A GENERAL FREE-VOLUME BASED THEORY FOR THE DIFFUSION OF LARGE MOLECULES IN AMORPHOUS POLYMERS ABOVE TG .2. MOLECULAR SHAPE DEPENDENCE

The free volume based theory of Mauritz and Storey for the diffusion of extraordinarily large and elongated molecules in rubbery polymers has been generalized in this work to include large molecules of any shape, regular or irregular. As in the original theory, the diffusant molecular hopping displacement is but a fraction of the molecular dimension along a given direction of motion. The accounting for an increase in entropy accompanying the liberation of one penetrant translational degree of freedom after adjacent hole creation, by random thermal fluctuation, has also been retained from our original theory. The basic improvement to the original restrictive model was affected by considering the elementary penetrant diffusive hopping vector to be the vector sum of component displacements along the instantaneous directions of the three principal axes passing through the molecular center of mass. The theory predicts that, for a fixed penetrant molecular volume, compacted structures migrate slower than corresponding extended structures, in qualitative accord with available experimental evidence. Present model shortcomings and suggestions for future improvement are outlined. © 1990, American Chemical Society. All rights reserved.