Destruction and Reorganization of Physically Cross-Linked Network of Thermoplastic Polyurethane Depending on Its Glass Transition Temperature

Abstract

The mechanical behavior and morphological variation of thermoplastic polyurethanes (TPUs) with different hard segment contents (HSCs) deformed at different temperatures are investigated using tensile machine, small-angle X-ray scattering (SAXS), and wide-angle X-ray scattering (WAXS) techniques. The average constraint molecular weight ⟨Mc⟩ between the hard domains, calculated from the stress–strain curve, increases with the increasing deformation temperature and decreases with the increasing HSC. The increased ⟨Mc⟩ is also confirmed by the increased long spacing calculated from the SAXS peak. The intensity increase of the SAXS peak at high deformation temperature indicates that the induced hard domains exhibit higher density. The WAXS patterns indicate the induction of crystal form III at the deformation temperature above the Tg of the hard domains. The starting true stress of the generation of form III lies in the interval between 20 and 35 MPa. The orientation degree of chain segments at high temperature is lower as compared to deformation at low temperature. The fracture mechanism of TPUs deformed above and below the Tg of the hard domain is different. The conclusion is a quite useful guidance for TPU processing.