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A comprehensive analysis of microstructural changes associated with hydration of Poly(lactic acid) (PLA) and their effect on the macroscopic properties has been provided. Although water absorbed in PLA is extremely small (~0.5% by weight or 1 water molecule per 30 PLA monomer units), we found significant increase in the kinetics of physical aging and crystallization in the presence of water. The fact that water has such a strong effect on PLA structure is attributed to the polar nature of water, strong intermolecular interactions present in PLA and their changes in the hydrated state. Using vibrational spectroscopy, features associated with bound water or free water were found for PLA with different hydration levels. The structural rearrangements were correlated to the presence of free liquid water disrupting the intermolecular interactions in PLA. The effect of these structural rearrangements on the mechanical behavior of PLA in different environments such as wet and submerged state was determined. The elastic modulus surprisingly increased by as much as 10% upon hydration at 37 oC as compared to the dry ones. However, based on DSC and X-ray diffraction data, no increase in the degree of crystallinity was found. Instead the increase in modulus was attributed to the extremely fast aging process that occurred when PLA was hydrated. This densification process resulted in a higher cohesive energy that can be characterized by a rise in Tg (~5 degrees Celsius) and a ~30% decrease in the intensity of the sub-Tg β transition peak. Time resolved FTIR spectroscopy in the near-IR frequency region showed different populations of water molecules having specific interactions with the carbonyl group of PLA. It was concluded that water forming intermolecular bridges between the PLA chains also contributed to the increase in elastic modulus. Interestingly, water had an opposite effect on the mechanical properties of semicrystalline PLA where the elastic modulus decreased by ~10% upon hydration. In semicrystalline PLA, in addition to the perturbing effects being limited by the constraints introduced by the crystalline domains, absorbed water exhibited bulk water like features, leading to decrease in the modulus. |
