No abstract

Colloidal interactions between particles in a dispersion can be tuned by grafting polymeric chains onto the surface of the particles. The affinity between the polymeric chains and the continuous-phase liquid controls the strength of these interactions. In our system the polymerliquid affinity is strongly influenced by temperature, and as a result, dramatic changes occur in the dispersion microstructure on heating. The system is an aqueous dispersion of polystyrene ͑PS͒ particles bearing grafted poly͑ethylene oxide͒ ͑PEO͒ chains of low molecular weight ͑ ϳ 2000͒. At room temperature, water is a good solvent for PEO chains, and the dispersion is a stable, low-viscosity sol. As temperature is increased, water becomes a progressively worse solvent for PEO. Beyond a temperature T c there is a sharp transition in microstructure from a stable sol to a volume-filling gel. The sol-gel transition is reversible and the transition temperature T c can be pinpointed using tan ␦ versus temperature plots. Remarkably, T c is more than 100°C lower than the temperature for PEO ͑2000͒ in water, i.e., the gelation occurs under significantly better-thanconditions. T c is independent of particle concentration, but is strongly influenced by the graft density of PEO chains on the particles. The higher the graft density, the higher the T c for gelation; conversely, at very low graft density, the samples are gels even at room temperature. Above T c , the elastic modulus (G Ј) of the gels reveals a power law dependence with particle volume fraction ͑͒, i.e., G Ј ϰ n. The power law exponent n is independent of the PEO graft density, implying that the various gels have a similar microstructure. We suggest that gelation is the result of a weak secondary minimum in the interparticle potential that can develop in the case of short stabilizing moieties and moderate solvent conditions.

A novel hydrophilic macromer adapted for chemical grafting has been synthesized. It consists of methyl-end-capped poly(ethylene glycol) functionalized with a urethane terminus. Dispersion polymerization of styrene in an alcohol/water medium in the presence of the macromer allows chemical grafting of the macromer to the surfaces of the developing polystyrene (PS) latex particles. Scanning and transmission electron microscopies confirm the formation of spherical, submicron polystyrene particles. Transmission electron microscopy of films prepared from the latex particles also permits direct visualization of the stabilizing macromer layer grafted on the PS particle surfaces. Data acquired from proton nuclear magnetic resonance reveal a direct correlation between the concentration of macromer in the reaction mixture and the amount grafted to the latex particles. Rheological techniques are employed to (i) discern the stabilization efficacy of the macromer and (ii) identify correlations between latex and flow characteristics.

A urtique urethane llnkage that permits chemical grafting of poly(ethy1ene oxide) (PEO) linear chains to the surfaces of polystyrene (PS) latex particles has been developed. Chemically grafting the functionalized hydrophilic PEO macromers to the PS particle surface allows the latex to be polymerically stabilized in a water-based medium. Advantages of the urethane linkage include the high yield of the macroiner synthesis and the hydrolytic stability of the final latex. Rheological experiments are used to examine both processing behavior and interparticle interactions fixlatex systems with different amounts of grafted PEO. Dynamic rheological experiments reveal that, at high macromer concentrations, the grafted PEO layer is effecti\,e in shielding the attractive interactions of the core PS particles that lead to flocculation. However, at low macromer concentrations, strong interactions are seen even at low particle weight fractions, indicating the presence of a flocculated system. Steady shear rheological evaluations show that the latex systems possessed suitable flow behavior for coating applications, even at relatively high particle weight fractions. Experimental steady shear data is utilized in conjunction with the Krieger-Dougherty equation to determine the size of the PEO stabilizing layer. The stabilizing layer thickness decreases as particle concentration increases, indicating a compressible system. Finally, the relationship between the strength of interparticle interactions and PEO graft density is gauged from the dependence of the power-law exponent of the elastic modulus on particle concentration.