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.     

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.     

The Lyotropic Smectic Phase on the Flory Lattice

Abstract

The Flory lattice approach for rod-like molecules is extended to encompass the position-restricted (smectic-A) phase. The phase equilibria conditions are obtained from equity of relevant chemical potentials, together with minimization conditions of the Gibbs potential with respect to the system orientational (Flory disorder index, y) and positional (translational disorder index, ζ in the smectic phase) order.     

Structure and Packing in Smectic E and Smectic A Phases in the Series of 4-n-Alkyloxy-4′-alkanoylbiphenyls

By means of X-ray investigations the structure of the S_E and S_A-phases of the title compounds has been studied. The lattice parameters of the orthorhombic S_E structure are given. The thickness of the smectic layers is nearly the same in the S_E and S_A phases and agrees well with the length of the molecules in their most stretched form. That suggests a similar conformation of the alkyl chains in both phases. A model of the packing in the S_A phase is discussed which is based on a dense packing with antiparallel orientation of molecules with a non-symmetrical shape.     

The Crystalline state and the Mesophases of Cholesteryl Oleyl Carbonate

The preparation of pure crystalline cholsteryl oleyl carbonate is described. The melting point, the mesophase transition temperatures, and the enthalpy of the melting process are recorded, and a method of maintaining cholesteryl oleyl carbonate in a pure state over extended periods of time is suggested.     

Structure,Stability and Response to the Electric Field of Antiferroelectric Smectic Materials

Abstract

The structure and switching of five antiferroelectric mesogens are studied by the constant temperature-pressure molecular dynamics simulation. The conformations of the end chains in the smectic phase are compared with those at an isolated single molecule estimated by the method of molecular orbitals, where the bending angle of the chiral chain as well as the angle of the achiral one is shown to be enhanced. The tilt angles obtained here are in well agreement with the experimental ones. The response of the anticlinic alignment to the transverse electric field is also tested.     

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.     

Structure and Correlations in Smectic B,F and I Phases

The question of which of the known smetic phases exhibitis long range 3 dimensional order is first discussed briefly, The major theme of the paper is the nature of the structure and correlations in S_B, S_F, and S₁ phases. Results are presented about the temperture dependence of the interlayer stacking arrangement and the existence of strong layer undulations in crystalline S_B phases and about the existence of a S_B phase with very weak positional correlations between layers and short range order within the layers.

The raltion between the S₁ and S_F structures is established for the first time: both consist of uncorrelated layers in which the molecules are tilted relative to the layer normal and have limited positional correlations within layers. Both have 3-d long range bond orientational order of the C-centered monoclinic lattices. They differ in the direction of tilt of the molecules relative to the pseudo-hexagonal packing in the plane normal to the long axes: in S_F the tilet is directed towards an edge of the hexagon, while in S₁ it is directed towards an apex. These results enable previously described phases to be classified (or reclassified) as S₁ phases and emphasise the existence of more ordered phases with similar tilt to the S₁ phase but otherwise analagous to S_G and S_H—we call these S_G and S_H. Finally a preliminary repor is given of X-ray structural work which shows that the recently postulated S₁ and S_K phases do not exist.     

A Theoretical Study of Molecular Ordering of Some Smectic Liquid Crystals

Abstract

A theoretical investigation of molecular ordering of smectic C liquuid crystals C₈H₁₇O-C₆H₄-C₆H ₄-COO-C₆H₄-OC₈H₁₇ { A }, C₉H₁₉O-C₆H₄-COO- C₆H₄-COO-C₆H₄-NO₂ { B } and its binary mixtures { A+B } has been carried out by the method based on the Rayleigth-Schrodinger perturbation theory. The mutual arrangement of two molecules corresponding to the minimum of the total interaction energy between them U_pair has been established. The curves of the dependence of U_pair and its various contributions on displacement of molecules from each other along the molecular long X-axis and angle θ between molecules' long axes has been also computed in the minimum point vicinity. The obtained results make it possible to determine the peculiarities of the structural organization of molecules, as well as to construct a model of the structure of A and B compounds in different phases taking into account the most probable packing of molecules.     

The Molecular and Crystal Structure of p-methyl-phenyl-glyoxyacid-p-chloranilide

Using X-ray crystal structure analysis of the title compound the positions of all the atoms (including all H atoms) in the monoclinic unit cell with the parameters a = 13.90₀ Å, b = 5.13₈ Å, c = 17.95₆ Å and β = 91.05° and the space group P 2₁/n were localized. The existence of an intramolecular N—H(N)…︁ O(1)-bridge was inferred, whose H-bridge acceptor predominantly is the π-electron density of the carbonyl group. At the same time the H(N) atom participates in the intermolecular bridge N H(N)…︁ O′(1) to the symmetry-equivalent neighbouring molecule. H-bridge-like interactions of the two phenyl hydrogen atoms H(6) and H(15) to the carbonyl oxygen O(2) have also been inferred from the torsion angles.     

Molecular Relaxation in hi Ghly Ordered Orthogonal Smectic SA,SB and SE Phases

Dielectric relaxation and calorimetric investigations were perforemed on different smectic phases. Discountinuities of the critical frequency were found at both S_A-S_B and S_B-S_E phase transitions. S_A and S_E structures can be regarded as two-dimensional liquid and two-dimensional crystal, respectively. The low frequency dielectric dispersion presented here demonstrates the difference btween the S_E and a three-dimensional cyrstal where this phenomena is absent.     

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.     

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.     

The Crystal and Molecular Structure of Mesogenic Malonates

The crystal and molecular structures of diethylene-6-[4-(4′-nitrophenylazo)phenoxy]dodecylmalonate (12 PhNO₂) and -propylmalonate (3 PhNO₂) were determined with CuK_α to a = 19.235(5) Å, b = 7.620(1) Å, c = 22.400(4) Å; α = 90°, β = 103.27(1)°, γ = 90°; space group P2₁/c for 12 PhNO₂ and a = 6.797 (6) Å, b = 7.756 (8) Å, c = 22.74(2) Å; α = 81.48(7)°, β = 88.29(7)°, γ = 68.10(9)°; space group P1 for 3 PhNO₂. Refinement with 1130 observed reflections (> 3σ) for 12 PhNO₂ and 1615 for 3 PhNO₂ leads to R (12 PhNO₂) = 5.8% and R(3 PhNO₂) = 4.4%. The molecular conformation in the crystalline state is the most extended trans conformation for both compounds. On rapidly cooling from the isotropic melt, 12 PhNO₂ shows a smectic A-phase. In agreement with prior investigations on polar mesogens, the molecules arrange in an antiparallel packing. The azo-mesogen is nearly coplanar with the alkoxy spacer up to the branching atom. The low molecular weight malonates investigated may serve as model compounds for side-group liquid crystalline polymers.     

The Crystal and Molecular Structure of 1,18-dibromooctadecane

The crystal and molecular structures of 1,18-dibromooctadecane are determined precisely by X-ray diffraction method. The results obtained are as follows: empirical formula, C₁₈H₃₆Br₂, formula weight Mr = 412.27, crystal system, monoclinic, space group, P2₁/n, lattice parameters, a = 5.496(1), b = 5.403(1), c = 34.374(4) Å, β = 94.50(1)°, volume of unit cell, V = 1017.6(3) ų, Z value, Z = 2, calculated density, D_x = 1.346 g/cm³, wave length of X-ray, λ(CuKα) = 1.5418 Å, absorption coefficient, μ(CuKα) = 49.64 cm⁻¹, F(000) = 428.00, temperature, T = 293 ± 1 K, R factor, R = 0.052, weighted R factor, R_w = 0.078 for 1019 unique observed reflection [I > 3σ (I)]. There are no unusual bond distances or angles. The structure obtained here is quite similar to that of 1,12-dibromododecane.     

Investigation of the Cholesteryl Oleate Mesophases by X-Ray Diffraction

The following phase transitions occurring in the cholesteryl oleate were investigated by X-ray diffraction: solid-isotropic liquid, isotropic liquid-cholesteric, cholesteric-smectic.

The sample purity was 98%. Strong pretransitional effects were observed at the solid-isotropic liquid phase transition and at the cholesteric-smectic phase transition.

At this last transition the longitudinal coherence length ξ_∥ appears to diverge as the critical temperature is approached in the cholesteric phase, whereas the transversal coherence length ξ_τ increases at the critical temperature in the smectic phase in an abrupt way. It seems from the temperature dependence analysis of the angular position of the low angle peak that the smectic phase is a smectic A phase and that a progressive melting of chains occurs at the temperature increases.