Journal
POLYMER
Volume 52, Issue 3, Pages 814-821Publisher
ELSEVIER SCI LTD
DOI: 10.1016/j.polymer.2010.12.042
Keywords
Poly(l-lactic acid); Polymorphic Phase; Crystallization
Categories
Funding
- Direct For Mathematical & Physical Scien
- Division Of Materials Research [820506] Funding Source: National Science Foundation
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Poly(lactic acid) samples rich in alpha' or alpha crystals have been characterized using spectroscopic and thermal methods. Cryogenic infrared and Raman spectroscopy were used to probe the differences in chain conformation and packing. Compared to the alpha crystal, the alpha' crystal has weakened specific carbonyl and methyl interactions. Experimental spectroscopic analysis in conjunction with simulation studies have shown that the alpha' crystal has uniform conformational disorder in the C-alpha-C torsion angle. This disorder in chain conformation and packing leads to different crystalline forms with different stabilities. The difference in thermal stability was quantified by measuring enthalpic change at melting for both crystalline forms. Significantly different values for the two crystalline forms were obtained (Delta H degrees(m)(alpha') = 57 +/- 3 J g(-1) and Delta H degrees(m)(alpha) = 96 3J g(-1)). The transformation from the less stable a' to the more stable a phase has been characterized. This analysis provides an explanation to the double melting peaks usually found in PLLA samples. (C) 2010 Elsevier Ltd. All rights reserved.
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