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Effects of crystallinity and molecular weight on crack behavior in crystalline poly(L-lactic acid)



The crack behavior and spherulitic morphology in melt-crystallized poly(L-lactic acid) (PLLA) were found to be molecular weight (MW) and crystallinity dependent, along with other key factors. With increasing MW in PLLA, the size of spherulites, band spacing of ring-banded spherulites, and degree of crystallinity decreased, whereas cracks were increasingly less likely to occur. Multiple types of cracks, that is, circumferential and/or radial cracks, were massively present in low-MW PLLA (PLLA-11k), which had a high crystallinity. Upon cooling, in PLLA-11k at most crystallization temperatures (Tc's), cracks formed, and the crack patterns were dependent on the lamellar morphology within the spherulites. Hexagonal, rather than circular, cracks occurred spontaneously during the cooling process of PLLA of a medium-MW grade (PLLA-120k) in PLLA film samples crystallized only at high Tc (135–138°C) and cooled to ambient temperature. However, no cracks of any types at all were present in PLLA films of high enough MWs (PLLA-152k and PLLA-258k) upon either slow air cooling or quench cooling when the samples were dipped into liquid nitrogen. Apparently, cooling-induced contraction differences in different directions were invalid or not sufficient to address the complex cracking behavior in PLLA. In addition, for PLLA-11k with a substantially high crystallinity, cracks were so prone to occur that even cover constraint imposed another factor in determining the crack and ring-band patterns. More plausible mechanisms and correlations between the cracks, MW, crystallinity, spherulite size, and spherulite lamellar patterns of PLLA were analyzed in detail and proposed in this study. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.