Brian Vincent

Polymers at interfaces

Preface. Polymers in solution. General features of polymers at interfaces. Experimental methods. Theoretical methods. Homopolymer adsorption. Adsorption of copolymers. Electrostatic effects: charged surfaces and polyelectrolyte adsorption. Terminally-attached chains. Fluid interfaces. Depletion. Interactions in the presence of polymers. Appendices. Glossary of symbols. Key word index.

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.

Depletion flocculation in dispersions of sterically-stabilised particles (“soft spheres”)

Abstract A new approach is described for calculating the depletion interaction in systems of sterically-stabilised particles (“soft spheres”) plus added free polymer. By combining the depletion interaction with the steric interaction, the overall interparticle pair potential is calculated. Variations in the amount of free polymer required to induce flocculation is predicted for these systems, in particular as a function of the nature of the solvent (through the χ parameter) and the properties of the adsorbed polymer layer (through the thickness and the average segment volume fraction). Some experimental results are presented for the system silica—g—polystyrene plus solvent plus free polystyrene, which confirm, at least in a qualitative way, the predictions of the new theory.

Experimental aspects of polymer adsorption at solid/solution interfaces.

Several recent review articles have been concerned with the topic of polymers at interfaces from the theoretical standpoint. This reflects the extensive effort made in this area over the last 10 – 15 years. However, new experimental techniques for studying polymers at interfaces have also begun to appear in recent years; so have better defined model systems. This article is therefore directed more to a survey of these experimental aspects of the subject. However, a short review of the current state of the theory is given first as background and to define concepts. In the following chapter, details of the modern experimental methods are given. The last chapter comprises an extensive comparative review of results obtained using these techniques with model systems, covering homopolymers, copolymers and polyelectrolytes.

The effect of free polymer on the stability of sterically stabilized dispersions

Abstract Further experimental work is presented on the weak flocculation behavior that is observed when free polymer is added to the continuous phase of a sterically stabilized dispersion. In particular, equilibrium aspects of the flocculation are described. In addition, a first-approach theory is presented to account, in a semiquantitative manner, for the observed trends in behavior. The theory predicts the instability point φ ps † (i.e., the minimum added polymer concentration at which flocculation is first observed), but does not account for the restabilization point (φ ps ‡ ), although this is accounted for qualitatively. The observed trends of φ ps † with the molar mass of the added polymer and the particle volume fraction are predicted by the theory.

Colloidal copolymer microgels of N-isopropylacrylamide and acrylic acid: pH, ionic strength and temperature effects

Aqueous colloidal microgels have been prepared, based on poly(N-isopropylacrylamide)[poly(NIPAM)], cross-linked with bisacrylamide, containing 5% w/w acrylic acid (AAc) as a comonomer. Transmission electron micrographs of the microgels show that the copolymer microgels are monodisperse spheres. The size of the microgel particles containing 5% AAc has been studied, using dynamic light scattering, as a function of pH (3–10), ionic strength (10–4–10–1M NaCl) and temperature (20–75 °C). The hydrodynamic diameter of the copolymer microgels decrease both with increasing ionic strength (at pH 6 and 25 °C) and reversibly with increasing temperature at pH values of 2.6, 3.4 and 6.5. However, under isothermal conditions in 10–3M NaCl at 25 °C, the hydrodynamic diameter increases in going from pH 3.3–9.4. In addition, the temperature-induced conformational changes in the polymer chains have been followed using high-sensitivity differential scanning calorimetry (HSDSC). A comparison is made with the behaviour of poly(NIPAM) microgel particles, not containing AAc. Explanations are offered to account for the pronounced difference in the physico-chemical properties observed.

Suspension polymerisation to form polymer beads

Abstract The conventional method employed for the production of large beaded particles is suspension polymerisation. A review of suspension polymerisation is presented, with particular reference to variations in chemical composition of the component phases. Both oil-in-water and water-in-oil systems are considered, as are the relevant methods of droplet stabilisation for such systems. Factors governing droplet stability and particle size and morphology are discussed. New developments including the use of continuous type reactors are also included.

Aqueous dispersions of electrically conducting monodisperse polypyrrole particles

Stable, aqueous dispersions of polypyrrole latex particles were prepared by a dispersion polymerization technique using ferric chloride as the initiator and partially hydrolyzed poly(vinyl acetate) as the stabilizer. Monodisperse particles in the approximate size range from 100 to 150 nm diameter were obtained. At the optimum PVA:pyrrole mass ratio (corresponding to the “knee” in the PVA adsorption isotherm on polypyrrole particles), the electrical conductivity of the particles is ∼5 Ω −1 cm −1 .

The determination of very small electrophoretic mobilities in polar and nonpolar colloidal dispersions using phase analysis light scattering

An apparatus is described that can determine electrophoretic mobilities of polar and nonpolar colloidal dispersions down to 1 × 10−12 m2 s−1 V−1, with typical resolutions of 0.5 to 5%, depending on the nature of the dispersion being studied. The diffusion coefficient and settlingconvection velocity of the sample may be determined simultaneously with the electrophoretic mobility in real time. The technique, phase analysis light scattering (PALS), is based upon classical laser-Doppler electrophoresis, but employs signal processing of the time domain phase information within the scattered light signal, rather than analysis of its frequency spectrum. This allows much smaller electric field strengths to be employed, thereby alleviating the usual heating problems associated with electrophoretic studies of nonpolar dispersions. PALS measurements of typical aqueous latex dispersions with large mobilities and some nonpolar dispersions with very small mobilities are presented to illustrate the versatility of this technique.

Phase separation in dispersions of weakly-interacting particles in solutions of non-adsorbing polymer

Abstract Phase separation may be induced in dispersions of hydrophobic silica particles in cyclohexane upon the addition of non-adsorbing polymer (polydimethylsiloxane). The origin of this effect is ascribed to the depletion interaction. Depending on the particle size and molecular weight of the polymer, the analogues of the molecular gas/solid, gas/liquid and liquid/solid co-existing phase regions may each be observed. It is demonstrated that using relatively simple, mean-field theories for both the fluid state (random phase approximation or adhesive sphere model), and also the solid state (cell model), all these transitions may be predicted. Although the quantitative agreement between experiment and theory is not exact, qualitatively all the observed trends in behaviour are accounted for.