Roles of Conformational and Configurational Defects on the Physical Aging of Amorphous Poly(lactic acid)

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.