PIEZOELECTRICITY AND VISCOELASTIC CRYSTALLINE DISPERSION IN HIGHLY ORIENTED POLY(VINYLIDENE FLUORIDE)

In the reported experiments, the effects of poling temperature on piezoelectricity and its thermal stability were investigated on the basis of the thermal molecular motion associated with the crystalline region. This was done by using a film of highly oriented poly(vinylidene fluoride) containing form-I crystals. The film was prepared by a zone-drawing apparatus of the forced-quenching type. The piezoelectric stress constant e//3//1 is a monotonically increasing function of the poling temperature which becomes steeper above ca. 320 K and again at ca. 400 K. The degree of orientation of the crystal b axis generated by poling also increases more steeply with poling temperature above ca. 320 K and again at 400 K. These temperatures correspond, respectively, to the crystalline dispersion temperature at 11 Hz, designated as alpha //c, and the initiation temperature T//p//m of large-scale molecular motion corresponding to premelting of form-I crystals. Thus the effect of poling temperature on piezoelectricity closely reflects the molecular motion in form-I crystals. The annealing temperature T prime //a which e//3//1 decreases to 70% of that of unannealed sample by annealing a poled sample increases with the poling temperature and again this increase is steeper above poling temperatures of ca. 320 K and ca. 400 K.