(336d) Non-Equilibrium Sorption and Anomalous Diffusion of Water in Glassy Polymers

For the development of advanced barrier plastics, a more thorough understanding of the impact of moisture on glassy polymers is of great interest. In this study, the sorption and diffusion of water in two glassy polymers, poly(methyl methacrylate) (PMMA) and poly(styrene) (PS), was measured over a wide range of water vapor activities and temperatures using several experimental techniques, including quartz spring microbalance (QSM) and in situ time-resolved Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy. At temperatures below the glass transition, the same type of anomalous diffusion was observed in both polymers. This phenomenon is the result of diffusion and polymer relaxation occurring on a similar time scale, where the polymer relaxes in response to a sorption-induced stress. With FTIR-ATR spectroscopy, both water diffusion and polymer relaxation were measured simultaneously through unique and separate molecular signatures for the polymer backbone and water. These results will be presented along with the results from a diffusion-relaxation model that captures both the diffusion coefficient of water and the stress-induced relaxation time of the polymer. Additionally, the non-equilibrium or pseudo-equilibrium sorption of water in both polymers was compared, where the uptake of water in PS is an order of magnitude lower than PMMA. Interestingly, FTIR-ATR spectroscopy reveals that the states of water in PS are also significantly different than PMMA. This data was compared along with temperature-dependent diffusivities to understand the impact of the states of water on sorption and diffusion.