Theory of nematic-smectic phase separation in thin twisted liquid crystal cells

Recently it has been shown experimentally by the authors that a highly twisted thin nematic cell at low temperatures can separate into a smectic A region in the middle of the cell surrounded by twisted nematic layers at the boundaries. In this case the twist is expelled into the nematic layers and the nematic-smectic A transition temperature is strongly depressed. We present a thermodynamic theory of such a phase transition in a twisted nematic cell, taking into account that the smectic A slab inside the nematic cell can be stable only if the decrease of free energy in the smectic region overcomes the increase in distortion energy of the twist deformation in the nematic layers plus the energy of the nematic-smectic A interface. In such a system the equilibrium thickness of the smectic A slab corresponds to the minimum of the total free energy of the whole cell, which includes all the bulk and surface contributions. Existing experimental data are at least qualitatively explained by the results of the present theory. This opens a unique possibility to study the properties of the nematic-smectic interface which is perpendicular to the smectic layers.     

Effect of the temperature range of the nematic phase on the induction of a twist grain boundary phase in non-chiral liquid crystals

Recent experimental results suggest that the twist grain boundary phase may be induced in non-chiral smectic liquid crystals by confining the material in a twist cell in which the nematic directors at the two surfaces are perpendicular to each other. However, the effect of the temperature range of the nematic phase on the induction of the twist grain boundary (TGB) phase has not been studied to date. We have performed experiments on non-chiral liquid crystals having the nematic-smectic A (N-SmA) phase transition sequence and confined in a twist cell. It is observed that materials with a second order N-SmA phase transition show characteristics of a TGB phase, but a material with a first order N-SmA transition does not.     

Effect of the temperature range of the nematic phase on the induction of a twist grain boundary phase in non-chiral liquid crystals

Recent experimental results suggest that the twist grain boundary phase may be induced in non-chiral smectic liquid crystals by confining the material in a twist cell in which the nematic directors at the two surfaces are perpendicular to each other. However, the effect of the temperature range of the nematic phase on the induction of the twist grain boundary (TGB) phase has not been studied to date. We have performed experiments on non-chiral liquid crystals having the nematic-smectic A (N-SmA) phase transition sequence and confined in a twist cell. It is observed that materials with a second order N-SmA phase transition show characteristics of a TGB phase, but a material with a first order N-SmA transition does not.     

A simple electro-optic technique for studying the electroclinic effect in the smectic A* phase

The critical behaviour of the electroclinic response in the chiral smectic A* phase in the vicinity of the second-order smectic A* to smectic C* phase-transition temperature has been investigated using a new electro-optic technique. The temperature variation of the electroclinic coefficient, the relaxation frequency and the coefficient of the quartic term in the tilt angle in the Landau free energy expansion have been studied. The electroclinic coefficient diverges with decreasing temperature as the smectic A* to smectic C* phase-transition temperature is approached with a critical exponent, as predicted in the mean field Landau theory. The measured quartic coefficient varies strongly with temperature, contrary to the usual assumptions of the mean field Landau theory.     

Electroclinic effect around a smectic A-chiral nematic phase transition

We present measurements of the dynamics of the electroclinic effect around a smectic Achiral nematic transition. The phenomenon has been studied between 1 kHz and 1 MHz, extending by one order of magnitude the frequency range of previous studies. The results have unambiguously allowed us to distinguish two additive mechanisms in the generation of the optical tilt. A model to explain the physical origin of both mechanisms is presented. The first mechanism (fast) is the only bulk process intrinsic to the material and behaves normally at the transition, in the sense that both the magnitude of the tilt as well as the relaxation time have the expected temperature dependence. On the other hand, the second mechanism (slow) is not properly an electroclinic effect, but a surface-mediated effect driven by elastic forces. This explains the apparent anomalous behaviour of the phenomenon reported in the literature one decade ago.     

Electroclinic effect around a smectic A-chiral nematic phase transition

We present measurements of the dynamics of the electroclinic effect around a smectic Achiral nematic transition. The phenomenon has been studied between 1 kHz and 1 MHz, extending by one order of magnitude the frequency range of previous studies. The results have unambiguously allowed us to distinguish two additive mechanisms in the generation of the optical tilt. A model to explain the physical origin of both mechanisms is presented. The first mechanism (fast) is the only bulk process intrinsic to the material and behaves normally at the transition, in the sense that both the magnitude of the tilt as well as the relaxation time have the expected temperature dependence. On the other hand, the second mechanism (slow) is not properly an electroclinic effect, but a surface-mediated effect driven by elastic forces. This explains the apparent anomalous behaviour of the phenomenon reported in the literature one decade ago.     

Phase Transition and Structural Investigations in 50.5, 50.6 and 50.7

We report X-ray diffraction, density, ultrasonic velocity and refractive index studies in the N(p-n-pentyloxy benzylidene) p-n-alkylaniline compounds, viz. 50.5, 50.6 and 50.7. The nematic-smectic A (NA) transition is found to be weak first order in 50.6 while it is second order in 50.5 and 50.7. The salient features observed are cybotactic clusters in the nematic phase in all the compounds, molecular tilt which was inferred due to the end alkyl chains tilt causing orienta-tional disorder (smaller orientational order parameters 's' than expected) in smectic A phase, smectic F phase and large tilt angle variation in smectic C phase in a small temperature range in 50.5. The observed results are discussed in the light of available data in other n0.m compounds.     

A study of the electroclinic effect in the smectic A* phase of mixtures with strongly chiral alkoxybiphenyl-phenyl carboxylate dopants

A detailed investigation has been reported of the electroclinic behaviour in the smectic A* phase of eleven mixtures made up of a commercial achiral smectic C host and strongly chiral alkoxybiphenyl-phenyl carboxylate dopants. A new technique was used to measure the induced tilt angle as a function of electric field and temperature. For all the mixtures, the electroclinic response followed a Curie-Weiss type temperature dependence for measurements performed well away from the smectic A* to smectic C* phase transition temperature. The strength of the electroclinic response was evaluated for each mixture by determining the temperature independent ratio k/a (i.e. the electroclinic coupling susceptibility, k divided by the susceptibility coefficient controlling the induced tilt, a). Analysis of the results showed that k/a of the mixtures was dependent on the type and position of the electronegative or polar substituents that affected the net dipole moment of the chiral dopants. In fact, the value of k/a was largest when fluorine was replaced by hydrogen in the lateral position and at the chiral centre. Furthermore, relatively short alkyl chains (e.g. C₆H₁₃) at the chiral centre were preferred to longer ones (e.g. C₁₀H₂₁) for a larger electroclinic response.     

Tricritical point induced by smectic ordering of a nematic gel

The authors study volume phase transitions of a nematic gel immersed in a liquid crystal (LC) solvent, which shows a second-order nematic-smectic A phase transition (NST). Combining Flory's elastic energy [Principles of Polymer Chemistry (Cornell University Press, Ithaca, 1953)] for a swelling of the gel with the McMillan model [Phys. Rev. A 4, 1238 (1971)] for smectic ordering, the authors calculate the equilibrium swelling of the gel and smectic order parameters as a function of temperature. The authors take into account an attractive interaction parameter c between the gel and LC solvents. On increasing the value of the coupling constant c, a second-order NST of the gel is changed to a first-order one and a continuous volume phase transition of the gel is changed to a discontinuous one. The authors find a tricritical point of the gel induced by smectic ordering.     

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     

X-ray investigation of the nematic and reentrant nematic phases of N-[(4-n-octyloxybenzoyloxy)-salicilidene]-4'-cyanoaniline

We have performed an X-ray scattering study of the nematic-smectic A and reentrant nematic-smectic A phase transitions in N-[(4-n-octyloxybenzoyloxy)-salicilidene]-4'-cyanoaniline (OOBOSCA). A diffractometer with a linear position sensitive detector was used. The results show that the smectic phase in OOBOSCA is of the A_d type with an interlayer spacing incommensurate with the molecular length L; d ∽ 1·2L. In the reentrant nematic phase two types of fluctuation modes were found. One of them corresponds to the monolayer wavevector q₁ ∽ 2π/L, and the other is due to the partial bilayer fluctuations with the wavevector q₂ ∽ 0·8q₁. The temperature dependences of the interlayer spacing, X-ray scattering intensity and longitudinal correlation length for both types of layering in the reentrant nematic phase are presented. The change of the fluctuation regime from S_Ad, to S_Cd type with decreasing temperature in the reentrant nematic phase of OOBOSCA was found. The results are discussed on the basis of models with competing order parameters. The influence of alkyl chain flexibility on the stability of a partial bilayer smectic phase is also considered.     

Phase Transition and Structural Investigations in 50.5, 50.6 and 50.7

Abstract

We report X-ray diffraction, density, ultrasonic velocity and refractive index studies in the N(p-n-pentyloxy benzylidene) p-n-alkylaniline compounds, viz. 50.5, 50.6 and 50.7. The nematic-smectic A (NA) transition is found to be weak first order in 50.6 while it is second order in 50.5 and 50.7. The salient features observed are cybotactic clusters in the nematic phase in all the compounds, molecular tilt which was inferred due to the end alkyl chains tilt causing orienta-tional disorder (smaller orientational order parameters ‘s’ than expected) in smectic A phase, smectic F phase and large tilt angle variation in smectic C phase in a small temperature range in 50.5. The observed results are discussed in the light of available data in other n0.m compounds.     

Fast switching ferroelectric side-chain liquid-crystalline polymer and copolymer

New ferroelectric side-chain liquid-crystalline polymers, a copolymer and a homopolymer, with siloxane backbone and a triaromatic mesogen as the side group have been synthesized. The materials exhibit a chiral smectic C phase over a large temperature range extending to room temperature. They possess high values of spontaneous polarization: 105 nC cm^-2 for the homopolymer and 180 nC cm^-2 for the copolymer. The electro-optic switching time in the chiral smectic C phase is extremely fast (150 μs). In the smectic A phase, an electroclinic effect with switching times less than 100 μs and with field induced tilt angles of 18° is observed.     

A Driven Voltage-Controlled Reversible Electrooptic Effect in a Smectic A Phase II. PVA Anchoring

The driven voltage-controlled reversible electrooptic effect in a smectic A phase with a preceding nematic phase twisted by SiO anchoring, previously discovered (cf. the previous paper) has been studied with polyvinyl alcohol (PVA) anchoring. The unique feature of this novel electrooptic effect in smectic A phases is that it reverses by relaxing when the electric field is removed. For the case of PVA anchoring the reversibility of the effect only occurs near the smectic A-nematic phase transition in the smectic A phase. The temperature region of the reversibility of this effect can be widened with a higher voltage. A comparison with the case of a reversible electrooptic effect in a smectic A with a preceding twisted nematic phase and SiO anchoring is made. The possible application of this electrooptic effect in liquid crystal displays with storage is discussed briefly.     

A Driven Voltage-Controlled Reversible Electrooptic Effect in a Smectic A Phase II. PVA Anchoring

Abstract

The driven voltage-controlled reversible electrooptic effect in a smectic A phase with a preceding nematic phase twisted by SiO anchoring, previously discovered (cf. the previous paper) has been studied with polyvinyl alcohol (PVA) anchoring. The unique feature of this novel electrooptic effect in smectic A phases is that it reverses by relaxing when the electric field is removed. For the case of PVA anchoring the reversibility of the effect only occurs near the smectic A-nematic phase transition in the smectic A phase. The temperature region of the reversibility of this effect can be widened with a higher voltage. A comparison with the case of a reversible electrooptic effect in a smectic A with a preceding twisted nematic phase and SiO anchoring is made. The possible application of this electrooptic effect in liquid crystal displays with storage is discussed briefly.     

X-ray investigation of the nematic and reentrant nematic phases of N-[(4-n-octyloxybenzoyloxy)-salicilidene]-4′-cyanoaniline

We have performed an X-ray scattering study of the nematic-smectic A and reentrant nematic-smectic A phase transitions in N-[(4-n-octyloxybenzoyloxy)-salicilidene]-4′-cyanoaniline (OOBOSCA). A diffractometer with a linear position sensitive detector was used. The results show that the smectic phase in OOBOSCA is of the A_d type with an interlayer spacing incommensurate with the molecular length L; d ∽ 1·2L. In the reentrant nematic phase two types of fluctuation modes were found. One of them corresponds to the monolayer wavevector q ₁ ∽ 2π/L, and the other is due to the partial bilayer fluctuations with the wavevector q ₂ ∽ 0·8q ₁. The temperature dependences of the interlayer spacing, X-ray scattering intensity and longitudinal correlation length for both types of layering in the reentrant nematic phase are presented. The change of the fluctuation regime from S_Ad, to S_Cd type with decreasing temperature in the reentrant nematic phase of OOBOSCA was found. The results are discussed on the basis of models with competing order parameters. The influence of alkyl chain flexibility on the stability of a partial bilayer smectic phase is also considered.     

Non-symmetric chiral isoflavone dimers: synthesis,characterisation and mesomorphic behaviour

A new series of non-symmetric chiral isoflavone-based liquid crystalline dimers, α-(2-methylbutyl-4′-(4″-phenyloxy)benzoate)-ω-(3-(4′-decyloxyphenyl)-4H-1-benzopyran-4-one-7-oxy)alkanes, with 3–12 carbon atoms in the alkyloxy spacer, have been synthesised. A pronounced odd–even effect for the phase transition temperatures upon varying the spacer length was observed. The short dimers exhibited monolayer smectic A (SmA) and smectic C (SmC*) phases while for longer homologues a chiral nematic (N*) phase was found. The temperature range of the nematic phase was broadened with elongation of the alkyl spacer. Stabilisation of the nematic phase resulted from competition between the monolayer and intercalated smectic structures. The SmA–SmC* phase transition was second order for all studied compounds with a cross over to the de Vries type behaviour for the shortest homologue.     

Determination of elastic constants of a binary system (7CPB+9.CN) showing nematic,induced smectic Ad and re-entrant nematic phases

The temperature variation of the splay and bend elastic constants of a binary system exhibiting nematic-induced smectic A_d and re-entrant nematic phases measured by electric field-induced Freedericksz transition method has been reported. As bend deformation is not permitted in the smectic A phase, bend elastic constant (K₃₃) could only be determined in the nematic and re-entrant nematic phases. In both the nematic phases, the splay elastic constant has the same order of magnitude and does not show any pretransitional effect. However, in the induced smectic A_d phase, the value of K₁₁ is about one to two orders higher than that in the nematic phases. The bend elastic constant shows a strong pretransitional effect near the nematic–smectic and smectic–re-entrant nematic phase transitions. The influence of the presence of the induced smectic phase is observed even in those mixtures which have no induced smectic phases. As the smectic phase is approached, the ratio K₃₃/K₁₁ increases rapidly and diverges to infinity.     

Odd-even effects in the phase transition behaviour of novel U-shaped liquid crystals

A homologous series of U-shaped dimeric liquid crystals in which two mesogenic groups are connected via catechol has been prepared and the effects of terminal alkyl chains, alkyl spacers and core structures on the transition properties investigated by means of optical microscopy and differential scanning calorimetry. The phase sequence exhibits a pronounced odd-even effect as the length and parity of the spacers is varied, in which the even members favour the nematic and smectic C phases, whereas the odd members favour the nematic and smectic A phases. We discuss the transition behaviour of the U-shaped compounds in terms of molecular shape.