Macroscopically homogeneous deformation in injection molded polypropylene induced by annealing studied with microbeam X-ray scattering

In a previous study (Shinohara et al., Macromolecules, 2012, 45, 1398-1407), we have found that the annealing of injection molded polypropylene makes its macroscopic deformation fairly homogeneous despite the existence of a non-oriented core layer composed of the spherulite structure. In this study, to elucidate the mechanism for the homogeneous deformation behavior induced by annealing, we conduct a scanning microbeam x-ray scattering measurement of injection molded polypropylene under uniaxial stretching and analyze the structural change at each layer for the first time. It is observed that a plastic deformation of the crystals at each layer of the annealed sample is suppressed compared to that of an as-prepared sample. Furthermore, in the highly oriented layers of the annealed sample, homogeneous cavitation accompanied by the formation of the void with a narrow size distribution is observed. We conclude that the homogeneous cavitation induces strain hardening in the oriented layers, and that the hardened oriented layers dominate the total deformation behavior.