Molecular dynamics of a ferroelectric smectogen in its smectic phases by means of 2H NMR spectroscopy

In this work the dynamic behaviour of the ferroelectric liquid crystal (-)-(S )-[4-(2-methylbutyloxycarbonyl)phenyl] 4-n-heptylbiphenylcarboxylate (MBHB) in its smectic A (SmA), unwound chiral smectic C (uSmC*) and chiral smectic C (SmC*) phases has been studied by means of ²H NMR spectroscopy. Zeeman (T _1Z) and quadrupolar (T _1Q) spin-lattice relaxation times have been analysed to extract dynamic parameters (diffusion coefficients and activation energies). The small step rotation diffusion model in the uniaxial approximation has been used to describe overall spinning and tumbling motions, and the strong collision model to describe the internal reorientations of the aromatic fragment. Relaxation data in the SmC* phase have been analysed by using a theoretical approach. The dynamic features obtained in the smectic phases of this mesogen are here presented and discussed in comparison with the results obtained in other ferroelectric liquid crystals, focusing on the fast regime of motions.     

Studies of translational diffusion in the smectic A phase of a Gay-Berne mesogen using molecular dynamics computer simulation

Molecular dynamics computer simulations are used to determine the self-diffusion coefficients for a Gay-Berne model mesogen GB (4.4,20,1,1) in the isotropic, nematic and smectic A phases along two isobars. The values of the parallel and perpendicular diffusion coefficients, D(parallel) and D(perpendicular), are calculated and compared in the different phases. For the phase sequence isotropic-smectic A, D(perpendicular)*> or =D(parallel)* over the whole smectic A range with the ratio D(parallel)*/D(perpendicular)* decreasing with decreasing temperature. At a higher pressure, a nematic phase is observed between these two phases and we find that D(parallel)*>D(perpendicular)* throughout the nematic region and the inequality D(parallel)*>D(perpendicular)* remains on entering the smectic A phase. However, the ratio D(parallel)*/D(perpendicular)* decreases with decreasing temperature within the smectic A range and eventually this ratio inverts such that D(perpendicular)*>D(parallel)* at low temperatures. The temperature dependence of the parallel diffusion coefficient in the smectic A phase for this model mesogen is compared to that predicted by a theoretical model for diffusion subject to a cosine potential.     

New 1,3-dioxan-type ionic liquid crystal compounds having a large dielectric constant

New pyridinium-type thermotropic ionic liquid crystal materials having a 1,3-dioxan ring in its central core: N-substituted-4-(5-alkyl-1,3-dioxan-2-yl)pyridinium bromides (6) were synthesized. The mesomorphic behaviour of these compounds and dielectric constant perpendicular to the molecular axis were measured. The principal features of these compounds are that they exhibit a smectic A phase over a wide temperature range including room temperature - for example 6f: g -9 SmA 181 I (°C)-and they have a large dielectric constant perpendicular to the molecular axis.     

Evidence for a tilted smectic phase in Anopore membranes by dielectric spectroscopy

Collective relaxation processes are completely undetectable in a ferroelectric liquid crystal confined in porous Anopore membranes, as a result of perfect orientation of the smectic layers perpendicular both to the long axis of the pores and the direction of the measuring electric field. In the ferroelectric liquid crystal – Anopore composite only one relaxation process, assigned to rotation of the molecule around the molecular short axis, appears throughout all smectic phases. The temperature dependence of the relaxation frequency and of the dielectric strength of this process also shows no irregularity at the point of polarization sign reversal. The temperature dependence of the relaxation frequency follows the Arrhenius law with an activation energy slightly higher in the ferroelectric SmC* phase. Analysis of the non‐linear changes of temperature dependence of the dielectric strength at the SmA–SmC* phase transition enables one to obtain the temperature dependence of the tilt angle of the molecules in the SmC* phase in the Anopore membrane. Dielectric measurements confirm the existence of the tilted smectic phase in Anopore cylindrical channels with no tilt anomaly at the point of polarization sign reversal.     

New 1,3-dioxan-type ionic liquid crystal compounds having a large dielectric constant

New pyridinium-type thermotropic ionic liquid crystal materials having a 1,3-dioxan ring in its central core: N-substituted-4-(5-alkyl-1,3-dioxan-2-yl)pyridinium bromides (6) were synthesized. The mesomorphic behaviour of these compounds and dielectric constant perpendicular to the molecular axis were measured. The principal features of these compounds are that they exhibit a smectic A phase over a wide temperature range including room temperature - for example 6f: g -9 SmA 181 I (°C)-and they have a large dielectric constant perpendicular to the molecular axis.     

DTA, X-ray and dielectric relaxation studies of 14CB at atmospheric and elevated pressures

The pressure-temperature phase diagram of 4'-tetradecyl-4-cyanobiphenyl (14CB) up to 220 MPa (2.2 kbar) and between 320-400 K was established using DTA. The temperature range of the smectic A (SmA) phase slightly increases with pressure. The layer spacing d at 1 atm was determined as a function of temperature using X-ray diffraction. It was related to the molecular length l by the ratio d/l ~ 1.4. The dielectric relaxation measurements in the isotropic and smectic A d phases of 14CB at 1 atm were performed in the frequency range 10 kHz-3 GHz. Contributions from both principal rotational motions, i.e. around the short and long molecular axes, were separated. The relaxation measurements under high pressure in the SmA phase covered the low frequency process. The longitudinal relaxation time τl , characterizing the molecular reorientations around the short axis, was analysed with respect to the pressure and temperature dependences, giving activation volumes, Δ# V = RT ( ∂ln τl / ∂p ) T , and activation enthalpies, Δ# H = RT ( ∂ln τl / ∂T -1 ) p , respectively. Surprisingly, all the activation quantities characterizing the rotational motions of 14CB molecules under different conditions are nearly the same as those determined recently for the much shorter homologue, 8CB. This indicates that the 14CB molecule is in fact relatively short due to conformational motions of the alkyl tail.     

Electroclinic effect in the chiral smectic A and cholesteric phases at the proximity of a N*–SmA–SmC* multicritical point

We studied the electro-optic and dielectric properties of three pure ferroelectric liquid crystal materials (C10, C11 and C12) of the same series exhibiting cholesteric (N*), smectic A (SmA) and chiral smectic C (SmC*) phases. From electro-optic investigations, the tilt angle and spontaneous polarisation were determined as a function of temperature. In the dielectric measurements carried out without a dc bias field, we studied the soft-mode relaxation in the SmA phase. From experimental data and using the results of a Landau model, we evaluated the soft-mode rotational viscosity and the electroclinic coefficient in the SmA phase. A soft-mode like mechanism was also observed in the N* phase for compounds with shorter chains (C10 and C11). This relaxation process is not detected for the homologue with a longer chain (C12). The observation of this mechanism is related to smectic order fluctuations within N* phase whose amplitude is increased when approaching the SmC*–SmA–N* multicritical point.     

EPR anisotropy induced by a magnetic field in the smectic A phase of a mesogenic Cu(II) complex derived from a Schiff's base

The Q-band electron paramagnetic resonance (EPR) spectrum of the frozen smectic phase of the mesogenic compound bis[N -(4'-n-pentoxyphenyl)-4-n-decyloxysalicylaldiminate] copper(II) is studied. An anisotropic EPR spectrum is observed if freezing from the SmA phase is done in the presence of a magnetic field; the sample tends to orient with the director perpendicular to the magnetic field. The relative intensities of the EPR signal of this exchange coupled system are discussed in relation to the orientational distribution of the molecules. The results appear to be compatible with a biaxial magnetic susceptibility in the SmA phase. This suggests that an ordering of the short axes of the molecules in the plane perpendicular to the director in the SmA mesophase may take place.     

Amplitude and phase fluctuation modes in the smectic Cα* phase of an antiferroelectric compound

Dielectric measurements have been carried out on the chiral smectic C_α (SmC_α*) phase of a MHPOBC analogous compound. Two relaxation modes have been observed in this phase for planar orientation of the molecules. One process has been observed at frequency lower than that of the soft mode of the chiral smectic A (SmA*) phase. This relaxation process is connected with the helicity of the SmC_α* phase. In the high‐frequency region, another relaxation process has been observed in the SmC_α* phase for which bias field dependence is similar to that of the soft mode at the SmA*–SmC* phase transition. The experimental observations are in agreement with a recently proposed dielectric theory for the SmC_α* phase and theoretical dielectric results obtained by numerical simulations. Thus, we report here experimental verification of two theoretically predicted dielectric modes in the SmC_α* phase.     

Soft mode and related behaviour in the SmA and SmC* phases of a ferroelectric liquid crystalline polymer by dielectric spectroscopy

Anomalous dielectric relaxation behaviour is observed in the ferroelectric liquid crystalline polymer (viz. ferroelectric copolysiloxane (R)-COPS 11-10) around the ferroelectric SmC* to paraelectric SmA phase transition. Measurements have been performed on sample of thickness ~10 mum in indium-tin-oxide coated cell in the frequency range 10 Hz to 13 MHz. With increase of temperature, a gradual shift of the soft mode frequency towards the higher frequency side was observed, while a decrease in the relaxation strength was seen with the corresponding increase in temperature. The shifts of the soft modes in the SmC* and SmA phase are considered to be due to change in the viscosity of the polymer, as an increase in viscosity increases fluctuations of the coupling between the dipoles in the network even far from the paraelectric-ferroelectric phase transition. Application of a bias field causes a shift of the critical frequency towards the higher frequency side, while the dielectric strength ( Δε) decreases under the bias field. The Cole-Cole fitting parameters obtained from the best fit of the dielectric constant data are found to be consistent with other similar materials. Another relaxation mode (molecular mode) was also observed which comes into play in both the smectic phases (SmC* and SmA) and contributes to the dielectric permittivity.     

Soft mode and related behaviour in the SmA and SmC* phases of a ferroelectric liquid crystalline polymer by dielectric spectroscopy

Anomalous dielectric relaxation behaviour is observed in the ferroelectric liquid crystalline polymer (viz. ferroelectric copolysiloxane (R)-COPS 11-10) around the ferroelectric SmC* to paraelectric SmA phase transition. Measurements have been performed on sample of thickness ～10 μm in indium-tin-oxide coated cell in the frequency range 10 Hz to 13 MHz. With increase of temperature, a gradual shift of the soft mode frequency towards the higher frequency side was observed, while a decrease in the relaxation strength was seen with the corresponding increase in temperature. The shifts of the soft modes in the SmC* and SmA phase are considered to be due to change in the viscosity of the polymer, as an increase in viscosity increases fluctuations of the coupling between the dipoles in the network even far from the paraelectric-ferroelectric phase transition. Application of a bias field causes a shift of the critical frequency towards the higher frequency side, while the dielectric strength (δε) decreases under the bias field. The Cole-Cole fitting parameters obtained from the best fit of the dielectric constant data are found to be consistent with other similar materials. Another relaxation mode (molecular mode) was also observed which comes into play in both the smectic phases (SmC% and SmA) and contributes to the dielectric permittivity.     

Dielectric spectroscopy of a smectic liquid crystal

Complex dielectric spectroscopy (frequency range 5 Hz–13 MHz) has been used to analyse the frequency, temperature and bias‐field dependences of the molecular dynamics of a very high‐spontaneous‐polarization ferroelectric liquid crystalline material exhibiting SmA, SmC* and unknown SmX smectic phases. Different smectic phase transition temperatures have been observed from the study of the temperature dependence of the dielectric strength and the relaxation frequency. The phase transition temperatures (crystalline to isotropic phases) have also been described very accurately from the temperature‐dependent symmetric and asymmetric shape parameters of the relaxation function and also the dc conductivity. In a planar aligned cell, two symmetric modes (Goldstone mode and domain mode) have been observed in both the SmX and SmC* phases. One asymmetric mode (X‐mode) observed in the SmC* and SmA phases could be related to the interaction of dipoles of the ferroelectric liquid crystals being affected by the surface of the cell. The soft mode, which usually appears very close to the SmC*–SmA phase transition was not observed until the bias field was applied. The second order nature of the SmC*–SmA phase transition was revealed.     

Anomalous behaviour in the SmA*–SmCA* pre‐transitional regime of a chiral swallow‐tailed antiferroelectric liquid crystal

The temperature‐ and electric field‐dependent dielectric relaxation and polarisation of a new chiral swallow tailed antiferroelectric liquid crystal, i.e. 1‐ethylpropyl (S)‐2‐{6‐[4‐(4′‐decyloxyphenyl)benzoyloxy]‐2‐naphthyl}propionate (abbreviated as EP10PBNP), were investigated. The electric field‐induced dielectric loss spectra of EP10PBNP revealed electroclinic and anomalous dielectric behaviour in the chiral smectic A (SmA*)–chiral antiferroelectric smectic C (SmC_A*) pre‐transitional regime. From an analysis of thermal hysteresis of the dielectric constant, electric field‐induced polarisation and dielectric loss spectra, the appearance of a ferrielectric‐like mesophase is observed followed by an unstable SmC_A* phase in the SmA*–SmC_A* pre‐transitional regime.     

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 (SmC_A*) 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 SmC_A* 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 SmC_A* phase.     

2H NMR Spectroscopic Investigation of para-Nitroazobenzene Liquid Crystals

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Dielectric spectroscopy of a smectic liquid crystal

Complex dielectric spectroscopy (frequency range 5 Hz-13 MHz) has been used to analyse the frequency, temperature and bias-field dependences of the molecular dynamics of a very high-spontaneous-polarization ferroelectric liquid crystalline material exhibiting SmA, SmC* and unknown SmX smectic phases. Different smectic phase transition temperatures have been observed from the study of the temperature dependence of the dielectric strength and the relaxation frequency. The phase transition temperatures (crystalline to isotropic phases) have also been described very accurately from the temperature-dependent symmetric and asymmetric shape parameters of the relaxation function and also the dc conductivity. In a planar aligned cell, two symmetric modes (Goldstone mode and domain mode) have been observed in both the SmX and SmC* phases. One asymmetric mode (X-mode) observed in the SmC* and SmA phases could be related to the interaction of dipoles of the ferroelectric liquid crystals being affected by the surface of the cell. The soft mode, which usually appears very close to the SmC*-SmA phase transition was not observed until the bias field was applied. The second order nature of the SmC*-SmA phase transition was revealed.     

Temperature and bias-field dependences of dielectric behaviour in the antiferroelectric liquid crystal, (R)-MHPOBC

Temperature and bias-field dependences of dielectric behaviour in the antiferroelectric liquid crystal, R-MHPOBC, were investigated (see also previous paper). There are mainly two relaxation modes in the SmC*alpha and SmC* phases: one behaves as the soft-mode, which shows significant slowing down in the SmA* and SmC*alpha phases; the other one appears at lower frequencies and changes the dielectric strength remarkably, especially in the SmC* phase, which is considered to relate to the azimuthal phase-fluctuation of molecules in the parallel tilt sequences of the smectic layers. These two modes show different bias field dependences in different C* subphases. In the SmC*A phase, two other types of relaxation mode were observed, which are probably due to the in-phase and anti-phase azimuthal angle fluctuations of molecules in the anti-tilt pairs.     

Synthesis and characterization of 6-[4-(trans -4-pentylcyclohexyl)phenoxy]hexyl acrylate homopolymer and copolymers

The compound 6-[4-(trans -4-pentylcyclohexyl)phenoxy]hexyl acrylate (2) was prepared and homopolymerized. The block copolymer and copolymer of 2 with styrene were synthesized by photopolymerization and solution techniques, respectively. These polymers were characterized by IR and ¹H NMR spectra and size exclusion chromatography. Polarizing optical microscopy (POM) and X-ray diffraction (XRD) studies revealed that these polymers exhibited smectic A (SmA) phases. POM showed that the homopolymer showed a higher order SmA phase than did the block copolymer and copolymer. After magnetically forced alignment the samples exhibited similar optical texture but the domain size of the liquid crystalline phase increased. Differential scanning calorimetry, POM and XRD data suggest that the SmA domain size decreased in the order hompolymer > block copolymer > copolymer.     

Hole mobility and lifetime in a smectic liquid crystalline photoconductor of a 2-phenylnaphthalene derivative

We have investigated hole transport properties in the smectic mesophases of a 2-phenylnaphthalene derivative 6-(4'-octylphenyl)-2-dodecyloxynaphthalene in detail by using time-of-flight technique. The transient photocurrents were measured in liquid-crystal cells with various thickness from 5 to 700 microm. They were well defined and nondispersive in the smectic A (SmA) phase up to 500 microm and in the smectic B (SmB) phase within the entire thickness employed, while they exhibited an exponential decay in the SmA phase at 700 microm. The mobilities in the SmA and SmB phases were constant in each mesophase irrespective of the cell thickness, and were 2.5 x 10(-4) and 1.7 x 10(-3) cm2V s, respectively. The hole lifetimes were determined to be 10 ms and longer than 5 ms for the SmA and SmB phases, respectively. We discuss the origin of these lifetimes from the two points of view, i.e., hole trapping by a trace amount of existing impurities and recombination with negative ionic charges. We conclude that impurities are mainly responsible for the present hole lifetime test.     

Ferroelectric liquid crystals derived from isoleucine I. Synthesis and characterization

A series of ferroelectric liquid crystals (FLCs), 4 (3-methyl-2-halopentanoyloxy)- 4-hexyloxybiphenyls (3M2XPHOB, X=F for fluorine, C for chlorine, B for bromine) and their racemates (3M2XPHOB-R), were synthesized and characterized. The FLCs contain a chiral tail comprised of alpha -halo acids which are derived from L-isoleucine (DL-isoleucine for the racemates). The mesogens were characterized by high-resolution 1H and 13C NMR and their phase behaviour was studied by optical microscopy and differential scanning calorimetry. The chloro and bromo derivatives show both chiral smectic C (SmC*) phases and smectic A (SmA) phases, while the fluoro derivatives exhibit only a SmA phase. The spontaneous polarization of 3M2CPHOB and 3M2BPHOB were measured in the respective SmC* phases; that of the fluoro derivative was inferred by extrapolating its concentration dependent polarization in an achiral SmC solvent, a racemic mixture of 3M2BPHOB.