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Morphological features governing dimensional stability of variously processed poly(lactic acid) have been investigated. The exotherm observed near the glass transition temperature (Tg) 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.

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Infrared and Raman spectra of the PLA α-crystal were analyzed. Detailed analysis is only possible by using highly oriented electrospun fiber mat. Infrared dichroic analysis of large two-dimensional PLA crystals was also conducted. By combining the two dichroic analyses, accurate band assignments for an α-crystal were achieved for the first time. This analysis is only consistent with a 10/3 helical chain conformation, as opposed to the more simplistic 3/1 helix generally used. Infrared and Raman activities of the PLA 10/3 single-chain helix were derived and correlated to the orthorhombic α-crystal. The orthorhombic structure, previously suggested from X-ray studies, was confirmed by vibrational analysis. Poorly understood spectral features, such as the unexpectedly large number of carbonyl stretching vibrations, are attributed to crystal field splitting. An additional Raman component has its origin due to Fermi resonance interactions.

The roles of poly(lactic acid) chain conformation and configuration on the enthalpy relaxation kinetics of amorphous poly(lactic acid) were examined. Enthalpic relaxation data, which were scaled to the same supercooling from the initial fictive temperature, were taken for three types of the polymer containing various d-lactyl monomers (5.7%, 13.0%, 50%) to assess the effects of configurational defects. The kinetics data were very similar from sample to sample. The effects of configurational defects were assessed using the generalized Kohlrausch−Williams−Watts (KWW) equation solved by the Tool−Narayanaswamy−Moynihan (TNM) equation. The major effect of increasing the d-lactyl contents was to lower the PLA glass transition temperature, thereby accelerating the kinetics of enthalpic relaxation. Configurational defects showed no significant effect on the other KWW/TNM fit parameters (x, Δh, ln A). A slightly larger KWW β stretched exponential parameter is observed for greater (50% d) than for lower (5.7% d) amount of d-lactyl monomer, although these differences are just within the experimental error. Raman spectroscopy showed that conformation does not change appreciably during physical aging.