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

摘要

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 T-c determined by capacitance C(T) and loss factor Tan Delta(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 theta(T) inferred critical field exponent beta(1). Secondary order parameter investigated as spontaneous polarization P-s in SmC* inferred Mean field exponent beta(2). Dipole moment (mu) is resolved as longitudinal (mu(l)) and transverse (mu(t)) components. Reorientation of mu to the field analyzed. Relaxation studied at different temperatures in SmC*; SmBcryst and SmG phases inferred a low-frequency (LF f(R)similar to 100Hz) and another high frequency (HF f(R)similar to 100kHz) reorientation processes. LF relaxation in SmC* phase is influenced by dc bias field to suggest polarization helix formed by coupling of field mu(t). Threshold field (V-th) for polarization switching estimated from the data of field variation of loss epsilon(max)''(E). LF relaxation in SmC* is identified with Goldstone mode (GM) manifested as the suppressed epsilon ''(max)(E) with field. HF relaxation in SmC* is identified with Soft Mode (SM) by its decreasing trend of f(R) towards T-IC*. Activation energies (E-a) estimated from Arrhenius behavior of f(R)(T) agree with reports. Field influence on GM manifests through shift of loss epsilon ''(max)(E) and f(R)(E). Activation energy E-a is estimated from the shift of f(R)(E) in terms of equivalent temperature plots. Dispersion, epsilon '' versus epsilon ' in LC phases infers a non-Debye distribution of relaxation times. Cole-Cole plots inferred large shift of static permittivity epsilon(o). Data of dielectric relaxation parameters, i.e., relaxation frequency f(R), relaxation time tau(R), activation energy E-a, loss maxima epsilon ''(max), static permittivity (epsilon(o)), high frequency permittivity epsilon(infinity), dielectric strength Delta epsilon and distribution (alpha) parameter are presented. Dielectric strength analyzed for SM in SmC* phase confirms Ferro-Electric Curie-Weiss behavior.