Multiple relaxation in uniaxially stretched P(VDF-TrFE) films after crosslinking

Abstract

Uniaxial stretching has been well reported to be a robust technique to significantly enhance the dielectric performance of poly(vinylidene fluoride) (PVDF) based ferroelectric polymers. That is mostly ascribed to the contribution of the aligned ferroelectric domains located in the crystalline phase in previous investigation. In an effort to give a clear understanding of the relationship between the multiple orientations in different phases and the dielectric performance of poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)), in this contribution, the films of P(VDF-TrFE) bearing certain internal double bonds are cast from solution followed by crosslinked with benzyl peroxide (BPO). The crosslinked films are stretched at ambient temperature followed by annealed at elevated temperature. By monitoring the shape evolution and the changing of dielectric performances, the orientation and relaxation of polymer chains in amorphous phase and crystal domains have been carefully correlated to the varied dielectric performance under altered electric fields. It has been shown that the orientation of polymer chains in amorphous phase shows great improvement onto the ferroelectric transition and the resultant dielectric properties of these ferroelectric polymers. That might be attributed to the small ferroelectric domains generated in the amorphous phase induced by the extension strength, which exhibits a ferro-to para-electric (F-P) transition temperature at 45 °C. The coupling force among the ferroelectric domains (coupling I) and the aligned polymer chains (coupling II) are responsible for the well maintained shape and extension of the stretched samples at ambient temperature. As annealing temperature increases, the extension is gradually recovered for the weakened coupling forces, so does the enhanced dielectric properties. The work reveals that besides the ferroelectric domains in crystal phase, the ferroelectric grains may be generated in amorphous phase by stretching and lead to great contribution to the dielectric property of the PVDF based ferroelectric polymers.