Dislocation Movement Induced by Molecular Relaxations in Isotactic Polypropylene

The thermal stability of deformation-induced dislocations was investigated in polypropylene (PP) during annealing by means of in-situ X-ray diffraction using synchrotron radiation. The samples were cold rolled to high strains (e = 1.2) in order to introduce a high number of dislocation lattice defects and immediately stored in liquid nitrogen afterward. Then, stepwise annealing was applied from −180 °C up to above the melting temperature (165 °C) while synchrotron X-ray diffraction patterns were recorded at each step. The resulting low noise, high angular resolution diffraction patterns were evaluated using multireflection X-ray profile analysis (MXPA), revealing parameters such as the dislocation density and the thickness of the crystalline lamellae as a function of the annealing temperature. Two significant decreases of the dislocation density were found at annealing temperatures of about 10 and 85 °C. These distinct changes in the dislocation density could be identified as the mechanisms of β- and α-re...