S. Urban

X‐ray studies on the crystalline E phase of the 4‐n‐alkyl‐4′‐isothiocyanatobiphenyl homologous series (nBT, n = 2–10)

X‐ray diffraction measurements were carried out on nine members of the isothiocyanatobiphenyl homologous series (nBT) in their smectic E (SmE) phase, with the aim of estimating the layer thickness and deriving orthorhombic unit cell parameters for the SmE phase. The data obtained are discussed in relation to molecular reorientation motion around the short axis in that phase.

X‐ray studies of the layer thickness in smectic phases

X‐ray investigations of nine smectogenic substances exhibiting the smectic Ad, A1 and crystalline E phases were performed at various temperatures. X‐ray patterns yielded the layer thickness d (Ad, A1 phases) and orthorhombic unit cell parameters (E phase). The layer thickness of the Ad phase in 4′‐n‐alkyl‐4‐cyanobiphenyls (nCBs) has different temperature coefficients for shorter (n = 8–10) and longer (n = 12–14) members, which is explained as resulting from two competing effects: a weakening with temperature of the intermolecular association energy that favours an increase in d, and the increasing number of conformers which reduces the molecular length. A small anisotropy of the thermal expansivity in the smectic phases was found by comparing the linear quantity d(T) with the linearized bulk characteristic of the system, V −3(T), where V = 1/ρ is the specific volume, ρ is the density. Differences between the slopes of the two quantities are less in the case of the A1 phase of two nDBTs (5‐n‐alkyl‐2‐(4′‐isothiocyanatophenyl)‐1,3‐dioxanes). The present X‐ray data and recent results of studies of the low frequency relaxation process in these compounds (under atmospheric as well as elevated pressures) give a consistent picture of molecular reorientations around the short axes in the smectic phases.

Comparison of the Dielectric Relaxation Data for the Isotropic and Nematic Phases of Three Homologous Series: nCb, nOCB and nPCH, (n =5÷8)

Abstract Results of dielectric studies of three homologous series: n-alkyl-cyanobiphenyls - nCB, n-alkyloxy-cyanobiphenyls - nOCB, and trans-4-(4′-n-alkyl-cyclohexyl)-cyanobenzene - nPCH, with n from 5 to 8, are presented. The parameters characterizing the rotational motions of the molecules around their short axes in the nematic and isotropic phases were derived (dielectric increments, relaxation times, activation enthalpies, retardation factors, nematic potentials, order parameters). Comparative analyses of the parameters in respect to the lengths of the terminal groups and the structures of the molecular cores are given.

Thermodynamic scaling and the characteristic relaxation time at the phase transition of liquid crystals.

The longitudinal relaxation time tau of a series of alkyl-isothiocyanato-biphenyls (nBT) liquid crystals in the smectic E phase was measured as a function of temperature T and pressure P using dielectric spectroscopy. This relaxation time was found to become essentially constant, independent of T and P, at both the clearing point and the lower temperature crystalline transition. tau(T,P) could also be superposed as a function of the product TV(gamma), where V is the specific volume and gamma is a material constant. It then follows from the invariance of the relaxation time at the transition that the exponent gamma superposing tau(T,V) can be identified with the thermodynamic ratio Gamma=- partial differential log(T(c)) partial differential log(V(c)), where the subscript c denotes the value at the phase transition. Analysis of literature data on other liquid crystals shows that they likewise exhibit a constant tau at their phase transitions. Thus, there is a surprising relationship between the thermodynamic conditions defining the stability limits of a liquid crystalline phase and the dynamic properties reflected in the magnitude of the longitudinal relaxation time.

The synthesis and properties of fluoro-substituted analogues of 4-butyl-4′-[(4-butylphenyl)ethynyl]biphenyls

The synthesis of 4-butyl-4′-[(4-butyl-2,6-difluorophenyl)ethynyl]biphenyl and its higher fluorinated analogues is presented and discussed. Correlations between molecular structure and mesomorphic properties for presented compounds as well as other known from the literature analogues have been drawn. The dielectric study of four synthesised compounds and their mixtures are presented and discussed. Trifluoro-substituted analogues are trade off between low dielectric anisotropy of difluorinated compounds and lower clearing points of tetrafluorinated ones.

Dielectric properties of ten three-ring LC fluorosubstituted isothiocyanates with different mesogenic cores

Results of the static and dynamic dielectric investigations of 10 liquid crystalline compounds consisting of three rings (phenyl or cyclohexyl) with the n-propyl and isothiocyanato terminals, fluoro lateral substitutes, and different bridging groups (CH2CH2 and/or COO) are presented. These compounds form the nematic phase in a broad temperature range. The dielectric parameters are analyzed in relation to the chemical and dipole structures of the molecules. The influence of different structural elements such as rings, bridging groups and fluorosubstitution on the dielectric properties is discussed. The relaxation time and activation enthalpy characterizing the molecular rotations around the short axes were determined. In the nematic phase, the compounds having two fluorine atoms at the lateral positions exhibit a crossover of the principal permittivity components within the megahertz frequency range. This makes them interesting from the application point of view as the so-called dual frequency materials.

Mesomorphic, dielectric, and optical properties of fluorosubstituted biphenyls, terphenyls, and quaterphenyls

Compounds with moderate and large negative dielectric anisotropy (Δɛ) are very attractive liquid crystal (LC) for vertical alignment mode (VA). Materials with such properties can be achieved by lateral substitution of a polar group into a mesogenic molecule. We synthesized some new LC materials with a negative value of Δɛ, a moderately high birefringence (Δn), and a low viscosity. The mesomorphic and physical behaviour of the novel biphenyls, terphenyls and quaterphenyls fluorosubstituted in the rigid core and also with fluorinated alkyl and alkoxy chains are investigated. The prepared series of four LC compounds are promising for new LC mixtures for various applications. Examples of nematic mixtures with Δɛ∼−3.25 will be presented.

Orientational Order of Difluorinated Liquid Crystals : A Comparative 13C-NMR, Optical, and Dielectric Study in Nematic and Smectic B Phases

Structural and orientational order properties of 3Cy2CyBF2 and of 5CyCy2BF2 have been investigated by means of (13)C-NMR, optical, and dielectric spectroscopy methods. In the case of NMR, order parameters have been independently obtained from the analysis of either (13)C-(19)F dipolar couplings or (13)C chemical shift anisotropies, both measured from (13)C-{(1)H} NMR static spectra. The assignment of the (13)C resonances has been carried out thanks to the comparison with solution state spectra and DFT calculations, and the relevant geometrical parameters and (13)C chemical shift tensors needed to derive orientational order parameters have been calculated by DFT methods. In the analysis of (13)C-(19)F dipolar couplings, empirical corrections for vibrations and anisotropic scalar couplings have been included. Dielectric measurements have been performed over a broad frequency range for two orientations of the nematic director with respect to the measuring field. At low frequencies (static case) a positive dielectric anisotropy has been determined, which has enabled the calculation of the order parameters according to a well-tested procedure. At high frequencies the dielectric anisotropy changes its sign, a property which can be useful in designing a dual addressing display. The nematic order parameter determined from optical, dielectric, and NMR methods have been compared: their trends with temperature are very similar, apart from some slight shifts, and were analyzed by Haller and Chirtoc models. The differences among the results obtained by the four methods have been discussed in detail, also with reference to the assumptions and approximations used in each case, and to the results recently reported for similar fluorinated nematogens. The presence of a non-negligible order biaxiality has been related to the presence of a CH2CH2 bridging group, linking one cyclohexylic unit with either the other cyclohexyl or the phenyl ring.

PVT measurements on n -octyl-isothiocyanato-biphenyl (8BT) at elevated pressures

For the first time pressure-volume-temperature (PVT) measurements for the crystal-like smectic E phase have been performed. The phase diagram of 4′-n -octyl-4-isothiocyanatobiphenyl 8BT has been recently established using differential thermal analysis up to 250 MPa. 8BT exhibits a splitting of the clearing line above 170 MPa. PVT data have been measured in the same pressure range for temperatures between 313 and 393 K. Volume and enthalpy changes accompanying the clearing line of 8BT are also presented. The configurational part of the entropy change at the CrE-I transition of 8BT amounts to ≈ 60%. Using the PVT data and recently published dielectric relaxation results, the isochoric activation energy was calculated (giving ≈ 50% of the activation enthalpy); this is compared with analogous results for other liquid crystals.

Terahertz and optical properties of nematic mixtures composed of liquid crystal isothiocyanates, fluorides and cyanides

Several simple nematic eutectic liquid crystal mixtures composed of different two and three ring isothiocyanato compounds as well as mixtures consisting of analogous cyano and fluoro compounds were formulated. The optical properties of the thereby obtained mixtures, i.e. refractive indices, birefringence and absorption coefficients αe and αo are measured in the frequency range between 0.5 and 2.5 THz and in the visible range at 589 nm. Our results show that two mixtures can be identified, which seems particularly promising for applications in the terahertz (THz) range: those consisting only of isothiocyanates show the highest birefringence at THz frequencies while those consisting of isothiocyanates and cyanides have the smallest dichroic ratio over a broad THz range. We also found that in most cases, the absorption coefficients of the isothiocyanato compounds fulfill the relationship αe > αo while for cyano and fluoro compounds, αo > αe is observed as a dominating trend. Furthermore, we show that by carefully selecting constituents of both isothiocyanato and cyano compounds, we can achieve a mixture for which the values of αo are in the same order as for αe for THz frequencies below 2 THz.