C. Legrand

On the molecular interpretation of the dielectric relaxation of nematic liquid crystals

This paper presents the results of studies of the dielectric relaxation of nematic 6CHBT obtained for different values of the angle between the directions of the macroscopic orientation of the sample (director n) and the probing electric field E. Analysis of the evolution of the relaxation spectrum from e*‖(ω) (E‖n) to e*⊥(ω) (E⊥n)allows one to explain the hitherto existing inconsistency in the molecular interpretation of the spectra. A model of the molecular dynamics in the oriented nematics is proposed.

Liquid crystals applications to R.F. and microwave tunable components

Abstract We present an experimental procedure for dielectric characterization of liquid crystals in the frequency range 103 – 1010 Hz. This procedure is based an open-end strip line measuring cell and a 2D S.D.A. electromagnetic simulation software. Effective permittivities and tensor permittivity elements are determined for a commercial nematic liquid crystal. An application to an electrically tunable microwave phase-shifter is also presented. Transmission coefficient phase variations of about 0.5%/cm length are obtained per GHz showing the liquid crystals potentialities for such applications.

Dielectric Dispersion in the S*Cα Phase of an Antiferroelectric Liquid Crystal

Abstract We have performed dielectric measurements in the S*Cα phase of an antiferroelectric liquid crystal. Without superimposition of a DC bias, one relaxation process is detected in this phase. This process goes continuously from the SA soft mode to the S* C Goldstone mode, with linear temperature dependence of the reversed dielectric strength and of the critical frequency. The S* Cα phase is clearly detected since a break in slopes is observed at the SA - S* Cα and the S*Cα-S* C phase transitions. This relaxation process is connected with the helicity of the S* Cα phase since it is strongly modified under bias. At high bias, one relaxation process is also observed which behavior is similar to that of the soft mode at a SA-S* C phase transition.

Dielectric evidence of an electroclinic effect in the cholesteric phase near a N * -SmA-SmC * multicritical point

Chiral liquid crystal materials (n = 10 and 11 homologues of a biphenyl alkyloxybenzoate series) exhibiting the ferroelectric smectic C phase (SmC*), the smectic A phase (SmA) and the cholesteric phase (N*) have been studied by structural, electro-optical and dielectric methods. In the structural study, helical pitch, tilt angle and polarization measurements show that the N* -SmC* and the N* -SmA phase transitions appear near a N* -SmA-SmC* multicritical point. In the dielectric measurements, we have studied the soft mode in the SmC* and SmA phases. We have detected a new relaxation mechanism in the N* phase of these compounds. The temperature dependence of this relaxation process is similar to that of the classical soft mode observed in the SmA phase. This dielectric relaxation process is attributed to an electroclinic effect in the N* phase near a N*-SmA-SmC* multicritical point. From the experimental data (structural and dielectric studies), we have evaluated the soft mode rotational viscosity, the α -coefficient of the free energy and the electroclinic coefficient for the SmA and N* phases.

Influence of the proximity of a N*–SmA–SmC* multicritical point on the electroclinic effect in the cholesteric phase

Chiral liquid crystal material (C12 homologue of biphenyl benzoate series) exhibiting the cholesteric (N*), smectic A (SmA) and ferroelectric smectic C (SmC*) phases have been studied by structural, thermodynamic, electrooptical and dielectric investigations. The helical pitch, tilt angle and spontaneous polarization have been determined. In the dielectric measurements, we have studied the soft mode in the SmC* and SmA phases. From experimental data, we have evaluated the soft‐mode rotational viscosity and the electroclinic coefficient in the SmA phase. All results are discussed and compared with previous studies performed on other homologues of the same series. The main result is that the relaxation process detected in the N* phase for the C8, C10 and C11 homologues and explained as a soft‐mode‐like mechanism, is not observed for C12. This corroborates the idea that this mechanism is related to the appearance of smectic order fluctuations within N* phase, the amplitude of which is increased when approaching the SmC*–SmA–N* multicritical point.

Electro-optical and dielectric characterizations of the Goldstone mode relaxation in ferroelectric chiral smectic C liquid crystals

We report structural (helical pitch), electro-optical (tilt angle and spontaneous polarization) and dielectric (Goldstone mode) investigations of ferroelectric liquid crystals (FLCs) exhibiting the chiral smectic C phase (SmC(*)). All these characterizations were performed on two pure FLCs showing the SmC(*)-SmA-N(*) phase sequence and having small pitch, high spontaneous polarization and a large relaxation frequency. We have determined the Goldstone rotational viscosity and the twist elastic constant in the SmC(*) phase from the helical pitch, tilt angle, polarization, dielectric strength experimental data and from the relaxation frequency of the Goldstone-mode relaxation. An Arrhenius-type behaviour of the Goldstone rotational viscosity was obtained and the corresponding activation energies were evaluated.

Some Properties of Nematic Liquid Crystal E7/Acrylic Polymer Networks

Polymer Dispersed Liquid Crystal films were elaborated by photopolymerization using ultraviolet radiation of liquid crystal/monomer mixtures. Three acrylic difunctional propyleneglycol based monomers were used; differing only by their chain lengths in terms of their molecular weight. Infrared spectroscopy investigation made it possible to obtain the monomer conversion rates, in order to determine the effect of the presence of liquid crystal on the kinetics of polymerization and phase separation. The characterization by linear dielectric spectroscopy of the monomers and polymers was carried out as a function of temperature in the frequency range from 20 Hz to 1 MHz.

Coplanar Liquid Crystal Reconfigurable Phase-Shifters

A coplanar liquid crystal phase-shifter is presented and characterized in the frequency range of 5–40 GHz. The interest of this structure lies in the technological realization which is very simple. The phase-shift variation is of 0.2°/cm/GHz for an attenuation of 2.5 dB/cm at 40 GHz. These performances are in agreement with those predicted in a preliminary theoretical study.

Study of molecular dynamics in nematic and isotropic phases of 4-isothiocyanatophenyl 4-hexyl-bicyclo[2,2,2]octane-1carboxylate by dielectric spectroscopy

Abstract Dielectric relaxation study for the liquid crystal 4-isothiocyanatophenyl 4-hexylbicyclo[2,2,2]octane-1carboxylate in the nematic and isotropic phases has been carried out in the frequency range from 1 kHz to 1 GHz. Two relaxation processes, described by the Debye functions, have been observed in the isotropic phase as well as for the permittivity component parallel to the director in the nematic phase. These two processes are related to the rotation of the permanent dipole moment around the short and long molecular axes. The height of the potential barrier which hinders the rotation of the liquid crystal molecules around the short axis in the presence of the nematic potential, and the order parameter of the liquid crystal investigated have been estimated from the relaxation time values in the nematic and isotropic phases.

Ferroelectric Liquid Crystals Composed of Banana-Shaped Thioesters

Thermotropic liquid crystals composed of rod like molecules are known as calamitic liquid crystals. Flatten molecules of some organic compounds form discotic liquid crystals. Both kinds of compounds may exhibit nematic phase. Calamitic mesogens may also form lamellar smectic structures being so important for living systems, whereas discotic molecules make columnar mesophases. Banana-shaped (bow-shaped or bent-core) ferroelectric liquid crystals have been discovered in the last decade of the 20-th century (Noiri et al., 1996). Since then there has been a great interest in their dielectric and electrooptic properties due to potential applications (Sekine et al., 1997; Link et al., 1997; Pelzl et al., 1999). At the beginning some of the bent-core compounds were not chemically stable enough as to study them experimentally during cooling and heating runs in long lasting experiments (Wrobel et al., 2000). It came out that bent-core achiral thioesters are very stable materials showing either B1 or B2 phase (Rouillon et al., 2001; Ossowska-Chruściel, 2007, 2009). In this article we present complementary studies on B1 and B2 phases of 1,3-phenylene bis{4-[(4-alkoxybenzoyl)-sulfanyl]benzoates} (in short: nOSOR) having achiral symmetric bent-core molecules shown in Fig. 1.