Studies of 4,4′-diphenylmethane diisocyanate (MDI)/1,4-butanediol (BDO) based TPUs by in situ and moving-window two-dimensional correlation infrared spectroscopy: Understanding of multiple DSC endotherms from intermolecular interactions and motions level

In situ infrared spectroscopy and moving-window two-dimensional (MW2D) correlation spectroscopy were employed to study the multiple endotherms observed in differential scanning calorimetry (DSC) curves for linear thermoplastic polyurethanes (TPUs). A 4,4′- diphenylmethane diisocyanate (MDI)/1,4-butanediol (BDO) polyester based TPU with a hard segment content of 45 wt % was used in this study. Temperature-dependent infrared absorbance of H-bonded N–H stretching peak before and after 170 °C annealing both showed three stages, in which the absorbance slopes showed different variation trends with temperature increase. Correspondingly, two boundary temperatures existed between these stages. The lower boundary temperatures coincided with the onset of endotherms above 100 °C in the corresponding DSC curves. While the higher boundary temperatures were located around 200 °C and almost independent of the annealing effect. These distinct absorbance slopes reflected the interaction changes between N–H and CO groups with temperature increase and revealed the endotherms origin from the intermolecular interactions level. MW2D autocorrelation spectra exhibited the enthalpy relaxation of amorphous hard segments, which was consistent with the endotherms below 100 °C observed by DSC. MW2D correlation spectra also revealed the specific orders of temperature response during hydrogen bonds dissociation, and indicated free N–H group might play a key role as the starting point of temperature response during the dissociation. The micro-crystalline structures of hard segments and morphologies of these samples before and after annealing were characterized by Wide-angle X-ray Diffraction (WAXD) and Atomic Force Microscopy (AFM) respectively. Finally, a schematic evolution containing factors influencing the endotherms behaviors was presented.