Theoretically guided design of efficient polymer dielectrics

Abstract

We analyze the nature of energy storage in polymer dielectrics, with a focus on the case of polymers containing permanent electric dipoles. We note that the stored energy resides largely in the potential energy of distortion of molecular bonds. In the case of a model polymer dielectric in which the permanent electric dipoles lie perpendicular to the chain axis, we study how the density of cross-linked sites, at which the chain is prevented from rotating, affects the stored energy. If the length of segment of chain that is free to rotate is large, the dipoles rotate freely in unison with little bond strain, and hence little stored energy. If the segment length is small, then the large distortion of bond angles necessary for dipole rotation makes this rotation small, and there is again little stored energy. An optimum cross-link density can thus be found at which the stored energy is maximized. The interaction between dipoles is evaluated, and found to be potentially most significant at high dipole density, but also greatly dependent on the structural details of the polymer. For the model considered here, which reflects some of the characteristics of polyvinylidene fluoride or its copolymers, the optimum energy storage is found within a very narrow range of densities of cross links. The Clausius-Mossotti instability can lead to a significant reduction in stored energy density in some circumstances. V C 2014 AIP Publishing LLC.