Reexamination of the microphase separation in MDI and PTMG based polyurethane: Fast and continuous association/dissociation processes of hydrogen bonding

Microphase separation and hard segment packing in polyurethanes remain areas of active research interest in order to optimize their performances. In this work, a commercial thermoplastic polyetherurethane (1180A, BASF) is used as a model system to investigate the packing of hard segments during microphase separation. Although DSC studies show several transitions during heating and cooling, no crystal structure is detected by XRD. Nevertheless, these endothermic and exothermic peaks should belong to the complicate hard/soft segment interaction and related structures. Time and temperature dependent FTIR spectra prove fast hydrogen bonding association/dissociation phenomena in the current system. We infer that such fast process can induce loosely or irregular packing of hard segments during cooling. On the other hand, the continuous change of degree of microphase separation with temperature implies different hard domain structures and stabilities at different temperatures. At high temperature, only hard domains with large size and compactly packed segments can survive. This is because the thermodynamic stability of hard domains is decided by the strength of hydrogen bonding and the size of hard domain, similar with classical description of 'nucleation'. Morphological information obtained by fitting the SAXS curves proves the validity of the above model. Moreover, the significant difference in mechanical properties before and after thermal treatment can be explained by the microstructure model.