microgel pelton

Upload: fatin-damia-muhammad

Post on 06-Apr-2018

243 views

Category:

Documents


1 download

TRANSCRIPT

  • 8/3/2019 microgel pelton

    1/33

    Advances in Colloid and Interface Science .85 2000 1 33

    Temperature-sensitive aqueous microgelsRobert Pelton U

    McMaster Centre for Pulp and Paper Research, Department of Chemical Engineering, McMaster Uni ersity, Hamilton, Ontario, Canada L8S 4L7

    Abstract

    An account of the preparation and characterization of temperature-sensitive aqueous .microgels based on poly N -isopropylacrylamide was first published in 1986. Since then

    there has been a steady increase in the number of publications describing preparation,characterization and applications of temperature-sensitive microgels. This paper reviews theimportant developments in the area of temperature-sensitive aqueous microgels over the

    .last decade. Although most of the work involves gels based on poly N -isopropylacrylamide ,other polymers are also considered. Core shell latex particles exhibiting temperature-sensi-tive properties are also described. 2000 Elsevier Science B.V. All rights reserved.

    Keywords: Thermal sensitive microgels; Temperature-sensitive microgels; Latexes; Colloids; Swelling;Polymer surfactant interactions

    Contents

    1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22. Microgel synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

    2.1. Homogeneous gels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52.1.1 Novel synthetic methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

    2.2. Core shell microgels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .92.3. Non-NIPAM temperature-sensitive microgels . . . . . . . . . . . . . . . . . . . . . .103. Microgel properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

    3.1. Gel structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .113.2. Gel swelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13

    U Tel.: q 1-905-525-9140 ext. 27045; fax:q 1-905-528-5114. . E-mail address: [email protected] R. Pelton

    0001-8686r 00r $ - see front matter 2000 Elsevier Science B.V. All rights reserved. .PII: S 0 0 0 1 - 8 6 8 6 9 9 0 0 0 2 3 - 8

  • 8/3/2019 microgel pelton

    2/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 332

    3.2.1. Swelling theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .133.2.2. Swelling measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .143.2.3. Swelling in other solvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16

    3.3. Surface activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18

    3.4. Rheological properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .183.5. Electrical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .193.6. Gel surfactant interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .233.7. Colloidal stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24

    4. Microgel applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .274.1. Biotechnology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .274.2. Controlled uptake and release . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .294.3. Other potential applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30

    5. Closing remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31

    1. Introduction

    Aqueous microgels are an important part of water-borne polymer technologies.Cosmetics, coatings, food, and industrial processing industries employ aqueousmicrogels to modify rheological properties, to retain water, and for many otherreasons. Commodity microgels include those based on starch, cross-linked

    . w xpoly sodium methacrylate 1 , and a variety of gums. However, the focus of thisreview is on a new class of synthetic aqueous microgels whose properties displayextreme temperature sensitivity in water. Most of these systems are based on

    . .poly N -isopropylacrylamide polyNIPAM or related copolymers. The structure of NIPAM is shown in Fig. 1.

    In the absence of a universal definition of microgels, this review is restricted todispersions with average diameters ranging between 50 nm and 5 m. Thepreparation and characterization of much larger gel particles has been described in

    w x.the literature e.g. see Park and Hoffman 2 . This review also includes core shellgel structures in which the shell has gel properties in water whereas the core madeof water insoluble polymer such as polystyrene. The distinction between core shellgels and latex particles coated with hydrophilic polymer is not clear. For example,polystyrene particles coated with low molecular weight grafted polyethylene glycol .PEG would not, in normal circumstances, be considered a microgel. On the other

    hand, the properties within the PEG coating are those of a gel and the particles arew xtemperature-sensitive 3 . In this review, no attempt is made to resolve these

    definitions. Instead, the core shell systems reviewed are those which seem to havebeen prepared specifically to display temperature-sensitive behavior in waterirrespective of thickness or volume fraction of the gel layer.

    Thermo-responsive aqueous colloidal microgels form an interesting subset of polymer colloids the gels have properties in common with water-soluble

  • 8/3/2019 microgel pelton

    3/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 33 3

    Fig. 1. Monomers employed in temperature-sensitive microgel synthesis.

    polymers, water-swollen macro gels, and water-insoluble latex particles. Like water-soluble polymers, the properties of microgels depend upon the subtle balanceof polymer r polymer vs. polymer r water interactions. Like macroscopic aqueousgels, colloidal microgels are characterized by a degree of swelling, an averagecross-link density and characteristic time constants for swelling and shrinking. Likehydrophobic polymer colloids, colloidal microgels can be prepared to havemonodisperse particle size distributions; microgels can be characterized by stan-dard colloidal techniques including electrophoresis, dynamic light scattering, rhe-ology and electron microscopy. Microgels can be flocculated by salt or flocculantaddition.

    In 1978, Philip Chibante, a high school summer student, with aspirations tobecome a dentist, prepared the first reported temperature-sensitive aqueous mi-

    w xcrogel under the authors supervision 4 . The microgel was a monodisperse, .colloidal dispersion based on cross-linked poly N -isopropylacrylamide , herein

    called polyNIPAM, a polymer which has a lower critical solution temperature . w xLCST in water of 32 C 5,6 . All the microgel properties were sensitive functionsof temperature over the range 15 50 C. A detailed description of microgel proper-ties and characterization is given in subsequent sections. However, as a preview,the essential temperature-sensitive properties are illustrated in Fig. 2. At roomtemperature, the gels have a high water content, a low refractive index difference

  • 8/3/2019 microgel pelton

    4/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 334

    Fig. 2. A schematic illustration of the factors influencing microgel colloidal stability below and abovethe VPTT.

    with water, and a few electrically charged groups on the chain ends. By contrast, at

    elevated temperatures the particle volume is 10-times less; the density of electri-cally charged groups is higher and the refractive index difference with water isgreater.

    Since 1986 there have been many publications describing the preparation,characterization and application of temperature-sensitive microgels, most based onpolyNIPAM. This activity reflects the current interest in switchable or intelligent

    w xmaterials 7 9 . The recent advances in temperature-sensitive microgels are re-w x viewed. This work not only updates previous reviews 10 13 but also attempts to

    identify areas where more research is required. . N -Isopropylacrylamide NIPAM is the major building block for temperature-

    sensitive microgels. The monomer is available from specialty chemical distributors.

    .With a structure see Fig. 1 close to acrylamide, many of the properties of NIPAMare similar to those of acrylamide. In aqueous solution it undergoes rapid freeradical polymerization in water to give high molecular weight polymers at rates

    w xsimilar to that of acrylamide 5,14 16 . Like acrylamide, NIPAM is a suspectedcarcinogen and neurotoxin, however, unlike acrylamide, NIPAM has an intenseodor so monomer contamination is easy to detect.

    Gels, either micro or macro, are temperature-sensitive if most of the polymer inthe gel network displays has temperature-sensitive phase behavior in the swellingsolvent. A linear polymer that displays cloud point behavior when heated can becross-linked to give a temperature-sensitive gel network. Upon heating such a gel,one observes the gel to shrink by expelling water over a narrow temperature range,

    .usually called the olume phase transition temperature VPTT . PolyNIPAM-basedaqueous microgels are temperature-sensitive because polyNIPAM has a lowercritical solution temperature of 32 C see Schilds excellent review of the

    w xproperties of linear polyNIPAM 6 . Fig. 3 shows a phase diagram for polyNIPAMin water. The cloud point temperature is not very sensitive to concentration over

    w xmost of the range of water contents 6 . Indeed, there is a tendency in the literatureto equate experimentally determined cloud point temperatures to the lower critical

  • 8/3/2019 microgel pelton

    5/33

  • 8/3/2019 microgel pelton

    6/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 336

    w x

    Fig. 4. Transmission electron micrograph of polyNIPAM microgels 4 .

    from the center-to-center spacing of the ordered disks in Fig. 4. The shape of thedehydrated spheres was confirmed to be a rather flat cap by Crowther and Vincentw x27 using scanning electron microscopy an example is shown in Fig. 5.

    Microgel particle formation occurs by homogenous nucleation that is knownw xsometimes to give latex dispersions with a narrow particle size distribution 28 .

    According to this mechanism a water-soluble sulfate radical initiates a water-solu-ble NIPAM monomer which then grows in solution until it reaches a critical chainlength after which the growing chain collapses to become a colloidally unstable

    precursor particle. The precursor particles follow one of two competing processes.Either they deposit onto an existing colloidally stable polymer particle or theyaggregate with other precursor particles until they form a particle sufficiently large

    .to be colloidally stable. At the polymerization temperature 50 70 C which is wellabove the LCST, the growing polyNIPAM microgel particles are colloidallystabilized by electrostatic stabilization originating from sulfate groups introducedby the persulfate initiator. In the case of surfactant-free styrene polymerization,

  • 8/3/2019 microgel pelton

    7/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 33 7

    Fig. 5. Scanning electron micrograph of polyNIPAM microgels taken at an angle to emphasize thew xshape of the dehydrated particles 27 .

    the relatively hydrophilic sulfate terminated chains concentrate at theparticle r water interface so the charge density increases with particle size which inturn means the colloidal stability should increase as the particles grow. Thus,aggregating precursor particles eventually achieve a diameter where the particle iscolloidally stable and a new latex particle is formed.

    Microgel polymerizations can yield a significant fraction of sol i.e. linear or.slightly branched polymer . The sol fraction must be removed to generate the very

    ordered arrays shown in Figs. 4 and 5. Centrifugation, followed by decantation andredispersion in clean water is an effective cleaning procedure. The microgelparticles readily redisperse after room temperature centrifugation, showing no

    indication of coagulation.PolyNIPAM microgel preparation do not always yield monodisperse colloidal

    particles. Too much BA cross-linker or dirty monomer can give unstable suspen-sions. Indeed, some approaches to the synthesis of porous macroscopic tempera-

    w xture-sensitive gels seem to involve the aggregation of colloidal microgels 29 .w x Analogous to surfactant-free polystyrene latex 17,86 , cationic polyNIPAM

    microgels can be prepared by employing a positively charged free radical initiator . w xsuch as 2,2 -azobis- 2-amidiopropane dihydrochloride 4,30 32 . The resulting

    microgels have covalently bonded cationic initiator fragments. The concentrationof cationic groups in the microgels can be increased by copolymerization with a

    w x . w xcationic monomer 32 , by binding sorption of a cationic surfactant 30 or by the

    Mannich reaction of acrylamide containing microgels to give ionizable aminew xgroups 33 .

    PolyNIPAM microgels must contain a cross-linking comonomer such as BA toprevent the gel from dissolving in water at low temperature. Furthermore, withinlimitations, it is possible to incorporate most other water soluble vinyl monomers.

    w xThe original example is acrylamide 4 . The upper limit to the content of a thirdmonomer is the requirement that the polymer not be soluble in water at the

  • 8/3/2019 microgel pelton

    8/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 338

    Fig. 6. The influence of SDS on the average particle size of polyNIPAM latex. Volume averagew xdiameters were determined by dynamic light scattering at 50 C 15 .

    polymerization temperature. In the case of acrylamide it is difficult to incorporatemore than 10 wt.%.

    Many other water-soluble monomers have been copolymerized with NIPAM inw x w x

    thermo-sensitive microgels. These include N -acryloylglycine 34 , acrylic acid 35 ,w x2-aminoethylmethacrylate hydrochloride 36 see Fig. 1.It is difficult to prepare small microgels by surfactant-free methods because

    there is simply not enough available charge to stabilize high concentrations of small particles. The surface charge density of a polyNIPAM microgel latex was

    w xfound to be 3.8 eq. r g 37 which is about two orders of magnitude lower than acorresponding surfactant free polystyrene latex. Indeed, sometimes it is difficult to

    w xget good surfactant-free dispersions of any size. McPhee et al. 37 first showed that . .the presence of low sodium dodecyl sulfate SDS concentrations up to 4 mmol

    gave robust preparations. The average particle size in water at 25 C decreased by afactor of 10 when the SDS concentration was increased from 0.2 to 4 mmol. Fig. 6

    .shows the diameter of shrunken 50 C particles as a function of SDS concentra-w xtion used to prepare the microgels 15 . The fitted line on the double logarithmic

    w xy 0.71plot predicts D SDS where D is the diameter. Although employing SDSextends the latitude of microgel synthesis, it does introduce difficulties. SDS binds

    .to polyNIPAM see subsequent sections and thus can be difficult to remove.

    2.1.1. No el synthetic methodsHeating a solution of linear polyNIPAM through the LCST can also form

    polyNIPAM particles. For example, Fig. 7 shows that the particle size of thepolyNIPAM dispersions increases linearly with the cube root of the total polymer

    w xconcentration 38 . The linear relationship implies that the number concentrationof particles produced was independent of polyNIPAM concentration. The particlesappear to be electrostatically stabilized against coagulation. The electrophoreticmobilities for the dispersions varied from y 1 = 108 m2r Vs to y 4 = 108 m2r Vs.Similarly, cationic particles form from phase separated solutions of cationic copo-

    w xlymers of NIPAM 39 . Upon cooling the polyNIPAM particles revert to polymersolutions. However, one could envision a process whereby polyNIPAM chains wereheated to give colloidal particles that were then cross-linked, perhaps by condensa-tion chemistry, to give microgels.

  • 8/3/2019 microgel pelton

    9/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 33 9

    2.2. Core shell microgels

    Core shell gels consist of a water-insoluble latex particle coated with a gel layer.The composite particles have properties characteristic of both the core and the

    .shell. The core dominates light scattering turbidity behavior whereas the colloidstability is determined by the hydrogel shell. For example, latex with temperature-sensitive colloidal stability can be achieved with a polyNIPAM shell this will bedescribed in more detail in the section dealing with colloidal properties.

    The first reported preparation of polyNIPAM coated core shell particles was byw xPelton 40 who described both the one-shot surfactant-free preparation of

    polystyrene r polyNIPAM gels and the grafting of polyNIPAM onto existing latexparticles. Unlike the homogenous microgel preparations, these polymerizations areconducted below the LCST so that polyNIPAM does not phase separate. Similarpolystyrene-co-polyNIPAM surfactant-free latexes are obtained when starting witheither NIPAM monomer or with polyNIPAM. This reflects the fact that NIPAM

    polymerizes much more quickly than does styrene in aqueous media. Thus, most of the NIPAM is converted to polyNIPAM at extremely low styrene conversions.w xMakino et al. 41 refined Peltons procedure with a two-stage approach. The

    first stage was a one-shot surfactant-free polymerization with styrene and NIPAM.In the second stage, the surface gel layer was expanded by adding more NIPAMmonomer and a nonionic water-soluble free radical initiator.

    w xZhu and Napper 42,43 employed a two-stage procedure for the preparation of polystyrene-polyNIPAM core shell particles. In the first stage styrene was addedslowly to an aqueous polyNIPAM solution, presumably at room temperature, in thepresence of a redox initiator. The objective of this stage was to produce a solublepolyNIPAM-co-styrene copolymer, however, the polymer from this stage was notisolated. In the second stage the remainder of the styrene was added quickly to yield a latex.

    w xDuracher et al. 44,45 investigated the preparation and properties of cationicpolystyrene-polyNIPAM core shell particles. Both cationic initiator and amino-ethylmethacrylate introduced positively charged groups. The cationic monomer

    Fig. 7. Influence of polymer concentration on the average diameter of polyNIPAM particles formed byw xheating polymer solutions 37 .

  • 8/3/2019 microgel pelton

    10/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 3310

    gave smaller particles, however a variety of particle morphologies were obtained,depending upon the polymerization conditions.

    w xTakeuchi et al. 46 prepared polystyrene-core polyNIPAM-shell particles by aninteresting macromonomer approach. The macromonomer had approximately 24

    .NIPAM residues with a methacrylamido end group see Fig. 1 . The core shellparticles were prepared by aqueous emulsion polymerization of styrene, macro-monomer and SDS at 70 C. The surfactant was necessary because at 70 C thepolyNIPAM would have no capability to stabilize the latex. Presumably, thesurfactant would not have been necessary if the preparations had been conductedat room temperature with a redox initiator. The resulting latex showed thetemperature-sensitive colloid stability characteristics of a polyNIPAM-coated parti-cle. The macromonomer approach is appealing because it should exclusivelyproduce terminally attached polyNIPAM chains to give a brush structure on thesurface. In contrast, the other reported methods produce more complicated

    w xpolyNIPAM surface gels. More recently, Chen et al. 47 employed the macro-

    monomer approach to prepare polystyrene-core polyNIPAM-shell particles which were used to support platinum catalysts.Many of the potential biotechnological applications of polyNIPAM microgels

    require the presence of reactive functional groups to act as coupling sites. Zhou etw xal. 26 prepared cyano functionalized gels by a two-stage polymerization. Conven-

    tional polyNIPAM microgel was prepared in the first stage and NIPAM, BA, andacrylonitrile were polymerized at 60 C in the second stage. A two-shot procedure was also used. The resulting microgels were monodisperse. Shell recipes with 50and 75 wt.% acrylonitrile produced particles with raspberry morphologies in theSEM.

    w xYasui et al. 48 described a particularly elegant route to core shell particlesbearing reactive groups. Carboxyl terminated low molecular weight, 2500 and

    11 000 Da, polyNIPAM chains were condensed onto the surface of poly styrene-.co-divinylbenzene-co-glycidyl methacrylate particles by carbodimide coupling

    chemistry. The enzyme trypsin was coupled to the free polyNIPAM chain ends.

    2.3. Non-NIPAM temperature-sensiti e microgels

    Most of the work discussed in this review involves microgels based on N -isopro-pylacrylamide. However, related monomers also yield polymers which have a lowercritical solution temperature in water and a few of these monomers have been used

    w xto make temperature-sensitive hydrogels. Duracher et al. 49 reported the prepa-ration and characterization of microgels based on N -isopropylmethacrylamide see

    .Fig. 1 . Monodisperse microgels were prepared under the same conditions aspolyNIPAM microgels, however the polymerization times were five times longerfor isopropylmethacrylamide. This observation is consistent with the fact that thepropagation rate constant for acrylamide is 22 times greater than that of

    w xmethacrylamide 49 . A consequence of the low reactivity of isopropylmethacryl-amide was that BA cross-linker was consumed much more quickly than N -isopro-pylmethacrylamide. This in turn could lead to a significant content of linear

  • 8/3/2019 microgel pelton

    11/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 33 11

    .poly N -isopropylmethacrylamide . The authors identified conditions to minimizethis problem.

    Since N -isopropylmethacrylamide contains one more methyl group than doesNIPAM, one might expect the polymer to have lower cloud point temperature thanpolyNIPAM in water but this is not the case. Reported cloud point temperatures

    .for poly N -isopropylmethacrylamide range between 39 and 47 C; based on opticalw xdensity measurements the value seems to be 45 C 49 . The incorporation of BA to

    give microgels caused the optical transmittance vs. temperature plots to broadenand shift towards lower temperature with increasing BA content. This may reflect adistribution of polymer structures arising from a batch polymerization of BA with N -isopropylmethacrylamide.

    w xLowe et al. 50 have prepared temperature-sensitive microgels based on N -ethylacrylamide. The precipitation polymerizations were conducted at 90 C, above

    .the phase separation temperature 78 C for the homopolymer, yielding monodis-perse dispersions. The transition was less abrupt than for polyNIPAM. In contrast

    to the behavior of polyNIPAM gels, the DSC scans over the transition weredependent upon concentration, not reversible, and repeated measurements on thesame sample were not identical.

    As with the homogeneous gels, only a few temperature-sensitive, core shell gelsnot based on polyNIPAM have been described. Most of the work has been done in

    w xHarma Kawaguchis laboratory in Japan. For example, they 51 described thesurfactant-free polymerization of N -acryloylpyrrolidine copolymers with styrene.Spherical particles were obtained for particles containing more than 80 wt.%, andthe resulting particles showed temperature-sensitive colloidal stability and elec-trophoretic mobility. Swelling data were not given, however, and in view of the high

    .styrene content the particles are more likely to be poly N -acryloylpyrrolidinestabilized polystyrene particles than homogeneous microgels.

    w xIn a related study Hosino et al. 52 compared the properties of shells, onpolystyrene cores, based on N -acryloylpyrrolidine with those based on N -

    .acryloylpiperidine see Fig. 1 . The pyrrolidine-based particles were more hy-drophilic displaying temperature sensitivity at 50 C compared with 5 C for thepiperidine shell particles.

    .Poly N -vinylisobutyramide , an isomer of polyNIPAM in which the positions of .the nitrogen and carbonyl are exchanged see Fig. 1 , has an LCST of 40 C.

    w xSerizawa et al. 53 , reported the preparation of monodisperse polystyrene latex .stabilized with a shell of poly N -vinylisobutyramide which was added to the

    polymerization as a macromonomer. The authors claim the particles are colloidallystable above the VPTC, which is difficult to explain because the particles wereprepared with a nonionic initiator; thus they should have little surface charge.

    3. Microgel properties

    3.1. Gel structure

    Most of the reported microgel preparations involve batch polymerization. Fig. 8

  • 8/3/2019 microgel pelton

    12/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 3312

    . w xFig. 8. Conversion curves for NIPAM and methylenebisacrylamide BA 15 . Note that 9 mM SDS wasemployed in the polymerizations.

    shows the consumption of NIPAM and BA cross-linker as functions of time andw x

    temperature 15 . BA is consumed more quickly than NIPAM indicating that theparticles are unlikely to have a uniform composition. Indeed, it seems reasonableto speculate that there exists a zone of relatively high cross-link density in eachparticle. Since polyBA is more hydrophilic than polyNIPAM at elevated tempera-ture, one might further speculate that at least part of the high cross-link densityzone is situated on the particle r water interface.

    A number of techniques have been applied to characterize polyNIPAM mi-crogels, and the results of some of this work give insight into gel particle

    w xmorphology. Fujimoto et al. 54 treated homogeneous microgels with a fluorescentprobe and measured emission intensity and maximum wavelength as functions of temperature. Both parameters showed large changes between 30 and 35 C, and theprobe indicated a more hydrophobic environment in the shrunken gels. In thesame paper they compared fluorescent quenching by nitromethane in hydrogels with linear polyNIPAM at 40 C. The quenching efficiency was much lower in thehydrogel indicating that the fluorescent probe was less accessible to the ni-tromethane in the cross-linked network. Linear polyNIPAM at 40 C can be presenteither as colloidally dispersed particles or as a macroscopic coacervate phase. Although the paper did not reveal the physical state of the linear polyNIPAMduring the fluorescent quenching studies, the results are interesting and this workindicates that fluorescent quenching may be a good tool for microgel characteriza-tion.

    A number of interesting techniques have been used to investigate macroscopicgels and may have a role in future microgel characterization. Positron annihilation

    w xlifetime spectroscopy was used by Sousa et al. 55 to measure the free volume inmacroscopic polyNIPAM-co-AM gels. The free volume decreased by nearly 30% when NIPAM was replaced with AM. The explanation for the difference is notclear.

    Raman microscopy has been used to image the pore structure in polyNIPAMmacrogels; however, the resolution of this technique seems to be too low to be of

    w xinterest in microgel characterization 56 .

  • 8/3/2019 microgel pelton

    13/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 33 13

    Small angle neutron scattering shows much promise for the characterization of microgels. The only microgel work published thus far is a study of the structure of

    w xbound surfactant 57 . A series of papers describing SANS characterization of w xcharged polyNIPAM macrogels has been published by Shibayama et al. 58,59 .

    Scattering originates from both spatial variations of gel structure and thermaldensity fluctuations

    3.2. Gel swelling

    3.2.1. Swelling theoryTemperature-sensitive swelling is the most spectacular property of polyNIPAM

    gels, and this has received a lot of attention for both micro and macro gels. Thedriving force and the equilibrium extent of swelling should be the same for a500-nm microgel as for a 5-cm macrogel. On the other hand, the dynamic aspectsare sensitive to the size of gels. Microgels achieve steady-state swelling in less than

    a second when the temperature is changed whereas macrogels can take a very longtime because shrinking of the exterior layer prevents water transport from theinterior.

    Although the thermodynamic theory of gel swelling is a classical subject, therehave been a number of recent theoretical articles aimed at the polyNIPAM system.

    w xLele et al. 60 applied an extended lattice theory that accounts for hydrogenbonding, to published polyNIPAM macrogel swelling data. Their calculationsindicate that the bound water content above the volume phase transition tempera-ture is 0.4 gr g. Their approach distinguishes between hydrogen-bonded bound water and other bound water. The same paper also shows that 1H-NMR static line width measurements change dramatically around the VPTT.

    w xPrausnitzs group 61 has applied semi-empirical extended Flory Huggins theoryto predict the volume phase transitions for polyNIPAM macrogels. Some of themodel parameters were obtained from the experimental properties of linearpolyNIPAM solutions. The predicted VPTT was about 1 C higher than the LCST

    w xof linear polyNIPAM. Choi et al. 34 employed a similar theory to predict the .swelling behavior of poly NIPAM-co- N -acryloylglycine-co-BA gels. The computed

    swelling vs. temperature curves was slightly broader than the experimental data.w xInomata et al. 62 interpreted polyNIPAM macrogel swelling by one of

    Prausnitzs earlier models. The gels were swollen by equilibration with aqueouspolyethylene glycol that allowed calculation of the osmotic pressure. The interpre-tation of polyNIPAM gel swelling behavior is aided by the observation that the BAcross-linker has little influence on the polyNIPAM r water interaction. Thus, thedriving force for swelling can be estimated from the properties of linear poly-NIPAM solutions, whereas the gel elasticity opposing swelling comes mainly fromthe network topology, which depends upon BA concentration.

    Semi-empirical extensions to Flory Huggins theory are available to predictmicrogel swelling. However, the number of required parameters is high. For

    w xexample, Choi et al. 34 requires nine for their treatment of copolymer gels, andnone of the theories seem to have dealt with the non-uniformity of real gels.

  • 8/3/2019 microgel pelton

    14/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 3314

    3.2.2. Swelling measurementsPolyNIPAM microgels can be prepared with a particle size distribution of

    .narrow dispersity in the submicron size range. Thus, dynamic light scattering DLSis a very convenient technique for measuring swelling, and there have been manymicrogel papers with DLS data. However, swollen particle diameters often ap-proach 1000 nm that is outside the sensitive range for DLS. Furthermore, swellingchanges are usually computed as volume changes, so the DLS diameters must becubed which limits the accuracy of the volumetric data.

    DLS measurements give particle size, whereas the concentrations of polymer and water in the gel are usually the required quantities. Therefore, more information isrequired to relate particle volume to polymer or water content of the microgel.Some have avoided this problem by reporting swelling volume or diameter ratioscalculated by comparing particle size to a reference particle size. However, if theaverage mass of polymer per gel particle is known, then the diameters are easilyconverted to absolute measures of swelling. A few approaches have been applied to

    w x

    the measurement of absolute water content in microgels. McPhee et al. 37centrifuged polyNIPAM microgels to give an ordered, packed bed of particles, which was iridescent, indicating ordered packing. The water content of the bed wascalculated from the bed mass before and after drying.

    Another approach is to measure the particle size at a temperature above theVPTT and assume a water content based on macrogel data, such as that given by

    w xDong and Hoffman 63 . For example, consider a microgel with a particle diameterof 100 nm at 45 C and a corresponding diameter of 200 nm at 20 C. If we assumethat the mass concentration of water in the particles at 45 C is 25%, then thecorresponding concentration of water in the particles at 20 C is 0.8912. Note thatthis calculation is based upon the assumptions that: the density of polyNIPAM is

    3 w x1269 kgr m 60 , the polyNIPAM density is independent of temperature over thisrange; and, that there is no excess volume of mixing. If the actual water content is

    .30% instead of 25% at 45 C i.e. an error of 20% , the corresponding water .content of the particles at 20 C is 0.8996. Thus, a large 20% error in the estimate

    of the water content at 45 C corresponds to only a 0.94% error in the estimated water content at 20 C. It seems therefore that this is a robust method forestimation of microgel water content in swollen gels.

    The water content in swollen polyNIPAM microgels has been determined byw xcombining the results of two light scattering techniques 23 . Intensity light scatter-

    ing was used to give a molecular weight of the polymer per particle and dynamiclight scattering to give the corresponding hydrodynamic volume per particle.

    The first published DLS data for polyNIPAM microgels as functions of tempera-w xture and electrolyte concentration were by Pelton et al. 64 . Their results, shown in

    Fig. 9, indicate that from 10 C to approximately 30 C the diameter decreases bylinearly by approximately one-quarter whereas from 30 to 35 C the diameterdecreases by another quarter in monovalent electrolyte. The VPTT is usually takenas the steepest portion of the diameter vs. temperature curve. In 0.1 M CaCl the2low temperature swelling was less than in more dilute KCl. Furthermore, theVPTT was lower in the presence of 0.1 M CaCl this is consistent with the work2

  • 8/3/2019 microgel pelton

    15/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 33 15

    Fig. 9. Average particle diameter of polyNIPAM microgel dispersions determined by dynamic lightw xscattering at three electrolyte concentrations. Data from Pelton et al. 64 .

    w xof Park and Hoffman 65 who have documented the influence of electrolytes onthe VPTT of macrogels. Above 35 C the average diameter in 0.1 M CaCl 2dramatically increased due to the aggregation of the shrunken polyNIPAM mi-crogel particles.

    The extent to which a gel can swell is limited by the presence of cross-links . w xusually based on BA see Fig. 1 for chemical structure . McPhee et al. 37

    published microgel diameter vs. temperature curves for a series of microgels with varying BA content an example of their results are shown in Fig. 10. As

    expected, the swelling ratios i.e. the diameters at 20 C divided by the diameters at.40 C decreased with increasing BA content. Note the differences in diameters at

    high temperature in Fig. 10 do not reflect the influence of BA on water content

    above the VPTT, but instead reflect the influence of BA on the particle nucleationduring latex synthesis. The lowest BA concentration gave the smallest particles athigh temperature and thus gave the highest particle concentration during gelsynthesis. The swelling data were interpreted by Florys gel theory. McPhee and

    w xcolleagues results were recently confirmed by Kratz and Eimer 25 and Oh andw xBae 66 who both neglected to mention the McPhee paper and some of the other

    earlier work.w xWus group in Hong Kong 23,67 employed dynamic light scattering to compare

    the temperature sensitivity of polyNIPAM microgels to linear polyNIPAM. VPTTfor the gel was slightly greater than the lower critical solution temperature forlinear polyNIPAM and the transition was less sharp for the microgel. The rate of

    change of microgel diameter with a step change in temperature was given in thesame paper. Approximately 10 ms were required for swelling or shrinking, however,the authors state that this reflects the time scale of the temperature change. Thus,microgel response seems to be very fast indeed.

    The same group has investigated the swelling dynamics of macroscopic polyw xNIPAM film formed from microgel latex 22 . The gels shrink rapidly and then the

    released water evaporates. In would be interesting to compare drying kinetics in

  • 8/3/2019 microgel pelton

    16/33

  • 8/3/2019 microgel pelton

    17/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 33 17

    Fig. 11. The moisture adsorption isotherm for freeze-dried microgel. Data calculated from results of w x Agbugba et al. 70 .

    w x

    point temperature or the VPTT to values greater than in water. Winnik et al. 71proposed that the phase behavior was because of the preferential adsorption of w xmethanol on the polymer chains. McPhee et al. 37 reported polyNIPAM microgel

    swelling vs. temperature curves for up to 35% methanol. These results arew xcompared with the LCST behavior of linear polyNIPAM 71 in Fig. 12. With the

    .exception of the highest methanol concentration 35% v r v the LCST of the linearpolymer corresponded to the temperature at which the microgel vs. diameter curvehad the largest negative slope. This comparison emphasized the general trend thatthe swelling behavior of the microgels is dominated by the interaction of thepolyNIPAM segments with the solvents.

    w xZimehl and Mielke 20,74 report isothermal swelling curves for polyNIPAMmicrogels as functions of the concentration of methanol, ethanol and the twoisomers of propanol. Both cationic and anionic microgels were studied. Theyshowed very interesting differences in the response of cationic and anionic mi-crogels to propanol. However, the microgels were prepared with surfactants that were not removed. The propanol results should be repeated with clean microgels.

    w xFig. 12. Comparison of cloud point temperatures for polyNIPAM 71 with VPTT for polyNIPAMw xmicrogels 37 .

  • 8/3/2019 microgel pelton

    18/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 3318

    w xCrowther and Vincent 27 reported polyNIPAM microgel swelling behavior asfunctions of temperature and the concentration of methanol, ethanol and isopro-panol. Insufficient data were presented to give VPTT values as functions of alcoholcontent nor was any attempt made to relate the results to the significant body of macrogel swelling results and theory.

    In summary, polyNIPAM gels show interesting behavior in mixed alcohol watersolutions. Like the water-swelling behavior, there is no evidence of any significantdifferences between the steady-state degree of swelling for microgels and macrogelsas functions of temperature and alcohol content. Of course the microgels willapproach steady-state swelling much more quickly.

    3.3. Surface acti ity

    .Linear poly N -isopropylacrylamide is surface active aqueous solutions have asurface tension of approximately 42 mJ r m2 and this value is not very

    w x

    temperature-dependent through the LCST of the polymer solution 6,75 . Thereare many examples of surfactants and water-soluble polymers that lower thesurface tension of water. On the other hand, there are only a few examples of surface-active particles and these are usually very hydrophobic particles suspended

    w xon a Langmuir trough 76,77 .PolyNIPAM microgels in water share the features of both colloidal particles and

    soluble polymers, so we were curious to know whether microgels were surfaceactive. Fig. 13 shows aqueous surface tensions for polyNIPAM microgel suspen-

    w xsions as functions of time 78 . A variety of gel morphologies was obtained by varying the amount BA cross-linker and the type polymerization batch or semi-

    .batch . The drawings beside the curves indicate the target morphology. In all casesthe microgels lowered the surface tension of water to values close to valuesobtained with linear polymer. The time required to achieve a steady state surfacetension increased with the particle cross-linking. Direct evidence for surfaceactivity was obtained by examining water surfaces in an environmental scanningelectron microscope. Fig. 14 shows one of the images obtained for the most

    w xcross-linked microgels 79 . Clearly, the particles formed an ordered array at theair r water interface.

    3.4. Rheological properties

    The volume of aqueous polyNIPAM gels is a sensitive function of temperature.Therefore, one would expect the rheological properties of polyNIPAM microgel

    w xsuspension to be sensitive to temperature. Kiminta et al. 21 have reported therheological behavior of homogeneous polyNIPAM microgels as functions of tem-perature, shear rate and concentration. The elastic modulus decreased by an orderof magnitude between 28 and 50 C reflecting the decrease in the effective volume

    w x w xfraction of dispersed gel phase. Lowe et al. 50 and Zimehl and Mielke 20,74have published rheological data that show the expected decrease in viscosity withincreasing temperature.

  • 8/3/2019 microgel pelton

    19/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 33 19

    Fig. 13. The influence of microgel morphology on surface tension vs. time. Data taken from Zhangw xand Pelton 78 .

    3.5. Electrical propertiesThe electrostatic properties of aqueous colloids influence colloidal stability and

    colloid interactions with dissolved polymers and surfaces. PolyNIPAM microgeldispersions have fascinating electrical properties due to the presence of covalentlybonded electrically charged groups originating from the initiator. McPhee et al.

    Fig. 14. Environmental Scanning Electron Micrograph of polyNIPAM microgels at the air r waterw xinterface 79 . With permission of American Chemical Society.

  • 8/3/2019 microgel pelton

    20/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 3320

    Fig. 15. The influence of temperature on the electrophoretic mobility of polyNIPAM microgel latex.w x . y 1 6 .Data taken from Pelton et al. 64 . Parameters: Eq. 1 e s 8.8 = 10 coulomb; Eq. 2 N s 4.5

    .mmol, f s 6 3 nm; and, Eq. 3 N s 4.5 mmol, s 0.5r nm.

    w x37 reported charge contents for a persulfate initiated polyNIPAM microgel to be3.8 eq. r g of which three-quarters were carboxyl groups and one-quarter weresulfate groups. These charge contents are two orders of magnitude lower than atypical surfactant-free polystyrene latex because the NIPAM propagates to muchlonger chain lengths than styrene so there are many less chain ends bearinginitiator fragments.

    Fig. 15 shows the first reported electrophoretic mobility data for polyNIPAMlatex as a function of temperature and electrolyte concentration. At room tempera-ture the mobility is nearly zero reflecting the low charge density in the particles,

    whereas above the VPTT the mobility increases by an order of magnitude. Sincethe electrical charges are covalently bonded, the total charges per particle areconstant. On the other hand, the density of charges increase through the VPTTbecause the charges must be distributed through 1 r 10 the volume available belowthe VPTT.

    A few efforts have been made to model the temperature-dependent elec-trophoretic mobility results such as those shown in Fig. 15. A simple model,

    . w xsummarized by Eq. 1 64 , relates the electrophoretic mobility of the microgels, ,to particle radius, r , and the number of charges per particle, . The otherparameters are: e is the charge of an electron; is the viscosity; and is theDebye Huckel parameter. This equation was based on the assumptions that: all

    the charges are located on the exterior surface of the microgel; the charge densityis related to the potential by the Helmholtz equation s ; the surface o r potential equals the zeta potential; and, the electrophoretic mobility is related tothe zeta potential by the Smoluchowski equation. If the microgel diameter isknown as a function of temperature, then only , the total number of charges per

    .particle is needed to calculate the electrophoretic mobility with Eq. 1 . The .mobility vs. temperature data were fitted by Eq. 1 using the corresponding gel

  • 8/3/2019 microgel pelton

    21/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 33 21

    .diameter r temperature data Fig. 9 . The computed curve is compared with theexperimental data in Fig. 15. Note, that the noise in the computed curves arisesfrom the noise in the gel diameter vs. temperature data used in the calculations.The fitted value of , expressed as a specific charge content based on drypolyNIPAM was 36 eq. r g which is approximately 10 times higher than McPheesw x .37 experimental value of 3.8 eq. r g. Therefore, Eq. 1 gives a reasonableprediction of experimental behavior, however, the predicted gel particle chargecontents are too high.

    y e .s 124 r

    .The applicability of Eq. 1 is questionable since there is no reason to assume thatall the electrical charges are located on the exterior surface of the microgel

    particles, particularly in the highly swollen state at low temperature. Two more .realistic models have been published and are now compared. Eq. 2 was given byw xBuscall et al. 80 to describe the electrophoretic mobility of a polyelectrolyte

    particle based on the theory of Hermans and Fujita, where f is the friction factor'per repeat unit, b s fN r and N is the number of electrically charge groups per

    unit volume.

    y e 3 3 q 3 2 b q 2 b2 q b3 .s 23 2 f 3 q 3 b

    .Eq. 2 was fitted to our data assuming a friction factor calculated from the Stokesequation for a sphere of 3 nm, the water content of the polyNIPAM gel particles was 20% by mass. Experimental diameter vs. temperature results shown in Fig. 9 were used to calculate the temperature-dependent volumetric charge densitytogether with an assumed specific charge content of 4.5 eq. r g. The predictedelectrophoretic mobility vs. temperature curve is compared with the experimental

    .data in Fig. 15. The mobility values computed with Eq. 2 were a little lower than .the data at low temperature and higher than the data at high temperature. Eq. 2

    .predicted the same mobilities as did Eq. 1 up to 32 C, the VPTT.w xOshima et al. 81 presented an alternative gel layer model for electrophoresis of

    polyNIPAM microgels. Their model is described by the following equations.

    . . r q r zen o r o m Don .s q 32 . .1r q 1r m

    where N is the volumetric concentration of charge, is a softness parameter withunits of meter, is the valency of the counter ions, n is the supporting electrolyteconcentration, is the Debye parameter, and the remaining terms are given by the

  • 8/3/2019 microgel pelton

    22/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 3322

    following equations

    1r 2 1r 22 2 kT zN zN 2 n zN .s ln q q 1 q 1 y q 1 40

    5 5 / / e 2 n 2 n zN 2 n 01r 22 kT zN zN .s ln q q 1 5DON 5 / e 2 n 2 N

    1r 42 zN .s 1 q 6 m /2 n

    . Although this set of equations is a little more complicated than Eq. 2 , this is alsoa two parameter model, N and . As before, N was calculated on the assumptionthat the specific charge content of the gel was 4.5 eq. r g whereas wasconsidered to be an adjustable, temperature-independent parameter. The curve

    . 8predicted by Eq. 3 is also shown in Fig. 15 assuming s 5 = 10 r m. This modelgave a slightly better prediction of the experimental results than did the previous

    . .models. Note, Eqs. 1 and 3 gave very similar predictions about the VPTT.w xNabzar et al. 32 reported electrophoretic mobilities as functions of electrolyte

    concentration for polystyrene-core, polyNIPAM-co-aminoethylmethacrylate hydro-chloride shell latexes. They claim that Oshimas theory was unable to fit theirresults for a single pair of values for N and . Their data are compared with the

    .predictions of Eq. 3 in Fig. 16. Neither theoretical curve fits the experimentalresults.

    In summary, polyNIPAM microgels provide an extensive database of elec-trophoretic mobility values as functions of temperature, and electrolyte and surfac-

    Fig. 16. Comparison of experimental and theoretical mobility data as functions of electrolyte. The dataw x . .taken from Nabzar et al. 32 Latex DD4 Fig. 6 and the calculations are based on Eq. 3 . N s 0.52

    mol r l.

  • 8/3/2019 microgel pelton

    23/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 33 23

    tant concentration. It seems unlikely that electrophoresis will provide detailedmicrostructure information about the gels. Instead, gel data seem to be a toughtest for electrokinetic models.

    3.6. Gel surfactant interactions

    The temperature sensitivity of polyNIPAM and related materials arises from thefact that the polymer has considerable hydrophobic character. This same character-

    w xistic also evokes strong interactions with surfactants in water. Eliassaf 82 was thefirst to report the interaction of aqueous surfactant with polyNIPAM. Schild and

    w x.Tirrell see references in Schild 6 have investigated the influence of surfactantchain length. In general, both cationic and anionic surfactants bind to polyNIPAM whereas non-ionic polymers seem not to. The cloud point of linear polyNIPAMincreases with anionic surfactant binding but is less influenced by cationic surfac-tants.

    Surfactants also bind to polyNIPAM microgels. Fig. 17 shows a binding isothermw xfor SDS to polyNIPAM microgel 58 . One of the advantages of studying surfactantbinding to microgels compared with binding to linear polymers is that microgels areeasily separated by centrifugation which, in turn, allows direct analytical determi-nation of the amount of bound surfactant. Of course, less direct methods such as

    w x w x w xconductivity 15 , rheology 83 , and NMR 22 also give information about surfac-tant binding.

    Microgel swelling dramatically increases with bound surfactant. Fig. 18 shows . w xmicrogel swelling as functions of sodium dodecyl sulfate SDS concentration 30

    w xand these results have been verified by others 22,23,42 . At all temperatures SDSbinding gave increased swelling and also tended to raise the VPTT. Careful work with state-of-the-art DLS measurements indicates two inflection points whenpolyNIPAM microgel radius is plotted as a function of temperature in the presence

    w xof SDS 23 . The dramatic influence of surfactant on the VPTT may limit the utility

    Fig. 17. Binding isotherm for SDS onto polyNIPAM microgels at 23 C. Data taken from Mears et al.w x57 .

  • 8/3/2019 microgel pelton

    24/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 3324

    w xFig. 18. The influence of SDS on the swelling of polyNIPAM microgels 30 . With permission of the American Chemical Society.

    of temperature-sensitive gels in formulated products with high surfactant concen-trations.

    In general, discussions of the interactions of polyNIPAM interactions with SDShave drawn on the body of published work on SDS binding to polyethyleneoxidew x84 . In particular, it is often assumed that polymer-bound surfactant is present as

    w x w xrather large micelles 22 . Mears et al. 57 reported neutron scattering studies in which the polyNIPAM gel was contrast matched with the water r D O mixture and2scattering off deuterated SDS was observed. Surprisingly, the neutron resultsindicated that the bound surfactant was present as very small aggregates containingless than six surfactant molecules. Thus, polymer bound micelles do not appear toexist in the polyNIPAM microgel system it would be of interest to determine if this conclusion also holds for linear polyNIPAM.

    Finally, in very recent work we have reported isothermal calorimetric titrationw xresults 85 . In this technique, SDS is slowly added to a microgel suspension while

    recording the heat effect. Fig. 19 shows the heat effect with SDS addition tomicrogel above and below the VPTC. The binding of SDS to polyNIPAM microgel was endothermic below the VPTC and exothermic above it. Heat effects wereobserved at even the lowest SDS addition levels which indicates that the critical

    .aggregation concentration CAC for SDS binding to polyNIPAM is below 17 mmolor there is no CAC.

    3.7. Colloidal stability

    Temperature-sensitive colloidal stability has many potential applications includ-ing coating, papermaking, selective wetting, etc. For example, latex rubber glovesare made by dipping a mold into a concentrated latex suspension. The mold iscoated with coagulant that induces latex deposition and coalescence. A potentialproblem with this technology is that residual coagulant can remain inside the glove.On the other hand, if the rubber latex had temperature-sensitive colloidal stability,

  • 8/3/2019 microgel pelton

    25/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 33 25

    Fig. 19. Isothermal titration of polyNIPAM with SDS below and above the VPTT. Data taken fromw xWang et al. 85 .

    rubber gloves could be made by dipping hot glove molds into a cold latex bath.w xPolyNIPAM coated latexes exhibit temperature dependent colloidal stability 40 .

    Aqueous colloids are considered stable if they do not aggregate in the time scaleof interest. Colloid stability depends upon the balance of van der Waals attraction, which causes aggregation, and steric or electrostatic forces that oppose aggrega-tion. Below the VFTT, polyNIPAM microgels are colloidally stable at low and highelectrolyte concentration, whereas above the VPTT, the microgels are only stable

    w xin low electrolyte concentrations. For example, Pelton and Chibante 4 showedthat microgel suspensions in 0.1 M CaCl were colloidally stable until the tempera-2

    .ture was raised above 34 C see Fig. 9 . This behavior is now explained by standardconcepts of colloid science.

    .Below the VPTT the particles are swollen and thus mainly water see Fig. 2 .Under these conditions, the van der Waals attractive forces are relatively small.Furthermore, it seems reasonable to propose that polymer tails extend from the gelstructure to act as steric stabilizers further enhancing colloid stability. Note that for

    surfactant free polyNIPAM microgels, the electrophoretic mobility is low see Fig.. w x15 64 indicating that electrostatic repulsion does not contribute to colloidal

    stability below the VPTT. At elevated temperatures the water content of the gels is reduced giving a higher

    density and thus a greater Hamaker constant than at low temperature. The greaterthe Hamaker constant, the greater are the attractive van de Waals forces tendingto aggregate the gels. Above the VPTT any polyNIPAM tails on the microgelsurface will be collapsed upon the particle surface and thus not contributing tocolloidal stability. On the other hand, at low ionic strength the electrophoretic

    .mobility of polyNIPAM microgels is high see Fig. 15 indicating that electrostaticstabilization is operative. CaCl induces coagulation above the VPTT because the2electrical double layer is compressed giving low electrostatic interactions. Theelectrical charges in surfactant-free polyNIPAM are sulfate and carboxyl groupsthat originate from the ionic free radical polymerization initiator. Presumably,

  • 8/3/2019 microgel pelton

    26/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 3326

    microgels prepared from non-ionic initiators would be colloidally unstable abovethe PVTT without electrolyte addition. This remains to be proven.

    PolyNIPAM colloidal microgels exhibit extreme colloid stability below the VPTT,w xhowever they can be aggregated by polymer addition. Snowden and Vincent 18

    showed that sodium polystyrene sulfonate gave depletion flocculation. Similarly,polyNIPAM- shell polystyrene-core latexes can be flocculated by very high molecu-

    lar weight cationic polyacrylamide copolymers Cong and Pelton, unreported.results .

    w xIslam et al. 31 reported the temperature-sensitive heteroflocculation of anionicpolystyrene latex with cationic polyNIPAM microgel the mixed dispersion wascolloidally stable at low temperature and it aggregated above the VPTT. Anunexpected observation was that flocs formed at high temperatures could beredispersed at low temperature only when the pH was 10 or greater. This could bedue to a decrease in electrostatic attraction either induced by increased ionicstrength or, more likely, to the alkaline hydrolysis of cationic amidine groups.

    w xThere are many parallels between this work and that of Deng et al. 39 , whoreported the aggregation of anionic colloidal titanium dioxide with cationic linearcopolymers of polyNIPAM. Above the cloud point temperature, the copolymer waspresent as phase separated colloidal particles with an average diameter of 87 nm.Like the more recent Islam and Snowden results, cationic polyNIPAM particlesheteroflocculated with the anionic TiO . Below the cloud point, dilute soluble2cationic polyNIPAM copolymer also induced aggregation of the polystyrene latexpresumably by a bridging mechanism. This behavior is in contrast to the behavior

    w xof cationic microgels which did not aggregate latex below the VPTT 31 . Pre-sumably the water swollen cationic microgel had too low a cationic content to giveadsorption and thus bridging flocculation below the VPTT.

    w xZhu and Napper 42,43 used dynamic light scattering to study the aggregationkinetics of polystyrene-core polyNIPAM-shell latex aggregation. Aggregation wasinitiated by either increasing temperature at constant ionic strength or by increas-ing ionic strength at constant temperature. The fractal dimensions of the aggre-gates were estimated from the slopes of log log plots of average aggregate size vs.time. The behaviors of PEG and polyNIPAM stabilized polystyrene were com-pared. For PEG, the fractal dimension of the aggregated latex was independent of the electrolyte concentration whereas the polyNIPAM latex showed an increase infloc fractal dimension with electrolyte concentration. This was explained in termsof the attractive interactions between polyNIPAM globules on the latex surface.

    w xPolyNIPAM often displays specific ion effects. Zhu and Napper 43 showed that

    the electrolyte concentration required to generate a given floc fractal dimensionfollowed the trend NaCl - NaBr - NaNO - NaI - NaSCN. According to the3authors, above the VPTT flocculation is induced by an attraction between polyNIPAM shells on neighboring particles driven by the release of bound waterpresent below the VPTT. Specific ion effects arise from the ions changing theamount of bound water released with heating. The authors supported thesearguments by illustrating a correlation between the salt concentration required to

  • 8/3/2019 microgel pelton

    27/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 33 27

    give a specific fractal dimension to relative effects of the same series of salts onaqueous solution viscosity.

    w xNabzar et al. 32 recently reported the aggregation kinetics of polystyrene-core,polyNIPAM-co-aminoethyl methacrylate-shell latexes. Stability ratios, based onturbidity measurements were presented as functions of electrolyte concentrationboth below and above the VPTT. In general, the results were consistent withprevious work and are consistent with the expected behavior of a electro-stericallystabilized colloid below the VPTT and an electrostatically stabilized colloid at hightemperature. Combined Hamaker constants were calculated from the coagulationkinetics using classical models. Modern approaches to obtaining Hamaker con-

    .stants or functions involve the application of Lifshitz theory to spectroscopic data.No such analysis has been done for polyNIPAM as a function of temperature or water content.

    4. Microgel applications

    4.1. Biotechnology

    The first reported investigation of the interaction of biological molecules withw xpolyNIPAM microgels was by Kawaguchi et al. 19 who described the tempera-

    ture-dependent sorption and desorption of human globulin. Fig. 20 shows thatthe maximum protein binding occurs at low pH and 40 C which is above the VPTT.Presumably binding was driven by hydrophobic interactions between the proteinand isopropyl groups on the gel. When sorption occurred at 40 C, desorption couldbe induced by lowering the temperature to 25 C. However, the extent of desorptiondepended on the length of time used in the sorption step. The longer the sorptiontime at high temperature, the lower the amount of protein which desorbed at a lowtemperature. This possibly indicates that the protein slowly entered the dehydratedgel structure above the VPTT.

    In a subsequent article the same group compared the interactions of fourw xproteins with polyNIPAM microgel and polystyrene latex 87 . Protein binding to

    polyNIPAM microgel was higher at 40 C than at 25 C. However, all four proteinsshowed a higher degree of adsorption onto polystyrene latex than on polyNIPAMat 40 C. The activity of peroxidase desorbed from polyNIPAM was greater thanthat of the enzyme desorbed from polystyrene. Thus polyNIPAM had a lowertendency than polystyrene to denature the enzyme.

    The studies, summarized above involved the physical sorption of proteins topolyNIPAM microgel. A number of recent reports have addressed the properties of

    w xenzymes covalently bonded to microgels. Shiroya et al. 33 grafted trypsin andperoxidase to polyNIPAM microgels. The enzyme activity went down when temper-ature was raised above the VPTT. This was explained by decreased substratediffusion rates and by trapping of the enzyme in the surface layer. On the otherhand, temperature-independent enzyme activities were obtained when the enzymes

  • 8/3/2019 microgel pelton

    28/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 3328

    Fig. 20. Influence of pH and temperature on human gamma globulin adsorption onto polyNIPAMw xmicrogels. Adapted from Kawaguchi et al. 19 .

    were attached to the microgels via PEG spacers. Presumably the spacers isolate theenzymes from the temperature-sensitive polyNIPAM domains.

    A particularly clever approach to obtaining temperature-sensitive enzyme activ-w xity was described by Yasui et al. 48 who attached trypsin to latex surfaces with

    .polyNIPAM linear chains MW s 2500 or 11 000 . PolyNIPAM chains not bearingenzymes surrounded the tethered enzymes. The system had two characteristictemperatures. The surface polyNIPAM chains underwent a coil-to-globule transi-tion at ; 30 C whereas the enzyme terminated chains had a transition tempera-ture of 38 C. The main observation was that enzyme activity increased when thetemperature was raised above 30 C. The explanation is illustrated in Fig. 21. At lowtemperature the enzyme is surrounded by polyNIPAM which inhibits substratetransport, whereas above 30 C the enzyme-free surface polyNIPAM chains arepresent as compact globules leaving the enzyme terminated chains to extend intosolution with unrestricted access to substrate which gave rapid kinetics.

    w xGranulocytes are cells that attack foreign bodies. Achiha et al. 88 compared the .interaction of granulocytes with micro spheres latexes including polystyrene,

    polystyrene-core polyNIPAM-shell, polyNIPAM microgel, and acrylamide microgelparticles. The polystyrene spheres interacted strongly at both 20 and 37 C asevidenced by oxygen consumption, strong cell r latex adhesion leading to phagocyto-

    .sis engulfment of particles . By contrast the hydrophilic acrylamide-based mi-crogels showed little interaction at both temperatures. PolyNIPAM microgels orcore shell spheres displayed temperature-sensitive interaction with the granulo-cytes. Below the VPTT the polyNIPAM particles were relatively inert, similar tothe acrylamide based gels, whereas at 37 C the polyNIPAM spheres interactedstrongly with the granulocytes.

    One of the most promising applications of polyNIPAM microgels is as supportmaterials for biological testing. Pichots group has described the coupling of

    . w xoligodeoxyribonucleotides ODN to polyNIPAM microgels 89 . The microgels

  • 8/3/2019 microgel pelton

    29/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 33 29

    Fig. 21. Schematic illustration of temperature dependent enzyme activity for polyNIPAM tetheredenzymes. A key feature of the effect is that the cloud point temperature of enzyme terminated

    w xpolyNIPAM is 38 C compared with 32 C for polyNIPAM chains without terminal enzyme 48 .

    have an affinity for specific DNA that is determined by the ODN structure. Highersensitivities were obtained with polyNIPAM supports than with polystyrene. Ironi-cally this application does not directly employ the temperature sensitivity of themicrogels. Instead, the key property of the particles is their hydrophilic r hydro-phobic nature coupled with the fact that they can be prepared as uniform spheres.

    4.2. Controlled uptake and release

    The temperature-sensitive uptake or release of chemicals has been one of themost intensively investigated potential applications for both microgels andmacrogels. As discussed in earlier sections, microgels, unlike macrogels, exhibit very rapid swelling or shrinking in response to temperature change. Thus, tempera-ture-triggered drug or chemical release is possible.

    w xSnowden 90 showed that fluorescein labeled dextran bound to polyNIPAM

    microgels below the VPTT and was subsequently released when the temperature . was raised above the VPTT. Dextran has no affinity for poly ethylene glycol so it

    seems unlikely that it would interact with polyNIPAM. Thus, the dextran bindingpossibly was due to hydrophobic interactions between the fluorescein label and

    w xpolyNIPAM. In a subsequent publication, Snowden and Booty 91 measured thebinding of nitrate ions onto cationic microgels and there was no release of nitrateupon heating. Also investigated was the sorption of acetylsalicylic acid and

  • 8/3/2019 microgel pelton

    30/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 3330

    aluminum citrate onto anionic polyNIPAM microgels. Two-thirds of the materialbound at room temperature was removed by heating to 40 C. No mechanism ormodeling was given to explain the results.

    One would expect the diffusion rate of small molecules into or out of polyNIPAM microgels should be lowest at temperatures above the VPTT because of the relatively high concentration of polymer chains. This is illustrated in the results

    w xof Kato et al. 92 who studied the diffusion rate of ascorbic acid into compositeparticles consisting of a polyNIPAM shell around a core of cytochrome c embed-ded in a matrix of polystyrene-co-2-ethylhexyl methacrylate. The initial diffusionrate of ascorbic acid into the microgel spheres is shown in Fig. 22 as a function of temperature. The rates were much faster below the VPTT of the shell. Further-more, transport was slower when the polyNIPAM shell was cross-linked.

    In summary, the mass transport rate of solutes through polyNIPAM microgel will be faster below the VPTT where the gel has a high water content and a loweffective viscosity. On the other hand, solutes that undergo hydrophobic bonding will be released above the VPTT because solute r NIPAM interactions are replacedby NIPAM r NIPAM interactions. A note of caution-solute binding usually modifiesthe VPTT.

    4.3. Other potential applications

    It seems reasonable to expect some unique applications for uniform, tempera-ture-sensitive microgels. Some possibilities are now considered.

    Fig. 22. The initial rate of ascorbic acid transport into polyNIPAM-coated cytochrome c particles as aw xfunction of temperature. Data taken from Kato et al. 92 .

  • 8/3/2019 microgel pelton

    31/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 33 31

    w xSnowden et al. 93 have a European patent application for the use of polyNIPAM microgels to modify the permeability of oil reservoirs. In this applicationthe microgels aggregate to form precipitates at temperatures above the VPTT.

    w xChen et al. 47 described the preparation of Pt catalysts supported onpolystyrene-core, polyNIPAM-shell particles. The polyNIPAM chains retarded themigration and agglomeration of the 1.5-nm Pt particles. Because of this effect,catalysts were more long-lived than those supported on bare polystyrene latex.Furthermore, the catalytic activity was temperature-dependent. Reaction rates were lower above the LCST of the polyNIPAM shell. Presumably, the collapsed

    w xshell inhibited interactions between the catalyst and substrate 94 .Finally, Ashers group has shown that arrays of temperature-sensitive microgels,

    such as shown in Fig. 4 give temperature-sensitive Bragg diffraction which mayw xhave applications in non-linear optics 24 .

    5. Closing remarks

    The fascinating properties of temperature-sensitive microgels are likely to cont-inue to generate activity in the literature. Indeed, there are a few obviousopportunities for further work. There have been few attempts to characterize themorphology distributions within and among particles. Morphological characteriza-tion is likely to lead to semi-batch and other techniques for controlling gelmorphology to give more exotic particle morphologies. However, in my view thebiggest opportunity is the identification of new applications for temperature-sensi-tive microgels. Indeed, a motivation for writing this review was the belief that itmight stimulate new applications.

    Acknowledgements

    My work in this area has been performed by a talented group of students andpostdoctoral fellows. I am indebted to Karinne Chan, Philip Chibante, Yulin Deng,Wayne McPhee, Sara Mears, Michael Tam, Shirley Wu and Ju Zhang.

    Referencesw x .1 M.S. Wolfe, C. Scopazzi, J. Colloid Interface Sci. 133 1989 265 277.w x .2 T.G. Park, A.S. Hoffman, J. Polym. Sci. 30 1992 505 507.w x .3 F. Hoshino, T. Fugimoto, H. Kawaguchi, Y. Ohtsuka, Polymer J. 19 1987 241 247.w x .4 R.H. Pelton, P. Chibante, Colloids Surf. 20 1986 247 256.w x . .5 M. Heskins, J.E. Guillet, J. Macromol. Sci. Chem. A2 8 1968 1441 1455.w x .6 H.G. Schild, Prog. Polym. Sci. 17 1992 163 249.w x .7 Y. Osada, S.B. Ross-Murphy, Sci. Am. May 1993 82 87.w x . .8 I.Y. Galaev, Russian Chem. Rev. 64 5 1995 471 489.w x .9 R. Dagani, C and EN, June 9 1997 26 30.

    w x .10 R. Pelton, X. Wu, W. McPhee, K.C. Tam, in: J.W. Goodwin, R. Buscall Eds. , Colloidal PolymerParticles, Academic Press, 1995, pp. 81 99.

  • 8/3/2019 microgel pelton

    32/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 3332

    w x .11 H. Kawaguchi, in: M. Yalpani Ed. , Biomedical Functions and Biotechnology of Natural and Artificial Polymers, ATL Press, 1996, pp. 157 168.

    w x .12 H. Kawaguchi, in: R. Arshady Ed. , Microbeads, Microcapsules and Liposomes, STN Books,London, 1998, pp. 233 248.

    w x13 M.J. Murray, M.J. Snowdon, The preparation, characterization, and applications of colloidal .microgels, Adv. Colloid Interface Sci. 54 1995 73 91.

    w x .14 W.C. Wooten, R.B. Blanton, H.W. Coover, Jr., J. Polym. Sci. XXV 1957 403 412.w x .15 X. Wu, R.H. Pelton, A.E. Hamielec, D.R. Woods, W. McPhee, Colloid Polym. Sci. 272 1994

    467 477.w x .16 G. Bokias, A. Durand, D. Hourdet, Macromol. Chem. Phys. 199 1998 1387 1392.w x .17 J.W. Goodwin, R.H. Ottewill, R. Pelton, G. Vianello, D. Yates, Br. Polym. J. 10 1978 173 180.w x .18 M.J. Snowden, B. Vincent, J. Chem. Soc, Chem. Commun. 1992 1103 1105.w x .19 H. Kawaguchi, K. Fujimoto, Y. Mizuhara, Colloid Polym. Sci. 270 1992 53 57.w x .20 M. Mielke, R. Zimehl, Prog. Colloid Polym. Sci. 111 1998 74 77.

    w x . .21 D.M. Ole Kiminta, P.F. Luckham, S. Lenon, Polymer 36 25 1995 4827 4831.w x .22 C. Wu, S. Zhou, J. Polym. Sci., Part B Polym. Phys. 34 1996 1597 1604.w x .23 C. Wu, S. Zhou, S.C.F. Au-Yeung, S. Jiang, Die Ang. Makro. Chemie 240 1996 123 136.w x .24 J.M. Weissman, H.B. Sunkara, A.S. Tse, S.A. Asher, Science 274 1996 950 959.w x .25 K. Kratz, W. Eimer, Ber. Bunsenges Phys. Chem. 102 1998 848 854.w x .26 G. Zhou, A. Elaissari, Th. Delair, C. Pichot, Colloid Polym. Sci. 276 1998 12, 1131 1139.

    w x .27 H.M. Crowther, B. Vincent, Colloid Polym. Sci. 276 1998 46 51.w x28 Gilbert, Emulsion Polymerization: A Mechanistic Approach, Academic Press, London, 1995.w x .29 T. Gotoh, Y. Nakatani, S. Sakahara, J. Appl. Polym. Sci. 69 1998 895 906.w x .30 K.C. Tam, S. Ragaram, R.H. Pelton, Langmuir 10 1994 418 422.w x .31 A.M. Islam, B.Z. Chowdhry, M.J. Snowden, J. Phys. Chem. 99 1995 14205 14206.w x .32 L. Nabzar, D. Duracher, A. Elaisssari, G. Chauveteau, C. Pichot, Langmuir 14 1998 5062 5069.w x .33 T. Shiroya, N. Tamura, M. Yasui, K. Fujimoto, H. Kawaguchi, Colloids Surf. B 4 1995 267 274.w x .34 H.S. Choi, J.M. Kim, K. Lee, Y.C. Bae, J. Appl. Polym. Sci. 69 1998 799 806.w x .35 M.J. Snowden, B.Z. Chowdhry, B. Vincent, G.E. Morris, J. Chem. Soc, Faraday Trans. 92 24

    .1996 5013 5016.w x .36 F. Meunier, A. Elaissari, C. Pichot, Polym. Advanced Technol. 6 1994 489 496.w x .37 W. McPhee, K.C. Tam, R. Pelton, J. Colloid Interface Sci. 156 1993 24 30.w x .38 K. Chan, R. Pelton, J. Zhang, Langmuir 15 1999 4018 4020.w x .39 Y. Deng, H. Xiao, R. Pelton, J. Colloid Interface Sci. 179 1996 188 193.w x .40 R.H. Pelton, J. Polym. Sci. 26 1988 9 18.w x41 K. Makino, S. Yamamoto, K. Fujimoto, H. Kawaguchi, H. Oshima, J. Colloid Interface Sci. 166

    .1994 251 258.w x . .42 P.W. Zhu, D.H. Napper, Phys. Rev. E. 50 2 1994 1360 1366.w x .43 P.W. Zhu, D.H. Napper, Colloids Surf. A 98 1995 93 106.w x .44 D. Duracher, F. Sauzedde, A. Elaissari, A. Perrin, C. Pichot, Colloid Polym. Sci. 276 1998

    219 231.w x .45 D. Duracher, F. Sauzedde, A. Elaissari, C. Pichot, L. Nabzar, Colloid Polym. Sci. 276 1998 10,

    920 929.w x46 S. Takeuchi, M. Oike, C. Kowitz, C. Shimasaki, K. Hasegawa, H. Kitano, Makromol. Chem. 194

    .1993 551 558.w x .47 C.W. Chen, M.Q. Chen, T. Serizawa, M. Akashi, Chem. Commun. 1998 831 832.w x .48 M. Yasui, T. Shiroya, K. Fujimoto, H. Kawaguchi, Colloids Surf. B: Biointerfaces 8 1997

    311 319.w x . .49 D. Duracher, A. Elaissari, C. Pichot, J. Polym. Sci., Part A Polym. Chem. 27 12 1999

    1823 1837.w x .50 J.S. Lowe, B.Z. Chowdhry, J. Parsonage, M.J. Snowden, Polymer 39 1998 1207 1212.w x .51 H. Kawaguchi, F. Hoshino, Y. Ohtsuka, Chem. Rapid Commun. 7 1986 109 114.w x .52 F. Hoshino, M. Sakai, H. Kawaguchi, Y. Ohtsuka, Polym. J. 19 1987 383 387.w x .53 T. Serizawa, M.Q. Chen, M. Akashi, J. Polym. Sci. Part A: Polym. Chem. 36 1998 2581 2587.

  • 8/3/2019 microgel pelton

    33/33

    ( ) R. Pelton r Ad ances in Colloid and Interface Science 85 2000 1 33 33

    w x .54 K. Fujimoto, Y. Nakajima, M. Kashiwabara, H. Kawaguchi, Polym. Int. 30 1993 237 241.w x .55 R.G. Sousa, R.F.S. Freitas, W.F. Magalhaes, Polymer 39 1998 3815 3819.w x .56 R. Appel, W. Xu, T.W. Zerda, Z. Hu, Macromolecules 31 1998 5071 5074.w x .57 S.J. Mears, Y. Deng, T. Cosgrove, R. Pelton, Langmuir 13 1997 1901 1906.w x .58 M. Shibayama, K. Kawakubo, F. Ikkai, M. Imai, Macromolecules 31 1998 2586 2592.w x .59 F. Ikkai, M. Shibayama, C.C. Han, Macromolecules 31 1998 3275 3281.w x .60 A.K. Lele, M.M. Hirve, M.V. Badiger, R.A. Mashelkar, Macromolecules 30 1997 157 159.w x . .61 T. Hino, J.M. Prausnitz, Polymer 39 14 1998 3279 3283.w x .62 H. Inomata, K. Nagahama, S. Saito, Macromolecules 27 1994 6459 6464.w x .63 L. Dong, A.S. Hoffman, J. Controlled Release 30 1990 21 31.w x .64 R.H. Pelton, H.M. Pelton, A. Morphesis, R.L. Rowell, Langmuir 5 1989 816 818.w x .65 T.G. Park, A.S. Hoffman, Macromolecules 26 1993 5045 5048.w x .66 K.S. Oh, Y.C. Bae, J. Appl. Polym. Sci. 69 1998 109 114.w x .67 C. Wu, Polymer 39 1998 4609 4619.w x .68 L. Dong, A.S. Hoffman, J. Controlled Release 15 1991 141 152.w x .69 H.G. Schild, D.A. Tirrell, J. Phys. Chem. 94 1990 4352 4356.w x .70 C.B. Agbugba, B.A. Hendriksen, B.Z. Chowdhry, M.J. Snowden, Colloids Surf. A 137 1998

    155 164.w x .71 F.M. Winnik, H. Ringsdorf, J. Venzmer, Macromolecules 23 1990 2615 2616.w x .72 H.G. Schild, M. Muthukumar, D.A. Tirrell, Macromolecules 24 1991 948 952.

    w x73 F.M. Winnik, M.F. Ottaviani, S.H. Bossmann, M. Garcia-Garibay, N.J. Turro, Macromolecules 25 .1992 6007 6017.

    w x .74 M. Mielke, R. Zimehl, Ber. Bunsenges. Phys. Chem. 102 1998 1 7.w x .75 J. Zhang, R. Pelton, Langmuir 12 1996 1611 1612.w x .76 A. Doroszkowski, R. Lambourne, J. Polym. Sci. Part C 34 1971 253 264.w x .77 M.J. Garvey, D. Mitchell, A.L. Smith, Colloid Polym. Sci. 257 1979 70 74.w x .78 J. Zhang , R. Pelton, Colloids Surf. 1999 to appearw x .79 J. Zhang, R. Pelton, Langmuir 1999 in press.w x .80 R. Buscall, T. Corner, I.J. McGowan, in: Th Tadros Ed. , Effect of Polymers on Dispersion

    Properties, Academic Press, New York, 1982, p. 379.w x81 H. Oshima, K. Makino, T. Kato, K. Fujimoto, T. Kondo, H. Kawaguchi, J. Colloid Interface Sci.

    .159 1993 512 514.w x .82 J. Eliassaf, J. Appl. Polym. Sci. 22 1978 873 874.w x .83 K.C. Tam, X.Y. Wu, R.H. Pelton, J. Polym. Sci., Part A Polym. Chem. 31 1993 963 969.w x .84 E.D. Goddard, Colloids Surf. 19 1986 255 300.w x .85 G. Wang, R. Pelton, J. Zhang, Colloids Surf. 1999 unpublished.w x .86 J.W. Goodwin, R.H. Ottewill, R. Pelton, Colloid Polym. Sci. 257 1979 61 69.w x .87 K. Fujimoto, Y. Mizuhara, N. Tamura, H. Kawaguchi, J. Intelligent Mater. Syst. Struct. 4 1993

    184 189.w x .88 K. Achiha, R. Ojima, Y. Kasuya, K. Fujimoto, H. Kawaguchi, Polym. Adv. Technol. 6 1995

    534 540.w x .89 Th. Delair, F. Meunier, A. Elaissari, M.H. Charles, C. Pichot, Colloids Surf. A: 153 1999

    341 353.w x .90 M.J. Snowden, J. Chem. Soc., Chem. Commun. 1992 803 804.w x91 M.J. Snowden, M.T. Booty, Spec. Publ.- Roy. Chem. Soc. Encapsulation Controlled Rel. 138

    .1993 141 147.w x .92 T. Kato, K. Fujimoto, H. Kawaguchi, Polym. Gels Networks 2 1994 307 313.w x .93 M.J. Snowden, B. Vincent, J.C. Morgan, UK Patent Application GB 262 2 117A 1993 .w x .94 C.W. Chen, M. Akashi, Langmuir. 13 1997 6465.