Nonequilibrium Sorption of Water in Polylactide
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The sorption of water in polylactide (PLA) was measured at various vapor activities (0-0.85) and temperatures using a number of experimental techniques, including quartz spring microbalance (QSM), quartz crystal microbalance (QCM), and in situ time-resolved Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy. Additionally, a prediction of the water sorption isotherm in PLA was obtained with the use of the nonequilibrium lattice fluid (NELF) model, where an excellent agreement between the model prediction and experimental sorption data was observed at different experimental temperatures (all below the glass transition temperature of PLA) for water vapor activities less than 0.65. Results from in situ time-resolved FTIR-ATR spectroscopy revealed that water is present predominately as dimers in PLA at water vapor activities less than 0.65, and the presence of larger hydrogen-bound water clusters was observed at water vapor activities >0.65. This provides a rationale for the deviation between the NELF model prediction and water sorption data at high vapor activities, where the NELF model does not account for the strong self-association interactions present in large hydrogen-bound water clusters. Furthermore, non-Fickian sorption kinetic behavior was observed with all the experimental techniques, and this highlights the nonequilibrium nature of the water-glassy polymer system and provides insight into the variability in the sorption isotherms reported in the literature. 2012 American Chemical Society.
