| Emphasizing on the effect of molecular structure, the issues surrounding the thermal stability amorphous and crystalline states of poly(lactic acid), or PLA, are explored. Enthalpic relaxation, which correlates with physical aging, is investigated for PLA of different tacticities, and we find that a decreased number of configurational defects in the polymer backbone leads to a smaller Kohlrausch-Williams-Watts exponent. On the other hand, specific volume, or equivalently density, does not relate simply to enthalpic stability when comparing the alpha and stereocomplex forms of PLA crystals. Although the a crystal has the higher density, molecular interactions, as inferred from vibrational spectroscopy and molecular modeling, are stronger in the stereocomplex, a trend consistent with a higher enthalpy of fusion. The methyl-methyl and carbonyl-carbonyl interactions are the main contributors to the alpha crystal thermal stability, whereas the methyl-methyl and carbonyl-to-alpha-hydrogen interactions are the important interactions for the thermal stability of the stereocomplex. In addition, good correlation between the post-Tg exotherm and fiber shrinkage can be explained using spectroscopic and calorimetric means. We find that fiber thermal stability is not achieved when crystallizable chains remain largely uncrystallized. During processing, if crystallization is not completed before vitrification sets in, fiber shrinkage will take place, followed by crystallization enhanced due to pre-existing crystallites from processing. |
