Achieving very high fraction of β-crystal PVDF and PVDF/CNF composites and their effect on AC conductivity and microstructure through a stretching process

It is well known that the ferroelectric performance of poly (vinylidene fluoride) (PVDF) is caused by its β-crystal structure, which can be efficiently induced through a stretching process applied to the PVDF. Though numerous PVDF nanocomposites have been reported on, there is still a lack of studies on how the stretching process affects the phase transformation in PVDF nanocomposites. In this study, the effects of stretching on the crystalline structures and alternating current (AC) conductivity of PVDF nanocomposites with different concentrations (up to 5.0 wt.%) of CNFs were investigated. Results revealed that the stretching process is not only an effective approach to produce β-crystal from pure PVDF, but also for CNF/PVDF composites. The extremely high phase transformation from α- to β-crystal (⩾96%) is maintained for the nanocomposites with above 1.0 wt.% CNFs. The AC conductivity of CNF/PVDF composites remarkably decreases when the resultant percolation threshold is raised from 1.0 to 4.2 wt.% CNFs after stretching. This is attributed to the reduced crystallinity induced by the phase transformation from α- to β-PVDF as well as the CNF re-orientation.