Microgels at the Oil/Water

Interface, Poster Presentation at 27th Conference of the European Colloid and

Interface Society, Sofia, Bulgaria, September 1th

– 6th

, 2013.


Pich. Behaviour of Thermo-sensitive Copolymer Microgels at the Oil/Water

Interface, Poster Presentation at IUPAC 10th International Conference on

Advanced Polymers via Macromolecular Engineering, Durham, UK, August 18th

– 22nd

, 2013.

“behavior of temperature-responsive microgels at...

Documents