Polarization of thick polyvinylidene fluoride/trifluoroethylene copolymer films

Thick (800 μm) polyvinylidene fluoride/trifluoroethylene (P(VDF-TrFE)) copolymer films for transducer applications are poled under applied voltage at elevated temperatures. By using different heat treatments, poling temperatures, and poling time, we are able to prepare a uniformly poled film with a single resonance peak at 1.3 MHz, or a nonuniformly poled film with two resonances (1.3 and 2.6 MHz), or a film with bimorph structure with a single resonance at 2.6 MHz. The nonuniform polarization which arises from charge injection from the cathode is checked by the pressure wave propagation method. The polarization mechanisms in these thick films are expected to be similar to those previously reported for thin films. The results obtained in this work may lead to practical applications because they suggest a means for controlling transducer frequency by poling.