Anomalous dipolar flexoelectric effect in a nematic main-chain polymer.

Abstract

An anomalously large flexoelectric effect is predicted in the nematic phase of thermotropic main-chain polymers, which have a defined orientation of subsequent monomers bonded on the chain. We identify two different ranges of order parameters. When the persistence length of the chain in the direction parallel to the order parameters is larger than a monomer length but not too long (small order parameter), the longitudinal flexoelectric coefficient ${\mathit{f}}_{\mathrm{\ensuremath{\parallel}}}$ is proportional to this exponentially large persistence length, assuming that the polymers are many persistence lengths long. This result corresponds to the simple estimation that relates flexoelectric coefficients to the Frank elastic constants: ${\mathit{f}}_{\mathrm{\ensuremath{\parallel}}}$\ensuremath{\sim}${\mathit{K}}_{\mathit{s}}$. When the nematic ordering approaches its saturation (strongly extended chains), the flexoelectric effect is essentially independent of the persistence length and is either determined by a competition between this effect and the tendency of the chain segments to align antiparallel, or the system is ferroelectric in the absence of deformations.