Deformation and molecular chain evolution of poly(vinylidene fluoride-trifluoroethylene) copolymer films under combined electrical and mechanical loads

Abstract

Deformation and molecular chain evolution under combined electrical and mechanical loads are characterized for poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer films with 55 and 68 mol % VDF content [termed P(VDF-TrFE) 55∕45 and P(VDF-TrFE) 68∕32, respectively]. In both copolymer films, the direction of the applied electric field with respect to the stress field direction has significant influence on the stress-strain behavior and the evolution of molecular chain structure. When the electric field is applied parallel to the tension direction, the molecular chains have the tendency to convolute, which prevents the films from acquiring a more oriented fibrillar structure compared with film structures under pure mechanical tensile stress. Consequently, the yield strength is enhanced while the fracture strain is reduced. When the electric field is applied along the film thickness direction, it causes randomly oriented dipoles in the films to orient preferentially along the direction of the external electric field. The combination of the mechanical tensile load and the electric field in the film thickness direction contributes to a well aligned and closely packed molecular chain structure, and consequently both UTS and fracture strain are increased. The VDF content dependence on the stress-strain behavior is discussed.