Preparation and assessment of piezo- and pyroelectric poly (vinylidene fluoride-hexafluoropropylene) copolymer films

Films of poly(vinylidene fluoride-hexafluoropropylene) copolymer [P(VDF-HFP)] were cast from a dimethylsulfoxide (DMSO)/acetone solution of Solef((R))85-15 P(VDF-HFP) copolymer powder grade 21508, Undrawn and uniaxially drawn cast copolymer films were investigated with respect to their piezo- and pyroelectric properties. Quasistatic charge integration was employed for the determination of the poling-field dependence of the piezoelectric d(31) and pyroelectric p(3) coefficients. In addition, the thermal stability of the pyroelectric effect was studied with a combination of thermally stimulated discharge (TSD) and temperature-modulation techniques. Cast copolymer films could withstand electrical poling fields of up to 400 MV/m. The maximum values of d(31) = 30 pC/N and p(3) = 49 muC/(m(2)K) for uniaxially drawn samples are similar to those found on commercial PVDF films and much higher than those on pure PVDF films cast from solution. Samples kept for 5 min at 150 degreesC still exhibit 30%-40% of the initial pyroelectric effect [up to around 20 muC/(m(2)K)]. After this annealing step, no further decay of the pyroelectric coefficient could be observed during storage at 120 degreesC for several hours.