Using Successive Self-Nucleation and Annealing to Detect the Solid–Solid Transitions in Poly(hexamethylene carbonate) and Poly(octamethylene carbonate)

Abstract

Solid–solid transitions in poly(hexamethylene carbonate) (PC6) and poly(octamethylene carbonate) (PC8), denoted the δ to α transition, have been investigated, using self-nucleation and the successive self-nucleation and annealing (SSA) technique. The SSA protocol was performed in situ for thermal (differential scanning calorimetry (DSC)), structural (wide-angle X-ray scattering (WAXS)), and conformational (Fourier transform infrared spectroscopy (FT-IR)) characterization. The final heating after SSA fractionation displayed an enhanced (compared to a standard second DSC heating scan) endothermic and unfractionated peak signal at low temperatures corresponding to the δ to α transition. The improved (i.e., higher enthalpy and temperature than in other crystallization conditions) δ to α transition signal is produced by annealing the thickest lamellae made up by α and β phase crystals after SSA treatment. As thicker lamellae are annealed, more significant changes are produced in the δ to α transition, demonstrating the transition dependence on crystal stability and, thus, on the crystallization conditions. The ability of SSA to significantly enhance the observed solid–solid transitions makes it an ideal tool to detect and study these types of transitions. In situ WAXS reveals that the δ to α transition corresponds to a change in the unit cell dimensions, evidenced by an increase in the d-spacing. This implies a more efficient chain packing in the crystal, for both samples, in the δ phase (lower d-spacing at low temperatures) than in the α phase (higher d-spacing at high temperatures). The chain packing differences are explained through in situ FT-IR measurements that show the transition from ordered (δ phase) to disordered (α phase) methylene chain conformations.