The use of NMR linewidths to study b-axis distributions in poled and unpoled PVDF

Abstract

Reorientation of molecular dipoles in a strong electric field is an important contributor to the generation of piezo‐ and pyroelectricity in polymers. An understanding of the mechanisms involved and a knowledge of the extent to which dipoles are reoriented is clearly a desirable goal. Strong piezoelectric properties can be conferred on polyvinylidene flouride (PVDF) in the crystallographic β form for which the molecular dipoles are colinear with the crystallographic b axis. The β polymorph is usually generated through mechanical deformation by uniaxial drawing and/or rolling. The resulting material is spatially highly anisotropic at the molecular level. Anisotropy in the observed NMRlinewidth of deformed polymers, monitored as a function of the angle between the sample draw or machine direction and the magnetic field H0 has been a source of useful information on the spatial distribution of molecules. Here, molecular distributions, specifically distributions of b axes about the poling direction, have been studied in poled and unpoled PVDF by observing linewidthanisotropy both as a function of draw direction alignment and poling axis alignment with respect to H0. The tendency for b axes to orient towards the direction of the applied electric field in the poling process is evident but ultimate alignment is by no means achieved. Our results are in broad agreement with earlier x‐ray studies on comparable materials and it would appear, in addition, that there is still significant b‐axis alignment, at large angles to the poling direction in the two poled samples investigated by NMR. The agreement achieved between theory and experiment justifies the initial assumption of (nπ/3)‐fold rather than (nπ)‐fold dipole rotation. Our study clearly demonstrates the unusual sensitivity of the NMRlinewidth to b‐axis distribution in β‐form PVDF and provides measures of the means 〈cos 2δ〉 and 〈cos 4δ〉, where δ is the angle between poling and b axes.