Proton Spin-Lattice Relaxation Dispersion Study of the Ferroelectric Smectic Phases in Dobambc

The frequency dispersion of the proton spin-lattice relaxation rates T^?1 ₁ in the ferroelectric smectic phases of DOBAMBC has been determined in the MHz region and analyzed in terms of the order fluctuations, self-diffusion, and rotational contributions to T^?1 ₁. In the smectic C* phase the main rate determining contribution are order fluctuations with a T^?1 ₁ ∝ ν^?1/2 _L frequency dependence, whereas in the smectic H* phase the translational diffusion, which is slower than in the smectic H* phase, determines the dispersion of T^?1 ₁.     

A Chevron Model of the Electroclinic Effect across the SA*-SC* Phase Transition in a SSFLC

Abstract

We here present a thorough model of the electroclinic effect across the S_A*-S_C* phase transition, being the first to incorporate the vertical chevron structure. This structure has previously been observed in the S_C* phase of surface-stabilised FLC (SSFLC) devices, and some experimental evidence suggests that it may be present in the electroclinic effect [1–3]. Excellent agreement is achieved when comparing this model with previous experimental results of optical tilt through the A-C* transition for a SSFLC cell with homogeneous alignment.     

Smectic C-to-Nematic Transition in p-n-Heptyloxybenzylidene-p-aminobenzoic Acid

The results on the phase transitions in p-n-heptyloxybenzylidene-p-aminobenzoic acid are reported. The microscopic and X-ray studies show that the compound goes over from smectic C to nematic phase directly. The transition entropies and specific heat are determined using differential scanning calorimetry. The translational order parameter is determined using low angle X-ray intensity measurements. The tilt angle in smectic C phase is found to be 56° and is independent of temperature. The X-ray data also lead to the conclusion that the compound exists as a dimer.     

New chiral liquid crystal materials based on menthol: Synthesis and phase behavior

To study structure-mesomorphism relationships of chiral materials based on menthol, a series of chiral compounds containing menthyl group were synthesized. Their chemical structures, formula, and the purity were characterized by Fourier transform infrared (FTIR), proton nuclear magnetic resonance (¹H NMR), and elemental analyses. The corresponding phase behavior was investigated with differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). The effect of the mesogenic core, flexible spacer, and terminal groups on the mesophase formation and type was discussed. The compound 4-menthyloxyacetoxybenzoyloxy-4′- (6-bromohexyloxy)biphenyl showed the smectic A (S_A) and cholesteric phases on heating cycle, and a cubic Blue Phase, chiral smectic C (S_C^*), S_A and cholesteric phases on cooling cycle. However, the compound 4-menthyloxyacetoxybenzoyloxy-4′-(4-bromobutoxy)biphenyl showed enantiotropic S_C^*, S_A and cholesteric phases, moreover, the platelet texture of a cubic Blue Phase was also observed on cooling cycle. The methacrylate monomer with six flexible methylene units and three phenyl rings as mesogenic core only showed cholesteric phase, while the methacrylate monomer with four methylene unit and three phenyl rings showed the S_C^* and cholesteric phases. However, when the terminal methacryloyloxy groups in the monomers were substituted by thiophenylacryloyloxy groups, the corresponding monomers all show no mesophase. With increasing the rigidity of mesogenic core or decreasing the flexible methylene spacer length, the corresponding melting temperature (T_m) or isotropic temperature (T_i) increased.     

The Structure and Unusual Optical Textures of Smectic C2 Phases of New Tail-to-Tail Twins

New tail-to-tail twins with alkyl or perfluorinated spacers were synthesized. The mesomorphic properties were studied by polarizing microscopy and X-ray diffraction measurements. It was found that the smectic C phase is a Sc₂ phase with alternating tilt and an intercalated structure. The alternating tilt gives rise to an unusual schlieren texture which exhibits not only singularities with the strength s = ±1 but also those with s = ±1/2 and s = ±3/2.     

High-Resolution X-ray Study of the Smectic A-Smectic B Phase Transition and the Smectic B Phase in Butyloxybenzylidene Octylaniline

We have carried out a high resolution X-ray study of the smectic phases of Butyloxybenzylidene Octylaniline. We find that the phase previously identified as Smectic-B in this material is crystalline with in-plane order extending over at least 1.4 μm. The in-plane Bragg peaks are accompanied by anomalously strong diffuse scattering that can be described by a form 1/(q ²⊥ + γ² q ² _z). Unless the elastic constant C₄₄ is more than an order of magnitude smaller than previously reported values of ～ 10⁸ ergs/cm³ the diffuse scattering can not be due to acoustic phonons. The crystalline-B to Smectic-A melting transition is strongly first order with no observable pre-transition effects on either side of the transition.     

Stimulation of the Electrohydrodynamic Instability in the Smectic a Phase

The electrooptical properties of 4-n-octyl-4′-cyano-biphenyl (8CB) in the smectic A phase were investigated. The electrohydrodynamic instability in the smectic A phase was observed, when a small quantity of any homologue (C₇, C₈, C₉) of 4-n-alkoxyben-zoic acid was adzed to this material. After the electrical field is turned off, the stable focal-conic texture is formed. A reverse focal-conic to homeotropic transition can be as well induced by means of an electric field.     

Lifshitz-Like Behaviour in Ferroelectric Liquid Crystals

In order to explain the anomalous behaviour of the critical magnetic field and the helical pitch in the vicinity of the Λ-line and the Lifshitz point in the ferroelectric liquid crystal DOBAMBC a model involving not only chiral but also non-chiral biquadratic coupling between the polarization and the tilt has been proposed. The model predicts an “S - like” dependence of the modulation wave vector close to the smectic C* - smectic A transition. High temperature resolution measurements of the helical pitch and the intensity of the diffraction sattelites indeed support the proposed model and seem to show that the pitch is finite at T_c. In such a case the Lifshitz field H_L is finite too.     

Energy criteria for orientation of smectic C* liquid crystals in electrooptic elements

Energy analysis of the types of orientation of smectic C* liquid crystals is performed. Criteria for the type and quality of their orientation in electrooptic elements with surfaces of orientants of different nature are revealed. An energy parameter of the optical quality of planar orientation of a smectic C* liquid crystal is proposed, which determines the contrast ratio of electrooptic elements independent of the type and nature of the smectic C* liquid crystal and the orientant.     

The Microhardness and the Toughness Anisotropy of Paratellurite Single Crystals in the Tr — 650 °C Temperature Range

The temperature hardness (H_v) and toughness (K_1C) dependence of paratellurite (100), (010), (001) planes is investigated in the T_r −650 °C range. The correlation between the anisotropy degree of Young modulus and H_v is not found. The presence of 11 kind H_v and K_1C anisotropy is shown.     

The Order-Disorder Phase Transition in Liquid Crystals as a Function of Molecular Structure. I. The Alkyl Cyanobiphenyls

The pretransitional behaviour of a homologous series of mesomorphic compounds, the alkyl cyanobiphenyls (CNρρC _n H _{2n + 1}) for n = 5–12, has been studied as a function of temperature by light scattering in the isotropic phase. The higher homologues, n = 8–12, exhibit a smectic A phase that becomes increasingly important at the expense of the nematic phase for increasing n and for n = 10 and 12 no nematic phase is observed. We have shown from light scattering and differential scanning calorimetry measurements that the presence of the smectic A phase changes markedly the pretransitional behaviour, and gives rise to two distinct pretransitional regions. The first region is for temperatures greater than ～3°C above the clearing temperature T_c where the systems exhibit a typical pretransitional behaviour adequately described by the phenomenological Landau-de Gennes model of a second order phase transition with an intervering first order transition at T_c . The constants of this model for this region are given and show a distinct odd-even effect. The second region is close to T_c where a strong divergence from the already critical behaviour is observed. This secondary divergence is observed to be a function of the alkyl chain length and its conformation, and is attributed to coupling between smectic A like layers and the orientational order parameter in the isotropic phase.     

Anomalous Smectic Mesomorphism of 4,4′-di-n-Pentyloxyazoxybenzene

Calorimetric and optical texture studies reveal the existence of a previously unrecognised, stable, highly ordered and enantiotropic smectic phase of C₅ (4,4′-din-ent yloxasoxybensene). This represents a distinctly anomalous behaviour as the next five homologs, C₆ - C₁₀, do not exhibit such a highly ordered smectic phase. It is shown that configurationdl melting of the alkyl end-chains can largely account for the observed thermodynamic parameters of the solid-smectic transition of C₅.     

Experimental Determination of the Curvature-Induced Reduction in the Smectic A-Nematic Transition Poin

On the basis of an analogy between smectic A-nematic and superconductor-normal metal transitions, de Gennes has predicted that a twist or bend distortion should reduce T _AN with respect to that of a curvature-free sample. We report measurements of the reduction of T _AN on samples prepared in two different geometries. In the first, a magnetically induced twist distortion has been used to obtain preliminary data on 8 OCB and CBOOA samples. In the second, a wedge-shaped twisted nematic cell has been used to get quantitative data on the reduction of T _AN of CBOOA as a function of twist distortion.     

Twist Grain Boundary Phases in the Binary Mixtures of Smectic and Cholesteryl Compounds

Abstract

The binary mixture of Terephthal-bis-p-n-hexadecylaniline (TB16A) and cholesteryl nonanoate (CN) exhibits I - N* - TGB _A - TGB _c*- Sm A - Sm I - Sm F - Sm G - K phases in the concentration region between 95 to 98% of TB16A at higher temperature range. The 90% of TB16A exhibits I - N* - TGB _A - TGB _c - Sm A - Sm I - Sm F - K phases. The mixtures of concentration between 50 to 80% of TB16A exhibits I - Sm A - Sm I - K phases. The frustrated blue phase is also observed in the mixtures of lower concentration of TB16A in CN at higher temperature. Optical, DSC and X-ray studies were carried out for the identification of the different phases. The phase diagram of the Chen-Lubensky model in this context is discussed.     

Electrical Conductivity of Liquid Crystal Mixtures with Induced Smectic Phases

The electrical conductivity of the two liquid crystal mixtures N-(4-ethoxybenzylidene)-4′-amino benzonitrile (PEBAB)/4,4′-di-n-hexyloxy-azoxybenze (HEXOAB) and 4-n-heptyloxy-4′-cyanobiphenyl (7 CBP)/HEXOAB, which exhibit induced smectic A phases is investigated. In the smectic phases, the conductivity anisotropy of the PEBAB/HEXOAB mixtures is negative at the lowest PEBAB concentrations; this behaviour is usually expected for a smectic layer structure. With increasing PEBAB concentration the anisotropy increases and becomes positive. Possibly, this is an indication for a growing double layer structure, which was observed in polar smectic phases. In the nematic phases the conductivity anisotropy of the pure HEXOAB is considerably reduced by adding a relatively small amount of the polar component. An addition of 10 mole% 7 CBP reduces the anisotropy ratio of the electrical conductivity to V = K_∥/K ₁ ≈ 0.2, which probably is the lowest value observed in a nematic phase so far. Besides the negative conductivity anisotropy, these mixtures also exhibit a positive anisotropy of the dielectric constant. They thus fulfil the conditions for inverse dynamic scattering.     

The Smectic A and the Smectic C Phase: A Coherent Molecular Picture

A molecular field theory of the smectic A and smectic C phase is presented which is based on an interaction derived in terms of the molecular polarizability, its anisotropy and the components of the static molecular quadrupole tensor. It is shown that this attractive interaction strongly depends on the anisotropy of the excluded molecular volume. The results obtained explain the stability of the A phase, the appearance of the tilt in the C phase and the A-C phase transition, without resource to adjustable parameters which indeed have no molecular significance.     

Viscoelastic Parameters of a Side-Chain Nematic Polymer: I—Results of a Proton NMR Study

The reorientational behaviour in a static magnetic field of a nematic siloxane side-chain polymer is investigated by proton NMR. At variance with what occurs in main-chain and lyotropic nematic polymers, the return to equilibrium of a tnonodomain whose mean director has been rotated at an angle α to the field, is found to be homogeneous for all angles a between 0 and π/2. The usual nematic order parameter and the static (on the proton NMR time scale) order parameter are deduced from analysis of the equilibrium lineshapes. The twist viscosity γ₁ is deduced from the study of time dependent lineshapes following the rotations α. The temperature dependence of γ₁ is discussed in terms of the Vogel equation γ₁ = B S ^β exp(E _β/R(T - T _o}). It is not possible to discriminate between β = 1 or 2, but in both cases, the freezing temperature of the director T_o is found to be non-zero. A nematic to smectic pretransitional effect on γ₁ is observed. The information concerning the viscoelastic parameters contained in the value of the static order parameter is also discussed.     

New chiral imines based on S-α-phenylethylamine and S-α-benzylethylamine in induced cholesteric and smectic mesophases: II. Behavior of chiral dopants in ester liquid-crystal systems

The temperature dependences of the pitch of induced helical structures in the ester liquid-crystal systems containing hydroxyphenyl benzoate derivatives and chiral N-aroyloxybenzylidene-S-α-phenylethylamines or N-aroyloxybenzylidene-S-α-benzylethylamines have been measured. The twisting power of chiral dopants is calculated and the temperature dependence of the helical pitch in the induced cholesteric (N ^*) and smectic C ^* mesophases is quantitatively characterized. The parameters A = dT _is/dC that characterize the effect of dopants on the thermal stability of the N ^* phases are determined from the concentration dependences of the cholesteric-isotropic transition temperature T _ts for the n-hexyloxyphenyl-n-butylbenzoate-chiral dopant systems. The results obtained are compared with the data for the 4-n-pentyl-4′-cyanobiphenyl (5CB)-based cholesteric systems induced by the chiral dopants under consideration.