Temperature-dependent polymorphic crystalline structure and melting behavior of poly(butylene adipate) investigated by time-resolved FTIR spectroscopy

The polymorphic crystalline structure and melting behavior of biodegradable poly(butylene adipate) (PBA) samples melt-crystallized at different crystallization temperatures were studied by differential scanning calorimetry (DSC) and fourier transform infrared (FTIR) spectroscopy. The crystalline structure and melting behavior of PBA were found to be greatly dependent on the crystallization temperature. By comparison of the FTIR spectra and the corresponding second derivatives between the α- and β-crystal of PBA, the spectral differences were identified for the IR bands appeared at 1485, 1271, 1183, and 930 cm⁻¹ and the possible reasons were presented. Especially, the 930 cm⁻¹ band was found to be a characteristic band for the β-crystal. Combining the DSC data with the analysis of normalized intensity changes of several main IR bands during the melting process, the melting behaviors of the α- and β-crystal were clarified in detail. It is demonstrated by the in situ IR measurement that the β-crystalline phase would transform into the α-crystalline phase during the melting process, and the solid-solid phase transition from the β- to α-crystal was well elucidated by comparing the intensity changes of the 1170 and 930 cm⁻¹ bands. The dependence of the β- to α-crystal phase transition on the heating rate was revealed by monitoring the intensity ratio of the 909 and 930 cm⁻¹ band. It was suggested that at the heating rate of 0.5 or 1 °C/min, the percent amount of the transformed α-crystal from the β-crystal was much higher than that at the higher heating rate. The β-crystal transforms into the α-crystal incompletely at the higher heating rate because of the less time available for the phase transition. In addition, the β- to α-crystal phase transition was further confirmed by the IR band shifts during the melting process.