Blend- and Surface-Assisted Foaming of Polymers with Supercritical Carbon Dioxide

Abstract

This thesis involves development of novel micro- and nanocellular foamed polymers using judicious polymer processing strategies with supercritical carbon dioxide (scCO₂). Apart from serving as a processing aid in the form of a transient plasticizer, scCO₂ is a powerful blowing agent to manufacture foamed plastics. It is also a viable replacement for the harmful chemical blowing agents such as chlorofluorocarbons, hydrofluorochlorocarbons and perfluorocarbons that are still prevalent in the foaming industry. This thesis focuses on improving our fundamental understanding of polymer foaming with scCO₂ to create novel materials that cannot be synthesized otherwise using traditional foaming technologies. In particular, we aim at creating new polymer foaming paradigms via either judicious polymer blending or introduction of surfaces to the polymer matrix.

Novel experimental apparatus, utilizing both a continuous extrusion process and a batch process, have been designed and constructed to study various foaming applications with scCO₂. We show that microcellular foams (foams with pores on the order of 10 μm) containing semicrystalline polymers can be generated continuously by blending with a compatible amorphous polymer. Blends of miscible poly(vinylidene fluoride) (PVDF) – poly(methyl methacrylate) (PMMA) blends yield vastly improved microcellular morphologies compared to PVDF alone. We find that blend miscibility, viscosity reduction facilitated by scCO₂ and reduction or elimination of crystalline melting point of the polymer blend are key factors in producing these materials.

The latter part of this dissertation investigates the feasibility of a scCO₂-based foaming procedure to generate micro and nanoporous thin polymer films. Our experimental findings reveal that controlling scCO₂ diffusion from film surfaces is the critical factor towards realizing uniform porosity in polymer films. We use a combination of physical constrains on film surfaces and introduction of interfaces via addition of a nanoscale filler or a tailored non-ionic surfactant to generate controlled foamed structures. Foaming experiments are conducted on 100 μm thick PMMA films with a variety of additives including colloidal silica (particle diameter of 10-12 nm), Zonyl fluorosurfactants, block and graft copolymer of PMMA with a CO₂-philic group such as a fluoropolymer (1,1-dihydroperfluorooctyl methacrylate) and a siloxane (poly (dimethyl siloxane)) in the presence of CO₂. The addition of a low molecular weight block copolymer (PMMA-b-PFOMA) with a CO₂-soluble block (PFOMA) and a polymer-miscible block (PMMA) are found to provide the highest increase in cell nucleation densities and smallest cell sizes.