Increase In Orientational Order Research Articles

Design of liquid crystals with ‘de Vries-like’ properties: the effect of an ethynyl spacer in the core structure

ABSTRACTWe investigate the origins of ‘de Vries-like’ liquid crystalline behaviour by introducing an ethynyl spacer in the core of the tricarbosilane-terminated 5-phenylpyrimidine mesogens QL16-6 and QL17-6. The rationale for this structural change is based on the assumption that an ethynyl spacer would create more free volume in the core sub-layer and therefore decrease the orientational order parameter S2 in the SmA phase. The tricarbosilane-terminated mesogens WL41-5 and WL42-6 with a 5-(phenylethynyl)pyrimidine core in either a normal or inverted orientation were synthesised, and their mesomorphic and ‘de Vries-like’ properties characterised using polarised optical microscopy, differential scanning calorimetry, birefringence measurements, small–angle (SAXS) and 2D X-ray scattering. Reduction factors R derived from SAXS and optical tilt angle measurements suggest that neither WL41-5 (R = 0.49) nor WL42-6 (R = 0.80) exhibit ‘de Vries-like’ properties. The S2(T) profiles show an increase in orientational order with decreasing temperature and a sharp discontinuity at the SmA-SmC transition, which is consistent with ‘de Vries-like’ behaviour. However, the Leff(T) profiles suggest an increase in interdigitation that reduces the positive effect of increasing S2 in compensating for the molecular tilt.

Orientational Fluctuations Near the Smectic A to Smectic C Phase Transition in Two “de Vries”‐Type Liquid Crystals

On the basis of thorough analysis of 2D X-ray diffraction patterns from smectic monodomains, we examine the influence of orientational fluctuations on the weakly first-order smectic A (SmA) to smectic C (SmC) transitions in two nonchiral organosiloxane "de Vries"-type liquid crystals. We find that these materials exhibit very large molecular tilt fluctuations with magnitudes of up to 35°--thus larger than the average tilt itself. This is essential to understand the underlying molecular mechanism behind the practical absence of smectic layer contraction in these materials: in the SmA phase, the nematic order parameter is very low (molecular fluctuations correspondingly high), and the expected layer shrinkage at the SmA to SmC transition is almost fully compensated by the increase in orientational order, as the fluctuations diminish with decreasing temperature. In addition to the general tilt fluctuations, we observe intrinsic soft-mode fluctuations. They have a λ-shaped temperature dependence that peaks at the SmA-SmC transition with a maximum amplitude of about 2°.

Conformation of Single Polymer Chain in Rubbed Thin Film Observed by Fluorescence Imaging

The conformation of poly(methyl methacrylate) (PMMA) chains in a thin film after the rubbing process was investigated through the direct observation of the single chains by scanning near-field optical microscopy (SNOM) and excitation polarization modulation microscopy (EPMM). The rubbing at room temperature hardly changed the dimension on the whole chain scale in spite of the increase in orientational order on the segmental scale. The increase in the chain dimension along the rubbing direction was observed in the film rubbed at the higher temperature, which showed a surface morphology with fine groove. The extension ratio of the whole chain in the rubbed film was much smaller than that in the uniaxially stretched film. This indicates that the rubbing process mainly induces the conformational change on the length scale of the monomer unit rather than for the whole chain.

Effect of Increase in Orientational Order of Lipid Chains and Head Group Spacing on Non Steroidal Anti-Inflammatory Drug Induced Membrane Fusion

Membrane fusion is a key event in many biological processes. The fusion process, both in vivo and in vitro, is induced by different agents which include mainly proteins and peptides. For protein- and peptide-mediated membrane fusion, conformational reorganization serves as a driving force. Small drug molecules do not share this advantage; hence, drug induced membrane fusion occurring in absence of any other fusogenic agent and at physiologically relevant concentration of the drugs is a very rare event. To date, only three drugs, namely, meloxicam (Mx), piroxicam (Px), and tenoxicam (Tx), belonging to the oxicam group of non steroidal anti-inflammatory drugs (NSAIDs), have been shown by us to induce fusion at very low drug to lipid ratio without the aid of any other fusogenic agent. In our continued effort to understand the interplay of different physical and chemical parameters of both the participating drugs and the membrane on the mechanism of this drug induced membrane fusion, we present here the effect of increase in orientational order of the lipid chains and increase in head group spacing. This is achieved by studying the effect of low concentration cholesterol (<10 mol %) at temperatures above the chain-melting transition. Low concentration cholesterol (<10 mol %), above the gel to fluid transition temperature, is mainly known to increase orientational order of the lipid chains and increase head group spacing. To isolate the effect of these parameters, small unilameller vesicles (SUVs) formed by dimyristoylphosphatidylcholine (DMPC) with an average diameter of 50-60 nm were used as simple model membranes. Fluorescence assays were used to probe the time dependence of lipid mixing, content mixing, and leakage and also used to determine the partitioning of the drugs in the membrane bilayer. Differential scanning calorimetry (DSC) was used to study the effect of drugs in the presence of cholesterol on the chain-melting temperature which reflects the fluidization effect of the hydrophobic tail region of the bilayer. Our results show contradictory effect of low concentration cholesterol on the fusion induced by the three drugs, which has been explained by parsing the effect of orientational order and increase in head group spacing on the fusion process.

Energy landscape view of phase transitions and slow dynamics in thermotropic liquid crystals

Thermotropic liquid crystals are known to display rich phase behavior on temperature variation. Although the nematic phase is orientationally ordered but translationally disordered, a smectic phase is characterized by the appearance of a partial translational order in addition to a further increase in orientational order. In an attempt to understand the interplay between orientational and translational order in the mesophases that thermotropic liquid crystals typically exhibit upon cooling from the high-temperature isotropic phase, we investigate the potential energy landscapes of a family of model liquid crystalline systems. The configurations of the system corresponding to the local potential energy minima, known as the inherent structures, are determined from computer simulations across the mesophases. We find that the depth of the potential energy minima explored by the system along an isochor grows through the nematic phase as temperature drops in contrast to its insensitivity to temperature in the isotropic and smectic phases. The onset of the growth of the orientational order in the parent phase is found to induce a translational order, resulting in a smectic-like layer in the underlying inherent structures; the inherent structures, surprisingly, never seem to sustain orientational order alone if the parent nematic phase is sandwiched between the high-temperature isotropic phase and the low-temperature smectic phase. The Arrhenius temperature dependence of the orientational relaxation time breaks down near the isotropic-nematic transition. We find that this breakdown occurs at a temperature below which the system explores increasingly deeper potential energy minima.

On the origin of high optical director tilt in a partially fluorinated orthoconic antiferroelectric liquid crystal mixture

We have investigated the orthoconic antiferroelectric liquid crystal mixture W107 by means of optical, X-ray and calorimetry measurements in order to assess the origin of the unusally high tilt angle between the optic axis and the smectic layer normal in this material. The optical birefringence increases strongly below the transition to the tilted phases, showing that the onset of tilt is coupled with a considerable increase in orientational order. The layer spacing in the smectic A* (SmA*) phase is notably smaller than the extended length of the molecules constituting the mixture, and the shrinkage in smectic C* (SmC*) and smectic Ca* (SmCa*) is much less than the optical tilt angle would predict. These observations indicate that the tilting transition in W107 to a large extent follows the asymmetric de Vries diffuse cone model. The molecules are on average considerably tilted with respect to the layer normal already in the SmA* phase but the tilting directions are there randomly distributed, giving the phase its uniaxial characteristics. At the transition to the SmC* phase, the distribution is biased such that the molecular tilt already present in SmA* now gives a contribution to the macroscopic tilt angle. In addition, there is a certain increase of the average tilt angle, leading to a slightly smaller layer thickness in the tilted phases. Analysis of the wide angle scattering data show that the molecular tilt in SmCa* is about 20° larger than in SmA*. The large optical tilt (45°) in the SmCa* phase thus results from a combination of an increased average molecule tilt and a biasing of tilt direction fluctuations.

Liquid crystalline behavior of octylcyanobiphenyl confined to submicron-size randomly connected porous glasses

We report a study of the orientational order via deuterium NMR and high resolution ac calorimetry on octylcyanobiphenyl (8CB) confined to randomly oriented and interconnected macroporous (1000 \AA{} mean pore size) and microporous (100 \AA{} mean pore size) glasses. In the macroporous glass, the nematic to isotropic $(N\ensuremath{-}I)$ phase transition retains the bulklike weakly first order nature. The nematic phase is characterized by an NMR powder pattern spectra, as expected for a director distribution with no preferred spatial direction, and it coexists with an isotropiclike component. The orientational order is slightly suppressed from the bulk's and there is no sudden increase in orientational order due to the onset of the smectic phase suggesting a weaker coupling between nematic and smectic order parameters. Further, due to the confining size, the smectic-$A$ to nematic transition is considerably suppressed and broadened. In the microporous glass, the orientational order behavior is reminiscent of that in Vycor glass. No phase transitions are detected, rather, there is a continuous evolution of local orientational order. Interporous and intraporous interactions are important.

Melting dynamics of a plasma crystal

PLASMAS have long been regarded as the most disordered state of matter; nevertheless, a set of colloidal particles introduced into a charge-neutral plasma can spontaneously exhibit ordered crystalline structures1,2—so-called 'plasma crystals'. Such systems, which reach equilibrium very rapidly and can be easily tuned between their ordered and disordered states, are ideally suited for investigating the processes underlying the solid-to-liquid phase transition. Here we report the results of experiments on 'flat' plasma crystals (with thicknesses of only a few lattice planes) which suggest that the melting transition occurs through two fundamental intermediate stages. On melting, the crystal first enters a state characterized by islands of crystalline order, about which streams of particles flow. The crystalline regions then dissolve as the vibrational energy of the system increases, but this is accompanied by a temporary increase in orientational order before the system finally enters a disordered, liquid state. The unexpected 'vibrational' phase, characterized by enhanced orientational order, might arise as a consequence of the mixed two- and three-dimensional nature of the flat plasma crystals. Alternatively, it may indicate the existence of a new intermediate state in melting transitions more generally.

A lattice dynamics study of a Langmuir monolayer of monounsaturated fatty acids

A Monte Carlo dynamics (MCD) scheme has been applied in a study of the effects of unsaturated double bonds on the internal conformational dynamics and orientational order of hydrocarbon chains arranged in a monolayer on the surface of an impenetrable interface. The MCD algorithm makes use of the high coordination {2 1 0} lattice for the representation of both sp3 and sp2 valence states of the carbon atoms. The chain dynamics are considered to arise from a superposition of local conformational rearrangements. The simulations reproduced the principal features of the experimentally observed order parameter profiles of the C–H bonds on taking into account the intramolecular conformational energy of the molecules and excluded volume effects. The results show that the introduction of a rigid, planar, unsaturated segment enhances the orientational order in the monolayer. The extent of the enhancement is larger for the trans unsaturated chains than for the cis unsaturated ones. The increase in orientational order is accompanied by a marked increase in the effective rotational correlation times, indicating that the unsaturated segments undergo slow and restricted motion. In addition, the C–H bonds of the saturated chain segment between the cis double bond and the headgroup of the chain undergo slower motions than the corresponding vectors in the saturated and trans unsaturated chains. This arises from the anchoring of the headgroup at the impenetrable monolayer interface.

Invited Lecture. Molecular order and motion in nematogens from pulsed dynamic N.M.R. A comparative study of model compounds and parent liquid-crystal polymers

Abstract Model compounds of semiflexible liquid-crystal polymers (LCPs), specifically deuteriated at various positions of the mesogenic units and aliphatic chains, have been studied by multipulse dynamic N.M.R. techniques. Analysis of the various experiments, employing a density-matrix treatment based on the stochastic Liouville equation, provides new information about the dynamic organization of the different systems. The results, referring to monomers and dimers, are discussed in relation to the properties of the parent LCPs. The pronounced increase in orientational order from the monomers to the dimers can be rationalized by an intramolecular order transfer via highly extended spacers, in agreement with observations for the parent LCPs. A strong dependence of the nematic order on the parity of the spacer (even-odd effect) supports this concept. However, long-range orientational order of both model compounds is completely lost upon crystallization. Thus, despite the fact that dimers already achieve unus...

Excess heat capacities and orientational order in systems containing hexadecane isomers of different molecular structure

Using the Picker flow microcalorimeter, excess heat capacities have been obtained at 25°C throughout the concentration range for 2,2-dimethylbutane,n-hexane, and cyclohexane each mixed with a series of hexadecane isomers of increasing degrees of orientational order, as determined by depolarized Rayleigh scattering. The isomers are 2,2,4,4,6,8,8-heptamethylnonane, 6-, 4-, and 2-methylpentadecane, andn-hexadecane. Thec values are negative, increasing rapidly in magnitude with increase of orientational order, and are not predicted by the Prigogine—Flory theory which neglects order. Values ofc are obtained at 10, 25, and 55°C for cyclohexane +6-, 4-, and 2-methylpentadecane which with other literature data lead to the temperature dependence of the thermodynamic excess functions for cyclohexane solutions of the five C16 isomers. The excess enthalpy and entropy vary with the C16 isomer and with temperature, but the corresponding variation of the excess free energy is small, indicating a high degree of enthalpy-entropy compensation. This is consistent with a rapid decrease with temperature of orientational order in the C16 isomers.