R. Dhar

Study of different modes of dielectric relaxation in a newly synthesized material exhibiting ferro-, ferri- and antiferro-electric phases

Employing thermodynamical, optical and comprehensive dielectric studies, we have investigated a freshly synthesized chlorinated MHPOBC analogue antiferroelectric liquid crystal; it exhibits different chiral SmC sub-phases, viz. SmCα*, SmC* and SmCγ*, at considerably lower temperature in comparison with MHPOBC, and a wide range room temperature antiferroelectric SmCA* phase. Dielectric spectroscopy investigations under planar molecular anchoring were carried out in the frequency range 1u2009Hz to 10u2009MHz. The data measured for low and high frequency regions were corrected by the method of fitting operation of the theoretical equations. Seven different modes of dielectric relaxation in the different phases were identified. Soft mode relaxation was identified in the SmA* and SmCα* phases. The Goldstone mode, characteristic of the ferroelectric phase, was observed in the SmC* and SmCγ* phases. Two characteristic peaks are seen in the spectrum of the SmCA* phase: one, at high frequency, corresponds to fluctuation where the molecules in neighbouring layers are rotating in opposite phase; the other, at low frequency, arises due to phase fluctuation in the helicoidal superstructure.

Low frequency dielectric spectroscopy of two room temperature chiral liquid crystal mixtures

Dielectric spectroscopy of two room temperature chiral liquid crystal mixtures (W-96 and W-97) have been carried out in the frequency range of 100u2009mHz–10u2009MHz. Low frequency dielectric relaxation modes corresponding to collective behavior of molecules (Goldstone- and soft-modes) in the SmC* phase have been found to be masked by the ionic conductance. Two slow modes of dielectric relaxation due to the ionic conductance have been detected (below 15u2009Hz) in planar-aligned samples. It has been observed that the effect of ionic conductance decreases with the number of thermal annealing cycles on the materials. With large number of thermal annealing cycles it has been possible to wipe out the effects of ionic conductance and then to detect other weak modes of dielectric relaxation which are otherwise masked.

Dielectric spectroscopy of unsymmetrical liquid crystal dimers showing wide temperature range TGBA and TGBC* phases

The electrical properties of frustrated twist grain boundary (TGB) phase are a matter of curiosity. Some studies have indicated the existence of soft and Goldstone modes in TGBA and TGBC* phases respectively. However, the experimental results are still not very conclusive. In the present work, we report dielectric studies of wide temperature range TGBA and TGBC* phases of an optically active dimeric compound 4‐n‐decyloxy‐4′‐(cholesteryloxycarbonyl‐1‐butyloxy) chalcone in the frequency range of 1 Hz to 35 MHz for the planar and homeotropic anchoring of the molecules. Two different relaxation processes have been detected for the planar anchoring of molecules in the TGBA and TGBC* phases. The soft mode like behaviour is obtained due to tilt fluctuation of molecules in the megahertz region for both TGBA and TGBC* phases. Goldstone mode like behaviour due to phase fluctuation of molecules has been detected for the TGBC* phase in the low frequency region (∼200–300 Hz). Activation energies for DC conductivity have also been determined for various phases of the material.

Electro-optical and dielectric relaxation studies of an antiferroelectric liquid-crystal mixture (W-132A)

Electro-optical properties and dielectric relaxation phenomenon have been investigated for antiferroelectric liquid-crystal mixture W-132A exhibiting SmA*, SmC*, and wide temperature range (∼0–48°C) induced SmCA* phases. Spontaneous polarization, tilt angle, response time, viscosity, relaxation frequency, dielectric strength, and distribution parameter have been determined as a function of temperature. Switching process has been studied in the antiferroelectric SmCA* phase with the help of transmission intensity-voltage characteristics. Impedance spectroscopy of the material under the planar anchoring conditioning of the sample has been carried out in the frequency range of 1Hz–10MHz. Usual soft-mode dielectric relaxation in the SmA* phase and Goldstone mode in the SmC* phase have been observed. A tendency of the soft mode to penetrate into the SmC* phase and that of the Goldstone mode into the SmCA* phase have been observed. The dielectric spectrum of the SmCA* phase exhibits two absorption peaks, namely...

Temperature and bias electric field dependence of dielectric parameters of ( S )-(+)-4-(1-methylheptyloxycarbonyl) phenyl-4’-(3-butanoiloxyprop-1-oxy)biphenyl-4’-carboxylate

Temperature and bias electric field-dependent dielectric behaviour of an antiferroelectric liquid crystal compound was investigated in the frequency range 1 Hz to 10 MHz. The investigated compound possessed paraelectric (SmA*) and antiferroelectric ( ) phases, in addition to a monotropic hexatic antiferroelectric smectic ( ) phase. The dielectric study showed the presence of a soft mode in the SmA* phase. In the phase, two weak relaxation modes were observed for a planar orientation of molecules. These two modes were related to antiferroelectric ordering and the helical structure of the phase. Two modes, related to the hexatic order of the phase, were also observed and characterized in terms of bond-orientational order and coupling to the molecular tilt direction. With respect to non-collective modes, rotation around the short axis of molecules was detected in the SmA* and phases for a homeotropic alignment.

Study of different modes of dielectric relaxation in the ferro- and antiferroelectric phases of a newly-synthesized highly tilted chiral liquid crystal material 6H6B

Frequency- and temperature-dependent dielectric spectroscopy of a newly-synthesised AFLC compound in its planar orientation has been carried out in order to determine the different dielectric parameters and analyse the various modes of dielectric relaxation of the different phases. The compound is (S)-(+)-4′-(1-methylheptyloxycarbonyl)biphenyl-4-yl 4-(6-heptanoyloxyhex-1-oxy)benzoate (acronym 6H6B). It possesses paraelectric SmA* and ferroelectric SmC* phases, in addition to a wide-temperature-range antiferroelectric phase (∼23°C). The SmA* phase shows a soft mode of relaxation related to the amplitudons. Its relaxation frequencies are highly temperature-dependent and decrease with decrease in temperature. The dielectric strength is small (∼4–5), but increases with decrease in temperature, and follows the Curie law. The ferroelectric SmC* phase shows a Goldstone mode of relaxation related to the phasons. Its relaxation frequency is almost invariant (∼1–2u2009kHz) with temperature. The dielectric strength of the Goldstone mode is very large (∼130) and slowly increases with decrease in temperature; it shows a maximum just above the SmC*–SmCA* transition temperature. Two modes of dielectric relaxation have been clearly observed in the antiferroelectric phase. The lower-frequency mode observed in the kHz range has been assigned as the PL mode, whereas the higher-frequency mode observed at about 100u2009kHz has been assigned as the PH mode. Most of the observed relaxations follow a generalised Cole–Cole equation.

Switching and electrical properties of ferro‐ and antiferroelectric phases of MOPB(H)PBC

Switching and dielectric relaxation phenomena were investigated for an antiferroelectric liquid crystal, 4,4‐(1‐methyloctyloxycarbonyl)phenyl]‐4′‐[3‐(butanoyloxy)prop‐1‐oxy]biphenyl carboxylate, exhibiting chiral smectic A (SmA*), smectic C (SmC*) and antiferroelectric (SmCA*) phases. Spontaneous polarisations, rotational viscosities, relaxation frequencies, dielectric strengths and distribution parameters were determined as a function of temperature. The electric field required for saturation of the spontaneous polarisation increased with a decrease in temperature. In the SmA* phase, only one relaxation mechanism was observed that behaves as soft mode. Two relaxation processes were detected in the SmC* phase. A high‐frequency relaxation process invariant at 2.2 kHz was due to a Goldstone mode, but the origin of low‐frequency relaxation process (1–20 Hz) is unclear; however, it may belong to an X‐mode. The dielectric spectrum of the SmCA* phase exhibits two absorption peaks separated by two decades of frequency. The low‐frequency peak is related to the antiferroelectric Goldstone mode, whereas the high‐frequency peak originates from the anti‐phase fluctuation of the directors in the anti‐tilt pairs of the SmCA* phase.

Temperature Dependence of Dielectric Behaviour of Mhpb(H)Pbc Antiferroelectric Liquid Crystal

Temperature dependence of dielectric behavior of antiferroelectric liquid crystal, MHPB(H)PBC, has been investigated in the frequency range of 1 Hz to 10 MHz. The experimental results have been analyzed using a generalized Cole–Cole expression. There is mainly one relaxation mechanism in SmA* phase, which behaves as soft mode. Two dielectric relaxation processes have been observed in SmCA* phase, one at about 50 kHz and the other at about 400 kHz. Dielectric strengths of both modes remain constant throughout the SmCA* phase, while relaxation frequencies of both modes decrease with decrease in temperature. These two observed modes have been assigned to the in-phase and antiphase azimuthal fluctuations of the director of molecules in the antitilt pairs.

Dielectric relaxation studies in ferro-, ferri- and antiferro-electric phases of (S)-4′-octyloxybiphenyl-4-carboxylic acid 3-chloro-4-(1-methylheptyloxycarbonyl)phenyl ester

Temperature-dependent dielectric spectroscopy of (S)-4′-octyloxybiphenyl-4-carboxylic acid 3-chloro-4-(1-methylheptyloxycarbonyl)phenyl ester (chlorinated MHPOBC analogue) has been carried out in the frequency range from 1u2009Hz to 10u2009MHz for sample thicknesses 5, 10 and 20u2009µm during heating and cooling runs. This compound shows a rich and complex dielectric response, with several active modes in each subphase. Goldstone-mode relaxation has been observed for , and subphases; however, another relaxation mode (apparently a soft mode) has also been observed in these subphases. It has been found that the dielectric strength corresponding to a relaxation mode increases with increasing thickness of the sample, whereas the relaxation frequency remains invariant. The activation energy of the molecular rotation about their short axes has been determined. It has been found that the molecular reorientation about the short axes is more hindered in chiral smectic C (tilted) subphases than in orthogonal phase.

Twisted Grain Boundary Phases in the Binary Mixtures of 3ß-Chloro-5-Cholestene and 4-N-Decyloxybenzoic Acid

From the thermodynamical, optical texture and dielectric studies of the binary mixtures of 3β-chloro-5-cholestene (ChCl) and 4-n-decyloxybenzoic acid (DOBA), the phase diagram has been drawn. It has been observed that low concentrations of ChCl (1 to 7 mol%) in DOBA induce various types of twisted grain boundary (TGB) submesophases, whereas higher concentrations induce a smectic A (SmA) mesophase. Various optical textures of the TGB phases under different conditions of molecular anchoring have been observed. Weak transitions related with TGB phases have been detected from the temperature dependence of dielectric permittivity. The observed phase diagram of ChCl-DOBA binary system is in complete conformity with the theoretically predicted mean-field phase diagram derived by Renn within the framework of the chiral Chen-Lubenski model