Gas Solubility in and Foamability of Various Forms of Syndiotactic Polystyrene

A high pressure microbalance was used to measure solubility of CO ₂ at 35°C and pressures to 55 atm in amorphous, meso, and α-crystalline forms of syndiotactic polystyrene, sPS. Diffusion coefficient of CO ₂ in the various samples were derived from sorption kinetics. When normalized with respect to 100% amorphous content, the solubility and diffusivity of CO ₂ was found to be highest in the semi-crystalline α-form, followed by the meso and the amorphous forms. This rather unusual observation was attributed to the existence of microvoids in α-sPS and meso-sPS, and due to the finite solubility of gas in the rigid amorphous phase associated with these two forms. All three forms of sPS were foamed at various temperatures in the range 50 to 295°C to give microcellular structures. The cell walls in foams obtained from amorphous sPS showed unusual, starburst type structure. The density of meso-sPS foams first decreased and then increased with foaming temperature due to the transformation of the mesophase to the α-phase with increasing temperature. The microvoids in α-sPS and meso-sPS are conjectured to serve as the nucleating sites rather than the often suggested phenomenon of heterogeneous nucleation induced by the crystallites.