Philip L. Taylor, Case Western Reserve University
We have introduced a simple model, and have examined how the various parameters of the model influence the amount of energy that can be stored. We have clarified a general and fundamental fact about how energy is stored in a polymer chain exposed to an electric field, namely that it resides in the potential energy due to the distortion of the molecular configuration. We have studied a detailed model in which a crystalline polymer consists of an array of parallel chains held perpendicular to an applied electric field. Permanent electric dipoles of fixed magnitude are associated with some of the individual monomers. We then suppose that each chain in this array is free to rotate about its axis without steric hindrance. This might be considered as an approximate representation of a copolymer of polyvinylidene fluoride with some larger monomers, such as chlorotrifluoroethylene, that would keep the chains apart. We assumed that the chains are sparsely crosslinked in a way that prevents rotation at the linking site. Our idealized model is then one in which chain segments of N + 1 monomers, each carrying a dipole moment, are pinned at their ends. The moments are initially parallel, and at an angle to the direction along which the electric field is to be applied. The potential energy is due to the N Hookean springs, each of spring constant K, that act between adjacent monomers.
The energy density was evaluated for various N as a function of the maximum field Eb that can be applied without breakdown of the sample. For N = 2, the energy increased only slowly with Eb. As the number of monomers N per pinning site was increased, it was observed that the curvature with which the energy grows also increased, but the field at which the energy density started to saturate decreased. The optimum value of N thus decreases with increasing breakdown field. We also examined a sparse array of chains, and found that in this more complex example the ferroelectric instability occurs for N > 8, whereupon the energy density becomes negative at zero field. It is thus seen to be highly disadvantageous for energy storage if the material is in the regime where the Clausius-Mossotti transition has occurred. This requires careful choice of the number of monomers between pinning sites.
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