The retardation effects of lamellar slip or/and chain slip on void initiation during uniaxial stretching of oriented iPP.

Cavitation is frequently encountered during the deformation of semi-crystalline polymers. Although abundant works have been performed to reveal the cavitation behavior in detail, the relationship between void formation and the plastic deformation of crystalline phase is still unclear. In this work, by in-situ synchrotron X-ray scattering, the void formation and the slip process in crystalline phase during uniaxial stretching of oriented isotactic polypropylene (iPP) were investigated. Results proved clearly that lamellae slip or/and chain slip presents a retardation effect on void formation. As the Deviation Angle (DA, defined as the angle between stretching direction and lamellae normal) was 0°, voids were initiated at a Hencky strain (ε H) of 0.04 and the longitude direction of voids was perpendicular to lamellae normal. Meanwhile, no slip process happened when voids were induced. As DA was 15°, lamellae slip took place once the stretching was started and voids were formed at ε H of 0.09. When DA was further enlarged to 30° and 45°, both lamellae slip and chain slip could be found as voids were induced. The ε H of voids formation were 0.29 and 0.31, respectively. Interestingly, it is worth noting that as DA was increased from 0° to 45°, although the orientation degree of voids was decreased, the longitude direction of voids stayed perpendicular to the normal of lamellae. When DA was enlarged further to 90°, voids were formed at ε H of 0.1 and the longitude direction of voids coincided with lamellae normal. Additionally, neither lamellae slip nor chain slip existed once voids appeared. Combining the value of micro-strain of lamellae stacks and crystal lattice, it is proposed that voids were formed either in the amorphous phase on the normal side of lamellae (0° ≤ DA ≤ 45°) or in the amorphous phase on the lateral side of lamellae (DA = 90°). Image 1 • Void formation and slip processes in oriented isotactic polypropylene (iPP) were studied by in-situ synchrotron X-ray scattering. • Lamellae slip or/and chain slip presents a retardation effect on void formation. • Combining the value of micro-strain of lamellae stacks and crystal lattice, it is proposed that voids were formed either in the amorphous phase on the normal side of lamellae or in the amorphous phase on the lateral side of lamellae. [ABSTRACT FROM AUTHOR]