A BROAD-LINE NUCLEAR-MAGNETIC-RESONANCE STUDY OF STRESS-INDUCED CRYSTALLINITY CHANGES IN POLY(VINYLIDENE FLUORIDE)

Many mechanisms have been proposed to explain the piezoelectric response of poly(vinylidene fluoride) (PVDF), but none of these are adequate to explain why d32 is so much less than d31. Although it has been suggested that this difference could relate to the anisotropy of Poisson's ratios, direct measurement of the latter show that this explanation is not correct. The other viable explanation is that there are changes in crystallinity under stress. This possibility is tested in the present investigation using broad-line nuclear magnetic resonance measurements to estimate the change in crystallinity when drawn oriented samples are subjected to a tensile stress applied parallel or perpendicular to the draw direction. The results show that the crystallinity increases for a stress applied parallel to the draw direction and decreases for a stress applied perpendicular to the draw direction. The crystallinity changes give a positive contribution to d31 and a negative contribution to d32, which can explain the relatively low figure for d32 compared with d31.