Solid–Solid Crystal Transitions (δ to α) in Poly(hexamethylene carbonate) and Poly(octamethylene carbonate)

Abstract

Poly (hexamethylene carbonate) (PC6) and poly (octamethylene carbonate) (PC8) were studied under different crystallization conditions. Using differential scanning calorimetry (DSC), a new solid-solid transition, denoted α to δ transition, was detected at low temperatures (<RT) in both PC6 and PC8 samples. The α to δ transition was represented by exothermic (i.e., α to δ) (−6 °C (PC6) and −20 °C (PC8)) and endothermic peaks (i.e., δ to α) (15 °C (PC6) and 28 °C (PC8)), during cooling and heating DSC scans, respectively. Isothermal tests revealed that this solid-solid transition depends on the specific thermal history, since it is not observed at isothermal temperatures higher than room temperature. Still, it is detected in the subsequent cooling and heating scans. Wide-angle X-ray scattering (WAXS) and Fourier-transform infrared spectroscopy (FT-IR) experiments were performed at identical conditions to those by DSC. WAXS experiments showed lower d-spacings in the δ phase than in the α one, corresponding to a unit cell shrinkage, explained by a more efficient packing of the methylene groups in the δ phase. The δ phase is also characterized, according to FT-IR experiments, by more ordered conformation of the methylene groups (i.e., reflected in the appearance of a new absorption band) compared to the less ordered conformation in the α phase.