Crystallization of poly(hexamethylene carbonate)-co-poly(hexamethylene urethane) segmental block copolymers: From single to double crystalline phases

Abstract

The crystallization behavior of aliphatic poly(hexamethylene carbonate)-co-poly(hexamethylene urethane) (PHCU) segmental block copolymers was studied through non-isothermal differential scanning calorimetry (DSC) experiments at different rates, successive self-nucleation and annealing (SSA), in-situ wide-angle X-ray diffraction (WAXD), and in-situ Fourier transform infrared spectroscopy (FTIR). The crystallization and melting temperatures show linear dependence on the hexamethylene urethane content ([HU]) in PHCU copolymers with high [HU], as well as the similarities of the crystalline structure, indicated an isomorphic-like cocrystallization behavior. However, such behavior was discarded because of the sequential crystallization and sequential melting found by cooling and heating the samples at 1 °C/min, and the cold-crystallization peaks by heating the samples at 10 °C/min, specially at low [HU]. Also, bimodal SSA profiles at low [HU] (≤30 mol%) were found. The fractions at low temperatures could be assigned to the polycarbonate (PC)-rich phase, and those at high temperatures to the polyurethane (PU)-rich phase. The bimodal SSA profile was evident in the PHCU with 10 mol% [HU] (i.e., PHCU-10), which was selected for further analysis. The in-situ WAXD and in-situ FTIR results corroborated the sequential crystallization and melting of the PC- and PU-rich crystals in the PHCU-10 and can be extrapolated to [HU] as high as 30 mol%. The PHCU copolymers had a complex and rate-dependent crystallization behavior, characterized by a single crystallization of the PU-rich phase at high [HU] and double phases including crystallization of both PU- and PC-rich at low [HU].