V-shaped switching in binary mixtures of an achiral swallow-tailed material with the antiferroelectric liquid crystal (S)-MHPOBC

An achiral swallow-tailed material, 2-propylpentyl 4-(4'-decyloxybiphenyl-4-carbonyloxy)benzoate, p, showing SmA and SmC_alt phases was prepared for mixing (by weight percentage) with an antiferroelectric liquid crystal, (S)-MHPOBC, m, for the study. The binary mixture p15/m85 using (S)-MHPOBC (85%) as a host doped with achiral material (15%) resulted in a phase sequence SmA-SmC*-SmC*_A. The electro-optic response of this mixture in the ferroelectric SmC* phase displayed V-shaped switching, while that in the antiferroelectric SmC*_A phase displayed a double hysteresis switching. The mixture p85/m15 possessed SmA* and SmC*_A phases; V-shaped switching was found in the antiferroelectric SmC*_A phase of this mixture. These optical phenomena implied that a binary mixture containing a larger amount of achiral swallow-tailed material and/or possessing relatively lower polarization favours the occurrence of V-shaped switching in the antiferroelectric phase. The results of this work also suggested that thresholdless V-shaped switching in chiral smectic liquid crystals can be achieved by mixing an achiral swallow-tailed material with an antiferroelectric liquid crystal.     

Hysteresis-free electro-optical switching in conductive ferroelectric liquid crystals: experiments and modelling

The hysteresis-free electro-optical switching, or so-called V-shaped, regime has been studied in a commercial ferroelectric liquid crystal (FLC) mixture having a smectic C* phase with a very small value of spontaneous polarization. The FLC was introduced into commercial EHC cells with thin aligning layers. In such cells V-shaped switching could be observed only at very low frequencies, less than 1?Hz. However, when the same material is strongly doped with a conductive impurity, its conductivity markedly increases and hysteresis-free switching is observed over a wide range of applied frequencies and voltages. Experimental results are in good agreement with computer modelling carried out as part of this work. The modelling takes into account all the important parameters of smectic C* FLC: non-polar anchoring conditions, possible bookshelf and chevron structures, the capacitance of the aligning layers and the conductivity of a FLC. The last two factors appear to be the most crucial for hysteresis-free switching in the smectic C* phase.     

Hysteresis-free electro-optical switching in conductive ferroelectric liquid crystals: experiments and modelling

The hysteresis-free electro-optical switching, or so-called V-shaped, regime has been studied in a commercial ferroelectric liquid crystal (FLC) mixture having a smectic C* phase with a very small value of spontaneous polarization. The FLC was introduced into commercial EHC cells with thin aligning layers. In such cells V-shaped switching could be observed only at very low frequencies, less than 1 Hz. However, when the same material is strongly doped with a conductive impurity, its conductivity markedly increases and hysteresis-free switching is observed over a wide range of applied frequencies and voltages. Experimental results are in good agreement with computer modelling carried out as part of this work. The modelling takes into account all the important parameters of smectic C* FLC: non-polar anchoring conditions, possible bookshelf and chevron structures, the capacitance of the aligning layers and the conductivity of a FLC. The last two factors appear to be the most crucial for hysteresis-free switching in the smectic C* phase.     

V-shaped switching in binary mixtures of an achiral swallow-tailed material with the antiferroelectric liquid crystal (S)-MHPOBC

An achiral swallow-tailed material, 2-propylpentyl 4-(4′-decyloxybiphenyl-4-carbonyloxy)benzoate, p, showing SmA and SmC_alt phases was prepared for mixing (by weight percentage) with an antiferroelectric liquid crystal, (S)-MHPOBC, m, for the study. The binary mixture p/15/m85 using (S)-MHPOBC (85%) as a host doped with achiral material (15%) resulted in a phase sequence SmA-SmC*-SmC*_A. The electro-optic response of this mixture in the ferroelectric SmC* phase displayed V-shaped switching, while that in the antiferroelectric SmC*_A phase displayed a double hysteresis switching. The mixture p85/m15 possessed SmA* and SmC*_A phases; V-shaped switching was found in the antiferroelectric SmC*_A phase of this mixture. These optical phenomena implied that a binary mixture containing a larger amount of achiral swallow-tailed material and/or possessing relatively lower polarization favours the occurrence of V-shaped switching in the antiferroelectric phase. The results of this work also suggested that thresholdless V-shaped switching in chiral smectic liquid crystals can be achieved by mixing an achiral swallow-tailed material with an antiferroelectric liquid crystal.     

Synthesis and mesomorphic properties of chiral liquid crystals derived from (S)-lactic acid with 3-pentanol

The chiral swallow-tailed liquid crystals, 1-ethylpropyl (R)-2-[4-(4'-alkoxybiphenylcarbonyloxy)-phenoxy]propionates, EPmPBPP (m = 8-12), were prepared by using chiral (S)-lactic acid with 3-pentanol as starting materials. Mesophases and their corresponding transition temperatures were determined by polarizing microscopic textures and DSC. The results showed that all the chiral materials exhibited enantiotropic BP, N*, TGBA*, SmA*, and SmC* phases. Spontaneous polarization, dielectric constant and electro-optical response for the materials in the ferroelectric SmC* phase were investigated. It was noted that the electro-optical response of transmittance versus applied voltage obtained from the ferroelectric phase of material EPmPBPP (m = 10) displayed V-shaped switching, while that of other materials displayed the typical characteristics of ferroelectric hysteresis switching or U-shaped switching.     

Synthesis and mesomorphic properties of chiral liquid crystals derived from ( S )-lactic acid with 3-pentanol

The chiral swallow-tailed liquid crystals, 1-ethylpropyl (R)-2-[4-(4′-alkoxybiphenylcarbonyloxy)-phenoxy]propionates, EPmPBPP (m = 8?12), were prepared by using chiral (S)-lactic acid with 3-pentanol as starting materials. Mesophases and their corresponding transition temperatures were determined by polarizing microscopic textures and DSC. The results showed that all the chiral materials exhibited enantiotropic BP, N*, TGBA*, SmA*, and SmC* phases. Spontaneous polarization, dielectric constant and electro-optical response for the materials in the ferroelectric SmC* phase were investigated. It was noted that the electro-optical response of transmittance versus applied voltage obtained from the ferroelectric phase of material EPmPBPP (m = 10) displayed V-shaped switching, while that of other materials displayed the typical characteristics of ferroelectric hysteresis switching or U-shaped switching.     

Contrast ratio and switching of zigzag defect-free surface stabilized FLCD by photoinduced alignment

The contrast ratio and electro-optical switching behaviours of surface stabilized ferroelectric liquid crystal cells with alignment layers of a photodegradable polyimide prepared by polarized ultraviolet light irradiation have been investigated. The higher contrast ratio was obtained for a zigzag defect-free sample obtained by the photo-induced alignment method than by the rubbing method. The switching behaviours of the FLC, such as spontaneous polarization and response time, were measured. Higher spontaneous polarization and faster response times were obtained with cells prepared by the photoinduced alignment method. In addition, response times of the order of microseconds, which are fast enough to realize high quality moving images with video-frame rate, were obtained.     

Contrast ratio and switching of zigzag defect-free surface stabilized FLCD by photoinduced alignment

The contrast ratio and electro-optical switching behaviours of surface stabilized ferroelectric liquid crystal cells with alignment layers of a photodegradable polyimide prepared by polarized ultraviolet light irradiation have been investigated. The higher contrast ratio was obtained for a zigzag defect-free sample obtained by the photo-induced alignment method than by the rubbing method. The switching behaviours of the FLC, such as spontaneous polarization and response time, were measured. Higher spontaneous polarization and faster response times were obtained with cells prepared by the photoinduced alignment method. In addition, response times of the order of microseconds, which are fast enough to realize high quality moving images with video-frame rate, were obtained.     

Photomechanically induced ferroelectricity in smectic liquid crystals

We report new photomechanical effects in the ferroelectric liquid crystal SCE13 doped with a photoisomerizing guest azo dye. Low concentrations of dye (∼5 per cent wt:wt) are shown to cause an isothermal, reversible disruption of smectic phases when the system is illuminated with low power density (∼ 1 mW cm^-2) UV light. In the case of a sample initially in the S*_c phase, this results in a fall in the magnitude of spontaneous electrical polarization (P_s) and changes in electro-optic switching characteristics. If the sample is illuminated in the S_A phase, the electroclinic switching decreases. In contrast to this, when systems containing higher concentrations of dye (≥ 10 per cent wt: wt) are UV illuminated in the S_A phase, a reversible, isothermal transition to a biphasic S*_c/isotropic state occurs. In this case, the P_s is seen to rise from zero in the S_A phase to a finite value(∼2 nC cm^-2) in the biphasic mixture and hysteresis occurs in the electro-optic switching. When these higher dye concentration mixtures are held initially in the S*_c phase and UV illuminated, a more complicated variation of P_s occurs with the sample again undergoing a transition to a biphasic S*_c/isotropic state. Possible mechanisms for the transition are discussed.     

Giant surface electroclinic effect in a chiral smectic A liquid crystal

We report the observation of a very large surface electroclinic effect in the smectic A* phase of a chiral liquid crystal. In planar-aligned cells of enantiomerically pure W415, the smectic A* phase grows in from the isotropic state with the layer normal rotated ψ = - 24° from the rubbing direction, a consequence of the surface electroclinic tilt θ_s of the director. The sign of θ_s depends on the molecular handedness, with θ_s ≡ 0 in the racemate, and increasing linearly with moderate enantiomeric excess before saturating as ee → 1. A uniform layer structure can be achieved using cross-rubbed alignment layers, in which case thin cells of W415 in the smectic C* phase display V-shaped (analogue) electro-optic switching.     

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.     

Use of a gelator in a ferroelectric liquid crystal: pitch compensation and nanofibres

A new azobenzene-containing gelator for liquid crystals, AG2, was synthesized and used to prepare a ferroelectric liquid crystal (FLC) gel. The FLC gel shows interesting features. On cooling from the isotropic phase into the N * phase, the dissolved AG2 acts as a chiral dopant and has a compensation effect on the helical pitch of the N * phase. With 0.5 wt% of AG2 in the FLC host, a homogeneous alignment of the FLC molecules is formed in the N * phase, ensuring the bulk alignment in the SmC * phase, even on quenching the mixture from the isotropic phase. This alignment under fast cooling contrasts sharply with the slow cooling rate required for the alignment of a pure FLC. After formation of the bulk alignment, the aggregation of AG2 occurs in the lower temperature SmA or SmC * phases, and the gelator molecules self-assemble into nanometer-sized fibres (about 100nm diameter) that are aligned and located between the smectic layers. As the gelator is microphase-separated from the FLC in the SmC * phase, it exerts little disruption on the electro-optic properties of the FLC cell.     

New ferroelectric liquid crystalline materials containing one and two lactate groups attached to the molecular core

Several new liquid crystalline materials containing one, two or three chiral centres and having one or two lactate groups in the molecular core have been synthesized. Most of the materials show the blue phase, chiral nematic phase, paraelectric smectic A phase and orthogonal hexatic smectic B phase; some possess the ferroelectric SmC* phase. A study of the mesomorphic properties has been performed using differential scanning calorimetry, optical microscopy and X-ray diffraction. The thickness of the smectic layers and the value of the average distance between the long axes of neighbouring molecules were determined. In the SmC* phase, the temperature dependence of spontaneous polarization, spontaneous tilt angle and helical pitch was measured. The influence of the number of lactate groups on mesogenic behaviour has been established.     

Collective and molecular processes in a new class of ferroelectric liquid crystals

Electro-optic and dielectric studies have been performed for a thioester (with a chiral chain incorporating the lactic acid ester moiety), which exhibits a monotropic room temperature ferroelectric S_c* phase. Moreover, there are some hints that this substance also shows the recently discovered helicoidal S_A* phase. The dielectric and electro-optic measurements have been carried out using ITO coated glass plates (d = 10 μm). Planar alignment was obtained by means of an AC electric field (v = 1Hz and V_p-p = 70V). The tilt angle and spontaneous polarization temperature dependences show that the S_A*-S_c* transition is of second order. Soft mode behaviour in the pretransitional region also confirms this result. The monotropic S_c* phase exhibits a rather strong Goldstone mode (GM) with pronounced temperature dependence of the dielectric parameters. The molecular reorientation about the long axis has been found for both principal alignments. The homeotropic effective alignment was obtained by applying a magnetic field of 1-2 T to a thick sample (c. 100μm) placed in a specially designed cell for anisotropic high frequency measurements. The reorientation about the long axis gives a broad spectrum centred at c. 100MHz. This spectrum shows minor changes at the S_A*-S_c* transition.     

A possible structural model of the SmC*beta phase

The so-called chiral smectic C-beta (SmC*beta) phase has been reported as distinct from the SmC* phase in several materials that exhibit antiferroelectric liquid crystal mesophases and subphases. The SmC*beta phase is known to be chiral, tilted and to exhibit ferrielectric switching, but no structure had been suggested which explains these effects. This paper presents a possible structure for the SmC*beta phase which can explain the ferrielectric properties. The model is proposed on the basis of complementary optical, electric and X-ray diffraction studies of a chiral liquid crystal. The layer spacing, optical and steric tilt and spontaneous polarization over the temperature range of the SmC*beta phase are described. The complementary experimental techniques used reveal the occurrence of inversion phenomena, on which this model is based.     

Alignment modulation of azobenzene-containing liquid crystal systems by photochemical reactions

Recent progress in alignment modulation of azobenzene-containing liquid crystal systems by photochemical reactions has been reviewed by dividing the modulation methods into two types: phase transitions (order–disorder change) and change of liquid crystal directors (order–order change). First, photochemical phase transitions and alignment changes of liquid crystals in guest/host mixtures and polymers are summarized. Then, alignment control of liquid crystals by linearly polarized light and photoactive surface layers is discussed. Finally, recent applications of alignment change and photochemical phase transitions of liquid crystals in holographic technology and photomechanical effects are introduced. In addition, future possible applications for a variety of practical devices, such as display devices, optical switching and reversible optical image storage, are mentioned.     

Light exposure dependence of molecular orientation of glassy polyfluorene layers formed on photo-aligned polyimide films

We have investigated the molecular orientation of glassy poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) layers formed on photo-aligned polyimide films with different in-plane anisotropy. The polyimide contains azobenzene in the backbone structure (Azo-PI), allowing us to control the in-plane anisotropy of the film by varying linearly polarized light (LP-L) exposure. The glassy PFO layers (30 nm thick) were obtained by annealing the samples at the liquid crystalline phase of PFO and then quenching them to room temperature. The degree of alignment of PFO was assessed by the polarization ratio of photoluminescence (PL). The PL polarization ratio increased rapidly with the LP-L exposure, and it reached 10 at 2.8 J/cm². Beyond this LP-L exposure, it became almost constant around 10.4. This PL polarization ratio was much higher than the absorption dichroic ratio of the underlying Azo-PI film. This result suggests that the degree of alignment of PFO is determined by its liquid crystalline nature. The saturation dependence of the degree of alignment is very useful for fabricating alignment patterns by a simple photo-mask exposure method. We have succeeded in fabricating 3 μm line-and-space alignment patterns of PFO.     

A bilayer model of the double hysteresis loop in antiferroelectric liquid crystals

We investigate, both analytically and numerically, a simple model of the field induced double hysteresis loop in AFLC materials. This bilayer model of the bulk of an AFLC describes the free energy in terms of polar and non-polar interactions due to surface alignment, the electric field/dipole interaction in each layer and the dipole/dipole interaction between the layers. The static hysteresis loop is found analytically and the stability of each analytic solution is investigated. The dynamic switching characteristics are found numerically and then investigated as the system parameters and electric field characteristics are changed.     

Synthesis and ferroelectric properties of chiral liquid crystals derived from (S)-1-propyloxy-2-propanol

A new optically active chiral moiety, (S)-1-propyloxy-2-propanol, was designed and synthesized by the treatment of 1-propanol with (S)-propylene oxide under basic conditions. Its derivatives, the (R)-1-propyloxy-2-propyl 4-[4-(4-alkoxyphenyl)phenoyloxy]benzoates, PPmPPB (m = 8-12), were prepared for the investigation of mesomorphic properties. All of the chiral materials displayed enantiotropic SmA* and SmC* phases, and the shorter alkyl chain members (m = 8-11) displayed an additional unidentified SmX* phase. The switching current, spontaneous polarization, tilt angle, dielectric constant and electro-optical response for the materials in the ferroelectric SmC* phase were measured. The electro-optic responses in polyimide film-coated homogeneously aligned cells exhibit thresholdless, V-shaped switching in the ferroelectric phase.     

Evidence for a tilted smectic phase in Anopore membranes by dielectric spectroscopy

Collective relaxation processes are completely undetectable in a ferroelectric liquid crystal confined in porous Anopore membranes, as a result of perfect orientation of the smectic layers perpendicular both to the long axis of the pores and the direction of the measuring electric field. In the ferroelectric liquid crystal – Anopore composite only one relaxation process, assigned to rotation of the molecule around the molecular short axis, appears throughout all smectic phases. The temperature dependence of the relaxation frequency and of the dielectric strength of this process also shows no irregularity at the point of polarization sign reversal. The temperature dependence of the relaxation frequency follows the Arrhenius law with an activation energy slightly higher in the ferroelectric SmC* phase. Analysis of the non‐linear changes of temperature dependence of the dielectric strength at the SmA–SmC* phase transition enables one to obtain the temperature dependence of the tilt angle of the molecules in the SmC* phase in the Anopore membrane. Dielectric measurements confirm the existence of the tilted smectic phase in Anopore cylindrical channels with no tilt anomaly at the point of polarization sign reversal.