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 process. Beyond the strain of 0.95, the long period and the crystal size remain nearly unchanged. During further stretching, the newly formed lamellae serve as the anchoring points for polymer chains in the amorphous phase. The extension of the polymer chains in between lamellae triggers the strain hardening behavior. On the other hand, annealing significantly decreases the critical strain for voids formation and increases the voids number but restricts the void size. For those samples annealed at a temperature lower than 90 °C, voids are formed between strain II and strain III, and voids are oriented in the stretching direction once they are formed. However, for those samples annealed at a temperature higher than 105 °C, voids are formed between strain I and strain II. In this case, voids are initially oriented with their longitudinal axis perpendicular to the stretching direction and then transferred along stretching direction via void coalescence. Additionally, the formation of voids influences neither the critical strains for lamellae deformation nor the final long period, the orientation of polymer chains, or the crystal size. The final long period, the orientation of polymer chains in the crystal, and the crystal size are determined only by the stretching temperature through melting–recrystallization.