Isothermal crystalline polymorphs of poly(L-lactic acid) by FTIR coupled with two-dimensional correlation spectroscopy and perturbation-correlation moving-window two-dimensional analysis

The evolutions of the crystalline polymorphs of poly(L-lactic acid) (PLLA) at 85 degrees C and 145 degrees C were respectively studied by Fourier transform infrared (FTIR) spectroscopy coupled with two-dimensional correlation spectroscopy (2DCOS) and perturbation-correlation moving-window two-dimensional (PCMW2D) analysis in the carbonyl stretching band region (1820-1720 cm(-1)). The perturbation region was divided into a few sub-regions by PCMW2D based on the spectral variations. Further 2DCOS analyses were implemented on these sub-regions. Four stages were identified for crystallization at 85 degrees C, in which the transformation of amorphous PLLA to alpha'-PLLA was found in the initial stage (0-30 min), while some of a'-PLLA was also changed to alpha-PLLA in the growth stage (30-150min). For isothermal crystallization at 145 degrees C, the amorphous and the intermediate PLLAs were first changed to the crystalline forms in the initial period (0-30 min), then alternate changes between alpha-, alpha'- and intermediate phases occurred in the other periods with the extension of crystallization. (c) 2018 Elsevier B.V. All rights reserved.