Spontaneous polarization, tilt angle, hysteresis and low-frequency relaxations in hydrogen bonded chiral liquid crystal dimer: M*SA:9OBA

Abstract

Influence of low-frequency (2-Hz-10MHz) AC field on LC phases exhibited by chiral hydrogen bonded liquid crystal dimer, viz., M*SA:9OBA is investigated. Phase transition temperatures Tc determined by capacitance C(T) and loss factor TanΔ(T) agree with the earlier report. Induced surface polarization reflected through capacitance C(E) exhibited hysteresis in behavior SmC* phase. Growth of primary order parameter in SmC* phase investigated by tilt angle θ(T) inferred critical field exponent β1. Secondary order parameter investigated as spontaneous polarization Ps in SmC* inferred Mean field exponent β2. Dipole moment (μ) is resolved as longitudinal (μl) and transverse (μt) components. Reorientation of μ to the field analyzed. Relaxation studied at different temperatures in SmC*; SmBcryst and SmG phases inferred a low-frequency (LF fR∼100Hz) and another high frequency (HF fR∼100kHz) reorientation processes. LF relaxation in SmC* phase is influenced by dc bias field to suggest polarization helix formed by coupling of field μt. Threshold field (Vth) for polarization switching estimated from the data of field variation of loss εmax″(E). LF relaxation in SmC* is identified with Goldstone mode (GM) manifested as the suppressed ε″max(E) with field. HF relaxation in SmC* is identified with Soft Mode (SM) by its decreasing trend of fR towards TIC*. Activation energies (Ea) estimated from Arrhenius behavior of fR(T) agree with reports. Field influence on GM manifests through shift of loss ε″max(E) and fR(E). Activation energy Ea is estimated from the shift of fR(E) in terms of equivalent temperature plots. Dispersion, ε″ versus ε′ in LC phases infers a non-Debye distribution of relaxation times. Cole-Cole plots inferred large shift of static permittivity εo. Data of dielectric relaxation parameters, i.e., relaxation frequency fR, relaxation time τR, activation energy Ea, loss maxima ε″max, static permittivity (εo), high frequency permittivity ε∞, dielectric strength Δε and distribution (α) parameter are presented. Dielectric strength analyzed for SM in SmC* phase confirms Ferro-Electric Curie-Weiss behavior.