Polarized, shaped material of copolymer of vinylidene fluoride

#The present invention relates to a piezoelectric polymer material having a high piezoelectric performance even at a frequency in a high frequency range, and more particularly, a polarized, shaped bodies comprising a copolymer of vinylidene fluoride for use as elements in an ultrasonic transducer, the ultrasonic transducer provided with the elements being superior to the conventional ultrasonic transducer provided with conventional elements in sensitivity of transmitting and receiving ultrasonic waves.

#As a piezoelectric polymer substance, for instance, poly(vinylidene fluoride) and a copolymer of vinylidene fluoride have been reported, respectively in Japanese Patent Publications Nos. 45-83771 (1970) and 50-29159 (1975), and it has been reported in Japanese Patent Publication No. 51-23439 (1976) that these polymer and copolymer have a piezoelectric performance even at a frequency in a high frequency region.

#Furthermore, in Japanese Patent Application Laying Open No. 53-26995 (1978), it has been disclosed that a binary copolymer of vinylidene fluoride and trifluoroethylene is a piezoelectric polymer material.

#FIG. 1 shows a longitudinal cross section of an ultrasonic transducer provided with an element comprising the polarized, shaped body, of which the ultrasonic sensitivity is to be measured, and

#FIG. 2 shows a block diagram of a circuit for examining the ultrasonic sensitivity of the ultrasonic transducer by pulse-echo method.

#The present invention relates to polarized, shaped material showing an electromechanical coupling factor (k.sub.t) of larger than 0.21 and comprising a copolymer of 40 to 87 mol %, preferably 65 to 80 mol % of vinylidene fluoride, 10 to 40 mol %, preferably 15 to 30 mol % of trifluoroethylene and 3 to 20 mol %, preferably 5 to 15 mol % of vinyl fluoride.

#Although the terpolymer in the copolymer of the polarized shaped bodies according to the present invention is desirable, a small amount of one or more than one fluorine-containing monomers such as tetrafluoroethylene, hexafluoropropylene, monocholorotrifluoroethylene, etc. may be added as the monomeric component(s). In any case, wherein, at any rate, the respective amounts of vinylidene fluoride, trifluoroethylene and vinyl fluoride are out of the above-mentioned respective ranges, the value of k.sub.t of the element prepared by the copolymer becomes smaller than 0.20 thereby causing the reduction of the sensitivity of the ultrasonic transducer to which the polarized body of such a copolymer is applied, or the softness of the original copolymer is lost. Accordingly, the monomeric composition of the copolymer must be in the range shown above. As the shaped body, for instance, films and tubes may be mentioned as the representatives thereof, and in addition, concave films are preferably used to flat films because of the better sensitivity of the former.

#The polarized, shaped bodies are prepared by known methods, i.e., that for polymerizing vinylidene fluoride et al, that for molding the thus produced copolymer of vinylidene fluoride into shaped articles and that for subjecting the thus shaped bodies of the copolymer of vinylidene fluoride to polarization. It is desirable to thermally treat the shaped bodies according to the present invention thereby improving the electromechanical coupling factor (k.sub.t) before or during subjecting the shaped body to the treatment for polarizing at a temperature between the temperature which subtracts 5.degree. C. from the crystalline secondary transition temperature of the copolymer of vinylidene fluoride and the melting temperature of the copolymer of vinylidene fluoride.