Bending‐beam study of water sorption by thin poly(methyl methacrylate) films

Abstract

AbstractA bending‐beam technique has been employed to study the effects of film thickness (7–55 μm) and rate of cooling during film preparation (∼ 6°C/h to a dry ice quench) on sorption characteristics of water by poly(methyl methacrylate) films coated on thin fused quartz beams (˜ 84 μm thick). In each experiment, the curvature of a polymer coated beam exposed to liquid water was monitored as a function of time by a low power laser pointer. With the use of a transport model which considers the sorption process as the linear superposition of contributions from Fickian diffusion and a first‐order polymer molecular relaxation process, the beam curvature data were analyzed to determine the governing transport kinetics and associated transport parameters such as water diffusion coefficient and relaxation rate constant. From curvature analysis for thin films (7–13 μm in thickness), it was found that water diffusion proceeds at early times in a Fickian‐like manner with a diffusion constant of 2–4 × 10−9 cm2/s. At later times, significant relaxation contributions lead to non‐Fickian diffusion behavior, an effect that is more pronounced as the film thickness or sample cooling rate decreases. In addition, sorption of water was found to reduce the film stress (initially tensile at ˜ 108 dyn/cm2) at a rate that increases with sample cooling rate. The high initial film stress not present in free‐standing films may account for the relatively higher diffusion coefficient (∼ 2 × 10−8 cm2/s) found here for very thick (55 μm) PMMA coatings. Because the bending‐beam technique uses coated samples, it is especially well suited for studying penetrant transport into polymer coatings.