Analysis of the complex thermal behavior of poly(L-lactic acid) film. I. Samples crystallized from the glassy state

The melting behavior of poly(L-lactic acid) film crystallized from the glassy state, either isothermally or nonisothermally, was studied by wide angle X-ray diffraction (WARD), small angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), and temperature-modulated differential scanning calorimetry (TMDSC). Up to three crystallization and two melting peaks were observed. It was concluded that these effects could largely be accounted for on the basis of a "melt-recrystallization" mechanism. When molecular weight is low, two melting endotherms are readily observed. But, without TMDSC, the double melting phenomena of high molecular weight PLLA is often masked by an exotherm just prior to the final melting, as metastable crystals undergo melt-recrystallization during heating in the DSC. The appearance of a double cold-crystallization peak during the DSC heating scan of amorphous PLLA film is the net effect of cold crystallization and melt-recrystallization of metastable crystals formed during the initial cold crystallization. Samples cold-crystallized at 80 and 90 degrees C did not exhibit a long period, although substantial crystallinity developed. (c) 2006 Wiley Periodicals,