Structural changes and crystallization kinetics of polylactide under CO2 investigated using high‐pressure Fourier transform infrared spectroscopy

The structural changes and crystallization kinetics of polylactide (PLA) during cold crystallization under CO2 at 80 °C were studied using in situ high‐pressure Fourier transform infrared (FTIR) spectroscopy. The FTIR spectra show that PLA can crystallize under air and CO2, and some differences are observed. In the second‐derivative spectra, the 1220 cm−1 band is only found for PLA crystallized under CO2, and the tt conformer of PLA crystallized under CO2 is located at 1749 cm−1, while that of PLA crystallized under air is located at 1751 cm−1. From wide‐angle X‐ray diffraction, only the α′‐crystal is observed when PLA is crystallized under air, whereas the α‐crystal appears when crystallized under CO2. The crystalline‐sensitive bands at 921 and 1458 cm−1 were used to analyze the crystallization kinetics of PLA. When PLA crystallizes under air, the 1458 cm−1 band changes faster than the 921 cm−1 one; when it crystallizes under CO2, the result reverses. This suggests that CO2 hinders interchain interactions while promoting the helix conformation.