Volume Change of Glassy Polymers by Sorption of Small Molecules and Its Relation to the Intermolecular Space

The volume changes caused by the sorption of H2O, Ar, N2, CO2, CH4 and Acetone in Bisphenol-A polycarbonate and of CO2 in different substituted polycarbonates and in Kapton were measured in a dilatometer at room temperature. The partial molar volumes of the small molecules are much smaller than the ones obtained in the liquid or rubbery state of polymers. It is a special feature of the glassy state that the partial molar volume increases as concentration increases. Both findings are explained by a model developed recently, where the volume of the site occupied by small molecules is related via elastic distortions to the solution energy into this site. Assuming a spherical shape of site volumes and a Gaussian distribution of the volumes yields an average value of 33 to 38 Å3/site and a width of about 10 Å3. These quantities vary in a systematic manner with the glass transition temperature and the nature of side groups of the polymer. The results are compared with values calculated from life times of o-positronium and values of the d-spacings from X-ray.