Achiral diacrylate with an anticlinic smectic phase

We present an achiral diacrylate that shows a phase transition from the synclinic smectic C phase (SmC) to the anticlinic smectic C (SmCalt). This last phase has been previously reported only for swallow-tailed or dimeric compounds. Our studies using differential scanning calorimetry, X-ray diffraction, polarizing optical microscopy and broad band dielectric spectroscopy suggest the existence of this mesophase in this bifunctional smectogen.     

Novel hockey‐stick mesogens with the nematic,synclinic and anticlinic smectic C phase sequence

New hockey‐stick mesogens have been synthesised and their mesomorphic properties studied. The molecular structure consists of a laterally substituted central naphthalene unit prolonged by two non‐symmetrical arms, both of which contain ester units of identical orientation. The shorter arm possesses only one and the longer arm three benzene rings. Nematic, synclinic smectic C and anticlinic smectic C phases were identified and their physical properties studied. No banana‐type ordering producing dipolar phases has been found.     

Achiral bent-core azo compounds: observation of photoinduced effects in an antiferroelectric tilted smectic mesophase

We report the synthesis and physical properties of four novel bent-core compounds, containing an azo linkage and alkyl terminal chains. It is found that, except for the lowest homologue, all the materials are liquid crystalline, exhibiting columnar or tilted polar smectic phases. In one of the compounds a reversible photoinduced variation of the spontaneous polarization of the antiferroelectric state was observed.     

Synthesis,properties and crystal structure of chiral semiperfluorinated liquid crystals with ferro and anticlinic smectic phases

A new chiral and semiperfluorinated series with ferro and anticlinic properties has been synthesized and characterized. The mesomorphic behaviour has been established on the grounds of both microscopic observations and DSC measurements. The non-chiral intermediate ethyl 4-semiperfluorinated alkyloxybenzoates exhibit SmA phases, unusual for compounds with a single phenyl ring. The final derivatives display SmA, SmC* and in several cases SmC*A phases. The longer fluorinated chains favour the SmA and SmC* phases at the expense of the SmC*A phase. Electro-optical measurements were carried out with the classical SSFLC geometry. The spontaneous polarization and tilt angle at saturation are higher than those of the hydrogenous homologues, around 140 nCcm-2 at 40 degrees C. One compound of the series, the 4,4,5,5,6,6,7,7,8,8,8-nonafluoroheptyloxy derivative, C36H35O7F9, Mx=750.6 g mol-1, crystallizes in the triclinic system, space group P1 with four independent molecules in the asymmetric unit (Z=4). The molecules are arranged in a head-to-tail fashion with two molecules oriented in the same direction and the two others in the opposite direction. They give rise to sheets with a smectic C-like arrangement. The final reliability factors were R=0.117 and wR=0.134; the goodness of fit was S=1.366.     

Achiral bent-core azo compounds: observation of photoinduced effects in an antiferroelectric tilted smectic mesophase

We report the synthesis and physical properties of four novel bent-core compounds, containing an azo linkage and alkyl terminal chains. It is found that, except for the lowest homologue, all the materials are liquid crystalline, exhibiting columnar or tilted polar smectic phases. In one of the compounds a reversible photoinduced variation of the spontaneous polarization of the antiferroelectric state was observed.     

Synthesis, properties and crystal structure of chiral semiperfluorinated liquid crystals with ferro and anticlinic smectic phases

A new chiral and semiperfluorinated series with ferro and anticlinic properties has been synthesized and characterized. The mesomorphic behaviour has been established on the grounds of both microscopic observations and DSC measurements. The non-chiral intermediate ethyl 4-semiperfluorinated alkyloxybenzoates exhibit SmA phases, unusual for compounds with a single phenyl ring. The final derivatives display SmA, SmC* and in several cases SmC*A phases. The longer fluorinated chains favour the SmA and SmC* phases at the expense of the SmC*A phase. Electro-optical measurements were carried out with the classical SSFLC geometry. The spontaneous polarization and tilt angle at saturation are higher than those of the hydrogenous homologues, around 140 nCcm-2 at 40 degrees C. One compound of the series, the 4,4,5,5,6,6,7,7,8,8,8-nonafluoroheptyloxy derivative, C36H35O7F9, Mx=750.6 g mol-1, crystallizes in the triclinic system, space group P1 with four independent molecules in the asymmetric unit (Z=4). The molecules are arranged in a head-to-tail fashion with two molecules oriented in the same direction and the two others in the opposite direction. They give rise to sheets with a smectic C-like arrangement. The final reliability factors were R=0.117 and wR=0.134; the goodness of fit was S=1.366.     

New hockey stick compounds with a lateral methyl group showing nematic,synclinic and anticlinic smectic C phases

New hockey stick-shaped mesogens with a lateral methyl group inserted between the m-alkyloxy-chain and the azomethine connecting group, 4-(3-n-alkyloxy-2-methyl-phenyliminomethyl) phenyl-4-n-alkyloxycinnamates have been synthesised. The interesting feature of these hockey stick-shaped mesogens is the appearance of the nematic phase in addition to two polymorphic tilted smectic phases – the synclinic smectic C (SmC_s) and the anticlinic SmC_a phase. Physical characterisation of these mesogens has been carried out using polarising optical microscopy, differential scanning calorimetry, optical birefringence, X-ray diffraction (XRD), electro–optical and dielectric spectroscopy measurements. The SmC_s–SmC_a phase transition is accompanied by only a small calorimetric signal but causes a pronounced change in the optical textures. Orientational order parameters determined from the birefringence measurements have been compared with those estimated from XRD patterns. Dielectric measurements point to the formation of small ‘soft’ ferroelectric clusters responsible for a low frequency absorption range.     

Twist deformation in anticlinic antiferroelectric structure in smectic B2 imposed by the surface anchoring

A simplified model is used to estimate the energies of observed structures in the smectic B₂ phase composed of bent-shaped molecules. An approximate twist solution connecting anticlinic antiferroelectric structure in the sample bulk with synclinic ferroelectric order induced at the sample surfaces by anchoring is proposed and the elastic and anchoring energies of this structure are determined. It is shown that uniform, twisted and partly twisted (mixed) anticlinic antiferroelectric structures can coexist with nearly the same energies.     

Interfacial layer interactions: their effects on synclinic and anticlinic smectic mesophase behaviour in liquid crystals

Through the use of bulky cyclic terminal groups, anticlinic smectic C phases have been observed in 2-methylbutyl materials based on the MHPOBC motif.     

Structure of the smectic D phase

Abstract

The so-called ‘smectic’ D phase of 4′-n-octadecyloxy-3′-cyanobiphenyl-4-carboxylic acid (one of only four materials known to exhibit this phase) has been shown unambiguously by X-ray diffraction to be characterized by a primitive cubic space group. The space group is either P23 or Pm3, and the lattice parameter a ₀ is 86 Å. It is shown that data from previous studies of this phase may be re-intepreted to be consistent with these findings. In view of these conclusions it is clearly inappropriate to refer to the phase as smectic.     

A re-entrant smectic C phase in a sequence smectic C-oblique columnar-smectic C-nematic

A new six-ring double-swallow-tailed compound is presented which exhibits a re-entrant smectic C phase in the sequence smectic C-oblique columnar-smectic C-nematic. This unusual sequence detected by observation of the optical textures and by X-ray investigations is also confirmed by the phase diagrams with homologous compounds.     

Textural transformations of a smectic phase with asymmetric banana-shaped molecules

Materials consisting of asymmetric banana-shaped molecules may form a smectic C_G phase having C₁ symmetry. We have studied textural transformations in a smectic phase of an asymmetric chlorine-substituted banana-shaped material under electric and mechanical fields. We observed two novel features that have not been observed so far on corresponding materials with symmetric banana-shaped molecules. These observations, however, could be explained by the same arguments as were used for the symmetric molecules. Although our studies do not exclude the possibility that the material has C₁ symmetry, we suggest that the chlorine molecules are positioned arbitrarily and the bulk has C₂ symmetry.     

Nano-segregated structures of hydrogen-bonded mesogens with perfluorinated moieties: cubic phase formation and first order smectic A to smectic C phase transition

Two series of mesogenic compounds having both a perfluorinated substituent and a hydrogen bonding active site were synthesized and their phase behavior investigated. Due to the chemical architecture of these materials exhibiting amphiphilic character, structures of nano-segregation are expected to form. We found a thermotropic cubic phase with Ia3d symmetry in one of the acid/base hydrogen-bonded complexes, which is a nano-segregated structure. Moreover materials exhibiting a first order smectic A to smectic C phase transition were found, which was ascertained by differential scanning calorimetry measuring a large latent heat, and X-ray diffraction experiments observing abrupt changes of physical properties at the phase transition, i.e. the tilt angle, the intensity and the half-width of the small angle reflection. This first order phase transition occurs due to the frustration of nano-segregated structures of lamellar phases.     

Textural transformations of a smectic phase with asymmetric banana-shaped molecules

Materials consisting of asymmetric banana-shaped molecules may form a smectic C_G phase having C₁ symmetry. We have studied textural transformations in a smectic phase of an asymmetric chlorine-substituted banana-shaped material under electric and mechanical fields. We observed two novel features that have not been observed so far on corresponding materials with symmetric banana-shaped molecules. These observations, however, could be explained by the same arguments as were used for the symmetric molecules. Although our studies do not exclude the possibility that the material has C₁ symmetry, we suggest that the chlorine molecules are positioned arbitrarily and the bulk has C₂ symmetry.     

Nano-segregated structures of hydrogen-bonded mesogens with perfluorinated moieties: cubic phase formation and first order smectic A to smectic C phase transition

Two series of mesogenic compounds having both a perfluorinated substituent and a hydrogen bonding active site were synthesized and their phase behavior investigated. Due to the chemical architecture of these materials exhibiting amphiphilic character, structures of nano-segregation are expected to form. We found a thermotropic cubic phase with Ia3d symmetry in one of the acid/base hydrogen-bonded complexes, which is a nano-segregated structure. Moreover materials exhibiting a first order smectic A to smectic C phase transition were found, which was ascertained by differential scanning calorimetry measuring a large latent heat, and X-ray diffraction experiments observing abrupt changes of physical properties at the phase transition, i.e. the tilt angle, the intensity and the half-width of the small angle reflection. This first order phase transition occurs due to the frustration of nano-segregated structures of lamellar phases.     

Computer simulation of diffuse X-ray scattering by an aligned smectic C-like nematic phase

We report a new method for the quantitative analysis of diffuse X-ray scattering by some disordered smectic phases. Computer simulations of diffuse X-ray scattering are compared with actual diffraction patterns of a series of oriented liquid crystalline polymers TDI-C _m C _n . The simulation shows that both Bragg peaks (spots) and equidistant streaks (diffuse scatterings) are diffracted by one smectic C-like structure in which each of the molecular chains is displaced from its mean position by a random distance Δ _z , along the chain axis (z). This is a first type of disorder. By assuming a Gaussian distribution of the disordered displacements, the mean value of Δ _z has been determined from the position and intensity of the streaks and Bragg peaks for the polymers.     

Benzoxazine-epoxy thermosets with smectic phase structures for high thermal conductive materials

ABSTRACT

To further increase the intrinsic thermal conductivity (TC) of polybenzoxazine, a series of benzoxazine-epoxy thermosets (s-PBEI) were obtained through the sequential curing of a smectic phase epoxy monomer (s-EP) and a bifunctional benzoxazine monomer (BZ) in the presence of imidazole. The results show that s-PBEI exhibits a smectic mesophase. The formation mechanism of the smectic phase is reaction-induced phase separation caused by the preferential curing of s-EP. Owing to the increment of the liquid crystalline structure content, the TC of s-PBEI increases with increasing s-EP content. The TC of s-PBEI55 containing equal weight of BZ and s-EP reaches 0.30 W m^?1 K^?1, which is higher than that of n-PBEI55, a benzoxazine-epoxy thermoset with nematic phase structures. Additionally, the TC, glass transition temperature, and 10% weight loss temperature of s-PBEI64 containing 60 wt% BZ and 40 wt% s-EP are 0.28 W m^?1 K^?1, 216°C, and 334°C, respectively, indicating its potential applications in electronic packaging, LED lighting, and other fields requiring a high TC resin matrix.     

Ultrafast, electric field-induced fingers in an anticlinic liquid crystal

Propagating fingers of synclinic liquid crystalline phase were observed to invade the anticlinic phase for applied electric fields E larger than a characteristic threshold field E_th. The front velocity was found to be highly non-linear in E, with enormous velocities of at least 10 cm s^-1, and perhaps as high as 400 cm s^-1 for the maximum applied field. These are by far the largest velocities ever observed for a liquid crystal. The results are discussed theoretically, including the possibilities of a field-dependent molecular interaction coefficient and shear thinning.