Shear-induced nonisothermal crystallization of two grades of PLA

Shear-induced nonisothermal crystallization of two commercial polylactides (PLAs) differing in optical purity was studied. The molten polymers were sheared at selected temperatures (T-S) and subsequently cooled. The crystallization was followed by a light depolarization method, whereas the specimens were analysed ex-situ by DSC, 2D-WAXS and SEM after etching. It was found that the effect of shear, especially on the crystallinity developed during post-shearing cooling, intensified with a decrease of T-S from 160 to 146 degrees C, and with increasing shear rate and strain. Moreover, the effect of shear on PLA1.5 with D-lactide content of 1.5% was stronger than PLA2.8 with 2.8% of D-lactide, although maximum crystallinity of both polymers was practically the same. A decrease of cooling rate from 30 to 10 degrees C/min increased crystallinity of both PLAs, except for those shearing conditions which induced high crystallinity even during faster cooling. Although SEM examination revealed some row-nucleated forms, no significant crystal orientation was detected by 2D-WAXS, indicating that, under the experimental conditions, the shear induced predominantly point-like nuclei. (C) 2016 Elsevier Ltd. All rights reserved.