Diffusion in Glassy Polymers

Abstract

It is well known that the diffusion of organic vapors or liquids in glassy polymer often fails to obey Fick’s Law. Both experimental results and theoretical explanations of the “anomaly” were summarized by Park in 1968.¹ By and large, the interpretations proposed at that time still appear to offer a sound basis, at least qualitatively, for the understanding of the complex phenomena. In the meantime, interest in this subject was further stimulated by the work of Alfrey, Gurnee and Lloyd.² In their elegant paper, a limiting case of non-Fickian behavior of liquid diffusion in glassy polymers was characterized by the following features. (1) As the solvent molecules penetrate the polymer, a sharp advancing boundary separates the inner glassy core from the outer swollen shell. The existence of a sharp boundary, however, is not a sufficient criterion for non-Fickian diffusion.3 (2) At temperatures well below the glass temperature of the unswollen polymer, the distance of penetration, or the weight gain, increases linearly with time, i.e., the boundary between the glassy core and the swollen shell advances at a constant velocity, v. This limiting case was designated by Alfrey, et al., as case II diffusion.²