Stability Of Liquid Crystal Phases Research Articles

On structural organisation and crystallisation/vitrification phenomena in low molecular weight glass-forming liquid crystals

ABSTRACT Results of the dielectric spectroscopy, differrential scaning calorimetry, adiabatic calorimetry, X-ray diffraction and infrared spectroscopy studies concerning the formation of glasses with some degrees of long-range order and the cold crystallisation phenomenon in about 15 liquid crystal compounds are discussed. Based on the temperature dependence of molecular dynamics, the liquid-like N, N*, SmC* and SmC*A phases have been identified as the fragile glass formers, while the solid-like SmI*A, SmBcr, SmE and SmG phases and the orientationally disordered and conformationally disordered crystals as the strong ones. Features of cold crystallisation in the fragile and strong glass-forming phases are presented. A comparison of cold and melt crystallisations is made. The role of molecular chains in the stability of liquid crystal phases is emphasised. We also discuss the quasi-binary model of the liquid crystal order in the smectics.

Stereochemistry, Conformational Dynamics, and the Stability of Liquid Crystal Phases

Although discotic liquid crystal dimers have been widely targeted as organic semiconductors and as LC-glass formers, the role of conformational dynamics on the self-assembly of these flexible mesogens remains poorly understood. In an effort to probe this effect, we investigated the impact of linker stereochemistry on the phase behavior of discotic liquid crystalline dimers. Diastereomeric dibenzo[a,c]phenazine diesters were prepared from (2R,3R)- and meso-2,3-butanediol. While both dimers form columnar phases, the meso-isomer had a clearing temperature (Tc) that was 31 °C higher than that of its chiral diastereomer. Conformational analysis via DFT calculations, 1H-NMR, and DOSY experiments indicated that both compounds adopt predominantly extended conformations but that the meso-dimer shows a stronger preference to unfold in solution. To probe how conformation alters phase stability, we prepared derivatives in which catechol and hydroquinone act as rigid linkers that lock the dimers in a folded or an extended conformation, respectively. The diester of hydroquinone possessed a Tc that was nearly 100 °C higher than the catechol derivative, consistent with a model where extended conformations stabilize the LC phase. Extended dimers also exhibited higher transition enthalpies at the Tc, an indication that their columnar phases are more ordered than folded structures.

Impact of Intermediate and Terminal Groups on the Thermal Stability of Bent-Core Liquid Crystals

A series of new mesogenic chalcone, azomethine and azo compounds were successfully synthesized. A chalcone was prepared via condensation of isophthaldehyde with a p-hydroxyacetophenone in a basic medium, Schiff bases was prepared by condensation isophthaldehyde with a p-hydroxyaniline in acidic medium. While azo compound was obtained by a reaction of m-diaminobenzen with NaNO2 and phenol, and the generated compound was reacted with alkyl bromide. The structures of such compounds were confirmed by FT-IR and 1H-NMR spectrometer. The liquid crystal phases were studied by Differential Scanning Calorimeter (DSC) and polarizing optical microscope with heating. A thermal stability of liquid crystal phases were increased by increasing the length of aliphatic chain to smectic phases and decreasing in the thermal stability of nematic phase. It has been observed that the thermal stability of liquid crystal in chalcone compounds were higher than others, azo and azomethine compound

Impact of Intermediate and Terminal Groups on the Thermal Stability of Bent-Core Liquid Crystals

A series of new mesogenic chalcone, azomethine and azo compounds were successfully synthesized. A chalcone was prepared via condensation of isophthaldehyde with a p-hydroxyacetophenone in a basic medium, Schiff bases was prepared by condensation isophthaldehyde with a p-hydroxyaniline in acidic medium. While azo compound was obtained by a reaction of m-diaminobenzen with NaNO 2 and phenol, and the generated compound was reacted with alkyl bromide. The structures of such compounds were confirmed by FT-IR and 1 H-NMR spectrometer. The liquid crystal phases were studied by Differential Scanning Calorimeter (DSC) and polarizing optical microscope with heating. A thermal stability of liquid crystal phases were increased by increasing the length of aliphatic chain to smectic phases and decreasing in the thermal stability of nematic phase. It has been observed that the thermal stability of liquid crystal in chalcone compounds were higher than others, azo and azomethine compound. http://dx.doi.org/10.25130/tjps.23.2018.014

O-Carborane derivatives for probing molecular polarity effects on liquid crystal phase stability and dielectric behavior

A series of mesogenic derivatives ofo-carborane was synthesized, their properties were analyzed by thermal, optical and XRD methods, and results were compared with those of isostructuralp-carborane and benzene analogues.

Inductive effect for the phase stability in hydrogen bonded liquid crystals, x-(p/m)BA:9OBAs

A new series of hydrogen bonded liquid crystal (HBLC) complexes, made up with substituted benzoic acids (BAs) and nonyloxy benzoic acid, viz., x-(p/m)BA:9OBAs are reported for x = F, Cl, Br and –CH3 substituted at para (p) or meta (m) positions of BA moiety. Proton nuclear magnetic resonance (1H-NMR) spectrum confirms the HBLC complex. Infra red (IR) spectrum confirms linear, double and complementary type of hydrogen bonding (HB) between x-(p/m)BAs and 9OBA. The liquid crystal (LC) phases are characterised by polarisation optical microscopy (POM) and differential scanning calorimetry (DSC) techniques. x-(p/m)BA:9OBA exhibit N, C and G LC phase variance. HB induces tilted phases and enhances LC phase stability. The influence of configuration, size, electronegativity, electron directing capacity and inductive nature of substituent (x) is investigated for the stability of LC phases. An overview of the LC phase data indicates predominant ‘negative inductive effect’ in HBLCs with electron withdrawing substituents. Inductive effect operates effectively for para substitutions. Results are discussed in the wake of reports in other HBLCs.

Liquid Crystal System as Molecular Machinery: Investigation of Dynamic Impedance Matching between Molecular Core and Terminal Groups Using Rotor-Bearing Model

Liquid crystal phase stability is examined for liquid crystal terphenyls and their fluorinated derivatives, and some general rules are proposed that relate phase stability to molecular structures. A mechanical model is applied to explain the phase behaviours of these compounds. We postulate that liquid crystal phases are micromachine systems consisting of an ensemble of molecular rotors. The molecular core and terminals are the rotor and axes, respectively. The terminals also function as bearings for the subsequent layer's molecular rotors. Phenomena of the thermal stability of phase and polymorphism are interpreted in terms of basic mechanical and dynamical parameters: centre of gravity, eccentricity, moment of inertia and mass distribution. The importance of dynamic impedance matching between parts, rotors, axes and bearings, is emphasized. Fluorination effects on liquid crystallinity are explained in a unified manner on a mechanical and dynamical basis.

Dopant Effects on Electro-Thermo-Optical Characteristics in Matrix-addressed Smectic-A Liquid Crystal Displays

Abstract Liquid crystal (LC) materials doped with non-mesomorphic or chiral substances are used for the matrix-addressed LC displays based on electro-thermo-optical effect. The influence of phase-transition temerature on driving condition and contrast ratio is investigated. Non-mesomorphic dopants which decrease greatly the thermal stability of LC phase expands the allowance of the driving condition and increases the contrast ratio. The chiral dopants also cause similar effects. However, the effects are not caused by the decrease of the thermal stability but by the twist force of chiral dopants. The minimum signal voltage to make a transparent state is not changed by non-mesomorphic dopant, whereas it increases with increasing the concentration of the chiral dopant. On the basis of our investigation, LC blends with the non-mesomorphic or chiral dopants have been developed. They have not only a wide allowance of driving condition and a high contrast ratio but also a high writing speed.