Smectic Liquid Crystal Phases Research Articles

Electroclinic effect and modulated phases in smectic liquid crystals.

We explore the possibility that the unusually large electroclinic effect observed in the smectic-A phase of a ferroelectric liquid crystal arises from the presence of an ordered array of disclination lines and walls in a smectic-C* phase. If the spacing of these defects is in the subvisible range, this modulated smectic-C* phase would be similar macroscopically to a smectic-A phase. The application of an electric field distorts the array, producing a large polarization, and hence a large electroclinic effect. We show that with suitable elastic parameters and sufficiently large chirality, the modulated phase is favored over the smectic-A and helically twisted smectic-C* phases. We propose various experimental tests of this scenario.

Smectic liquid crystals from supramolecular guanidinium alkylbenzenesulfonates.

A homologous series of guanidinium alkylbenzenesulfonates from ethyl to tetradecyl were synthesized and characterised. Their thermotropic polymorphism was investigated by polarizing optical microscopy, differential scanning calorimetry, and dilatometry. The structure of the smectic liquid crystal phases obtained at high temperature with the compounds from octyl to tetradecyl was analysed by X-ray diffraction. The supramolecular assembling of the ionic species inside the smectic layers was investigated by infrared spectroscopy.

Interlayer structures of the chiral smectic liquid crystal phases revealed by resonant X-ray scattering.

The structures of the liquid crystalline chiral subphases exhibited by several materials containing either a selenium or sulphur atom have been investigated using a resonant x-ray scattering technique. This technique provides a unique structural probe for the ferroelectric, ferrielectric, antiferroelectric, and SmC(*)(alpha) phases. An analysis of the scattering features allows the structural models of the different subphases to be distinguished, in addition to providing a measurement of the helical pitch. This paper reports resonant scattering features in the antiferroelectric hexatic phase, the three- and four-layer intermediate phases, the antiferroelectric and ferroelectric phases and the SmC(*)(alpha) phase. The helicoidal pitch has been measured from the scattering peaks in the four-layer intermediate phase as well as in the antiferroelectric and ferroelectric phases. In the SmC(*)(alpha) phase, an investigation into the helical structure has revealed a pitch ranging from 5 to 54 layers in different materials. Further, a strong resonant scattering signal has been observed in mixtures of a selenium containing material with as much as 90% nonresonant material.

Light shutters from antiferroelectric liquid crystals of bent-shaped molecules

We present novel scattering-type displays using antiferroelectric smectic phases of liquid crystals of bent-shaped molecules. There can be two distinct states (‘racemic’ and ‘chiral’) that work in opposite ways. The racemic structure is scattering in the OFF state and is optically clear under electric fields. The chiral structure is transparent at zero fields and is scattering in the field ON state. These two structures may be reversibly interchanged, implying their use in devices that consume energy only during switching from one stable state to the other.

X-Ray Diffraction Investigation of Oleic Acid under High Pressure

Pressure has a specific influence on the light scattering in those liquids in which all molecules have an unsaturated double bond C=C. Strong changes of intensity of transmitted and scattered light in oleic acid were observed during its transition to the high-pressure phase. In this paper, local order in liquid of oleic acid has been studied using the X-ray diffraction photographic Laue method. Experiments were run at room temperature under atmospheric pressure as well as under pressures up to 0.25 GPa. At pressures above 0.2 GPa a strong increase of ordering of molecules, similar to a smectic liquid crystal phase, has been observed. This confirms results obtained previously by studies of light scattering under high pressure.

Evidence from infrared dichroism, x-ray diffraction, and atomistic computer simulation for a “zigzag” molecular shape in tilted smectic liquid crystal phases

Infrared dichroism (IR) and atomistic computer simulation are employed to probe molecular shape in smectic liquid crystal phases where the optic axis is tilted relative to the layer normal. Polar plots of absorption profiles due to core (phenyl, C-C) and tail (alkyl or methylene, CH2) vibrations in the tilted synclinic (smectic-C) phase of a variety of materials show the phenyl (core) IR absorbance symmetry axes to be consistently tilted at larger angle from the layer normal than the alkyl or methylene (tail). This suggests that, on average, the tails are less tilted than the cores. Furthermore, we find that optic axis tilt angle is close to the core tilt angle measured by IR dichroism, as expected, since liquid crystal birefringence arises primarily from the cores. These results are in accord with the "zigzag" model of Bartolino, Doucet, and Durand. However, we find that only a small fraction of the tail, the part nearest the core, is tilted, and only this part contributes significantly to layer contraction upon molecular tilt.

Racemic and chiral structures of banana-shaped smectic liquid crystals viewed as one-dimensional piezoelectric crystals

A smectic liquid-crystal phase formed by achiral banana-shaped molecules (CB2) is studied as one-dimensional piezoelectric crystals with fluidity of tilted-C2V anisotropy in the other two spatial dimensions. Variation of the elastic-piezoelectric free energy of the system predicts the existence of racemic (R), left-, and right-handed chiral (H) antiferroelectric equilibrium structures, a result in agreement with the observation by Link et al. [Science 278, 1924 (1997)]. The electric-field-induced ferroelectric state is shown to be a special C2 symmetry, which confirms the measurements by optical second harmonic generation in a CB2 by Macdonald et al. [Phys. Rev. Lett. 81, 4408 (1998)]. The hysteresis loop of switching polarization is also discussed, and the molecular tilt angle dependence of spontaneous polarization is obtained as a function of the elastic and the piezoelectric constants.

Chirality transfer in ferroelectric liquid crystals.

This Account describes how concepts used in the fields of host-guest chemistry and chiral molecular recognition may be used to explain the unique chiral induction behavior of molecules with atropisomeric biphenyl cores when doped into a two-dimensionally ordered smectic liquid crystal phase formed by rod-shaped molecules with a phenylpyrimidine core structure.

Flexoelectricity and piezoelectricity: the reason for the rich variety of phases in antiferroelectric smectic liquid crystals.

The free energy of antiferroelectric smectic liquid crystals which takes into account polar order explicitly is presented. Steric, van der Waals, piezoelectric, and flexoelectric interactions to the nearest layers, and dipolar electrostatic interactions to the nearest and to the next-nearest layers, induce indirect tilt interactions with chiral and achiral properties, which extend to the third- and to the fourth-nearest layers. Although the strength of microscopic interactions changes monotonically with decreasing temperature, the effective interlayer interactions change nonmonotonically and give rise to a nonmonotonic change of the modulation period through various phases. Increased chirality changes the phase sequence.

Observation of a soft smectic liquid-crystal phase in a mixture showing V-shaped switching.

We present the experimental observation of a soft smectic liquid crystal phase in a three-component liquid crystal mixture reported to exhibit V-shaped switching. Unlike the layer compression modulus B of the usual smectic phase, B of the soft smectic phase has a peculiar temperature and frequency dependence. It was found that this characteristic feature is one of the main driving force to realize an ideal V-shaped switching.

Liquid Crystal Optical Phase Modulators for Beam Steering

ABSTRACTBeamsteering using liquid crystals can be achieved with refractive or diffractive implementations. The common thread in these various structures is that the liquid crystal is employed as an optical phase modulator. Either nematic or smectic liquid crystal phases can be used to shift the phase of light and steer an optical beam. Various liquid crystal optical phase modulating schemes will be described. Examples include polarization independent and quasi-achromatic modulators. Model predictions and experimental results demonstrating the optical phase modulation and beamsteering made possible using different liquid crystal based designs will be presented.

Helical filamentary growth in liquid crystals consisting of banana-shaped molecules

The formation of coils is common in nature when achiral symmetry breaking occurs. Here we describe spectacular examples of single, double and triple coils observed in smectic liquid crystal phases of achiral banana-shaped molecules. Such molecules form chiral smectic phases due to two symmetry-breaking instabilities: polar molecular packing, and molecular tilt. The appearance of helical filaments at the isotropic-smectic transition is therefore a direct indication of the achiral symmetry-breaking of the smectic structures. The number of observed left- and right-handed domains is equal, reflecting the achiral nature of the constituent molecules. Our studies indicate that the helical filaments consist of concentric smectic layers. The coiling stabilizes the growth process and suppresses the penetration of molecules from the isotropic phase, leading to moving of the tip with constant speed.

Roto-translational diffusion of biaxial probes in uniaxial liquid crystal phases

We discuss the problem of roto-translational diffusion of a rigid biaxial molecule dissolved in a uniaxial smectic liquid crystal phase. We examine distorted rod and disklike molecules and show how biaxiality and roto-translational coupling can produce significant effects on some of the correlation functions and spectral densities most useful in analyzing experimental observables.

Pseudo-lattice vibrations in smectic phase of liquid crystals: studies on small wave number Raman spectra of 4-alkyl-4′-cyanobiphenyl

Small wave number Raman spectra of 4-C n H 2 n +1-4′-cyanobiphenyl ( nCB) were measured for n=8,9,10,12. Sharp Raman bands are observed in common below about 10 cm −1 for the smectic A phase of these compounds. These bands are assigned to intermolecular vibrations along the direction perpendicular to the smectic layers each of which comprises two sublayers. Existence of such definite intermolecular vibrations has been reported previously by the present authors. We called them pseudo-lattice vibrations in smectic liquid crystals. Apparent force constant and damping factor of these vibrations are smaller for a larger n. These facts imply that for a larger n, the structural ordering in a sublayer is better, but the interaction between sublayers is smaller.

Microscopic origins of heliclinic phases in smectic liquid crystals.

In a previous article [Phys. Rev. E 60, 1799 (1999)], the authors considered a model Landau free energy that explained the ferriclinic phases of chiral smectic liquid crystals as a series of short period helical modulations. In this paper we begin with a physically more realistic, more microscopic interlayer free energy and show how our previous work can be derived using only simple short-ranged interactions. We then discuss what additional information this provides about the Landau coefficients used previously to construct the phase diagram for the heliclinic phases of chiral smectic liquid crystals. Finally, we investigate a means for explicitly including chirality in our model.

Electric-field-induced chiral separation in liquid crystals.

Recently it was shown that smectic liquid crystal phases formed by achiral banana-shaped molecules are chiral. Films of such phases generally contain both homochiral (consisting of layers of the same chirality) and racemic (the chirality alternates in subsequent layers) domains. So far it has not been clear how to control the overall chirality of the sample. By observing the effects of the application of suitable electric fields, it was noted that chirality of a film of banana shaped molecules was interchanged between homochiral and racemic. We present dielectric, electro-optic, and polarization current measurements on both the racemic and chiral states. The observations indicate that the synclinic states have minimum free energies. The observed dielectric modes can be explained by a simple model assuming that the antiferroelectric state is very weak.

Helical single-lamellar crystals thermotropically formed in a synthetic nonracemic chiral main-chain polyester

Phase structures and transformation mechanisms of nonracemic chiral biological and synthetic polymers are fundamentally important topics in understanding their macroscopic responses in different environments. It has been known for many years that helical structures and morphologies can exist in low-ordered chiral liquid crystalline (LC) phases. However, when the chiral liquid crystals form highly ordered smectic liquid crystal phases, the helical morphology is suppressed due to the crystallization process. A double-twisted morphology has been observed in many liquid crystalline biopolymers such as dinoflaggellate chromosomes (in Prorocentrum micans) in an in vivo arrangement. Helical crystals grown from solution have been reported in the case of Bombyx mori silk fibroin crystals having the beta modification. This study describes a synthetic nonracemic chiral main-chain LC polyester that is able to thermotropically form helical single lamellar crystals. Flat single lamellar crystals can also be observed under the same crystallization condition. Moreover, flat and helical lamellae can coexist in one single lamellar crystal, within which one form can smoothly transform to the other. Both of these crystals possess the same structure, although translational symmetry is broken in the helical crystals. The polymer chain folding direction in both flat and helical lamellar crystals is determined to be identical, and it is always along the long axis of the lamellae. This finding provides an opportunity to study the chirality effect on phase structure, morphology, and transformation in condensed states of chiral materials. [S0163-1829(99)01042-5].

A solid-state13C-NMR study of the mesophases of PEIM-9 (poly(4,4′-phthaloimidobenzoylnonamethylene-oxycarbonyl))

On cooling from the melt, poly(4,4′-phthaloimidobenzoylnonamethyleneoxycarbonyl) (PEIM-9) forms a monotropic, smectic liquid crystal phase (Ti = 369.2 K). The main driving force for this mesophase formation is the attainment of nanophase separation of the mobile nonamethylene spacer from the geometrically rigid, but irregular, phthaloimidobenzoyl group, coupled with partial conformational ordering of the CH2 groups (about 20% of the CH2 groups attain trans-conformations). It is shown by nuclear magnetic resonance that PEIM-9 consists at room temperature of two motionally distinguishable components. One is the liquid crystal that remains mobile to its glass transition temperature (Tg ≈ 323 K), the other a more rigid crystal with a large degree of conformational disorder. In this crystal phase (Tm ≈ 415 K) the conformationally disordered nonamethylene spacer has a similar amount of disorder than in the liquid crystal phase and the phthaloimidobenzoyl group is also not fully ordered. Even after long-term annealing, all molecules remain conformationally disordered, but Tm increases to about ≈ 437 K.

Twist modulated phases in chiral smectic liquid crystals.

By considering short period helical planar modulations about the layer normal, we construct a model free energy for the ferriclinic phases observed in chiral smectic liquid crystals. We then use this free energy to construct the phase diagram for our model. The resulting phases are compared with the experimentally observed smectic-C* subphases (ferroclinic, antiferroclinic, and heliclinic). A strong coupling is found between the ferroclinic q=2pi/a and the heliclinic q=2pi/3a modes. This coupling was not considered in previous models. The resulting additional stability of this "locked in" phase is discussed.

Molecular Dynamics Simulation of Smectic Phases in Pentamethyl-Disiloxane-Terminated Mesogens

Molecular dynamics simulations have been used to investigate ensembles of molecules that show smectic liquid crystal phases. We have shown that there is a high tendency for penta methyldisiloxane-terminated mesogens to form assembled dimers, which are then associated into well-defined layers. By applying tilted 3D-boundary conditions we have been able to simulate the formation of a tilted smectic C phase.