Critical Strains for Lamellae Deformation and Cavitation during Uniaxial Stretching of Annealed Isotactic Polypropylene

The lamellae deformation and cavitation behavior of annealed isotactic polypropylene during uniaxial stretching are comprehensively investigated by in situ synchrotron small-angle X-ray scattering and wide-angle X-ray scattering. We reveal how lamellae deformation occurs in the time scales of elastic deformation, intralamellar slip, and melting–recrystallization, separated by three critical strains which are only rarely influenced by annealing. Strain I (0.1) marks the end of elastic deformation and the onset of intralamellar slip, beyond which the crystallinity decreases gradually. Strain II (0.45) signifies the start of the recrystallization process, from where the long period in the stretching direction begins to decrease from its maximum and the polymer chains in the crystal start to orient along the stretching direction. The energy required for melting arises from the friction between the fragmented lamellae produced by intralamellar slip. Strain III (0.95) denotes the end of the recrystallization pr...