Orientation dependent of stretching-induced cavitation behavior of Poly(vinylidene fluoride-co-hexafluoropropylene)

This study elucidates the orientation-dependent cavitation dynamics in poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) through in situ X-ray scattering analysis of cast films featuring controlled lamellar architectures. By engineering two distinct lamellar orientations—normals parallel versus perpendicular to the tensile axis—we demonstrate that cavitation initiates exclusively when lamellae are orthogonally aligned to the stretching direction. The critical onset strain for cavitation exhibits annealing-temperature sensitivity, decreasing systematically from 8.60 % to 6.60 % with increasing annealing temperature. X-ray scattering evidence reveals a sequential structural evolution: lamellar fragmentation precedes cavity nucleation, followed by dimensional progression from nanoscale voids to micrometer-scale defects. Positron annihilation lifetime spectroscopy indicates that amorphous free volume coalescence during lamellar disruption further facilitates cavity expansion within the amorphous phase. Our findings establish that lamellar fracturing dominates in highly oriented PVDF-HFP systems, providing new insights for designing mechanically robust porous polymer architectures.