PURPOSE: To produce the titled element having a high piezoelectric property, by polarizing an unstretched vinylidene fluoride resin sheet cooled and molded under a specified condition with a high direct-current voltage.
CONSTITUTION: A vinylidene fluoride resin is melt-extruded and then cooled and molded by means of a cooling roll kept at a temperature ≤60°C to obtain an unstretched sheet comprising a II-form (α-form) crystal and having a turbidity ≤3mm-1. This unstretched sheet is unidirectionally or biaxially stretched at room temperature W150°C and a draw ratio of 3W6 to obtain a stretched film containing an orientated I -form (β-form) crystal. This film is polarized in the temperature range of -30°CW the m.p. by applying thereto a direct current electric field below the dielectric breakdown voltage of this film (100W2,500kv/cm) in the direction crossing with the surface of the film at a nearly right angle through electrodes.
EFFECT: It is possible to improve the electric field orientability of the dipoles of the orientated I -form (β-form) crystal.
NAKAJIMA KENJI
ICHIHARA YOSHIJI
