| Thermal analysis is one of the most commonly used techniques to characterize the structure and properties of semicrystalline polymers. 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 complex 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). A major goal of the research was to sort out the various phenomena and to understand the mechanisms that produced them.; 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 by the recrystallization exotherm. It was concluded that PLLA double melting originates mainly from the melt-recrystallization of metastable crystals. For samples cold-crystallized above 140°C, it is due to a dual lamellar population present in the samples. For crystallization below 120°C, a proposal in the literature that PLLA double melting is caused by the formation and subsequent transformation of beta-phase could not be ruled out.; A unique double "cold-crystallization" peak was observed when amorphous PLLA was heated at 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.; The equilibrium melting temperature, Tm°, and the equilibrium heat of fusion, DeltaHm° are very important quantities whose values vary widely in the literature. Analysis of our data extract that T m° = 207+/-3°C and DeltaHm° = 90+/-4 J/g, which are comparable to some of the existing literature results. |
