Piezoelectricity in-Uniaxially Drawn Films

Piezoelectricity in drawn films was studied by Nakamura and Wada (1971) for PVDF and by Cohen and Edelman (1971) for PVDF and PVC. 

The piezoelectricity of PVDF film increases in order of magnitude when the film is roll-drawn. Crystals of a-form (or type II) in the original 

In the drawn film in which the c-axis of the crystal is oriented along the draw-axis (z-axis) and a, b-axes are randomly oriented around the z-axis, the polarization P, along the x-axis of the film is not expected with elongation in the plane yz. The observed piezoelectricity of roll-drawn PVDF film, therefore, may be ascribed to the embedded charge as will be evidenced in the following. 

The piezoelectricity of roll-drawn PVDF exhibits a strong anisotropy which has been investigated by Nakamura and Wada (1971). The result is illustrated in Fig. 24. The e-constant is greatest when the film is stretched along the draw direction, in contrast to the anisotropy in Fig. 9. The 

As would be expected from Eq. (29) (eint=0 in this ease), the anisotropy should be ascribed to the electrostriction constant k. It should be noted that e and k in Eq. (29) refer to the elongational strain along the x-axis, Su whereas e in Fig. 24 is related to the elongational strain S in the yz-plane. If the film is assumed to be elastically isotropic, 

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