Nematic Range Research Articles

End Group Effect on the Thermal and Gas Chromatographic Properties of Some para-Substituted Non-Symmetrical Nematogens

Non-symmetrically substituted nematic liquid crystals with different moieties as terminal substituents at one end and n-heptyloxy group at the other end were synthesized and characterized. The common structural features of the compounds are that they consist of the same terminal substituent at one end. The five present nematogens structurally related nematogens are compared to evaluate the effect of the type of end group on the thermal and analytical properties. The compounds were characterized by elemental analysis, FTIR, 1H-NMR spectroscopy and chemical ionization mass spectrometry. The materials’ thermal behavior and phase characterization were investigated by differential scanning calorimetry and polarizing optical microscopy. All the compounds are mesomorphic in nature, showing the nematic phase over a wide temperature range. Differential scanning calorimetry showed that the transition temperatures as well as the nematic range are strongly influenced by the type of end group attached at only one end of the nematic core molecule. Gas chromatographic properties of the compounds were determined and shown to have excellent separation capabilities of various classes of compounds, including polycyclic aromatic hydrocarbons (PAH), substituted benzenes and volatile organic compounds present in the essential oil. The separation mechanism of only some isomers was discussed in detail and, however, a relative basis for given isomers, taking into account the type of chemical bonding, molecular shape and size, the intermolecular forces (IMFs) and other factors has been proposed.

Thermal and optical characterization of liquid crystal 4′-hexyl-4-biphenylcarbonitrile/4-hexylbenzoic acid mixtures

We present the thermal and optical properties of binary mixtures formed from hydrogen-bonded liquid crystal 4-hexylbenzoic acid (6BA) and 4-hexyl-4′-biphenylcarbonitrile (6CB) mesogens. Phase transition temperatures and enthalpy values are evaluated by differential scanning calorimetry (DSC) and phases identified by polarized optic microscopy (POM). The experimental results obviously show that the 6BA/6CB binary mixtures exhibit nematic and smectic phases. The most interesting result is that although the smectic phase is not observed in pure components 6BA and 6CB, it is observed in their some binary mixtures. The thermal properties like phase peak temperatures, enthalpy changes and thermal span of binary mixtures are affected by depending on the mixture ratio. The nematic range increase in the binary mixture compared to the individual mesogen, and also the phase transition temperature values and the nematic thermal stability factor increase as heating rate increases. Furthermore, the calculated activation energy values show that the reorientation of the molecules during the phase transitions of the mixture occurs on an orderly basis.

Effect of the hockey-shaped mesogen as an additive on the critical behaviour at the smectic A to nematic phase transition

We present systematic experimental data on high resolution measurements of temperature dependence of optical birefringence (Δn) in a binary liquid crystalline system comprising calamitic 4-cyano-4′-octyloxybiphenyl (8OCB) and a hockey stick-shaped compound, 4-(3-decyloxyphenyliminomethyl) phenyl 4-decyloxycinnamate (SF7). The hockey stick-shaped mesogen shows a smectic A (SmA) phase in addition to two polymorphic tilted smectic phases. From an analysis of the Δn data, a quotient Q(T) has been derived which exhibits power law divergences both above and below the smectic A-nematic (SmA-N) phase transition. A noticeable shrinkage in width of the nematic range has been observed for a small increase in concentration of the angular mesogenic dopant. In the vicinity of the SmA-N phase transition, the values of effective critical exponent (α′), characterizing the critical fluctuation at this transition, have appeared to be in between those predicted by the 3D-XY and tricritical hypothesis. Furthermore, with increasing hockey stick-shaped molecular concentration, the SmA-N transition shows a tendency to be driven towards a first order nature.

Effect of polar intermolecular interactions on the elastic constants of bent-core nematics and the origin of the twist-bend phase.

A molecular theory of both elastic constants and the flexoelectric coefficients of bent-core nematic liquid crystals has been developed taking into account dipole-dipole interactions as well as polar interactions determined by the bent molecular shape. It has been shown that if polar interactions are neglected, the elastic constants are increasing monotonically with the decreasing temperature. On the other hand, dipolar interactions between bent-core molecules may result in a dramatic increase of the bend flexocoefficient. As a result, the flexoelectric contribution to the bend elastic constant increases significantly, and the bend elastic constant appears to be very small throughout the nematic range and may vanish at a certain temperature. This temperature may then be identified as a temperature of the elastic instability of the bent-core nematic phase which induces a transition into the modulated phases with bend deformations like recently reported twist-bend phase. The temperature variation of the elastic constants is qualitatively similar to the typical experimental data for bent-core nematics.

Quantification of photoinduced order increase in liquid crystals with naphthopyran guests.

Photoinduced order-increasing phase transitions can occur in dye-liquid crystal mixtures when the photoproduct of the excitation of the dye molecules is more compatible with the liquid crystalline medium than the initial dye species. A detailed investigation of the photoinduced changes of the phase behavior and optical properties of mixtures of liquid crystals with naphthopyran guests upon exposure to light at 365 nm is presented here. In these guest-host systems, the nematic-to-isotropic phase transition temperature is increased upon irradiation. We show that the nematic range can be extended up to 2.9 °C by illumination in 5CB (4-n-pentyl-4'-cyanobiphenyl) liquid crystal mixtures. The order parameter is significantly increased by illumination at all temperatures within the nematic range and the changes are larger at higher concentrations of the guests. In particular, the illuminated guest-host mixtures exhibit order parameters close to those of the neat liquid crystal host at the same temperature relative to the clearing point. An improved understanding of the photophysical processes taking place at the molecular level in these material systems can inform the design of photoresponsive materials and enhance their potential utility in optical or photonic devices.

High performance liquid crystals for vehicle displays

We report three liquid crystal (LC) mixtures with a wide nematic range (−40°C to ~100°C), small visco-elastic coefficient and low activation energy for vehicular displays. Physical properties at different temperatures were characterized. These LCs greatly improve the performance of different display devices in a car: 1) for head-up projection using liquid-crystal-on-silicon with an average gray-to-gray (GTG) response time less than 1 ms at an elevated temperature. 2) The average GTG response time is maintained at ~10 ms for fringing field switching LCD at T = 0°C and also ~10 ms for twisted nematic LCD at T = −20°C.

Low mid-infrared absorption tolane liquid crystals terminated by 2,2-difluorovinyloxyl: synthesis, characterization and properties

New 2,2-difluorovinyloxyl terminated tolane liquid crystals with wide nematic phase range, high birefringence and low mid-infrared absorption are reported.

Synthesis, mesogenic and dielectric properties of fluorosubstituted isothiocyanatoterphenyls

The synthesis, mesomorphic and physical properties of 14 homologous series of laterally fluorinated 4ʹ-alkyl-4-isothiocyanatoterphenyls were described. Influence of the number of fluorine atoms and their position in the terphenyl core on the phase transition temperatures, nematic range, dielectric and optic anisotropy as well as bulk viscosity were discussed. The compounds with the most optimal properties for formulation of nematic mixtures were selected, and mixture examples with low viscosity and medium or high birefringence were presented.

High-resolution birefringence study at the nematic-smectic A phase transition of ZnS nano particle dispersed octyloxycyanobiphenyl

We have studied the effect of ZnS nano particles with different concentrations on the nematic to smectic A (N-SmA) phase transition of a thermotropic liquid crystal octyloxycyanobiphenyl (8OCB). These effects are probed using a high resolution optical birefringence measurement. A decreasing trend of the nematic range of 8OCB has been observed up to a certain limit with increasing concentrations of the guest nano particles. A simple power law analysis has been carried out to extract the critical exponent (α) at the N-SmA phase transition, which allows one to investigate the critical behaviour near the phase transition as well as the universality class. The critical exponent (α) at the N-SmA phase transition strongly indicates the change in the coupling strength between the nematic and smectic A order parameters.

Large Twist Elastic Constant in Diphenylacetylene-Core-Based Liquid Crystals

We determine elastic constants K11, K22, and K33 in two diphenylacetylene-core-based liquid crystals with azo and ester linkages. As we have already reported (Adv. Mater. (2014) 26, 1918), K33 is extremely large (sub-nN) compared with K11 (pN) in the azo compound, since cybotactic smectic (Sm) clusters exist in whole the nematic (N) phase range. As a natural consequence, K22 is expected to be also large. We confirm that K22 is several tens pN, which is smaller than K33 but larger than K11 by one order of magnitude, K33>>K22>>K11. This is the first compound, which has a larger K22 than K11. In the ester compound, which has no clusters in the N phase, all the elastic constants are in a pN range, and are in the usual order, K33>K11>K22. It is also important to note that not only K33 and K22 but also K11 show a pretransitional increase when the N-SmC phase transition temperature is approached.

Low absorption chlorinated liquid crystals for infrared applications

We report a wide nematic range and low absorption loss chlorinated liquid crystal mixture, designated as IR-M2, for mid-wave infrared applications. IR-M2 is quite transparent in the 3.8-5.0 μm window while keeping a high birefringence (Δn~0.194) in the infrared region and a modest dielectric anisotropy. For long-wave infrared applications, we propose another high Δn chlorinated cyanoterphenyl compound.

Synthesis and properties of difluoromethyleneoxy-bridged liquid crystals terminated by 2,2-difluorovinyloxy group

New difluoromethyleneoxy-bridged liquid crystals with a 2,2-difluorovinyloxy terminal group have been synthesised and characterised. The mesomorphic properties, dielectric anisotropies, birefringence and rotational viscosities were evaluated. The results show that the replacement of fluorine atom with 2,2-difluorovinyloxy as a terminal group brings about significant superiorities of nematic phase range (80.3°C over 60.8°C), dielectric anisotropy (26.68 over 25.83) and rotational viscosity (175 mPa·s over 360 mPa·s). All of the characteristics make them to be practically useful candidates for the application of low power consumption and rapid response devices.

Thermal characterisation of binary mixture of some supramolecular liquid crystals

The thermal characterisation of binary mixture formed from hydrogen-bonded supramolecular liquid crystals 4-hexylbenzoic acid (6BA) and 4-octyloxybenzoic acid (8OBA) was investigated by differential scanning calorimetry (DSC) and phases identified by polarised optic microscopy (POM). The phase diagram of the mixture was obtained from theoretical calculations, and binary mixture with a maximum nematic range according to the phase diagram was produced. The DSC and POM results clearly indicate that the produced liquid crystal mixture displays liquid crystalline properties, and the nematic range of 6BA/8OBA eutectic mixture is wider than the nematic ranges of both liquid crystals 6BA and 8OBA. Also with increasing the heating rate from 5 to 20 °C min−1, the phase transitions occur in a shorter time. As a result, the 6BA/8OBA mixture does not loss its liquid crystalline properties against sudden temperature changes.

Fast-response infrared phase modulator based on polymer network liquid crystal

We report a high birefringence terphenyl liquid crystal mixture, designated as M3, for infrared phase modulation with special emphasis on mid-wave infrared (MWIR). In addition to high birefringence, M3 exhibits excellent UV stability, modest dielectric anisotropy, and a very broad nematic range. The high birefringence enables a thin cell gap to be used for achieving a 2π phase change while maintaining a high transmittance (T>98%) in the MWIR region. To achieve fast response time, we employed a polymer network liquid crystal using M3 with 2π phase change at λ = 4μm and 3.6-ms response time. This response time is about 100X faster than that of a nematic LC phase modulator.

Highly fluorinated liquid crystals with wide nematic phase interval and good solubility

A series of new liquid crystal compounds with both ethylene and difluoromethyleneoxy linking groups have been synthesised and their physical properties were measured. The compounds exhibit lower melting points and broader nematic mesophase temperature range than that of corresponding four-rings compound. Their anisotropic properties are Δε of about 25 and Δn of 0.135. The investigation shows that their solubilities are much better than that of reference compound. A mixture made by equivalent mass ratio of the compounds with side chain of ethyl, propyl and pentyl is found to keep nematic phase from 12°C°C to 109°C, which is rarely observed in highly fluorinated liquid crystals.

Three Nematogen Azobenzene-Based Stationary Phases for Capillary GC: Synthesis and Comparative Study

In this paper, three laterally substituted liquid crystals termed LCC1, LCC3 and LCC4 were synthesized. Characterization of these substances was carried out by 1H NMR and 13C NMR; they possess similar core and differ in their lateral substituents. Their thermal stability was conducted by thermogravimetric analysis and showed decomposition beginning at 212, 233 and 264 °C related to LCC1, LCC3 and LCC4, respectively. Thermal properties were determined by differential scanning calorimetry, thermomicroscopy and inverse gas chromatography. The three novel compounds exhibited a nematic mesophase at a defined range of temperatures, which varied from 81 to 55 °C. Lengthening of the lateral substituent seemed to affect the nematic range by decreasing it. Fused-silica capillary columns (30 m × 0.32 mm) were employed for the analytical study. Numerous mixtures of components of various properties were injected. Particular attention was given to the selectivity towards the close-boiling isomers. These investigations indicated that lateral attached groups had considerable effect on the thermal and chromatographic properties of the liquid crystal compounds. These interesting results may guide in improving the development of new type of liquid crystals with desired properties for a widespread application.

Synthesis and phase transition behaviours of new non-symmetric liquid crystal dimers

A new series of non-symmetric liquid crystal dimers N-(4-(n-(4-(benzothiazol-2-yl)phenoxy)alkyloxy)-4-nitro-benzylideneimine) containing benzothiazole and benzylideneaniline as the mesogenic core units connected by a flexible alkyl spacer, -(CH2)n-, with n ranging from 2 to 6, 8, 10 and 12 have been prepared. Their molecular structures were proposed via spectroscopic techniques. Mesomorphic properties were studied using differential scanning calorimetry and polarising optical microscopy. The results showed that the clearing temperatures decreased upon lengthening of the alkyl spacers. Whilst even member with the shortest alkyl spacer (n = 2) exhibited nematic phase, the rest of even members (n = 4, 6, 8, 10 and 12) possessed smectic and nematic properties. Nematic phase range gradually decreased from n = 2 to n = 12. Odd members are belonged to non-mesogenic compounds except for n = 5 which exhibited monotropic (metastable) nematic phase.

Low loss liquid crystals for infrared applications

Seven fluorinated and chlorinated terphenyl compounds intended for mid-wave infrared (MWIR) applications are synthesised and two eutectic mixtures formulated, and their physical properties evaluated. In addition to low absorption, some desirable properties for MWIR applications include wide nematic range, high birefringence, large dielectric anisotropy and low viscosity. The fluorinated terphenyl mixture exhibits a relatively low absorption in the vicinities of λ ~ 3 μm, but a fairly strong overtone appears in the 4–5 μm region. To suppress these overtone absorptions, short-chain chlorinated terphenyl compounds are proven to be useful.

Thermal and mesomorphic properties of ternary mixtures of some hydrogen-bonded liquid crystals

The thermal and mesophase behaviour of ternary mixtures formed from hydrogen-bonded nematic liquid crystals 4-hexylbenzoic acid, 4-(octyloxy)benzoic acid and 4-(decyloxy)benzoic acid were investigated by differential scanning calorimetry (DSC) and phases identified by polarised optic microscopy (POM). The DSC and POM results clearly indicate that the produced mixtures display liquid crystalline properties. The phase transition temperature values obtained in the DSC experiments are in line with the POM observations. Investigation of the thermal properties of ternary mixtures reveals that the nematic range of the produced mixtures is affected in a way that is dependent on both the mixture ratio and the difference in the alkyloxy chain lengths. Furthermore, the phase transition temperature values of ternary mixture increase with increasing heating rate, and the calculated activation energy values show that the reaction arising during the phase transitions of the mixture is regular.

Synthesis of fluorinated terphenyl liquid crystals with 3-propylcyclopentane end group

High performance liquid crystal (LC) displays require nematic mixtures with low rotational viscosity to reduce the response time and large dielectric anisotropy to reduce the threshold voltage. Furthermore, liquid crystals (LCs) with low melting temperature and high clearing temperature are particularly attractive for automobile and outdoor display applications. In this article, we report a series of new fluorinated terphenyl LCs with 3-alkylcyclopentane end group. The terphenyl mesogen is designed for high Δn value, and the lateral fluorine atom is used to reduce the melting temperature. In comparison with the terphenyl analogue containing a 4-alkylcyclohexyl end group, discussed in previous works, the obtained LCs show wider nematic temperature range and smaller rotational viscosity because 3-alkylcyclopentyl moiety is introduced. The effect of alkyl chain length and the lateral fluoro substituent on the mesomorphic properties of the obtained LC is discussed. The results show that the nematic temperature range increases with the alkyl chain length increase. The position of the lateral fluoro substituent also plays an important role; LC with the fluoro substitution on the central benzene ring provides the widest nematic temperature range. In conclusion, we have synthesised a new series of LCs with low melting point, wider nematic temperature range and high Δn value, which is promising in formulating eutectic mixture for display application.