Specific deformation behavior of isotactic polypropylene films under a multiaxial stress field.

Abstract

The specific deformation behavior of crystalline polymer films, namely unoriented crystallized isotactic polypropylene (<i>it</i> PP) films, was investigated under a multiaxial stress field. Changes in the aggregation structure of the films were investigated during the bulge deformation process using <i>in situ</i> small-angle X-ray scattering, wide-angle X-ray diffraction (WAXD) measurements, and polarized high-speed-camera observations. The films had a thickness of approximately 10 μm. The <i>it</i> PP films were fixed at the hole of a plate, then bulge deformation was applied using N₂ or He gas pressure, and stress-strain curves were then calculated from the applied pressure and bulge height. Yielding was observed in the stress-strain curves. Below the yield point, <i>in situ</i> WAXD measurements revealed that the crystal lattice expanded isotropically at the center, edge, and bottom of the bulge hole. Above the yield point, a craze started to form slightly near the center, and crazes formed in various directions with a further increase in strain, while the crystal lattice expanded uniaxially along the circumference at the edge and bottom. Crazes oriented in various directions merged and lost birefringence, indicating a change to the isotropic orientation. The different directions of the crazes indicated several directions of stress. In other words, even if multiaxial deformation is applied to a crystalline <i>it</i> PP film, the string-shaped crystalline polymer chain structure produces local anisotropic uniaxial stress.