Deformation of Stereoirregular Isotactic Polypropylene across Length Scales. Influence of Temperature

The development of a highly oriented fiber morphology by stretching low stereoregular isotactic polypropylene (iPP) samples is studied at two different temperatures. The structural and morphological transformations occurring during deformation are followed in real time by collecting in situ wide (WAXS) and small-angle (SAXS) X-ray scattering data. WAXS analysis reveals that the disordered γ form initially present in the samples gradually transforms by stretching into the oriented mesomorphic form of iPP at room temperature and in well-oriented crystals of α form at 60 °C. SAXS analysis indicates that regardless of the stretching temperature, a cascade of events occurs at mesoscale during deformation, consisting in interlamellar separation and lamellar reorientation at low deformations, followed by formation of chevron-like textures, lamellar fragmentation, and cavitation. Moreover, different fibrillary morphologies develop at the different temperatures. In fact, the fibrillary morphology that develops at room temperature is characterized by rod-like fibrillary entities containing mesomorphic aggregates separated by amorphous regions, placed at uncorrelated longitudinal and lateral distances. Instead, the morphology that develops at 60 °C consists of a fibrillary network for the more stereoregular sample and of rod-like fibrillary entities for the less stereoregular sample, and both morphologies include well-oriented crystals of α form alternating with amorphous regions which are periodically stacked along the stretching direction and are placed at uncorrelated lateral distance. We argue that the difference in structural arrangement of amorphous and crystalline regions at mesoscale in the stretched samples are mainly due to the increase of the mobility of the chains in the amorphous matrix at higher temperature, which facilitates the relaxation of the amorphous phase and the changes in the relative arrangement of the crystalline domains.