Dielectric and electro-optical studies of a ferroelectric copolysiloxane

Abstract

This paper reports an extensive study of the static (spontaneous polarization, ${\mathit{P}}_{\mathit{s}}$, tilt angle \ensuremath{\theta}, and pitch) and dynamic properties of a ferroelectric liquid-crystalline polymer. It is shown that a ferroelectric liquid-crystalline polymer (diluted polysiloxane) exhibits physical behavior similar to that of a low-molar-mass liquid crystal in terms of contrast ratio, pitch of the helix, and relationship between ${\mathit{P}}_{\mathit{s}}$ and \ensuremath{\theta}. The degenerated collective modes for the director rotation are shown to split into two modes at the Sm-A--Sm-${\mathit{C}}^{\mathrm{*}}$ phase transition temperature using dielectric spectroscopy. Both frequency and the inverse dielectric strength show critical slowing down behavior in the vicinity of the Sm-A--Sm-${\mathit{C}}^{\mathrm{*}}$ phase transition temperature. The dynamical behavior and the relative dependence of the dielectric strength on temperature for the soft mode in Sm-${\mathit{C}}^{\mathrm{*}}$ and Sm-A phase is consistent with the influence of a significant biquadratic coupling compared to its linear coupling between ${\mathit{P}}_{\mathit{s}}$ and \ensuremath{\theta}. The material parameters of the generalized Landau model for the diluted polysiloxane liquid-crystalline polymer have been found by fitting the experimental data to the mean-field theory.