A GENERAL FREE-VOLUME BASED THEORY FOR THE DIFFUSION OF LARGE MOLECULES IN AMORPHOUS POLYMERS ABOVE TG .1. APPLICATION TO DI-NORMAL-ALKYL PHTHALATES IN PVC

The standard free volume theory of the diffusion of small molecules in amorphous polymers above Tg has been extended to include the fundamental motions of comparatively large molecules such as plasticizers. In addition to accounting for liberated penetrant translational entropy, a new feature consisting of an elementary displacement of the molecule that is but a fraction of its long dimension is introduced. An effective diffusion coefficient is derived for elongated molecules which is the result of averaging over probable penetrant jumps from zero up to the entire molecular length. While a necessary plasticizer efficiency parameter presently must be determined by experiment, the theory requires a minimal input database of readily accessible quantities and involves no adjustable parameters. An application of the rough theory to the di-n-octyl and di-n-decyl phthalate in PVC systems shows encouraging results. © 1990, American Chemical Society. All rights reserved.