Influences of Physical Aging on Enthalpy Relaxation Behavior, Gas Permeability, and Dynamic Mechanical Property of Polylactide Films with Various D-isomer Contents

We report the isothermal enthalpy relaxation behavior, gas permeability, and dynamic mechanical properties of melt-quenched amorphous polylactide (PLA) films with various D-isomer contents (1, 4, and 9 mol%) as a function of the aging temperature (T-a) and time (t(a)). It was found that the enthalpy relaxation peak area (Delta H-relax) and glass transition temperature (T-g) of all PLA samples aged at a given T-a increased initially and then remained relatively unchanged with increasing t(a). The experimental Delta H-relax vs. t(a) plots were fitted well with the Cowie-Ferguson equation incorporating the Kohlrausch-Williams-Watts (KWW) function. A comparison of the PLA samples aged at a similar degree of subcooling (Delta T = T-g-T-a) showed that the ultimate Delta H-relax was somewhat higher for a PLA film with a lower D-isomer content. In addition, the oxygen permeability of all PLA films decreased linearly with increasing t(a), which is due to the densification of glassy PLA films during physical aging. The dynamic mechanical data demonstrated that for the aged PLA films, the storage moduli around glass transition region increased significantly and the loss moduli peak corresponding to the glass transition region shifted to higher temperatures and became narrower.