Morphological study on thermal shrinkage and dimensional stability associated with oriented poly(lactic acid)

Morphological features governing dimensional stability of variously processed poly(lactic acid) have been investigated. The exotherm observed near the glass transition temperature (T,) is one of the most sensitive indicators reflecting structural changes as a function of temperature and time. A combination of spectroscopic and calorimetric techniques was used to analyze the amorphous chain conformation of deformed PLA samples. We found that nucleation-enhanced crystallization is the major contributor to the exotherm, not shrinkage. Our analysis suggests the existence of two amorphous regions. One is characteristic of the deformed structure. The other is consistent with a relaxed isotropic conformation distribution. Results showed that the existence of deformed amorphous chains is necessary for any fiber shrinkage. But above a critical crystallinity of 40%, a rigid crystalline network prevents shrinkage, even for highly deformed chains. In addition, the previously reported value of tg't fraction in the amorphous region (f(a) = 0.80) was validated in the current study.