Mesomorphic phase formation of plasticized poly(l-lactic acid)



The effects of the crystallization temperature, Tc, on the crystal structure as well as its thermal behavior of plasticized poly(l-lactic acid) were investigated by means of wide-angle X-ray diffraction (WAXD), Fourier-transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC). PLLA blended with succinic acid-bis[2-[2-(2-methoxyethoxy)ethoxy]ethyl] ester (SAE) showed clear difference in Tc dependence of crystalline form compared to PLLA homopolymer. PLLA with 26 wt % SAE crystallized into orthorhombic α form for Tc above 80°C, while a peculiar disordered structure (mesophase) was obtained for Tc at 40°C. A detailed FTIR analysis of the mesophase of PLLA, focusing on the intra- and inter-chain interaction in the structure, indicated that mesophase had a large degree of disorder in 10/3 helical conformation as well as its packing manner of disordered 10/3 helical chain. Upon heating, mesophase showed a steep exothermic peak at 80°C in DSC thermogram, indicating the phase transformation from mesophase to a form crystal. FTIR results showed that the degree of interchain interaction of C=O in PLLA started to decrease above 60°C, followed by steep increase at 80°C due to the recrystallization into a form. Melt-recrystallization process in mesophase-α transformation was clarified. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 39762.