Phase transition of nematic to smectic C through an intermediate boundary induced smectic A structure

An intermediate phase has been observed at the transition nematic to smectic C where a smectic A structure induced by the boundaries and a nematic phase exist simultaneously. The temperature interval of existence of the intermediate phase is of the order of one tenth of degree. Several conditions are necessary for its appearance: the latent heat L of the phase transition nematic-smectic C of the liquid crystal should be sufficiently small (L < k_BT_c); the nematic phase should undergo a bend deformation by the boundaries (possibly a twist one as well) and the transition nematic-smectic C should be carried out as slower as possible.     

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.     

Effect of Nitro Terminal Substituent on the Liquid Crystalline Characteristics of Ester Mesogens: p-Nitrophenyl-p'-n-Alkoxycinnamates

The homologous series p-nitrophenyl-p'-n-alkoxycinnamates is a medium melting homologous series; the first two members are non-mesogens and do not exhibit even monotropy. The nematic phase begins from the third derivative in monotropic condition; from the fourth member enantiotropic mesomorphism commences. Polymesomorphism begins from the sixth member with the appearance of smectic mesophase and ends at the seventh member. The rest of the homologues up to the octadecyl are purely smectogens. Thus the homologous series is predominantly smectogenic even though -NO₂ group is reported to be enhancing nematic character.^' Within the polymesomorphic region the nematic mesophase is homeotropic; however, it shows a clear threaded texture where it is the only mesophase shown. The N-I transitions do not exhibit the usual alternation in transitions which is a rare phenomenon. The N-I transition curve is unusually neither rising nor falling though a slight curve is seen; it merges with the S-I curve which shows a slight rising tendency as the alkyl chain length is increased. The S-N transition curve steeply rises and seems to be merging with the S-I curve. The mesomorphic range is good; the nature of the curve can predict the latent N-S or I-N transitions. All smectic orientations are of focal conic smectic A texture except in the case of sixth and seventh homologues where homeotropic nature prevails.     

The influence of the boundary conditions on the character of the phase transition nematic-smectic C

The possibility is investigated for inducing the intermediate smectic A structure in substances with nematic and smectic C phases by the boundary forces. Different geometries are used: homeotropic, homogeneous and tilted under a different angle toward the supporting substrates of the liquid crystal cell. At homeotropic and at tilted orientation where the molecules tilt toward the supporting substrates is different from the molecule tilt in the smectic layer, an intermediate smectic A structure is observed at the transition nematicsmectic C in a narrow temperature interval. In this case the phase transition nematicsmectic C extends and transforms into phase transition of second order. At homogeneous orientation and at tilted orientation of an angle equal to the molecule tilt in the smectic layer, the transition nematic-smetic C carried out in two stages, as partially it begins on the surface still in the nematic phase and terminates at the temperature of the phase transition N-Sc. The influence of the boundary forces depends essentially on the energy of the phase transition N-S_C of the substances.     

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.     

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.     

Effect of polarity of smectic Ad molecules on the induction of the nematic phase in binary mixtures of smectics A1 and Ad

The phase diagrams are determined of binary mixtures composed of 4-n-alkylbiphenylates 4-cyanobiphenyl (n-CBB)-smectics A₁, for n < 8 and esters of 4-(trans-4′-n-alkylcyclohexyl)benzoic acid (10CPCHB, 10NPCHB, 10FOPCHB)-smectics A_d. The effect is studied of the smectic layer spacing ratio and polarity of the components on the induction of the nematic phase in the tested series of compounds. The virtual N S_Ad phase transition points in series n-CBB are estimated and the effect is discussed of the stability of the smectic phase and polarity of the molecules of the mixture components on the width and position of the nematic gap.     

Magnetic Field Dependence of Laser Light Scattering through CBOOA in Nematic Phase

In the nematic phase of homeotropically aligned liquid crystals which have the smectic A phase at lower temperature, interference rings have been observed above some threshold magnetic field H_c which nearly coincides with the ocurrence of Freedericksz transition, only for the light polarized parallel to the direction of magnetic field. This interference ring disappears with increasing magnetic field above second threshold H_s . The origin of these interference rings is explained tentatively by a periodic deformation of bulk directors.

The bend elastic constant of CBOOA estimated from the measurements of the interference ring as a function of temperature, diverges with the critical exponent (0.5 ± 0.02) near the smectic A-nematic transition temperature and agrees well with a recent suggestion of McMillan based on the mean field theory.     

The Dielectric Anomaly Near the Transition from the Smectic A* to Smectic C* Phase and Visco-Elastic Properties of Ferroelectric Liquid Crystals

A pyro-electric technique is developed which allows the measurement of the dielectric response near the A*-C* phase transition in ferroelectric liquid crystals. The temperature dependence of the elastic modulus K _θ(T) corresponding to the molecular tilt in smectic layers is calculated from the experimental data. Direct pyro-electric measurements of the relaxation time for the spontaneous polarization P _s and the data on K _θ(T) allow us to calculate the temperature behaviour of the twist-viscosity γ₁(T) for the smectic C* phase. The curves γ₁(T) are compared for the smectic C* and the nematic phase of the same compound and the dependence of the twist viscosity on the molecular tilt angle in the C* phase is investigated. The results of the dielectic measurements are discussed in terms of the mean-field approximation.     

Dielectric relaxation in nematic,smectic A and C phases in the MHz region

The dielectric properties of four 4-n-alkyloxyphenyl 4-n-alkyloxybenzoates with nematic, smectic C and partly additional S_A phases were investigated. In 2 substances for ϵ′ reproducible values also in the smectic phases resulted allowing to conclude on a good orientation in the samples. In all 4 substances the dielectric relaxation in the MHz region was measured and the respective activation energies were determined. The molecular mobility in the S_C phases is higher than in the S_A phases, however comparable with that of the nematic phases. In all 3 phase types a single relaxation mechanism exists, which is explained as rotation of the molecules around a short axis.     

Multicomponent Mixtures of A1 Polar Smectics with the Nematic Reentrant Phase

Two series of ternary systems were investigated. The first series composed of mixtures (90BCAB + 80BCAB) and 10TPCHB, the second one composed of 80BCAB and mixtures (10TPCHB + 9TPCHB). There was observed the destabilization of the smectic A₁ phase followed by a nematic gap on the one side of the diagram and the extension of the smectic phase and the nematic reentrant phase on the other. The factors affecting the stability of smectic A phase and of the resulting S_Ad and N_re ones in these mixtures were determined.     

Spontaneous homeotropic alignment of nematic liquid crystals induced by a double-armed side chain liquid crystalline polymethacrylate

A liquid crystalline polymethacrylate having two cyclohexylphenyl mesogens on its side-chain (PMG0) was synthesized by the radical polymerization of the corresponding methacrylate monomer (MG0). PMG0 exhibited a smectic A phase around room temperature showing a focalconic fan texture in POM observations and a broad diffusion in a wide angle range of its WAXD profile. Upon slow cooling at ?1 °C min^?1 from the isotropic melt, PMG0 exhibited spontaneous homeotropic alignment between two native glass surfaces. A mixture of the commercial nematic liquid crystal (ZLI-4792) containing 1.0 wt% PMG0 also exhibited a homeotropic alignment, which easily covered hydrophilic surfaces such as glass or polyacrylamide.     

Density and Ultrasonic Velocity Measurements in Hexyloxybenzylidene Phenylazoaniline

The temperature variation of density and ultrasonic velocity of the liquid crystal hexyloxybenzylidene phenylazoaniline are reported. The density across the smectic A—smectic B transition is more predominant than the other transitions. The density variation with temperature and the calculated thermal expansion coefficients suggest that the transitions isotropic liquid—nematic, nematic—smectic A and smectic A—smectic B are of first order. Anomalous behaviour of ultrasonic velocity is observed across the isotropic liquid—nematic transition and prominent dips in velocity are observed at the nematic—smectic A and smectic A—smectic B transitions. The adiabatic compressibility (β _ad ) Rao number (R _a ) and molar compressibility (B) are estimated using the experimental density and ultrasonic velocity.     

Phenomenological model of phase transitions between isotropic, nematic, and smectic phases in liquid crystals

The existence of the biaxial smectic phase SmA _b has been proved in the model of the Landau potential with two (nematic and smectic) interacting order parameters.     

Molecular Arrangement in Mesophases of Some p-n-Alkoxybenzoic Acids

The properties of an even homologous series of alkoxy benzoic acids have been investigated in the crystalline and mesomorphic states by means of thermal microscopy, x-ray scattering and dilatometry.

Structural parameters such as inter-lamellar distance d, and the molar volume V_m were studied as a function of the number n of methylene units of the alcoxy radical.

In the crystalline state and the smectic C state the variation of d and of V_m with n is perfectly linear. The angle of tilt of the paraffinic chains α′ and of the aromatic stems φ was determined in the crystalline state. It was found that both moieties have the same magnitude of the tilt. In the smectic C state the value of α′ was found to be 25° as compared to 60.7° in the crystalline state indicating a “pulling up” of hydrocarbon tails when passing from the crystalline state to the smectic state. The passage from crystalline into the smectic state is also accompanied by a strong increase in V_m. Analysis of data indicates that this increase is entirely due to the aromatic part of the molecule while the aliphatic chains have the same V_m as in the crystalline state. It is concluded that while in the crystalline state the cohesion of the phase is insured to a large extent by dimerized, hydrogen bonded molecules, in the smectic C state the cohesion is insured by attraction between extended aliphatic chains. Evidence is also given for the formation of cybotactic groups in the nematic phase of octyloxy benzoic acid.     

Orientational order of dissolved molecules in the nematic,smectic A and smectic B phases of 4-n-hexyloxybenzylidene-4′-n-hexylaniline

The orientational order S of a dichroic dye dissolved in a liquid crystal has been determined in the nematic, smectic A and smectic B phase of 4-n-hexyloxybenzylidene-4′-n-hexylaniline. The used method is based on the absorption measurement of unpolarized light in a homeotropic oriented liquid crystalline layer and in an isotropic layer. The orientational order of the dissolved molecules was compared with the order parameter of the liquid crystalline solvent.     

Calorimetric Study of Crystalline and Liquid-Crystalline Polymorphism in the Homologues of Terephthal-bis-n-alkylaniline

The phase sequences of the homologues with n (the number of carbon atoms in the terminal alkyl chains) from 2 to 8 are obtained. At low temperatures, i) several crystalline modifications are found in each of the members with n = 4, 5, and 6 and ii) the monotropic transitions between crystal and smectic phase are observed in almost all the members. At intermediate temperature, iii) between the smectic H and C phases, the smectic F phase is found in the homologues with n ≥ = 5 by morphological observation or by a rheological method. The constancy of the temperature of the S_H(or S_F) – S_c transition with respect to “and broadening of the temperature range of S_H with increasing n in the expense of nematic phase are worthy of note. The even-odd effect of the end chain length n is observed very distinctively on the crystal melting and clearing point. Proton NMR measurement reveals a large mobility in the terminal alkyl chains in the crystal of the member with n = 6.     

Complex zigzag flexoelectric walls in the Schlieren textures of MBBA: possibility for determination of two basic Schlieren textures with singular or non‐singular surface points

The flexo‐dielectric behaviour of a homeotropic MBBA nematic layer has been experimentally studied. Asymmetric strong‐weak anchoring of the homeotropic nematic layer was achieved by treating the glass plates – one of them with lecithin ensuring the strong anchoring and the other with usual soap ensuring the weak anchoring. The application of a dc voltage with a sufficient amplitude led to the appearance of a complex texture consisting of gradient flexo‐dielectric deformations including Schlieren texture with many singular points and zigzag flexoelectric walls. The application of additional orienting a.c. voltage brought clarification of the Schlieren texture resembling that of the smectic‐C liquid crystal. Inversely, the application of an ac voltage across the homeotropic nematic layer led to formation of a nice Schlieren texture. The additional application of a d.c. voltage created complex zigzag gradient flexoelectric walls which connected the singular points in the Schlieren texture. In this way, one can determine for the first time how many points in the initial Schlieren texture are singular and how many points are non‐singular. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.