Application of a generalised thermodynamic model to study of the ferroelectric properties of DOBAMBC and DOBA-1-MPC

Abstract

The temperature variations of tilt angle, spontaneous polarisation, helical pitch, Goldstone mode rotational viscosity and twist elastic constant of the ferroelectric smectic C (SmC*) phase near the smectic A (SmA)-SmC* transition of the ferroic mesogens DOBAMBC and DOBA-1-MPC were calculated using a thermodynamic model based on an extended Landau expansion of the free-energy density. The model free-energy density of the system is written in terms of order parameters (tilt vector ξ, polarisation P and wavevector q of helical pitch) involving eleven mean-field coefficients including bilinear, biquadratic and flexoelectric couplings between ξ and P and coefficients due to coupling between ξ and q. The values of these coefficients were determined by fitting the results of calculation with the experimental data for the pitch, tilt angle and spontaneous polarisation. A detailed analysis of the relative contribution of each individual term appearing in the free-energy density expansion is presented. Taking these values of parameters, in addition to tilt angle, spontaneous polarisation and pitch, we calculate the Goldstone mode rotational viscosity and twist elastic constant as a function of temperature. The theoretical results agree well with the experimental data for both mesogens. © 2008 Taylor & Francis.