EFFECT OF GEOMETRY ON SOLVENT FRONT PENETRATION IN GLASSY-POLYMERS

This paper resolves the fundamental issue whether a penetrating solvent front will maintain a constant rate throughout a radially symmetric polymer sample, such as a sphere or a cylinder, under conditions where the front penetrating behaviour in a sheet sample of identical polymer composition and comparable dimension is completely Case II. Similar concerns on the effect of geometry in systems with known Fickian and non-Fickian sorption behaviour are also addressed here. These issues are dealt with by first establishing a theoretical framework with relevant front penetration behaviour simulated for different geometries, based on available solutions to the corresponding moving boundary problem. This is followed by presenting, for the first time, experimental results comparing swelling front movement in sheet and spherical bead samples of identical polymer composition and morphology. In this regard, our experimental data on water penetration in poly(2-hydroxyethyl methacrylate) (PHEMA) and methanol penetration in poly(methyl methacrylate) (PMMA) show excellent agreement with the predicted trends. Both the experimental and theoretical results presented here reveal that only the initial stage of the solvent penetration in radially symmetric geometries exhibits the same transport characteristics as in sheet samples. This is followed by an intermediate transition region with an apparent linear front movement prior to an accelerated front penetration towards the core. In addition to demonstrating that such front acceleration is a natural outcome of the radially symmetric geometry, our results also suggest that it occurs independent of the transport mechanisms involved.