a piezoelectric polymeric material having utility in ultrasonic transducers is disclosed which consists essentially of a copolymer of about 65 to 82 mol percent of vinylidene fluoride and about 35 to 18 mol percent of ethylene trifluoride . the polymeric material is formed into a film of the desired thickness and then heated to a temperature greater than about the crystal phase transition temperature and less than about the melting - point temperature to produce a heat - treated film . the heat - treated film is poled in an electrical field of from about 50 to about 1 , 500 kilovolts per centimeter to give the film a thickness extensional electromechanical coupling factor of at least 0 . 05 and preferably greater than 0 . 2 .