New insights into contribution of aromatic ring versus aliphatic ring to thermal transition temperatures of heat resistant polyamides: A comparison study of PA 10T and t-PA 10C

Abstract

In order to understand the differences in the contribution of aromatic (p-phenylene) ring versus aliphatic (trans-1,4-cyclohexylene) ring to the thermal transition temperature of heat-resistant polyamides, a comparison study is both experimentally and computationally implemented on a typical aromatic-heat-resistant polyamide PA 10T containing p-phenylene moieties and a typical alicyclic-heat-resistant polyamide t-PA 10C containing trans-1,4-cyclohexylene moieties. DSC, WAXD, solid state-NMR, computational simulations, and in-situ FTIR were utilized in sequence to reveal the differences in thermal performance, crystal structure, chain conformation, cohesive energy, interchain interaction, and melting process of PA 10T and t-PA 10C. t-PA 10C displays 30 °C-higher Tm and 50 °C-higher Tg than PA 10T. As indicated by the hydrogen bonding dissociation process, the enthalpy change dominates the difference in Tm as the gap in the entropy change between PA 10T and t-PA 10C is negligible. Computational simulation indicates that the energy-favorable hydrogen bonding conformation and dense trans-1,4-cyclohexylene-associated chain packing endow the formation of stronger interchain interactions in t-PA 10C which results in larger cohesive energy and greater cohesive energy density. The higher melting enthalpy change in the melting process derived from in-situ FT-IR experiment also verifies the simulated energy results. Henceforth, the question that WHY can the building block change, from rigid planar aromatic unit (PA 10T) to conformation-switchable nonplanar aliphatic unit (t-PA 10C), in the two polyamides induce so large improvement in Tg and Tm is systematically answered.