Transient microstructure of low hard segment thermoplastic polyurethane under uniaxial deformation

Microstructure evolution of a low hard segment (< 10 mol %) thermoplastic polyurethane (LHS-TPU) has been followed by in-situ wide-angle X-ray (WAX) and small-angle X-ray scattering (SAX) with a focus on elucidating peculiar microstructural changes during uniaxial deformation (lambda = 1-3.5). For the LHS-TPU, the hard segments, due to their low content and chemical structure, do not crystallize but form glassy regions that act as physical cross-links. Two types of soft segment crystallites are resolved upon elongation via DSC, SAX, and WAX experiments. Phase I consists of a small amount of initial crystallites (<2%) that function similar to conventional PU hard segment domains, deforming at small uniaxial strains (lambda = 1-2) to a chevron-type morphology, which exhibit equatorial 4-point patterns in SAX. Phase II evolves at higher deformations (A > 2) due to strain-induced crystallization. Phase II exhibits a conventional meridional 2-point pattern along the deformation direction with lamellar crystallites aligning in the plane normal to the deformation. WAX, SAX, and DSC confirm that both phases coexist over a small strain window (lambda = 1.9-2.5), demonstrating the independent nature of the two crystalline phases. These findings indicate that the LHS-TPU in this study is similar to poly(butylene adipate) (PBA) in its morphological and structural behavior. This is further substantiated by NMR, which reveals that the LHS-TPU consists of 90% soft segments, which are identified as PBA via crystal structure analysis of a highly aligned fiber. The soft segments in the LHS-TPU dominate the morphology and the X-ray patterns upon deformation.