[en] Resonant x-ray diffraction was carried out at the Se K edge in thick free-standing films of a selenophene liquid crystalline material, revealing detail of the structure of the ferro-, ferri-, and antiferroelectric phases. The ferrielectric phase was shown to have a three-layer superlattice. Moreover, the structure of a lower temperature hexatic phase was established. For the antiferroelectric phase, investigations were also carried out in a planar device configuration. The device allowed resonant scattering experiments to be carried out with and without the application of an electric field and resonant data are compared with electro-optic measurements carried out on the same device. (c) 2000 American Institute of Physics

No abstract available

No abstract available

[en] We apply an anisotropic version of the polymer reference interaction site model (PRISM) integral equation description of flexible polymers to analyze athermal liquid crystallinity. The polymers are characterized by a statistical segment length, σ_o, and by a physical hard-core thickness, d, that prevents the overlap of monomers on different chains. At small segment densities, ρ, the microscopic length scale d is irrelevant (as it must be in the universal semidilute regime), but becomes important in concentrated solutions and melts. Under the influence of the excluded volume interactions alone, the chains undergo a lyotropic, first-order isotropic-nematic transition at a concentration dependent upon the dimensionless ''aspect ratio,'' σ_o/d. The transition becomes weaker as d→0, becoming second order, as has been previously shown. We extend the theory to describe the transition of rigid, thin rods, and discuss the evolution of the anisotropic liquid structure in the ordered phase. (c) 2000 American Institute of Physics

[en] We report polarization-analyzed, resonant x-ray diffraction at the sulfur K edge performed upon free-standing liquid-crystal films. Our studies of the thiobenzoate liquid-crystal enantiomer 10OTBBB1M7 yield the polarization states of resonant satellite peaks arising from characteristic superlattices in the chiral smectic-C (SmC^*) variant phases, including the antiferroelectric SmC_A^*, ferrielectric SmC_FI1^* and SmC_FI2^*, as well as SmC_α^*. The observed polarizations agree with the clock model of chiral smectic-C variants, and rule out other proposals made to date for these structures. Data from the 10OTBBB1M7 racemate also support the clock model. Our resonant diffraction results from a thiophene liquid-crystal compound reveal the same superlattice periodicities seen in corresponding antiferroelectric and ferrielectric phases of 10OTBBB1M7. (c) 1999 The American Physical Society

[en] We investigate the structure and thermodynamics of anisotropic polymer fluids, focusing on the nematic phases of flexible polymers. The chains interact only through monomer-monomer excluded-volume interactions. As a function of an externally provided alignment along a fixed nematic director, we calculate the anisotropic pair correlation function, and demonstrate the existence of two density correlation lengths, ξ_{(perpendicularsign)} and ξ_z, controlling transverse and longitudinal density fluctuations, respectively. We allow the possibility that the chains align either along the director (nematic conformations), or are anti-aligned in a ''discotic-like'' configuration. The cohesive contribution to the free energy is established in a high-temperature approximation, and its sensitivity to the orientation of the chains is probed. Our approach is not limited to homogeneous liquid crystalline phases, but applies in any circumstance when the orientation of otherwise disordered polymers is the physically controlling effect, e.g., confinement in thin films or pores, shear-alignment of flexible polymers, or straining a cross-linked rubber network. (c) 2000 American Institute of Physics

[en] Recent experiments have found that a mechanically distorted blue phase can exhibit a primary linear electro-optic (Pockels) effect [F. Castles et al., Nat. Mater. 13, 817 (2014)]. Here, it is shown that flexoelectricity can account for the experimental results and a model, which is based on continuum theory but takes into account the sub-unit-cell structure, is proposed. The model provides a quantitative description of the effect accurate to the nearest order of magnitude and predicts that the Pockels coefficient(s) in an optimally distorted blue phase may be two orders of magnitude larger than in lithium niobate

No abstract available

[en] The electric properties of cholesteryl laurate and cholesteryl caprilate mixtures (75:25%, 50:50%, 25:75% by weight) are investigated. Activation energies for conduction were determined from the Arrhenius plots ln I f(1/T), as well as temperature intervals characteristic for different mesophases. It was shown that these mixtures exhibit a storage effect associated to a negative resistance in a certain temperature range. (author)

No abstract available