Phase transitions and soft elasticity of smectic elastomers

Smectic-C elastomers can be prepared by cross-linking, e.g., liquid crystal polymers, in the smectic-A phase followed by a cooling through the smectic-A to smectic-C phase transition. This transition from D(infinityh) to C(2h) symmetry spontaneously breaks rotational symmetry in the smectic plane as does the transition from a smectic-A to a biaxial smectic phase with D(2h) symmetry. We study these transitions and the emergent elasticity of the smectic-C and biaxial phases in three related models and show that these phases exhibit soft elasticity analogous to that of nematic elastomers.     

Photopyroelectric investigation of binary mixtures of liquid crystals in contact preparations

The photopyroelectric (PPE) method is proposed as a sensitive technique to study a binary mixture of liquid crystals in a contact preparation. The photothermal signal is generated while scanning the contact preparation. The crystal (K)/smectic-A (S _A), smectic-A/nematic (N), and nematic/isotropic (I) interphase boundaries are detected. The displacement of these boundaries due to the variation of the temperature is monitored. Received: 13 Oktober 1998 / Accepted: 7 July 1999 / Published online: 3 December 1999     

Bend-induced melting of the smectic-A phase: analogy to a type-I superconductor

Using an atomic force microscope to nanopattern a substrate for liquid crystal alignment, a bend distortion is imposed on a liquid crystal. In regions of large bend the smectic-A phase melts into the nematic phase, and the width of the melted region is measured as a function of temperature. The results are consistent with type-I superconducting (nematic-smectic-A) behavior, wherein a large magnetic field (bend or twist distortion) induces an order to disorder transition. A model that accounts for non-mean-field behavior is presented.     

Computer simulation study of a flexible-rigid-flexible model for liquid crystals

Small molecules that form liquid crystals typically consist of a rigid core with flexible tails on one end or on both ends. To date, most computer simulation studies have used completely rigid models such as hard spherocylinders: cylinders, characterized by their length/diameter ratio L/D, with hemispherical end caps. We have studied a model consisting of spherocylinders with L/D = 4, with a flexible tail attached to each end. The tails are ‘ideal’ in the sense that they have no volume. Using Monte Carlo simulations the phase behaviour of this model was studied and, for comparison, the behaviour of hard spherocylinders with L/D = 4 without tails was studied as well. The addition of the tails is found to stabilize the smectic-A phase at a lower pressure, and the nematic phase disappears. In the smectic-A and crystal phases, the smectic layers are further apart when tails are added. The structure of the layers and the smectic-A–crystal transition pressure change only a little. For both models close to melting the crystal consists of ordered layers, but there is almost no correlation between particle positions in neighbouring layers. In fact, the layer coupling is so weak that in a long simulation the layers are found to glide over each other. As the pressure is increased the crystal gradually becomes more ordered and the crystalline layers ultimately ‘lock’ into place.     

Theory of the incommensurate smectic-A phases in dimesogenic liquid crystals

A mechanism assuming a mismatch between the basic smectic periodicity and the amplitude and phase modulations periods of the mass density wave order parameter is proposed, for describing the incommensurate smectic-A structure found in dimesogenic liquid crystals. The different sequences of phases found in this family of compounds are described theoretically.PACS: 61.30.Cz Molecular and microscopic models and theories of liquid crystal structure - 64.70.Md Transitions in liquid crystals     

Nuclear magnetic resonance study of the smectic-cholesteric phase transition in a dimesogenic liquid crystal

Nuclear magnetic resonance (NMR) spectroscopy has provided a unique opportunity to study the characteristic smectic-A to chiral nematic phase transition in a dimesogenic liquid crystal (“KI-5S”). The order parameters in the liquid crystalline phases were obtained from the ²H NMR quadrupole splitting and ¹³C NMR chemical shift measurements, manifesting a first-order smectic-nematic phase transition.     

Confinement-induced orientational order in a ferroelectric liquid crystal containing dispersed aerosils

The study of the smectic-A to chiral smectic-C(*) phase transition of the liquid crystal S-(+)-[4-(2(')-methyl butyl) phenyl 4(')-n-octylbiphenyl-4-carboxylate] (CE8) containing dispersed hydrophilic aerosils reveals novel properties, important to understanding quenched disorder and confinement in ferroelectric liquid crystals. Smectic layer compression leads to a distribution of transition temperatures inducing smearing of the macroscopic data across the transition. A pronounced confinement-induced pretransitional tilted order is observed.     

Elliptic phases: a study of the nonlinear elasticity of twist-grain boundaries

We develop an explicit and tractable representation of a twist-grain-boundary phase of a smectic-A liquid crystal. This allows us to calculate the interaction energy between grain boundaries and the relative contributions from the bending and compression deformations. We discuss the special stability of the pi/2 grain boundaries and discuss the relation of this structure to the Schwarz D surface.     

Electroclinic liquid crystals with large induced tilt angle and small layer contraction

Optical and x-ray scattering studies of a chiral, organosiloxane smectic-A liquid crystal indicate a large field induced optical tilt of up to 31 degrees accompanied by a very small contraction of the smectic layers. This result suggests that the molecules have a nonzero tilt even with no applied field, and that the primary effect of the field is to induce long range order in the direction of the molecular tilt.     

Experimental evidence of the Halperin-Lubensky-Ma effect in liquid crystals

The orientational order parameter decrease, due to nematic director macroscopic fluctuations, is obtained by birefringence measurements around the nematic <--> smectic-A transition temperature of the liquid crystal 4'-n-octyl-4-cyanobiphenyl. The measured nematic order reduction shows the same amplitude as the secondary order parameter discontinuity at the transition providing direct evidence of the Halperin-Lubensky-Ma effect. The importance of nematic director thermal fluctuations on the character of the transition is revealed as their quenching by an electric field of approximately 20 V/micrometer restores the second order character of the transition.     

Field-induced realignment of a smectic nanodroplet in an external magnetic field: A numerical investigation

The field-induced realignment of a smectic-A phase is in principle a complicated process involving the director rotation via the interaction with the field and the layer rotation via the molecular interactions. Time-resolved X-ray scattering experiments have revealed major phenomena concerning the maintenance of the integrity of the smectic-A layer structure during the alignment process. In order to obtain a deeper insight into this process, we have carried out a dissipative particle dynamics study of the realignment kinetics of a nanodroplet of a smectic-A liquid crystal suspended in an isotropic fluid following a switch in the direction of an applied magnetic field. The strength of the mesogen-field interaction is small compared to the inter-molecular interactions. The reaction of the smectic configuration to the field switch was found to depend on the balance between the inter-molecular interactions stabilising the formation of the smectic layering and the interaction of the mesogens with the external field. It is found that the rotational behaviour of the smectic layers under the influence of an external magnetic field arises from a combination of stochastic translational displacements and rotational motions of the centres of mass of the mesogens in the nanodroplets. The simulations indicate that X-ray scattering and NMR experiments monitoring the orientational order are sensitive to different aspects of the realignment process.     

Low-frequency electrodynamic instability in the smectic-A phase of a liquid crystal

An instability, induced by constant and alternating electric fields, is observed and studied in the planar texture of the smectic-A phase of a liquid crystal. The effect of the parameters of the applied field and of the material on the characteristics of the instability is studied. The mechanism for the appearance of the instability is linked to the nonuniform distribution of the electric field over the thickness of the liquid crystal.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 82–86, October, 1985In conclusion, the authors thank V. G. Chigrinov for useful discussion and interest in this work.     

Transition from smectic-A to crystalline phase in the mean field approximation

We present here a smectic-A to crystalline phase transition curve for liquid crystals, in addition to the phase diagram previously reported which includes the isotropic, nematic, and smectic-A phases calculated in the mean field approximation.     

Study on optical characterization of TGB and reentrant smectic-A phases in binary mixture of two liquid crystals

In this work, our investigation is to study the optical and thermal properties of the binary mixture of cholesteric and nematic compounds, namely, cholesteryl nonanoate and p-methoxybenzylidene-p-ethylaniline, which exhibits a very interesting liquid crystalline twisted grain boundary (TGB) phase and reentrant smectic-A phase. The chiral liquid crystalline TGB phases and reentrant smectic-A phases have been observed at different concentrations and at different temperatures. The existence of TGB and reentrant smectic-A phases is confirmed by differential scanning calorimetry and optical microscopic studies. The variation of optical anisotropy has been discussed. The helical pitch of the cholesteric phase has also been discussed.     

Surface ordering transitions at a liquid crystal-solid interface above the isotropic smectic-A transition

The degree of orientational order induced by confining cylindrical surfaces is monitored via deuteron nuclear magnetic resonance linesplitting and linewidth above the smectic-A to isotropic phase transition. The orientational order strongly depends on the length of the surfactant coupling molecule, on the surface coverage, and on the liquid crystal. Continuous and stepwise growth of orientational order and surface-induced orientational order transitions found in the isotropic phase are explained in terms of a simplified model of surface-induced layering and molecular self-diffusion.     

Electro-optical Kerr effect in the isotropic phase of the two antiferroelectric liquid crystal mixtures

We have experimentally studied for the first time the electro-optical Kerr effect and the pre-transitional behavior in the isotropic phase of two antiferroelectric liquid crystal mixtures, W-232 and W-204D, composed of rod-like ester molecules exhibiting the direct smectic-A to isotropic (SmA-I) phase transition. The Kerr law has been confirmed for the two compounds and the variation of inverse Kerr constant with temperature above the smectic–isotropic transition temperatures were determined. Both the mixtures with very broad antiferroelectric phase around room temperature have similar sequence of the phases (i.e., Cr-SmC*A-SmC*-SmA-I). Although, the pre-transitional behavior is usually complex in the isotropic phase of the chiral smectic liquid crystal compounds, the investigated compounds showed a similar behavior compared to that of nematic–isotropic behavior.     

Smectic-A*-smectic-C* transition in a ferroelectric liquid crystal without smectic layer shrinkage

The smectic layer spacing of a nonfluorinated ferroelectric liquid crystal (FLC) compound with almost no shrinkage and only minor tendency to form zigzag defects was characterized by small angle x-ray diffraction. The material lacks a nematic phase. The smectic-A*-smectic-C* phase transition was studied by measuring the thermal and electric field response of the optical tilt and the electric polarization. These properties are described very well by a Landau expansion even without introduction of a higher-order Theta(6) term. This result suggests a pure second-order phase transition far from tricriticality and differs considerably from the typical behavior of the A*-C* transition in most FLC materials.     

Effect of nonmesogenic impurities on the order of the nematic to smectic-A phase transition in liquid crystals

By means of adiabatic scanning calorimetry, we have studied the effect of the nonmesogenic solutes cyclohexane (CH) and biphenyl (BP) on the nematic-smectic-A phase transition in the liquid crystal octylcyanobiphenyl (8CB). For all concentrations of BP studied, the transition remains second-order. For 8CB+CH, however, crossover from second-order to first-order is observed at a tricritical point of the mole fraction x of CH around 0.0460. The difference between the two systems and the crossover is explained in terms of a mean-field free energy density expression including coupling terms of x with the nematic and smectic-A order parameters.     

Smectic order parameters via liquid crystal NMR spectroscopy: Application to a partial bilayer smectic A phase

Solute molecules were dissolved in the liquid crystal 4-cyano-4??-n-octyloxybiphenyl (8OCB), known to form a partial bilayer smectic-A phase. Through measurement of solutes?? and solvent??s orientational order parameters via nuclear magnetic resonance spectroscopy, and their analysis via a statistical thermodynamic density functional theory, values of the solvent??s positional order parameters and solutes?? positional-orientational distribution functions were obtained. Near to the transition to the nematic phase, the main positional order parameter of the smectic liquid crystal turned out to be comprised in the interval 0.4?C0.6, though the quality of the fittings assuming the phase as nematic all across the temperature range investigated was only slightly worse. This may be ascribed to the looseness of the partial bilayer smectic structure. Solutes were found to preferentially lie in those regions where liquid crystal molecule terminal chains are located.