composite films formed by cellulose nanocrystals and latex ... · np aq of 1.25 and 1.67 wt%. (c)...
TRANSCRIPT
i
Composite Films Formed by Cellulose nanocrystals and
Latex Nanoparticles: Optical, Structural, and
Mechanical Properties
By:
Brandon McRae Vollick
A thesis submitted in conformity with the requirements for the degree of Master of Science
Graduate Department of Chemistry
University of Toronto
© Copyright by Brandon McRae Vollick 2017
ii
Composite Cellulose Films Containing Crosslinked, Soft, Latex
Nanoparticles: Optical, Structural, and Mechanical Properties Brandon McRae Vollick
Master of Science
Graduate Department of Chemistry
University of Toronto
2017
Abstract
This thesis describes the preparation of iridescent, birefringent, composite films composed of
cellulose nanocrystals (CNCs), latex nanoparticles (NPs) and a NP crosslinker; hexanediamine (HDA).
First, aqueous suspensions were prepared with varying quantities of CNCs, NPs and HDA before
equilibrating for one week. The cholesteric (Ch) phase was then cast and dried into a film. The optical,
structural and mechanical properties of the film was analyzed.
Second, films with identical compositions of CNCs, NPs, and HDA were fabricated in three
different ways to yield films of different morphology, (i) fast drying of an isotropic suspension, yielding
an isotropic film, (ii) slow drying of an isotropic suspension, yielding a partially Ch films, (iii) slow drying
of an equilibrated suspension, yielding a highly Ch film. The optical and mechanical properties of the
films was analyzed.
iii
Acknowledgments
My heartfelt gratitude and appreciation goes to my supervisor Professor Eugenia Kumacheva
for her constant guidance and dedication to my work. I am thankful for her kind and patient
mentorship throughout my graduate studies.
Additionally, I thank Héloïse Thérien-Aubin for her guidance in the early stages of my research,
and Ilya Gourevich for his electron microscopy wisdom.
Finally, I would like to sincerely thank the entirety of the Kumacheva research group for their
support, advice, and genuine friendship throughout this experience.
iv
Table of Contents
Abstract ..................................................................................................................................... i
Acknowledgments .................................................................................................................... ii
Table of Contents ...................................................................................................................... iii
List of Tables ............................................................................................................................. vi
List of Figures ............................................................................................................................ vii
Chapter 1: Introduction .......................................................................................................... 1
1.1 Cellulose nanocrystal suspensions ......................................................................... 1
1.1.1 Cellulose in nature ................................................................................ 1
1.1.2 Cellulose nanocrystals .......................................................................... 3
1.1.3 Colloidal and Liquid crystalline properties of CNC suspensions ........... 5
1.2 Cellulose nanocrystal films ..................................................................................... 7
1.2.1 Film formation and modification .......................................................... 7
1.2.2 Additives to CNC films .......................................................................... 8
1.2.3 Applications of films ............................................................................. 10
1.3 Summary ................................................................................................................. 14
1.4 References .............................................................................................................. 15
Chapter 2: Materials and Methods ..................................................................................19
2.1 Materials 19
2.1.1 Cellulose nanocrystals ......................................................................................... 19
2.1.2 Latex nanoparticles .............................................................................................. 19
2.2 Methods .................................................................................................................. 20
2.2.1 Transmission electron microscopy of cellulose nanocrystals ................. 20
2.2.2 Effect of hexanediamine on cellulose nanocrystal electrokinetic potential
..........................................................................................................................20
v
2.2.3 Synthesis of latex nanoparticles .............................................................. 20
2.2.4 Characterization of latex nanoparticles .................................................. 21
2.2.5 Film preparation (for experiments described in Chapter 3) ................... 21
2.2.6 Film preparation (for experiments described in Chapter 4) ................... 22
2.2.6.1 Equilibrium (EQU) ..................................................................... 22
2.2.6.2 Partial (PAR) .............................................................................. 22
2.2.6.3 Isotropic (ISO) ........................................................................... 22
2.2.7 Fluorescence spectroscopy ..................................................................... 23
2.2.8 Attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR)
.......................................................................................................................... 23
2.2.9 Characterization of extinction of the composite films ............................ 23
2.2.10 Circular dichroism spectroscopy............................................................ 24
2.2.11 Confocal fluorescence microscopy ........................................................ 24
2.2.12 Electron microscopy characterization of film structure ........................ 24
2.2.13 Mechanical analysis of composite films ................................................ 25
2.2.14 Determination of latex glass transition temperature ........................... 25
2.3 References .................................................................................................. 26
Chapter 3: Composite Cholesteric Nanocellulose Films with Enhanced Mechanical Properties
.............................................................................................................................27
3.1 Introduction ............................................................................................................ 28
3.2 Latex nanoparticles ................................................................................................. 30
3.3 Properties of Cellulose Nanocrystals ...................................................................... 33
3.4 Effect of Crosslinker on Cellulose Nanocrystals and Latex Nanoparticles ............. 34
vi
3.5 Suspensions of Cellulose Nanocrystals and Latex Nanoparticles ......................... 36
3.6 Optical properties of composite CNC-latex films ................................................. 39
3.7 Microstructure of the composite films ................................................................. 42
3.8 Mechanical properties of the composite films ..................................................... 45
3.9 Discussion.............................................................................................................. 46
3.10 Conclusion ............................................................................................................ 47
3.11 References ........................................................................................................... 48
Chapter 4: Composite films formed by cellulose nanocrystals and latex nanoparticles: effect of film
morphology on optical and mechanical properties ................................................................ 50
4.1 Introduction .......................................................................................................... 51
4.2 Latex nanoparticles .............................................................................................. 53
4.3 Film fabrication ..................................................................................................... 53
4.4 Film structure ........................................................................................................ 56
4.5 Optical properties of the composite films ............................................................ 58
4.6 Mechanical properties .......................................................................................... 60
4.7 Discussion.............................................................................................................. 63
4.8 Conclusion ............................................................................................................. 65
4.9 References ............................................................................................................ 66
Chapter 5: Conclusions and Outlook ...................................................................................... 68
5.1 Conclusions ........................................................................................................... 68
5.2 Outlook ................................................................................................................. 70
5.3 References ........................................................................................................... 71
vii
List of Tables
Chapter 1 Introduction
Table 1.1. Optimized conditions for production of stable aqueous colloid of CNCs ............... 4
Chapter 3 Composite Cholesteric Nanocellulose Films with Enhanced Mechanical Properties
Table 3.1. Quantities of each reactant used for batch synthesis of latex NPs by emulsion
polymerization. ......................................................................................................................... 31
Table 3.2. Formulations of mixed NP/CNC suspensions used for film preparation. The cumulative mass
of each suspension was 20 g. ................................................................................................... 37
Chapter 4 Composite films formed by cellulose nanocrystals and latex nanoparticles:
effect of film morphology on optical and mechanical properties
Table 4.1. Composition, annealing conditions, and thickness for ISO, PAR, and EQU films .... 55
viii
List of Figures
Chapter 1 Introduction
Figure 1.1 (a) Repeat unit of cellulose biopolymer. (b) Transmission electron microscopy image of CNCs.
Scale bar is 400 nm. (c) Cationic functionalization of cellulose with epoxypropyltrimethylammonium
chloride ..................................................................................................................................... 3
Figure 1.2 Schematic representation of the dispersion of the electrostatic envelope surrounding a CNC
upon sonication ........................................................................................................................ 7
Figure 1.3 Surrounding of CNC with the zwitterionic surfactant resulting in increased Ch pitch. Inset:
Electrostatic interaction between cationic region of DMAPS and negatively charged sulphate group on
CNC surface ............................................................................................................................... 9
Figure 1.4 Structural design of electro-optical light shutter with CNC film embedded within the low
molecular weight liquid crystal................................................................................................. 12
Figure 1.5 Production of mesoporous carbon (a) and mesoporous titania (b) starting from CNCs and
silica .......................................................................................................................................... 13
Figure 1.6 Biomimetic composite film formed by coassebly of CNCs and Upy-functionalized polymer with
high hydrogen bonding ability .................................................................................................. 14
Chapter 3 Composite Cholesteric Nanocellulose Films with Enhanced Mechanical Properties
Figure 3.2 Polymerization reaction used for the synthesis of latex nanoparticles ................. 30
Figure 3.3. Relationship between the total molar monomer (M) to surfactant (SA) ratio and the average
diameter of the resultant nanoparticles after emulsion polymerization ................................ 31
Figure 3.4. (a) Representative distribution in hydrodynamic diameters of latex NPs. (b) Representative
distribution in ζ-potentials of latex NPs ................................................................................... 32
Figure 3.5. Differential scanning calorimetry curves for non-crosslinked (a) and crosslinked (b) latex NPs
with the Tg denoted by the vertical dotted lines ...................................................................... 33
Figure 3.6. Histograms of CNC size distribution. (a) Variation in CNC length, based on the analysis of 300
individual CNCs. (b) Variation in CNC dimeter based on the analysis of 134 individual CNCs . 34
Figure 3.7. Variation in the electrokinetic potential of CNCs, plotted as a function of the molar
concentration of HDA in the suspension. The concentration of CNCs was 1.25 wt% ............. 35
Figure 3.8 Crosslinking of latex nanoparticles with hexane diamine (a) A crosslinking reaction between
the acetoacetate groups of the latex NPs and HDA. (b) ATR-FTIR absorbance spectra of films formed
from an HDA-free dispersion of latex NPs (blue), latex NPs crosslinked with HDA (red), and from a
ix
mixture of latex NPs, CNCs and HDA (green). (c) zoomed in segment of the spectrum of films formed
from the mixture of latex NPs, CNCs and HDA ......................................................................... 36
Figure 3.9. Phase separation of latex-CNC NP suspensions. (a) Photographs of suspensions containing 5
wt% of CNCs and 1.67 wt% of latex NPs at 0.004CHDAaq0.9 wt% (increasing right-to-left), following 1
week equilibration. (b) Variation in the fraction of Ch phase, plotted as a function of CHDAaq for CNP
aq of
1.25 and 1.67 wt%. (c) Photographs of mixed suspensions containing 5 wt% of CNCs, CHDAaq=0.04 wt%
and 0CNPaq2.5 wt% (increasing from left-to-right), following their 1 week equilibration. (d) Variation in
the fraction of Ch latex-CNC phase, plotted as a function of CNPaq .......................................... 38
Figure 3.10. (a) Calibration plot used to relate NP fluorescence to CNPaq for a given composite CNC/NP
suspension. (b) Relationship between CNPaq
initial and CNPaq
Ch after 7-day equilibration of the mixed
suspension of latex NPs and CNCs ............................................................................................ 39
Figure 3.11. (a) Left: latex-free CNC film, middle: latex-free CNC film at CHDAs=0.7 wt%, right: crosslinked-
latex CNC film at CNPs=24 wt% and CHDA
s=0.7 wt% and (b) Extinction spectra of composite CNC at CNPs=10
wt% and varying CHDAs; (c) Extinction spectra of composite CNC films at CHDA
s=0.7 wt% and varying CNPs;
(d) Variation in λmax of the composite CNC films at varying concentrations of HDA and latex NPs
.................................................................................................................................................. 41
Figure 3.12. (a) CD spectra of composite CNC films at CHDAs=0.7 wt% and varying CNP
s; (b) CD spectrum
of composite CNC films at CNPs=10 wt% and varying CHDA
s. (c) Variation in CD intensity, plotted vs. CHDAs
for 8≤CNPs≤ 24 wt% ................................................................................................................... 42
Figure 3.13. Confocal fluorescence microscopy images (top), and polarized optical microscopy images
(bottom) of composite CNC-latex films at CHDAs=0.7 wt% and CNP
s of 10 wt% (a, c) and 24 wt% (b, d)
.................................................................................................................................................. 43
Figure 3.14. Scanning electron microscopy images of the cross-sectional area of composite CNC films at
CNPs of (a) 10 wt% and (b) 24 wt% and CHDA
s = 0.7 wt% ........................................................... 44
Figure 3.15. Representative tensile stress-strain curves for composite CNC/latex NP films with varying
CNPs and CHDA
s=0.7 wt%, with the corresponding tensile strength (TS) and the modulus of toughness (MT)
.................................................................................................................................................. 45
Chapter 4 Composite films formed by cellulose nanocrystals and latex nanoparticles: effect of film morphology on optical and mechanical properties
Figure 4.1. Preparation of ISO, PAR and EQU films .................................................................. 54
Figure 4.2. Photographs of the composite latex NP/CNC films prepared by different methods. Scale bar
is 1 cm ....................................................................................................................................... 56
Figure 4.3. Scanning electron microscopy images of the cross-sectional area of ISO, PAR and EQU films.
Scale bar is 5 μm ....................................................................................................................... 57
x
Figure 4.4. UV-Vis (a) and CD (b) characterization of ISO, PAR and EQU films. A minimum of four
measurements were taken within the central 1 cm2 area of each film and the results were averaged
.................................................................................................................................................. 59
Figure 4.5. Peak width at half-height for (a) extinction and (b) CD spectra for PAR and EQU composite
CNC films ................................................................................................................................... 60
Figure 4.6. Representative tensile stress-strain curves for EQU, PAR and ISO films. Insets: SEM images
of the cross-sectional fracture surface for EQU (top) and ISO (bottom) films. Scale bars are 2 µm
.................................................................................................................................................. 61
Figure 4.7. Mechanical properties of ISO, PAR and EQU films. The Young’s modulus, E, is shown on the
left Y-axis. The modulus of toughness, MT, and the tensile strength, σ, are shown on the right Y-axis.
Statistical analysis was performed using Sigmaplot version 12 (Systat Software Inc., CA) for one-way
ANOVA and Tukey tests ............................................................................................................ 62
1
Chapter 1
Introduction
1
1.1 Cellulose nanocrystal suspensions.
1.1.1 Cellulose in nature
Cellulose nanocrystals (CNCs) have drawn significant attention in recent years, due to their
natural abundance, relatively low cost, high mechanical strength and ease of surface functionalization1–
3. Cellulose naturally occurs in plants4, some forms of algae5 and is secreted by some forms of bacteria6.
Cellulose is the most abundant biopolymer on Earth1 and, as a result, is relatively inexpensive. The use
of cellulose is highly appealing for a wide range of applications such as paper products, textiles, food
additives, and packaging materials.
Cellulose is an organic homopolymer with a molecular weight in the range from 27,000 to
900,000 and a degree of polymerization from 90 to 3000. It consists of β-1-4-linked anhydro-D-glucose
units, where each unit is rotated 180o with respect to the preceding unit7 (Figure 1.1, a). A directional
asymmetry exists on each cellulose chain where one end consists of a hemiacetal unit (known as the
reducing end) and the other has a pendant hydroxyl group (nonreducing end).
Biosynthesis of cellulose occurs within protein complexes embedded in the plasma membrane of cells.
In this process, individual cellulose molecules are spun together to form nanofibrils which agregate to
form cellulose fibers8. These microfibrils provide rigidity to the cellular wall, offering structural support
2
and protection to the living organism. Cellulose molecules exist in different polymorphs, depending on
the molecular orientation and hydrogen bonding within cellulose nanofibrils.1 Six different cellulose
polymorphs (I, II, IIII, IIIII, IVI, IVII) can be obtained from different cellulose sources, and by using
different extraction methods, or post-extraction treatments.2 The aggregation of cellulose molecules
into fibers occurs due to a combination of van der Waals forces between cellulose chains and
intramolecular and intermolecular hydrogen bonds1 between OH groups on each glucose subunit.
Periodic amorphous regions exist in cellulose fibers, which form by defects during biosynthesis, as well
as a result of chain dislocations caused by internal strain resulting from fiber twisting and bending.1
These amorphous domains are less chemically resilient than crystalline regions and can be selectively
broken down by oxidative and hydrolytic processes.9,10 This process is used to isolate needle-like
nanoscopic crystalline cellulose fibrils called cellulose nanocrystals (CNCs). These CNCs can range in
length from 25 to 3000 nm and in width from 3 to 70 nm.11,12 Figure 1.1 b, shows CNCs imaged using an
electron microscope.
3
Figure 1.1 (a) Repeat unit of cellulose biopolymer. (b) Transmission electron microscopy image of CNCs.
Scale bar is 400 nm. (c) Cationic functionalization of cellulose with epoxypropyltrimethylammonium
chloride.
1.1.2 Cellulose nanocrystals
A variety of methods have been used to break down the amorphous domains within the cellulose
nanofibrils. The most commonly employed method involves the prolonged application of heat under
acidic conditions. The acid hydrolyzes the β-linkage between glucose rings. Sulphuric acid yields a
negatively charged sulphate half-ester group and hydrochloric acid yields a hydroxyl group13 on the CNC
surface. The use of phosphoric acid14, yielding phosphate half-ester surface groups, and hydrobromic
acid15, yielding negative hydroxyl groups, has also been used however these methods are not common.
The CNC surface charge originating from negative SO3- groups is tunable by adjusting the reaction
conditions with sulphuric acid.16 Longer hydrolysis times lead to CNCs with higher surface charge17,18,
however, longer times, as well as higher temperatures, lead to cellulose degradation and yield shorter
4
CNCs.11 Bondeson et al.17 reported optimized reaction conditions to achieve a high-yield of
monodisperse CNCs in a short amount of time. This group used microcrystalline cellulose (MCC) as their
starting material and analyzed the effects of sulphuric acid concentration, reaction temperature,
reaction time, and sonication time on the final yield and dimensions of the resulting CNCs. Their
optimized conditions are outlined in Table 1.
Table 1.1. Optimized conditions for production of stable aqueous colloid of CNCs17
MCC conc.
(g/100 mL)
H2SO4 conc. %
(w/w)
Temperature (oC) Time (min) Sonication time
(min)
Yield (% initial
weight)
Dimensions
length/width
10.2 63.5 44.0 130.3 29.6 30 200-400 nm/10 nm
Starting materials used to produce CNCs comprise a broad range of materials, with common
sources including cotton18, hemp19, sisal20, ramine21, flax22, bleached softwood12 and hardwood23 pulps,
sugar beet pulp24, tunicates25 and bacterial cellulose26. The degree of crystallinity of the cellulose within
each material is highly variable (e.g. 96 % for hemp fibers and 67 % for Norway spruce).27 The raw
material is broken down in an initial processing step (cut, ground or turned to pulp) and then treated
with acid. Usually the material is discoloured by bleaching before or after hydrolysis. The acid is then
neutralized and the material is washed by dialysis before being dispersed by ultrasonication. In addition,
Yang et al.28,29 successfully produced CNCs by oxidation of softwood pulp with sodium periodate,
followed by sodium chlorite in mildly acidic conditions.
An important parameter that governs the colloidal stability and viscosity of the CNC suspension
(discussed in section 1.1.3) is the surface charge of the CNCs. As discussed above, hydrolysis with
sulphuric acid yields negative SO3- groups on the CNC surface (with a typical ζ-potential30 down to -50
mV), while hydrolysis with HCl yields neutral OH groups, which can subsequently be converted to SO3-
5
by sulphuric acid treatment16. Another commonly employed surface modification technique is the use
of 2,2,6,6-Tetramethylpiperidine 1-oxyl (TEMPO) to oxidize the hydroxyl methyl groups on the surface
to their carboxylic form31–33. This “TEMPO-mediated oxidation” selectively reacts with the primary
alcohols on the CNC, thus introducing negative surface charges onto the CNCs and providing a site for
chemical functionalization. Alternatively, a positive charge can be introduced to the CNC surface by
functionalization of surface OH groups with ammonium-containing compounds, for example,
epoxypropyltrimethylammonium chloride (EPTMAC)34 (Figure 1.1, c)
1.1.3 Colloidal and liquid crystalline properties of CNC suspensions
One of the truly unique features of CNCs is their ability to form cholesteric (Ch) liquid crystalline
phases in aqueous suspensions8. This structure consists of planar CNC layers, where the CNCs in each
layer are parallel to each other and each layer is rotated with respect to the previous. This structure
maximizes translational entropy of the particles.35 Individual CNCs have a right-handed corkscrew
morphology (confirmed by small angle neutron scattering36,37) which results in a chirality in the
electrostatic layer surrounding each CNC. In a suspension, these charged layers interact with each other,
resulting in the self-assembly of CNCs into a left-handed Ch liquid crystal1. Since repulsive forces are
necessary for colloidal stability, CNCs prepared by HCl hydrolysis do not form a liquid crystal since their
surface contains only neutral hydroxyl groups8.
Generally, an aqueous suspension of CNCs phase-separates into an isotropic top phase and a Ch
bottom phase when the CNC volume fraction reaches a threshold value38. The threshold volume fraction
depends on the charge density and ranges from ~1 to 10 %(w/w)1. The Ch pitch (the distance for a 360o
rotation across the twisting CNC layers) decreases as CNC concentration in the suspension increases due
6
to higher CNC-CNC proximity and a larger steric influence between CNCs. The pitch in aqueous
suspensions can vary1,2 from < 1 up to ~80 µm. The equilibrium between isotropic and Ch phases is
sensitive to the ionic strength of the aqueous medium. An increase in electrolyte concentration results
in a decrease in the volume fraction of Ch phase and a decrease in Ch pitch. Ions shield the electrostatic
repulsion between CNCs by forming an electrostatic double-layer around each CNC, thereby partially
neutralizing their surface charge. This affects the electrostatic double-layer of each CNC, resulting in a
smaller inter-CNC distance39. Additionally ultrasonication of a CNC suspension removes the shielding
layer of the CNC from the CNC surface, resulting in a stronger electrostatic repulsion between CNCs and
an increase in Ch pitch40 (Figure 1.2). It was theorized that cationic CNCs functionalized with EPTMAC
(Figure 1.1, c) would also form a Ch phase however this could not be experimentally verified34.
7
Figure 1.2 Schematic representation of the dispersion of the electrostatic envelope surrounding a CNC upon
sonication.40
1.2 Cellulose nanocrystal films
1.2.1 Film formation and modification
Upon evaporation of the Ch liquid crystalline suspension, planar CNC layers approach each other
and the Ch pitch dramatically reduces until evaporation is complete. A solid film formed by water
evaporation from a cast CNC suspension has a Ch pitch on the same length scale as the visible spectrum
of light. The film is birefringent and iridescent due to the interaction of circularly polarized light with
the periodic Ch structure41. The perceived wavelength of light, λ, of the CNC film depends on the Ch
pitch, as described by the Bragg’s equation: nλ=2dsinθ, where the length of the repeating periodic
8
structure, d, corresponds to the distance for a 180o rotation along the Ch director. This distance
corresponds to one-half of the Ch pitch, that is, P/2. The refractive index, n, varies slightly depending
on the degree of crystallinity of the CNCs and θ is the angle of incident light (where a beam perpendicular
to the film is 90o).
Properties that affect the pitch of the aqueous CNC suspension such as the ionic strength of the
aqueous medium and the CNC aspect ratio have a corollary effect on the resultant solid film. For
example, the reduced pitch in an electrolyte-rich Ch-CNC suspension results in a film with reduced pitch
and blue-shifted reflectance40. Additionally, a more hydrophilic substrate (steel) will facilitate adhesion
of CNCs to the substrate and result in smaller Ch pitch in the CNC film, while the opposite effect is
observed for hydrophobic surfaces (Teflon).42 Lastly, an increase in drying temperature resulted in an
increase in the Ch pitch43 of the solid film, which was explained by an increase in thermal motion and an
increase in average CNC-CNC distance.
1.2.2 Additives to CNC films
One significant drawback of CNC films is their brittle nature and propensity to crack.44 Much
research has been done to mitigate this limitation by the addition of polymers and low-molecular weight
plasticizers to aqueous CNC suspensions prior to water evaporation. Guidetti et al.45 investigated the
use of the zwitterionic surfactant dimethylmyristylammoniopropane-sulfonate (DMAPS) at DMAPS:CNC
mass ratios between 0 and 1, to generate flexible, iridescent CNC films (Figure 1.3). The authors found
that the presence of DMAPS did not interfere with the interaction between CNCs or the Ch order but
the film flexibility and wavelength of reflection increased with increasing DMAPS content.
9
Figure 1.3 Surrounding of CNC with the zwitterionic surfactant resulting in increased Ch pitch. Inset: Electrostatic
interaction between cationic region of DMAPS and negatively charged sulphate group on CNC surface.45
Water-soluble polymers such as poly(ethylene glycol),46 (PEG) poly(vinyl alcohol)47 (PVA) and
sodium polyacrylate46 have been successfully combined with CNC suspensions and cast to yield
iridescent films. The problem is that the use of these polymers was limited to specific concentration
ranges (up to 40 wt% PVA, up to 10 wt% PEG) in which the formation of Ch structures was uninhibited.
Above these concentrations, steric, electrostatic, and/or van der Waals forces disrupt the electrostatic
forces between CNCs and the Ch order is destroyed. The use of hydrophobic polymers as an additive to
CNC suspensions was reported by Cheung et al.48 In this work, CNCs were neutralized with NaOH prior
to dispersion in polar organic solvents. Iridescent films were successfully created using CNCs and
polystyrene, poly(methyl methacrylate), polycarbonate or poly(9-vinylcarbazole), however this method
once again, was limited to the use of specific polymer/solvent combinations and required additional CNC
modification steps.48
Latex nanoparticles (NPs) enable the use of a wide range of polymers and both hydrophilic and
hydrophobic dispersability in water. The use of NPs also adds the advantage of unique colloidal
interactions that result in interesting film morphologies upon film formation.49,50 Similar to a NP-free
10
CNC suspension, a mixture of CNCs and hard spheres reaches thermodynamic equilibrium at which the
translational entropy of each particle-type is maximized.35 For the case of hard rods and hard spheres,
this equilibrium is achieved when the rods and spheres undergo phase separation into a rod-rich phase
and a sphere-rich phase.51 For the case of CNCs and spherical NPs, complete phase separation is
kinetically inhibited by viscous forces, however, local phase separation can occur, resulting in small NP-
rich and CNC-rich domains.30 Since the viscosity of the suspension increases when water is evaporated,
a gel-state is reached and the non-equilibrium state of the mixture can be kinetically trapped. At this
point, evaporation occurs more slowly and the arrangement of particles within the Ch-CNC phase does
not change significantly.2 Factors such as NP size, surface charge, and glass transition temperature affect
the degree of phase separation between CNCs and spherical NPs (see section 3.1). Smaller particles fit
between Ch-CNC layers easier and a negative surface charge allows them to coexist without losing
colloidal stability. Soft (low Tg) spheres are malleable and deform to increase translational entropy
within the Ch CNC liquid crystal.
1.2.3 Applications of films
Cellulose nanocrystals are naturally abundant, and thus have a relatively low cost. This
advantage, in combination with high mechanical strength and ease of surface functionalization, has led
to great interest in CNC applications such as drug delivery,52 ion exchange membranes,53 recyclable
substrates,54 and the reinforcement of thermoplastics.55,56 The liquid crystalline properties of CNC
suspensions has led to many applications that take advantage of their unique Ch properties. Zhang et
al.57 took advantage of the hydrophilicity of cellulose and produced films that swell at high levels of
11
humidity in the ambient environment. This swelling resulted in an increase in Ch pitch and a shift in film
colour from blue to orange, thus making a humidity sensor.
Ping et al.58 investigated the use of CNC films for encryption of banknotes as an anti-
counterfeiting measure owing to the tunability of film extinction and the added feature of selectivity
towards left-handed polarized light. The researchers also reported successful combination of CNC films
with fluorescent “TINOPAL” as an additional encryption strategy.
Bardet et al.59 developed a process for the fabrication of iridescent, naturally derived, pigments
from CNCs by grinding iridescent films into a powder. To overcome the water-solubility of the resultant
powder, the sulphate ester groups were removed by vacuum overdrying. These pigments were treated
with aqueous NaCl up to 0.25 M to increase the ionic strength of the suspension and inhibit uptake of
water into the CNC powder (which would result in a loss of iridescence).
Geng et al.60 were able to improve cellulose-based electro-optical devices by using CNCs (as
opposed to cellulose fibers) as the cellulose component. A Ch-CNC film was introduced into a nematic,
low molecular weight liquid crystal (4-cyano-4’-pentylbiphenyl) (Figure 1.4). Upon application of an
electric field, polarized light entering the cell underwent circular rotation due to the Kerr effect. This
rotation allowed the light to pass through a second polarizer on the other side. The birefringence of the
CNC film assisted the rotation of the light such that less voltage was needed to achieve the light-
transmitting “ON” state. Additionally, the presence of the CNC film reduced the time taken to reach the
“ON” state after voltage was applied.
12
Figure 1.4 Structural design of electro-optical light shutter with CNC film embedded within the low molecular
weight liquid crystal.60
The Ch self-assembly of CNCs in films has been used to induce a Ch, periodic structure in many
materials. It has been used as a template to produce mesoporous silica;61 an important material for
photonic applications. The process involved combination of CNCs with a silica precursor, Si(OEt)4, which
was subsequently hydrolyzed to form a homogeneous dispersion. The synthesis of silica did not inhibit
the formation of a Ch phase, and upon drying, a Ch-CNC/SiO2 film was produced. Shopsowitz et al.62
went one step further with the Ch-CNC/SiO2 material by converting the embedded cellulose material to
carbon by exposure to high heat. Subsequent etching of silica with NaOH yielded mesoporous carbon62
(Figure 1.5, a). This material has applications as a molecular sieve, in electrochemical double-layer
capacitors, lithium ion batteries, catalyst supports and in field-effect transistors.62
Alternatively, Shopsowitz et al.63 removed the cellulose from the mesoporous CNC/silica
structure discussed above by repeated acid hydrolysis. The result was a porous silica structure which
was filled with TiCl4 followed by calcination and removal of silica by NaOH etching (Figure 1.5, b). The
result was mesoporous titania with potential applications as dye-sensatized solar cells, photocatalysts,
gas sensors and batteries.
CNC
13
Figure 1.5 Production of mesoporous carbon62 (a) and mesoporous titania63 (b) starting from CNCs and silica.
In addition, Ch arrangements of protein-based materials are present in biological organisms such
as in the scales of the fish: Arapaima gigas. A Ch arrangement of collagen fiber-sheets containing
hydroxyapatite nanocrystals provides unique resistance to tensile strain induced by piranha attacks.64
Under tensile stress the majority of fibers rotate towards the tensile axis to distribute the load across
the entire length of the fiber, thereby resisting deformation. Similarly, the exoskeleton of the crab is
composed of composite fibers of chitin and proteins arranged in a Ch structure65, resulting in an isotropic
response to stress. This has inspired research into biomimetic materials using CNCs. Wang et al.47
reported the use of Ch-CNC layers embedded in a PVA polymer matrix to achieve a material of improved
strength and toughness. More recently, Zhu et al.66 went one step further and coassembled CNCs with
ureidopyrimidinone (UPy)-containing polymer. This polymer has a high propensity for hydrogen
bonding with four potential donor-acceptor sites (Figure 1.6) resulting in Ch-CNC materials with the
greatest stress-response and pliability to date.
14
Figure 1.6 Biomimetic composite film formed by coassebly of CNCs and Upy-functionalized polymer with high
hydrogen bonding ability.66
1.3 Summary
Cellulose nanocrystals are naturally abundant, have low cost, and are highly versatile materials.
They are primarily produced from selective acid hydrolysis of the amorphous regions of native cellulose
fibers, where the use of H2SO4 yields negatively charged sulphate ester groups on the CNC surface.
These negative charges result in electrostatic repulsion between CNCs in aqueous suspensions and
hence a stable colloid. Due to the inherent chirality of individual CNCs, at a critical concentration in an
aqueous suspension, CNCs form a Ch liquid crystal. The properties of these liquid crystals are highly
sensitive to the presence of electrolytes in the suspension, the temperature of the suspension, the CNC
dimensions and the CNC surface charge. Upon evaporation of water, the Ch structure of the liquid crystal
is maintained in the solid film resulting in birefringence and iridescence. These films are quite brittle
and thus additives such as surfactants, polymers, and NPs have been added as plasticizers. This thesis
describes how the use of polymer (latex) NPs can affect the resultant properties of CNC films.
15
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19
Chapter 2
Materials and Methods
2
2.1 Materials
2.1.1 Cellulose nanocrystals
A 4.37 L aqueous 12.2 wt% suspension of CNCs was purchased from the University of Maine
Process Development Centre. A portion of this suspension was decanted into a 1 L Nalgene bottle,
diluted to 9 wt% and stored at 4 oC. This suspension was used for the preparation of composite films.
2.1.2 Latex nanoparticles
Butyl acrylate (BuA, ≥99%), 9-vinyl anthracene (VA, 97%), 2-(methacryloyloxy)ethyl
acetoacetate (MAEAA, 95%), potassium persulphate (KPS, ≥99%), hexamethlene diamine (HDA, 98%),
sodium dodecyl sulfate (SDS, 92.5-100.5%), dialysis tubing (cellulose, 12-14 kD) and activated
aluminum oxide were purchased from Sigma-Aldrich Canada. BuA was distilled at 60oC under vacuum
before use and MAEAA was filtered using an aluminum oxide column. Water was obtained through a
Millipore Quantum TEX treatment system.
20
2.2 Methods
2.2.1 Transmission electron microscopy of cellulose nanocrystals
A 20 μL pipette was used to deposite three separate droplets of a 0.5 wt% aqueous CNC
suspension on a Ted Pella Inc. 400 mesh Cu grid. The droplets were allowed to dry completely before
imaging a minimum of 300 CNCs on the grid using a Hitachi H-7000 Transmission Electron Microscope.
Size measurements were made post-imaging using ImageJ software.
2.2.2 Effect of hexanediamine on cellulose nanocrystal electrokinetic potential
The effect of HDA on the ζ-potential of CNCs was determined by adding HDA at a concentration
of 12, 2.4, or 1.2 mM to a 1.25 wt% CNC suspension. The pH was first measured with VWR Analytical
pH test strips. To measure the ζ-potential the suspensions were first diluted five-fold with deionized
water and analyzed by dynamic light scattering using a Malvern Zetasizer Nano-ZS.
2.2.3 Synthesis of latex nanoparticles
Latex NPs were synthesized by emulsion polymerization1 by copolymerizing BuA (12.83 mL, 89.5
mol %), MAEAA (1.91 mL, 10 mol %) and VA (0.10 g, 0.05 mol %) under nitrogen atmosphere in 55 mL of
water at 80 oC using SDS (0.11g) as a surfactant and KPS (0.14g) as the initiator. First, SDS was dissolved
with stirring in 55 mL of water while the co-monomers (BuA, MAEAA, VA) were combined with stirring
in a separate vial. Once the VA had completely dissolved, the solutions were combined and stirred at
200 rpm at 80oC under nitrogen for 30 min. In a separate vial, KPS was dissolved with stirring in 5 mL of
water and purged with nitrogen for 30 min. The KPS solution was then added to the monomer emulsion
dropwise over the course of 10 min and the reaction was stirred at 200 rpm at 80oC under nitrogen
21
overnight. The suspension was then cooled and vacuum-filtered through Whatman filter paper.
Following filtration, the dispersion of latex NPs was purified by dialysis against deionized water, with
water change daily for 7 days.
2.2.4 Characterization of latex nanoparticles
The hydrodynamic radius and the electrophoretic mobility of latex NPs were characterized by
dynamic light scattering using a Malvern Zetasizer Nano-ZS. Aggregation of latex NPs in a 4.2 wt%
suspension was examined in the presence of 0.9 wt% HDA by measuring the hydrodynamic radius of the
NPs immediately after synthesis and after 7-day dialysis. The glass transition temperature, Tg, of latex
NPs in non-crosslinked and crosslinked films was determined under a nitrogen atmosphere using a
differential scanning calorimeter (DSC) model Q100 from TA Instruments. Dynamic DSC measurements
were carried out at a ramp rate of 10 oC/min-1 in the temperature range of -75 to 100 °C to obtain heat-
flow curves of the samples.
2.2.5 Film preparation (for experiments described in Chapter 3)
A thin layer of Sylgard 184 Silicone Elastomer mixed with a curing agent was cast on the bottom
of a 60×15 mm polystyrene Petri dish and cured at 70 oC for 4 h. To increase its hydrophilicity and to
improve wetting with the mixed NP/CNC/HDA suspensions, the resultant elastomeric film was exposed
to air plasma (500 mTorr, 45W, 5 min) in a Harrick Expanded Plasma Cleaner. Mixed latex NP/CNC
suspensions were prepared at a CNC concentration of 5 wt% and latex NP concentrations, CNPaq, of
1.25, 1.67, 2.5 and 5 wt%. A crosslinker, HDA, was added to the mixed suspension at a concentration,
CHDAaq, of 0, 0.38, 0.76, 3.8, and 7.6 mM. The mixed suspension had a total mass of 20 g. The mixed
suspension was equilibrated for 7 days, which resulted in its phase separation into an isotropic (top)
22
phase and an anisotropic (bottom) Ch-CNC phase. A portion (3 mL) of the Ch-CNC phase was extracted
with a micropipette and cast onto a plasma-treated elastomeric film in a Petri dish. Composite films
formed at 28 oC and ~90% relative humidity for 7 days and were subsequently removed from the
silicone substrate for further characterization. Crosslinked CNC-free latex films were prepared by
solution-casting a 1.67 wt% latex dispersion mixed with 0.38, 0.76, 3.8, or 7.6 mM of HDA onto an
elastomer substrate and drying them overnight under ambient conditions.
2.2.6 Film preparation (for experiments described in Chapter 4)
2.2.6.1 Equillibrium (EQU) – A 45g aqueous suspension was prepared in a 50 mL falcon
centrifuge tube consisting of 5 wt% CNCs, 1.67 wt% NPs, and 3.8 mM HDA. The crosslinker was added
last and the suspension was vortex-mixed before allowing 7 days to equilibrate. Two 3 mL portions of
the Ch phase were then extracted from the bottom of the tube and cast into two 60x15 mm
polystyrene Petri dishes. Composite films formed at 28 oC and ~70 %RH for 10 days and were stored in
a 70 %RH environment thereafter.
2.2.6.2 Partial (PAR) – A 6 g suspension was prepared in a 10 mL glass vial consisting of 5 wt%
CNCs, 0.7 wt% NPs and 3.8 mM HDA. The crosslinker was added last and the suspension was vortex-
mixed before two 3 mL portions of the mixed suspensions were extracted and cast into two separate
60x15 mm polystyrene Petri dishes. Composite films formed at 28 oC and ~70 %RH for 10 days and
were stored in a 70 %RH environment thereafter.
2.2.6.3 Isotropic (ISO) – A 6 g suspension was prepared in a 10 mL glass vial consisting of 5 wt%
CNCs, 0.7 wt% NPs and 3.8 mM HDA. The crosslinker was added last and the suspension was vortex-
mixed before two 3 mL portions of the mixed suspensions were extracted and cast into two separate
23
60x15 mm polystyrene Petri dishes. Composite films formed at 25 oC and ~16 %RH overnight and were
stored in a 70 %RH environment thereafter.
2.2.7 Fluorescence spectroscopy
The concentration of latex NPs in the Ch phase was determined using fluorescence
spectroscopy (Cary Eclipse Fluorescence Spectrophotometer, Varian) at the excitation wavelength of
360 nm and emission wavelength of 411 nm in a 10x10x30 mm quartz cuvette. Calibration standards
were prepared by 100-fold dilution of the mixed suspensions at a CNC concentration of 5 wt% and
latex NP concentrations, CNPaq, of 0.2, 0.4, 1, 2, and 5 wt%.
2.2.8 Attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR)
Characterization of the crosslinking reaction between the acetoacetate groups of the latex NPs
and HDA was carried out for a crosslinked latex NP film and a crosslinked latex NP/CNC film using ATR-
FTIR experiments on a Bruker Vertex 70 spectrometer with a 1.85 mm diameter diamond crystal. A
non-crosslinked latex NP film was used as a control system.
2.2.9 Characterization of extinction of the composite films
Extinction spectra of the composite NP/CNC films were acquired using Varian Cary 5000 UV-Vis-
NIR Spectrophotometer. The films were placed orthogonally to the incident beam path with a
transmission area of 2 cm2. The measurements were taken at five different locations within the central
1 cm-diameter region of the film at wavelengths in the spectral range from 400 to 900 nm.
24
2.2.10 Circular dichroism spectroscopy
A Jasco-810 Spectropolarimeter was used to characterize the circular dichroism (CD) of the
composite films. A film area of 6 mm2 cut from the central region of the composite film was placed
orthogonally to the beam path and analysed in a spectral range from 400 to 850 nm at low sensitivity
and a scan speed of 100 nm/min with a 4 s response time.
2.2.11 Confocal fluorescence microscopy
A Zeiss LSM 710 NLO Confocal Microscope was used to examine the distribution of fluorescent
latex NPs in the composite films by placing the film perpendicular to the beam path and imaging it at a
depth of ~30 μm within the central 1 cm diameter of the film at an excitation wavelength of 420 nm. A
minimum of 8 images of each film were taken and image analysis was conducted using ImageJ
software where at least 20 measurements were made per film.
2.2.12 Electron microscopy characterization of film structure
A scanning electron microscope equipped with a QUANTA field emission gun 250 and a bright
field/dark field scanning transmission electron microscope detector was used to image the cross-
sectional area of the composite films at a sample temperature of -22 oC. Prior to imaging, the films
were freeze-fractured under liquid nitrogen and subsequently, coated with 10 nm-thick gold film. A
minimum of 6 samples and 60 images were taken for each film.
25
2.2.13 Mechanical analysis of composite films
Static-force tensile strength tests were conducted in using a TA Q800 Dynamic Mechanical
Analysis instrument (TA Instruments). Composite films were prepared with the concentration of latex
NPs in the films, CNPs, of 0, 8, 10, 15, or 24 wt% and the concentration of HDA in the films, CHDA
s, of 0.7
wt%. Films were cut into 30 × 2 mm strips for analysis and clamped lengthwise for testing. Prior to the
measurements, the films were maintained at a temperature of 20 °C and a relative humidity of 70% for
7 days. The films were tested at 25 °C with 3 N/min ramp. The tensile stress was calculated as force
per unit area, where the dimensions of the film were measured with a caliper with a precision of ±0.02
mm. For each composition, at least, three films were tested. The tensile strength was determined as
the ultimate strength of the film before failure and the toughness was determined as the area under
the corresponding stress/strain curve.
2.2.14 Determination of latex glass transition temperature
The values of Tg of non-crosslinked and crosslinked latex NPs were measured by dynamic
differential scanning calorimetry (DSC) using a model Q100 calorimeter from TA instruments. Dynamic
DSC measurements were carried-out at a ramp rate of 10°C/min from -60 to 150 °C. Two 1.67 wt% NP
suspensions were prepared, one with 3.8 mM HDA and the other free of HDA. These suspensions
were cast in a 60x15 mm polystyrene Petri dish and dried in ambient conditions before analysis. At
least three film samples were tested for each formulation.
26
2.3 References
1 R. G. Gilbert, Emulsion Polymerization: A Mechanistic Approach, 1995.
27
Chapter 3
Composite Cholesteric Nanocellulose Films with Enhanced Mechanical Properties
The following work is based on the article published in Chem. Mater., 2017, 29, pp 789–795. It
was done in collaboration with post-doctoral fellow, Pei-Yu Kuo from the research group of Professor
Ning Yan in the Department of Forestry, University of Toronto. Pei-Yu assisted me with mechanical
characterization and differential scanning calorimetry of latex and composite films. The nanoparticle
synthesis and crosslinking methods were devised by Dr. Héloïse Thérien-Aubin. I conducted the
synthesis of latex nanoparticles, produced composite films and executed all the necessary
experimentation and characterization.
Cellulose nanocrystals (CNCs) form cholesteric films that are brittle and prone to cracking. This
chapter describes the preparation, structure and properties of composite cholesteric films formed from
CNCs and soft reactive latex nanoparticles (NPs). Composite films exhibited a self-stratified morphology,
with lateral cholesteric CNC-rich layers and isotropic latex NP-rich layers. The films retained their
photonic properties and exhibited significantly enhanced mechanical properties. In comparison with
latex-free CNC films, composite films had 60 % higher toughness but did not compromise their tensile
strength. The combination of photonic performance and improved mechanical properties of the
composite nanocellulose films expands the range of applications of these materials for the fabrication
of optical devices.
28
3
3.1 Introduction
The limitation of cholesteric (Ch) cellulose nanocrystal (CNC) films is their poor mechanical
properties: the films are brittle and prone to cracking, which limits their applications as advanced optical
materials. To overcome this drawback, water-soluble polymers such as polyethylene glycol1 and
polyvinyl alcohol2 have been added to aqueous CNC suspensions to form composite CNC-polymer films
with enhanced flexibility and preserved Ch structure, however the use of hydrophilic polymers narrowed
the range of applications of the composite films. Functionalization of CNCs with polystyrene or the use
of surfactants enabled the formation of Ch-CNC phases in non-polar organic solvents,3,4 thus making
possible the addition of hydrophobic polymers; yet, the formation of Ch films from such suspensions has
not been reported. In another approach, Ch-CNCs carrying neutralized acidic groups formed Ch films
from a suspension in dimethylformamide, a polar organic solvent.5 Subsequent introduction of
polystyrene into the CNC suspension yielded composite Ch-CNC films, however their mechanical
properties have not been reported.
An alternative approach to composite CNC-polymer materials relies on the use of CNCs and latex
nanoparticles (NPs). Latex NPs can be synthesized with a broad range of compositions, dimensions,
morphologies, and surface chemistries,6-8 which would broaden the range of applications of CNC-
polymer composite materials. Importantly, both hydrophilic and hydrophobic latex NPs can be
introduced in aqueous CNC suspensions. Composite materials based on CNCs and styrene-butadiene
rubber latex,9 polyvinyl acetate latex,10 and isoprene rubber11 have been reported. In these materials,
29
latex films were reinforced with CNCs, which implied that a relatively low fraction of CNCs (not exceeding
20 wt%) in the films. As a result, these composite materials did not exhibit a Ch structure.
Recently, our group has developed composite CNC-latex films at CNC volume fractions up to 60
vol%.12 These films had a Ch structure and exhibited photonic crystalline properties and circular
dichroism. The formation of Ch films was favored when latex NPs had a small size, negative charge and
a low glass transition temperature, Tg. Importantly, at the temperature of film formation the NPs
deformed and spread throughout the Ch phase. This promising approach to composite CNC-latex films
was however limited by the low Tg of latex NPs, which led to film tackiness (or stickiness), and low
structural integrity.12 The mechanical properties of the films had not been explored.
In the present work, we report a comprehensive study of the optical, structural and mechanical
properties of composite films formed from CNCs and reactive low-Tg latex NPs. Hexanediamine (HDA)
was used to crosslink poly(butylacrylate-co-2-(methacryloyloxy)ethyl acetoacetate)) NPs. The
copolymer was covalently labeled with the fluorescent marker 9-vinylanthracene, thus enabling the
characterization of the NP distribution in the films using fluorescence microscopy, along with
independent characterization of the Ch phase by polarized optical microscopy.
Notably, in previous works for the preparation of composite NP-CNC films, the NPs were mixed
with the CNC suspension prior to its phase separation into an isotropic and Ch phase and the mixed
suspension was immediately cast to form a film.12–14 In the suspension, a large fraction of latex NPs
partitioned in the isotropic phase. In the present work, a different approach to film preparation was
undertaken. An isotropic mixed suspension of CNC, latex NPs and a crosslinker was allowed to
equilibrate for 1 week, thus enabling NP and crosslinker partition in the Ch phase under close-to-
equilibrium conditions. Subsequently, composite films were prepared from the Ch-CNC phase containing
30
latex NPs and the crosslinking agent. The resultant films self-stratified into layers of Ch CNC-rich phase
and isotropic latex NP rich phases and were iridescent and birefringent. These films exhibited a marked
increase in toughness without a significant decrease in tensile strength, in comparison with latex-free
CNC films.
3.2 Latex nanoparticles
Latex NPs were synthesized by emulsion polymerization by copolymerizing butyl acrylate (BuA),
2-(methacryloyloxy)ethyl acetoacetate (MAEAA) and 9-vinyl anthracene (VA) in the quantities given in
Table 3.1. The polymerization was initiated by potassium persulfate, BuA was chosen because of the
low Tg of the corresponding polymer (Tg = -54 oC)15, MAEAA was selected as a crosslinker and VA was
selected as a fluorescent marker to aid in NP visualization in the films. Sodium dodecyl sulphate (SDS)
was used as an anionic surfactant to promote colloidal stability with of the NPs. Figure 3.2 shows the
reaction used for NP synthesis. The molar ratio of the comonomer mixture (BuA, VA and MAEAA)-to -
SDS surfactant was varied to tune the diameter of latex NPs in the NP-CNC suspension. Small NP size
was desirable to favor NP inclusion within the Ch-CNC phase and suppress phase-separation between
the CNCs and NPs during equilibration (described above).
Figure 3.2. Polymerization reaction used for the synthesis of latex nanoparticles.
31
Figure 3.3 shows that with increasing the molar ratio of monomer mixture (M) to surfactant
(SA), the size of NPs increased. Using this calibration, latex NPs with a targeted average diameter of
~50 nm were synthesized in 10 syntheses. The optimized formulation to achieve this NP size is given in
Table 3.1.
Figure 3.3. Relationship between the total molar monomer (M) to surfactant (SA) ratio and the
average diameter of the resultant nanoparticles after emulsion polymerization.
Table 3.1. Quantities of each reactant used for batch synthesis of latex NPs by emulsion
polymerization.
Water Sodium dodecylsulphate
Butyl Acrylate
2-(methacryloyloxy)ethyl acetoacetate
9-vinylanthracene
Potassium persulphate
mass (g) 110 0.11 - - 0.10 0.14
vol (mL) 110 - 12.83 1.91 - 5 mol% (aq)
mmol - 0.38 89.5 10 0.5 0.52
32
The latex NPs had the average hydrodynamic diameter and electrokinetic potential (ζ-potential)
of 47 ± 6 nm and -48 ± 8 mV, respectively (Figure 3.4). The effect of HDA addition on latex NP size and
ζ-potential was examined at a NP concentration, CNPaq = 2.5 wt% and HDA concentration, CHDA
aq = 3.8
mM in the latex dispersion. No change in NP size was observed after 3 days, however a 30 mV increase
in electrokinetic potential was observed (thus it became more positive in the presence of HDA). Despite
this change, no latex NP aggregation was observed.
Figure 3.4. (a) Representative distribution in hydrodynamic diameters of latex NPs. (b) Representative
distribution in ζ-potentials of latex NPs.
The values of Tg for non-crosslinked and crosslinked latex NPs in the films were -35oC and -8oC,
respectively (measured by DSC, Figure 3.5). Since in the presence of CNCs, the amount of HDA available
for latex crosslinking reduced (see below), for composite crosslinked latex-CNC films we assumed the
range of -------35 ≤ Tg ≤ -8 oC, that is, below the temperature of film formation.
(a) (b)
33
Figure 3.5. Differential scanning calorimetry curves for non-crosslinked (a) and crosslinked (b) latex
NPs with the Tg denoted by the vertical dotted lines.
3.3 Properties of Cellulose Nanocrystals
The average length and width of the cellulose nanocrystals was determined by transmission
electron microscopy (TEM). The CNCs had an average length of 150 ± 50 nm (n=300) and width of 21 ±
6 nm (n=134). The histogram of measurements is presented in Figure 3.6. Due to the presence of
sulphate half-ester groups on their surface, the CNCs had a ζ-potential of (-47 ± 10 mV).12
(a) (b)
34
Figure 3.6. Histograms of CNC size distribution. (a) Variation in CNC length, based on the analysis of
300 individual CNCs. (b) Variation in CNC dimeter based on the analysis of 134 individual CNCs
3.4 Effect of Crosslinker on Cellulose Nanocrystals and Latex Nanoparticles
In order to investigate the interaction between CNCs and HDA, we examined the change in the
-potential of the CNCs in the presence of HDA in the latex-free suspension. The value of the -potential
of the CNCs decreased with increasing CHDAaq, until the pH of the suspension surpassed the value of ~11,
the pKa of HDA16 (Figure 3.7). Thus neutralization of the negatively charged CNCs by protonated HDA
occurred in the CNC suspension and the amount of HDA available for NP crosslinking reduced.
(a) (b)
35
Figure 3.7. Variation in the electrokinetic potential of CNCs, plotted as a function of the molar
concentration of HDA in the suspension. The concentration of CNCs was 1.25 wt%
The crosslinking of latex NPs with HDA was characterized by Attenuated Total Reflectance
Fourier transform Infrared Spectroscopy (ATR-FTIR) by acquiring the IR spectra of three types of films,
namely, films prepared from an HDA-free latex NP dispersion, from a mixture of HDA and latex NPs, and
from a mixture of HDA, latex NPs and CNCs. In Figure 3.8 b, new peaks emerged at 1605 and 1653 cm-1
in the spectrum of the film formed from latex NPs crosslinked with HDA (red spectrum), in comparison
with the non-crosslinked latex film (blue spectrum). The peak at 1605 cm-1 was attributed to an N-H
bend of HDA, and thus could not be used for the characterization of the crosslinking reaction, however
the peak at 1653 cm-1 corresponded to the C=C stretch of the enamine, confirming that HDA had reacted
with the acetoacetate comonomer of the NPs. Figure 3.8 c, shows a close-up of the spectrum in the
range of 1500–1800 cm-1, which was acquired for the CNC-latex film obtained in the presence of HDA.
The appearance of the peak at 1653 cm-1 in the spectrum of this film (Figure 3.8 c) suggested that the
crosslinking reaction between HDA and the acetoacetate comonomer took place in the CNC-latex
mixture. Note that a peak at 1640 cm-1 corresponded to the O-H bend of water absorbed by CNCs.
36
Figure 3.8 Crosslinking of latex nanoparticles with hexane diamine (a) A crosslinking reaction between
the acetoacetate groups of the latex NPs and HDA. (b) ATR-FTIR absorbance spectra of films formed
from an HDA-free dispersion of latex NPs (blue), latex NPs crosslinked with HDA (red), and from a
mixture of latex NPs, CNCs and HDA (green). (c) zoomed in segment of the spectrum of films formed
from the mixture of latex NPs, CNCs and HDA.
3.5 Suspensions of Cellulose Nanocrystals and Latex Nanoparticles
The formulations used in CNC/NP film preparation are summarized in Table 3.2. Following 7-
day equilibration, phase separation took place in the mixed suspensions of CNCs NPs and HDA into a
NP/CNC isotropic top phase and a NP/Ch-CNC bottom phase. Fluorescence of the Ch-CNC phase at
λexc=365 nm, corresponding to anthracene-labeled NPs, confirmed the partition of the latex into the Ch
37
phase. All suspensions listed in table 3.2 had a pH 7. The bottom Ch-CNC phase was subsequently used
for film preparation.
Table 3.2. Formulations of mixed NP/CNC suspensions used for film preparation. The cumulative
mass of each suspension was 20 g.
Suspension CNC wt% NP wt% HDA mM H2O (g)
A1 5 0 0 19.0
A2 5 0 0.38 19.0
A3 5 0 0.76 19.0
A4 5 0 3.8 19.0
A5 5 0 7.6 19.0
B1 5 1.25 0 18.8
B2 5 1.25 0.38 18.8
B3 5 1.25 0.76 18.8
B4 5 1.25 3.8 18.8
B5 5 1.25 7.6 18.8
C1 5 1.67 0 18.7
C2 5 1.67 0.38 18.7
C3 5 1.67 0.76 18.7
C4 5 1.67 3.8 18.7
C5 5 1.67 7.6 18.7
D1 5 2.5 0 18.5
D2 5 2.5 0.38 18.5
D3 5 2.5 0.76 18.5
D4 5 2.5 3.8 18.5
D5 5 2.5 7.6 18.5
E1 5 5 0 18.0
E2 5 5 0.38 18.0
E3 5 5 0.76 18.0
E4 5 5 3.8 18.0
E5 5 5 7.6 18.0
With an increasing HDA concentration, in the original NP/CNC suspension, CHDAaq, the volume
fraction, of the Ch phase, Ch, reduced until complete disappearance (Figure 3.9 b). This effect was
attributed to the shielding of inter-CNC electrostatic interactions essential for the formation of the Ch
phase17,18 by protonated HDA. With an increasing concentration of latex NPs, CNPaq, in the initial
38
suspension prior to its phase separation from 0 to 2.5 wt% (corresponding to the NP concentration in
the Ch phase from 0 to 1.1 wt %, respectively), an increase in Ch was observed (Figure 3.9 c and d). This
effect was ascribed to the incorporation of the negatively charged NPs within the Ch phase, thereby
increasing the inter-CNC distance, in agreement with earlier report.12
Figure 3.9. Phase separation of latex-CNC NP suspensions. (a) Photographs of suspensions containing 5
wt% of CNCs and 1.67 wt% of latex NPs at 0.004CHDAaq0.9 wt% (increasing right-to-left), following 1
week equilibration. (b) Variation in the fraction of Ch phase, plotted as a function of CHDAaq for CNP
aq of
1.25 and 1.67 wt%. (c) Photographs of mixed suspensions containing 5 wt% of CNCs, CHDAaq=0.04 wt%
and 0CNPaq2.5 wt% (increasing from left-to-right), following their 1 week equilibration. (d) Variation in
the fraction of Ch latex-CNC phase, plotted as a function of CNPaq.
To quantify the amount of NPs in the Ch phase, CNPaq
Ch, relative to the initial concentration of
NPs in the original NP/CNC suspension, CNPaq
initial, a calibration graph was created to relate the
fluorescence intensity of a NP suspension to the concentration of latex NPs, CNPaq. Figure 3.10 a,
39
shows that the fluorescence intensity of a NP suspension increased linearly with CNPaq. Thus, we
related the concentration of latex NPs added to the CNC suspension, CNPaq
initial, to the concentration of
latex NPs partitioning in the Ch phase, CNPaq
Ch after phase separation (Figure 3.10, b). The slope of the
calibration curve, 0.38, denotes that the concentration of NPs in the Ch phase is approximately 38% of
what was originally added to the suspension.
Figure 3.10. (a) Calibration plot used to relate NP fluorescence to CNPaq for a given composite CNC/NP
suspension. (b) Relationship between CNPaq
initial and CNPaq
Ch after 7-day equilibration of the mixed
suspension of latex NPs and CNCs.
3.6 Optical properties of composite CNC-latex films.
Films were prepared by extraction and casting of the Ch bottom phase of equilibrated
suspensions onto a poly(dimethylsiloxane) (PDMS) substrate in a 5 cm diameter polystyrene petri dish.
The cast suspensions were dried over 10 days at 70 %RH. A slow drying process gave the CNCs and NPs
time to equilibrate to the evaporation-induced decrease in water content. Composite films prepared
from the mixed suspension of CNCs, latex NPs and HDA were iridescent and did not exhibit the
(a) (b)
40
“tackiness” observed in previous work.12 The concentration of latex NPs and HDA in the composite latex-
CNC films were determined using a calibration graph (described above) and elemental analysis by
quantification of nitrogen content, respectively. The CNPaq of 1.25, 1.67, 2.5, and 5 wt% in the initial
composite suspension corresponded to a CNPs of 8, 10, 15, and 24 wt%, respectively, in the composite
film, while CHDAaq of 0.38, 0.76, 3.8, and 7.6 mM corresponded to CHDA
s of 0.07, 0.14, 0.7 and 1.4 wt%,
respectively, in the film.
Figure 3.11 a, shows photographs (left-to-right) of a pure CNC film, a latex-free CNC film
containing 0.7 wt% HDA and a composite crosslinked CNC-latex film. With addition of HDA and
crosslinked latex NPs to CNPs=24 wt%, film iridescence in the visible range decreased, which together
with spectroscopic and structural characterization of the film (see below), reflected a disruption of the
Ch structure. The optical properties of the composite films were characterized by UV-visible
spectroscopy. Figure 3.11 b and c, shows that composite CNC-latex films exhibited an extinction peak in
the spectral range from ~650-800 nm, corresponding to the stop band of the transmitted light. The
wavelength of the maximum extinction, λmax, consistently decreased with increasing CHDAs, due to
reduced inter-CNC repulsion during film formation, thereby leading to a smaller Ch pitch (Figure 3.11 b).
Conversely, λmax increased at a higher CNPs (Figure 3.11 c), due to entrapping of the latex NPs between
the CNCs in the Ch phase, thus increasing the pitch of the CNCs in the film.13 At the highest CNPs=24 wt%,
the stop band was not observed. These trends remained consistent for the films prepared at varying CNPs
and CHDAs (Figure 3.11,d).
41
Figure 3.11. (a) Left: latex-free CNC film, middle: latex-free CNC film at CHDAs=0.7 wt%, right: crosslinked-
latex CNC film at CNPs=24 wt% and CHDA
s=0.7 wt% and (b) Extinction spectra of composite CNC at CNPs=10
wt% and varying CHDAs; (c) Extinction spectra of composite CNC films at CHDA
s=0.7 wt% and varying CNPs;
(d) Variation in λmax of the composite CNC films at varying concentrations of HDA and latex NPs.
Composite films exhibited a positive CD peak in the range of ~650-850 nm, characteristic of the
left-handed Ch structure. Figure 3.12 a, shows representative CD spectra of the films with CNPs=15 wt%,
a weak red-shift in λmax was observed, however the CD intensity did not significantly change.At CNPs=24
wt%, however, a notable decrease in CD signal was observed, correlating with the disappearance of a
stop band in the visible region in Figure 3.11 c. Figure 3.12 b, shows that with increasing CHDAs, the CD
peak consistently blue-shifted and decreased in intensity. Figure 3.12 c, shows the general relationship
for the composite films, with a weak variation in CD intensity with increasing CNPs (except for CNP
s=24
wt%) and an overall decrease in CD with increasing CHDAs.
42
Figure 3.12. (a) CD spectra of composite CNC films at CHDAs=0.7 wt% and varying CNP
s; (b) CD spectrum
of composite CNC films at CNPs=10 wt% and varying CHDA
s. (c) Variation in CD intensity, plotted vs. CHDAs
for 8≤CNPs≤ 24 wt%.
3.7. Microstructure of the composite films.
Confocal fluorescence microscopy and polarized optical microscopy imaging were performed on
the composite films with varying latex NP content (at CHDAs=0.7 wt%.). Dual-mode imaging enabled the
characterization of the distribution of the Ch CNC-rich regions and highly fluorescent latex NP-rich
regions. Figure 3.13 a and b, show representative confocal fluorescence microscopy images with an
increasing average size of NP-rich domains at higher CNPs, which implied a stronger phase separation
between the CNC-rich and a latex NP-rich phases.
43
(a) (b)
(d) (c)
Polarized optical microscopy images showed bright birefringent CNC-rich regions at CNPs=10 wt%,
which became longer and larger in films with CNPs=24 wt% (Figure 3.13 c and d, respectively).
Importantly, latex-rich regions in Figure 3.13 c and d, exhibited birefringence, while CNC-rich regions in
Figure 3.13 a and b, exhibited fluorescence, indicating that phase separation between the CNCs and
latex NPs was not complete and each phase contained the counterpart component. As we show below,
this feature was beneficial for the mechanical properties of the composite films.
Figure 3.13. Confocal fluorescence microscopy images (top), and polarized optical microscopy images
(bottom) of composite CNC-latex films at CHDAs=0.7 wt% and CNP
s of 10 wt% (a, c) and 24 wt% (b, d).
Scanning electron microscopy was used to image the cross-sectional area of the composite films
on a smaller length scale. At CNPs=10 wt%, a layered structure of the film cross-section, characteristic of
the Ch order, was observed (Figure 3.14 a). The Ch structure was interrupted with “islands” of NP-rich
44
domains. At CNPs=24%, the NP-rich domains became larger and more abundant, due to the higher
content of latex NPs in the film and stronger phase separation between the CNCs and NPs. The average
half-pitch for the films shown in Figure 3.14 a and b, was 239 ± 27 nm and 290 ± 15 nm, respectively. A
larger half-pitch at higher CNPs was expected due to a larger NP fraction partitioned in the Ch-CNC phase,
thus leading to an increased inter-CNC distance. This effect was in agreement with the red-shift of
extinction and CD peaks in Figures 3.11 and 3.12. We note that the composite latex-CNC films had a
stratified structure with planar Ch-CNC layers and close-packed latex NP layers, as opposed to a uniform
distribution of hydrophilic polymers added as a solution to the precursor CNC suspension prior to film
preparation.2
Figure 3.14. Scanning electron microscopy images of the cross-sectional area of composite CNC films at
CNPs of (a) 10 wt% and (b) 24 wt% and CHDA
s = 0.7 wt%.
45
3.8. Mechanical properties of the composite films.
The effect of latex NPs on tensile strength and toughness of the composite latex-CNC films was
investigated by comparing stress-strain behavior of films with varying CNPs (Figure 3.15). In this study,
the toughness was characterized as the total area under the stress-strain curve in tensile tests. The
addition of NPs to the CNC films at the concentration of up to CNPs= 15 wt% increased the modulus of
toughness (MT) from 74 to 120 kJ·m-3, with a decrease in tensile strength (TS) from 33 to 29 MPa. At
CNPs=24 wt%, the structural integrity of the composite film greatly diminished and the modulus of
toughness, MT, was only ~58 kJ·m-3.
Figure 3.15. Representative tensile stress-strain curves for composite CNC/latex NP films with varying
CNPs and CHDA
s=0.7 wt%, with the corresponding tensile strength (TS) and the modulus of toughness (MT).
Tukey's range test19 was used to determine whether the change in mechanical film properties
was significant , that is, p≤0.05. The test confirmed that the change in tensile strength for the composite
films at 0CNPs15 wt% was not significant (p>0.05), which implied that in this concentration range of
CNPs, its tensile strength of the films was not compromised. More importantly, the addition of latex NPs
46
to the CNC films resulted in a significant (p=0.02) increase in film modulus of toughness. Thus, the latex
NPs acted as a toughening agent. Under stress, the stress was concentrated close to the soft latex
inclusions, resulting in their deformation and expansion, and thus energy dissipation.20 At CNPs=24 wt%,
the film became too soft and both the toughness and tensile strength decreased significantly.
3.9 Discussion
The preparation of the composite latex-CNC films was carried out from the separated Ch phase
of the CNC suspension containing latex NPs and HDA. This mixture did not show evidence of phase
separation after 7 day equilibration, however upon evaporation of water during film formation, the
concentration of the latex NPs and CNCs increased and the mixture underwent further phase separation
into an isotropic and Ch phases.12,14 As a result, the resulting film contained laterally aligned CNC-rich
Ch regions and isotropic latex-rich regions, in agreement with our earlier work.12 The introduction of
latex NPs into the films did not significantly disrupt the extent of order in the CNC-rich Ch regions (up to
CNPs =15 wt%) and led to a red-shift in the stop-band and CD signal, in agreement with Bragg's law, due
to partition of the NPs between the CNC layers.
Addition of HDA to the latex-CNC mixture resulted in inter-particle and intraparticle latex
crosslinking, although the contribution of each effect could not be determined. As the result of
crosslinking, non-tacky composite films amenable to mechanical characterization were obtained. Based
on the results of SEM imaging, in the films, latex NPs were sufficiently soft to coalesce. With HDA
addition the ability of CNCs to form Ch films reduced, most probably, due to the neutralization of the
negatively charged CNCs by protonated HDA.16 Nevertheless, for 8CNPs15 wt% up to CHDA
s=0.7 wt%,
the composite films retained their iridiscence, birefringence, and CD properties.
47
The addition of crosslinked low-Tg latex NPs resulted in a significant (by 60%) increase in
toughness of the composite films, in comparison with latex-free CNC films. At the same time the tensile
strength of the composite films was not compromised. Such a combination of tensile strength and
toughness is a very favorable feature, as generally, improving one property comes at the expense of
another. The contribution of the layered structure21 of the latex-CNC films, as opposed to the uniform
distribution of water-soluble polymers in composite CNC-based films2,22 is yet to explored. Finally, the
loss of the mechanical integrity of the composite films at CNPs=24 wt% coincided with a diminution of its
optical properties. Thus at the optimized 8CNPs15 wt% the mechanical properties of the composite
films were significantly improved without compromising in their optical properties.
3.10 Conclusion
We developed the preparation of crosslinked composite latex-CNC films that possess
birefringence, iridiscence, and CD properties and enhanced mechanical properties. Addition of latex NPs
up to CNPs=15 wt% resulted in a red-shift in extinction and CD spectra of the films and did not significantly
affect the structure of the Ch phase of the composite films. In contrast, the amount of crosslinker
introduced in the films (up to 1.4 wt%) was limited by the disruption of the Ch structure of the films.
Film toughness was increased by 60 % due to the addition of latex NPs, without compromising film
tensile strength, in comparison with latex-free CNC films. The combination of photonic performance and
improved mechanical properties of the composite latex-CNC films expands the range of applications of
these materials, in particular, for the fabrication of optical devices.
48
3.11 References
1 R. Bardet, N. Belgacem and J. Bras, ACS Appl. Mater. Interfaces, 2015, 7, 4010–4018.
2 B. Wang and A. Walther, ACS Nano, 2015, 9, 10637–10646.
3 J. Yi, Q. Xu, X. Zhang and H. Zhang, Polymer (Guildf)., 2008, 49, 4406–4412.
4 L. Heux, G. Chauve and C. Bonini, Langmuir, 2000, 16, 8210–8212.
5 C. C. Y. Cheung, M. Giese, J. A. Kelly, W. Y. Hamad and M. J. Maclachlan, ACS Macro Lett., 2013, 2,
1016–1020.
6 Z. Chen, Y. Zhang, Y. Liu, L. Duan, Z. Wang, C. Liu, Y. Li and P. He, Prog. Org. Coatings, 2015, 86,
79–85.
7 C. St Thomas, R. Guerrero-Santos and F. D’Agosto, Polym. Chem., 2015, 6, 5405–5413.
8 V. Bagalkot, M. A. Badgeley, T. Kampfrath, J. A. Deiuliis, S. Rajagopalan and A. Maiseyeu, J. Control.
Release, 2015, 217, 243–255.
9 P. K. Annamalai, K. L. Dagnon, S. Monemian, E. J. Foster, S. J. Rowan and C. Weder, ACS Appl.
Mater. Interfaces, 2014, 6, 967–976.
10 F. López-Suevos, C. Eyholzer, N. Bordeanu and K. Richter, Cellulose, 2010, 17, 387–398.
11 M. Nagalakshmaiah, N. El Kissi, G. Mortha and A. Dufresne, Carbohydr. Polym., 2015, 136, 945–
954.
12 H. Thérien-Aubin, A. Lukach, N. Pitch and E. Kumacheva, Nanoscale, 2015, 7, 6612–6618.
49
13 H. Thérien-Aubin, A. Lukach, N. Pitch and E. Kumacheva, Angew. Chem. Int. Ed. Engl., 2015, 54,
5618–22.
14 Y. Li, J. J. Suen, E. Prince, E. M. Larin, A. Klinkova, H. Thérien-Aubin, S. Zhu, B. Yang, O. D.
Lavrentovich and E. Kumacheva, Nat. Commun., 2016, 7, 12520.
15 Sigma Aldrich, 1999, 52–53.
16 D. Perrin, Dissociation constants of organic bases in aqueous solution, IUPAC Chem Data Ser: Suppl
1972, Buttersworth, London, 1972.
17 J. P. F. Lagerwall, C. Schütz, M. Salajkova, J. Noh, J. Hyun Park, G. Scalia and L. Bergström, NPG
Asia Mater., 2014, 6, e80.
18 Y. Habibi, L. A. Lucia and O. J. Rojas, Chem. Rev., 2010, 110, 3479–3500.
19 G. E. P. Box, W. G. Hunter and J. S. Hunter, Statistics for Experimenters An Introduction to Design,
Data Analysis, and Model Building, John Wiley and Sons, Inc, 1978.
20 A. A. Collyer, Rubber Toughened Engineering Plastics, 1994.
21 S. E. Naleway, M. M. Porter, J. McKittrick and M. A. Meyers, Adv. Mater., 2015, 27, 5455–5476.
22 Y. P. Zhanga, V. P. Chodavarapua, A. G. Kirka and M. P. Andrews, Sensors Actuators, B Chem.,
2013, 176, 692–697.
50
Chapter 4
Composite films formed by cellulose nanocrystals and latex nanoparticles: effect of film morphology on optical and
mechanical properties
The following is based on work submitted to Nanoscale. It was performed in collaboration
with post-doctoral fellow, Pei-Yu Kuo from the research group of Professor Ning Yan in the
Department of Forestry, University of Toronto. Pei-Yu assisted me with mechanical
characterization and differential scanning calorimetry of latex and composite films. The
nanoparticle synthesis and crosslinking methods were devised by Dr. Héloïse Thérien-Aubin. The
3D schematic (Figure 4.1) was created by Moien Alizadehgiashi of the Kumacheva group. I
conducted the synthesis of latex nanoparticles, produced composite films and executed all the
necessary experimentation and characterization.
To enhance the properties of cholesteric films formed from cellulose nanocrystals (CNCs),
different types of additives are introduced in the composite films, however the relationship
between the film microstructure and properties is not well-understood. We used composite CNC-
polymer films with the same composition but different morphology to explore the effect of the
film structure on their optical and mechanical properties. Composite films were prepared from
CNC suspensions mixed with negatively charged, soft, reactive latex nanoparticles. Film
morphology - from highly disordered to cholesteric films - was controlled by different film
preparation methods. Films with the highest cholesteric order exhibited strong extinction and
51
circular dichroism properties, and had a higher Young's modulus and lower modulus of toughness
than isotropic latex-CNC films. This work underlines the importance of film structure on its optical
and mechanical properties and provides guidance for the preparation of CNC-based
nanocomposite films with targeted properties.
4
4.1 Introduction
The morphology of composite latex NP/Ch-CNC films is largely pre-determined by
the state of the mixed precursor suspension.1 Generally, composite Ch-CNC films are
prepared by mixing an isotropic CNC suspension with a desired additive (e.g., a surfactant,
a polymer, or NPs), casting a liquid film and slowly evaporating water to allow for the
formation of the Ch structure in the solid film.2–6 It has been established that when latex
NPs are used as an additive, phase separation in the mixed precursor suspension lead to
the formation of NP-rich isotropic regions and CNC-rich Ch regions, in a process driven by
the increase in free volume and translational entropy of the CNCs and latex
nanospheres.7,8 In NP/CNC films, phase separation leads to the formation of planar
isotropic latex NP-rich layers embedded in the CNC-rich Ch regions.
In another approach, prior to film casting an isotropic latex NP/CNC suspension is
allowed to equilibrate to achieve close-to-complete phase separation into an isotropic NP-
52
rich phase and a Ch CNC-rich phase.16 The NP/Ch-CNC phase is then separated and used
to prepare a solid film under slow water evaporation conditions.9 Notably, equilibration
of mixed NP-CNC suspension is a time-consuming process that takes 7-10 days.
In contrast with the two methods of Ch film preparation described above, fast
water evaporation with no equilibration of the mixed suspension can be more time-
efficient however it suppresses the formation of the Ch phase and may yield amorphous
films.1
By using different methods of latex NP/CNC film preparation, it is possible to
change film structure without changing its composition, and in this manner, examine the
role of film morphology on its optical and mechanical properties. In addition,
understanding of the advantages and limitations of each method may lead to a more time-
efficient film formation process.
This Chapter describes the results of comparative experimental studies of the
structure, optical properties and mechanical performance of latex NP/CNC films with the
same composition but different morphologies (the latter was controlled by the method of
film preparation). Composite Ch-CNC films comprising latex NPs were prepared from a
mixed suspension of CNCs, latex NPs (with low glass transition temperature, Tg), and
hexanediamine (acting as a crosslinker of the NPs). We show that equilibration of the
NP/CNC suspension prior to film casting leads to the inclusion of the NPs in the Ch film
regions, increase in Ch-pitch and overall, greater film uniformity. These films have
excellent optical properties and exhibit a ~50% increase in Young’s modulus and a 73%
53
decrease in toughness, in comparison with disordered NP-CNC films with the same
composition.
4.2 Latex nanoparticles
Latex NPs had an average hydrodynamic diameter and electrokinetic potential (ζ-
potential) of 47 ± 6 nm and -48 ± 8 mV, respectively. We stress that the utilization of
negatively changed latex NPs was important, since electrostatic interactions between
negatively charged CNCs and positively charged latex NPs resulted in the formation of an
isotropic colloidal gel. The Tg of the crosslinked NPs was -8 oC, thus film formation occurred
at a temperature above the Tg of the latex.
4.3 Film fabrication
Figure 4.1 illustrates schematically the methods of latex NP/Ch-CNC film
preparation. The ISO composite films were prepared by mixing an isotropic CNC
suspension with latex NPs and hexanediamine (HDA), immediately casting the liquid
suspension and drying overnight at RH=16% (Figure 4.1, top row). The rational was that
rapid film formation should suppress CNC organization in the Ch manner. In the PAR
method (Figure 4.1, middle row), a mixed isotropic suspension of CNCs, latex NPs and HDA
was cast as a liquid film and dried at 70 %RH for 10 days. We expected that during this
time, the latex NP-CNC suspension would phase separate into a CNC-rich Ch regions and
isotropic NP-rich layers. In the EQU method, a mixed isotropic suspension of CNCs, latex
54
NPs, and HDA was equilibrated for 7 days to achieve close-to-complete phase separation
into the isotropic NP-rich top phase and the CNC-rich Ch bottom phase. The bottom
NP/Ch-CNC phase was then separated, cast as a liquid film and dried at 70 %RH for 10
days. For such films, we expected the highest degree of the Ch order and the least phase-
separation.
Figure 4.1. Preparation of ISO, PAR and EQU films.
55
Table 4.1 summarizes the compositions and the conditions of film preparation.
Importantly, all films had the same composition. Since in EQU films, only 39% of latex NPs
partition into the Ch-CNC phase9 , the concentration of latex NPs in the original NP/CNC
suspension was increased to 1.67 wt%. The film thickness decreased from the ISO to PAR
to EQU films.
Table 4.1. Composition, annealing conditions, and thickness for ISO, PAR, and EQU films.
Film ISO PAR EQU
CCNC in suspension (wt%) 5 5 5*
CNP in suspension (wt%) 0.7 0.7 0.7
HDA (vol%) 0.05 0.05 0.05
Drying time (days) 1 10 10
RH, % 16 70 70
CNP in film (wt%) 10 10 10
CCNC in film (wt%) 90 90 90
Film thickness (μm) 70±7 54±4 45±4
* In EQU films, the concentration of CNCs could be 0.2% higher, since the Ch-CNC phase has a higher content of CNCs than the isotropic phase.10
Figure 4.2 shows representative photographs of the composite NP/CNC films
prepared by the ISO, PAR and EQU methods. All the films were flexible and not prone to
cracking, in comparison with NP-free CNC films. The PAR and EQU films exhibited
structural green and red colours, respectively, suggesting Ch periodicity of the CNC
arrangement in the composite film.1 The lack of colour for the ISO films indicated that
they were isotropic.
56
Figure 4.2. Photographs of the composite latex NP/CNC films prepared by different
methods. Scale bar is 1 cm.
4.4 Film structure
The morphology of the cross-section of the composite films was examined using
scanning electron microscopy (SEM). Figure 4.3 shows representative structures of the
films prepared by the ISO, PAR and EQU methods. The EQU films had a periodic, layered
structure characteristic of Ch CNCs and a small fraction of disordered regions aligned in
the plane of the film. NP-rich domains were observed in the PAR film at the interface
between the Ch regions and the disordered regions. The PAR film had a smaller fraction
of Ch-layered domains than the EQU film and contained isotropic regions. The average
pitch of the Ch regions in the PAR films was ~170 nm, in comparison with ~300 nm
measured for the EQU films. The ISO film showed no Ch periodicity or NP-rich regions,
that is, the cross-sectional area was completely isotropic.
57
Figure 4.3. Scanning electron microscopy images of the cross-sectional area of ISO, PAR
and EQU films. Scale bar is 5 μm.
58
4.5 Optical properties of the composite films
The optical properties of the composite NP/CNC films were analysed by ultraviolet-
visible (UV-vis) and circular dichroism (CD) spectroscopy. Figure 4.4 shows the UV-Vis and
CD spectra of the composite films. An extinction peak (a stop band) was observed for the
PAR and EQU films (Figure 4.4a). The periodicity of the Ch structure resulted in Bragg’s
diffraction11 according to the equation, nλ=2dsinθ, where the reflected wavelength of
light, λ, is proportional to the spacing d (equal to the half-pitch, P/2). Since the EQU and
PAR films had the same composition and thus the same average refractive index n, the
red shift for the EQU films resulted from a larger pitch (as shown in Figure 4.3), which
explained the difference in structural colours of the EQU and PAR films in Fig. 2. A greater
P value in the EQU films suggested a greater NP inclusion within the Ch-CNC regions,
thereby increasing the inter-CNC distance (as in Figure 4.3). Furthermore, the bandwidth
at half-height of the extinction peak for the EQU films was 8 nm narrower than that for
the PAR films (Figure 4.5), suggesting higher control of the Ch pitch throughout the EQU
films. The ISO films did not exhibit any extinction peaks, since these films lacked Ch
structure, consistent with the absence of structural colour (Figure 4.2).
59
Figure 4.4. UV-Vis (a) and CD (b) characterization of ISO, PAR and EQU films. A minimum
of four measurements were taken within the central 1 cm2 area of each film and the
results were averaged.
Similarly, in Figure 4.4b, the CD spectra of EQU and PAR films exhibited a CD peak,
consistent with the left-handed Ch structure of the films.12,13 The intensity of the CD peak
for the EQU films was higher and its width the half-height was smaller by 5 nm than for
the PAR film, corresponding to a more uniform Ch pitch throughout the film. A red-shift
in the CD peak observed for the EQU films suggested increase in the Ch pitch, resulting
from an increased quantity of NPs within the CNC layers. We note that a second small,
broad peak appeared in the EQU and PAR spectra towards the red end of the CD spectrum.
60
This peak could reflect the existence of larger-pitch regions in the composite film,
however this effect is under investigation.
Figure 4.5. Peak width at half-height for (a) extinction and (b) CD spectra for PAR and
EQU composite CNC films.
4.6 Mechanical properties
Next, we examined the mechanical properties of the ISO, PAR and EQU films by
tensile tests (Figure 4.6). The stress-strain curves of the EQU and PAR films revealed a
more brittle failure characteristic, with a lower strain value and a less pronounced plastic
region, than the ISO films. The fracture surface or the path of crack propagation in the
cross-section of the EQU films featured distinctive, step-wise cleavage of the Ch-CNC
(a) (b)
61
layers in the Ch phase (Figure 4.6, top inset). In contrast, the fracture surface in the cross-
section of the ISO films (Figure 4.6, bottom inset) displayed a random distribution of deep
valleys and protrusions, that is, the crack path was less defined due to higher plastic
deformation in these films.
Figure 4.6. Representative tensile stress-strain curves for EQU, PAR and ISO films.
Insets: SEM images of the cross-sectional fracture surface for EQU (top) and ISO
(bottom) films. Scale bars are 2 µm.
Figure 4.7 shows the summary of the mechanical properties of EQU, PAR and ISO
films. The Young’s modulus, E, calculated as the slope of the linear section of the tensile
stress-strain curve in Figure 4.6, was significantly higher for the EQU and PAR films than
for the ISO films (p<0.001, EQU=PAR>ISO). More specifically, 57 and 39% higher E value
62
was measured for the EQU and PAR films, respectively, in comparison with the ISO films.
The variation in the modulus of toughness, MT, determined as the total area under the
tensile stress-strain curve before film fracture,14 showed a trend opposite to the variation
in E for the EQU, PAR and ISO films; that is, an overall ~78% decrease from ISO to EQU
films (p<0.001, EQU=PAR<ISO).
The tensile strength, σ, calculated as the applied stress when the film failed,
reduced by 15 % for the EQU films, in comparison with ISO films, however, high standard
deviation resulted in no statistically significant difference (p=0.581) among the three
films. Notably, PAR films had intermediate values of E, MT and σ, in comparison with EQU
and ISO films.
Figure 4.7. Mechanical properties of ISO, PAR and EQU films. The Young’s modulus, E, is
shown on the left Y-axis. The modulus of toughness, MT, and the tensile strength, σ, are
shown on the right Y-axis. Statistical analysis was performed using Sigmaplot version 12
(Systat Software Inc., CA) for one-way ANOVA and Tukey tests.
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4.7 Discussion
The method of preparation of the ISO, PAR and EQU films has a prominent effect
on their morphology. As shown in Figure 4.3, the ISO films had a disordered structure,
with soft latex NPs uniformly distributed throughout the film. The lack of order in these
films resulted in their featureless extinction and CD spectra in the spectral range from 400
to 600 nm. The EQU films had a highly ordered, periodic Ch structure. Most of the latex
NPs resided within the Ch-CNC layers, thereby increasing the pitch of the Ch structure.
The PAR films exhibited a morphology that combined the features of the ISO and EQU
films: the Ch regions alternated with disordered planar layers. Latex NPs resided mostly
in the disordered CNC-regions and isotropic, latex NP-rich layers. We note that the
partition of additives in defects of topological liquid crystalline materials is a well-
established effect5.
The close-to-perfect Ch structure and increased pitch of the EQU films resulted in
a reduced width at half-height and red-shift of extinction and CD peaks in the
corresponding spectra, in comparison with the PAR films, thus the spectra of the EQU and
PAR films correlated with film morphology.
The thickness of the composite films decreased with an increasing fraction of Ch
regions from ISO to PAR to EQU films, owing to the higher density of the crystalline
domains.20 Even though the Ch pitch was smaller in the PAR film than in the EQU films,
the contribution of the isotropic regions of the films was prevalent and governed the
variation in film thickness.
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Since all the films had an identical composition, the significant differences in their
mechanical properties of the films was a consequence of differences in film structure. The
morphology and mechanical properties of the PAR films were intermediate between the
structures and properties of the ISO and EQU films, and thus in the discussion below we
focus on the properties of ISO and EQU films.
A significantly higher value of the Young’s modulus, E, of the EQU films than of the
ISO films was attributed to the Ch-CNC structure of the EQU films. In the EQU films, the
CNCs were packed into ordered Ch-CNC regions, leading to a reduction in film thickness
and thus film densification, compared to the ISO films (Table 4.1). An increased density
resulted in a stiffer material and increased E. In addition, in the EQU films, the latex NPs
were mostly confined between the Ch-CNC layers. Confinement between solid surfaces
could restrict polymer chain mobility and lower the extent of polymer collective
movement,13,26 thus increasing the apparent stiffness of the confined latex NPs. The ISO
films had lower density and limited NP confinement between CNCs, which resulted in a
lower E.
The lower value of MT for the EQU films, relative to the ISO films, stemmed from
the weaker inelastic deformation of the latex NPs confined between the Ch-CNC layers in
the EQU films, thereby making these films less ductile. Confinement of the latex NPs
within the Ch-CNC layers in the EQU films resulted in a lower effective MT of the polymer,
and thus reduced toughness of the composite EQU film. In the ISO films, the latex NPs
were randomly distributed between the individual CNC crystals, thus undergoing
significantly larger deformation than in the EQU films, that is, larger strain at fracture.
65
Generally, the tensile strength, σ, is highly sensitive to existing weak spots that are
prone to failure initiation. Therefore, any structural flaw, whether it is due to CNC or latex
or the CNC/latex interface, would have a strong influence on the σ value. It could be
expected that in the ISO films, the larger plastic deformation of the latex polymer could
have contributed to lower the overall tendency for crack initiation in the film thereby
leading to a higher average σ. Nevertheless, unlike E and MT, the difference in tensile
strength, σ, was not statistically significant among the three types of films.
4.8 Conclusion
We report our findings on the relationship between the structure of the composite
CNC-latex films and their optical and mechanical properties. For CNC-latex films with the
same composition, the variation in morphology was achieved by using different methods
of film preparation. Film formation from the NP-loaded Ch-CNC suspension yielded EQU
films with a close-to-perfect Ch structure and latex NPs embedded between the Ch-CNC
layers. Fast drying of an unequilibrated CNC-NP suspension yielded disordered ISO films,
while slow drying of the same suspension resulted in stratified PAR films comprising Ch-
CNC regions and NP-rich disordered planar layers. Due to the latex NPs confined between
the Ch-CNC layers, EQU films had a larger Ch pitch and red-shifted extinction and CD
peaks, in comparison with PAR films.
The morphology of the films had a strong effect on their mechanical properties.
High stiffness and reduced toughness of EQU films was ascribed to the restricted
coordinated movement of the latex polymer, due to its confinement between the Ch-CNC
66
layers. Isotropic latex-CNC films, by contrast, had lower stiffness and increased toughness.
PAR films had the morphology and the mechanical properties that were intermediate
between the ISO and EQU films.
This study demonstrates the impact of film microstructure on their optical and
mechanical properties and provides guidelines for the preparation of nanocomposite
films with tailored properties.
4.9 References
1 J. P. F. Lagerwall, C. Schütz, M. Salajkova, J. Noh, J. Hyun Park, G. Scalia and L. Bergström,
NPG Asia Mater., 2014, 6, e80.
2 R. Bardet, N. Belgacem and J. Bras, ACS Appl. Mater. Interfaces, 2015, 7, 4010–4018.
3 I. M.-Z. Qingkai Meng, Compos. Sci. Technol., 2015, 120, 1–8.
4 H. Thérien-Aubin, A. Lukach, N. Pitch and E. Kumacheva, Angew. Chem. Int. Ed. Engl.,
2015, 54, 5618–22.
5 R. Xiong, Y. Han, Y. Wang, W. Zhang, X. Zhang and C. Lu, Carbohydr. Polym., 2014, 113,
264–71.
6 N. L. Garcia de Rodriguez, W. Thielemans and A. Dufresne, Cellulose, 2006, 13, 261–270.
7 G. a Vliegenthart and H. N. W. Lekkerkerker, J. Chem. Phys., 1999, 111, 4153.
8 M. Adams, Z. Dogic, S. L. Keller and S. Fraden, Nature, 1998, 393, 349–352.
9 B. Vollick, P.-Y. Kuo, H. Thérien-Aubin, N. Yan and E. Kumacheva, Chem. Mater., 2017,
789–795.
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10 Y. Li, J. J. Suen, E. Prince, E. M. Larin, A. Klinkova, H. Thérien-Aubin, S. Zhu, B. Yang, O. D.
Lavrentovich and E. Kumacheva, Nat. Commun., 2016, 7, 12520.
11 W. H. Bragg, Phys. Rev., 1913, 18, 428–438.
12 C. Schutz, M. Agthe, A. B. Fall, K. Gordeyeva, V. Guccini, M. Salajkova, T. S. Plivelic, J. P. F.
Lagerwall, G. Salazar-Alvarez and L. Bergstrom, Langmuir, 2015, 31, 6507–6513.
13 X. M. Dong, T. Kimura and D. G. Gray, Langmuir, 1996, 12, 2076–2082.
14 S. Lampman, Characterization and Failure Analysis of Plastics, 2003.
15 B. Wang and A. Walther, ACS Nano, 2015, 9, 10637–10646.
16 A. Dufresne, Curr. Opin. Colloid Interface Sci., 2017, 29, 1–8.
68
Chapter 5
Conclusions and Outlook
5
5.1 Conclusions
In this thesis, I described the preparation of crosslinked composite latex-CNC films that
possessed birefringence, iridescence, and CD properties, as well as enhanced mechanical
properties. In addition, I reported my findings on the relationship between the structure of the
composite CNC-latex films and their optical and mechanical properties. By varying the ratio
between the concentrations of NPs, crosslinker and CNCs in the films, I achieved variation in their
optical and mechanical properties. Furthermore, for CNC-latex films with the same composition,
I created films with a variation in morphology by using different preparation methods.
To investigate the effect of crosslinked nanoparticles on the optical, structural and
mechanical properties, 50 nm-diameter, soft, negatively charged latex NPs were combined in
varying mass ratios with an aqueous suspension of CNCs, followed by addition of varying amounts
of a NP-crosslinker, hexanediamine. Addition of latex NPs up to CNPs = 15 wt% resulted in a red-
shift in extinction and CD peaks of the films and did not destroy the Ch structure of the films. In
contrast, the amount of crosslinker introduced in the films (up to 1.4 wt%) was limited by the
disruption of the Ch structure of the films. Film toughness was increased by ~60 % due to the
69
addition of latex NPs at a concentration of 15 wt% in the films, in comparison with latex-free CNC
films, without compromising film tensile strength.
Subsequently, the effect of film morphology on the optical and mechanical properties of
the film was investigated by maintaining a constant composition of the NP-CNC films, and varying
the film preparation method. We generated films with three different morphologies. Film
formation from the NP-loaded Ch-CNC suspension yielded EQU films with a close-to-perfect Ch
structure and most of the latex NPs embedded between the Ch-CNC layers. Fast drying of an
unequilibrated CNC-NP suspension yielded disordered ISO films, while slow drying of the same
suspension resulted in stratified PAR films comprising Ch-CNC regions and NP-rich disordered
planar layers. Due to the latex NPs confined between the Ch-CNC layers, EQU films had a larger
Ch pitch and red-shifted extinction and CD peaks, in comparison with PAR films.
The morphology of the films had a strong effect on their mechanical properties. High
stiffness and reduced toughness of EQU films was ascribed to the restricted coordinated
movement of the latex polymer, due to its confinement between the Ch-CNC layers. Isotropic
latex-CNC films, by contrast, had lower stiffness and increased toughness. PAR films had the
morphology and the mechanical properties that were intermediate between the ISO and EQU
films.
70
5.2 Outlook
The combination of photonic performance and improved mechanical properties of the
composite latex-CNC films expands the range of applications of these materials for the
fabrication of optical devices and materials. This thesis demonstrates the impact of film
composition and film microstructure on the resultant optical and mechanical properties and
provides guidelines for the preparation of nanocomposite films with tailored properties.
Knowledge of how latex NPs and HDA affect film properties enables fabrication of films with a
targeted wavelength of polarized light reflection by fine-tuning film composition. In addition, the
intensity of reflectance vs. the mechanical resilience can be optimized depending on the material
application.
A significant result of this research is the ability to alter the mechanical and optical
properties of a composite CNC film without changing film composition. Very simple alterations
in film preparation (drying humidity, equilibration time) result in significant changes in the final
film. Hypothetically, only a single production apparatus is required to create CNC film with a
variety of characteristics, by only altering equilibration time, and ambient drying humidity.
Furthermore, there has been a growing interest in fabrication of biomimetic materials via
layer-by-layer deposition of various materials. Attempts to mimic the “brick and mortar”
structure of nacre include alternating layers of TiN and Pt1 or montmorillonite clay tablets with
poly(diallyldimethylammonium chloride) polyelectrolytes2 or sodium carboxymethyl cellulose.3
In addition, Khan et al.4 used layer-by-layer deposition of CNC/polymer films to fabricate a
biomimetic photonic actuator. By relating the microstructure and composition of composite CNC
71
films, this thesis provides valuable insight into how the layer-by-layer composition of composite
CNC films impacts the resultant optical and mechanical properties. Variation in the Ch pitch, the
existence of NP-rich islands, and the effect of isotropic vs. Ch regions, significantly affect film
characteristics. This knowledge can be applied in layer-by-layer fabrication of biomimetic
photonic materials.
5.3 References
1 H. D. Espinosa, J. E. Rim, F. Barthelat and M. J. Buehler, Prog. Mater. Sci., 2009, 54, 1059–
1100.
2 Z. Tang, N. A. Kotov, S. Magonov and B. Ozturk, Nat. Mater., 2003, 2, 413–418.
3 P. Das, S. Schipmann, J. M. Malho, B. Zhu, U. Klemradt and A. Walther, ACS Appl. Mater.
Interfaces, 2013, 5, 3738–3747.
4 M. K. Khan, W. Y. Hamad and M. J. Maclachlan, Adv. Mater., 2014, 26, 2323–2328.