Sorption, Diffusion, and Dilation in Linear and Branched Polycarbonate–CO ₂ Systems in Relation to Solid State Processing of Microcellular Foams

The sorption and diffusion of CO ₂ in linear and branched polycarbonates were investigated using a high-temperature, high-pressure gravimetric technique. The branched structure does not affect gas solubility, but affects the gas diffusivity in the sub-T _g region, with CO ₂ diffusing faster through the branched structure than the linear one. Both solubility and diffusivity depend linearly on pressure at temperatures above T _g , but demonstrate complicated pressure dependence at temperatures below T _g . In the sub-T _g region, volume change of the polymer matrix accompanying the gas dissolution was measured using a dilatometer. The plasticized T _g s obtained from dilation and solubility measurements agreed with those obtained directly by high-pressure DSC. In addition, a plasticization related densification phenomenon was observed in the sub-T _g region. The competing effects of dilation and densification suggest significant implications in terms of solid state processing of microcellular foams.