W. H. De Jeu

The Determination of the Elastic Constants of Nematic Liquid Crystals

Abstract The elastic constants K11 and K33 are calculated from the threshold field He associated with the Frederiks transition in a magnetic field of a uniform planar and a horneotropic layer, respectively. The elastic constant K22 is determined indirectly using a twisted planar layer.The errors associated with the determination of Hc are critically evaluated. In particular the anchoring conditions at the boundaries are found to have an important influence on Hc. Results for all three elastic constants of PAA and MBBA are given. In general our values for K11 and K33 are substantially higher than those reported in the literature.

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 disto...

Physical studies of nematic azoxybenzenes. I. Magnetic susceptibilities and the order parameter

The anisotropy of the magnetic susceptibility and the density of the series of nematic p,p′‐di‐n‐alkyl and p,p′‐di‐n‐alkoxy azoxybenzenes are reported as a function of temperature. Assuming that the molecules are effectively axially symmetric the anisotropy of the susceptibility is a direct measure of the nematic order parameter S. The results for S agree reasonably well with the predictions from Maier and Saupe’s theory of the nematic phase. With increasing chain length both the order parameter and the relative volume change at the nematic–isotropic transition alternate parallel to the nematic–isotropic transition temperature and the transition entropy.

Nematic Phenyl Benzoates in Electric Fields: I. Static and Dynamic Properties of the Dielectric Permittivity

Abstract A discussion is given of the dielectric properties of some nematic phenyl benzoates with the structure For n = 2 the dielectric anisotropy Δ∊=∊‖– ∊⊥ is found to be positive. For n = 1, however, the sign depends on the para substituents In particular, replacement of an alkyl group by an alkoxy group shifts Δ∊ towards negative values. The frequency dependence of ∊‖ shows a relaxation at relatively low frequencies (kliz range), while ∊⊥ remains constant. The activation energy for this relaxation is 0.4 till 0.5 eV for all the compounds studied. In case of a positive static Δ∊ the relaxation leads to a change of sign of Δ∊ which is of interest for the study of electrohydrodynamic instabilities.

Influence of smectic order on the static dielectric permittivity of liquid crystals

In some p, p′‐di‐n‐alkyl‐azoxybenzenes, an anomalous decrease of e‖ and increase of e⊥ is observed when the nematic phase is cooled to below the nematic/smectic‐A transition temperature. This is attributed to dipole‐dipole interaction, which becomes especially important for neighboring molecules in the same smectic layer. The Kirkwood‐Frohlich theory for the dielectric properties of anisotropic polar liquids is used in order to account for these interactions.

Dependence of the clearing temperature on alkyl chain length in nematic homologous series

Abstract In homologous series of nematic compounds with relatively high clearing temperatures, these temperatures are known to decrease with increasing chain length. Recently, however, series with relatively low clearing temperatures have been synthetized in which these temperatures increase with increasing chain length. In both cases the clearing temperatures alternate within a series. A qualitative explanation is given for these facts. The discussion is based on an expression for the free energy in which both the attractive dispersion forces and the excluded volume affects are taken into account.

Molecular Structure and Nematic Liquid Crystalline Behaviour

Abstract In this review some experimental results on the variation of the nematic-isotropic transition temperature TN1 with molecular structure will be evaluated with the aid of expressions for TN1 from molecular-statistical theories. The effect of terminal substituents on TN1 and some typical trends in the values of TN1 for the members of homologous series can be understood from changes in the anisotropic dispersion forces as discussed in Maier and Saupes theory. The effects of some other structural changes in the molecules on TN1, as for example the variation of central groups, cannot be explained on this basis. Probably it is necessary to take into account as well the molecular repulsions due to the excluded volume.

X-ray diffraction of the smectic phases of N-(p-n-heptyloxybenzylidene)-p′-n-pentylaniline

Abstract Using X-ray diffraction the various smectic phases of N-( p - n -heptyloxybenzylidene)- p ′- n -pentylaniline can be classified (with increasing temperature) as S H , S B , S C and S A , respectively. The long molecular axes are probably tilted with respect to the normal to the layers in the S A , S C and S H phases, and perpendicular to the layers in the S B phase.

Distortion of a twisted nematic liquid crystal by a magnetic field

Abstract The critical magnetic field H c for distortion of a twisted nematic structure is measured as a function of the sample thickness d . Except for values of d H c · d is constant, in agreement with the theoretical predictions by Leslie.