Structural Changes and Crystallization Dynamics of Poly(<scp>l</scp>-lactide) during the Cold-Crystallization Process Investigated by Infrared and Two-Dimensional Infrared Correlation Spectroscopy

Abstract

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 CO 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 CO 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.