Variable-temperature Fourier-transform infrared studies of poly(l-lactic acid) in different states of order: A 2DCOS and PCMW2D analysis

•Variable-temperature FT-IR measurements of PLLA in two different states of order.•Unusual wavenumber shift of the crystallinity-sensitive ν(CO) band of PLLA.•Detection of an intermediate state of order of PLLA.•Interpretation of the variable-temperature structural changes by 2DCOS and PCMW2D.

Variable-temperature Fourier-transform infrared (FT-IR) spectra of a predominantly amorphous and a semi-crystalline poly(l-lactic acid) (PLLA) film were measured between 30 °C and 170 °C in order to investigate their temperature-dependent structural changes as a function of the initial state of order. For an in-depth analysis of the spectral variations in the carbonyl stretching band region (1803–1722 cm−1) two-dimensional correlation spectroscopy (2DCOS) and perturbation-correlation moving-window two-dimensional (PCMW2D) analyses were applied. Significant spectral changes were observed during heating of the amorphous PLLA sample whereas the semi-crystalline specimen showed only slight band shifts as a function of the external perturbation. The PCMW2D results suggested that for efficient 2DCOS analyses the heating process should be split up in two temperature intervals. These analyses then provided information on the recrystallization of the amorphous regions, the presence of an intermediate state of order and a sequence scenario for the observed spectral changes.