(645d) Non-Equilibrium Sorption and Diffusion of Water In Polylactide: Experiments and Model Predictions

Polylactide (PLA) is actively being pursued as a sustainable replacement to widely used commodity plastics because it is produced from renewable sources and readily degrades into benign natural small molecule products. For many applications (e.g., medical implants, packaging), an accurate assessment and fundamental understanding of water sorption and diffusion in PLA is of great interest. Previous investigations report low water solubilities in PLA (e.g, < 1 wt%). However, there are only a few reports on water sorption and diffusion in PLA and the values reported vary among these studies.

In this study, sorption and diffusion of pure water in amorphous PLA films were measured over a wide range of vapor activities and temperatures using several experimental techniques, including quartz spring microbalance (QSM), quartz crystal microbalance (QCM), and time-resolved Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy. Non-Fickian behavior (diffusion-relaxation phenomena) was observed by all three techniques, while FTIR-ATR spectroscopy also provides information about the distribution of the sates of water and water transport mechanisms on a molecular level. This non-Fickian behavior highlights the non-equilibrium nature of the water-glassy polymer system and provides insight into the variability in the sorption isotherms reported in the literature.

In addition to experimental measurements, a purely predictive non-equilibrium lattice fluid (NELF) model was applied to predict the sorption isotherms of water vapor in amorphous PLA. The NELF model accurately predicts the experimental results over a wide range of temperatures and water vapor activities as high as 0.7. Interestingly, at different water vapor activities, different states of water were observed with FTIR-ATR spectroscopy. Water dimers were exclusively observed at activities < 0.7, while larger hydrogen-bound water clusters were noticed at activities > 0.7. The deviation between the NELF model prediction and the experimental results at high water vapor activities can be ascribed to the states of water, where the NELF model does not accurately account for the self-association of water.