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1.
Relaxation frequencies fr characterising the molecular rotation around the short axes (flip-flop movement) of 45 three- and four-ring polar compounds with high birefringence (mainly cyclohexyl, phenyl and biphenyl tolanes and quaterphenyls terminated with F, OCF3, NCS group) have been measured. These compounds exhibit a large positive dielectric anisotropy Δε = ε ? ε in the nematic phase and relatively low fr values (in the range of dozens kHz). The dependence of fr upon molecular structures and physical properties of systems (molecular rigid core length, polarity and viscosity) is discussed. Mixtures for dual frequencies addressing display with the crossover frequency fco of the order of ~1 kHz have been prepared by joining the above compounds with a mixture consisted of two, three and four fluoro-substituted in the lateral position tolanes and phenyltolanes showing negative dielectric anisotropy. It was stated that relaxation frequency is correlated mainly with their length and polarity.  相似文献   

2.
In this study, the twist elastic constant (k22) of liquid crystals (LCs) was accurately measured using capacitance method. The constant can be obtained on the basis of accurate measurement of other LC parameters, such as parallel and vertical dielectric constants (ε// and ε), splay and bend elastic constants (k11 and k33), and rotational viscosity coefficient (γ1). First, by using dual-cell capacitance method and an LC cell capacitance model to measure ε// and ε, k11 and k33 can be obtained from the threshold voltage determined from the voltage–capacitance curve of the parallel-aligned nematic LC layer and the comparison between the experimental and theoretical results based on the Frank elastic theory, respectively. In addition, γ1 can be obtained from the measurement of the dynamic response in the parallel-aligned nematic cell. Finally, k22 can be accurately determined using the threshold voltage of the twisted nematic LC cell. By adopting the above method, the measured k22 for LC E7 was 6.7 × 10?12 N. The proposed method is more rigorous and yields a more accurate measurement result than the other methods reported in the literature.  相似文献   

3.
The use of liquid crystals (LCs) as anisotropic solvents is desired for various potential applications and usually for other organic and inorganic compounds. In this work, solvent polarity parameters are obtained using a spectroscopic method for four LCs with a range of high and low dielectric anisotropy (?ε). Solvatochromic polarity parameters for these LCs were defined via Kamlet–Abboud–Taft polarity functions characterizing different temperatures and phases, isotropic and anisotropic, and using the Reichardt’s dye and 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio) phenolate standard probe. The investigated polarity parameters reveal the effects of LC media on the photo-physical behaviour of solute molecules in isotropic and anisotropic media. Subsequently, a new LC polarity parameter (Zo) is introduced as an overall matrix anisotropy polarity parameter to characterize variation between isotropic and anisotropic phases. The values of Zo are sorted from higher to lower dielectric anisotropies (?ε).  相似文献   

4.
A novel capacitor with high dielectric constant (ε) has been developed by blending poly(vinylidene fluoride) (PVDF) with polyamide (PA11). The blends show high dielectric constants (εblend = 40), which give better frequency stability (1 MHz), and excellent mechanical properties. Based on certain volume fractions, the measured dielectric constants (ε blend ) were found to exceed those of the corresponding polymers, in contrasted to conventional composites, where εpolymerA < εcomposite < εpolymerB. SEM investigations suggest that the enhanced dielectric behavior originates from significant interfacial polymer‐polymer interactions. DSC and XRD demonstrate that blending PA11 with PVDF affects the crystalline behavior of each component. However, the PA11/PVDF blends exhibit a slightly high dielectric loss (tanδ ≈ 0.17), which is a great disadvantage to a capacitor. Adding a copolymer of styrene and maleic anhydride decreased the dielectric loss (tanδ ≈ 0.057) and increased the dielectric constant (εblend = 60). Our findings suggest that the high‐ε polymeric blends created represent a novel type of material that is flexible and easy to process, of relatively high dielectric constant, of high breakdown strength and, moreover, is suited to applications in flexible electronics.

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5.
ABSTRACT

In order to produce liquid crystal (LC) with high birefringence (Δn) in the THz band, eight LC monomers were designed and synthesized. We formulate two mixtures (LC S200-2 and E7-2) by adding eight LC monomers to two commercial LCs (S200 and E7), respectively. The dielectric constants of the mixture LCs were measured using the frequency selective surface (FSS) in the region from 90 to 140 GHz. We compare the simulated results with the experimental measurements and determine the dielectric constants of the LCs. The dielectric anisotropy (Δε) of the nematic S200-2 and E7-2 LC mixtures are 16.4% and 16.3% higher than that of the commercial S200 and E7 LCs, respectively. The results show the frequency tunability of the two LC mixtures is greater than that of the commercial LCs.  相似文献   

6.
The dielectric investigations of porous synthetic silica gel modified with polyaniline (PANI) and polyethylene glycol (PEG) polyblend at various concentrations are demonstrated in this paper. By using the chemical oxidative process to embed polyaniline (PANI) and polyethylene glycol (PEG) into a silica matrix, conducting gel nanocomposites were synthesized. For various dopant concentrations, the dielectric permittivity (ε′), D.C. conductivity (σdc), loss tangent (tanδ) and dielectric loss (ε″) were investigated. The samples were characterized using differential thermal analysis/thermogravimetric analysis, Fourier transform infrared spectroscopy and high-resolution transmission electron microscopy. Depending on the co-blend content, PANI-PEG modified silica structures produce nanoparticles ranging in size from 9.9 to 48.1 nm. The variation of DC conductivity (σdc) with PANI/PEG content shows Maxwell-Wagner Sillars (MWS) effect confirming the role of the conjugation and the structural order.  相似文献   

7.
《中国化学》2018,36(3):199-205
We report a new small molecular acceptor, ITIC‐OEG, which is based on indacenodithieno[3,2‐b]thiophene and 1,1‐(dicyanomethylene)‐3‐ indanone including oligoethyleneglycol (OEG) side‐chains. ITIC‐OEG was found to have higher dielectric constant (εr=5.6) than that of a reference molecule of ITIC with normal alkyl substituents (εr=3.9). The dielectric constant of medium influences significantly the exciton binding energy and the resulting charge separation and recombination. The optical, electrochemical and morphological properties of ITIC‐OEG and its photovoltaic characteristics were investigated by blending with a semi‐crystalline donor polymer, PPDT2FBT, with comparison to those of ITIC. ITIC‐OEG shows more red‐shifted absorption and stronger crystalline packing than ITIC. However, the lower photovoltaic performance (with 1.58% power conversion efficiency, PCE) was measured for PPDT2FBT:ITIC‐OEG, compared to PPDT2FBT:ITIC (5.52% PCE). The incompatibility between PPDT2FBT and ITIC‐OEG (due to high hydrophilic nature of OEG chains) resulted in poor intermixing with large domain separation over 300 nm, showing inefficient charge separation and significant charge recombination. Therefore, to investigate the effect of dielectric constant of the materials on the charge separation and recombination, the blend morphology of the PPDT2FBT:ITIC‐OEG should be optimized first by improving their miscibility and phase separation.  相似文献   

8.
Magnetic and dielectric loss have been systematically controlled by using two kinds of high-permeability particles with different electrical conductivity (Sendust, Permalloy), and the effect of the magnetic and dielectric loss on the noise-absorbing properties has been investigated. Noise-absorbing sheets are composite materials of magnetic flake particles of high aspect ratio dispersed in a polymer matrix. The frequency dispersion of the complex permeability was almost the same for the Sendust and Permalloy composite specimens. However, the complex permittivity of the Permalloy composite (ε r′ ≈ 250, ε r″ ≈ 50) was much greater than that of the Sendust composite (ε r′ ≈ 70, ε r″ ≈ 0). Enhancement of the space-charge polarization between the highly conductive Permalloy particles results in the high values of ε r′ and ε r″. Due to the large dielectric loss of the Permalloy composite (in addition to its inherent magnetic loss), the bandwidth of noise absorption is increased, especially in the lower frequency region. Dielectric loss should be considered in the design of broad-bandwidth noise-absorbing composites.  相似文献   

9.
A time-domain reflectometry technique has been used to measure complex dielectric permittivity ε*(ω) = ε?(ω) ? ″(ω) of 1-propanol–dioxane, 2-propanol–dioxane, aniline–dioxane, N-methylaniline–dioxane and N,N-dimethylaniline–dioxane mixtures in the frequency range of 10 MHz to 30 GHz. The complex permittivity spectrum has been fitted with a single relaxation time with a small amount of Davidson–Cole behaviour. The least squares fit method has been used to obtain the static dielectric constant (ε0), relaxation time (τ), Bruggeman factor and Kirkwood correlation factor. The Luzar theoretical model is used to compute the binding energies and average number of hydrogen bond between co-solvent–co-solvent and co-solvent–dioxane molecules.  相似文献   

10.
11.
Measurements of the complex permittivity, ε*?=?ε′ – ″, within the frequency range 200 Hz to 10 MHz for 15 laterally fluoro-substituted terphenyls have been conducted. In most cases the substances exhibited the nematic phase over a broad temperature range. All substances were characterised by negative dielectric anisotropy, and are potentially useful for vertical alignment mode systems. The static permittivity tensor components have been analysed in relation to the dipole structure of the molecules. Dielectric relaxation processes observed in the liquid crystalline (LC) and solid rotator (R) phases (obtained by slow cooling of the samples) are characterised by calculation of the relaxation times and activation barriers. The rotation motions around the short axes are typical for LC phases, whereas rotations about the long axes, accompanied in some cases by internal motions, are present in the R phase.  相似文献   

12.
In this work, self‐assembly method was used to improve the dielectric constant of triblock copolymers. A series of ABA triblock copolymers with a defined length of poly(n‐butyl acrylate) (PBA, B block) segment and different lengths of liquid crystalline (LC) poly[11‐(4‐cyano‐4′‐biphenoxy)undecyl methacrylate] (P11CBMA, A block) segments were synthesized by using the atom transfer radical polymerization method. The well‐defined triblock copolymers P11CBMAmb‐PBAnb‐P11CBMAm possess three different B/A ratios (n = 50, m = 17, 43, 53). Due to the supramolecular cooperative motion effect, the copolymers can form worm‐like microstructure (WLC = 52.8%), cylinder‐like nanostructure with P11CBMA phase embedded in PBA matrix (WLC = 73.9%), and wide stripe structure with LC domains distributed unevenly in a continuous PBA matrix (WLC = 77.7%) after annealed at 160°C (above Ti) under N2 for 24 h, respectively. In order to study the influence of microphase separated morphology of triblock copolymer on the dielectric properties, solvent annealing was also used to develop various nanostructures. After thermal or solvent annealing, the dielectric constants of block copolymers increased dramatically while their loss factors remained the same. For different block copolymers, the dielectric constants increased with the increase of the LC block length. For diverse treatments, dielectric permittivities of samples varied widely with different nanostructures. The results show that the dielectric constants of block copolymers could be tuned by the block ratios and the self‐assembled microstructures. These findings will inspire researchers using self‐assembly method to design and develop novel flexible materials with high dielectric permittivity. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

13.
Three stereoselective syntheses and the physicochemical properties of trans,trans‐5‐(4‐ethoxy‐2,3‐difluorophenyl)‐2‐(4‐propylcyclohexyl)tetrahydropyran, which is an important liquid‐crystal compound with a large negative dielectric anisotropy (Δε=?7.3), are described. The key step in the construction of the trans‐2,5‐disubstituted tetrahydropyran ring in the first approach involved a benzylic cation mediated intramolecular olefin cyclization of a 2‐allyloxy‐1‐arylethanol derivative. The second method included the Et2Zn‐induced 1,2‐aryl shift of a bromohydrin obtained from a hetero‐Diels–Alder reaction, followed by stereoselective bromination. The third approach utilized the hetero‐Diels–Alder reaction of trans‐4‐propylcyclohexanecarboxaldehyde and a 2‐aryl‐3‐(trimethylsilyl)oxy‐1,3‐butadiene, followed by stereoselective protonation. From results obtained by using a quantum chemical calculation method, the reason why the target compound shows a large negative Δε value is discussed.  相似文献   

14.
We present here the evidence for the origin of dc electrical conduction and dielectric relaxation in pristine and doped poly(3‐hexylthiophene) (P3HT) films. P3HT has been synthesized and purified to obtain pristine P3HT polymer films. P3HT films are chemically doped to make conducting P3HT films with different conductivity level. Temperature (77–350 K) dependent dc conductivity (σdc) and dielectric constant (ε′(ω)) measurements on pristine and doped P3HT films have been conducted to evaluate dc and ac electrical conduction parameters. The relaxation frequency (fR) and static dielectric constant (ε0) have been estimated from dielectric constant measurements. A correlation between dc electrical conduction and dielectric relaxation data indicates that both dc and ac electrical conductions originate from the same hopping process in this system. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1047–1053, 2010  相似文献   

15.
The Merck nematic mixture E49 exhibits a large nematic interval (0–100 °C) and a large dielectric anisotropy. Both of these features make E49 interesting for applications and basic physics. Unfortunately, no systematic measurements of the material constants of this mixture and their temperature dependence have been reported in the literature. In this paper we report experimental measurements of the splay and bend elastic constants (K 11 and K 33) of the ordinary and the extraordinary refractive indices (n ort and n par) at the wavelength λ?=?632.8 nm and of the two elastic constants parallel and orthogonal to the director (εpar and εort) at the frequency ν?=?5?kHz. The temperature dependence of all of these parameters is found in the temperature range 25–99 °C. The measurements of the elastic constants are performed using both a dielectric and an optical method simultaneously on the same nematic sample. The results obtained using the two methods are in a satisfactory agreement between them within the estimated experimental uncertainty. The ordinary and the extraordinary indices are measured using the prism method.  相似文献   

16.
An in situ dielectric measurement for atactic poly(methyl methacrylate) (at‐PMMA) was performed under high‐pressure CO2 under various pressures and temperatures. The at‐PMMA has the acetate side group with a large dipole moment. In the glassy state, a local relaxation process (β‐process) can be observed using dielectric measurement. In the rubbery state, the micro‐Brownian motion of main chain (α‐process) occurs, and the β‐process changes into αβ‐process coordinated with the α‐process. The dielectric loss (ε″) spectrum of at‐PMMA in the glassy state is asymmetric because of the density fluctuation for the amorphous structure. The loss peak frequency shifted to higher frequencies, and the relaxation strength increased with increasing CO2 pressure. In the glassy state, the shape of ε″ spectrum became more symmetric with increasing CO2 pressure. These show that the molecular mobility enhanced by the plasticization effect of CO2 allows the dipolar side groups in the high‐density region to contribute to the relaxation process. We also found that the apparent activation energy decreased under high‐pressure CO2. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2951–2962, 2005  相似文献   

17.
Nα-L -Iysine with a 2-carboxy-4, 6-dinitrophenyl (Dncp) haptenic group on the ε-amino function is a potent anaphylactogen in the guinea pig. We prepared Nε -Dncp- Nα-benzoly-L -Iysinamide and Nε-Dncp-Nα-benzoyl-L -lysyl-1-aminopropane where the carboxyl group of Iysine is blocked. Both compounds were non-elections of anaphylaxis.  相似文献   

18.
Abstract— This study was undertaken to further investigate the way in which the counter anion controls the Λmax of the absorption spectrum of compounds similar to N-retinylidene-n-butylammonium salts (NRBA). The following relationship had been found: ΔE =ΔEo -F d0e2/εd2; here ΔE is the observed excitation energy, e the charge on the electron, ε the dielectric constant, d0 a constant and d the distance between centers of opposite charge as estimated from crystallographic radii. Resonance theory implies that ΔEo should be of the same numerical value as the corresponding carbonium ion which can be generated readily from the corresponding alcohol. The C22SB analog of NRBA was prepared and then converted to the halide salts. The Δmax of these salts was determined in several halohydrocar-bon solvents, and ΔEo was determined by least squares for each solvent. The average value of ΔEo was found to be 653 nm, while the Λmax, for the carbonium ion was previously found to be 644 nm. The results are supportive of previous work.  相似文献   

19.
The dielectric properties of a series of semiconducting polyazophenylenes were studied as a function of temperature and molecular weight in the temperature range 293–600°K and for molecular weights between 5,100 and 62,800 at a constant frequency of 1 kHz. The compounds studied included poly-2,4-diaminotoluene, poly-2,4-diaminoanisole, and poly-2,5-diaminotoluene. The dielectric properties are presented in the usual way in terms of a complex dielectric constant ε* = ε′—jε″. Activation energies of relaxation processes were evaluated from the areas and widths of the dielectric loss factor, ε″, against reciprocal temperature at constant frequency. The dielectric activation energies were found to be roughly equal to the activation energies from the DC conductivity. This indicates that the conduction mechanism is based on rotational movements of molecules or parts of molecules.  相似文献   

20.
The dielectric properties of mixed‐oxide CaMn7O12 are reported. This compound exhibits a high dielectric constant at room temperature (ε′r> 104), for frequencies up to 104 Hz, values that make it very attractive for potential applications provided that its rather high losses can be minimized. Complex plane analysis of the obtained data reveal an important extrinsic contribution to these high ε′r values; and also that the intrinsic dielectric constant of this material is ε′r,∞ ≈ 30, a value rather high for this type of compounds, that could be related to the electronic process of charge‐ordering present in this oxide below 440 K.  相似文献   

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