A phenomenological model for the undulating twist grain boundary-C* phase

We propose a simple phenomenological model which is able to account for the various twist grain boundary (TGB) phases, including the recently discovered undulating twist grain boundary-C* (UTGB_C*) phase. In the UTGB_C* phase, the smectic C* (SmC*)-like blocks and the grain boundaries separating them undulate to form a two-dimensional square lattice perpendicular to the TGB helix axis. We treat the grain boundaries separating adjacent smectic blocks as interfaces with an anisotropic interfacial tension. At moderate chiral strengths we find a TGB_A-TGB_C-SmC* sequence. As the chiral strength is increased this goes to the sequence TGB_A-UTGB_C*-SmC*. Such sequences have been observed experimentally.     

Free-standing films of twist grain boundary TGBA and UTGBC* liquid crystals studied by fluorescence confocal polarizing microscopy

The director structures, meniscus profile, and defects in free-standing films of the twist grain boundary TGB_A and UTGB_C* liquid crystals were studied. The films were characterized by a combination of polarizing microscopy and fluorescence confocal polarizing microscopy. Five principal regions of meniscus were distinguished. When film thickness in the meniscus is much smaller then the TGB pitch, there is no difference between the free-standing films of TGB and ordinary smectic A liquid crystals. When the film thickness is larger than the TGB pitch, filamentary texture is observed. The 3D director pattern of the filaments are similar to the ground state director fields of TGB_A and UTGB_C* liquid crystals. In the intermediate thickness region of the meniscus, when the film thickness and TGB pitch are commensurate, a unique radial pattern is observed. Based on the fluorescence confocal polarizing microscopy studies of the director field, we propose a model for the 3D director structure in this part of the meniscus.     

Unsymmetrical dimers possessing a cholesteryl ester moiety and a difluoro-substituted biphenyl core: synthesis and mesomorphic behaviour

Some new unsymmetrical dimers consisting of a cholesteryl ester moiety, covalently linked to either a 4′-(1,3-difluoro-4-n-octyloxy) biphenyloxy or a 4′-(2,3-difluoro-4-n-decyloxy) biphenyloxy through odd-even parity paraffinic central spacers, have been synthesized and investigated for their mesomorphic behaviour. Except for one, all the dimers exhibit enantiotropic smectic A, twist grain boundary (TGB) and chiral nematic mesophases. Five of the eight unsymmetrical dimers synthesized show a chiral smectic C (SmC*) phase. Interestingly in some of the compounds the SmC* exists over a wide temperature range including room temperature. Among the eight compounds, a dimer having a C₇ central paraffinic spacer and a C₈ alkoxy terminal chain shows an enantiotropic twist grain boundary with SmC* blocks (TGB_C*) phase. It appears that the variation in the length of the spacer has a remarkable influence on the phase transition temperatures as well as on the mesomorphic behaviour.     

Stable frustrated phases in chiral liquid crystals derived from optically active (R)- and (S)-3-ethylmercapto-2-methylpropionic acids

Highly optically pure (R)- and (S)-3-ethylmercapto-2-methylpropionic acids were synthesized by using optically active (D)- and (L)-2,10-camphorsultams as chiral auxiliaries, respectively. Their derivatives, (R)- and (S)-EMMPNmB (m=6-12), were prepared for investigation. Microscopic texture observations demonstrated that the materials possess three stable frustrated phases: BP, TGB_A* and TGB_C* phases. Interestingly, it was found that the N* phase behaves as an intermediary phase between BP and TGB_A* phases in a rather narrow temperature range (calc. 0.5-1.4°C). A study of the racemic mixture, (±)-EMMPNmB (m=10), indicated that the chirality of the molecule could suppress the formation of smectic phases in the heating process. An increase of alkyl chain length favoured the formation of the TGB phases particularly, in accompaniment with a change of TGB phases from monotropic to enantiotropic. Moderate maximum P _S values (calc. 14-19 nC cm^-2) and apparent tilt angle (calc. 20°) were obtained for the TGB_C* phase in a surface stabilized ferroelectric liquid crystal geometry.     

A specialized apparatus for the study of liquid crystals at high pressures

Abstract

A pressure system specially designed for the study of liquid crystal materials at high pressures up to 4kbar is presented. The pressure system is based on a hydrostatic screw injector and uses either oil or gas as the pressure transmitting medium. The type of measurements which can be performed with the instrument include polarized microscopy, optical spectroscopy, electrooptic and electrical measurements. The different measurements performed place specific constraints on the design of the apparatus and the pressure cells, and details are given. A preliminary study of the smectic A modification of the twist grain boundary phase (TGB_A) at high pressures is presented. The pressure versus temperature phase diagram shows (i) a negative gradient of the TGB_A/isotropic phase boundary line and (ii) that the TGB_A phase does not exist at pressures above about 250 bar. Following Lubensky's analogy between the TGB_A phase and type II superconductors, the disappearance of the TGB_A phase at high pressure may imply that the Landau-Ginsburg parameter K decreases with pressure.     

Liquid crystal dimers containing Cholesteryl and Triazole-containing mesogenic units

ABSTRACT

New liquid crystals categorised as cholesteryl dimers have been successfully synthesised through the reaction between cholesteryl 4-(prop-2-ynyloxy)benzoate moieties with n-azido(cholesteryloxy-carbonyl)alkane. All the dimers display enantiotropic mesophases. Whilst the odd-numbered dimers exhibit chiral nematic (N*), twisted grain boundary (TGB) and chiral smectic C (SmC*) phases, the even-numbered members from the same series show chiral smectic A and C. A detailed inspection on mesophase reveals that the chiral centres and the bent conformation of the odd-numbered members are essential for the induction of TGB phase. However, upon decreasing the temperature, the ratio of the transition temperatures (T_SmC*-SmA*/T_SmA*-I) is found to be 0.95, which indicate the second order transition according to the McMillan’s molecular theory. In addition, the X-ray diffraction study supports the presence of the smectic A phase on the even members rather than the N* by the appearance of the Bragg diffraction peaks at 190°C. A comparison study with the other analogues in which the cholesterol entity is substituted by azobenzene or biphenyl tails has been carried out to assess the relationship between the molecular structure and mesomorphic behaviour.     

Unsymmetrical dimers possessing a cholesteryl ester moiety and a difluoro-substituted biphenyl core: synthesis and mesomorphic behaviour

Some new unsymmetrical dimers consisting of a cholesteryl ester moiety, covalently linked to either a 4'-(2,3-difluoro-4-n-octyloxy) biphenyloxy or a 4'-(2,3-difluoro-4-n-decyloxy) biphenyloxy through odd-even parity paraffinic central spacers, have been synthesized and investigated for their mesomorphic behaviour. Except for one, all the dimers exhibit enantiotropic smectic A, twist grain boundary (TGB) and chiral nematic mesophases. Five of the eight unsymmetrical dimers synthesized show a chiral smectic C (SmC*) phase. Interestingly in some of the compounds the SmC* exists over a wide temperature range including room temperature. Among the eight compounds, a dimer having a C₇ central paraffinic spacer and a C₈ alkoxy terminal chain shows an enantiotropic twist grain boundary with SmC* blocks (TGB_C*) phase. It appears that the variation in the length of the spacer has a remarkable influence on the phase transition temperatures as well as on the mesomorphic behaviour.     

Cholesterol-based dimeric liquid crystals: synthesis and mesomorphic behaviour

Chiral non-symmetric dimeric liquid crystals consisting of a cholesteryl ester moiety as chiral entity and a biphenyl aromatic core, interconnected through n-butyl (C₄) or n-pentyl (C₅) parity alkylene spacers, have been synthesized and investigated for their liquid crystalline properties. All the dimers exhibit enantiotropic mesophases. The first member of the dimers having the C₄ central spacer exhibit only the chiral nematic (N*) mesophase, while the higher homologues also show smectic A (SmA) and twist grain boundary (TGB) mesophases. The dimers of the other series containing the C₅ central spacer also have stable SmA, TGB and N* mesophases, except for the first which does not show the TGB phase. Both series of compounds show a weak odd-even effect with terminal alkyl chain substitution, while the spacer length has a marked influence on the phase transition temperatures.     

Chiral dopant induced twist grain boundary phases

A diarylethane alpha-chloro ester was mixed with a chiral dopant of high helical twisting power at variable concentration to study its capability for induction of twist grain boundary phases. With increasing concentration of the chiral dopant, TGBA* and TGBC* phases were observed with a rather broad region of existence. In contrast to homeotropic alignment, planar boundary conditions seem to enhance the phase stability of the TGB phases, resulting in metastable TGB states and phase coexistence with the respective smectic phases. The phase growth of SmA*/SmC* out of the TGBA*/TGBC* state was studied under isothermal conditions as a function of temperature and concentration of the chiral dopant.     

Chiral dopant induced twist grain boundary phases

A diarylethane alpha-chloro ester was mixed with a chiral dopant of high helical twisting power at variable concentration to study its capability for induction of twist grain boundary phases. With increasing concentration of the chiral dopant, TGBA* and TGBC* phases were observed with a rather broad region of existence. In contrast to homeotropic alignment, planar boundary conditions seem to enhance the phase stability of the TGB phases, resulting in metastable TGB states and phase coexistence with the respective smectic phases. The phase growth of SmA*/SmC* out of the TGBA*/TGBC* state was studied under isothermal conditions as a function of temperature and concentration of the chiral dopant.     

A chiral material with a new phase sequence: twist grain boundary smectic C phase-smectic blue phases

A new phase sequence: twist grain boundary smectic C (TGBC) to smectic blue phases (BP_Sm) is observed in a chiral compound (S)- or (R)-1-methyloctyl 3'-fluoro-4'-(3-fluoro-4-hexadecyloxybenzoyloxy)tolane-4-carboxylate. It is the first time that a TGBC phase has been found to occur under smectic blue phases in the absence of the twist grain boundary smectic A (TGBA) phase. These phases are characterized by polarizing optical microscopy, differential scanning calorimetry and X-ray scattering.     

A chiral luminescent liquid crystal with a tolane unit

We have synthesised a chiral liquid crystal with a tolane unit that exhibits intense fluorescence in both solution and the solid states. The liquid crystal can form the enantiotropic twist-grain boundary A (TGB_A*) and Blue II phases, and the helical pitch of the TGB_A* phase decreases with an increasing temperature.     

Twist grain boundary states in non-chiral smectogen-chiral dopant

The liquid crystalline systems studied consisted of non-chiral mesomorphic esters [the eutectic mixture of 4-n-hexyloxyphenyl 4-n-octyloxybenzoate and 4-n-octyloxyphenyl 4-n-hexyloxybenzoate] and structurally similar chiral dopants (N-arylidene derivatives of S-1-phenyl- and S- 1-benzyl-ethylamine). Twist grain boundary phases occur between the cholesteric and smectic C* or smectic A phases in all the investigated systems. The different structures of these TGB phases (TGBA and TGBC) are proved by small angle X-ray scattering and textural studies. The concentration and temperature ranges of the TGB phases are defined by the twisting power of the chiral dopants and their own mesomorphic peculiarities. The experimental dependences of TGB phase temperature range on cholesteric helical twist are influenced by a ratio change of the optically active and racemic forms of the dopant at a constant total concentration. The results obtained are discussed within Renn's theory.     

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.     

A bent-core dopant-induced smectic A* twist state

The phase behaviour of a commercial calamitic ferroelectric liquid crystal mixture, doped with different mesogenic and non-mesogenic bent-core molecules was investigated through polarising microscopy, optical measurements and quenched growth. A twisted smectic structure, similar but not equivalent to a twist grain boundary (TGB) phase, and absent in the neat FLC mixture, was verified. The twisted smectic state can only be observed on cooling and its stability depends on the rate of temperature decrease, which indicates a kinetically governed behaviour. Further, the growth dynamics of the low temperature uniform SmA* bookshelf structure is dominated by viscosity instead of free energy density, as would be expected for a true thermodynamic phase transition. The investigations signify the chiral induction capability of achiral, bent-core dopant molecules and we believe that the observed behaviour represents the onset of TGB formation at very large pitch. It can thus give valuable information for the fundamental physical understanding of twist grain boundary phase formation.     

A bent‐core dopant‐induced smectic A* twist state†

The phase behaviour of a commercial calamitic ferroelectric liquid crystal mixture, doped with different mesogenic and non‐mesogenic bent‐core molecules was investigated through polarising microscopy, optical measurements and quenched growth. A twisted smectic structure, similar but not equivalent to a twist grain boundary (TGB) phase, and absent in the neat FLC mixture, was verified. The twisted smectic state can only be observed on cooling and its stability depends on the rate of temperature decrease, which indicates a kinetically governed behaviour. Further, the growth dynamics of the low temperature uniform SmA* bookshelf structure is dominated by viscosity instead of free energy density, as would be expected for a true thermodynamic phase transition. The investigations signify the chiral induction capability of achiral, bent‐core dopant molecules and we believe that the observed behaviour represents the onset of TGB formation at very large pitch. It can thus give valuable information for the fundamental physical understanding of twist grain boundary phase formation.     

Experimental study of a SmA*-SmC*A phase transition

We have performed a detailed experimental study of the electroclinic behaviour in the SmA* phase above a SmA*-SmC*_A phase transition. The tilt angle and polarization were measured as a function of the applied a.c. electric field, and the dielectric constant was obtained under different values of the bias field (0≤E _d.c.≤ 3 V μm^-1). In the region of linear regime, the behaviour observed for this SmA*-SmC*_A phase transition is very similar to the one previously described for SmA*-SmC* phase transitions. The experimental results obtained under high bias field are in good agreement with the predictions of the simple theoretical model considered.     

Influence of the bias field on dielectric properties of the SmCA* in the vicinity of the SmC*–SmCA* phase transition

In a large class of smectic mixtures prepared at our University, the phase transition between chiral ferroelectric smectic C (SmC*) and chiral antiferroelectric smectic C (SmC_A*) phases can be observed on cooling. Under bias field the temperature of the phase transition SmC*?SmC_A* decreases (ca. 100°C in the investigated mixture). The transition is called: unwound SmC*?twisted SmC_A* phase transition. The Goldstone mode in SmC* phase is reduced by a direct current field while two modes (P_H and P_L) in the SmC_A* phase are amplified. The amplitude of the fast X mode observed in the SmC_A* phase is reduced. The aim of this paper is to show how parameters of the modes in SmC_A* phase (calculated from Cole–Cole model) change with bias voltage—when twisted structure in SmC_A* phase is gradually unwound. The character of the modes observed in SmC_A* is discussed. A new effect is shown: a high value of dielectric loss is detected in the unwound SmC* phase, which is very close to SmC_A*.     

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.