saunders microgels

Advances in Colloid and Interface ScienceŽ .80 1999 1]25

Microgel particles as model colloids: theory,properties and applications

Brian R. Saundersa,U , Brian Vincentb

aDepartment of Chemistry, Uni ersity of Adelaide, North Terrace, Adelaide, 5005, AustraliabSchool of Chemistry, Uni ersity of Bristol, Cantock’s Close, Bristol, BS8 1TS, UK

Abstract

This review presents an overview of the literature concerning microgel particles withemphasis on work performed during the past 5 years. Microgel particles are cross-linkedlatex particles that are swollen in a good solvent. The particles are conveniently prepared by

Ž .surfactant-free emulsion polymerisation SFEP and may be viewed as sterically stabilisedparticles without a core. The narrow particle size distribution combined with the inherentsteric stabilisation of particles prepared by SFEP makes them ideal model systems for the

Ž . Ž .study of solution-dependent phenomena e.g. osmotic de-swelling . The poly NIPAMŽ .NIPAM s N-isopropylacrylamide microgel system is considered in detail in terms of

Ž .swelling, rheological, small-angle neutron scattering SANS and kinetic data. The reviewconcludes with a discussion of the internal structure for microgel particles and considerationof areas for further research. Q 1999 Elsevier Science B.V. All rights reserved.

Keywords: Microgel particles; Synthesis; Properties; Internal structure

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22. Applications involving microgel particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53. Microgel particle synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64. Particle swelling theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

U Corresponding author.

0001-8686r99r$ - see front matter Q 1999 Elsever Science B.V. All rights reserved.Ž .P I I: S 0 0 0 1 - 8 6 8 6 9 8 0 0 0 7 1 - 2

( )B.R. Saunders, B. Vincent r Ad¨. Colloid Interface Sci. 80 1999 1]252

5. Properties of microgel particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146. Osmotic de-swelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167. Stability of dispersion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188. Swelling control by addition of alkanols: co-non-solvency . . . . . . . . . . . . . . . . . . 199. Small-angle neutron scattering investigations of microgel particles . . . . . . . . . . . . 20

10. Internal structure of microgel particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2111. Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

1. Introduction

A microgel particle is a cross-linked latex particle which is swollen by a goodw xsolvent. The term ‘microgel’ was first introduced by Baker 1 . However, Staudinger

w xand Husemann were the first to prepare microgel particles 2 . They polymerisedDVB at high dilution in a good solvent to achieve swollen cross-linked polymer

w xparticles. Microgel particles were the subject of a review in 1995 3 . A largeamount of work has been performed on new microgel systems in recent years andsome of the older systems have been re-investigated using advanced techniques ornovel methods. This work aims to bring the reader up to date with the currentposition of microgel research.

The structures of the monomers commonly used to prepare microgels appear inŽ . Ž .Fig. 1 the abbreviations employed in this paper are listed in Table 1 . Poly NIPAM

is the most well studied water-swellable microgel system. The best example of anorganic swellable microgel is PS; these particles are swollen by aromatic solventsŽ .e.g. toluene . The ionic microgel particles prepared to date frequently containcarboxylate groups derived from acrylic acid or methacrylic acid. There is consider-able scope for variation of microgel properties by changes in the structure of the

w xmonomers. For example, N-vinyl-n-butyramide 4 is isomorphous with NIPAMand is a good candidate for microgel preparation. Variations of this type will allow

Ž .fine control of the lower critical solution temperature LCST .Ž .Poly NIPAM undergoes thermally induced de-swelling when the solution tem-

Ž .perature is increased above the LCST 328C in water . Heskins and Guillet wereŽ . w xthe first to extensively study the LCST for linear poly NIPAM chains in water 5 .

Water behaves as a good solvent through hydrogen bonding with the amide groupsat room temperature. This hydrogen bonding with water is increasingly disruptedon heating causing water to act as a poorer solvent leading to gradual chaincollapse. Inter- and intra-polymer hydrogen bonding and polymer]polymer hy-drophobic interactions become dominant above the LCST.

Fig. 2 depicts a microgel particle dispersed in good and poor solvents. Theparticle is fully swollen or collapsed in good and poor solvents, respectively.

ŽMicrogel particles swell considerably in a good solvent depending on the.crosslinking level and the turbidity of dispersions is often observed to decrease

( )B.R. Saunders, B. Vincent r Ad¨. Colloid Interface Sci. 80 1999 1]25 3

Ž .Fig. 1. Structures of the monomers frequently used to prepare microgel particles. The monomers are aŽ . Ž . Ž . Ž . Ž .methylmethacrylate, b methacrylic acid, c styrene, d divinylbenzene, e acrylic acid, f N-isopro-

Ž . Ž .pylacrylamide, g N,N9-methylene bisacrylamide and h ethyleneglycol dimethacrylate.

dramatically on swelling. The exact structure of the interior of microgel particles isnot yet settled.

Ž .The volume fraction of dispersion occupied by microgel particles f is andimportant parameter for industrial applications and rheological studies. The value

Ž .of f is proportional to the volume fraction of polymer in the dispersion f andd pis independent of particle size. The relationship between f , f and f is simply:d p 2

fp Ž .f s 1d f2

where:

mp

rp Ž .f s 2p m mp sqr rp s

m , m are the mass of polymer and solvent, respectively; r and r are thep s p sdensities of polymer and solvent. f is the volume fraction of polymer in each2particle.

The most important property for microgel particles is the extent of swelling. The


Table 1Abbreviations used

Abbreviation Meaning

AA Acrylic acidAC AcrylamideAL AlcoholALHCl Allylamine hydrochlorideAMPS 2-Acrylamido-2-methylpropanesulfonateAN AcrylonitrileBA N,N9-methylenebisacrylamideBAlc Benzyl alcoholBC ButylcarbazoleBCA Butylcarbazole acetateBz BenzeneDMF DimethylformamideDVB DivinylbenzeneEA EthylacrylateEB EthylbenzeneHA Hydroxamic acidLCST Lower critical solution temperatureMAA Methacrylic acid

Ž Ž . .MADQUAT 2- Methacryloyloxy ethyl trimethylammoniumd-MEOD Deuterated methanolMeOH MethanolMMA MethylmethacrylateNIPAM N-isopropylacrylamide

Ž .PAA Poly acrylic acidPCS Photon correlation spectroscopy

Ž .PEG Poly ethyleneglycolŽ .d-PEO Deuterated poly ethylene oxide

PrOH 2-PropanolPS PolystyreneQAC Quarternary ammonium chlorideSDS Sodium dodecyl sulfateSFEP Surfactant-free emulsion polymerizationSty StyreneTBA Tert-butylmethacrylateTol TolueneTHF Tetrahydrofuran2VP 2-VinylpyridineW Water

extent of swelling is usually determined from changes in the hydrodynamic diame-ters measured using PCS. It is experimentally convenient to measure swelling

Ž .changes relative to the fully swollen hydrodynamic diameter d . The extent ofoŽ .particle de-swelling is expressed as the de-swelling ratio a which is simply:

Ž .3a s drd , where d is the measured hydrodynamic diameter at a given tempera-oŽ .3ture etc. The de-swelling ratio and f are related by: a s d rd rf where d ,2 c o 2 cŽis the diameter of the particles in the fully collapsed state i.e. particles contain no


Ž . Ž .Fig. 2. Diagram depicting a microgel particle in a poor a, x ) 0.5 and good b, x s 0 solvent,12 12respectively.

.solvent . This definition of a stems from the fact that it is difficult to obtain areliable value of d , for microgel particles, i.e. it is experimentally easier tocdetermine the de-swelling ratio, than the swelling ratio.

2. Applications involving microgel particles

The main applications involving microgel particles have been in the surfacecoatings industry. Microgel particle dispersions are shear thinning and provide

w xrheological control for automotive surface coatings 6 . The particles also havegood film forming properties and favour the alignment of added metallic flakesparallel to the substrate surface.

The original motive for employing microgel particles in surface coatings arosefrom US EPA regulations that required a decrease in the volatile component ofsurface coating formulations. This was achieved by increasing the total solids

Ž . w xcontent by decreasing the molecular weight of the linear polymer resin 7 ;however, this led to an unacceptably low viscosity. Microgel particles were added inorder to increase the dispersion viscosity. The microgel particles had the addedeffect of imparting a yield stress to the dispersion. Surface coating formulationsoften contain residual linear polymer which may affect microgel particle swelling.Our work has shown that dispersion of microgel particles in the presence of added

Žpolymer results in partial de-swelling of the particles by osmotic de-swelling see.below .

Microgel systems also show promise in the printing and pharmaceutical indus-tries. Microgel particles may be functionalised to yield photo-cross-linkable parti-cles. The high surface area and good surface-coating characteristics have allowedfunctionalised microgel particles to be used as printing offset plates with impressive

w xresults 8 .


Alternatively, microgel particles should have application as drug delivery systemsŽonce they have been designed to swell in the vicinity of the target sites lymph

.nodes etc. within the body. This micro-encapsulation technique relies on develop-ing sensitive trigger mechanisms whereby binding of the microgel surface groups to

Ž .specific sites on target cells e.g. cancer cells triggers particle swelling and releaseof the drug molecule from the particle interior.

3. Microgel particle synthesis

Four methods have been reported for the preparation of microgel particles:w x w xemulsion polymerisation 9,10 , anionic copolymerisation 11 , cross-linking of

w x w xneighbouring polymer chains 12 and inverse micro-emulsion polymerisation 13 .Emulsion polymerisation is a versatile technique which yields narrow particle sizedistributions. Emulsion polymerisation can be performed in the presence of added

Ž .surfactant conventional emulsion polymerisation or in the absence of addedŽ .surfactant surfactant-free emulsion polymerisation, SFEP . In the latter method,

Ž .the continuous phase must have a high dielectric constant e.g. water and ionicŽ .initiators are employed e.g. K S O . The charged polymer chains formed during2 2 8

polymerisation act as surfactant molecules and stabilise the growing particles.Conventional emulsion polymerisation enables preparation of very small mi-

Ž .crogel particles i.e. particle diameters less than ; 150 nm ; however, a problemwith this technique is the difficulty of completely removing residual surfactant.SFEP does not suffer from residual surfactant contamination. The seminal work on

w xSFEP was performed by Goodwin et al. 14 who used the technique to preparenon-swollen polystyrene latex particles. SFEP has been widely used for the prepa-

Ž . Ž . w xration of poly NIPAM , PS and poly MMA microgel systems 9,15,16 .Ž .SFEP is ideally suited to the preparation of poly NIPAM . Typically, NIPAM is

polymerised in the presence of persulfate and cross-linking monomer at a tempera-ture of ; 708C. The particles gradually swell when the temperature is decreasedwith the maximum degree of swelling occurring around 328C; this avoids the needto transfer the particles from a good to poor solvent after preparation.

Fig. 3 shows the salient features of SFEP. Thermal decomposition of the ionicŽ 2y.initiator S O initiates free-radical polymerisation. The oligomers produced are2 8

surface active and form nuclei when the length of the oligomers exceeds thesolubility limit of the solvent. The nuclei then undergo limited aggregation, thereby

w xincreasing the surface charge until electrostatic stabilisation is achieved 14 .Further particle growth occurs through absorption of monomer andror oligomericchains. This process results in a decrease in the concentration of oligomers tobelow the critical value required for particle formation. Polymerisation continueswithin the particles until another radical species enters the growing particle andtermination occurs. The key feature of SFEP is that the particle nucleation period

Ž .is very short of the order of minutes which ensures a narrow particle sizedistribution. The final particle size achieved by SFEP increases with electrolyte

w xconcentration and decreasing initiator concentration 14 .


Fig. 3. Mechanism for the preparation of microgel particles by SFEP. The steps shown are initiatorŽ . Ž . Ž . Ž . Ž .decomposition a , initiation b , propagation c , particle nucleation d , particle aggregation e ,Ž . Ž . Ž .particle growth in a poor solvent f and particle swelling in a good solvent g . The counter-cations

Ž . Ž .and the particle charges for steps f and g have been omitted for clarity. M represents a vinylmonomer.

Ž .For particles other than poly NIPAM , particle swelling is achieved by removalŽ .of the particles from the continuous phase e.g. by freeze-drying and re-dispersion

in a solvent with a good solvency for the polymer.Work performed at the University of Adelaide has shown that monodisperseŽ .poly NIPAM particles may be formed during SFEP in the absence of added

w xcross-linking monomer 17 . Thus, NIPAM appears to act as its own cross-linkingmonomer; however, the efficiency of cross-linking is clearly improved when cross-

Ž .linking monomers e.g. BA are employed.An alternative method used for the preparation of microgel systems involves

w xpolymerisation using a good solvent. Staudinger and Husemann polymerised 2dilute DVB solutions and obtained soluble products with low intrinsic viscosity.ŽBulk macrogel would have resulted if the monomer concentration employed was


such that the polymer concentration exceeded the critical overlap concentrationU . w xc . Antionetti and Rosenauer 18 re-investigated the DVB system and reported

w xbroad particle size distributions. Okay and Funke 19 used an analogous anionicpolymerisation method whereby 4-tert-butylstyrene was copolymerised with DVBin heptane to yield microgel particles. The size distributions for these productswere also broad.

The above examples reveal that particle formation using good solvents for thepolymer suffers from poor particle size uniformity. The primary reason for this is alack of electrostatic stabilisation during polymerisation; pendant vinyl groups areable to react with radical sites on neighbouring polymer chains. Network growthmay therefore occur by reaction with neighbouring particles at any time during thepolymerisation, resulting in broad particle size distributions. However, it is likelythat particles formed using this method have a relatively uniform distribution of

Ž .comonomers cf. SFEP because precipitation of high molecular weight chains doesnot occur.

w xSaito and Ishizu 20 have recently reported an elegant approach for theŽ .synthesis of microgel particles. Poly 2VP-b-sty-b-2VP copolymer micelles were

Žreacted with 1,4-di-idobutane in a hemi-solvent a solvent that was selective for the.polystyrene block . The cross-linking reaction transformed the flower-type micelles

into microgel particles with a particle size of 75]185 nm. The particle size rangereported indicates considerable aggregation of the block copolymer within micellesprior to cross-linking. The initial micelle size will determine the final microgelparticle size. It should be possible to produce monomolecular micelles by control ofcopolymer architecture using this method. The micelles could then be cross-linked

Žto yield nanogel particles. Nanogel particles are defined as cross-linked particles.with swollen particle sizes of less than 50 nm . The method discussed above should

produce monodisperse particle distributions since micelle particle distributions ofblock copolymers dispersed in hemi-solvents are usually narrow.

Neyret and Vincent have developed a new approach for the formation ofw xmicrogel particles; inverse microemulsion polymerisation 13 . The oil phase con-Ž .sisted of anionic 2-acrylamido-2-methylpropanesulfonate AMPS and cationic

Ž Ž . . Ž .2- methacryloyloxy ethyl trimethylammonium MADQUAT monomers in addi-tion to BA. The copolymerisation was initiated using UV irradiation and theproduct isolated and re-dispersed in aqueous electrolyte solution to yield polyam-pholyte microgel particles. The particles swelled in the presence of high electrolyteconcentrations as a result of screening of the attractive electrostatic interactionsbetween neighbouring chains.

Table 2 shows particle size data extracted from a representative selection of theliterature reports on microgel systems. SFEP is the most common method used to

Ž .prepare poly NIPAM microgel particles. Microgel particles based on PS andŽ .poly MMA are usually prepared by EP; although SFEP has also been used. EP

and SFEP typically yield microgel particles with diameters between 100 and 1000nm.

Ž .The most actively studied microgel system is poly NIPAM . NIPAM is commer-Ž .cially available and poly NIPAM particles are easily prepared. These particles


Ž .usually contain between 80 and 99% water in the swollen state 0.001 - f - 0.20 .2ŽThese particles are very sensitive to environmental changes as judged by

.swellingrde-swelling transitions .Ž . Ž .PS and poly MMA microgel particles have also been well studied Table 2 .

Ž .Poly MMA and PS particles swell in organic solvents. PS microgel particlesdispersed in EB and THF usually contain between 30 and 90 vol.% solventdepending on the preparation conditions.

Ž .Figs. 4 and 5 show TEM and SEM micrographs for poly NIPAM and PSw x Ž .microgel particles 15,16,39 . Poly NIPAM particles exhibit a pronounced tendency

to form ordered structures when deposited on TEM grids. A definite spacing is

Table 2aSummary of literature reports concerning microgel particles

dPolymer Solvent Microgel Prep Collapsed Swollen f Refs.2b ctype method particle size particle size

Ž . Ž .nm nm

Ž . w xPoly AC WrAL N PP 60]1000 21Ž . w xPoly AN DMF N EP 300 735 0.068 22Ž . w xPoly ALHCl W I PPC 22]26 12

w xPoly W I IMP 50 100]250 } 13ŽAMPSr

.MADQUATŽ . w xPoly EArMAA W I EP 60]100 140]600 0.08]0.005 23Ž . w xPoly MMA Bz N EP 205]230 355]480 0.11]0.20 24Ž . w xPoly MMA BCA N EP 135]205 0.07]0.28 25Ž . w xPoly MMA BAlc N EP 300 650 0.10 26

w xPoly W I EP 75]140 85]255 0.17]0.69 27Ž .MMArAA

w xPoly W I EP 40 28Ž .MMArAArHA

w xPoly W I SFEP 310 750 0.07 16Ž .MMArMAAŽ . w xPoly NIPAM W N SFEP 500 9Ž . w xPoly NIPAM W N SFEP 240 450 0.17 29Ž . w xPoly NIPAM W N SFEP 135 270 0.12 30Ž . w xPoly NIPAM W N SFEP 120 520 0.012 31Ž . w xPoly NIPAM W N SFEP 240 450 0.15 32Ž . w xPoly NIPAM W N EP 375 950 0.062 33Ž . w xPoly NIPAM W N SFEP 400 700 0.19 34Ž . w xPoly NIPAM W N SFEP 35Ž . w xPoly NIPAM W N SFEP 120 520 0.012 36Ž . w xPoly NIPAM W N SFEP 23]50 130]470 0.001]0.006 37Ž . w xPoly NIPAM W N SFEP 150 340 0.085 38Ž . w xPoly NIPAM W N SFEP 325]335 590]685 0.14]0.17 39Ž . w xPoly NIPAM W N SFEP 16Ž . w xPoly NIPAM W N EP 145 40Ž . w xPoly NIPAM W N SFEP 315 590 0.15 41Ž . w xPoly NIPAM W I EP 200]880 0.010]0.10 42


Ž .Table 2 Continued

dPolymer Solvent Microgel Prep Collapsed Swollen f Refs.2b ctype method particle size particle size

Ž . Ž .nm nm

w xPoly W I SFEP 300 700]900 0.04]0.08 43Ž .NIPAMrAA

w xPoly W I EP 30]100 140]425 0.005]0.045 44Ž .NIPAMrMAA

w xPS EB N EP 405]520 490]1150 0.098]0.63 10w xPS Various N EP 75]130 45w xPS Tol N EP 46w xPS EB N SFEP 345]355 570]660 0.16]0.22 15w xPS THF N EP 328]425 700 0.224 47w xPS W I EP 48w xPS Tol N FC 26]84 42]240 0.042]0.23 49w xPS Tol N FC 24]208 18w xPS W I FC 24]35 8

Ž . w xPoly styrAA Dioxan I FC 0.11]0.23 50Ž . w xPoly styrMAA THFrW I BC 60]80 51Ž . w xPoly styrQAC W I EP 250]380 52Ž . w xPoly styr2VP Tol N BC 75]185 20Ž . w xPoly styr2VP W I EP 150]200 600]1100 0.006]0.035 53

w xPoly W N SFEP 135 54Ž .styrmercaptoŽ . w xPoly TBA Heptane N AN 35]80 19Ž . w xPoly 4VP W NrI EP 250]700 0.035]0.28 55

a See Table 1 for abbreviations used for the polymers and solvents.b N, neutral; I, ionic.c EP, emulsion polymerization; SFEP, surfactant-free emulsion polymerization; IMP, inverse microemul-sion polymerization; BC, via block copolymers; AN, anionic polymerization; PP, percipitation polymer-ization; FC, free-radical copolymerization; PPC, post-polymerization cross-linking.d Volume fraction of polymer in swollen particles.

w xevident between the particles. This effect was first reported by Pelton et al. 9 andattributed to particle shrinkage during solvent evaporation.

Ž .The SEM data for poly NIPAM and PS microgel particles show that theŽparticles deformed to a ‘pancake’ structure during solvent evaporation see Fig.

.4a . Clearly, TEM data cannot be used to obtain accurate measurements of theparticle size for particles deposited from the swollen state. PCS data are moreaccurate in this regard.

A range of anionic microgel systems has also been investigated. The anionicmonomers most frequently incorporated are either AA or MAA. Microgel particlescontaining these monomers swell at high pH. This effect may be described in termsof internal electrostatic repulsion or, equivalently, the osmotic contribution frommobile counter-ions in the ionic particles. The swelling mechanism for ionicmicrogels is discussed in more detail below.

The overwhelming majority of the ionic microgel systems listed in Table 2 are


Ž .Fig. 4. Scanning electron micrographs of PS microgel particles deposited from ethylbenzene a andŽ .water b . The particles are viewed at an angle of 308 to the sample plane. The average size of the

Ž .particles shown in b was measured as 355 nm from TEM measurements.

Ž . w xanionic. However, a notable exception is poly styr4VP . Loxley and Vincent 53Ž .examined the pH-dependent swelling and swelling kinetics of the

Ž .poly styr4VP rwater system using PCS and stop-flow techniques. The particlesswell at low pH due to protonation of the 4VP group.

The data listed in Table 2 reveal that the number of microgel systems reported


Ž .Fig. 5. Transmission electron micrograph of poly NIPAM microgel particles. The average particlediameter was 450 nm.

Ž .to date is rather limited; the majority of the work relates to poly NIPAM and PSsystems. Clearly, it should be possible to prepare microgel particles using anyflexible, cross-linked polymer system.

4. Particle swelling theory

Flory’s theory of network swelling has been used to describe the swelling ofw xmicrogel particles in organic solvents 56 . A polymer network immersed in a good

solvent imbibes solvent in order to balance the solvent chemical potential insideand outside the gel network; the presence of cross-links restricts the extent ofswelling. Thus, swelling continues until the sum of the elastic forces betweencross-links is equal to the osmotic force. The extent of network swelling is usually

Ž . Ždescribed by the polymer volume fraction f obtained at equilibrium f s 1 in2 2. w xthe collapsed state . Flory’s theory leads to 56 :

3r5X¨ 1 Ž .f s 32 1½ 5Ž .V y xc 122

where X is the number of cross-links present within a collapsed network volume,V . The subscripts 1 and 2 refer to the solvent and network polymer, respectively;c¨ is the molar volume of the solvent and x represents the Flory solvent]poly-1 12


Ž .mer interaction parameter. The term xrV represents the average density ofccross linked units in the collapsed particle.

Microgel particles usually contain a high proportion of non-cross-linking polymerŽ . Ž .segments monomer B and a minor proportion of normally difunctional cross-lin-

Ž . Ž .king segments A ; the mole fraction of the latter X being typically less than 0.1.AIf it is assumed that the molecular weights of the A and B segments are the sameand that each mole of di-functional cross-linking monomer introduces two moles ofcross-linked units, it is readily shown that:

X 2 x rA B Ž .s 4V Mc B

where M and r are the molecular weight and density of the B segments,B BŽ . Ž .respectively. Substitution of Eq. 4 into Eq. 3 and assuming r s 1 leads to aB

simple expression describing the dependence of the volume fraction of the mi-crogel particles on the network composition and solvency:

3r52 x VA 1 Ž .f s 52 ž /n M12 B

Ž .where the excluded volume parameter, n s 0.5 y x . The polymer volume12 12Ž .fraction increases particles de-swell if the cross-link density increases or the

Ž .solvency becomes poorer n decreases .12Flory’s theory of network swelling has been applied to PS microgel particles

dispersed in ethylbenzene and was also extended to consider the effect of addedŽ . w xnon-adsorbing free polymer 15 . It was found that the ‘effective’ concentration of

incorporated cross-linking monomer was less than that actually used during thepreparation; hence there may have been a significant proportion unreacted,pendant, vinyl groups present in the microgel particles.

Ž .There are a number of problems with Flory’s theory of gel swelling: i it assumesa uniform cross-link density within the network. There is evidence that this is not

Ž . Ž .the case for many microgel systems see below . ii The concentration dependenceŽ w x .of x is not included Napper 57 , amongst others, has considered this problem .

w xThere is a particular problem here with aqueous gel systems 58,59 related tochanges in H-bonding and hydrophobic bonding, as the segmentrwater concentra-tion varies with swelling.

Ž .A more appropriate model though complex for describing the swelling ofŽ . w xpoly NIPAM networks has been employed by Lele et al. 60 . They applied the

Ž . Ž .lattice-fluid-hydrogen bonding theory LFHB to the swelling of poly NIPAMnetworks in aqueous alcohol solutions. The theory incorporated a number of

Ž .different hydrogen bonding interactions within the poly NIPAM -water-alcoholsystem. The extended LFHB model predicted re-entrant swelling for the system

Ž .and qualitatively described the experimental data. Poly NIPAM microgel particlesw xdispersed in water]alcohol mixtures also exhibit re-entrant swelling 41 and this is

discussed in more detail below.


As referred to earlier, microgel particles containing ionic co-monomers exhibitw x ŽpH-dependent swelling 23 . The contribution of strong acid or base, i.e. pH-inde-

. Žpendent groups from within microgel particles which have been extensively.dialysed against distilled water to the swelling of those particles is best viewed in

terms of the increase in internal osmotic pressure due to the mobile counter-ionscontained within the microgel particles. This balances the internal electrostaticrepulsion. The total charge of the microgel particles plus their counter-ions mustbe zero. It may be that some of the counter-ions are bound to charged sites,

Ž .effectively forming neutralised ion pairs. It is only the free mobile counter-ionswhich contribute to the osmotic pressure } at least to the ideal component; theremay be minor contributions from the ion-pairs to the non-ideal component of theosmotic pressure through small changes in the x-parameter.

The addition of inert electrolyte to a dispersion of microgel particles with strongacidrbase groups leads to de-swelling of those particles. A Donnan equilibrium isset up, whereby the free ions distribute themselves between the inside and outsideof the microgel particles. There is a net reduction in the osmotic pressuredifference between the inside and outside of the microgel particles, which may,alternatively, be viewed as additional screening of the electrostatic repulsion withinthe particles.

Ž .The case of microgel particles containing weak acid or base, i.e. pH-dependentgroups is more complex. The ionisation of the sample is governed by the pK , orapK of the groups concerned, but these parameters are functions of the localb

Ž .charge group chemistry a higher charge density suppresses ionisation and back-Ž .ground ionic strength screens local electrostatic repulsion . Irrespective of the

addition of any inert electrolyte, adjustment of pH itself inevitably leads to changesin background ionic strength.

Ž .Fig. 6 shows the effect of increasing pH for a poly MMArMAA microgelŽ . w xsystem the background ionic strength was not systematically controlled 16 . On

increasing the pH from low values to high values, the swelling extent increases asŽ .the weak carboxylic acid groups inside the microgel particles ionise. The rate of

increase levels off at higher pH values partly because the ionisation attains itsmaximum value; in addition, the increasing ionic strength screens the internalrepulsion, or, equivalently, reduces the osmotic pressure difference between theinside and outside of the microgel particles.

5. Properties of microgel particles

When a collapsed microgel particle swells in a good solvent, previously buriedsegments become accessible to the continuous phase. If the radius of the collapsedparticles is r and that of the polymer segments is r , then the total number ofc s

Ž .3 Ž .2segments per particle is r rr and the total on the surface is r rr . Hence, thec s c sratio of segments exposed to the continuous phase, in the swollen and collapsedstate, varies as r rr . Hence, for a collapsed particle with r s 150 nm andc s cr s 0.15 nm, the value for r rr is 1000. Thus, particle swelling results in as c s


Ž .Fig. 6. Effect of dispersion pH on the hydrodynamic diameter of poly MMArMAA microgel particles.

substantial increase in the accessible segments for a microgel particle. This resultshows that microgel particles have considerable potential for applications thatmake use of functionalised monomer units residing within the particles as well as

w xon the surface e.g. pollution control 43 and catalysis.Another beneficial feature of microgel particles is their rapid swellingrde-swell-

w xing kinetics in comparison to macrogels. Tanaka et al. 61 investigated thedependence of the swelling transition rate on particle size for spherical

Ž .poly acrylamide gels. A characteristic time, t , described the rate of swelling; trepresents the point where ; 75% of the total swelling has occurred. Tanaka et al.w x61 showed that t was proportional to the square of the collapsed particle radius,

Ž .r . Assuming this relationship is applicable to microgel particles r s 100 nm , thec cvalue of t for microgel particles is expected to be eight orders of magnitude faster

Ž .than for a macrogel r s 0.5 mm with the same composition. Microgel particlescrespond rapidly to changes in solvency.

The deformable nature of microgel particles has important implications for theirw xrheological properties. Buscall 62 reported that the rheological behaviour of

microgel particles is equivalent to that of hard particles with a thin, soft shell.Dilute microgel dispersions exhibit Newtonian flow, whereas concentrated disper-

w xsions are highly shear thinning 25 . When compared, at the same number concen-tration, to hard-sphere particles, swollen microgel particles greatly increase the

w xdispersion viscosity 37 ; due to the much larger effective hydrodynamic diameter ofthe swollen particles. It is for this reason that microgel particles have potentialapplication in the surface coatings industry as filler materials.

The electrophoretic mobility is an important property of microgel particles that


w xis frequently reported. Miller et al. 63 reported electrophoretic mobility measure-ments for an organic swellable microgel system. They measured mobilities ofapproximately y4 = 10y10 m2 sy1 Vy1 for PS microgel particles dispersed in

Ž . w xtoluene using phase analysis light scattering PALS . Snowden et al. 30 haveŽ .reported mobilities for poly NIPAM microgel particles dispersed in water in the

y8 2 y1 y1 w xrange y1 to y3 = 10 m s V . Oshima et al. 64 has developed a theorythat is applicable to soft particles with a hard core which may be adapted for

w xmicrogel particles 65 , with an assumed uniform space charge density. If thefrictional coefficient of the segments in the swollen microgel particles is known,then the space charge density may, in principle, be derived.

6. Osmotic de-swelling

The first report of an osmotic de-swelling mechanism for microgel particles wasw x Ž . Ž .by Sieglaff 45 in 1963 for the PS microgel rtoluenerPS free polymer system.

Sieglaff suggested that an ‘exclusion shell’ for PS free polymer would be producedaround the microgel particles; exclusion results when the polymer conformationsrequired to penetrate the particle interior become entropically unfavourable.

w xA theoretical model has been developed 15 that describes the change ofŽ .polymer volume fraction of the microgel particle f with volume fraction of2

Ž .excluded free polymer f in the continuous phase. The theory is based on Flory’s3theory of network swelling and incorporates the effect of the osmotic pressure dueto excluded free polymer on the solvent chemical potential, m . The condition that1m in the particle interior and in the continuous phase must equate at equilibrium1

w xyields the following equation 15 :

f M r M X r3 1 3 1 A B1 12 2 1r3Ž . Ž . Ž .y x f q s f y x y 2 f 613 3 2 12 22 2 ž / ž /r M r M1 3 1 B

The subscripts 1, 2, 3 and B refer to the solvent, polymer network, added freeŽ .polymer and non-cross-linking segments, respectively. M and r x s 1, 3 and Bx x

Ž .represent the molecular weight and density, respectively. Eq. 6 has been appliedŽ . Ž .to the PS microgel rEBrPS free polymer system and predicted osmotic de-swell-

w xing 15 . However, the experimental data were believed to have been affected bypartial penetration of PS free polymer into the microgel particle interior. Inaddition, the swellingrde-swelling of the PS microgel particles is believed to bekinetically limited, due to the high T value for PS. Both of the above factorsg

Ž .resulted in a smaller experimental value for f than predicted by Eq. 6 .2Ž .Fig. 7 shows the variation of the de-swelling ratio for poly NIPAM rwaterrPEG,

Ž . Ž . Ž .PS microgel rEBrPS free polymer and poly MMArMAA rwaterrPEG systemsas a function of the molecular weight of added free polymer at fixed values of f .3

Ž .The data show the greatest de-swelling for the poly NIPAM rwaterrPEG system.Ž .The poly MMArMAA particles have an electrostatic component which opposes


Ž . Ž . Ž . Ž . Ž .Fig. 7. Osmotic de-swelling of poly NIPAM B , PS l and poly MMArMAA ^ microgel particlesby addition of free polymer of different molecular weight. The free polymers used in the aqueous

Ž . Ž .systems f s 0.13 and non-aqueous system f s 0.19 were PEG and PS, respectively.3 3

de-swelling and is responsible for the modest osmotic de-swelling observed for thatsystem.

The rheological properties of microgel particle dispersions in the presence ofw xadded free polymer have been investigated by Racquois et al. 66 . The particles

were based on styrene and copolymerised with unspecified acrylic co-monomers. Areduction in viscosity was reported upon the addition of free polymer and this wasattributed to osmotic de-swelling of the particles } corresponding to a lower fdŽ Ž ..Eq. 1 .

Ž .Osmotic de-swelling of poly acrylate microgel particles in the presence of thew xsodium salt of PAA has been investigated by Kiefer et al. 67 using viscosity

measurements. The results implied that added PAA did, indeed, cause de-swellingof the microgel particles. Excluded PAA may be expected to induce de-swelling ofthe particles due to the osmotic pressure of the mobile ions associated with thepolyelectrolyte in addition to the contribution from excluded free polymer itself.However, low molecular weight PAA chains were believed to penetrate themicrogel particle interior.

PCS measurements of microgel particlerfree polymer systems are useful forcharacterising the swelling behaviour of microgel particles. In mixed particlerfreepolymer systems it is important to ensure that the scattered intensity from theparticles is at least an order of magnitude greater than that from the polymersolution. In calculating the hydrodynamic diameter from the measured diffusion

Ž .coefficient Stokes]Einstein equation the viscosity is taken to be that of thecontinuous polymer solution phase. Microgel particles usually have diameters atleast an order of magnitude greater than the surrounding free polymer chains, sothat the autocorrelation function may be deconvoluted to obtain the diffusioncoefficient for the microgel particles.


7. Stability of dispersion

The interaction between swollen microgel particles comprises contributions fromsteric and electrostatic terms. Microgel particles contain a large volume fraction ofsolvent in the fully swollen state. Thus, the effective Hamaker constant for theswollen particle is similar to the Hamaker constant for the solvent, resulting in anegligible van der Waals attraction between the swollen particles. Thus, in the fullyswollen state, dispersions of microgel particles are intrinsically stable. As de-swell-ing occurs so the van der Waals forces become increasingly more significant.

Ž .If charged groups are incorporated in the surface andror in the interior intothe particles during polymerisation then electrostatic interactions play a role in

Ž .determining the stabilisation. The surface groups on poly NIPAM particles areresponsible for the fact that these dispersions remain stable during synthesis, even

Ž .though the temperature is much higher than the LCST of poly NIPAM in water,such that the particles are de-swollen.

The importance of steric interactions to the stability of microgel dispersions isŽ .illustrated by the fact that poly NIPAM dispersions are stable in the presence ofŽ .high electrolyte concentrations such that electrostatic interactions are negligible

w xprovided the dispersion temperature is below the LCST 29 . In addition, PSŽ . w xmicrogel particles are swollen and stable in ethylbenzene 15 } even in the

Ž .presence of 20 wt.% free PS see below . These results show that steric stabilisationmakes a significant contribution to microgel dispersion stability. The magnitude of

Ž .the steric interaction depends on the polymerrsolvent interaction parameter xand is linked to the degree of particle swelling through f .2

The stability of particular dispersions to flocculation is conveniently examined byŽ Ž .measurements of the ‘n-value’. The n-value is the gradient of log optical density

Ž . Ž Ž . Ž ..vs. log wavelength plots n s ydlog O.D rdlog l and has a magnitude whichŽ .decreases with average particle size in the Rayleigh-Gans-Debye scattering region .

In order to examine depletion interactions of free polymerrmicrogel systems then-value for a range of systems was investigated. Fig. 8 shows the variation of the

Ž . Ž . Ž .n-value for poly NIPAM rwaterrPEG, PS microgel rEBrPS free polymer andŽ . w xpoly MMArMAA rwaterrPEG dispersions 15,16,39 . The data reveal thatŽ .poly NIPAM and PS particles flocculate at high volume fractions of PEG10K and

PS17K, respectively. The flocculation for both systems is attributable to depletioninteractions. Bridging interactions are not likely in either system under the condi-tions employed. However, a study involving adsorbing polymer may provide someinteresting information about the interaction between solution polymers andmicrogel particles as osmotic de-swelling may then occur simultaneously withbridging.

Free polymer chains below a certain critical molecular weight may diffusethrough the microgel particle pores into the microgel particle interior of a particlewith a uniform pore size distribution. In such cases, the particle may swell. Whenthe polymer chains are too large to penetrate the microgel particle interior,osmotic de-swelling results in collapse of the network. It is plausible that the pore

Žsize of the particles increases from the centre of the particle to the periphery see


Ž . Ž . Ž . ŽFig. 8. Variation of the n-values for poly NIPAMrwaterrPEG10K B , PS microgel rEBrPS17K free. Ž . Ž . Ž .polymer ^ and poly MMArMAA rwaterrPEG17K e with volume fraction of free polymer.

.below . Polymer chains below a certain size may be able to penetrate the pores atthe periphery. In that case, de-swelling of the particle core may occur to a greaterextent than the periphery. Osmotic de-swelling is expected to augment any non-uniform pore size distribution already present within the particles.

8. Swelling control by addition of alkanols: co-non-solvency

Ž .Linear poly NIPAM coils in water can be made to undergo a coil-to-globuletransition by addition of alkanols, even though the alkanols are good solvents for

w xthe polymer 68 . The phenomenon whereby a polymer has poor solubility in amixture of solvents that are individually good solvents for that polymer is referredto as co-non-solvency. This phenomenon is manifested as de-swelling for

Ž . w xpoly NIPAM microgel particles 16 and macrogels. The behaviour was firstw xreported for microgel particles by McPhee et al. 69 .

Ž .Fig. 9 shows the changes in the de-swelling ratio for poly NIPAM andŽ .poly MMArMAA microgel particles dispersed in aqueous PrOH solutions as a

Ž .function of 2-PrOH volume fraction f . The data for each system show de-swell-aing at moderate values of f and re-entrant swelling at higher values of f . Thea amechanism responsible for co-non-solvency most likely involves clathrate structure

w x Ž .formation 70 . In pure water the poly NIPAM network is fully swollen by theŽ .attractive water]polymer interactions hydrogen bonding . Addition of moderate

amounts of alcohol leads to the formation of transient structures involving PrOHŽmolecules which are encapsulated by locally ordered water molecules clathrate

.structure . Competition for water molecules occurs between these clathrate struc-


Ž . Ž . Ž .Fig. 9. The effect of added PrOH on the de-swelling ratio of poly MMArMAA B and poly NIPAMŽ .` microgel particles dispersed in aqueous PrOH solutions.

Ž .tures and poly NIPAM microgel particles; water molecules are removed from themicrogel particle interior causing de-swelling. The encapsulation of the alcoholmolecules breaks down at high volume fractions of PrOH leading to direct

Ž .solvation of the polymer chains hydrophobic interactions by the alcohol andre-entrant swelling.

9. Small-angle neutron scattering investigations of microgel particles

Neutron scattering results from a short-range repulsive interaction between theŽ .neutrons and nuclei of a material. The small-angle neutron scattering SANS

Ž Ž ..experiment involves the measurement of the scattered intensity I Q of a neutronŽ .beam with a wavelength, l, as a function of the scattering vector Q . The

Ž .scattering vector is related to the scattering angle u by:

4p uŽ .Q s sin 7ž /l 2

Ž .For dilute colloidal dispersions, such as microgels, the scattered intensity, I Q ,Ž . Ž Ž ..is a function of the contrast factor Dr , the particle form factor P Q and the

Ž .particle concentration N .p

22Ž . Ž . Ž . Ž .I Q s V N Dr P Q 8p p

where V is the particle volume. For spherical particles of uniform average density,pŽ .P Q is given by:

2Ž . Ž .sin Qr y Qrcos QrŽ . Ž .P Q s 3 93Ž .Qr


The contrast factor between the particles and the medium can be altered byŽ .selective isotopic substitution using deuterium in order to simplify interpretation

of SANS data for multicomponent systems. Consequently, SANS is ideally suitedfor studies of microgel particles in the presence of added free polymer.

Because of the presence of the cross-links, microgel particles do not have aw xuniform density at the length scales of the neutron wavelength. Mears et al. 40

Ž .were the first to examine poly NIPAM microgel particles in the presence of SDSŽ .sodium dodecyl sulfate using SANS. They assumed that the scattered intensityfrom the microgel particles obeyed the equation:

Ž .I 0Ly4Ž . Ž .I Q s AQ q 102 21 q j Q

Ž .where A and I 0 are constants and j is the correlation length which charac-LŽ .terises the mean separation between density fluctuations e.g. the pore size . Eq.

Ž .10 assumes that the scattering from microgel particles has components from theŽ . Ž‘whole particle’ first term and polymer in a solution-like environment second

. Ž .term . Mears et al. found that SDS caused swelling of the poly NIPAM particlesand that small polymer-bound aggregates of less than five monomer units formedwithin the particles. Presumably, SDS caused swelling due to the adsorption of the

Ž .surfactant molecules onto the poly NIPAM network resulting in electrostaticcontributions to the osmotic swelling.

SANS has also been employed to investigate the structural changes forŽ .poly NIPAM particles as a result of thermally induced de-swelling, osmotic

w xde-swelling and co-non-solvency 71,72 . Representative scattering data appear inFig. 10. The scattering profile for the pure microgel particles at 508C shows linear

Ž .behaviour with a gradient of y4.22 Porod scattering , whereas a shallowerŽgradient is observed at 258C. These results indicate a change from collapsed hard

. Ž .sphere particles to swollen diffuse particles upon decreasing the temperature toŽ .below the LCST. The data for the poly NIPAM particles in the presence of added

d-PEO18K or d-MEOD exhibit smaller gradients over most of the Q range. Thescattering profiles for the latter systems are similar over the Q range 0.008]0.02.

Ž .Poly NIPAM particles subjected to osmotic de-swelling and co-non-solvency ap-pear to have a more diffuse network structure than pure particles heated to 508C.The thermally de-swollen particles have sharp boundaries over the entire Q range.

10. Internal structure of microgel particles

The internal structure of microgel particles determines their swelling propertiesand is of considerable importance. The most important structural information isthe distribution of the cross-linking monomer as a function of distance from theparticle center. Although the theories of microgel swelling, described above,

Ž .assume uniform swelling i.e. a uniform cross-link density the situation is highlyunlikely. A more probable situation, for microgel particles prepared by emulsion


Ž .Fig. 10. Small-angle neutron scattering profiles for poly NIPAM microgel particles obtained under aŽ . Ž .variety of conditions. Measurement for the pure microgel system was made at 258C = and 508C B .

Ž .Data were also recorded at room temperature in the presence of d-MeOD f s 0.53, I and ofaŽ .d-PEO18K f s 0.13, v .3

polymerisation, is that the cross-link density decreases from the center of theparticles toward the periphery. There is some support for this viewpoint from the

w x Ž . Žwork of Nieuwenhuis et al. 24 . They examined poly MMA MMA s.methylmethacrylate microgel particles dispersed in benzene and reported a varia-

tion in the microgel particle diameters measured using a range of techniques. Itwas concluded that the particles contained an inhomogeneous cross-link distribu-tion.

w xMcPhee et al. 69 measured the efficiency of cross-linking monomer incorpora-tion into growing particles during SFEP and concluded that a large proportion ofthe cross-links were incorporated during the initial growth of the particles. This isnot surprising as the solubility of a polymer chain decreases with increasingmolecular weight and addition of cross-linking monomer facilitates a substantialincrease in its length. Accordingly, attainment of uniformly swollen microgelparticles may be more likely using polymerisation in a good solvent. The improvedhomogeneity of the structure would be at the expense of a monodisperse particle

Ž .size distribution see Section 3 .

11. Outlook

Microgel particles have excellent potential for application. This is evidenced byŽ .applications involving microgel particles in the surface coatings industry paint .


ŽFuture applications for microgel particles are expected to include catalysis micro-. Ž .gel particle supported catalysis and micro-encapsulation pharmaceutical industry .

There is considerable scope for enhanced drug delivery using core-shell microgelparticles. For example, future generations of microgel particles may containcis-platin and a cross-linked shell that undergoes spontaneous swelling when incontact with cancer cells. Delivery systems for cisplatin that specifically targetcancer cells are urgently required.

Frictional drag-reduction offers considerable promise for reducing energy lossesw xduring turbulent flow of fluids in pipes 73 . Addition of linear polymers to the

w xcontinuous phase has resulted in considerable drag reduction 74 . However, theshear stability of the chains is poor with shear induced chain scission resulting indiminished drag reduction as a function of time. Microgel particles are deformableand are promising candidates for drag reduction. The shear induced deformationof the network would be distributed over the entire network reducing chainscission and improving shear stability. This approach is being pursued at theUniversity of Adelaide.

Another area for future application of microgel particles is in water purification.Ž . w xPoly NIPAM-co-acrylic acid microgel particles take up heavy metal ions 43 .

However, the efficiency of uptake is low due to the restricted proportions of acrylicacid that can be used during preparation. Improvements are likely when microgelparticles containing high carboxyl contents are prepared. Moreover, it should bepossible to produce chelating monomers which will specifically bind target metalions.

There are a variety of possible applications for microgel particles. Some of theŽ .applications have already been achieved e.g. rheological control additives ; whereas

many more are yet to be realised. Microgel particles are versatile systems and areexpected to play an important role in pollution control and the surface coatingsand pharmaceutical industries in years to come. The understanding of microgelparticles has come a long way since their discovery and there is plenty of room forresearch into these unusual model colloids.

Acknowledgements

Ž .The authors would like to thank the EPSRC UK and the Paint ResearchŽ .Association UK for financial support. The co-non-solvency data supplied by

Ž .Helen Crowther University of Bristol is gratefully acknowledged.

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saunders microgels

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