In situ observation of smectic layer reorientation during the switching of a ferroelectric liquid crystal

Abstract

A thin plastic cell containing the ferroelectric liquid crystal ZLI3654 (4 μm) was placed edge-on to a pin-hole collimated horizontal X-ray beam. In this way, the smectic layers were brought into register. Subsequently, triangular voltage waves with various peak (V _p) values were applied across the cell and diffraction photos were obtained during the application of the alternating voltage. Up to V _p = ± 30 V, no significant movements in the initial tilted orientation of the smectic layers with respect to the surfaces (chevrons) could be observed. During the application of V _p = ± 32 V an increasing fraction of the smectic layers changed their initial tilt angle with respect to the cell surfaces to make larger tilt angles. At a slightly higher voltage, the layers became upright (bookshelf structure). Upon removing the voltage and short circuiting the cell, the quasi-bookshelf structure was sustained. The new method described here can be used in combination with a fast detector for time resolved experiments.     

A proton spin relaxation study of ferroelectric liquid crystals

Here we present and analyse N.M.R. measurements of the Larmor frequency dependence (dispersion) of the longitudinal proton spin relaxation time, T ₁(v), for two chiral ferroelectric mesogens (Merck IS-1912 and DOBAMBC) in the isotropic, smectic A and smectic C* phases, making use of fast field cycling techniques. Although in the low frequency range the relaxation times of IS-1912 are much shorter than those of DOBAMBC, the form of the dispersion profiles is not basically different for the two materials. This reveals contributions by smectic order fluctuations, self-diffusion and molecular rotations. The order fluctuation term, which means relaxation by collective molecular reorientations, is clearly seen by characteristic dispersion profiles in the kHz regime (T ₁ ～ v ¹ or T ₁ ～ v ^1/2), which disappear in the isotropic phase. Our results do not indicate significant dissimilarities between the main relaxation processes in the S_C and S*_C mesophases.     

Electro-optic response of an antiferroelectric liquid crystal in submicron cells

The dynamics of chiral smectic phases of antiferroelectric liquid crystal MHPOBC in a confined geometry has been analysed. Using an electro-optic response technique, the temperature dependences of the relaxation rates and electro-optic strengths of the elementary excitations in thin, planar aligned, wedge-type cells of thickness from 0.3 to 4 μm have been measured and compared with those for a 50 μm hometropically aligned cell. The effects of the confined geometry are the following. (i) The smectic C^* _γ phase does not exist in planar aligned cells with thickness less than 4 μm. Instead of this phase, we have observed the coexistence of the ferroelectric smectic C^* phase and the antiferroelectric smectic C^* _A phase over a very wide temperature range. (ii) The smectic C^* _α phase is stable at all measured thicknesses down to 0.3 μm. (iii) We have observed a decrease of the smectic A-smectic C^* _α phase transition temperature, proportional to the inverse of the cell thickness. (iv) Additional, thickness-independent phase modes have been observed above some critical value of the measuring electric field in all tilted phases.     

Electro‐optical behavior of ferroelectric liquid crystalline polyphenylene derivatives

Liquid crystalline (LC) polyphenylene derivatives, such as poly(para‐phenylene) (PPP), poly(meta‐phenylene) (PMP), poly(meta‐biphenylene) (PBP), and poly (meta‐terphenylene) (PTP) derivatives, were synthesized through substitution of fluorine‐containing chiral LC groups into side chains, with an aim to develop ferroelectric LC (FLC) conjugated polymers. All the polymers, except PTP, showed enantiotropic liquid crystallinities, where several types of mesophases were observed in both heating and cooling processes. Among them, PPP and PMP derivatives showed chiral smectic C (S_C*) phases responsible for ferroelectricity. In fact, they exhibited quick response to electric field, in spite of high viscosities inherent to polymers, giving rise to switching times of less than 1 s between two S_C states with reversely directed alignment. Hysteresis loops between the polarization and electric field were also observed for PPP and PMP. The spontaneous polarization (P_S) of PMP remained unchanged even after the electric field became zero, affording the residual polarization (P_R) whose value was the same as that of P_S. This indicates that PMP has a prospective memory function based on FLC nature. The present study is the first report for realizing a quick switching in macroscopic alignment using electric field and also for generating a potential memory function in π‐conjugated polymers. It is elucidated that unusual polymer main chains such as polyphenylenes can be used to prepare new ferroelectric polymers. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3591–3610, 2008     

Cone motion viscosity and optical second harmonic generation of ferroelectric liquid crystalline dendrimers

We report second harmonic generation in a ferroelectric liquid crystalline trimer and ferroelectric liquid crystalline dendrimers of first, second and third generation. Thin cells were filled with the compounds by capillary forces at elevated temperature, and cooled from the surface stabilized ferroelectric state to below the glass transition temperature, while kept in an electric field. The cone motion viscosity and the threshold electric field for unwinding of the helix axis of the chiral tilted smectic mesophases were studied separately at elevated temperature, and these data were used to optimize the preparation of the films. The measured response time was between 0.3 and 3ms, which corresponds to a cone motion viscosity between 0.5 and 50 Pa s. Second harmonic generation was studied both at elevated temperature with an electric field and at room temperature with and without electric field. The first generation dendrimer exhibited a strong increase in the second order non-linear optical response with time at room temperature. The d ₂₃-coefficient of this dendrimer was approximately four times larger than for the other macromolecules and was 0.045 pm V^-1. The relatively large d-coefficient of the first generation dendrimer is ascribed to crystallization, which improved the orientation of the molecular dipoles.     

Electric field induced birefringence in a ferroelectric liquid crystal

We have measured the d.c. electric field dependence of the birefringence and conoscopic images for the smectic C* phase of a partially racemized ferroelectric chiral smectic liquid crystal CE-8. The experiments were performed using 50mu m thick homeotropic cells with lateral electrodes which created a d.c. electric field parallel to the smectic layers. The observed field induced birefringence shows a characteristic step-like behaviour which is due to the stepby-step unwinding of the helical structure in a sample with finite dimensions along the helical axis. In conoscopic observations we observe that these steps are associated with moving disclination lines that traverse the sample in the direction of the smectic layers. The observed electric field dependence of the birefringence is discussed in terms of the soliton-like unwinding of helical smectic structures and compared with the predictions of the Landau theory. A qualitatively good agreement is obtained.     

Effect of the Magnetic Field on the Supramolecular Structure of Chiral Smectic C Phases: 2H NMR Studies

We present a theoretical and experimental ²H NMR study of the effect of external magnetic fields on the supramolecular organization of chiral smectic liquid‐crystalline mesophases, such as SmC* and re‐entrant SmC*. Three experimental cases in which the supramolecular helical structure of the smectic C* phase is unwound by a magnetic field (H), parallel to the helical axes of this phase, are discussed in detail. Unwinding of the helical structure is described by using a theoretical model based on the Landau‐de Gennes theory, which allows us to explain the transition temperatures among the SmA, SmC*, and uSmC* phases. The energy‐density behavior in the vicinity of the transitions and the value of the critical magnetic field H_C for unwinding the helical structure are discussed by applying this model to three ferroelectric smectogens ( MBHB , 11EB1M7 , ZLL7/* ), which are studied by ²H NMR spectroscopy at different magnetic fields (from 2.4 to 9.4 Tesla). Furthermore, the tilt angle of the three smectogens in the SmC* phase has been directly evaluated, for the first time, by comparing the quadrupolar splittings at different magnetic fields. In one case, ²H NMR angular measurements are used to obtain the tilt angle in the re‐entrant smectic C phase.     

Dielectric,viscosity, and pitch measurements of a ferroelectric liquid crystal exhibiting the phase sequence smectic M–smectic C

Abstract

A chiral liquid crystal compound exhibiting the ferroelectric smectic C phase and the recently discovered ferroelectric smectic M phase has been studied by measurements of the Goldstone-mode relaxation frequency and dielectric strength, the spontaneous polarization, the tilt angle and the helical pitch. The data allow the determination of the Goldstone-mode rotational viscosity and the pitch controlling elastic constant. The results indicate that the smectic M phase is characterized by a larger molecular order within the smectic layers compared to the smectic C phase confirming the assumption of a tilted hexatic structure for the smectic M phase.     

Ferroelectric smectic meso-phase formed by banana-shaped achiral liquid crystals

A new banana-shaped achiral molecule, 1,3-phenylene bis[4-(3-chloro-4-n-octyloxyphenyliminomethyl)benzoate] (PBCOB) has been synthesized, and its ferroelectric properties and homeotropic alignment investigated. The presence of a lateral chloro-substituent in the Schiff 's base moiety prevents the regular stacking of molecules and results in lowering the transition temperature and the degree of crystallinity of the switchable banana phase. Their smectic mesophases, including a switchable banana phase B₇, were characterized by differential scanning calorimetry, X-ray scattering and polarizing optical microscopy. Both the left- and right-handed helical domains are spontaneously formed upon cooling from the isotropic liquid to the switchable banana phase B₇. By X-ray study, the smectic phases showed a layer spacing of 38.1 Å, compatible with the end-to-end distance of the molecule with a bent conformation. Significantly, the smectic B7 phase exhibited a periodicity of 292 Å that corresponds to a helical structure with periodicity about 7.5 times 38.1 Å. The spontaneous polarization for PBCOB is about 50 nCcm^-2 and shows a temperature dependence. The ferroelectric lyomesophase of PBCOB showed a ferroelectric electro-optical switching range extending more than 50°C, switchable at room temperature.     

Enhancing the electro-optical properties of ferroelectric liquid crystals by doping ferroelectric nanoparticles

The effects on the physical and electro-optical properties of ferroelectric liquid crystals (FLCs) after the doping of a dilute suspension of ferroelectric nanoparticles (BaTiO₃) have been studied. Due to the permanent electric dipole moments of the ferroelectric nanoparticles, the spontaneous polarisation of FLCs with low doping concentration was about twice that of pure FLCs, in addition to a significant improvement in the dielectric properties, the response time and the V-shaped switching in the chiral smectic C (SmC?) phase. The results obtained point the way to an alternative for improving the applicability of FLCs without resorting to chemical synthesis.     

A coloured ferroelectric side chain polymer

Abstract

Coloured ferroelectric liquid-crystalline side chain copolymers containing 5 per cent and 15 per cent of an azo dye were synthesized and characterized by DSC, GPC and optical microscopy. Order parameters, S, of the azo compound exceeding 0·8 were measured in the frozen smectic phase for the 5 per cent copolymer. The copolymers exhibit fast electrooptic switching in the range of a few hundred microseconds to milliseconds in the S_c* phase. Both linear (i.e. electroclinic-like) and ferroelectric switching have been observed. Tilt angles of ～19° and spontaneous polarization of ～300nC cm^?2 have been recorded in the S_c* phase.     

Dielectric and electro-optical properties of Mn12-acetate and ferroelectric liquid crystal composite

The impact of single-molecule magnet (SMM), Mn₁₂-acetate, on the dielectric properties of ferroelectric liquid crystal (FLC) has been investigated by dielectric spectroscopy and electro-optical techniques in a dual electrode sample cell (DESC). The temperature-dependent dielectric studies on Mn₁₂-acetate/FLC composite have revealed the enhancement in the ferroelectric (smectic C*) to paraelectric (smectic A*) phase transition temperature by 3.5°C. The relaxation process corresponding to Goldstone mode in the ferroelectric phase of the composite is found to be slower compared to pure FLC sample. The electrical response for an input triangular wave shows the existence of one extra polarisation peak in Mn₁₂-acetate/FLC composite which is ascribed to the induced dipole moment in Mn₁₂-acetate molecule. The electro-optical texture of Mn₁₂-acetate/FLC composite revealed that the incorporation of SMM in FLC significantly improve the memory effect.     

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.     

Properties of new ferroelectric materials

A homologous series of chiral 4-(3-methylpentyl)benzenethio-4′-n-alkoxy-benzoates has been studied. These thioesters display a ferroelectric, chiral smectic C phase in addition to cholesteric and smectic A phases. A comparison is made between the thioester series and a phenylbenzoate, having the same molecular end group. The effect of the different central linkage on the transition temperature, and on the physical and ferroelectric liquid crystal (FLC) properties has been investigated. Several mixtures, containing these thioester components, were calculated and formulated to obtain room temperature chiral smectic C phases. Spontaneous polarization P_s values and electro-optical response times are correlated with chemical structures. Although these thioesters have very low P _s values, they are useful components for FLC mixtures because of their convenient chiral smectic C temperature ranges and their low viscosities.     

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.     

Surface effects on the polarization reversal of the ferroelectric liquid crystal 8SI*

Abstract

The activation field for the switching time (τ_1/2) in the truly ferroelectric liquid crystal 8SI* (CE8; ΔS-(+)-4-(2′-methylbutyl) phenyl-4′-n-octylbiphenyl-4 carboxylate) has been studied for various surface conditions of the transparent electrode (SnO₂), in order to understand the memory effect, the polarization reversal mechanism and its surface effect. We prepared five different surfaces, namely one coated with polyvinyl alcohol (PVA) and the other four by scratching N-times(N = 0, 10, 20 and 30) with a paste made of Cr₂O₃ powder (about 0·1 μm). The number and the area of scratches on the electrode surface obviously increase with N. The polarization reversal processes are influenced by this treatment. τ_1/2 decreases with increasing N, but no change is observed in the contrast of the light transmittance. Two different activation fields, E _a1, for the applied field E > E ₀ and E _a2 for E < E ₀ are observed, where E ₀ is a certain field depending on the sample. Here E _a1 is much larger than E _a2; typically E _a1, = 260 kV cm^?1 and E _a2 = 144 kV cm^?1 for N = 0. This means that nucleation for the higher field needs a large activation energy which therefore has strong barriers. In contrast with this, nucleation for the low field occurs easily. Changing a surface condition, E _a2 clearly decreases with increasing N although E _a1 is almost independent of the surface scratching. This may suggest that E _a1 corresponds to the activation field for the bulk pinning and nucleation, and E _a2 for the surface pinning. The PVA treatment gives an intermediate influence between N = 0 and 10.     

Polarisation-dependent dielectric processes in ferroelectric liquid crystals

The behaviour of dielectric relaxation process has been investigated in four ferroelectric liquid crystal (FLC) materials having different spontaneous polarisation (Ps) values. Ps effect on the permittivity in four different FLCs has been carried out in highly anchored sample cells around ~8 μm thick. It has been found that the main contribution to the dielectric permittivity in chiral Smectic C (SmC*) phase is due to Goldstone mode (GM) and partially unwound helical mode (p-UHM). In higher PS value FLC materials, the p-UHM process is found to dominate the dielectric properties. It has also been observed that p-UHM process is highly dependent on the probing ac voltage and temperature, whereas GM is found to be weakly dependent of probing voltage and temperature in SmC* phase of all the studied FLC materials. The influential contribution of p-UHM has exhibited the dielectric properties in its intrinsic frequency range making the materials suitable for futuristic display and photonics devices.     

Hydrogen bond-induced ferroelectric liquid crystals

Abstract

Hydrogen-bonded dimers of 4-alkoxy-4′-stilbazole homologues (C _n OSB, proton acceptors) and the mono-(2-methylbutyl)ester of terephthalic acid (MBTA, proton donor) behaved like conventional thermotropic mesogens. For chiral MBTA mixtures, chiral nematic and blue phases were observed with n = 1 and 2, and chiral smectic C phases were observed with n ≥ 5. Achiral phases were observed for mixtures with racemic MBTA as proton donor. Transition temperatures and enthalpies are similar for the chiral and achiral systems.     

An organic plastic ferroelectric with high Curie point

Plastic ferroelectrics, featuring large entropy changes in phase transitions, hold great potential application for solid-state refrigeration due to the electrocaloric effect. Although conventional ceramic ferroelectrics (e.g., BaTiO₃ and KNbO₃) have been widely investigated in the fields of electrocaloric material and catalysis, organic plastic ferroelectrics with a high Curie point (T_c) are rarely reported but are of great importance for the sake of environmental protection. Here, we reported an organic plastic ferroelectric, (−)-camphanic acid, which crystallizes in the P2₁ space group, chiral polar 2 (C₂) point group, at room temperature. It undergoes plastic paraelectric-to-ferroelectric phase transition with the Aizu notation of 23F2 and high T_c of 414 K, showing large entropy gain (ΔS_t = 48.2 J K⁻¹ mol⁻¹). More importantly, the rectangular polarization–electric field (P–E) hysteresis loop was recorded on the thin film samples with a large saturated polarization (P_s) of 5.2 μC cm⁻². The plastic phase transition is responsible for its multiaxial ferroelectric feature. This work highlights the discovery of organic multiaxial ferroelectrics driven by the motive of combining chirality and plastic phase transition, which will extensively promote the practical application of such unique functional materials.

An organic plastic ferroelectric, (−)-Camphanic acid, shows multiaxial ferroelectric feature and large entropy gain during the phase transition.