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In continuation of the investigations on chiral dopants derived from the aminoalcohols ephedrine and pseudoephedrine as chiral bivalent structure elements, a series of new agents bearing very different substituents has been synthesized. The products turned out to be suited for preparation of short-pitch chiral liquid crystalline (LC) phases and were characterized by measuring their helical twisting power (HTP) in nematic host mixtures. In particular cases, structural features on the molecular level of dopants in relation to host constitution will be discussed. On the other hand, macroscopic properties, such as dynamic and dielectric data of achiral host systems, have been considered to find out practically useful guidelines for tailoring chiral LC guest–host systems. The studies include recording of nuclear magnetic resonance (NMR) spectra to study the dependence of helicity induction on the configuration of stereogenic center. Further, some photostability tests of cholesteric mixtures reflecting light in the visible range have been undertaken with regard to exploiting these dopants in electro-optic liquid crystal devices such as cholesteric bistable displays. [ABSTRACT FROM AUTHOR]

Molecular mobility in liquid crystalline (LC) side-chain polyacrylates and polymethacrylates with cyanoazobenzene side mesogens and methylene spacers of different lengths was investigated by dielectric spectroscopy and thermally stimulated depolarization currents methods. For both series of LC polymers, near glass transition temperature (Tg), two relaxations of cooperative character related to segmental motion (α process) and reorientation of mesogens around their short axis (δ process) are observed. Cooperativity of these processes decreases with increasing length of side chains being higher for the α process. Good correlation between dielectric spectroscopy and thermally stimulated depolarization currents methods is obtained for the temperature transitions of the α and δ processes. [ABSTRACT FROM PUBLISHER]

This study focuses on the dielectric and phase behaviors of monomer/blue-phase-liquid-crystal (BPLC) composites before and after photopolymerization. At a fixed frequency of 1 kHz, where the dielectric behavior is dominated by the orientation of LC molecules, our experimental results indicate that the phase transition temperatures and, in turn, the temperature ranges of blue phases in BPLC cells can be distinguished by using the second-order derivative of dielectric permittivity with respect to the temperature. In considering the typical driving methods for display applications, the electro-optical performance of polymer-stabilized BPLC cells driven by in-plane and vertical-field switching is investigated and briefly compared. [ABSTRACT FROM AUTHOR]

The influence of the hydrophilic and hydrophobic aerosil nanoparticles density on the formation of the prenematic order close to the isotropic – nematic (I–N) phase transition is presented. The lowering of the activation energy, with the increase of aerosil nanoparticles density, suggests a change of the prenematic order in the isotropic phase. The temperature dependence of the static dielectric permittivity for 4–n–pentyl–4′–cyanobiphenyl (5CB) exhibits an anomaly near the I–N phase transition. With the increase of aerosil density, a substantial modification in the anomaly of the static permittivity was observed in the isotropic phase. Indicated changes are different for hydrophilic and hydrophobic aerosil as a result of different surface interaction between nematic molecules and aerosil nanoparticles. [ABSTRACT FROM AUTHOR]

In our study, the dielectric behaviour of the rod-like liquid crystal (6CHBT) doped with magnetic nanoparticles of spherical shape was investigated by means of dielectric spectroscopy in the frequency range from 20 Hz to 2 MHz. A low frequency dielectric dispersion in the nematic and isotropic phases of the pure liquid crystal (LC) has been assigned to the space charge polarization. After doping the host LC with the magnetic nano particles, a nearly Debye-like relaxation process was observed with the temperature dependence obeying the Arrhenius law. Considering a possible electric double layer formation on the particle surfaces, the detected relaxation process in the doped LC can be associated with the electric double layer polarization. The experimental results point out that in the measured frequency range the space charge and interfacial effects constitute the main dielectric response. Any anchoring effects were not observed and are therefore expected to appear in higher frequencies. [ABSTRACT FROM AUTHOR]

Chiral dopants were obtained by acylation of enantiomerically pure ephedrine and pseudoephedrine with promesogenic carbonyl reagents. The products have been investigated with respect to their chiral transfer ability on nematic host matrices characterized by extreme differences of the dielectric anisotropy. It has been found that the medium dependence of the helicity induction nearly disappears at reduced temperatures. Based on variable temperature1H NMR studies on monoacylated homologues, the estimated coalescence temperatures and free activation enthalpies for the hindered rotation around C‒N bonds could be correlated with the helical twisting power. Measurements by dielectric spectroscopy reveal the correlation between the molar mass of substituents linked to the chiral building block and the dynamic glass transition of corresponding chiral dopants. Furthermore, the effect of intramolecular and intermolecular hydrogen bonds has been studied by ATR-FTIR spectroscopy. [ABSTRACT FROM PUBLISHER]

Detail dielectric measurements were carried out on two different liquid crystalline (LC) samples formed by bent core (BC) molecules, showing B1 and B2 phases in the temperature range 100–150°C. The thermal variation of real part of dielectric permittivity (ϵ′) represents a tiny dip only in the saturated B1 phase which might arising from the dipole orientation disorder due to the inherent phase instability. The experimental data have been used to determine different relaxation parameters. Evidently only one relaxation mode has been detected in the dielectric relaxation spectrum for both phases, which is attributed to the long axis molecular rotation process. [ABSTRACT FROM AUTHOR]

This paper is concerned with the dramatic purification effect of porous metal–organic frameworks (MOFs) and regeneration of a degraded liquid crystal (LC) as leftover from a flat-panel production line. By soaking MOFs in the LC, the correlation between the recovery of electrical properties and the water-sorption capacity of MOFs is unambiguously revealed. Experimental evidence from the data of voltage-dependent capacitance, dielectric spectroscopy and voltage holding ratio strongly suggests that the primary contaminant in a LC leftover is moisture. This investigation paves the way for feasible recycling of deteriorated LC materials. The approach reported in this study can be applied to many fluids for purification and regeneration. [ABSTRACT FROM AUTHOR]

Polymer/liquid crystal (LC) films were elaborated by photopolymerization using UV radiation of mixtures made up of a LC, a monomer and a photoinitiator. Three acrylic difunctional tripropyleneglycol based monomers possessing the same chemical structures were studied. The monomer/LC films exhibit distinguished morphologies presenting specific LC domain sizes, which were observed by polarized optical microscopy. These results were correlated with the electro-optical responses of the corresponding polymerized/crosslinked systems. A detailed characterization by linear dielectric spectroscopy of monomers and corresponding polymers was carried out as a function of temperature in the frequency range from 20 Hz to 1 MHz. [ABSTRACT FROM PUBLISHER]

Dielectric properties of the orthoconic smectic liquid crystalline mixture W-129, which presents both antiferroelectric and ferroelectric smectic C phases, have been studied in the frequency range from 10 mHz to 100 kHz in planar aligned cells. Some important relaxation modes were detected in SmCa* and SmC* phases. Dielectric increments, distribution parameters and relaxation frequencies of these modes have been evaluated at different temperatures. From the study of dielectric spectra a great variety of relaxation responses emerges that could indicate the presence of different SmCa* subphases. [ABSTRACT FROM AUTHOR]

Changes in the dielectric properties of an electron beam irradiated antiferroelectric material (racemic mixture) filled dielectric cell have been determined by the dielectric spectroscopy. The material has phase sequence of I-SmC-SmCA-Cr. Goldstone mode could not be detected in the SmC phase due to the destruction of the helix. All the transition temperatures (except SmCA-Cr) and transverse component of the permittivity of the irradiated sample increase due to irradiation. Relaxation frequencies of the modes in the SmCA phase decrease due to irradiation. [ABSTRACT FROM AUTHOR]

The study of polymer properties requires the synthesis and analysis of simple model compounds. The aim of this work was the synthesis and investigation one of model compounds are needed to perform the physicochemical characteristics of linear, branched and dendrite polyesters. The description of relaxations processes of model oligoesters can make it easier to interpret the dependence between the useful properties of polyesters and the molecular structure of their macromolecules. Instrumental methods as NMR, FTIR and dielectric spectroscopy, calorimetry (DSC), thermogravimetric analysis and mechanical spectroscopy are used to this goal. The structure of the synthesized tetrakis(5-hydroxypentyl)piromelite (oligoester) was confirmed with the use of NMR, FTIR spectroscopy and elemental analysis. A detailed study of the relaxation properties using broadband dielectric spectroscopy and Dynamic Mechanical Thermal Analysis (DMTA) was also performed. A description of different dielectric and DMTA α-relaxation processes for the oligoester are indispensable to determine the relaxation properties of the suitable polyesters. For the synthesized oligoester glass temperature (Tg), relaxation time (τ) and activation energy (Ea) were determined from the results of DSC, DMTA and dielectric spectroscopy. [ABSTRACT FROM AUTHOR]

Dielectric relaxation behavior of a dichiral mesogenic liquid crystal material, viz. (R,S)-FPPY, having anomalous phase sequence has been studied over a wide range of frequency. Measured dielectric spectra show a new relaxation process, in addition to the usual Goldstone mode, in the SmC* phase, which has been assigned to the relaxation process originating from the dichirality of the compound. The overlapping of two molecular modes has been observed in the dielectric spectrum of both isotropic and optically isotropic (IsoX) phases. The present detailed dielectric spectroscopic studies also establish an interesting ferroelectric-like ordering in the unknown SmX* phase. [ABSTRACT FROM AUTHOR]

Dielectric permittivity measurements are carried out to investigate the effects of confinement and electric field on a chiral smectic liquid crystal in the smectic-[image omitted] ([image omitted]) phase for various cell thicknesses. The Sm-A* -[image omitted] transition temperature and the temperature range for which the [image omitted] phase is stable decrease with decreasing cell thickness. On reducing the cell thickness, the surface-induced mode appears, as a result of the influence of the cell substrates. This increases the dielectric strength of the [image omitted] phase. The distribution parameter of the relaxation process, α decreases significantly, and in the case of a thin cell, the decrease observed reflects a wide symmetric distribution of the relaxation process. On increasing the electric field, the dielectric strength decreases and the relaxation frequency increases in the [image omitted] phase. These are explained by the 'helical fracture' model, originally proposed for the Sm-C* phase. [ABSTRACT FROM AUTHOR]

Three different Antiferroelectric Liquid Crystals, synthesized in Military University of Technology were investigated by dielectric means. Some of them are orthoconic. The structures of studied compounds are very similar. The only one difference is presented in rigid core. The position and number of Fluorine atoms in rigid core changes. Very rich dielectric modes are detected with frequencies from 50 Hz up to 40 MHz. In this paper, dielectric spectroscopy of investigated materials is presented. [ABSTRACT FROM AUTHOR]

Within the range of 10-3-106 Hz, we investigated the frequency dependences of capacity C and resistance R of a 3 wt% solution of C60 with attached molecules of 1,8-octanediamine in a ferroelectric liquid crystal (FLC/C60-Ok). The measurements were carried out in darkness and on the exposure to focused white light. The basic feature of the electric properties of solutions of FLC/C60-Oc is a reduction of the ac conductivity σAC at the transition from the cholesteric to smectic phase. The measurements have shown that the ratio CP/CD (the subscripts D and P correspond, respectively, to measurements in darkness and under illumination) has a peak at a frequency of ≈ 3.5 Hz, and the ratio RD/RP increases monotonously up to a constant value within the range of 10-3-103 Hz. These data indicate that the volumetric photoelectric processes are more essential than the near-electrode ones in the solution FLC/C60-Oc as compared with the colloidal mixture C60 + C70/FLC. [ABSTRACT FROM AUTHOR]

A composite prepared from aerosil A380 and the liquid crystal (LC) 4-hexyl-4'-cyanophenyl benzoate (CP6B) was investigated by broadband dielectric spectroscopy in a large temperature range. The selected high silica density (ca. 7 g aerosil/1 g of CP6B) allows the observation of a thin layer (two-monolayer structure) adsorbed on the surface of the silica particles. For the composite one relaxation process is observed at frequencies much lower than that of the processes found for bulk CP6B. It is assigned to the dynamics of the molecules in the surface layer. The temperature dependence of its relaxation rates obeys the Vogel-Fulcher-Tammann law, which is characteristic for glass-forming liquids. The quasi 2D character of the observed glassy dynamics in the surface layer is discussed. The temperature dependence of the CP6B relaxation in the composite is compared with that of related hexylcyanobiphenyl molecules in the surface layer of aerosil composite with a similar concentration. [ABSTRACT FROM AUTHOR]

An achiral banana-shaped called F-490 liquid crystal has been examined by means of polarizing microscopy, differential scanning calorimetry (DSC) as well as X-ray diffraction (XRD) and dielectric spectroscopy methods. The phase diagram of the compound is proposed. The F-490 forms crystal-like phases the B4 and the B3 on heating and the banana-like antiferroelectric phase B2 as well the B3 and the B4 phases on cooling. Moreover, the formation of solid phase depends upon thermal history. Dielectric studies have been done with HP set-up in a wide frequency range of 0.01-1.5 MHz using LINKAM 5 μm cells. Dielectric dispersion as well as absorption were calculated from the data. On the base these data the complex rotational dynamics of banana-shaped molecule have been described. The layer thickness and the unit cell parameters of B-phases were determined on the base of XRD results. [ABSTRACT FROM AUTHOR]

DSC, polarizing optical microscopy and X-ray diffractometry were used to investigate the mesomorphic behaviour of 4-n-butyl-4'-acetyl-biphenyl, which resulted to exhibit a smectic E and, possibly, a smectic K phase. Additionally, 2H NMR spectroscopy was applied on a selectively deuterated sample to study the properties of alignment of the mesophases within a magnetic field, as well as to characterize molecular dynamics. Two motional processes were found to be active in the smectic E phase, namely fast π-flips of the benzene rings and molecular self-diffusion about the herringbone bidimensional lattice of the layers. No molecular overall diffusional motions were observed by either 2H NMR spectroscopy or dielectric relaxation measurements. [ABSTRACT FROM AUTHOR]

Banana-shaped liquid crystal called F-490 composed of achiral molecules has been examined by means of polarizing microscopy, differential scanning calorimetry (DSC) as well as optical (visible and UV) and dielectric spectroscopy methods. The results show phase diagram of this compound. It forms monotropic liquid crystal phase called B 2 which is antiferroelectric. Moreover the formation of solid phase depends upon thermal history. [ABSTRACT FROM AUTHOR]

Broadband dielectric spectroscopy was applied to study influence of the boundary conditions and layer thickness of liquid crystal confined to cylindrical pores on low frequency and high frequency relaxation processes. The low frequency measurements provide information on the relaxation of surface polarization that arises at liquid crystal–pore wall interface. The information about reorientational dynamics of molecular origin is obtained in high frequency experiments. The homeotropic alignment of molecules facilitates the investigation of the librational mode. The behavior of this mode is different from the behavior observed in investigations of relaxation due to reorientation of molecules around their short axis. [ABSTRACT FROM AUTHOR]

Confinement of 4n-octyl-4′-cyanobiphenyl to nanoporous molecular sieves with hexagonal structure of cylindrical pores or with cellular structure is studied. It was found by differential scanning calorimetry and broadband dielectric spectroscopy that composites with the pores completely filled with liquid crystal show phase transitions characteristic for bulk, while the partially filled samples do not show these transitions. Thermogravimetric, differential thermal analysis and in situ infrared spectroscopic investigations have indicated first time that the dealkylation and oxidation of the confined liquid crystal is quite possible inside the nanopores of these molecular sieves, at temperatures much lower than for the bulk. [ABSTRACT FROM AUTHOR]

We have investigated the effect of structure modification of various discotic liquid crystal side chain polymers on their phase transitions and the resulting dynamics. This has resulted in a better understanding of the structure of the various nematic phases that can be formed, in particular giving rise to the identification of supramolecular thermotropic nematic phases where the fundamental units comprise assemblies of molecules. In addition we have examined the dynamics of field-induced order parameters in the isotropic phase of calamitic side chain polymers using dielectric relaxation spectroscopy and dynamic Kerr effect measurements. [ABSTRACT FROM AUTHOR]

Polymer Dispersed Liquid Crystal films were elaborated by photopolymerization using ultraviolet radiation of liquid crystal/monomer mixtures. Three acrylic difunctional propyleneglycol based monomers were used; differing only by their chain lengths in terms of their molecular weight. Infrared spectroscopy investigation made it possible to obtain the monomer conversion rates, in order to determine the effect of the presence of liquid crystal on the kinetics of polymerization and phase separation. The characterization by linear dielectric spectroscopy of the monomers and polymers was carried out as a function of temperature in the frequency range from 20 Hz to 1 MHz. [ABSTRACT FROM AUTHOR]

Two liquid crystalline partially fluorinated compounds: (S)–(+)–4-(1-metyloheptyloxycarbonylphenyl) 4′-(6-perfluorobutanoyloxyhex-1-oxy) biphenyl-4-carboxylate, in short MHPP(9F)HBC and (S)–(+)-4-(1-metyloheptyloxy-carbonylbiphenyl) 4′-(6-perfluorobutanoyloxyhex-1-oxy) benzoate, in short P(9F)HPMHBC, exhibiting paraelectric, ferroelectric and antiferroelectric phases have been studied by dielectric spectroscopy and spontaneous polarization measurements. These two compounds have the same molecular weight but their terminal chains at para positions are interchanged. Influence of molecular structure on the phase diagram and dielectric behaviour is presented. Two relaxation process revealed in the antiferroelectric phase are interpreted as collective modes: anti-phase and in-phase fluctuation of phase and amplitude. The reorientation around the short molecular axis does not contribute to the dielectric spectra. [ABSTRACT FROM AUTHOR]