Effects of water sorption on poly(lactic acid)

When in contact with water, poly(lactic acid), PLA, undergoes severe physical changes. Hydrolysis is obviously the most expected and studied one, and it is followed by weight loss. However, other phenomena occur such as opacification and crystallization. Although these phenomena are sometimes reported in the literature, apart from hydrolysis which has been thoroughly studied at several temperatures, the other ones have not been deeply analyzed and their timescales have not been so far determined. This work aims at studying the physical changes induced by water sorption to PLA, by analyzing the modifications taking place in solid samples immersed in water for different times (starting from a few hours) and at temperatures close to the glass transition. The obtained results allow determining the timescales of the different phenomena and thus the possible cause-effect relationship among them. In particular, it is shown that opacity is the first phenomenon to become evident, on a timescale consistent with water sorption, and seems to be due to crazes induced by water. The second phenomenon to become evident is the crystallinity evolution, which is shown to be consistent with a heterogeneous nucleation on the crazes. These two phenomena take place before hydrolysis becomes significant, thus raising doubts about the fact that hydrolysis can be their cause.