Broadband dielectric relaxation study on a diluted solution of a ferroelectric liquid crystal by time domain reflectometry

Abstract

In this work, complex permittivity measurements on a diluted solution of the ferroelectric liquid crystal, 4-[(S,S)-2,3-epoxyhexyloxy]-phenyl 4-(decyloxy)-benzoate, which shows ferroelectric smectic C phase (S*_C), have been performed. Using time domain reflectometry, at frequencies between 10 MHz and 10 GHz, in the temperature range from 50°C down to 10°C for every 10°C, at 5, 10 and 30 wt.% in benzene we observe two independent relaxation processes around 150 MHz and 2 GHz. It is shown that the high frequency process is due to the internal molecular reorientations while the low frequency one is due to molecular orientation around the long molecular axis. It is concluded that the reorientation around the long axis is hindered and leads to a resultant macroscopic polarization.     

Temperature-dependent dielectric relaxation study of binary mixture using the time domain reflectometry method

Dielectric relaxation study of N,N-dimethylformamide (DMF) has been carried out with butylene glycol (BLG, i.e. 1,4-butanediol) at different temperatures. Time domain reflectometry in reflection mode has been used to measure the reflection coefficient in the frequency range from 10?MHz to 20?GHz. The dielectric parameters, static dielectric permittivity (ε ₀) and relaxation time (τ), have been obtained by Fourier transform and least squares fit methods. The experimental results show non-linear variation in dielectric permittivity, and relaxation time with volume fraction of BLG confirms the structural formation due to the intermolecular interaction between DMF and BLG. The variations in excess permittivity (ε^E ), excess inverse relaxation times (1/τ) ^E and Kirkwood correlation factors (g ^eff?;g?^f ) for the binary mixtures have also been reported in this article.     

A sub-hertz frequency dielectric relaxation process in a ferroelectric liquid crystal material

Dielectric studies of the first order phase transition of a ferroelectric liquid crystal material having the phase sequence chiral nematic to smectic C* have been performed using thin (2.5 mum) cells in the frequency range 0.01 Hz to 12 MHz. For planar alignment, one of the cell electrodes was covered with a polymer and rubbed. Optically well defined alignment was obtained by applying an a.c. field below the N*-SmC* transition. Charge accumulation was enhanced by depositing a thick polymer aligning layer for the alignment of the liquid crystal molecules. A sub-hertz frequency dielectric relaxation process is detected in smectic C*, in the chiral nematic and a few degrees into the isotropic phase, due to the charge accumulation between the polymer layer and the ferroelectric liquid crystal material. The effect of temperature and bias field dependences on the sub-hertz dielectric relaxation process are reported and discussed.     

A sub-hertz frequency dielectric relaxation process in a ferroelectric liquid crystal material

Dielectric studies of the first order phase transition of a ferroelectric liquid crystal material having the phase sequence chiral nematic to smectic C* have been performed using thin (2.5 mum) cells in the frequency range 0.01 Hz to 12 MHz. For planar alignment, one of the cell electrodes was covered with a polymer and rubbed. Optically well defined alignment was obtained by applying an a.c. field below the N*-SmC* transition. Charge accumulation was enhanced by depositing a thick polymer aligning layer for the alignment of the liquid crystal molecules. A sub-hertz frequency dielectric relaxation process is detected in smectic C*, in the chiral nematic and a few degrees into the isotropic phase, due to the charge accumulation between the polymer layer and the ferroelectric liquid crystal material. The effect of temperature and bias field dependences on the sub-hertz dielectric relaxation process are reported and discussed.     

Non-linear dielectric response of a ferroelectric liquid crystal

Dielectric spectroscopy is a very useful method for investigation of the structure and dynamics of liquid crystals. However, with few exceptions, most investigations have been only in the linear regime. In this note we present a simple method for extraction of the non-linear contributions to the dielectric constant and as an example give the results obtained for a ferroelectric liquid crystal.     

Estimation of entropy and enthalpy of a nematic liquid crystal using time domain reflectometry studies

Dielectric relaxation behavior of molecules in their pure form, a non-polar solvent, or mixtures of these substances at different microwave frequencies and over a range of temperatures and concentrations give an idea about inter- and intra-molecular forces. Such studies enable one to calculate thermodynamic parameters like change of activation energy for dipole orientation (??G*), enthalpy (??H*), and entropy (??S*) of activation. Such studies in liquid crystals give additional information about the molecular behavior in different phases of liquid crystal under study. An experimental investigation on verity of systems is necessary to draw quantitative conclusions regarding the system of the molecules which are not studied so as to examine if the results obtained are in favor or against, to the general conclusions already arrived at, in other systems. With this in view, the complex dielectric permittivity spectra of p-pentyl phenyl-p-propylbenzoate (PPPB) a nematic liquid crystal are obtained using Time Domain Reflectometry (TDR) method over a frequency range 10?MHz?C10?GHz at various temperatures covering both nematic and isotropic phases. The thermodynamic functions calculated using these measurements are presented and discussed in this article.     

The high frequency dielectric response of a ferroelectric liquid crystal

Abstract

The temperature and frequency dependences of the complex dielectric susceptibility of a ferroelectric liquid crystal near the smectic C*-smectic A phase transition have been calculated using the classical and generalized Landau models. It is shown that although the dielectric response of the S*_C phase consists generally of four modes (soft, Goldstone, and two high frequency polarization modes) only three bands appear in the dielectric loss spectrum of ferroelectric liquid crystals at the S_A–S*_C phase transition. These results are in agreement with recent experimental data.     

Preliminary communication Low frequency dielectric relaxation in the smectic C* phase of a ferroelectric liquid crystal

Dielectric measurements were made on the ferroelectric liquid crystal Felix 018/100 manufactured by Hoechst, Germany, over the temperature range 30 to 65degreeC (smectic C* phase), frequency range 0.1Hz to 100kHz, with bias voltages of 0, 1, 3 and 10 V, and in a dielectric cell with a spacing of 4 times the helical pitch. Plots of the dielectric loss versus log (frequency) show the usual monotonic increase in the loss with decreasing frequency, as well as the usual loss peak at approximately 1kHz. Plots of the log (dielectric loss) against log (frequency) at low frequencies, have slopes varying from -0.75 to -0.89 when the temperature increases from 30 to 65degreeC. Following the suggestion of Scaife, transforming the complex permittivity data to the complex polarizability of a sphere of unit radius in a vacuum, and plotting the loss polarizability against log (frequency), shows two distinct and separate loss peaks. The sums of the two loss peaks appear to be independent of temperature and bias voltage, even though both depend on these variables.     

Molecular dynamics study of the ferroelectric liquid crystal CI IPNOC by proton spin-lattice relaxation

Proton spin-lattice relaxation studies were carried out in the S_A and S*_C phases of the liquid crystal CI IPNOC using both conventional and fast field cycling NMR techniques. T₁ dispersion curves were obtained at two different temperatures for each mesophase covering frequencies from 10² to 3 × 10⁸ Hz. In both mesophases the T₁ data can be described assuming the presence of three different relaxation mechanisms, namely local molecular rotations, molecular self-diffusion and collective motions. The self-diffusion constant D₁ was evaluated for several temperatures and the activation energy associated with the diffusion process was obtained. The expected contribution of the soft-mode for the spin-lattice relaxation could not be separated from the contribution of other collective motions. The correlation times associated with the rotations around the molecular long axis and with the fluctuations of this axis were evaluated for both the S_A and the S*_C phases.     

Molecular dynamics study of the ferroelectric liquid crystal CI IPNOC by proton spin-lattice relaxation

Abstract

Proton spin-lattice relaxation studies were carried out in the S_A and S*_C phases of the liquid crystal CI IPNOC using both conventional and fast field cycling NMR techniques. T ₁ dispersion curves were obtained at two different temperatures for each mesophase covering frequencies from 10² to 3 × 10⁸ Hz. In both mesophases the T ₁ data can be described assuming the presence of three different relaxation mechanisms, namely local molecular rotations, molecular self-diffusion and collective motions. The self-diffusion constant D ₁ was evaluated for several temperatures and the activation energy associated with the diffusion process was obtained. The expected contribution of the soft-mode for the spin-lattice relaxation could not be separated from the contribution of other collective motions. The correlation times associated with the rotations around the molecular long axis and with the fluctuations of this axis were evaluated for both the S_A and the S*_C phases.     

Dielectric relaxation study of N -methyl formamide with glycols using time domain reflectometry technique

The dielectric relaxation study that is static dielectric permittivity (∈₀) and relaxation time (τ) of amide of N-methyl formamide (NMF) with increasing volume percent propylene glycol (PLG) and BLG has been carried out at different temperatures. The time domain reflectometry (TDR) technique has been used to measure reflection coefficient in frequency range of 10 MHz to 20 GHz. The dielectric parameters have been obtained by fitting experimental data with the Havriliak–Negami equation. The experimental observation shows that the static dielectric permittivity and relaxation time decreases with increasing temperature. The experimental observation also shows that the static dielectric permittivity decreases and relaxation time increases with increasing percentage volume of Propylene glycol (PLG) and Butylene glycol (BLG) in NMF. The nature of (?₀) and (τ) is same for the temperature ranges (20, 30, and 40°C). The thermodynamic parameters enthalpy (ΔH) and entropy (ΔS) of the binary mixture are also reported in this work.     

Spontaneous polarization,tilt angle and dielectric measurements on a ferroelectric liquid crystal

A new ferroelectric liquid crystalline compound 4-(p-butyloxy-2-chloro-tyrosinate)-2-hydroxy-benzylidene-4'-nonylaniline, abbreviated as BCTHBN, was characterized by various techniques. Phase sequence and transition temperature data were obtained by thermal microscopy and DSC. The values of spontaneous polarization, tilt angle, response times and viscosity, along with the wide thermal range of the SmC* phase, make this compound a candidate for applicational studies. Dielectric studies included variation of capacitance with temperature and calculation of activation energies in the soft and Goldstone modes.     

Dielectric relaxation of butyl acrylate—alcohol mixtures using time domain reflectometry

Dielectric relaxation measurements of butyl acrylate—alcohol mixtures at different concentrations and temperatures within the frequency range of 10 MHz to 10 GHz have been carried out using time domain reflectometry. Parameters such as the static permittivity, dielectric relaxation time, the Kirkwood correlation factor, the excess inverse relaxation time, and thermodynamic functions were determined and discussed to yield information on the molecular structure and dynamics of the mixture. The value of the dielectric properties decreases with increasing butyl acrylate concentration in alcohol and systematically varies with the length of alcohol alkyl chain. Negative values of the excess inverse relaxation time found for all concentrations and at all temperatures studied may indicate that the effective dipoles rotate slowly.     

Stressed states in domain switching of ferroelectric liquid crystal devices

In this paper we show that the director profile of a low pre-tilt surface stabilized ferroelectric liquid crystal passes through quasi-static stressed states during domain switching under direct drive conditions. Using polarized stroboscopic microscopy, we have observed two quasi-static transmission levels during a domain switching transition from dark to light. This is a result of the directors reorienting into stressed profiles both before and after the chevron interface has switched. By modelling the interaction between the elastic forces and the torque from the applied field, we have determined these voltage dependent director profiles and, by calculating their corresponding transmissivities, have shown very good agreement with the experimentally observed values.     

Optical guided-wave study of a homeotropically aligned ferroelectric liquid crystal

Abstract

The excitation of optical modes is used to study the optical tensor configuration in a thin ferroelectric liquid crystal layer, cooled from the initially homeotropically aligned nematic phase. By monitoring the angular dependent reflectivity for plane polarized radiation coupled into the guided modes in the smectic C* layer and subsequently fitting the recorded data to predictions from multilayer optics theory, the optical tensor configuration in the layer is fully evaluated. Iteratively modelling the full tilt/twist profile in the cell, progressively converging the predicted reflectivity to experimental data, gives a complete and very well specified picture of the optical tensor throughout the cell. By studying the cell at various temperatures, the temperature dependence of the tilt of the major axis of the optic tensor (which may be related to the cone angle if the smectic layers are parallel to the cell surface) has been established. The temperature dependent optical dielectric constants have also been obtained.     

Optical guided-wave study of a homeotropically aligned ferroelectric liquid crystal

The excitation of optical modes is used to study the optical tensor configuration in a thin ferroelectric liquid crystal layer, cooled from the initially homeotropically aligned nematic phase. By monitoring the angular dependent reflectivity for plane polarized radiation coupled into the guided modes in the smectic C* layer and subsequently fitting the recorded data to predictions from multilayer optics theory, the optical tensor configuration in the layer is fully evaluated. Iteratively modelling the full tilt/twist profile in the cell, progressively converging the predicted reflectivity to experimental data, gives a complete and very well specified picture of the optical tensor throughout the cell. By studying the cell at various temperatures, the temperature dependence of the tilt of the major axis of the optic tensor (which may be related to the cone angle if the smectic layers are parallel to the cell surface) has been established. The temperature dependent optical dielectric constants have also been obtained.     

A study of enthalpic relaxation of a liquid crystal

A liquid crystal, BL038, which was observed not to crystallize, has a glass transition at 215 K and a nematic to isotropic transition at 380 K. Samples aged below the glass transition at various temperatures T _a, exhibited an endotherm at the transition which developed with extent of ageing time, t _a. We attribute this endotherm to the relaxation of the glass towards the equilibrium liquid. The progress of the relaxation process was measured using differential scanning calorimetry. On subsequent reheating, the aged glass showed an apparent shift in the glass transition to higher temperatures. The endotherm was used to define the extent of enthalpic relaxation and the maximum value observed was found to increase initially then decrease, with the extent of undercooling from the glass transition temperature, Δ T, passing through a maximum for a Δ T = 15 K. From the temperature dependence of the relaxation times, an apparent activation enthalpy for the relaxation process of 85 ± 10 kJ mol^-1 was determined. The small value of the activation enthalpy compared with that found in the ageing of polymers reflects differences in the molecular species involved in relaxation processes.     

Low frequency dielectric relaxations of gold nanoparticles/ferroelectric liquid crystal composites

We present the characterisation and dielectric relaxation spectroscopy of a ferroelectric liquid crystal (FLC), namely KCFLC 7S. It was observed that the studied FLC material possesses the tendency of homeotropic alignment on glass substrates coated with indium tin oxide. A low frequency dielectric mode, along with the Goldstone mode, was observed in the SmC* phase of the FLC material. The low frequency mode became more dominant on doping gold nanoparticles into the FLC material. The occurrence of the low frequency mode was attributed to the ionisation–recombination-assisted diffusion of slow ions present in the FLC material. The behaviour of the relaxation frequency of the low frequency mode with applied dc bias and temperature was also demonstrated.     

Broadband dielectric spectroscopy on ferroelectric liquid-crystalline side group polymers

Broadband dielectric spectroscopy (10^-1 to 10⁷Hz, 100 to 450 K) has been applied, for the first time, to investigate the molecular dynamics of recently synthesized fast switching ferroelectric side group polymers. The softmode could be studied in detail in the S_c* phase and in the S₁* phase. At the transition a pronounced increase of the relaxational strength was found whereas its relaxational frequency remains constant The softmode is the molecular basis for the electroclinic effect which has high application potential for ferroelectric liquid crystal polymers.