In situ preparation of polyaniline coated fumed and precipitated silica fillers and their composites with nitrile rubber (Investigation on structure-property relationship)

In situ synthesis of polyaniline (PAni) coated pyrogenic or fumed silica (PCFS) and precipitated silica (PCPS) were carried out by the oxidative polymerization of aniline in presence of fumed silica (FS) and precipitated silica (PS). Both uncoated and PAni coated silica fillers were characterized through scanning electron microscope (SEM), infrared spectroscopy and thermo-gravimetric analysis (TGA) to evaluate particle morphology and physico-chemical character of coated and uncoated silica particles. Semi-conducting composites made from two different types of PAni coated silica fillers with NBR exhibit different trend in the variation of electrical properties under different temperature and pressure. These differences in electrical properties of two types of composites are mainly due to physico-chemical characteristics of filler particles as well as their distribution in the polymer matrix. This type of composites may be used as semi-conducting and ESD (electrostatic discharge) material.     

Enhancement of dielectric and electro-optical parameters of a newly prepared ferroelectric liquid crystal mixture by dispersing nano-sized copper oxide

ABSTRACT

Here, we present the effect of copper (II) oxide nanoparticles (nCuO) on dielectric and electro-optical parameters of a newly prepared ferroelectric liquid crystal (FLC) mixture, namely W302. The FLC mixture, comprising of pyrimidine compounds, was characterised through dielectric spectroscopy, differential scanning calorimetry (DSC), polarising optical microscopy (POM) and other electro-optical methods. The material parameters such as spontaneous polarisation, rotational viscosity, response time and tilt angle of W302 were found to be 14 nC/cm², 240 mPa.s, 150 µs and 28^?, respectively. The phase transition temperatures of W302 were observed through DSC and further confirmed by the dependence of dielectric loss factor in homogeneously aligned FLC sample with temperature. We also demonstrate the observance of a low-frequency dielectric relaxation mode due to the unwinding of the helix, called as partially unwound helical mode (p-UHM) along with Goldstone mode. The behaviour of p-UHM has been systematically studied with temperature and applied bias field. Further, dispersion of nCuO into host W302 has shown a significant increase in dielectric permittivity. Also, the p-UHM relaxation peak in the dielectric regime has disappeared with the incorporation of nCuO. These studies would be useful to fabricate better electro-optical devices for display, switching and beam steering applications. The formulation and characterization of a ferroelectric liquid crystal (FLC) mixture W302 composed of pyrimidine compounds is presented. Then, we observed the effect of copper (II) oxide nanoparticles (nCuO) on dielectric and electro-optical parameters of a newly prepared and characterized FLC mixture.     

Synthesis and properties of novel fluorinated epoxy resins based on 1,1-bis(4-glycidylesterphenyl)-1-(3′-trifluoromethylphenyl)-2,2,2-trifluoroethane

A novel fluorinated epoxy resin, 1,1-bis(4-glycidylesterphenyl)-1-(3′-trifluoromethylphenyl)-2,2,2-trifluoroethane (BGTF), was synthesized through a four-step procedure, which was then cured with hexahydro-4-methylphthalic anhydride (HMPA) and 4,4′-diaminodiphenyl-methane (DDM). As comparison, a commercial available epoxy resin, bisphenol A diglycidyl ether (BADGE), cured with the same curing agents was also investigated. We found that the BGTF gave the exothermic starting temperature lower than BADGE no mater what kind of curing agents applied, implying the reactivity of the former is higher than the latter. The fully cured fluorinated BGTF epoxy resins have good thermal stability with glass transition temperature of 170-175 °C and thermal decomposition temperature at 5% weight loss of 370-382 °C in nitrogen. The fluorinated BGTF epoxy resins also showed the mechanical properties as good as the commercial BADGE epoxy resins. The cured BGTF epoxy resins exhibited improved dielectric properties as compared with the BADGE epoxy resins with the dielectric constants and the dissipation factors lower than 3.3 and dissipation 2.8 × 10⁻³, respectively, which is related to the low polarizability of the C-F bond and the large free volume of CF₃ groups in the polymer. The BGTF epoxy resins also gave low water absorption because of the existence of hydrophobic fluorine atom.     

Formation of photo-functional spiroxazine monolayers and their dielectric constant determination using surface plasmon resonance

Spiroxazine are of considerable interest as photochromic materials because of their application. On the other hand, surface plasmon resonance (SPR) is a well-known optical method for measuring optical constants of thin film. In this study, photochromic materials were used as self-assembled monolayers (SAMs) of newly synthesized spiroxazine derivatives. We used Fresnel equation (four-layer model) to determine the precise dielectric constant () of the photochromic monolayers. Structure changes of spiroxazine derivatives under UV-light irradiation resulted in the change of optical constants, the dielectric constant and thickness. The obtained results indicated that the ring opening of photochromic spiroxazine can lead to the decrease in the dielectric constant and thickness.     

Decomposition of Toluene and Acetone in Packed Dielectric Barrier Discharge Reactors

The influences of TiO₂ catalytic material and glass pellet packing on the decomposition efficiency of toluene and acetone in air by dielectric barrier discharge (DBD) reactors were experimentally investigated in this study. The effects of both packing materials on the formation of byproducts such as CO and CO₂ were also evaluated. Experimental results indicate that the introduction of glass materials into the plasma zone of a wire-tube reactor would improve the decomposition efficiency of toluene and acetone compared to a nonpacked reactor. The apparent decomposition rate constant of a glass packed-bed reactor was 4.5–4.8 times greater than that of a nonpacked reactor. The results also indicate that the decomposition rate constant of toluene was approximately 2.6 times higher than that of acetone no matter which type reactor was utilized. The application of TiO₂ coated pellets in DBD reactors will enforce the hydrocarbon byproducts to further be oxidized to CO₂, notwithstanding, it will not significantly improve the performance of the reactors in the decomposition of toluene and acetone, and in the formation of CO. The results show that the best selectivity of CO₂ for acetone decomposition in a TiO₂ coated pellets packed-bed reactor was approximately 40% higher than that in a glass packed-bed reactor.     

Rheology of asphalts modified with glycidylmethacrylate functionalized polymers

Asphalt is known to be a colloidal suspension in which asphaltenes are covered by a stabilizing phase of polar resins and form complex micelles that are dispersed in the oily maltenic phase. In order to enhance its mechanical properties (e.g., in road paving), asphalts are often loaded with polymeric materials, thereby obtaining blends that can have different physical or chemical structures, depending on the composition of the added polymer. Asphalts modified by the addition of reactive ethylene terpolymers were prepared and their dielectric and rheological properties were measured both before and after a cure at high temperature. Even if it is not possible to determine the exact nature of the chemical interactions between asphalt and polymer, master curves obtained from dynamic data clearly show that during the cure the material tends to the behavior of a cross-linked network.     

Dielectric study on pure and KOH-doped tetrahydrofuran clathrate hydrates

Complex dielectric permittivities of pure and KOH-doped (x = 1.8 x 10^–4) tetrahydrofuran clathrate hydrates were measured in the temperature range 20–260 K and in the frequency range 20 Hz-1 MHz. The relaxation time of the water reorientational motion was found to decrease drastically as a result of the doping; e.g., the relaxation time of the doped sample was 10^–9 times shorter than that of the pure sample at 70 K. The activation enthalpy of the motion was reduced to 7.4 kJ mol^–1. On cooling the crystal, the value of decreased suddenly at the 62 K phase transition to the ₂ value of the pure sample and at the same time disappeared. No dispersion effect due to the guest reorientation was observed below the transition. These data indicate that both the host and guest molecules become ordered or, at least, change their mobility drastically. In the pure sample, a relaxation phenomenon of ₀₂ was found around the glass transition region. The relaxation times agreed well with those derived from the enthalpy of relaxation in a calorimetric study.Dedicated to Dr D. W. Davidson in honor of his great contributions to the sciences of inclusion phenomena.     

A novel microwave dielectric ceramic Ca2Zn4Ti16O38: Preparation and dielectric properties

A novel microwave dielectric powder with composition of Ca₂Zn₄Ti₁₆O₃₈ was synthesized through a citrate sol-gel process. The development of crystalline phases with heat-treating temperature for the gel derived powders was evaluated by using thermo-gravimetric analysis and X-ray powder diffraction analysis techniques. The pure phase of Ca₂Zn₄Ti₁₆O₃₈ with crichtonite crystal structure was obtained at relatively low temperature of 1000 °C. The synthesized powder has high reactivity and the dense ceramics with single crystalline phase were obtained at low sintering temperature of 1100 °C. Impedance spectroscopy and microwave dielectric measurements on sintered samples showed the present compound to be a modest dielectric insulator with excellent dielectric properties of ε_r∼47-49, Qf value ∼27,800-31,600 GHz and τ_f∼+45 to +50 ppm/°C. It shows comparable microwave dielectric properties to other moderate-permittivity microwave dielectrics, but much lower sintering temperature of 1100 °C.     

多场耦合作用下光热敏感介电凝胶的力电耦合变形行为

介电弹性体在电场作用下将产生形变,为获得高的变形能,需对介电弹性体施以较强的电场,而强电场的施加可能诱发力电耦合失稳导致失效。针对光热敏感介电凝胶力电耦合变形行为,基于热力学和连续介质力学理论建立力电耦合变形模型,分析了光强、温度以及预拉伸对光热敏感介电凝胶力电耦合变形行为的影响,结果表明:无预拉伸时,随着电场强度的增大,光热敏感介电凝胶最终发生力电失稳,光强越小、温度越低发生力电失稳时的临界电压越高;预拉伸可显著改善力电稳定性,施加等双轴预拉伸后,凝胶厚度方向的伸长率显著变小,电场强度随电位移增大而线性增大,未出现力电失稳现象。