All‐aromatic liquid crystal thermosets: New high‐performance materials for structural applications

Over the past two decades, there has been an increasing interest in all‐aromatic crosslinked liquid crystal (LC) systems for structural, electronics and coatings applications. In this paper we will give a brief overview of our work on oligomeric liquid crystals end‐capped with phenylethynyl reactive end‐groups. All reactive oligomers were synthesized using standard melt‐condensation techniques and the final products form homogeneous nematic melts over a wide temperature range (220–400?°C). The reactive LC oligomers could be cured at elevated temperatures (310–400?°C) to form true rigid‐rod nematic networks. As will be demonstrated, this oligomer approach allows us to design all‐aromatic nematic networks with a variety of favorable physical, mechanical and processing characteristics. Depending on the backbone chemistry, and end‐group concentration, we can access liquid crystal thermosets with Tg’s between 110–280?°C and storage moduli (E') of 2‐5 GPa. The oligomeric nature of our reactive LCs results in excellent melt processing characteristics (|η*| ? 1?Pa.s at 100?rad.s^?1) and allows for the processing of complex structures such as fiber‐reinforced composites. Based on our current results we are convinced that reactive all‐aromatic liquid crystals can be interesting alternatives over existing high‐performance polymers such as polyarylether ketones (PEEK and PEKK) polyphenylenesulfide (PPS) and polyetherimides (PEIs).     

Structure of a liquid crystal of 4-cyano-4′-n-octyloxybiphenyl

The structure of a liquid crystal of 4-cyano-4′-n-octyloxybiphenyl (C₂₁H₂₅NO) is determined by X-ray diffraction analysis. The compound crystallizes in the triclinic crystal system with unit cell parameters a = 7.322(1) Å, b = 12.693(3) Å, c = 20.393(2) Å, α = 92.45(1)°, β = 99.96(1)°, γ = 99.35(2)°, and space group $P\bar 1$ . The structure is solved by the direct method and refined to R = 0.057. Two independent molecules are located in the asymmetric unit. No short intermolecular contacts are observed in the crystal packing.     

Molecular and crystal structure of liquid crystalline p-octyloxyphenyl p′-pentyloxybenzoate

The crystal and molecular structure of p-octyloxyphenyl p′-pentyloxybenzoate C₅H₁₁-O-C₆H₄-C(O)-O-C₆H₄-O-C₈H₁₇, which forms a nematic mesophase upon melting, was determined by X-ray diffraction. There is one system of weak directional intermolecular C-H…π interactions responsible for the formation of the nematic phase in these crystals.     

Optical and dielectric properties of an isothiocyanato liquid crystal compound and comparison of its behavior with that of a homologous member

The properties of an isothiocyanato liquid crystal compound (code name S1) has been studied with a view to compare its behavior with that of another member of its homologous series (code name S5). Optical properties, such as refractive index, polarisability, and their anisotropies, have been evaluated with varying temperature and the orientational order experimentally determined from the study of polarisability anisotropy and compared with theoretically computed values. Dielectric studies have been conducted to determine not only the dielectric anisotropy but also the effective molecular dipole moment and its temperature dependence. The temperature variation of the angle of inclination (β) of molecular dipole moment with the director direction has also been determined and compared with that of S5. In order to determine the nature of molecular association in the mesophase, the molecular correlation factor (g) has been estimated from the measurements of dipole moments of compound in solution and compared with the values obtained for S5.     

Threshold switching in niobate glass — thick-film devices

Thick-film, niobate-glass threshold switches have been made in reducing gas atmospheres at high temperatures (1100–1400°C) using SiO₂ and B₂O₃ as glass formers. Their temperature characteristics are far superior to those of similar vanadate switches previously reported [1,2], and they are stable at low ON currents (< mA) if they are protected from atmospheric oxidation. They do not require forming.The process consists in pre-firing the printed, powdered glass in air and then reducing in argon/hydrogen gas. While the reducing conditions are not critical and virtually full reduction to tetragonal NbO₂ occurs, the pre-firing conditions in air are critical if high-resistance devices are to be obtained. Many devices can be made on a single alumina chip and the results of a statistical analysis are given.The electrical characteristics — measured, chiefly, using a double-pulse technique to examine the change in delay time with pulse separation, ON current, pulse width, etc. — are similar to those previously reported for the vanadate switches [2]. These measurements, together with the results of a microscope and electron-microprobe examination, indicate that a thermal mechanism of switching with filament or channel formation is the most likely. A steady-state computer simulation based on the thermal model indicates the temperatures to be expected during switching, and these are calculated to be less than the NbO₂ semiconductor-metal transition temperature of roughly 800°C, for all the cases examined.     

Polymorphic single crystal ↔ single crystal transformations in Rb0.975Cs0.025NO3

The crystal growth morphology of modifications IV, III, II, and I during the IV ↔ III ↔ II ↔ I polymorphic transformations in Rb_0.975Cs_0.025NO₃ has been investigated by optical microscopy. The equilibrium temperatures between phases IV and III (T ₀ = 393 ± 0.5 K), III and II (T ₀ = 421 ± 0.5 K), and II and I (T ₀ = 497 ± 0.5 K) of the crystal studied have been determined. It is established that these transformations are of the single crystal ↔ single crystal type and occur with the formation and growth of new-crystal nuclei in crystalline matrices.     

Effect of Cd1−xZnxS/ZnS core/shell quantum dot on the optical response and relaxation behaviour of ferroelectric liquid crystal

Dielectrics, polarizing optical microscopic and electro-optical measurements have been carried out on a core/shell quantum dot Cd_1?xZn_xS/ZnS dispersed ferroelectric liquid crystal (FLC). In the present study, quantum dots were dispersed into two different concentrations of 0.1 and 0.25 wt./wt.% in pure FLC. The electro-optical parameters of pure and QDs dispersed FLC were carried out as a function of applied voltage. A significant improvement in optical response time of QDs dispersed FLC system is one of the major finding of the present study which may be useful for fabrication of faster liquid crystal system.     

Liquid crystal–gold nanoparticle composites

Nanoparticles (NPs) have emerged as extremely promising materials to alter and improve the properties of liquid crystals (LCs) used, for example, in device applications. In this paper, we summarise recent work from our lab that aims to provide a fundamental understanding of structure–property and composition–property relationships governing LC–NP interactions, which may point to new directions for major improvements in the efficiency of LCs used in display applications. A variety of LC hosts (phases) doped with surface-functionalised gold NPs have been systematically studied ranging from one-dimensionally ordered nematic over two-dimensionally ordered smectic to three-dimensionally ordered columnar phases. Significant progress with respect to LC–NP interactions was made for NP-doped nematic phases. Here, the observation of an unusual texture for Au NP-doped nematic LCs, that is, the formation of birefringent stripe textures and the induction of homeotropic alignment of the nematic LC similar to chiral finger (or fingerprint) textures, provided the basis for numerous experimental studies using Au NPs with different core sizes and surface functionalities.     

Temperature variation of effective geometry parameter, αg,refractive indices and the determination of orientational orders in three room temperature nematic liquid crystal mixtures

Orientational order parameter, S and effective geometry, α_g are calculated from the refractive index in three nematic room temperature liquid crystal mixtures, blOO3, blOO6, and blO38. The method due to Kuczynski et al. used to determine S from birefringence. The effective geometry parameter is α_g = n_o/n_e. (n_o and n_e are the ordinary and extraordinary refractive indices of the liquid crystal material.) The behavior of α_g as a function of temperature and order parameter has also been reported. The S value calculated from α_g is identically equal to the value obtained from Kuczynski et al method, i.e., from birefringence, δn. Finally, dn_e/dT and dn_o/dT are obtained and the crossover temperatures, T_CO the characteristic temperature, which is in fact the temperature of minimum in the ordinary index of refraction in three mixtures for different wavelengths are calculated from dn_o/dT.     

Rapid growth of α-LiIO3 crystal

A fundamental possibility of significantly increasing the growth rate of α-LiIO₃ crystals without the formation of visible defects in them is demonstrated. The corresponding equipment is designed and a technique for obtaining intermediate-size samples is developed. The crystals grown with an average rate of up to 4 mm/days along the Z axis are of high enough quality to be used as materials of optical elements for laser frequency doubling. α-LiIO₃ crystals have been grown both in an open volume via natural faceting and in a rectangular habit, where growth occurs basically through the pyramid (101) face. It is shown that that amount of limiting impurities in the raw material used can be reduced and that the applicability of this material for rapid growth can be verified.     

Viscosity of Bi–Zn liquid alloys

Viscosity of Bi_100?xZn_x (x = 0, 8, 16, 50, 80, 85, 89, 100) liquid alloys has been measured by means of the oscillating crucible method. Temperature dependences of the viscosity coefficient have revealed anomalous behavior in the vicinity of the demixing curve. The maximum on background of Arrhenius type dependence of viscosity is most resolved for melt of critical concentration. The viscosity measurements results are analyzed from the point of view of concentration fluctuations. It is concluded that concentration–concentration fluctuations are responsible for the anomalous behavior of viscosity.     

Nonstoichiometric fluorides—Solid electrolytes for electrochemical devices: A review

The solid electrolytes with fluorine-ion conductivity that were revealed during the analysis of the phase diagrams of the MF _m -RF _n systems within the program of search for new multicomponent fluoride crystalline materials carried out at the Shubnikov Institute of Crystallography, Russian Academy of Sciences, are described. The most widespread and promising materials are the nonstoichiometric phases with fluorite (CaF₂) and tysonite (LaF₃) structures, which are formed in the MF₂-RF₃ systems (M = Ca, Sr, Ba, Cd, or Pb; R = Sc, Y, or La-Lu). These phases have superionic fluorine conductivity due to the anion sublattice disorder. The ionic conductivity of crystals of both structure types has been studied and the limits of its change with composition and temperature are determined. Nonstoichiometric fluorides are used as solid electrolytes in chemical sensors, fluorine sources, and batteries. The prospects of the use of fluorine-ion conductors in solid-state electrochemical devices, principles of their operation, and the problems of optimization of their composition are discussed.     

Molecular and crystal structure ofβ-isosparteine monoperchlorate

The crystal structure; determined by X rays, and the¹³C-NMR spectra of-isosparteine perchlorate are presented. The quinolizidine moieties are present in acis ring-juncture; all piperidine rings are in a chair conformation. The two nitrogen atoms are linked by a hydrogen bond of 2.704(5) Å. The proton at N(16) is also involved in a weak hydrogen bond with the O(2) of the perchlorate anion. The two quinolizidine moieties-protonated and unprotonated-differ only slightly in conformation; the protonation caused small but significant changes in the length of the bonds about the nitrogen atoms. The title compound was obtained by chemical transformations of 17-oxosparteine and protonation with an equimolar amount of HClO₄ in methanol.¹³C-NMR spectra in DMSO-²H₆ revealed the presence of the symmetricalcis-cis bis-quinolizidine structure of the salt in solution.