Determination of thermal transitions in polyesters by temperature-dependent FTIR spectroscopy

Temperature-dependent Fourier Transform Infrared Spectroscopy (FTIR-T) was employed to characterize thermal transitions in four semicrystalline biodegradable polyesters, poly(ε-caprolactone) (PCL), poly(butylene succinate) (PBS), poly(L-lactic acid) (PLLA), and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), and compared with Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA). The carbonyl stretching vibration ( ν C=O ) was monitored relative to a temperature-insensitive methylene deformation band ( δ CH2 ) using two metrics: a wavenumber ratio reflecting dipolar coupling changes and an intensity ratio strongly influenced by thermo-mechanical softening and interfacial penetration into the ATR crystal. FTIR-T detected the melting transition in all polymers, with derivative maxima at 62-63 °C for PCL, 119-120 °C for PBS, 173 °C for PLLA, and 185-186 °C for PHBV, closely matching the DSC melting peaks (62.3, 116.4, 174.1, and 175.8 °C, respectively). Wavenumber variations were consistent with structural disordering in PCL, PBS and PHBV, whereas PLLA exhibited negligible frequency shifts, suggesting limited changes in the local dipolar environment even near melting. Intensity changes increased sharply near melting and correlated with drops of several orders of magnitude in complex viscosity measured by DMA, supporting a predominantly thermo-mechanical contribution to the observed intensity variations. FTIR-T thus complements DSC by providing information consistent with molecular reorganization and interfacial rheological changes during polymer thermal transitions.