Motion mode of poly(lactic acid) chains in film during strain‐induced crystallization

How stress and temperature impact the movement of poly(lactic acid) (PLA) chains in the process of tensile film stretching was studied. The motion mode of chains was investigated through the study of the strain-induced crystallization and orientation through changes in the draw temperature (Td), draw ratio, and draw rate. The crystallinity and orientation degrees of the PLA films were measured by differential scanning calorimetry, Fourier transform infrared spectroscopy, and polarized optical microscopy. According to the competition between the orientation caused by the stretching and relaxation of chains under the temperature field, the motion modes of PLA chains during strain were divided into four types, modes I–IV. When Td was 100°C, the PLA chains acted in mode I, in which the relaxation rate of chains was so fast that no crystallinity or orientation could be obtained. Beyond a draw rate of 20 mm/min at a Td of 90°C, the type of chain movement changed from mode I to II. In mode II, only crystallites could be reserved after unloading. Chains in the PLA film moved in mode III at a Td of 80°C; then, both the crystallization and orientation were enhanced monophonically with increasing draw rate. Beyond the draw rate of 10 mm/min at a Td of 70°C, the orientation rate of chains was much faster than the relaxation one, and the motion mode transformed from mode III to IV. Then, obvious decreases in the crystallinity and orientation were observed with further increases in the draw rate; this resulted from the destruction of the crystallites. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 42969.