Crystallization studies of poly(L-lactic acid) nanocomposites

The thermal characteristics, isothermal crystallization kinetics, and structural changes of neat poly(L-lactic acid) (PLLA) and its nanocomposites during annealing were studied by using differential scanning calorimetry (DSC) and fourier transform infrared spectroscopy (FTIR). PLLA and its nanocomposite films were prepared at room temperature by solvent casting and annealed in the oven at various temperatures (Ta). It was found that the melting temperature (Tm) and degree of crystallinity increased with increasing Ta, attributed to lamella thickness increase. This implied degree of crystallinity increased as a function of Ta. The cold crystallization peak (Tcc) only showed for room temperature samples and disappeared at higher Ta as expected due to the crystallinity. Tcc was lowered with the addition of organically modified montmorillonite (OMMT) in PLLA. The isothermal crystallization kinetics study of PLLA and its nanocomposites was carried out using DSC at various Ta. It was demonstrated that the presence of nanoparticles in PLLA influences the crystallization kinetics of PLLA such as nucleation and crystal growth rate. Crystallization rate of PLLA nanocomposites increased as compared to neat PLLA. Rate of crystallization became slower with increasing Ta. Furthermore, the structural changes that occur during annealing were observed by using FTIR. It was found that FTIR regions from 1000-650 cm-1 and 1500-1250 cm-1 were very sensitive to the structural changes during the crystallization. In addition, the lack of band at 908 cm-1 and the presence of band at 922 cm-1 for annealed samples confirmed the formation of only the alpha-form.