Hydrogen bond breaking of TPU upon heating: understanding from the viewpoints of molecular movements and enthalpy

Abstract

Hydrogen bond breaking of TPU based on 4,4′-methylenediphenyl diisocyanate (MDI)/1,4-butanediol (BDO) upon heating was studied and elucidated from molecular movements and enthalpy. Two temperature regions of hydrogen bond breaking, region I (80–133 °C) and region II (133–169 °C), were determined via the combination of PCMW2D correlation with FTIR and DSC. The method of calculating the enthalpy of the hydrogen bond breaking was established via Van't Hoff plots. We also proposed a method of calculating the relative content of different hydrogen bonds. In region I, ΔH_h = 58.8 ± 0.5 kJ mol⁻¹ for N–H and CO, and ΔH_h = 37.2 ± 0.4 kJ mol⁻¹ for N–H and C–O–C groups. The content of hydrogen bonds generated by N–H and CO is 88.4%, and that of N–H and C–O–C is 11.6%. In region II, ΔH_h = 65.0 ± 1.1 kJ mol⁻¹ for N–H and CO, and ΔH_h = 73.0 ± 3.9 kJ mol⁻¹ for N–H and C–O–C groups. The contents of these two hydrogen bonds are 71.2% and 28.8%, respectively. The surprisingly high value of ΔH_h = 73.0 ± 3.9 kJ mol⁻¹ for N–H and C–O–C in region II is actually due to the stabilizing effect of the repulsion energy on hydrogen bonds at the interface. 2D correlation analysis was used to investigate the sequential order of the groups' movement involved in hydrogen bond breaking. In region I, the breaking of a small amount of hydrogen bonds between N–H and C–O–C at the interface first occurs, and then the breaking of irregular hydrogen bonds between N–H and CO in the TPU hard blocks dominates, resulting in the melting of the imperfect crystallinity in the hard blocks. In region II, the breaking of regular hydrogen bonds between N–H and CO in the perfect crystalline of the hard blocks first occurs, and is then followed by hydrogen bond breaking of N–H and C–O–C enhanced by the repulsion energy at the interface, leading to the order–disorder transition (ODT) of TPU.