Determination of bulk values of twist elasticity coefficient in a chiral smectic C* liquid crystal

A method is described for determination of bulk values of a twist elastic coefficient for smectic c-director in chiral smectic liquid crystals with a helical structure. The method was applied to 4-methylbutyloxy phenyl-4-octyloxy-benzoate (C8) in the chiral smectic C* phase. The measurements were performed using optical detection in a small deformation limit. In contrast to the usual methods, initial deformation of the helix (caused by strong surface interactions) was avoided by using homeotropic aligned thick samples. The critical temperature dependence of the measured coefficient was observed. The relation between the measured parameter and the smectic C order parameter is presented.     

Smectic polymorphism of 4-decyloxybenzylidene- 4′-alkyloxyanilines and their mixtures with polar standards of mesophases

This article describes liquid crystalline properties of the homologous series of 4-decyloxybenzylidene-4′-alkyloxyanilines. Based on the polarization microscopy (POM and TOA methods) and the calorimetric (DSC) measurements a rich polymorphism has been detected. Investigated compounds exhibit nematic, smectic A, smectic C, smectic B, smectic I and G (smectic G) mesophases with their characteristic configurations. The presence of these mesophases was confirmed by the miscibility method, using 4-heptyloxybenxylidene-4′-pentylaniline as a mesophase reference. Nine other smectic standards with different polarity have been applied in order to investigate their influence on the mesophase stability.     

Dielectric and calorimetric studies on 1BC1EPOPB feroelectric liquid crystal having a large spontaneous polarization

Experimental investigations on the ferroelectric liquid crystal, R-4′(1-butoxycarbonyl-1-ethoxy) phenyl 4-(4-octyloxy phenyl) benzoate (1BC1EPOPB) of large spontaneous polarization (P _S(+) = 240?nC?cm^?2), using dielectric and calorimetric techniques, are reported. The temperature range of 25.0–125.0°C has been chosen for dielectric measurements. Dielectric dispersion studies are carried out in the temperature range 45.0–75.0°C and in the frequency range 2?Hz to 2?MHz for the smectic A, smectic C* and smectic X phases. A new phase called ‘smectic X’ has been found around 56.3°C. The transition temperatures identified by the dielectric dispersion studies for different phases and those given by DSC techniques are in close agreement.     

Mechanical behavior of the smectic phases near the phase transitions

We present experimental data on the apparent layer complex rigidity of the smectic A, C, B phases of 40 P90B versus temperature at a fixed frequency of 50 Hz. Across the nematic → smectic A and smectic A → smectic C second order transitions, the modulus of the rigidity coefficient becomes very weak, and its phase undergoes a rapid variation of π, in a range of about 100 mK. At the first order smectic C → smectic $\text{B}$ transition, a corresponding discontinuity is observed. This behavior is tentatively explained in term of coupling of the nematic director (or defects) with the shear flow associated with the compression, and an associated viscoelastic relaxation.     

The bulk twist action on the single local monodomains of the smectic C liquid crystal

The angular dependence of the surface anchoring energies of the smectic C with temperature independent tilt angle (type C₁) on two surfaces, SiO _x /ITO/glass and holographic diffraction grating, were measured using a twist-cell method. The smectic C monodomains rotation, starting at a critical imposed bulk twist, revealed a surface anchoring breaking. The surface anchoring strengths of the SiO _x /ITO/glass and holographic grating were evaluated. A model for the bulk twist influence on the smectic C monodomain growth was suggested.     

Positron lifetimes in five phases of BEA

Positron lifetimes have been measured as a function of temperature in 4-butyloxybenzal-4′-ethylaniline (BEA). BEA has been previously reported to have two liquidcrystalline phases (smectic and nematic) with transition temperatures as follows: solid → smectic, 40.5° C; smectic → nematic, 51.0° C; and nematic → isotropic liquid, 65.5° C. Positron life time spectra were resolved into two components, with the shorter component τ₁ remaining approximately constant in all phases, and τ₂ exhibiting reversible changes at all of the above transitions. In addition, an irreversible discontinuity in the τ₂ lifetime was observed in the vicinity of 28° C, indicating the presence of a new phase (phase X) of BEA. The τ₂ andI ₂ values obtained for the various phases of BEA are: solid (1.25 nsec, 7.1%), phase X (1.97 nsec, 26.2%), smectic (2.36 nsec, 23.6%), nematic (2.72 nsec, 28.3%), and isotropic liquid (2.69 nsec, 29.8%).     

Comparative study of liquid-crystalline ordering in a monomer,linear polymer,and graft copolymer by the photon transmission technique

The photon transmission technique was used to study the phase transitions of a liquid crystalline acrylate monomer, 6-(4-cyanobiphenyl-4′oxy)hexyl acrylate (LC6), its homopolymer (PLC6) and its graft copolymer (GLC6) with polytetrahydrofuran grafts. The phase transitions were also confirmed by DSC and polarizing microscopy. We observed the phase transition sequence isotropic–nematic–smectic A–smectic C in the LC6 monomer. In PLC6 and GLC6 polymers, the nematic and smectic A phases appear dominant. The apparent nematic–smectic A transition is of first order in PLC6 and of second order in GLC6, with the transition temperature remaining the same. The effects of quenched random constraints introduced in GLC6 are consistent with the theory of quenched random interactions. The critical exponents were also evaluated.     

Synthesis and characterization of wide range mesogenic esters based on asymmetrical 2,5-disubstituted 1,3,4-thiadiazole

A homologous series of new 13 esters, 4-(5-(p-tolyl)-1,3,4-thiadiazol-2-yl)-phenyl-4-alkoxybenzoate, (IV_n), based on 1,3,4-thiadiazole core has been synthesized. The structures of these esters were confirmed by Fourier transform infrared spectroscopy, proton nuclear magnetic resonance and mass techniques. Their mesophases behavior was investigated with hot-stage polarizing optical microscope and differential scanning calorimetry. The thermal stability for most of these derivatives was measured by thermal gravimetric analysis. All the target esters showed enantiotropic mesomorphic behaviors with nematic and nematic/smectic C phases. The phase transition temperatures and liquid crystalline properties were affected by the nature of heterocyclic ring and the length of the alkoxy chain. Only the nematic phase was observed in the first 10 derivatives, (n = 1–10), while the last 3, (n = 12, 16 and 18) showed nematic and smectic C phases. These compounds demonstrated high liquid crystalline ranges, both in heating and cooling cycles. The mesomorphic results obtained were compared with the reported analogs of similar constituents.     

Hydrogen bonding and liquid crystallinity of low molar mass and polymeric mesogens containing benzoic acids: a variable temperature Fourier transform infrared spectroscopic study

The phase behaviour and mesomorphism of poly(4-(6-propenoyloxyhexyloxy)benzoic acid) (PPOHBA) and 4-pentyloxybenzoic acid (POBA) is studied using variable-temperature Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction. PPHOBA exhibits a smectic C phase and POBA, a nematic phase. The temperature dependence of the Fermi resonance bands associated with the hydroxyl groups and of the carbonyl stretching region in the FTIR spectra indicates that there is a dynamic equilibrium between monomers and open and closed dimers formed by hydrogen bonding between benzoic acid moieties. The nematic phase observed for POBA is linked to the anisotropic cyclic dimer, while an abrupt increase in the concentration of monomer drives isotropisation. In PPOHBA, hydrogen-bonded supramesogens promote smectic behaviour, while hydrogen-bonded crosslinks stabilise the lamellae. The increased viscosity arising from this dynamic crosslinking is offset by the flexibility of the acrylate backbone and alkyl spacers.     

Solid-state morphology of aromatic-aliphatic polyester crystallized from smectic liquid crystal

The solid-state morphology of poly(hexamethylene p,p'-bi-benzoate) (BB-6) and its dependence on the texture of the smectic liquid crystal state from which it crystallized were studied using electron microscopy, polarizing optical microscopy, and small-angle x-ray diffractometry. Thin films of BB-6 suitable for the microscopy were prepared by casting a hot solution of the polymer in tetrabromoethane onto mica at 150°C. The as-cast film of BB-6 was first heated to 260°C, maintained in the smectic range (210°C), and then cooled gradually to room temperature. A number of types of defect structures—such as Dupan cyclides, edge dislocation, tilt walls, and disclinations — were observed in the solid BB-6 formed from the smectic melt. On the basis of the electron microscopy and small-angle x-ray diffractometry, it is suggested that folded-chain crystals are formed during the crystallization from the smectic melt. Electron microscopic observations of the solid BB-6 and its orientation behavior under shear can be reasonably explained by assuming the presence of a precursory chain-folded layer structure in the smectic liquid crystal of BB-6.     

Effect of electric field on the rheology of ferroelectric liquid crystal MORA-8

Effects of electric field on the rheology, electrorheological effects, are investigated on ferroelectric smectic C* phase of 4-(6-methyl)octyl-resorcylidene-4′-octylaniline (MORA-8). When an electric field is applied in smectic C* phase, an increase of the shear stress is observed with its behavior depending on the shear rates; at higher shear rates the shear stress increases monotonously with the electric field strength, but at lower shear rates the shear stress increases monotonously until about 2 kV mm⁻¹, but above this field it discontinuously decreases to lower values, which is followed by an increase at higher electric fields. In close vicinity of the smectic C* to isotropic phase transition point, a field induced decrease of the shear stress is observed. These ER behaviors are discussed on the basis of the field induced orientational changes of the smectic layer and the director and a possible field induced structural change.     

Some recent studies of liquid crystals: A review

The following topics are reviewed briefly: (i) The first experimental determination of the decrease in the smectic A-nematic transition temperature brought about by a twist deformation, an effect which is the analog of the influence of a magnetic field on the superconductor-normal metal transition; (ii) the similarity between the hydrodynamics of cholesteric liquid crystals and superfluid hydrodynamics, and some of its consequences, e.g., the possibility of a temperature wave attending second sound in the cholesteric phase; and (iii) the nematicsmectic A-smectic C multicritical point.     

氢键诱导液晶的DSC和变温红外光谱研究

制备了两个分别基于4,4’-联吡啶(BPy)和丙基反式环己基苯甲酸(PCBA)以及BPy和丙基反式双环己基甲酸(PCCA)的氢键液晶复合物(PCBA-BPy和PCCA-BPy),结合使用偏光显微镜(POM),用差示扫描量热(DSC)和变温傅里叶变换红外光谱对它们的中间相性能和分子间氢键的热稳定性进行了表征。结果表明,这两个氢键复合物都具有由于分子间氢键作用而导致的近晶相,但它们中存在的氢键作用却不相同,在PCBA-BPy中,温度变化时,发生氢键结合的羰基的吸收峰的位置仅在发生晶型转变时有突变,而在PCCA-BPy中,温度变化时,其羰基的吸收峰的位置则基本不发生突变。另外,当温度高于它们的清亮点时,这两个氢键液晶复合物的分子间氢键都发生部分分解。     

Hunting for smectic C in calamitic azobenzene ionic liquid crystals with different cationic head groups

The tilted smectic C phase is a rather uncommon phase in ionic liquid crystals (ILCs), whereas the orthogonal smectic A phase is the most common phase in ILCs. We now present 2 new groups of mesogens with an azobenzene core that exhibit smectic C as well as smectic A phases. Their phase sequences and tilt angles were studied by polarizing microscopy, and their temperature‐dependent layer spacings and orientational order parameters were investigated by X‐ray diffraction. We present 1 new amidinium azobenzene mesogen that forms enantiotropic smectic C and A phases and another amidinium as well as 2 new guanidinium azobenzene mesogens that exhibit monotropic smectic C and enantiotropic smectic A phases. With this study, we show that azobenzene is indeed an SmC‐promoting group in ILCs. Comparing these results with our earlier results on azobenzenes with an N‐methylimidazolium head group (N Kapernaum et al, ChemPhysChem 2016, 17, 4116‐4123), we show that the aromaticity of the imidazolium head group plays an important role in the formation of smectic C phases.     

Molecular models for ferroelectric liquid crystals with conventional and anomalously weak layer contraction

A molecular theory of the ferroelectric smectic C* phase has been developed using the simple model of a chiral molecule composed of a uniaxial core and a pair of off-center nonparallel dipoles which determine molecular chirality and polarity. The interaction between uniaxial cores is modeled by a rather general effective potential which can be used to describe smectic materials with both conventional and anomalously weak layer contraction in the smectic C* phase. Spontaneous polarization, tilt, and layer spacing are calculated numerically as functions of temperature, and it is shown that the variation of the polarization generally deviates from that of the tilt angle. It is shown that this deviation is more pronounced in smectic materials tilting with low layer contraction which corresponds to existing experimental data. The model has been used to reproduce qualitatively the experimental data for polarization, tilt and layer spacing for two similar mixtures exhibiting conventional and anomalously weak layer contraction. The polarization and the tilt are also calculated in the case when the smectic A-smectic C* transition is characterized by the biaxial primary order parameter.     

Molecular order in bilayer,anticlinic and ferroelectric smectic C mesophases

In this work we present a proton NMR comparative study of the molecular order in the smectic C2 (), smectic C^* () and anticlinic smectic C phases ( and ) of two liquid crystalline compounds. The second moments of the experimental proton NMR spectra are well explained taking into account two different molecular orientations models. From the analysis of our experimental results it was possible to estimate the tilt angles for all the smectic C phases studied in this work. Received 15 September 1999 and Received in final form 18 January 2000     

Ultrasonic investigations of N- (p-n-pentyloxybenzylidene) p-n-butylaniline

The ultrasonic velocity in N-(p-n-pentyloxybenzylidene) p-n-butylaniline is measured as a function of temperature from the isotropic liquid phase to the smectic G phase. A mixed smectic phase (S_AG) for a temperature interval of 2°C is reported. The derived parameters such as adiabatic compressibility (β_ad). Rao number (R_n) and molar compressibility (B) are also reported.     

Induced Smectic X Phase Through Intermolecular Hydrogen-Bonded Liquid Crystals Formed Between Citric Acid and <Emphasis Type="Italic">p</Emphasis>-<Emphasis Type="Italic">n</Emphasis>-(Octyloxy)Benzoic Acid

Hydrogen-bonded liquid crystal (HBLC) is synthesized from citric acid (CA) and 4-(octyloxy)benzoic acid (8OBA) with different mole ratios. Fourier transform infrared spectroscopy (FT-IR) confirms the presence of hydrogen bond between CA and 8OBA. Nuclear magnetic resonance (NMR) spectroscopic studies validate the intermolecular complementary, cyclic type of hydrogen bond, and molecular environment in the designed HBLC complex. Powder X-ray diffraction analysis reveals the monoclinic nature of liquid crystal complex in solid phase. Liquid crystal parameters such as phase transition temperature and enthalpy values for the corresponding mesogenic phases are investigated using a polarizing optical microscope (POM) and differential scanning calorimetry (DSC). It is observed that the change in chain length and steric hindrance while increasing the mole ratio in HBLC complex induces a new smectic X (Sm X) along with higher-order smectic G (Sm G) phases by quenching of smectic C (Sm C). From the experimental observations, induced Sm X phase has been identified as a finger print texture. Also, Sm G is a multi-colored mosaic texture in 1:1, 1:2, and 1:3 mol ratios. The optical tilt angle, thermal stability factor, and enhanced thermal span width of CA + 8OBA complex are discussed.     

Density studies across the smectic G-nematic and nematic-lsotropic transitions

The densities of N(p-n-pentyloxy benzylidene) p-ethylaniline and N(p-n-hexyloxy benzylidene) p-ethylaniline are measured as a function of temperature from the isotropic liquid to the smectic G phase. Both the compounds exhibit enantiotropic smectic G and nematic liquid crystalline phases. The changes in density across the phase transformations and the thermal expansion coefficient confirm the order of the transitions as of first order. The particular importance of the smectic G to nematic transformation is apparent from the density jump across the transition. An estimate of the pressure dependence of the isotropic-nematic transition temperature is found to be in reasonable accord with the literature data.     

Change of thermotropic properties in liquid crystal with multiple phase transitions: Surface “aging” effect

The effect of surface “aging” on thermotropic properties of polymorphic mesogen, 4-butoxyphenylester of 4-decyloxybenzoic acid (BPEDBA) which exhibits the smectic G, smectic C, smectic A, and nematic mesophases has been investigated. Temperatures of direct and reverse phase transitions and temperature widths of the heterophase regions have been determined with high accuracy. The shift of the phase transition temperatures and change of the temperature widths of the heterophase regions under the influence of surface “aging” have been reported.