Permeation and plasticization behavior of glassy polymers: a WAXD interpretation

Abstract

Wide angle X-ray diffraction (WAXD) has been used to investigate the mechanism of plasticization in various glassy polymers. Polymer films were exposed to either CO₂, N₂ or n-hexane and the change in the average intersegmental spacing was measured by WAXD. The results have been used to rationalize various types of CO₂ permeability versus pressure profiles in glassy polymers reported in the literature. The increase in CO₂ permeability with pressure in the case of cellulose acetate (CA) and poly (methyl methacrylate) (PMMA) arises from the significant increase in the intersegmental spacing on exposure to high pressure CO₂. Smaller changes in intersegmental spacing are seen in the case of polycarbonate and various polysulfones. The change in the intersegmental spacing can be correlated with the T_g of the polymer prior to plasticization and also with the penetrant solubility in the polymer. The "conditioning" effect of CO₂ in the case of phenolphthalein based polysulfone has been attributed to changes in the segmental mobility, while the intersegmental chain spacing remains unchanged.