Diffusion in polymers with varying accessibility

This paper considers the problem of describing quantitatively the process of diffusion in polymers attended by plasticization of their amorphous phase. A consequence of such plasticization is that the volume accessible to the penetrant molecules continuously increases during the course of their sorption. Assuming that the rate of relaxation processes is far greater than that of diffusion, an equation has been generated to describe the sorption in a matrix with varying accessibility. A numerical solution of this equation, using experimental data for a polyamide-water system, demonstrates that sorption in a matrix with increasing accessibility is slower than sorption in a hypothetical matrix with constant accessibility and with the same proportion of the amorphous phase. At the same time, the concentration of the penetrant in the former case proves to be higher at any point in the polymer specimen. The results obtained are important for calculating the rate constants for chemical reactions that proceed in a polymer matrix in the diffusion-kinetic mode.