Nematic Phase Range Research Articles

Liquid crystals with multiple fluorinated bridges in the mesogenic core structure

A rational, building block oriented design approach gives rise to a new class of “super fluorinated” liquid crystals suitable for the newest generation of active matrix LCDs. These materials contain combinations of different fluorinated linking groups in their mesogenic core structure, resulting in a highly advantageous property profile including a broad nematic phase range, low rotational viscosity and high dielectric anisotropy. The large-scale synthesis of such complex materials became feasible only by some recent advances in the synthesis of α,α-difluoroether linkages.

New Tolan‐type Liquid Crystals with 1, 3‐Dithiane Ring

AbstractTwo new tolan‐type liquid crystalline (LC) materials, 2‐[4‐(4‐pentoxyphenyl)ethynylphenyl]‐5‐pentyl‐l,3‐dithiane (A) and 2‐[4‐(4‐pentoxy‐2, 3, 5, 6‐tetrafluorophenyl) ethynylphenyl]‐5‐pentyl‐1, 3‐dithiane (B) were synthesized and their mesomorphic behavior was determined. The 1, 3‐dithiane‐type compound A exhibits only nematic phase. The compound B, fluorosubstituted in the LC core, shows the same nematic phase range.

Liquid-Crystalline Compounds Consisting of Two Mesogenic Cores in Parallel Conformation

Behaviors and organization of novel liquid-crystalline compounds 1 and 2, which have an iminodicarbonyl moiety as a curved central junction and consist of two mesogenic units in parallel conformation, were investigated. Compound 2a showed a nematic phase which has extremely small transition enthalpy (0.045 kcal/mol) and a broad temperature range of pressure-induced nematic phase. The smectic A phase of 1b and 2b showed interdigitation of alkyl chains which originates in the molecular structure which has two cores with three alkyl chains in parallel conformation.

Fluorinated tolan-type nematic mesogens containing a 1,3-dioxan unit

Twenty new 2,5-disubstituted 1,3-dioxans were prepared, and their mesomorphic properties were investigated by optical polarizing microscopy and DSC; all of the compounds exhibit the nematic phase only. With fluorination in the mesogenic core, the melting and clearing points were reduced, but their nematic phase range only slightly shortened.

Synthesis and mesomorphic properties of 4-alkylamino-4'-substituted diphenyldiacetylenes

Various substituted aminodiphenyldiacetylenes of the type with X = C3H7, C5H11, F or NO2 and R, R' = H, CH3-C6H13 were synthesized and their mesomorphic properties determined. Semi-empirical and ab initio quantum chemical calculations using AM1, 421G and 631G* suggested that the amino group would increase the dielectric anisotropy and optical birefringence as compared to the alkyl chain. Mesomorphic properties were found to be poor with the maximum nematic phase range being 44.8°C and many of the compounds having no nematic phase. Both melting temperatures and enthalpies for those having nematic phases were too high to form good eutectic mixtures.

The synthesis of cholest-5-ene-3beta-carboxylates and a comparison of their mesomorphic behaviour with isomeric cholesterol esters with a reversed ester linkage

The syntheses of seven esters of cholest-5-ene-3beta-carboxylic acid are reported and the melting points, transition temperatures and mesophase morphologies of the esters are compared with those of the isomeric 3beta-cholesterol compounds which have the ester link reversed. For the examples reported, the cholest-5-ene-3beta-carboxlates always have significantly lower melting points, but the differences in clearing temperatures for the two series of esters are usually much less. Several of the new compounds give an increased chiral nematic phase range and an intense selective reflection of light. They therefore represent a novel type of chiral nematic material for use in thermochromic applications.

Binary Mixtures of Liquid Crystalline Ester Bismaleimides

Abstract A binary system consisting of a chlorohydroquinone-based ester bismaleimide (3-Cl), T m = 238°C, and a methylhydroquinone-based ester bismaleimide (3-Me), T m = 251°C, was investigated for the purpose of improving processability by widening the nematic phase range before polymerization. Calculations based on the Schroeder-van Laar equation predicted a system eutectic composition of 41% 3-Me monomer and a eutectic temperature of 202°C. Experiments found the eutectic composition at 35% 3-Me and the eutectic temperature at 218.5°C. Discrepancies between experimental results and theoretical predictions are likely due to error in measured heats of fusion either due to impurities in the samples or due to the reactive nature of the components being considered. Thermal cycling was also found to have a significant melting point depression effect. While significant depression of the system melting point was achieved, polymerization still occurred immediately after melting in all systems evaluated. All mixt...

A study of the phase behaviour of blends of side-chain liquid crystal polymers

The phase behaviour of mixtures of acrylate based side-chain liquid crystal polymers is examined. The constituent polymers differ only in the number of alkyl units in the coupling chain between the mesogenic units and the polymer backbone. Mixtures composed of such materials exhibit both a lower critical solution temperature in the isotropic phase range and a upper critical solution temperature in the nematic phase range. The miscibility gap for 50 50 mixtures is of the order of ∼30°C but there is minimal miscibility in the nematic phase range. This behaviour is observed for both mixtures involving nematic phases with the same sign of orientational coupling between the mesogenic units and the polymer chains and mixtures involving polymers with opposite signs of coupling. There is an asymmetry in the miscibility curve for a mixture containing polymers of opposite signs of coupling.

Creation of nematic and reentrant nematic phases in bi- and multi-component smectic mixtures

Abstract Experimental results referring to the transformation of smectic phases, mainly smectic A, into nematic and reentrant nematic phases are reviewed. A new explanation of some experimental results is proposed. Factors which are responsible for the depression of smectic phases in mixtures of polar mesogens are discussed and the possibility of forming mixtures with a broad temperature range of nematic phase from smectic compounds, which can be useful for liquid crystal displays (LCDs), is shown. A nematic gap observed in some cases between monolayer (SA1) or monolayer and partially bilayer (SAd) smectics results from the differences in the organization of the molecules in the smectic layers. It is concluded that polar phase from smectic A1 phases can be divided into two groups: (a) the first one is characteristic for compounds with the -NCS, -F, -CI, -I or ‒COC m H2m + 1 terminal group. The spacing of the smectic layer slowly expands with the increase in alkyl chain length and the structure of the smec...

A small-angle neutron scattering study of a semiflexible main-chain liquid crystalline copolyester

A main-chain semiflexible copolyester containing 4,4'-dihydroxy-a,ci'-dimethylbenzalazine as the mesogenic unit has been studied by small-angle neutron scattering (SANS). The polyester shows a nematic phase in the temperature range between 160 and 250 C. The polymer chain conformation, in the nematic phase range, was investigated for mixtures of deuterated and hydrogenous polymers, aligned using a magnetic field (BO = 0.6 T). Due to the occurrence of a transesterification reaction, polymer chains consisting of deuterated and nondeuterated blocks of different lengths are formed, as a function of annealing time. The average degree of polymerization and the radius of gyration of the blocks were measured from the neutron scattering profiles. Changes in chain anisotropy have been observed in the nematic phase range, as predicted by recent theories developed for semiflexible main-chain liquid crystalline polymers (LCPs). A persistence length of 120 A was measured in the temperature range from 175 to 196 OC for a mixture consisting of fully deuterated LCP and unlabeled polymer. The scattering from the pure polymers aligned in the mesophase was also investigated. The results suggest that the disclination structure associated with a domain structure, typical of liquid crystalline systems, may be observable by SANS.

Isothiocyanates and their Mixtures with A Broad Range of Nematic Phase

Abstract Results referring to liquid crystalline isothiocyanates especially as components of liquid crystal mixture were summarized. The outstanding feature of liquid crystalline isothiocyanates is the possibility to make mixtures with a wide range of nematic phase low viscosity and weak temperature dependence of viscosity. The compounds exhibit good chemical stability; they are especially resistant to humidity and UV radiation. Their thermal stability is not extremely high but this disadvantage is overcome by a suitable selection of the mixture components.

Rigid rod molecules as liquid crystal thermosets. II. Rigid rod esters

AbstractNine rigid rod ester monomers endcapped with maleimide, nadimide, and methylnadimide groups were prepared and studied by DSC and hot stage polarized light microscopy. All of the monomers showed thermotropic nematic liquid crystalline phases and could be thermally crosslinked in the nematic phase. The nematic texture was maintained in the crosslinked solid state. The nematic phase range was enlarged by B‐staging the monomeric compounds. Heating the monomers for a short period of time in the nematic phase lowered the crystal to nematic transition temperatures and increased the nematic to isotropic transition temperatures. A nonequilibrium phase diagram was proposed to explain the melting behavior of these reactive liquid crystal thermoset materials.

The Liquid Crystal Properties of 1 -(4-Cyanophenyl)–trans-4-n-dodecylcyclohexane (PCH-12)

Abstract 1-(4-Cyanophenyl)-trans-4-n-dodecylcyclohexane, a long chain member of the PCH series of liquid crystal materials has been prepared and its phase behavior established by optical microscopy and DSC. In comparison with the purely smectic (SA) biphenyl analogue (K36), PCH-12 maintains an enantiotropic nematic phase range of about 9°C above its monotropic smectic A phase, allowing a clearer comparison to be made between the smectic tendencies of the PCH and the K series.

Pressure Studies of Liquid Crystalline Transitions

Abstract Some of the important results emerging from our recent pressure studies of liquid crystalline transitions are presented. In particular, the following topics are discussed: (i) The relation between P m, the maximum pressure of smectic stability in reentrant mesogens, and the range of the nematic phase at atmospheric pressure vis-a-vis the molecular structure; (ii) Piezothermal studies in the vicinity of the A-N transition in 8 OCB; (iii) The influence of molecular ordering on the pressure behavior of the nematic-isotropic transition in reentrant nematic mixtures; and, (iv) The relation between the pressure behavior of the smectic A phase and the interdigitation of the molecules.

Materials aspects of thermally addressed smectic and cholesteric liquid crystal storage displays

New improved materials for smectic liquid crystal storage displays are described which have the advantages of image generation at lower laser power, independence of image generation over a wider range of laser scanning speeds, improved contrast, and near ambient temperature (24°C) storage. The electric erase characteristics, laser writing speeds, and contrast are demonstrated to be dramatically dependent on both the smectic liquid crystal and added dopants. Local electric erase is observed only for smectic liquid crystals having a nematic phase or incipient nematic range. Better contrast and a wider range of laser writing speeds are obtained when dopants are added which are soluble in the isotropic phase and insoluble in the smectic phase. Enhanced contrast (higher scattering center density) obtained by adding certain dopants is considered to result from enhanced nucleation in the bulk and stabilization of singularities resulting therefrom by reduction of the associated core and strain energies.