Multilevel crystalline structures and post-stretching induced structural evolutions of poly(vinylidene fluoride)-based cast films

Poly(vinylidene fluoride) (PVDF) films have been widely used as ferro- and piezoelectric materials, battery separators and filtration membranes. Blending with poly(methyl methacrylate) (PMMA) is an effective way to fabricate the PVDF-based films with high physical performances. However, the multilevel structures and structural evolutions of PVDF/PMMA films during processing are not well understood. Herein, we prepared the PVDF and PVDF/PMMA cast films and investigated their multilevel structures and structural evolutions during post-stretching. We find that the PVDF cast films with higher melt-draw ratio (RMD) possess raw-nucleated lamellar morphology, which endows the films with hard-elastic behavior. PVDF cast films exclusively form the α phase during casting. However, blending PVDF with PMMA results in the formation of PVDF β phase, which is favoured at a higher PMMA content (mPMMA) or larger RMD. The stress-accelerated crystallization and the PVDF/PMMA intermolecular interactions are responsible for the formation of PVDF β phase. In the post-stretching process, PVDF cast films undergo cavitation, fragmentation of lamellar skeleton, generation of new lamellae and α-to-β phase transition with the increase of strain. For PVDF/PMMA cast films, the cavitation is sensitive to the content of PVDF α phase and is supressed in the films containing solely β phase.