Twist Elastic Constant Research Articles

Properties of the liquid crystals of some biopolymers.

Nematic liquid-crystalline solutions of polypeptides such as PBLG and PELG orient in the direction of a very low static electric field as if the polypeptide molecules were a collection of rod-like molecular clusters having a huge, permanent electric dipole moment. This dipole moment originates from fluctuation of the distribution of polymer helices in both directions of the rod-like molecular cluster with roughly equal likelihood. The molecular cluster of PBLG is estimated to be composed of some 5 x 10(5) molecules and has dimensions of a few to several micrometers x 0.1 microns phi in methylene bromide when the degree of polymerization (mean) is 650. The nematic solutions also orient in the direction of a static magnetic field. The peptide groups of the polypeptide form planes which are parallel to the polymer helix and the magnetic-field orientation is due to the demagnetizing field produced in the peptide groups perpendicular to their planes by the ring current of the pi electrons. The formation of rod-like molecular clusters is absolutely necessary to invite the magnetic-field orientation as in the electric-field orientation. The anisotropy of the magnetic susceptibility has been estimated to be on the order of 10(-8) emu/g, and the twist elastic constant of the cholesteric structure on the order of 10(-7) dyn/cm. Derivatives of PBLG with long alkyl chains as pendant groups assume thermotropic liquid crystals, in which melted alkyl chains behave as a solvent in a similar manner as in lyotropic liquid crystals of the parent PBLG. An analogy of thermotropic liquid crystals of the modified polypeptides with those of discotic liquid crystals in the columnar phase has been suggested. Concentrated solutions of a double-stranded helix of poly(A).poly(U) and a triple-stranded helix of poly(A).2poly(U) form liquid crystals both of the nematic and the cholesteric types. The nematic solutions of these polyribonucleotide complexes orient perpendicular to the direction of a static magnetic field owing to the diamagnetic dipole moments induced in the purine or pyrimidine bases of the polyribonucleotides perpendicular to their planes, which are roughly perpendicular to the polymer helices. Any rod-like polymer has a good chance to form liquid crystals in which the occurrence of forces acting among the molecules is not necessary. The PLGA-Na in a neutral aqueous solution undergoes a "coil-to-alpha helix transition" when the polymer concentration is increased.(ABSTRACT TRUNCATED AT 400 WORDS)

Synthesis, Physical Properties and Structural Relationships of New, End-Chain Substituted Nematic Liquid Crystals

Abstract New nematic liquid crystalline phenylcyclohexanes with (i) a terminal side-chain vinylic group some distance from the rigid core and (ii) both polar and non-polar terminal side-chain groups, were synthesized. Investigations of their elastic, birefringence, dielectric and rotational viscous properties have shown several new odd-even effects. These are attributed to side-chain Simultaneous odd-even effects were found for δ, δn, k 33/k11 and K = k 11 + (k 33 - 2k 22)/4. Furthermore, unusually low twist elastic constants, k 22 were obtained for polar 6-alkenyls as well as for compounds exhibiting groups with permanent dipole moments in even side-chain positions.

The Twist Elastic Constant and Anchoring Energy of the Nematic Liquid Crystal 4-N-Octyl-4-Cyanobiphenyl

Abstract The twist elastic constant of the nematic liquid crystal 4-n-octyl-4′-cyano-biphenyl (8CB) and the azimuthal anchoring energy at the SiO-nematic interface have been measured by using the torsion pendulum technique. The twist elastic constant of 8CB is found to be systematically larger than that measured by the Freedericksz transition technique. The azimuthal anchoring energy is found to decrease rapidly as the nematic-isotropic transition temperature is approached. This behaviour is analogous to that already reported by us for the nematogen 5CB and can be interpreted by extending the Berreman model of the anchoring energy at a grooved interface.

Critical behaviour of a lattice model of the nematic to smectic-A transition

The critical behaviour of a lattice version of the de Gennes model of the nematic to smectic-A transition is studied by Monte Carlo simulations. Several features observed in experiments are qualitatively reproduced in the simulations. The model exhibits a continuous phase transition with the specific heat exponent α close to zero. The order-parameter correlation functions show anisotropic critical behaviour as the transition is approached form the nematic side. On the other hand, the observed behaviour of the correlation functions in the superconducting gauge strongly indicates an isotropic critical point for both small and large values of the splay elastic constant. The observed behaviour of the twist and bend elastic constants is consistent with this conclusion. The gauge-dependence of the correlation functions appears to be well-described by the decoupling approximation proposed by Lubensky and co-workers Presentation d'une version sur reseau du modele de Gennes pour la transition citee. Son comportement critique est etudie par des simulations suivant la methode de Monte Carlo

Periodic Fréedericksz transition for nematic-liquid-crystal cells with weak anchoring.

Lonberg and Meyer [Phys. Rev. Lett. 55, 718 (1985)] have recently discovered in polymer nematics the existence of a critical value rc=0.303 for the ratio r=K2/K1 between the twist and splay Frank elastic constants, below which a periodic splay-twist distortion is energetically favored with respect to the well-known aperiodic splay distortion which appears in a Freedericksz transition. In this paper the effect of weak-anchoring conditions on the critical r value and on the other parameters of interest is theoretically investigated. Analytical expressions between these parameters are given. It is shown that the periodic transition is not allowed in the range 0.5<r<2. Outside this range either the periodic or aperiodic transition appears, according to the surface treatment, the sample thickness, and the direction of the magnetic field. Hence the critical value of r can be controlled in a wide range

Freeze-Fracture Imaging of Ordered Phases of Tobacco Mosaic Virus in Water

Abstract Aqueous solutions of Tobacco Mosaic Virus are ideal systems for visualizing “molecular” distributions in nematic and crystalline phases by freeze-fracture electron microscopy. Nematic phases of TMV in water are highly oriented with measured order parameters of 0.93. Twist deformations appear to be most common in the micrographs, confirming experimental and theoretical evidence that the twist elastic constant is smaller than either the splay or bend constants. Both edge and screw disclinations are observed in TMV nematics at molecular resolution, and their overall configurations correspond closely to those predicted by continuum theory. Disclination cores have been visualized for the first time. The edge disclination core is small, no more than a single virus length wide. The virus reorient abruptly by 90° at the core, but appear to remain in the plane of the disclination line. The screw disclination core is several virus lengths in diameter and much more disordered. The virus twist out of the pla...

Molecular imaging of tobacco mosaic virus lyotropic nematic phases.

Aqueous solutions of tobacco mosaic virus are ideal systems for visualizing ``molecular'' distributions in the nematic phase by freeze-fracture electron microscopy. The order parameter in well aligned regions is measured to be greater than 0.9. Twist deformations appear to be most common, confirming that the twist elastic constant is smaller than either the splay or bend constants. Edge and screw disclinations are observed in tobacco mosaic virus nematics in configurations predicted by continuum theory. Disclination cores have been observed for the first time.

New ground state for the Splay-Fréedericksz transition in a polymer nematic liquid crystal.

We have discovered a new form of Fr\'eedericksz transition in which uniform splay distortion is replaced by a complex periodic twist-splay pattern having a lower critical field than that for uniform splay. Experimental evidence is presented along with an exact calculation of both the critical field and the initial form of the distortion as a function of the ratio of the splay and twist elastic constants.

Simultaneous magnetic-deformation and light-scattering study of bend and twist elastic-constant divergence at the nematic-smectic-A phase transition.

Ces mesures indiquent que le point critique nematique-smectique A est faiblement anisotrope, en desaccord avec les theories existantes

Determination of Twist Elastic and Viscotic Constants using Electric Field Dynamic Light Scattering

ABSTRACTWe report the first use, over an extended temperature range, of dynamic light scattering in the presence of electric fields as a means of determining the elastic and viscotic constants of a nematic liquid crystal. We have studied 4-cyano-4′-pentyl-biphenyl (5CB) over its entire nematic range and give values of the twist elastic constant (k22) and the twist viscosity (γ1) as a function of temperature. The data are compared with existing literature values and we conclude that the method is accurate and reliable.

Mixtures of Side Chain Polymeric and Monomeric Liquid Crystals: 2—Twist Elastic and Viscotic Constants

Dynamic light scattering has been used to measure the visco-elastic ratio k22/γ1 in a series of polymeric- monomeric nematic liquid crystal solutions. Solutions of a side chain polysiloxane copolymer, dissolved in pentyl cyanobiphenyl as the monomeric solvent, were studied up to a maximum concentration of 20% w/w. The results show that k22/γ1 decreases rapidly with increasing concentration. By applying an electric field during the measurements, it has been possible to study the behaviour of k22 and γ1 independently and show that most of the change in k22/γ1 can be attributed to γ1.

A new torsion pendulum technique to measure the twist elastic constant of liquid crystals

A new experimental method to measure the twist elastic constant of nematic liquid crystals is proposed. The method is based on a direct measurement of the torque which the nematic liquid crystal exerts on the surfaces when a magnetic field is applied. The reliability of this technique is tested by measuring the twist elastic constant of the nematic liquid crystal 4-n-pentyl-4'-cyanobiphenyl (5CB). The experimental results are compared with previous measurements performed by other authors Technique basee sur une mesure directe du couple exerce par le directeur sur les surfaces quand le nematique est soumis a un champ magnetique. Application a la mesure de K 22 du pentyl-4 cyano-4' biphenyle

Physical properties of nematic mixtures. II. Polar–nonpolar systems

The dielectric, diamagnetic, and elastic properties as well as the nematic–isotropic phase transition temperatures of terminal polar–nonpolar nematic mixtures have been studied. The properties of the mixtures are compared to those of their components. Deviations from the ideal behavior or the additivity rule have been observed and are partially attributed to the reduced degree of association of the polar compounds in these mixtures and to additional interactions between the polar and nonpolar molecules. Packing effects can also influence the physical properties of the mixtures. The bend elastic constant is more sensitive to these effects than the twist and splay elastic constants. Clearing points obtained by extrapolation are shown to strongly deviate (up to 40 °C) from the experimental values.

Electro—Optic and Dielectric Studies in Cholesteric Liquid Crystal Mixtures

Abstract In binary mixtures of cholesteryl chloride and cholesteryl alkanoates, the threshold field for the cholesteric to nematic transformation depends on the length of the alkanoate ester chain. The threshold field also depends on the pitch, the dielectric anisotropy and the twist elastic constant of the mixtures. This paper describes the experiments performed to establish the role of these three parameters in the observed variation of threshold field with alkanoate ester chain length in such binary mixtures. The temperature dependence of threshold field, dielectric anisotropy and pitch has been studied. From these experiments the relative response of the various mixtures to applied electric fields has been established. In most of the mixtures, temperature ranges over which pretransitional effects do not predominate, have been identified. Twist elastic constants of the mixtures in these temperature ranges have been calculated. It has been shown that both the intrinsic response of the mixtures to applie...

The determination of the elastic constants of a series of n-alkylcyanobiphenyls by anisotropy of turbidity

Measurement of the anisotropy of turbidity of aligned nematic liquid crystals in three selected geometries can yield the splay, twist, and bend elastic constants (K11, K22, and K33, respectively) which, along with the viscosity coefficients of the medium, govern director fluctuations that lead to a strong light scattering. We have utilized this procedure to determine the temperature dependence of the elastic constants of a homologous series of n-alkycyanobiphenyls (n-CB, where n=5, 6, 7, 8) in the nematic range. K33≳K11≳K22 at all temperatures for each member of the series. The absolute values of the moduli are consistently higher than those obtained by magneto- and electro-optic distortion methods (Frederik’s transitions). Odd–even effects are displayed as the number of methylene carbons in the alkyl chain increases. Distinctly larger K33 values for 8CB may be attributed to the short range smectic ordering in this liquid crystal, which has a lower temperature smectic-A phase.

On the Measurement of Elastic Constants in Nematic Liquid Crystals: Comparison of Different Methods

Abstract Different methods of measuring elastic constants of nematic liquid crystals by deformation of homogeneously and homeotropically oriented layers through external magnetic and electric fields, respectively, are critically examined. Possible sources of uncertainties are quantitatively explored. An approximative numerical fitting procedure is proposed allowing to take conductivity effects in electrically deformed layers up to large conductivities and/or temperatures into account. A novel, accurate and simple monitoring method to determine twist elastic constants is described. The measurements of the temperature dependence of the splay bend and twist elastic constants of the positive dielectric mixture R0-TN-403 are presented.

Elasticity and Orientational Order in Sometrans-p-n-Alkoxy-α-methyl Cyanophenyl Cinnamates

Abstract The splay (k 11), twist (k 22) and bend (k 33) elastic constants determined by the Freedericksz method and the orientational order parameters (s) derived from optical measurements in the nematic phase of six homologues of trans-p-n-alkoxy-α-methyl cyanophenyl cinnamates (n OMCPC) are reported. The data close to the nematic-isotropic transition point (T NI) are compared with T NI and the heats of transition (ΔH). The temperature-variation of elastic constants is discussed in terms of existing theories. The pretransitional increase in the twist and bend constants near the nematic-smectic A transition point (T NA ) of 10 OMCPC has also been analysed.

Distorted twisted nematic liquid-crystal structures in zero field

Leslie’s differential equations describing the orientation of the optic axis through a twisted nematic layer are solved for the case where there is a nonzero tilt bias angle. Three classes of solutions are investigated: the simple solution of constant tilt angle through the layer with uniform twist, symmetric solutions where the tilt angle in the middle of the layer has an extreme value, and antisymmetric solutions where the tilt angle in the middle of the layer is identically zero. These solutions and their corresponding elastic deformation energies are compared for typical values of the splay, twist, and bend elastic constants of nematic liquid crystals. For 90° twisted nematic layers the antisymmetric solution always has a higher elastic energy than the symmetric solution. This result explains why regions of reverse twist are suppressed in twisted nematic display devices which have finite tilt bias angles at both boundaries. Reverse-twisted domains can also be avoided by adding chiral dopants to the nematic mixture and by making layers having total twist angles of less than 90°. The effectiveness of these three techniques is discussed in terms of a comparison of the elastic energy difference between the oppositely twisted configurations.

Physical properties of new liquid-crystal mixtures and electrooptical performance in twisted nematic displays

Complete sets of measurements are reported of the elastic, dielectric, optical, and viscous properties of new liquid-crystal mixtures for twisted nematic displays. Their optical and capacitive threshold voltages were shown to be identical and were used to determine the twist elastic constants k 22 . The splay (k 11 ) and bend (k 33 ) elastic constants were measured using a novel phase sensitive capacitance bridge, The new mixtures consist of nematic and isotropic components belonging to different chemical classes. Large mesomorphic temperature ranges (T c > 100° C), fast electrooptical response, 1.5-V operation, and 3-V 1:3 multiplexability could be achieved with different eutectic mixtures. Good agreement was found between material properties of physically and chemically different mixtures and their static and dynamic electrooptical performance in twisted nematic displays.

Elasticity and Orientational Order in Some 4′-n-Alkyl-4-Cyanobiphenyls: Part II

Abstract The splay and twist elastic constants (k 11 and k 22) of pentyl, hexyl, heptyl and octyl cyanobiphenyls (5CB, 6CB, 7CB and 8CB respectively) have been determined by the Freedericksz transition method. The results are compared with the bend elastic constants (k 33) and order parameters (S) of these compounds reported by us in an ealier paper, as also with the heats of transition (δH) determined using a differential scanning calorimeter and the temperatures of transitions (T NI). T NI, and k 33 near T NI, exhibit alternation for all the four compounds. On the other hand, S, δH, and k 11 and k 22, near T NI exhibit alternation only for the first three compounds and increase between 7CB and 8CB. It is concluded that (ζ⊥/ζ∥), the ratio of the transverse to the longitudinal correlation lengths of the smectic A type cybotactic order increases between 7CB and 8CB.