Time dependence of piezoresistance for the conductor‐filled polymer composites

The piezoresistance and its time dependence of conductor‐filled polymer composites have been investigated. To reveal the origin of the time dependence of piezoresistance, the creep of the polymer matrix is also studied. Based on the interparticle separation change under the applied stress, a model has been developed to predict the piezoresistance and its time dependence. By analyzing this model, the influences of applied stress, filler particle diameter, filler volume fraction, matrix compressive modulus, potential barrier height, and the matrix creep behavior on the piezoresistance and its time dependence are interpreted quantitatively. These predicted results are compared with the experimental data obtained on the polymer composites filled with conductor fillers, and good agreements were obtained. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2739–2749, 2000     

Temperature dependence of the permittivity of water

In a previous work the temperature dependence of the permittivity of some pure normal alcohols was analyzed. The experimental values were fitted to a modification of Onsager’s equation. In this work, the static permittivity of water at different temperatures was measured and its values were fitted to the same equation used for alcohols. One parameter of the fittings agrees with the tendency observed for the alcohols and the other tends to the square of the refractive index. A good agreement was obtained between the experimental values and the fittings, this fact showing that water behaves, under this point of view, like an alcohol.     

Temperature dependence of benzene hydrogenation over sulfide catalysts

Non-Arrhenius temperature dependence of benzene hydrogenation was found for sulfide alumina-supported (Ni,Mo) and (Ni,W) catalysts under unsteady-state reaction conditions. It was shown that THE observed decrease in the catalyst activity at high temperature cannot be explained by the increasing role of the reverse reaction. The activity decrease was supposed to result from the catalyst reduction with the reaction mixture.     

Temperature dependence of sorptive deformation

A study has been made of the temperature dependence of deformation of CaA, CaX, and LaX zeolites when krypton or xenon is adsorbed, and also the deformation of activated carbon when dimethyl ether is adsorbed. It has been shown that the volume compressibility and the standard chemical potential of the adsorbate in a vacancy solution, within the interval of temperatures and pressures that were investigated, can be considered as linear functions of temperature. In this case, knowledge of the temperature dependence of sorptive deformation offers a means for calculating sorptive deformation curves for other temperatures.Institute of Physical Chemistry, Russian Academy of Sciences, 117915 Moscow. Institute of Petrochemical Synthesis, Russian Academy of Sciences. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 1, pp. 23–28, January, 1992.     

Temperature dependence of ultra-exothermic charge recombinations.

We measured the temperature dependence (from +32 to -50 degrees C) of charge-recombination rates between contact radical ion pairs in isopropyl ether. In the systems selected for this study, aromatic hydrocarbon cations are the electron acceptors and the fumaronitrile anion is the electron donor. Nearly quantitative electron transfers occur at all temperatures. The charge recombinations have excess exothermicities of -60 kcal mol(-1) and exhibit a very weak temperature dependence. Our observations emphasize the absence of solvent effects and the relevance of nuclear tunneling in charge recombinations.     

Effects of glass-transition temperature on properties of photorefractive polymer composite

In this study, photorefractive polymer composites were developed in which polycarbonate was doped with a dual-function dopant and a photocharge generation sensitizer. The dual-function dopant has the function of providing both charge transport and optical nonlinearity. The composites' photoconductivity and electrooptic coefficient were investigated experimentally. The effects of the glass-transition temperature (T_g), dual-function dopant content, and electric field on the composites' photorefractive properties were studied as well. The results show that the composites' photorefractive properties are enhanced with decreasing T_g, increasing dual-function dopant content, and increasing electric field. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3302–3306, 1999     

Temperature distribution in polymer samples during DSC-scans

Differential Scanning Calorimetry is frequently used for measurements of thermal properties on all kinds of substances. The temperature lag in the samples depend on the thermal properties and the thermal contact between sample and sample holder. In the paper, we discuss the temperature distribution in samples of comparatively low thermal conductivity, such as polymers. The purpose of this study is to pinpoint the substantial temperature differences that may occur in such bad conducting samples under different conditions. The calculations of the temperature gradients have been carried out by using a finite element software package.     

Temperature Dependence of the Dielectric Permittivity of Acetic Acid,Propionic Acid and Their Methyl Esters: A Molecular Dynamics Simulation Study

For most liquids, the static relative dielectric permittivity is a decreasing function of temperature, because enhanced thermal motion reduces the ability of the molecular dipoles to orient under the effect of an external electric field. Monocarboxylic fatty acids ranging from acetic to octanoic acid represent an exception to this general rule. Close to room temperature, their dielectric permittivity increases slightly with increasing temperature. Herein, the causes for this anomaly are investigated based on molecular dynamics simulations of acetic and propionic acids at different temperatures in the interval 283–363 K, using the GROMOS 53A6_OXY force field. The corresponding methyl esters are also considered for comparison. The dielectric permittivity is calculated using either the box‐dipole fluctuation (BDF) or the external electric field (EEF) methods. The normal and anomalous temperature dependences of the permittivity for the esters and acids, respectively, are reproduced. Furthermore, in the EEF approach, the response of the acids to an applied field of increasing strength is found to present two successive linear regimes before reaching saturation. The low‐field permittivity ε, comparable to that obtained using the BDF approach, increases with increasing temperature. The higher‐field permittivity ε′ is slightly larger, and decreases with increasing temperature. Further analyses of the simulations in terms of radial distribution functions, hydrogen‐bonded structures, and diffusion properties suggest that increasing the temperature or the applied field strength both promote a relative population shift from cyclic (mainly dimeric) to extended (chain‐like) hydrogen‐bonded structures. The lower effective dipole moment associated with the former structures compared to the latter ones provides an explanation for the peculiar dielectric properties of the two acids compared to their methyl esters.     

原位聚合法制备聚合物/纳米SiO_2复合材料的研究进展

纳米SiO_2改性聚合物制备的关键在于提高纳米粒子与聚合物基体的相容性及分散性;对纳米SiO_2进行不同的表面改性及选择合适的复合材料制备方法可以改变纳米粒子与聚合物基体的界面结合方式以及相容性和分散性,进而在不同程度上影响材料的性能.本文介绍了改性前后纳米SiO_2与聚合物基体的多种界面结合方式,对近年来利用原位聚合法制备聚合物/纳米SiO_2复合材料的研究现状和进展进行了综述.     

Novel syndiotactic polystyrene/BaTiO3-graphite nanosheets three-phase composites with high dielectric permittivity

Novel three-phase composites were prepared by embedding graphite nanosheets (GNs) and BaTiO₃ nanoparticles into syndiotactic polystyrene (sPS) matrix via a solution blending and flocculation method. The dependences of electric and dielectric properties of the resultant sPS/BaTiO₃-GNs composites on volume fractions of GNs (f_GNs) and frequency were investigated. The percolation theory was employed to explain the electric and dielectric behavior of sPS/BaTiO₃-GNs composite. It was found that the sPS/BaTiO₃-GNs composite showed an obvious insulator-conductor transition with a much low percolation threshold of f_GNs = 1.44 vol%. The dielectric permittivity of sPS/BaTiO₃-GNs composite reached as high as 51.8 at 100 Hz at percolation threshold, which was about 18 and 7 times higher than that of pure sPS and sPS/BaTiO₃ composite, respectively.     

Spherical filler-promoting thermally conductive pathway in graphite-containing polymer composites for high heat radiation

Graphite is an efficient and affordable filler for polymer composites, allowing the control of thermal conductivity. In comparison to other thermally conductive fillers, graphite is lightweight and flexible but affords anisotropic thermal conductivity. Herein, the control of thermal conductivity of graphite-containing polymer composite sheet using spherical polymer particles as additional fillers is described. The thermal conductivity in the through-plane direction (λ_t) of the composite sheet is enhanced by varying the composition ratio of the two fillers (flaky graphite and spherical particles), and optimizing the forming temperature and pressure. Graphite-containing (25 wt%) polymer composite sheet formed by compression at 150 °C and 10 MPa exhibits λ _t value of 0.66 W/m K. Upon mixing of polystyrene microspheres, λ _t is successfully increased. The maximum value of thermal conductivity for a composite sheet with 35 wt% of graphite and 50 wt% of spherical particles is 7.51 W/m K, at 180 °C and 10 MPa. The graphite-containing polymer matrix forms a sequentially connected network-like structure in the composite sheet. Excess polymer microspheres lead to the formation of void structures inside the composite sheet, reducing the thermal conductivity. Thermo-camera observations proved that the composite sheets with higher λ _t value showed comparably high heat radiations. © 2020 Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58, 607–615     

Moisture–absorption,dielectric relaxation,and thermal conductivity studies of polymer composites

In the search for new packaging materials for the electrical/electronics industry, three types of polymer composites have been studied. Silicone/boron nitride powders, polyurethane/alumina powders, and polyurethane/carbon fibers have all been synthesized to study the moisture–absorption kinetics, thermal conductivities, and the dielectric loss spectra under various levels of humidity. The water uptake data indicate that water molecules are absorbed not only by the polymer matrix, but also by the interfaces introduced by the fillers. For all materials, the dielectric relaxation spectroscopy shows the presence of a peak in the 175–200 K range, which is largely due to absorbed water. The silicone/boron nitride samples absorbed the least amount of moisture. Incorporating this result with the thermal conductivity data of the three types of polymer composites, it is concluded that silicone polymers embedded with boron nitride can best serve as the coating for the electronic devices that require heat dissipation and moisture resistance, in addition to electrical insulation. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2259–2265, 1998     

Ionic conduction in composite polymer electrolytes at subambient temperatures

The majority of investigations carried out on polymer(SINGLEBOND) salt systems have been on polyether electrolytes at moderate temperatures where such electrolytes exhibit macroscopic uniformity. Relatively little attention has been paid to the subambient temperature region where composite electrolytes based on polyethers exhibit much higher conductivities than their pure polyether electrolyte analogues. For all of the composite systems studied the conduction mechanism changes from one in which the ions are coupled to the polymer segmental relaxations to one in which the ions are decoupled and thermally activated ionic hopping produces higher conductivities than would be expected from ion-segmental coupling and higher than observed for the base polyether(SINGLEBOND) salt system. This change has been observed at temperatures between 10 and 80°C above the respective glass transition temperatures. The relationship between this interaction and these higher conductivities at subambient temperatures is explored and discussed. © 1996 John Wiley & Sons, Inc.     

A data base for polymer chromatography: Temperature dependence of interaction parameters in liquid adsorption chromatography

The temperature dependence of retention behaviour of polyethylene glycol (PEG) and its mono‐ and dimethyl ethers was studied on various RP columns in different mobile phases. The accessible volumes and the interaction parameters were determined from slope and intercept in a plot of the elution volumes of the oligomers of a polymer homologous series as a function of the difference of the elution volumes of consecutive oligomers. A quite different dependence of the interaction parameters was observed in the different mobile phases. While in methanol–water the interaction parameter decreases with increasing temperature, the opposite effect is observed in acetonitrile (ACN)–water. In acetone–water, the temperature dependence is almost negligible.     

Mechanical property evaluation of glass–jute fiber reinforced polymer composites

The natural fibers such as jute, coir, hemp, sisal etc. are randomly used as reinforcements for composite materials because of its various advantages such as low cost, low densities, low energy consumption over conventional fibers. In addition, they are renewable as well as biodegradable, and indeed wide varieties of fibers are locally available. In this study, glass–jute fiber reinforced polymer composite is fabricated, and the mechanical properties such as tensile, flexural and impact behavior are investigated. The materials selected for the studies were jute fiber and glass fiber as the reinforcement and epoxy resin as the matrix. The hand lay‐out technique was used to fabricate these composites. Fractured surface were comprehensively examined in scanning electron microscope (SEM) to determine the microscopic fracture mode. A numerical procedure based on the finite element method was then applied to evaluate the overall behavior of this composite using the experimentally applied load. Results showed that by incorporating the optimum amount of jute fibers, the overall strength of glass fiber reinforced composite can be increased and cost saving of more than 30% can be achieved. It can thus be inferred that jute fiber can be a very potential candidate in making of composites, especially for partial replacement of high‐cost glass fibers for low load bearing applications. Copyright © 2016 John Wiley & Sons, Ltd.     

Temperature Induced Interpenetration Suppression of a Couple of Azo‐based Isomers with a Flexible Second Ligand

Two new azo‐based coordination compounds with a flexible second ligand, crystal 1 (composed with [Zn(adc)(eda)]_n‐1) and crystal 2 (composed with {[Zn(adc)]_0.9568(eda)}_n‐1′ and {[Zn(adc)]_0.0414}_n) have been synthesized as a couple of isomers with different reaction temperature. Adc^2? and eda represent azobenzene‐4,4′‐dicarboxylic ion and 1,2‐ethanediamine, respectively. Single crystal X‐ray diffraction measurement indicates the interpenetration of the above crystals could be suppressed by simply tuning the reaction temperature, that is, the 5‐fold interpenetration which leads to no free volume left in lower temperature synthesized compound [Zn(adc)(eda)]_n‐1 could be reduced to a 4‐fold interpenetration in higher temperature synthesized {[Zn(adc)]_0.9568(eda)}_n‐1′ which results in free volume accounting for 3.6% (31.6 ų per unit cell volume) in crystal 2 . The synthesis, crystal structure and fluorescence spectra of crystals 1 and 2 have been discussed, together with the data of elemental analysis and X‐ray powder diffraction.     

Synthesis of a hexafluoropropylidene-bis(phthalic anhydride)-based polyimide and its conducting polymer composites with polypyrrole

A new electrically conducting composite film from polypyrrole and 4,4′-(hexafluoroisopropylidene)-bis(phthalic anhydride)-based polyimide was prepared. Pyrrole and the dopant ion can easily penetrate through the polyimide substrate and electropolymerize on the platinum (Pt) electrode due to the swelling of the polyimide on the metal electrode. The electrochemical properties of polypyrrole-polyimide (PPy/PI) composite films have been investigated by using cyclic voltammetry. The PPy/PI composite film is suitable for use as the electroactive material owing to its stable and controllable electrochemical properties. The electrical conductivity of composites falls in the range 0.0035–15 S/cm. Scanning electron micrograph, FTIR, and thermal studies indicate that PPy and PI form a homogeneous material rather than a simple mixture. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 3009–3016, 1997     

Temperature dependence of the interaction between water and sodium polymethacrylate in water–methanol mixture

The information for the interaction between water molecules and a polyelectrolyte was obtained from ²³Na-NMR studies for sodium polymethacrylate in water-methanol mixtures. The chemical shift and the line width of the ²³Na nucleus give the information on the immediate magnetic environment and the motion of the nucleus, respectively. The change of the chemical shift for ²³Na nucleus is investigated by decreasing the temperature and changing the mol fraction of water in the solvent. According to these results, the hydrogen bonding of water molecules and polyanions becomes weaker as the temperature of the system decreases, but the interaction increases when the temperature is below the freezing point for water in this system. This effect, due to the change of temperature, becomes higher as the water mol fraction increases in the solvent. © 1995 John Wiley & Sons, Inc.     

Fluorescence confocal microscopy as effective testing method of polypropylene fibers and single polymer composites

Single polymer composites are special type of composite made solely of one polymer. They are characterized by promising mechanical properties but the manufacturing process is very difficult to scale to mass production. In this paper a method of testing polymer fibers and single polymer composites was developed. The method provides fast and nondestructive detection of structural defects in fibers and delamination in single polymer composites, therefore it can be applied to quality control of the fibers as well as composites. The method based on the laser scanning confocal microscopy, that uses laser to scan the surface of the specimen and detect light of wavelength longer than laser beam. The results show that the luminescence occur only in the defected or delaminated spots of the sample. Raman and IR spectroscopies were used to analyze the source of the luminescence. The study indicates that the influence of impurities, additives and effects of chemical degradation of polymer doesn’t lead to luminescence of the specimen. As the explanation of the effect mechanical stress assisted photoluminescence was proposed.