Time‐Resolved SAXS/Stress–Strain Studies of Thermoplastic Polyurethanes During Mechanical Cycling at Large Strains

Abstract

Simultaneous small‐angle x‐ray scattering (SAXS) and force measurements have been recorded during tensile load/unload deformation cycles of two contrasting polyurethane elastomers. The elastomers comprise the same hard and soft chemical segments; in sample A, the length of the hard blocks is randomized, while, in sample B, the hard blocks are monodisperse. During the initial extension of sample A, the SAXS halo from the mesophase structure deforms to an ellipse with intensification on the meridian. On subsequent loading and unloading, the elliptical halo reverses but with an off‐axis intensification giving the pattern a four‐point character. In sample B, the halo transforms into a four‐point pattern during the first extension. The azimuthal positions of the four‐point maxima move systematically with changing load. However, the azimuthal movement of sample A is in the opposite sense to sample B. The positions of the maxima have been analyzed in terms of an affine deformation scheme of the spatial correlations of the hard‐block domains in real space. The movement of the four points in sample B is consistent with the affine deformation of a two‐dimensional, four‐point reciprocal lattice correlation. The contrasting movement of the maxima in sample A is explained in terms of the affine separation of particulate hard domains in which local spatial correlations between domains is represented by a linear, one‐dimensional lattice.