Controlled Foaming of Polymer Films through Restricted Surface Diffusion and the Addition of Nanosilica Particles or CO<sub>2</sub>-philic Surfactants

Synergistic use of surface barriers and nanoscale additives is investigated as alternate means by which to promote bubble nucleation in, and thus improve the porosity of, poly(methyl methacrylate) (PMMA) thin films (<i>i</i>) constrained between impenetrable plates, (<i>ii</i>) modified with either nanosilica particles, commercial short-chain fluorosurfactants, or designer CO<sub>2</sub>-philic block/graft copolymers, and (<i>iii</i>) exposed to high-pressure CO<sub>2</sub>. Resultant foamed films exhibit a vast array of micro/mesocellular morphologies in the presence of supercritical, as well as liquid, CO<sub>2</sub> and demonstrate that copolymer micelles afford better control over bubble nucleation (with pore cell densities, <i>N</i>, approaching 10<sup>12</sup> cells/cm<sup>3</sup>) relative to hard nonporous nanoparticles, which alone increase <i>N</i> by more than 2 orders of magnitude at low CO<sub>2</sub> pressures. Incorporation of these nanoscale additives in a surface-constrained polymer matrix serves to enhance foaming through concurrent restriction of CO<sub>2</sub> diffusion, heterogeneous nucleation of CO<sub>2</sub> bubbles, and/or reduced interfacial tension between PMMA and CO<sub>2</sub>.