Non-Fickian Diffusion of Water in Polylactide

The diffusion of water in polylactide (PLA) was measured at various external water vapor activities (0−0.85) and temperatures (25, 35, 45 °C) using three separate experimental techniques: quartz spring microbalance (QSM), quartz crystal microbalance (QCM), and time-resolved Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy. Non-Fickian sorption kinetic behavior was observed with all the experimental techniques used over the observed experimental time scales, which was a result of the nonequilibrium state of the glassy polymer. Two phenomena were observed over two distinct time periods (two-stage sorption kinetics), where early time data revealed diffusion driven by a water concentration gradient and later time data revealed diffusion driven by slow polymer relaxation or swelling due to additional water ingress over time. Diffusion coefficients and relaxation time constants were determined by regressing the early time data to a Fickian model and the data over the entire observed experimental time scale to a diffusion-relaxation model, respectively. Diffusion coefficients increased with temperature, were constant with activity and concentration of water in the polymer, and were similar among all three experimental techniques. The high Deborah numbers (relaxation time/diffusion time) determined from the diffusion-relaxation model confirm the observed two-stage non-Fickian behavior.