DIFFUSION OF SMALL MOLECULES IN GLASSY-POLYMERS

Small molecules in glassy polymers are considered to occupy sites with a distribution of free energies of dissolution. Then their diffusivit y depends on concentration and temperature in the same way as it has b een derived for hydrogen atoms in metallic glasses. For hydrogen it wa s shown that the tracer diffusion coefficient is proportional to the a ctivity coefficient of the solute atoms. The latter can be evaluated f rom measured data of sorption of the small molecules in the polymer. K nowing this quantity, the thermodynamic factor can be calculated and t he concentration dependence of the mutual diffusion coefficient is obt ained in excellent agreement with published experimental results. New experimental results are presented for the diffusion coefficient of CO 2 in Kapton and four polycarbonates (BPA-PC, BPZ-PC, TMBPA-PC, and TMC -PC) in the low CO2 pressure range of a few mbar up to 1 bar. The resu lts are in agreement with the model developed for hydrogen. The refere nce diffusion coefficient, which is a fitting parameter of the model t hat is independent of the distribution of free energies is smallest fo r the polycarbonate BPZ-PC having a high gamma-relaxation temperature. This correlation between the diffusion coefficient and the dynamics o f the polymer can be found for other substituted polycarbonates as wel l. (C) 1997 John Wiley & Sons, Inc.