Structural changes and crystallization dynamics of poly(L-lactide) during the cold-crystallization process investigated by infrared and two-dimensional infrared correlation spectroscopy

The structural evolution and crystallization dynamics of poly(L-lactide) (PLLA) during isothermal cold-crystallization process are studied by infrared (IR) spectroscopy and two-dimensional correlation analysis. In the C=O stretching region, the band shift to a higher wavenumber taking place during the crystallization of PLLA is attributed to the dipole-dipole interaction between the C=O groups. A detailed analysis is performed for the range of 1500-1000 cm(-1) where bands are highly overlapped. It was found that the 1458 cm(-1) band reflects the structural order of the CH3 group, and the band at 1109 cm(-1) is related to the C-O-C trans conformation in the crystalline phase of PLLA. The band at 1193 cm(-1) is sensitive not only to the structural adjustment of the C-O-C backbone but also to the structural order of CH3 group in crystalline phase. From the analysis of the difference spectra and 2D correlation spectra in the 1500-1000 cm(-1) region, it is shown that the structural adjustment of the CH3 group unambiguously precedes that of the ester group. Moreover, the detailed crystallization kinetic of PLLA is investigated by analyzing difference spectra.