Endotherm just above glass transition in uniaxially drawn poly(lactic acid)s films with various d-isomer contents

The endothermic heat just above glass transition for the drawn poly(lactic acid)s (PLAs) films with four different d-isomer contents (1, 4, 10, and 20 mol%: named as PLA1, PLA4, PLA10, and PLA20) was observed on differential scanning calorimetry (DSC) heating scan. The amount of endothermic heat increases with increasing the draw ratio, which means that the endotherm is undoubtedly caused by stretching the PLAs films. The endotherm is clearly seen in the drawn films of amorphous PLAs (PLA10 and PLA20). In the crystalline PLA4, the endothermic heat decreases with increasing crystallinity. Hence, it is considered that the endotherm is closely related to some events occurring in the amorphous phase of the drawn PLAs films. In order to verify an association between endothermic event and macroscopic shrinking deformation, two types of dimensional constraints during DSC measurement, which are effective in restraining the sample shrinkage, were imposed on a drawn PLA10 monofilament. The endothermic temperature of PLA10 with draw ratio of 4 moves from 62 °C for a free filament to 67 °C for an ends-fixed one. For a filament ends-fixed and very tightly rolled up with aluminum sheet, a sharp endothermic peak does not appear. Instead, a very broad endothermic curve ranging from 90 °C to 172 °C shows up, suggesting that the stretched chains in a filament with severe dimensional constraint relax slowly toward isotropic state over broad range of temperature. This result implies that the heat-absorption is contemporaneous with the macroscopic shrinkage of drawn filament. It was also visually confirmed that a drawn sample is shrunk above the endothermic peak. The conformational energy change during macroscopic shrinking deformation, determined by using Raman spectroscopic method, is comparable to the endothermic heat. Hence, the main origin of endothermic heat seems to be the conformational energy change during shrinking deformation.