Oil-continuous microemulsions mixed with an amphiphilic graft copolymer or with the parent homopolymer: Polymer–droplet interactions as revealed by phase behavior and light scattering

Abstract

Polymer–droplet interactions have been studied in AOT/water/isooctane oil-continuous microemulsions mixed with an amphiphilic graft copolymer, or with the parent homopolymer (AOT = sodium bis(2-ethylhexyl) sulfosuccinate). The graft copolymer has an oil-soluble poly(dodecyl methacrylate) backbone and water-soluble poly(ethylene glycol) side chains. Pseudo-ternary polymer/droplet/isooctane phase diagrams have been established for both the parent homopolymer and the graft copolymer, and the two types of mixture display entirely different phase behavior. The homopolymer–droplet interaction is repulsive, and a segregative phase separation occurs at high droplet concentrations. By contrast, the graft copolymer–droplet interaction is attractive: the polymer is insoluble in the pure oil, but dissolves in the microemulsion. A comparatively high concentration of droplets is required to solubilize even small amounts of polymer. Static and dynamic light scattering has been performed in order to obtain information on structure and dynamics in the two types of mixture. For optically matched microemulsions, with a vanishing excess polarizability of the droplets, the polymer dominates the intensity of scattered light. The absolute intensity of scattered light increases as phase separation is approached owing to large-scale concentration fluctuations. Dynamic light scattering shows two populations of diffusion coefficients; one population originates from “free” microemulsion droplets and the other from the polymer (for homopolymer mixtures) or from polymer–droplet aggregates (for mixtures with the graft copolymer). The graft copolymer forms large polymer–droplet aggregates with a broad size distribution, which coexist with a significant fraction of free droplets.