Low and high frequency dielectric spectroscopy on a liquid crystal with the phase sequence N*-SA-S*c

Broadband dielectric measurements on a multicomponent ferroelectric liquid crystal mixture have been performed. The alignment was homeotropic and the cell and sample holder were the same in the whole frequency range. Two relaxation processes have been observed in all liquid crystal phases with shapes given by the Havriliak-Negami and inverted Havriliak-Negami functions. Strengths and frequencies of both modes have been obtained for the different phases. The values of the latter and their activation energies allowed us to assign the low and high frequency mechanism to the molecular reorientation around the transversal axis and around the longitudinal axis, respectively. The behaviour of this high frequency mode does not show any jump in the S_A-S*_c phase transition, neither in the amplitude nor in the frequency, indicating that the appearance of the spontaneous polarization in the S*_c phase is not the consequence of the freezing of this mode.     

Linear electromechanical effect in a S*c polymer liquid crystal

A linear electromechanical effect, similar to that observed in low-molecular weight S*_c liquid crystals, is reported for a S*_c polymer liquid crystal. A 20 μm thick film of LCP1 (polymer liquid crystal, S*_c, supplied by BDH) was prepared that could be aligned by shearing. An alternating electric field was applied perpendicular and the lateral oscillating motion of the cover glass was measured and analysed. In addition to a linear response, vibrations with the frequency of the applied field, a quadratic response was also observed. The frequency, temperature and applied voltage dependences were measured. The response depends strongly on the alignment. The qualitative analysis of the effects indicates that the oscillations are due to viscous coupling between director reorientation and flow.     

Circular dichroism in ferroelectric and antiferroelectric liquid crystals

Liquid crystal induced circular dichroism (LCICD) measurements were made to investigate the pretransitional phenomena in the S_A phase just above the S_A-S*_C and S_A-S*_CA phase transitions of both the first and the second order. The pretransitional LCICD in S_A was observed in the second order phase transition to S*_c and S*_cA, suggesting the existence of a dynamic helical structure in S_A. Such behaviour disappears when the transition is of the first order. It seems that the handedness of the dynamic helix in S_A is the same as that in S*_c even when the lower temperature phase is S*_cA. This is explained as a result of a dominant contribution of ferroelectric soft mode.     

Hindered reorientation of chiral molecules around their long axes in the S*C phase

In this work, the high frequency dielectric relaxation process has been studied for the N*, S*_A and S*_C phases of the mixture ZLI 3654. To this end, time domain spectroscopy (TDS) was used to cover the frequency range from 10⁷ to 10¹⁰ Hz. The dielectric parameters obtained show that the high frequency spectrum is mostly influenced by the reorientation of molecules around their long axes. In addition, the latter process seems to be strongly hindered in the S*_C phase, because the necessary relaxation time is of the order of 1 ns, whereas for the N* and S*_A phases, the relaxation time is remarkably shorter and equal to 0.3 and 0.4 ns, respectively.     

Dye-containing S*c side-on/end-on copolymers

Mesomorphic copolysiloxanes, which combine 'normal' end-on linked chiral mesogenic units inducing S*_c phases and side-on fixed chromophores were prepared. The interplay of the different orientational tendencies of these moieties, which are perpendicular (end-on linked mesogenic groups) or parallel (side-on fixed chromophores) to the polymer chain, lead to a strong destabilization of the S*_c phase. However, copolymers with up to about 10 mol % of chromophores still show a smectic C* phase. FTIR measurements show that both moieties orient parallel to each other and perpendicular to the polymer chains. These copolymers are interesting to consider as coloured guest-host S*_c materials for coloured displays or as pyro-electric detectors.     

Photomechanically induced ferroelectricity in smectic liquid crystals

We report new photomechanical effects in the ferroelectric liquid crystal SCE13 doped with a photoisomerizing guest azo dye. Low concentrations of dye (∼5 per cent wt:wt) are shown to cause an isothermal, reversible disruption of smectic phases when the system is illuminated with low power density (∼ 1 mW cm^-2) UV light. In the case of a sample initially in the S*_c phase, this results in a fall in the magnitude of spontaneous electrical polarization (P_s) and changes in electro-optic switching characteristics. If the sample is illuminated in the S_A phase, the electroclinic switching decreases. In contrast to this, when systems containing higher concentrations of dye (≥ 10 per cent wt: wt) are UV illuminated in the S_A phase, a reversible, isothermal transition to a biphasic S*_c/isotropic state occurs. In this case, the P_s is seen to rise from zero in the S_A phase to a finite value(∼2 nC cm^-2) in the biphasic mixture and hysteresis occurs in the electro-optic switching. When these higher dye concentration mixtures are held initially in the S*_c phase and UV illuminated, a more complicated variation of P_s occurs with the sample again undergoing a transition to a biphasic S*_c/isotropic state. Possible mechanisms for the transition are discussed.     

Novel liquid crystalline structures of a chiral side chain polymer and its phase transitions

Two novel smectic bilayer structures have been identified in an enantiomerically enriched chiral side chain polymer containing the highly dipolar nitrile group at stereocentres. The structures were characterized by electron diffraction, electron microscopy, and X-ray diffraction. In both phases each smectic layer has a bilayer structure with backbones and spacers confined in a thin disordered region between two sublayers of mesogenic segments. One of the structures which we denote as CrE* has the unusual feature of having its side chains arranged parallel to the layer normal in spite of its enantiomeric bias and twisted nature. In the second structure side chains are tilted by 34.8° with respect to the layer normal and we denote this phase as CrH*_c In both structures each sublayer contains three different orientations of orthorhombic (CrE) or monoclinic (CrH*_c) lattices which are related to one another by rotations of ± 60° about the c-axis. In both the CrH*_c and the CrE* phases, lattices in each sublayer are regularly rotated about the c-axis by 5.9° relative to those in the adjacent sublayer. The observation of a chiral CrH phase is uncommon and in this specific case the structure is unique since the rotation between adjacent layers occurs about the sidechain axis (c-axis) (CrH*_c) and not about the layer normal (c-axis) (CrH*_c). We believe the system undergoes a change in molecular organization from CrH*_c to CrE* as a result of a chemical reaction which joins a fraction of the stereocentres through covalent bonds. With increasing temperature the CrE* structure was found to transform to a special orthorhombic untwisted smectic phase in which a = 31/2b, denoted here as CrE_h. The structure then transforms to a hexatic S_B phase and finally to a S_A phase at yet higher temperatures.     

Columnar phases of achiral vanadyl liquid crystalline complexes

Two liquid crystalline vanadyl complexes have been studied by frequency domain dielectric spectroscopy over the range 10 mHz to 13 MHz. The materials exhibit two or three columnar phases denoted Col_ro, Col_rd, and Col_hd that were identified by X-ray diffraction. In the higher temperature Col_rd phase, a relaxation process in the kHz range is observed that is attributed to the reorientation about the molecular short axis. A pronounced dielectric relaxation process shows up in the low temperature Col_ro phase at hertz and sub-hertz frequencies. This slow relaxation is assigned to reorientation of the molecular dipoles within the polar linear chains, which are aligned along the column's axis. Triangular wave switching studies at low frequency reveal processes inside the Col_ro phase which are most probably due to ionic/charges relaxations but a ferroelectric switching for an achiral discotic system cannot be ruled out completely. Below the Col_ro phase there is an orientationally disordered crystalline Cr_x phase with disordered side chain dipoles. A dielectric relaxation process connected with the intramolecular relaxation of the alkoxy side chains, similar to the beta-process of polymers, has been found in the lower temperature Cr_x phase.     

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.     

Fluorescent ferroelectric liquid crystalline copolyacrylates

Fluorescent liquid crystalline side chain polymers were synthesized by copolymerization of a ferroelectric monomer and 5 per cent of various blue fluorescent naphthalic imide dye comonomers. Those copolymers were characterized by DSC, X-ray, GPC and optical microscopy. In favourable cases, fast switching fluorescent ferroelectric polymers resulted, exhibiting high tilt angles (up to ∼ 34°) and spontaneous polarization values (up to ∼ 115 nC cm^-2) in the S*_c phase. One fluorescent copolymer shows orthogonal smectic phases exclusively due to the structure of the incorporated fluorescent comonomer. In this case a strong electroclinic effect and high induced tilt angles (12° 10 V μm^-1) have been observed in the S_a phase. Order parameters, S, of the dye moieties up to 0.64 were measured in the room temperature S_b phase for the copolymers     

Dielectric spectroscopy of the twist grain boundary phase and smectic-like behaviour in the Isotropic Phase

Dielectric properties of a novel liquid crystal with direct transition from isotropic to twist grain boundary (TGB) phase were studied. Measurements were performed over a wide range of temperature in the isotropic phase, TGB_A, TGB_C and SmC* phases. The Arrhenius plot of the relaxation frequency of the molecular process around the short axis shows a variation in the activation energy in the isotropic phase. This is attributed to smectic-like fluctuations in the isotropic phase. The dielectric relaxation strength in the TGB_C phase has anomalous behaviour in the vicinity of TGB_C-SmC* transition. The single cellective process observed in the TGB_C phase splits into two processes in the SmC* phase.     

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.     

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.     

Dielectric characterization of the B7 and a BX phase

The first clear experimental evidence of a dielectric relaxation in the B₇ phase is given. The associated molecular process is probably the reorientation of the bent-shaped molecules about the long molecular axis. It is also shown that the dielectric constant strongly increases at the phase transition I-B₇ pointing to a co-operative motion of ferroelectric clusters. Furthermore, a phase transition B₇-B_X is observed. The values of about 10 for the high frequency limit of the dielectric constants of the B₇ and B_X phases and the static dielectric constants of the isotropic phase are in agreement. Thus, in all the phases the main dipoles can reorient faster than 10^-8 s, i.e. the experimental limit. Only in the supercooled B_X phase were the dynamics of the transversal dipoles measured. Using atomic force microscopy, focal-conic domains modulated by parallel lines have been observed at room temperature.     

On the relationship between electro-optic and dielectric parameters of a smectic C* ferroelectric liquid crystal

A relationship between the electro-optic switching time and dielectric parameters of a S*_c ferroelectric liquid crystal (FLC) is obtained. This relationship is derived in terms of spontaneous polarization P_s, relaxation time τ_G and dielectric strength Δε_G of the Goldstone mode. It shows clearly that the switching phenomenon in FLCs is governed by the dielectric behaviour of the Goldstone mode. Based on the Landau model, the switching time has also been related to the material parameters of the FLC.     

The characteristic dielectric behaviour in ferroelectric liquid crystals at a phase transition and the contribution of the soft mode

The characteristic dielectric behaviour of ferroelectric liquid crystals with a large spontaneous polarization has been studied as functions of the D.C. bias field, frequency, cell thickness and applied pressure. Under the condition in which the contribution of the Goldstone mode is suppressed, a sharp peak in the temperature dependence of the dielectric constant is clearly observed at the transition between S_A and S*_C phases T_SC*sA. The relaxation of the soft mode is observed both in the S_A and S*_C phases by eliminating the contribution of the Goldstone mode under a D.C. bias field. Another relaxation is also observed in the S*_C phase around several kHz in addition to that of the soft mode and the Goldstone mode. The pressure effect on the soft mode was also studied.     

Helical structures induced by laterally-connected chiral twin molecules

A novel chiral twin material, (R)-bis[5-octyloxy-2-(4-octyloxyphenoxycarbonyl)phenyl] 3-methyladipate, has been prepared, where two mesogenic parts are connected laterally by a spacer possessing a chiral centre. A weaker helical structure, in particular in the chiral smectic C (S*_c) phase, was found to be induced by the laterally-connected twin material than by the analogous terminally-connected twin material. If laterally-connected chiral twin molecules prefer to stay in the smectic layer structure so that the two mesogenic parts exist in the same smectic layer, the twist interaction between adjacent layers cannot be produced by direct correlation of motion and directions of two mesogenic parts. Thus, the helical structure in the S*_c phase induced by laterally-connected chiral twin molecules becomes weak. An analogous laterally-branched 'monomeric' compound, (S)-5-octyloxy-2-(4-octyloxyphenoxycarbonyl)phenyl 3-methyl-pentanoate, has also been prepared, and the induced helical structures compared.     

The dielectric relaxation processes in an antiferroelectric liquid crystal under cylindrical confinement

In the cylindrical pore geometry of inorganic Anopore membranes the collective relaxation processes observed in a bulk antiferroelectric liquid crystal change considerably under confinement. The frequency degeneration of the soft and Goldstone modes present at the smectic A* (SmA*)-chiral smectic C (SmC*) phase transition in the bulk phase is removed under geometrical restrictions. The relaxation rate of the soft mode is strongly modified due to the deformation of the smectic layers in the curved geometry of the pores and is superimposed by the molecular relaxation process in the SmA* and SmC* phases. The soft mode in confinement splits into two relaxation processes, which are present through all other mesophases (SmC* and SmC_a*). One of them is nearly temperature independent and slightly decreases in frequency in the SmC_a* phase. This Goldstone-like process can be assigned to the highly deformed helical structure fluctuations. The second one exhibits the characteristic features for the molecular and soft mode relaxation processes depending on the temperature range. The biquadratic and the piezoelectric coupling between the tilt angle and spontaneous polarization are revealed in their temperature dependence.     

New ferroelectric liquid crystals having a trifluoromethyl group at the asymmetric centre and a three-ring heterocyclic core

New ferroelectric liquid crystals with a trifluoromethyl group at the asymmetric centre and a three-ring heterocyclic core have been synthesized. These compounds possessed a large spontaneous polarization and exhibited a S*_c phase over a wide temperature range. Their properties were largely affected by the structure of the spacer group and the core part.     

Director movement and potential function of surface stabilized ferroelectric liquid crystal cells by electrooptical investigations

Electrooptical investigations on a ferroelectric liquid crystal (DOBAMBC) were carried out on a well aligned sample in a 2 μm cell. On the basis of these measurements the complete average director movement during a switching process caused by an electric field can be calculated and will be discussed. The thermodynamic potentials at the S_A*-S_C* phase transition were determined from the experimental results with a modified Landau theory. They serve as a basis for a discussion of the phase transition and the influence of the surface orientation. Some preliminary results were obtained concerning the electrooptical behaviour of the S_H* phase.