Thermal and X-ray Analysis on the Origin of Double Melting Phenomena of Poly(L-lactic acid) Films

Thermal analysis is one of the most commonly used techniques to characterize the structure and properties of semicrystalline polymers. Unfortunately, the interpretation of thermal events is not always straightforward, but can be quite complicated. However, the complexity involved is often overlooked, leading to erroneous or, at least, questionable results and interpretations. In the present study we carry out an extensive investigation of the thermal behavior of poly(L-lactic acid), PLLA, films prepared under a wide range of crystallization conditions, including isothermal and non-isothermal crystallization from both the glassy state and the melt. The primary techniques used were differential scanning calorimetry (DSC), temperature modulated DSC (TMDSC), wide-angle x-ray diffraction (WAXD), and small angle x-ray scattering (SAXS). PLLA was known to exhibit complex thermal behavior, but there was disagreement in the literature about its interpretation. A major goal of the research was to sort out the various phenomena and to understand the mechanisms that produced them. This should provide a much better understanding of the thermal behavior of PLLA, but also would serve as an example of the complex behavior that can occur in semicrystalline polymers that could be useful in the interpretation of the results from other polymers.

Depending on the sample preparation conditions up to three crystallization peaks and two melting peaks could be observed during heating in the DSC. The occurrence of double melting is a function of crystallization temperature, crystallization time, heating rate, and molecular weight. It occurs under many experimental conditions, and it depends largely on the size and perfection of initial crystals, not the overall initial crystallinity, nor the completion of crystallization. It was found that the first endotherm was often obscured due to the close competition between melting and recrystallization processes. In general, crystallization treatments performed at temperatures over 120°C for prolonged time periods eliminate the double melting behavior, suggesting that such treatments produce crystals of sufficient size and perfection that they do not readily recrystallize during heat-up in the DSC. Based on such observations it was concluded that double melting in PLLA originates mainly from the melt-recrystallization of metastable crystals. However, a proposal in the literature that double melting in PLLA is caused by the formation and subsequent transformation of a different, metastable phase, rather than the a-phase, at crystallization temperatures below 120°C could not be ruled out on the basis of our results.

A unique double “cold-crystallization” peak was observed when amorphous PLLA was heated at a rate of 10°C/min. It was concluded to result from the sum of conventional cold-crystallization and the melt-recrystallization of the unstable crystals formed during the initial cold-crystallization.

Analysis of our data to extract the equilibrium melting temperature, T_m^o, and the equilibrium heat of fusion, DH_m^o, was also carried out. These are very important quantities whose values vary widely in the literature. Our results indicate that T_m^o = 207±3°C and DH_m^o = 90±4 J/g. These values are comparable to some of the existing literature results.