Polarisation switching and molecular relaxation behaviour of anthraquinone dye-dispersed polymer-stabilised ferroelectric liquid crystal composites

Guest–host polymer-stabilised ferroelectric liquid crystal (GH-PSFLC) composite films have been prepared with dispersion of small concentration (0.1, 0.25 and 0.5 wt%) of anthraquinone blue dye in PSFLC host matrix via a polymerisation-induced phase separation (PIPS) process. The variation in alignment and size of twisted fibril has been observed in the optical textures of the guest–host composites with different wt/wt ratio of anthraquinone dye. The electrical and dielectric properties of PSFLC mixture and its guest–host derivatives are studied. Our results showed that an optimum amount of dye concentration (0.1 wt%) enhances the dielectric permittivity as well as the spontaneous polarisation of the GH-PSFLC material in the SmC* phase.     

Effect of alkoxy-chain length proportionation on the mesophase behaviour of terminally di-substituted phenylazo phenyl benzoates

A series of 4-alkoxyphenylazo-4′-phenyl-4′′-alkoxybenzoates bearing two terminal alkoxy groups (I m + n), where m and n denote the number of carbon atoms in the alkoxy groups attached to the phenylazo and benzoate moieties, respectively, was synthesised, and the molecular structures were established via elemental analyses, IR, NMR and mass spectroscopy. First, the thermal transitions and mesophase characteristics of any two isomeric molecules, having the same molecular length but with differently proportionated alkoxy groups, were investigated by differential scanning calorimetry and polarised-light microscopy. All compounds investigated proved to have a wide smectic C and nematic liquid crystalline ranges, except for the lower homologous (I 6 + 8 and I 8 + 6) where they are purely nematogenic. Mesophases were confirmed by the miscibility method, using dodecyloxybenzoic acid as the mesophase reference. Second, the mesophase behaviours of binary mixtures of isomeric compounds, bearing the same total alkoxy-chain length (m + n) but differ in the individual values of m and n, were studied and discussed. It was found that the smectic C and nematic phases behaved ideally in their binary mixture with the 4-alkoxy benzoic acid, independent of either n or m.     

Phase behaviour and electro-optic characteristics of a polymer stabilized ferroelectric liquid crystal

The effects of adding a diacrylate monomer or its polymerized network to a ferroelectric liquid crystal have been characterized. The monomer lowers the temperatures of transition to the more ordered phases, whereas the polymer network phase separates into polymer rich and LC rich phases and has little effect on the LC phase behaviour. Ferroelectric polarization decreases comparably in both monomer and networked systems. As the network concentration increases, the size of LC domains decreases considerably. With low concentrations of polymer and, thus large LC domains, optical response and tilt angle remain fairly independent of polymer concentration, but as the polymer concentration increases, switching speed and tilt angle decrease dramatically. Polymerization rate maxima increase with monomer concentration until saturation of monomer in the liquid crystal is reached. The rate maxima then decrease as monomer must diffuse from monomer rich droplets. Double bond conversion during the polymerization is comparable for all monomer concentrations below 50 per cent.     

Fast switching response and dielectric behaviour of fullerene/ferroelectric liquid crystal nanocolloids

The electro-optical and dielectric responses of the fullerenes C₆₀-doped ferroelectric liquid crystal (FLC) nanocolloids are reported. Order parameter and phase transition temperature remain invariant as a function of varying dopant concentration (0.10 wt% to 0.50 wt%). Faster switching response of nanocolloids comparing to that of the non-doped FLC is manifested by increase in the localised electric field (around 76% increment for 0.50 wt%), while reduction in the spontaneous polarisation could be the result of anti-parallel correlation amid dopant and FLC dipoles. Decrease in dielectric constant, absorption strength, dielectric strength and rotational viscosity of FLC nanocolloids than that of non-doped FLC is the other consequence of C₆₀ doping. Goldstone-mode relaxation frequency is found to be increased with increasing doping concentration of C₆₀ in FLC.     

Liquid crystal tetramers: influence of molecular shape on liquid crystal behaviour

The synthesis and characterization of two homologous series of tetramers in which four mesogenic units are linked via three alkyl spacers are reported. Both series contain a hexamethylene central spacer while the length of the two outer spacers, n, is varied from three to 12 methylene units. The two series differ only in the substitution pattern around the inner two mesogenic units. The series in which one mesogenic unit is attached to the central spacer in the 4-position while the other is connected at the 3-position is referred to as the n-p6m-n series, while in the n-m6m-n series both inner units are attached in the 3-position. All the members of the n-p6m-n series exhibited a nematic phase while no liquid crystallinity was observed for the n-m6m-n series. The thermal behaviour of this series is compared with that of the n-p6p-n series and also with that of the corresponding series containing a pentamethylene central spacer. The trends observed are interpreted in terms of the average molecular shapes of these tetramers.     

A comparative study of intermolecular interactions in the crystal structures of phenyl/phenyl end‐capped oligoanilines

Three crystal structures have been analysed from the point of view of intermolecular interactions: N,N′‐di­phenyl‐1,4‐benzo­quinone di­imine, C₁₈H₁₄N₂, (I), its reduced form N,N′‐di­phenyl‐1,4‐phenyl­enedi­amine, C₁₈H₁₆N₂, (II), and N,N′‐di­phenyl‐1,4‐phenyl­enedi­ammonium bis(p‐toluene­sulfonate), C₁₈H₁₈N₂²⁺·2C₇H₇O₃S^?, (III), which contains fully protonated (II) with p‐toluene­sulfonic acid. The local molecular C_i symmetry is preserved in all three structures and the packing seems to be dominated by the mutual arrangement of the simple polyaniline oligomers in the different protonation states. In (I), the most significant molecular interactions are stacking forces, forming columns of mol­ecules along [001]. Close packing of the columns results in C‐centring of the structure. In (II), only van der Waals interactions can be observed. In the structure of (III), the p‐toluene­sulfonate ions serve as acceptors in relatively strong N—H?O hydrogen bonds. The N,N′‐di­phenyl‐1,4‐phenyl­enedi­ammonium cation intercalates between two anions related by a centre of symmetry.     

Aminolysis of 2,4-dinitrophenyl X-substituted benzoates and Y-substituted phenyl benzoates in MeCN: effect of the reaction medium on rate and mechanism

Second‐order rate constants (k_N) have been determined spectrophotometrically for the reactions of 2,4‐dinitrophenyl X‐substituted benzoates ( 1 a – f ) and Y‐substituted phenyl benzoates ( 2 a – h ) with a series of alicyclic secondary amines in MeCN at 25.0±0.1 °C. The k_N values are only slightly larger in MeCN than in H₂O, although the amines studied are approximately 8 pK_a units more basic in the aprotic solvent than in H₂O. The Yukawa–Tsuno plot for the aminolysis of 1 a – f is linear, indicating that the electronic nature of the substituent X in the nonleaving group does not affect the rate‐determining step (RDS) or reaction mechanism. The Hammett correlation with σ^? constants also exhibits good linearity with a large slope (ρ_Y=3.54) for the reactions of 2 a – h with piperidine, implying that the leaving‐group departure occurs at the rate‐determining step. Aminolysis of 2,4‐dinitrophenyl benzoate ( 1 c ) results in a linear Brønsted‐type plot with a β_nuc value of 0.40, suggesting that bond formation between the attacking amine and the carbonyl carbon atom of 1 c is little advanced in the transition state (TS). A concerted mechanism is proposed for the aminolysis of 1 a – f in MeCN. The medium change from H₂O to MeCN appears to force the reaction to proceed concertedly by decreasing the stability of the zwitterionic tetrahedral intermediate (T^±) in aprotic solvent.     

Optical guided-wave study of a homeotropically aligned ferroelectric liquid crystal

Abstract

The excitation of optical modes is used to study the optical tensor configuration in a thin ferroelectric liquid crystal layer, cooled from the initially homeotropically aligned nematic phase. By monitoring the angular dependent reflectivity for plane polarized radiation coupled into the guided modes in the smectic C* layer and subsequently fitting the recorded data to predictions from multilayer optics theory, the optical tensor configuration in the layer is fully evaluated. Iteratively modelling the full tilt/twist profile in the cell, progressively converging the predicted reflectivity to experimental data, gives a complete and very well specified picture of the optical tensor throughout the cell. By studying the cell at various temperatures, the temperature dependence of the tilt of the major axis of the optic tensor (which may be related to the cone angle if the smectic layers are parallel to the cell surface) has been established. The temperature dependent optical dielectric constants have also been obtained.     

Optical guided-wave study of a homeotropically aligned ferroelectric liquid crystal

The excitation of optical modes is used to study the optical tensor configuration in a thin ferroelectric liquid crystal layer, cooled from the initially homeotropically aligned nematic phase. By monitoring the angular dependent reflectivity for plane polarized radiation coupled into the guided modes in the smectic C* layer and subsequently fitting the recorded data to predictions from multilayer optics theory, the optical tensor configuration in the layer is fully evaluated. Iteratively modelling the full tilt/twist profile in the cell, progressively converging the predicted reflectivity to experimental data, gives a complete and very well specified picture of the optical tensor throughout the cell. By studying the cell at various temperatures, the temperature dependence of the tilt of the major axis of the optic tensor (which may be related to the cone angle if the smectic layers are parallel to the cell surface) has been established. The temperature dependent optical dielectric constants have also been obtained.     

Hysteresis and ferroelectric liquid crystal displays

One cause of the hysteresis in the voltage-transmittance curve is polarization in the alignment layers brought about by the spontaneous polarization of FLC molecules. The hysteresis was reduced by increasing the conductivity of the alignment layers. The structural changes on switching were examined by measuring time-resolved shifts of FT-IR peaks of selected groups. The shifts of selected groups in the IR peak can be attributed to changes in the local dielectric constants of the ambient environment during switching by molecular orbital calculations. The orientation on switching in the SmC* phase was found to be similar to that in the N* phase.     

Observing molecular motions of a fluorinated ferroelectric liquid crystal using fluorine-19 NMR

Fluorine-19 NMR spectra have been recorded for both the phases (cholesteric and smectic C*) of the ferroelectric liquid crystal 2,3-difluoro-4-octyloxybiphenyl-4'(-4'-oxycarbonyl-(S)-1-chloro-3-methylbutyl)benzoate, using a conventional high-resolution NMR probe. The magnitude of the dipolar coupling constant, D_FF0, was found to increase with decreasing temperature in the cholesteric phase with a sudden increase (of approximately 1 000 Hz) at the cholesteric-smectic C* transition, whereafter a more gradual change was observed. The order parameter, was S_FF, subsequently calculated for each temperature.     

Spontaneous polarization and electro-optic behaviour of a ferroelectric liquid crystal cell fabricated with polyimide Langmuir-Blodgett films

The spontaneous polarization and electro-optic response of ferroelectric liquid crystals (FLCs) were investigated in a cell fabricated with a polyimide alignment layer coated by the Langmuir-Blodgett method. The surface properties of the cured polyimide layers were monitored by contact angle measurement, and by FTIR spectroscopy and AFM for the orientation and surface roughness, respectively. The apparent spontaneous polarization of an FLC determined in a practical sandwich-cell depended on various conditions such as cell thickness, cooling rate from the smectic A to chiral smectic C phase, and deposition pressure. Electro-optic response and decay times of FLCs were also measured. Furthermore, the ions in the FLC mixture reduced the magnitude of the effective electric field, but had no effect at high frequency.     

Microphasic separation-induced enantiotropic liquid crystal behaviour single phenyl unit series based on the fluorophobic effect

The aim of this work was to demonstrate the possibility of obtaining enantiotropic liquid crystals containing a single phenyl group over a wide temperature range but without hydrogen bonding. We report the synthesis and characterization of several alkoxy monophenyl compounds linked by a thioester group to a 2-(perfluoro- n alkyl) ethyl chain. Compared with previous work using monophenyl cores, the synthetic pathway provides compounds is high yields from cheaper raw materials, i.e. 4-hydroxybenzoic acid. The mesomorphic properties were characterized using polarized light microscopy and differential scanning calorimetry. The effects of lengthening the substituents on the phenyl core were considered: varying the length of the alkyloxy groups had a marked effect, with the formation of enantiotropic mesomorphic phases with a maximum temperature range of 70°C. The temperature range of the mesomorphic phase is inversely proportional to the length of the alkyl chain and exhibits an odd-even effect. Decreasing the fluorinated chain length leads to a regular decrease in both the melting and clearing temperatures, giving room temperature transitions. This design approach using a single ring as the mesogenic core is of great interest in the development of low cost LC materials based on the fluorophobic effect.     

Microphasic separation-induced enantiotropic liquid crystal behaviour single phenyl unit series based on the fluorophobic effect

The aim of this work was to demonstrate the possibility of obtaining enantiotropic liquid crystals containing a single phenyl group over a wide temperature range but without hydrogen bonding. We report the synthesis and characterization of several alkoxy monophenyl compounds linked by a thioester group to a 2-(perfluoro-n alkyl) ethyl chain. Compared with previous work using monophenyl cores, the synthetic pathway provides compounds is high yields from cheaper raw materials, i.e. 4-hydroxybenzoic acid. The mesomorphic properties were characterized using polarized light microscopy and differential scanning calorimetry. The effects of lengthening the substituents on the phenyl core were considered: varying the length of the alkyloxy groups had a marked effect, with the formation of enantiotropic mesomorphic phases with a maximum temperature range of 70°C. The temperature range of the mesomorphic phase is inversely proportional to the length of the alkyl chain and exhibits an odd-even effect. Decreasing the fluorinated chain length leads to a regular decrease in both the melting and clearing temperatures, giving room temperature transitions. This design approach using a single ring as the mesogenic core is of great interest in the development of low cost LC materials based on the fluorophobic effect.     

On the relationship between electro-optic and dielectric parameters of a smectic C* ferroelectric liquid crystal

A relationship between the electro-optic switching time and dielectric parameters of a S*_c ferroelectric liquid crystal (FLC) is obtained. This relationship is derived in terms of spontaneous polarization P_s, relaxation time τ_G and dielectric strength Δε_G of the Goldstone mode. It shows clearly that the switching phenomenon in FLCs is governed by the dielectric behaviour of the Goldstone mode. Based on the Landau model, the switching time has also been related to the material parameters of the FLC.     

Study on the relationship between chromatographic retention behaviour and molecular structure

Summary With the molecular hydrophobic feature represented by Van der Waals molecular volume, the-hydrophilic parameter of substituting groups in mono-substituted benzenes was determined, in order to predict the retention of substituted benzenes in RP-HPLC. Thus it is possible to predict the retention of multi-functional substituted benzenes. The agreement between the calculated and the measured retention values shows that the proposed model to deal with retention of solutes is reasonable.     

A study on the effect of carboxylate ester and thioester linking groups in ferroelectric liquid crystal dimers

Two series of symmetrical liquid crystal twins possessing a chiral centre at the inner side of the molecules were synthesized and characterized. Structural effects on the mesomorphic and physical properties were investigated in terms of (a) variation of carboxylate and thioester groups linking the spacer and core, (b) variation in achiral chain length between two mesogenic units. The mesomorphic investigation reveals that these compounds exhibit a BP, N*, SmA* and SmC* mesophase sequence for the BDPNP series, and a SmA and SmC* sequence for the ABPNTP-n series. The P_s values were measured and a largest of 38.5 nC cm^-2 was observed for ABPNTP-12.     

The crystal structures,molecular spectra and thermal behaviour of carbamoylferrocene and ferrocenecarbonylhydrazide

Carbamoylferrocene (1) and ferrocenecarbonylhydrazide (2) were synthesised by modified literature procedures and studied by a combination of the conventional spectroscopic methods (IR, MS and NMR), X-ray crystallography and thermal analysis. The crystal structures of two new polymorphs of amide 1 and the structure of hydrazide 2 were determined, revealing extensive hydrogen bonding interactions of the polar ferrocene substituents and supportive π?$?$π stacking interactions. Whereas the thermal behaviour of hydrazide 2 was relatively complicated, thermolysis of amide 1 in the presence of air oxygen led to a complete elimination of the organic constituents in a single step under mild conditions affording essentially pure hematite (α-Fe₂O₃). Mössbauer spectra, powder X-ray diffraction and high-resolution transmission electron microscopy indicated the pyrolytic hematite to result in the form of random aggregates of well crystalline nanoparticles.