Tilt Angle, Spontaneous Polarization and LF Dielectric Relaxation Investigations in the Chiral Nematic and Smectic-C* Phases of a Hydrogen Bonded Ferroelectric Liquid Crystal (HBFLC)—11 bpa

Abstract

Investigations of tilt angle, spontaneous polarization and Low frequency (5 Hz - 10 MHz) dielectric relaxations are reported in a chiral hydrogen bonded liquid crystal, HBFLC, viz., 11 bpa. Isotropic to chiral nematic (IN*) and chiral nematic to chiral smectic-C (N*SmC*) phase transition temperatures determined by dielectric method agree with reports of TM and DSC. Temperature variation of order parameter (viz., primary order parameter tilt θ (T) and secondary order parameter spontaneous polarization P _S (T) relevant to SmC* phase) studied through critical exponents β ₁ and β ₂ agree with Mean-Field predictions. Influence of hydrogen bonding on the growth of relevant order parameters is discussed. Dielectric dispersion revealed a collective motion (i.e., Goldstone mode GM) around ∼ 100 Hz. Other two higher frequency relaxation processes (viz., Type–I and –II) observed in the N* and SmC* phases are explained on the basis of a molecular dipole model. The influence of bias field on G.M. is studied in SmC* phase. The off-centred non-Debye's dispersion is studied through the Cole-Davidson theory. HF relaxations of Type-I and –II are explained as reorientation of dipole distinctly configured on the molecular frame. Soft mode relaxation in SmC* phase follows ferroelectric Curie-Weiss law. Arrhenius shift of relaxation frequency f_R reveals higher activation energy for Type-II and lower energy for Type-I reorientation. The influence of hydrogen bonding on ferroelectricity in LCs is studied in terms of phase stability, tilt angle and polarization in SmC* phase in the wake of reports in other FE materials.

Keywords:

• Hydrogen bond
• liquid crystal
• tilt angle
• spontaneous polarization
• ferroelectric
• dielectric relaxations