Strain-induced molecular aggregation states around a crack tip in a segmented polyurethane film under uniaxial stretching

The development of molecular aggregation structures during stretching deformation and local structure mapping around a crack tip of a segmented polyurethane elastomer film were investigated by in situ simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) and by micro-beam WAXD, respectively. Both the hard segment (HS) domain orientation and the strain-induced crystallization of poly(ε-caprolactone) chains were tracked by the WAXD pattern due to the high crystallinity of the symmetric non-bulky aliphatic HS structure. Strain-induced crystallization occurred locally at the vicinity of the crack tip, and the crystallinity immediately decayed in the exterior, whereas the HS domain orientation was observed throughout the stretched film. The mechanisms underlying crack arrest and the mechanical strength of polyurethane elastomer films are associated with the local strain-induced crystallization at the crack tip, the mechanical stability of the HS domains, and the avoidance of local stress concentration through the HS domain orientation.