Joanna Czub

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.

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.

Dielectric studies of tolane derivatives exhibiting the E and K phases

Results of dielectric studies for three tolane derivatives containing the isothiocyanato (NCS) and alkyl or alkoxy groups in the terminal positions are presented. A relaxation process associated with molecular rotations around the short axes has been observed in the crystal‐like smectic phases (B, E, K). The K phase was characterized for the first time by dielectric spectroscopy. The results are discussed by taking into account the dielectric relaxation data obtained recently for the biphenyl analogues in the E phase. The available X‐ray data allow us to calculate the so‐called packing parameter which characterizes the free volume of the molecules in the unit cells.

Mesomorphic and dielectric properties of esters useful for formulation of nematic mixtures for dual frequency addressing system

Mesomorphic and dielectric properties of three homologous series of two and three ring fluorosubstituted esters are described. They are 4-cyano-3-fluorophenyl 4-alkyloxy-2-fluorobenzoates, 4-cyano-3-fluorophenyl 4-(4-alkylbenzoyloxy)-2-fluorobenzoates and 3-fluoro-4-cyanophenyl, or 3-fluoro-4-isothiocyanatophenyl or 3,4-difluorophenyl 4′-alkyl-2,3-difluoro-biphenyl-4-carboxylates. The compounds exhibit the nematic mesophase accompanied by the smectic A or smectic C mesophase in some cases. Most of them show strong dependence of the dielectric anisotropy Δε upon frequency, at low frequencies Δε reaches a value ∼200, while at high frequencies a small negative dielectric anisotropy appears.

High pressure studies of the static permittivity tensor components in the nematic phase of 6CB

The dielectric permittivity tensor components, εII and ε⊥, in the nematic phase of 6CB (4‐n‐hexyl‐4′‐cyanobiphenyl) were measured in the pressure range 0.1–130 MPa and the temperature range 12–58°C. The dielectric anisotropy, Δε(p, V, T) = εII ‐ ε⊥, was analysed in isothermal, isobaric and isochoric conditions taking into account the pVT data and the well known Maier and Meier equation. On that basis the nematic order parameter S(p, V, T) was determined. This was used to calculate the parameter Γ relating the interaction potential with the volume (density). Its value Γ = 4.1 agrees very well with other estimates.

Pressure–temperature phase diagrams for four higher members (nonyl–dodecyl) of the homologous series of 4′-alkyl-4-isothiocyanato-biphenyl ( n BT)

Differential thermal analysis (DTA) method was applied to study the pressure–temperature phase diagrams for four members of the 4′-alkyl-4-isothiocyanato-biphenyl (nBT) homologous series (n = 9–12). These compounds exhibit a smectic E polymorphism enriched by a smectic A phase for two longest members. Measurements were performed in the temperature range 25–110°C and the pressure range up to 165 MPa. For all substances studied the pressure-induced phases were established with one or two triple points in the examined pressure range. In addition, for the 11BT and the 12BT the phases existing under atmospheric pressure disappeared under relatively low pressures. The results are compared with recently studied smectogenic n-alkyl-cyanobiphenyl (nCB) compounds.

Dynamics of 4,4'-di-n-heptylazoxybenzene (HAB) studied using dielectric and 2H NMR relaxation measurements

Results of studies of 4,4′-di-n-heptylazoxybenzene (HAB) in the isotropic, nematic and smectic A phases are presented. Two experimental methods were employed: broad band dielectric spectroscopy and nuclear magnetic resonance spectroscopy. The complex dielectric permittivity, ϵ*(ω)=ϵ′(ω)−iϵ″(ω), was measured in the frequency range 1 kHz–4 GHz. This allowed two main relaxation processes to be separated in all the phases studied: the low frequency (l. f.) process connected with molecular reorientations around the short axes, and the high frequency (h. f.) process connected with the rotations around the long axes. The corresponding relaxation times and activation enthalpies were obtained. The l. f. relaxation time changes step-wise at the phase transitions, whereas the h. f. relaxation time passes smoothly through all the phases. The measurement of 2H spin-lattice relaxation times was carried out throughout the mesophase range at 61.38 MHz. These data were analysed together with the relaxation times measured at 10.00 and 46.04 MHz, available from previous studies. Using suitable theoretical models the principal components of the diffusional tensor, D ‖ and D ⊥, as well as the diffusion coefficients D R relative to the internal rotation of the phenyl rings, were determined. The results of both studies are compared and discussed.