An unusual chevron structure in an achiral smectic C liquid crystal device

The layer structure that occurs in an achiral smectic C liquid crystal device has been investigated as a function of temperature using the small angle X-ray scattering facility at the Synchrotron Radiation Source, Daresbury UK. The material studied shows a direct phase transition from the nematic to the smectic C phase. The layer structure proposed on the basis of the diffraction data is relatively complex, containing regions with chevron, quasi-bookshelf and curved structures. A rationale for the formation of the structure is presented, relying on both the phase transition characteristics of the system and the anisotropic layer elasticity in the smectic C phase. Qualitative analysis indicates that the layer constant A is greater than A 21, i.e. layer flexing is easier perpendicular to the plane of the director than parallel to it. It is also demonstrated that the surface chevron angle is several degrees different from the tilt angle of the smectic C phase at temperatures well below the smectic C to nematic phase transition. 12     

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 C_a* (SmC_a*) 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 SmC_a* is about 20° larger than in SmA*. The large optical tilt (45°) in the SmC_a* phase thus results from a combination of an increased average molecule tilt and a biasing of tilt direction fluctuations.     

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 C_a* (SmC_a*) 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 SmC_a* is about 20° larger than in SmA*. The large optical tilt (45°) in the SmC_a* phase thus results from a combination of an increased average molecule tilt and a biasing of tilt direction fluctuations.     

Systematic Variation of Length Ratio and the Formation of Smectic A and Smectic C Phases

The phase diagrams of four binary mixtures of chemically similar smectogenic mesogens differing only in molecular length are investigated. In these bidisperse systems the length ratio varies systematically. The phase diagrams show the stabilization of the smectic A and the destabilization of the smectic C phase with increasing length ratio as a general trend. Detailed small‐angle X‐ray diffraction and electro‐optic measurements revealed a decrease in smectic translational order and a continuous reduction of the tilt angle with increasing length difference. These surprising results are of general interest for the understanding of the structure and dynamics of smectic phases. The remarkably strong impact of the length difference on the smectic layer structure and the phase behavior is discussed from a mechanistic point of view taking into account sterical interactions. For the observed structural changes in these bidisperse smectics we propose pronounced out‐of‐layer fluctuations with increasing length difference as driving force, causing neighboring molecules within nearest layer into a smectic A‐like packing.     

Nature of smectic A*-C* phase transitions in a series of ferroelectric liquid crystals with little smectic layer shrinkage

The smectic layer spacing of two homologous series of ferroelectric liquid crystal compounds was characterized by small-angle x-ray diffraction and different degrees of smectic layer shrinkage on cooling from the SmA* into the SmC* phase were observed. The smectic A*-smectic C* phase transition was further studied by measuring the thermal and electric field effects on the optical tilt angle and the electric polarization. With decreasing length of the alkyl terminal chain the phase transition changes from tricritical exhibiting high layer shrinkage to a pure second-order transition with almost no layer shrinkage. This is explained by the increased one-dimensional translational order of the smectic layers, which seems to promote the "de Vries"-type [Mol. Cryst. Liq. Cryst. 41, 27 (1977)] smectic A*-C* phase transition with no or little layer shrinkage.     

Direct measurement of the twist penetration length in a single smectic A layer of colloidal virus particles

In the 1970s, deGennes discussed the fundamental geometry of smectic liquid crystals and established an analogy between the smectic A phase and superconductors. It follows that smectic layers expel twist deformations in the same way that superconductors expel magnetic field. We make a direct observation of the penetration of twist at the edge of a single isolated smectic A layer composed of chiral fd virus particles subjected to a depletion interaction. Using the LC-PolScope, we make quantitative measurements of the spatial dependence of the birefringence due to molecular tilt near the layer edges. We match data to theory for the molecular tilt penetration profile and determine the twist penetration length for this system.     

Dimeric liquid crystals with 5-(4-alkoxybenzoyloxy)tropone or 4-(4-alkoxybenzoyloxy)phenyl cores: evaluation of the tilt angles of the cores, spacers, and side chains

Two types of symmetric dimers with 5-(4-alkoxybenzoyloxy)tropone cores or with 4-(4-alkoxybenzoyloxy)phenyl cores were synthesized to evaluate the effect of the core structure and the length of the spacer on the mesomorphic properties. The former had smectic C phases whereas the latter had smectic C and F phases. Both types of dimer showed a remarkable odd-even effect on varying the spacer on the mesomorphic properties. Comparison of the thermal stability between them demonstrated that benzenoid twins are more stable than troponoid ones. The layer spacings of the smectic C phases were measured to determine the tilt angles of the core part, the spacer, and the side chains on changing the length of the spacer and the side chains. Troponoid dimers had a larger tilt angle of the core part of the smectic C phase than benzenoid twins, which lowered the thermal stability of the troponoid. Entropy changes of the smectic C phase to the isotropic liquid showed a contrast between troponoids and benzenoids. The former had smaller values and odd-even effects than the latter, which indicated that the former troponoids had a limited number of conformers in mesophases.     

Ferroelectric polysiloxane liquid crystals with 'de Vries'-type smectic A*-smectic C* transitions

We report preliminary results of optical and small angle X-ray scattering (SAXS) experiments on the smectic A*-smectic C* transition in two ferroelectric liquid crystalline polysiloxanes. Although the optical tilt angle in the SmC* phases reaches values up to 30°, temperature-dependent SAXS measurements clearly reveal that the smectic layer spacing is basically conserved during the A*-C* transition as well as in the subsequent C* phase. Connected with the A*-C* transition we further observed a significant increase in birefringence, hence reflecting an increase of orientational order. The practical absence of layer shrinkage and the enhanced orientational ordering are consistent with the de Vries diffuse cone model of smectic A-smectic C transitions.     

Optical guided-wave study of a homeotropically aligned ferroelectric liquid crystal

Abstract

The excitation of optical modes is used to study the optical tensor configuration in a thin ferroelectric liquid crystal layer, cooled from the initially homeotropically aligned nematic phase. By monitoring the angular dependent reflectivity for plane polarized radiation coupled into the guided modes in the smectic C* layer and subsequently fitting the recorded data to predictions from multilayer optics theory, the optical tensor configuration in the layer is fully evaluated. Iteratively modelling the full tilt/twist profile in the cell, progressively converging the predicted reflectivity to experimental data, gives a complete and very well specified picture of the optical tensor throughout the cell. By studying the cell at various temperatures, the temperature dependence of the tilt of the major axis of the optic tensor (which may be related to the cone angle if the smectic layers are parallel to the cell surface) has been established. The temperature dependent optical dielectric constants have also been obtained.     

Optical guided-wave study of a homeotropically aligned ferroelectric liquid crystal

The excitation of optical modes is used to study the optical tensor configuration in a thin ferroelectric liquid crystal layer, cooled from the initially homeotropically aligned nematic phase. By monitoring the angular dependent reflectivity for plane polarized radiation coupled into the guided modes in the smectic C* layer and subsequently fitting the recorded data to predictions from multilayer optics theory, the optical tensor configuration in the layer is fully evaluated. Iteratively modelling the full tilt/twist profile in the cell, progressively converging the predicted reflectivity to experimental data, gives a complete and very well specified picture of the optical tensor throughout the cell. By studying the cell at various temperatures, the temperature dependence of the tilt of the major axis of the optic tensor (which may be related to the cone angle if the smectic layers are parallel to the cell surface) has been established. The temperature dependent optical dielectric constants have also been obtained.     

Dielectric, viscosity, and pitch measurements of a ferroelectric liquid crystal exhibiting the phase sequence smectic M-smectic C

A chiral liquid crystal compound exhibiting the ferroelectric smectic C phase and the recently discovered ferroelectric smectic M phase has been studied by measurements of the Goldstone-mode relaxation frequency and dielectric strength, the spontaneous polarization, the tilt angle and the helical pitch. The data allow the determination of the Goldstone-mode rotational viscosity and the pitch controlling elastic constant. The results indicate that the smectic M phase is characterized by a larger molecular order within the smectic layers compared to the smectic C phase confirming the assumption of a tilted hexatic structure for the smectic M phase.     

Direct smectic A-smectic C*A phase transition

The anticlinic smectic CA phase belongs to the class of tilted smectic phases with an azimuthal angle alternating from one direction (theta=0) to the other (theta=pi) in successive layers. It occurs in general at lower temperature than the uniformly tilted smectic C phase, but may be obtained directly from the untilted smectic A phase. We use the chiral nCTBB9* series synthesized in this laboratory, in order to obtain a phase transition as close as possible to second order, as revealed by DSC. We measure the temperature behaviour of the birefringence and of the optical rotatory power across the transition in order to characterize the tilt angle. We finally study the optical response to a periodic electric field which excites separately the smectic C* and C*A soft modes. The main conclusion is that the only order parameter governing the critica Al behaviour of the phase transition is the tilt angle theta, as we get common divergence of both soft modes at the same temperature. This confirms previous high resolution calorimetric studies by Ema et al. that saw in MHPOBC an initial mean-field second order phase transition when the tilt appears, followed by sharp first order restructuring transitions between the tilted subphases.     

Direct smectic A-smectic C*A phase transition

The anticlinic smectic CA phase belongs to the class of tilted smectic phases with an azimuthal angle alternating from one direction (theta=0) to the other (theta=pi) in successive layers. It occurs in general at lower temperature than the uniformly tilted smectic C phase, but may be obtained directly from the untilted smectic A phase. We use the chiral nCTBB9* series synthesized in this laboratory, in order to obtain a phase transition as close as possible to second order, as revealed by DSC. We measure the temperature behaviour of the birefringence and of the optical rotatory power across the transition in order to characterize the tilt angle. We finally study the optical response to a periodic electric field which excites separately the smectic C* and C*A soft modes. The main conclusion is that the only order parameter governing the critica Al behaviour of the phase transition is the tilt angle theta, as we get common divergence of both soft modes at the same temperature. This confirms previous high resolution calorimetric studies by Ema et al. that saw in MHPOBC an initial mean-field second order phase transition when the tilt appears, followed by sharp first order restructuring transitions between the tilted subphases.     

The effect of chiral doping in achiral smectic liquid crystals on the de Vries characteristics: smectic layer thickness,electro-optics and birefringence

An addition of chiral dopant to two achiral smectic liquid crystals from a homologous series, by varying weight percentages with known low values of layer shrinkage, leads to chiral smectic-C* phase with a finite value of the spontaneous polarisation. The electro-optical response arising from changes in the induced apparent tilt angle brought about by a weak electric field in the SmA* phase gives rise to power law dependency on the reduced temperature. The critical exponent γ of the power law depends on the dopant concentration but its value is found to be greater than the typical value of 1.32. This implies that the short-range correlation extends from two dimensions to three dimensions in these materials in the SmA phase. The layer thickness of smectic layers in the guest–host system remains unaltered up to the 15 wt % addition of the chiral dopant to two achiral smectics. The system thus retains the low layer shrinkage of the achiral smectic as evidenced by measurements of the layer thickness from X-ray scattering and thickness measurements from optical interferometry. Results on the optical birefringence and the apparent tilt angle lead us to the conclusion of having successfully obtained chiral smectic materials for devices with de Vries characteristics by chiral doping.     

Dielectric,viscosity, and pitch measurements of a ferroelectric liquid crystal exhibiting the phase sequence smectic M–smectic C

Abstract

A chiral liquid crystal compound exhibiting the ferroelectric smectic C phase and the recently discovered ferroelectric smectic M phase has been studied by measurements of the Goldstone-mode relaxation frequency and dielectric strength, the spontaneous polarization, the tilt angle and the helical pitch. The data allow the determination of the Goldstone-mode rotational viscosity and the pitch controlling elastic constant. The results indicate that the smectic M phase is characterized by a larger molecular order within the smectic layers compared to the smectic C phase confirming the assumption of a tilted hexatic structure for the smectic M phase.     

De Vries smectic A phase formed by a liquid crystal side chain copolymer? A 13C NMR study

We study the orientation and order parameter of a liquid crystalline random side chain copolymer by ¹³C NMR. Evidence has previously been presented that this material forms a de Vries smectic A phase. The NMR data show that the molecular tilt angle in the smectic A phase is very small or zero and the smectic A–smectic C* transition is attributed predominantly to a change of the molecular tilt rather than azimuthal ordering. We discuss the NMR results in the context of earlier X‐ray and elastic characterizations of structurally similar materials.     

First Examples of de Vries‐like Smectic A to Smectic C Phase Transitions in Ionic Liquid Crystals

In ionic liquid crystals, the orthogonal smectic A phase is the most common phase whereas the tilted smectic C phase is rather rare. We present a new study with five novel ionic liquid crystals exhibiting both a smectic A as well as the rare smectic C phase. Two of them have a phenylpyrimidine core whereas the other three are imidazolium azobenzenes. Their phase sequences and tilt angles were studied by polarizing microscopy and their temperature‐dependent layer spacing as well as their translational and orientational order parameters were studied by X‐ray diffraction. The X‐ray tilt angles derived from X‐ray studies of the layer contraction and the optically measured tilt angles of the five ionic liquid crystals were compared to obtain their de Vries character. Four of our five mesogens turned out to show de Vries‐like behavior with a layer shrinkage that is far less than that expected for conventional materials. These materials can thus be considered as the first de Vries‐type materials among ionic liquid crystals.     

Dimeric liquid crystals with 5‐(4‐alkoxybenzoyloxy)tropone or 4‐(4‐alkoxybenzoyloxy)phenyl cores: evaluation of the tilt angles of the cores,spacers, and side chains

Two types of symmetric dimers with 5‐(4‐alkoxybenzoyloxy)tropone cores or with 4‐(4‐alkoxybenzoyloxy)phenyl cores were synthesized to evaluate the effect of the core structure and the length of the spacer on the mesomorphic properties. The former had smectic C phases whereas the latter had smectic C and F phases. Both types of dimer showed a remarkable odd–even effect on varying the spacer on the mesomorphic properties. Comparison of the thermal stability between them demonstrated that benzenoid twins are more stable than troponoid ones. The layer spacings of the smectic C phases were measured to determine the tilt angles of the core part, the spacer, and the side chains on changing the length of the spacer and the side chains. Troponoid dimers had a larger tilt angle of the core part of the smectic C phase than benzenoid twins, which lowered the thermal stability of the troponoid. Entropy changes of the smectic C phase to the isotropic liquid showed a contrast between troponoids and benzenoids. The former had smaller values and odd–even effects than the latter, which indicated that the former troponoids had a limited number of conformers in mesophases.     

Scanning tunnelling microscopic study of the interfacial order in a ferroelectric liquid crystal

We present scanning tunnelling microscopy images of a ferroelectric liquid crystal in its chiral smectic C phase. These images show evidence of pronounced interaction between the adjacent lateral dipoles located near the chiral centres of neighbouring molecules. The two dimensional structure at the graphite—liquid crystal interface is found to be highly ordered. Features of this crystal-like order are the pronounced dimer arrangement within a layer and a kinked conformation in which the aromatic core is tilted with respect to the chains. The core axis is tilted at 30° with respect to the layer normal in the plane of the interface. This tilt is very similar to the tilt angle of the bulk material.     

Cryo-TEM studies of two smectic phases of an asymmetric bent-core material

Cryo-TEM studies on two smectic phases of an asymmetric bent-core liquid crystal material are presented and compared to prior X-ray results obtained in bulk samples. While the bulk samples have layer-modulated structures, those modulations are not observable in the 100-nm-thick TEM samples, indicating surface-induced suppression of the layer modulations. The observed layer spacing is in agreement with the X-ray results in the lower temperature smectic phase, but distinctly larger in the higher temperature phase. This indicates surface-induced suppression of the director tilt. Cryo-TEM textures resolve the profiles of individual smectic layers at the scales down to few nanometres and reveal the presence of edge and screw dislocations, twist grain boundaries, small-angle and large-angle tilt grain boundaries.