Brian R. Saunders

Microgel particles as model colloids : theory, properties and applications

Abstract This review presents an overview of the literature concerning microgel particles with emphasis on work performed during the past 5 years. Microgel particles are cross-linked latex particles that are swollen in a good solvent. The particles are conveniently prepared by surfactant-free emulsion polymerisation (SFEP) and may be viewed as sterically stabilised particles without a core. The narrow particle size distribution combined with the inherent steric stabilisation of particles prepared by SFEP makes them ideal model systems for the study of solution-dependent phenomena (e.g. osmotic de-swelling). The poly(NIPAM) (NIPAM=N-isopropylacrylamide) microgel system is considered in detail in terms of swelling, rheological, small-angle neutron scattering (SANS) and kinetic data. The review concludes with a discussion of the internal structure for microgel particles and consideration of areas for further research.

Poly(NIPAM) microgel particle de-swelling: a light scattering and small-angle neutron scattering study

Small-angle neutron scattering (SANS) has been used to investigate structural changes during the de-swelling of poly(N-isopropylacrylamide) [poly(NIPAM)] microgel particles induced by temperature variation and the addition of free polymer [poly(ethylene oxide)]. The extent of particle de-swelling was characterized by photon correlation spectroscopy (PCS). Thermally-induced de-swelling of poly(NIPAM)/deuterated water dispersions occurred on increasing the temperature in the region of the lower critical solution temperature (LCST). The latter was found to be 34°C, which is 2°C higher than the value reported for poly(NIPAM) particles dispersed in water. The SANS data exhibit a Porod form (Q−4) of scattering in regard to the size of the colloidal particles. However, in the swollen state, the scattering measured at temperatures less than the LCST, also has a contribution from poly(NIPAM) chains in a solution-like environment (Ornstein–Zernike scattering). The trends observed in the SANS data as a function of temperature were simulated using a model comprised of Porod and Ornstein–Zernike scattering terms. The SANS data confirm earlier PCS measurements showing that addition of free polymer induces particle de-swelling. The SANS data obtained using added free polymer are the first examples of their type to be reported.

FACTORS AFFECTING THE SWELLING OF POLY(N-ISOPROPYLACRYLAMIDE) MICROGEL PARTICLES : FUNDAMENTAL AND COMMERCIAL IMPLICATIONS

Abstract A microgel particle is a cross-linked latex particle which is swollen by a good solvent. Particle swelling is intrinsically related to the nature of the interaction between the polymer and continuous phase. Microgel particles based on PNP [PNP=poly( N -isopropylacrylamide)] are particularly interesting since the parent homopolymer undergoes a coil-to-globule transition in water when the temperature increases above 32°C. In this work, PCS (photon correlation spectroscopy) and SANS (small-angle neutron scattering) are employed in a complementary manner to study the environmentally induced de-swelling of PNP particles. Further, we show that particle de-swelling may be induced at room temperature by addition of alcohols or excluded free polymer (i.e. non-adsorbing free polymer) to the continuous phase. (The extents of particle de-swelling observed using these additives are similar to those achieved by heating the pure microgel particles in water above 32°C.) Particle de-swelling in the presence of added alcohol or free polymer arises from “co-non-solvency” and osmotic de-swelling effects, respectively. Copolymerization of N -isopropylacrylamide (NP) with acrylic acid yields microgel particles whose diameters are sensitive to both pH and temperature. These particles adsorb Pb II ions from solution in a reversible manner. The latter property has potential application in water purification.

A creaming study of weakly flocculated and depletion flocculated oil-in-water emulsions

Abstract The creaming of weakly flocculated and depletion flocculated oil-in-water emulsions was investigated and the data compared with theory. The Stokes equation was applicable provided the average droplet diameter was less than 13 μm. The creaming data for emulsions containing excess added ionic surfactant (i.e. depletion flocculation) exhibited a minimum with respect to added ionic surfactant concentration which may be ascribed to formation of an emulsion gel phase. The oil phase employed in this work was kerosene. The emulsion preparation conditions simulated those used industrially for ‘Quick-break’ emulsions. Non-ionic surfactants of the type CxEy, (C and E represent alkyl and alcohol–ethoxylate moieties, respectively) and ionic surfactants (e.g. sodium dodecylsulfate (SDS) and cetyltrimethylammonium chloride (CTAC)) were employed. An extension of the Stokes equation was used to estimate the droplet diameter from the creaming rate data for the emulsions. The analysis suggested that the as-made emulsions were weakly flocculated. Theoretical analysis of depletion flocculation showed that the electrostatic exclusion region between micelles and droplets greatly increases the attractive interaction.

Poly(butyl methacrylate-g-methoxypoly(ethylene glycol)) and poly(methyl methacrylate-g-methoxypoly(ethylene glycol)) graft copolymers: preparation and aqueous solution properties

A series of water-soluble, amphiphilic graft copolymers has been prepared by free-radical copolymerization of methoxypoly(ethylene glycol) macromonomers, with either methyl methacrylate or butyl methacrylate as the comonomers, in water/ethanol solvent mixtures. Lower molecular weight copolymers were obtained by increasing the concentration of the initiator, azobisisobutyronitrile (AIBN), used in the polymerization reaction. However, the route used also led to the formation of significant quantities of tetramethylsuccinodinitrile, a toxic byproduct resulting from the cage reaction of AIBN. Static fluorescence measurements using pyrene as a probe, along with 1H NMR experiments, showed that the graft copolymers form aggregates in water at very low concentrations (approximately 0.01 g l(-1)) with the pendant hydrophilic graft chains forming a stabilizing shell around the hydrophobic backbone. An increase in the hydrophile-lipophile balance of the graft copolymers was found to lead to smaller aggregates with lower aggregation numbers and highly swollen hydrophilic shells, as revealed by small angle neutron scattering (SANS).

A new method for stabilising conducting polymer latices using short chain alcohol ethoxylate surfactants

This study involves the investigation of a new method for the preparation of colloidally stable poly(3,4-ethylenedioxythiophene) n(PEDT) dispersions. Our experimental data show for the first time that a relatively short-chain alcohol ethoxylate surfactant (C16E16, where C and E represent the methylene and ethylene oxide groups, respectively) can be used to stabilise a conducting polymer dispersion. The principle techniques used to study the PEDT dispersions are photon correlation spectroscopy, transmission electron microscopy and electrical conductivity measurements (of pelletised samples). Microanalytical data are also presented. The oxidant (ammonium persulfate, APS) oxidises the surfactant and monomer simultaneously during dispersion synthesis causing production of surfactant aggregates and conducting polymer particles, respectively. The oxidation results in control samples of surfactant solution becoming nturbid. The anionic surfactant aggregates adsorb onto the growing PEDT particles and provide electrosteric stabilisation for the PEDT dispersions. The effect of several key parameters on the hydrodynamic diameter for the PEDT particles are investigated, e.g., APS and surfactant concentration. Stable dispersions result only when critical concentrations of the APS and surfactant are exceeded. The electrical conductivity for PEDT samples increases with decreasing amount of surfactant used during preparation. A maximum conductivity of 0.016xa0S cm−1 was measured.

A new family of water-swellable microgel particles

In this study a new family of microgel particles is investigated which contain methylmethacrylate (MMA), ethylacrylate (EA), acrylic acid (AA), glycerol propoxytriacrylate (GPTA), and Emulsogen (Em). GPTA is a trifunctional crosslinking monomer, whereas Em is a polymerisable alcohol ethoxylate surfactant. TEM and PCS data reveal that the extent of microgel swelling originates from a pH-independent contribution (due to Em) as well as a pH-dependent contribution (due to AA). The major contribution to swelling comes from pH-independent swelling. Consideration of the equations governing particle swelling allows the effective pK(a) of the incorporated AA groups to be estimated. There is evidence of a shift of the pK(a) for the AA groups from 4.5 to ca. 9.5 when the microgel particles containing AA also contain Em. This suggests intraparticle hydrogen bonding between AA and ethylene oxide segments at low pH.

Thermally induced gelation of an oil-in-water emulsion stabilised by a graft copolymer

Oil-in-water (O/W) emulsions exhibiting reversible thermally ninduced gelation have been prepared using a graft (comb) copolymer ncontaining poly(N-isopropylacrylamide) [poly(NIPAM)] as the nbackbone and poly(ethylene glycol) methacrylate as the side chains.