SORPTION AND PARTIAL MOLAR VOLUME OF SMALL MOLECULES IN GLASSY-POLYMERS

The sorption of atoms (He, Ar) or molecules (N2, CO2, H2O, SO2, CH4, C2H6, C3H3) in polymers is described by assuming a continuous distribution of dissolution energies. The energy distribution arises from a volume distribution of the intermolecular space yielding different elastic distortion energies when occupied by solute molecules. On the basis of statistical fluctuations of the free volume and Eshelby's elasticity theory it will be shown that a Gaussian distribution of sorption energies is appropriate for small molecules in glassy polymers. The width of the Gaussian distribution can be calculated, and it agrees well with values obtained by fitting experimental sorption data. The entropy changes of the small molecules during sorption are about the same as for the gas/solid transition. The dependence of volume changes on gas pressure provides information on the size and distribution of intermolecular space.