Series Of Nematic Liquid Crystals Research Articles

Simulations of some physical parameters of homologous series of nBT and nCHBT at 0.3–20.0 THz

ABSTRACTIn this work, the terahertz (THz) absorption of homologous series of isothiocyanobiphenyls (nBT) and 4-(trans-4ʹ-n-alkylcyclohexyl)isothiocyanato-benzene (nCHBT) are simulated by using the density functional theory (DFT) method. Numerical results show that absorption of the tested nematic liquid crystals (NLCs) varies up to 0.04 in the broad range of 0.3–20.0 THz and takes maximum value in the range of 9–14 THz.In addition, the values of molecular polarisabilities (αo, αe) increase with the increase in the number of carbon atoms in the alkyl chain, while refractive indices (no, ne) decreases. The calculated birefringence (Δn) values for nCHBT are approximately twice as large as the birefringence value for nBT. It is crucial to examine these series of nematic liquid crystals at THz frequencies due to the role of soft matter in various switchable THz components.

Effect of Pyridine on the Mesophase of Teraryl Liquid Crystals: A New Series of Nematic Liquid Crystals Named 2-(4-Alkoxybiphen-4'-yl)-5-methylpyridines.

A new series of teraryl 2-(4-alkoxybiphen-4′-yl)-5-methylpyridines (nO-PPPyMe, n = 3–8) nematic liquid crystal compounds, bearing a biphenylene core and a picoline terminus, were synthesized using a short two-step reaction, and overall yields between 34% and 38% were obtained. Spectral analysis results were in accordance with the expected structures. The thermotropic behavior of the teraryl liquid crystal compounds was investigated through polarized optical microscopy and differential scanning calorimetry. All compounds exhibited a solely enantiotropic nematic phase at the medium–high temperature range of 162.4–234.2 °C. Furthermore, the results for the nO-PPPyMe series were analyzed relative to three other compound series, mO-PPPyCN (m = 2–8), iO-PPQMe (i = 3–8) and xO-PPyPMe (x = 1–10). Consequently, the effect of pyridine on the mesophase of teraryl liquid crystals was demonstrated.

含有2,3,5,6-四氟亚苯基的负性液晶合成及液晶性研究

Two kinds of fluorinated negative liquid crystal compounds were synthesized.Two series of nematic liquid crystals were measured by polarised optical microscopy with a heating stage and differential scanning calorimetry and their good mesomorphic properties were observed.For example,the negative liquid crystals introducing 1,4-perfluorophenylene to molecular structure of diphenyl acetylene have high birfringence and low rotational viscosity.The result shows that the liquid crystal compounds with ethylidene bridge could have low birfringence and low viscosity.The first type of compounds could be used in adaptive system like as application in construction of spatial light modulator;the second type of compounds could satisfy the requirements of a low viscosity component in TFT-LCD.Two target compounds were synthesized by sonogashira coupling reaction and then Pd/C catalytic hydrogenation.

Electroconvection with and without the Carr-Helfrich effect in a series of nematic liquid crystals.

Experimental electroconvection (EC) investigations on four homologs with negative dielectric anisotropy epsilona and conductivity anisotropies sigmaa of various signs (the relative sigma; a=sigmaa/sigma(perpendicular) ranges from -0.6 to +0.3 ) are reported. The homolog with positive conductivity anisotropy follows the predictions of the standard theoretical model, while the homolog with negative sigmaa shows a phenomenon, i.e., nonstandard electroconvection, not described by the Carr-Helfrich mechanism but manifesting in forms of stripes mainly parallel to the initial director, thus having a morphological similarity to the rolls of the standard EC. The remaining two homologs change the sign of their conductivity anisotropy with temperature that leads to a twofold character by crossing over from standard to nonstandard electroconvection. The patterns of both the standard and nonstandard EC have been characterized and compared.

Physical Studies of Homologous 4-Cyanophenyl Esters of 4′- n-Alkylcinnamic Acids

Abstract The mean polarizabilities of homologous series of nematic liquid crystals of 4-cyanophenyl esters of 4′- n-alkylcinnamic acids are evaluated by using the MLδP method. From these polarizabilities the mean diamagnetic susceptibilities of the liquid crystals are also calculated. The orientational order parameters of these compounds are also estimated using the Vuks and Neugebauer methods. The variation of the order parameter with temperature is discussed.

Polarizabilities and susceptibilities of nematic azoxybenzenes

The mean polarizabilities of homologous series of nematic liquid crystals of p,p' -di- n-alkyl azoxybenzenes are evaluated by using a modified Lippincott-δ-function potential model. 1 From these polarizabilities, the diamagnetic susceptibilities of the liquid crystals are calculated by using the method of Rao and Murthy. 2 The merits and limitations of the method are discussed.

Flow Alignment in Cyanobiphenyl Liquid Crystals

The cyanobiphenyl compounds 5CB (4-cyano-4'-n-pentyl-biphenyl) and the corresponding hexyl, 6CB, and heptyl, 7CB, form a homologous series of nematic liquid crystals. The flow of these liquid crystals in a torsional shear flow apparatus was studied. The shear induced alignment of the director field gives a corresponding induced birefringence, which is observed between crossed polarizers. At a given shear rate a nearly uniform flow alignment throughout the liquid crystal layer is obtained, except at two boundary layers where the director is changing rapidly in space, adjusting to the normal boundary conditions. The size of the boundary layers can be estimated by studying the way in which they decrease in thickness as the shear rate is increased. From the observed optical anisotropy we compute the flow alignment angle and the ratio of rotational viscosity coefficients for the three cyanobiphenyl liquid crystals as a function of temperature.

Physical studies of nematic azoxybenzenes. II. Refractive indices and the internal field

The ordinary refractive index and the birefringence are reported for two homologous series of nematic liquid crystals, the p,p′-di-n-alkyl and p,p′-di-n-alkoxy azoxybenzenes. It is found that the high-frequency dielectric anisotropy is proportional to the anisotropy of the magnetic susceptibility. For axially symmetric molecules this proportionality allows a simplification of the intricate problem of the internal field, which can be taken as independent of the anisotropy of the surroundings of a molecule. Representing a molecule by a homogeneously polarizable spheroid, an equation is derived which relates the anisotropic high-frequency dielectric permittivity to the anisotropic molecular polarizability, and which gives consistent results for both the solid state and the nematic phase.

The elastic constants of nematic liquid crystalline terminally substituted azoxybenzenes

The elastic constants K11, K22, and K33 of some members of two homologous series of nematic liquid crystals, the p,p′-di-n-alkyl and p,p′-di-n-alkoxy azoxybenzenes, are reported. Both K11 and K33 become greater with increasing alkyl chain length, while the ratio K33/K11 decreases along the series. For small alkyl chain length we find K33≳K11, for long chain length K33<K11, while for intermediate chain length K33?K11. In all cases K22 is the smallest of the three elastic constants, while K22/K11 is independent of temperature. The variation of K33/K11 with alkyl chain length can qualitatively be attributed to changes of correlation lengths parallel and perpendicular to the director. If the nematic distortion free energy is restricted to terms of the order S2, where S is the order parameter, theory predicts K33=K11. The experiments indicate that this equality holds approximately in some special cases, but in general higher order terms in S must be included to obtain a correct expression for the nematic distortion free energy.

Molecular Order and an Odd-Even Effect in an Homologous Series of Schiff-Base Nematic Liquid Crystals

Abstract The orientational order parameters of an homologous series Schiff-base nematic liquid crystals, p-alkoxybenzylidenebutylaniline Cn, H2n+1OC6H4CH=NC6H4C4H9 (n= 1 to 6), were obtained over a wide temperature range in 50-micron thick cells by means of optical anisotropy measurements. The ordering exhibits an odd-even alternation with respect to the number of carbon atoms n in the alkoxy CnH2n+1O− end group. These results provide the first experimental evidence of the odd-even molecular ordering effect near the transitions of an homologous series of nematic liquid crystals with non-equivalent end groups.

Molecular Ordering and Even-Odd Effect in a Homologous Series of Nematic Liquid Crystals

Order parameters have been measured in a homologous series of nematic liquid crystals, p-alkoxyazoxybenzenes, by /sup 13/C NMR. The ordering exhibits an even-odd alternation along the series, and a comparison of our data at the isotropic-nematic transition temperatures is made for the first time with a recent theory of Marcelja which explicitly accounts for end-chain interactions between the molecules. Some conclusions are drawn concerning molecular conformational motion and /sup 14/N spin relaxation in the nematic phase.

Dependence of birefringence threshold voltage on dielectric anisotropy in a nematic liquid crystal

The dielectric anisotropy Δε and threshold voltage Vth for electrically controlled birefringence have been measured for a series of nematic liquid crystals (binary mixtures of MBBA and PEBAB) with different negative Δε values. The threshold voltage varies as |Δε|−1/2, in agreement with theoretical predictions. The bend elastic constant K33 evaluated from these experiments is in order-of-magnitude agreement with recent measurements for pure MBBA.

Nematic Ordering of Rod-Like Molecules Interacting via Anisotropic Dispersion Forces as well as Rigid-Body Repulsions

A theory of nematic ordering in liquid crystals has been developed. The method of symmetry-breaking potential and a first-order cluster expansion technique for the partition function are employed. The obtained results agree with those of the Onsager-Isihara (for hard rods) and of the Meiar-Saupe (for dispersion forces) theories in respective limiting cases. The nematic order parameter is calculated as a function of temperature and density of liquids; the calculated values agree very well with the experimental ones on nematic para-azoxyanisole for the pressure range from 1 to 640 ber. The nematic-isotropic transition temperature T c is calculated as a function of the density and shape of molecules (length, breadth and their ratio). The theory provides good explanations, at least qualitatively, of behavior of T c for various homologous series of nematic liquid crystals.

Liquid Crystal Purification by Zone Refining

Abstract A series of nematic liquid crystals which had been solvent recrystallized was further purified by use of the technique of zone refining. In all cases purity increased. Purity was determined using differential scanning calorimetry. Purity differences when the zone refiner was used at mesophase temperatures and isotropic liquid temperatures were studied. A comparison of the results from this study with those of other investigators is given, and the implications concerning thermodynamic measurements on liquid crystals are discussed.

Nuclear magnetic relaxation in an homologous series of nematic liquid crystals

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTNuclear magnetic relaxation in an homologous series of nematic liquid crystalsC. R. Dybowski, B. A. Smith, and C. G. WadeCite this: J. Phys. Chem. 1971, 75, 25, 3834–3836Publication Date (Print):December 1, 1971Publication History Published online1 May 2002Published inissue 1 December 1971https://doi.org/10.1021/j100694a009RIGHTS & PERMISSIONSArticle Views25Altmetric-Citations19LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (327 KB) Get e-Alerts