Preparation and characterization of magnesium aluminate (MgAl2O4) spinel ceramic foams via direct foam-gelcasting

Lightweight MgAl₂O₄ spinel ceramic foams with high mechanical strength and good dielectric properties were prepared with a direct foam-gelcasting method using MgAl₂O₄ and TiO₂ (rutile phase, as sintering aid) powders. The effects of calcination temperature and foam volume on bulk density, apparent porosity, and on the mechanical and dielectric properties of the ceramic foams were investigated. Tailored porosity (75.14–82.46%), pore size (10–200 μm), dielectric constant (1.66–2.05), and compressive strength (4.0–14.3 MPa), were obtained based on the change of the foam volume in the foamed slurries, and the calcination temperature of porous ceramics. The compressive strength and dielectric constant of the as-manufactured spinel foam with a porosity of ~75.14% was as high as 14.3 MPa and 2.05, respectively. The spinel ceramic foam which had a porosity of 81.84% was prepared with a foam volume of 350 mL and a sintering temperature of 1500 °C, and exhibited heterogeneous pore structures, whereby large and open spherical cells involved in small circular windows on the internal walls with a mean pore size of ~66.26 μm and a grain size of ~8 μm. The experimental dielectric constant matches well with that calculated by the modified Bruggeman model. The dependence of the mechanical strength on the relative density can be represented by the Gibson and Ashby model. The fitted index values of the power relationship were 3.504 and 3.533, compared to the theoretical value of 1.5. The ceramic foam can potentially become a new type of electromagnetic wave-transmitting radome material due to its low dielectric constant (1.66–2.05) and dielectric loss (0.0026–0.006) values.     

Dielectric behavior of polystyrene foam at microwave frequency

Polystyrene foams have been generated and fabricated into differently shaped structures', by change of steaming period under constant impregnation time and solvent-nonsolvent composition. Optical photomicrographs of samples both plain and wax-copper-coated reveal uniform appearance, distinct grain-boundaries, and random cell size distribution. Dielectric measurements have been made on test specimens cut according to wave-guide size at 9.375 GHz X-band microwave frequency by short-circuited wave-guide method of Smith and Hippel modified by Dakin and Works. Dielectric constants are linear, on direct and semi-log scales in bulk-density and volume-fraction, obeying Weiner's inequalities. Formulae of Landau-Lifshitz, Beer, Maxwell-Wagner, Odelevsky, etc. have been tried. Data fit best with the logarithmic law of Lichtenecker and Rother. Specific polarization is also a true function of density. Dielectric constant vs bulk-density plots of foams resemble dielectric-constant vs fractional-density plots based on the theoretical derivation by Smith for polystyrene compacts, signifying that compacts containing closely-spaced oblong-spherical particles arc physically similar to foams having spherical gas inclusions in plastic structures. Tan δ lying in the range 0.002–0.0038 results from conformational polarization (β-relaxation at room-temperature for wide-angle torsional oscillations of side-groups with co-operative motion from wriggling chains). It is therefore possible for low-loss foam dielectrics suitable for micro-wave applications to be made by this method.     

Polyester resin as a compatibilizing agent for some polymeric blends

Dielectric and viscosity techniques were used to determine the degree of the compatibility of poly(methyl methacrylate)/polycarbonate, poly(methyl methacrylate)/ polystyrene, and polycarbonate/polystyrene blends in different ratios (25/75, 50/50, and 75/25 w/w). The effect of the addition of 5, 10, and 20% concentrations of the prepared polyester resin [poly(butylene terephthalate adipate)] on the compatibility of these blends was studied. The dielectric properties were measured over a frequency range (from 100 Hz to 100 kHz) at various temperatures covering the glass‐transition temperatures of the polymers used (from 30 to 170°C). It was found from the dielectric and viscosity measurements that the addition of 10% polyester to poly(methyl methacrylate)/polycarbonate, 20% polyester to poly(methyl methacrylate)/polystyrene, and 5% polyester to polycarbonate/polystyrene blends enhanced the degree of compatibility of such blends. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Novel dielectrics from IPNs derived from castor oil based polyurethanes

Three series of interpenetrating polymer networks (IPNs) based on a polyurethane (castor oil + toluene diisocyanate) with polystyrene, poly(methyl methacrylate), and poly(n-butyl methacrylate) were synthesized and characterized. Dielectric relaxation studies of these IPNs were carried out from ?150 to 100°C in the 100 Hz to 100 kHz range. The effects of structural variables such as composition, type of vinyl monomer, as well as the effect of interaction of the phases on the dielectric properties were studied. A certain degree of phase mixing was observed to exist in all series as detected by the variation of the glass-transition temperatures of the IPNs. Maxwell–Wagner–Sillars polarization at the interface of the two phases was observed. © 1994 John Wiley & Sons, Inc.     

Preparation and characterization of poly(methyl methacrylate)–lead silicate composites

Poly(methyl methacrylate)–lead silicate composites were prepared with different amounts of crystalline or amorphous PbO–SiO₂ binary composition (70 mole % PbO). The polymerization reaction was carried out in aqueous medium and in methanol–water mixture at 40°C using sodium bisulfite as initiator. The presence of lead silicate was found to increase the molecular weight of the poly(methyl methacrylate). The prepared composites were characterized by studying their shielding properties to γ radiation, mechanical hardness, dielectric constant, and thermal stability. It was found that lead silicate increases the absorbing power of the composites to γ radiation. This behavior was found to be dependent on the amount and the type of lead silicate (amorphous or crystalline). Gamma irradiation of the composites was found to cause chemical degradation of the poly(methyl methacrylate) leading to a decrease in mechanical hardness. Molecular weights of the poly(methyl methacrylate) for some of the composites were determined before and after irradiation viscosimetrically. Dielectric constants for some of the composites were determined at two different temperatures. The thermal stability of the composites was studied by means of an automatic thermogravimetric analyzer.     

Role of Cenosphere Addition on Dielectric Properties of Sisal Fiber-Polypropylene Composites

This research investigates the influence of addition of porous additives on dielectric constant of polypropylene. Composite composed of PP matrix with sisal fiber having cylindrical pores and cenospheres having spherical pore, presents low dielectric constant. A new relation concerning porosity is proposed by modifying the usual mixing rule to predict the dielectric constant of PP composite. This research presents the dielectric properties of sisal fiber-reinforced PP composites with and with out cenospheres. Treated and untreated cenospheres with different concentration were loaded in chopped sisal fiber-reinforced polypropylene. The loading of the polypropylene with the sisal fiber, increases the dielectric constant ε′ and improves the ac conductivity σ_ac. The effect of temperature on the dielectric spectrum of polypropylene composites was investigated in the frequency range ranging from 1–10 kHz. Sisal fiber-reinforced polypropylene composites having 20% sisal fiber with and without cenospheres were developed and electrical properties such as dielectric constant (?′), dissipation factor (tanδ) and ac conductivity (σ_ac) of these composites were determined. Dielectric constant, tan δ, and a.c. conductivity increases with increase in temperature at different frequencies.     

Dispersion polymerization of methyl methacrylate in supercritical carbon dioxide using a silicone‐containing fluoroacrylate stabilizer

Free radical dispersion polymerization of methyl methacrylate (MMA) was carried out in supercritical carbon dioxide (scCO₂) using poly{(heptadecafluorodecyl acrylate)‐co‐3‐[tris(trimethylsilyloxy)silyl]propyl methacrylate} (p(HDFDA‐co‐SiMA)) as stabilizer. Dry, fine powdered spherical poly(methyl methacrylate) (pMMA) particles with well‐defined sizes were produced. The resulting high yield of spherical and relatively uniform micron‐size pMMA particles was formed utilizing various amounts of p(HDFDA‐co‐SiMA) random copolymer. The particle diameter was shown to be dependent on the weight percent of the stabilizer added to the system. The effects of varying the concentration of stabilizer (1–7 wt%), reaction time (4–12 h) and pressure (15–35 MPa) upon the polymerization yield, molar mass and morphology of pMMA were investigated. Copyright © 2005 Society of Chemical Industry     

Effect of polymer structure on the morphologies and dielectric properties of nanoporous polyimide films

Low dielectric constant polyimide (PI) films have potential applications in integrated circuit. In this study, poly(methyl methacrylate), poly(ethylene oxide), and polystyrene as thermally labile materials were used as templates to generate PI films with nanopores by first mixing the polymer templates with the precursor of PI, poly(amic acid), followed by imidization of poly(amic acid) together with degradation of the polymer templates. The sizes of the formed pores, the thermal and dielectric constant of the nanofoamed PI films were studied and compared in detail. It is concluded that the dielectric constant of PI films using poly(ethylene oxide) as pore template is more stable because of the formation of uniform pores which is from the great accordance of imidization temperature of poly(amic acid) with the degradation temperature of poly(ethylene oxide). But that using poly(methyl methacrylate) as pore template is frequency dependent as the influence of inhomogeneous pores and PMMA residue from incompletely degradation of poly(methyl methacrylate). © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41480.     

High electromagnetic interference shielding effectiveness achieved by multiple internal reflection and absorption in polybenzoxazine/graphene foams

Electromagnetic interference (EMI) is an increasingly severe issue in modern life and high-performance EMI shielding materials are in desperate need. To achieve high EMI shielding effectiveness (EMI SE), a series of polybenzoxazine/graphene composites foams are developed using a simple sol–gel method. When the graphene loading increases from 1 to 20 wt%, the density of the composites foams drops from 0.4143 g/cm³ to 0.1654 g/cm³. Meanwhile, an electrically conductive path is formed at around 7 wt% of graphene. Below the percolation threshold, the dielectric constant increases with graphene content and composite foam with 5 wt% graphene shows dielectric constant of 10.8 (1 MHz). At the highest graphene content of 20 wt%, the electric conductivity reaches 0.02 S/cm, 10 orders of magnitude higher than pure polybenzoxazine foam. Benefiting from the high electrical conductivity and lightweight porous structure, the composite foam PF/20G delivers an EMI SE of 85 dB and a specific SE of 513.9 dB·cm³/g. Importantly, the EMI shielding is dominated by absorption attenuation, with PF/20G shows absorption ratio higher than 98% in the range of 8.4–11.0 GHz, which is believed to be caused by multiple internal reflection and absorption inside the conductive foam.     

Radiation-induced changes in the index of refraction,density, and dielectric constant of poly(methyl methacrylate)

Samples of commercially prepared poly(methyl methacrylate) (Lucite) were irradiated with 13-M.e.v. electrons in the dose range 0–11 Mrad, and the induced changes in the index of refraction, density, and dielectric constant were observed. An attempt is made to interrelate the observed changes theoretically.     

The melt elasticity of polymer blends: Polystyrene/poly(methyl methacrylate)

Samples of general-purpose polystyrene and poly(methyl methacrylate) were melt blended in a special mixer–extruder over the complete range of compositions from 100% polystyrene to 100% poly(methyl methacrylate). The blends were characterized for their melt rheological characteristics in a melt elasticity tester which measured their stress–strain behavior and strain recovery characteristics as a function of time. In addition, the blends were processed through a laboratory fiber spinning apparatus wherein the spinline tension was measured. Large maxima in the amount of recoverable strain, in the time for the strain recovery to finish, and in the melt tension were observed at a weight percent composition of 40% polystyrene and 60% poly(methyl methacrylate). The melt stress-strain curves showed double yield points at certain compositions. The results are discussed in terms of a model consisting of two interpenetrating continuous phases.     

Electrical conduction and dielectric properties of poly(methyl methacrylate)/perylene solar concentrators

Poly(methyl methacrylate) doped with fluorescent perylene dye was prepared by both radical polymerization of methyl methacrylate and solvent casting from polymer solutions. The samples were characterized by differential scanning calorimetry, Fourier transform infrared spectroscopy, electrical conductivity, and dielectric properties. Both conductivity and dielectric properties were measured in the temperature range 303–433 K and the frequency range 10³ to 5 × 10⁶ Hz. The results show that the direct‐current electrical conductivity increased by increasing dye content in solvent‐cast samples, whereas it decreased radically polymerized samples. The results of alternating‐current conductivity suggest electron hopping between filled and empty localized states. The study of dielectric properties showed two relaxation peaks corresponding to the dipole segmental and dipole group losses. Explanations based on the polymer free volume and acid–base interactions were proposed to examine the influence of the sample preparation and perylene dye concentration on the glass‐transition temperature and dielectric relaxation of the samples. The obtained results recommend the thermal and molecular stability of luminescent solar concentrator (LSC) matrices prepared by radical polymerization over those prepared by solvent casting. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 793–805, 2003     

Mie scattering from sphere structures in polymer blends

Polymer blends of transparent poly(methyl methacrylate) and polystyrene become opaque due to light scattering at the boundaries of the two polymers. The polymer blend is light brown when it is illuminated by white light. The coloring depends on the spherical domain structures existing in the polymer blend. The coloring was analyzed by using the rigorous Mie theory. The Mie results were compared with the semiempirical results previously reported by the authors. The wavelength dependence of theoretical scattering efficiencies on radii of scattering spheres from 0.05 to 1.2 μm was obtained for polystyrene spheres in poly(methyl methacrylate) matrix, and vice versa. The scattering at the short wavelength region is stronger than at the long wavelength region. The scattering efficiencies become almost constant in the visible wavelength region for sufficiently large spheres.     

Dipole moments of poly(allyl methacrylate) prepared by group transfer polymerization

Dipole moments of poly(allyl methacrylate) prepared by group transfer polymerization and of allyl methacrylate were determined from dielectric constant, refractive index increment and density measurements performed on their dilute benzene and carbon tetrachloride solutions within a temperature range of 25–60°C. Dipole moments ratios and temperature coefficient, d ln〈μ²〉/dT, where 〈μ²〉 is the mean-square dipole moment of the chain, were calculated. These results are compared with earlier results.     

Investigation on Self-assembled Blend Membranes of Polyethylene-block-poly(ethylene glycol)-block-polcaprolactone and Poly(styrene-block-methyl methacrylate) with Polymer/gold Nanocomposite Particles

A block copolymer of polyethylene-block-poly(ethylene glycol) and polycaprolactone was prepared via co-ordination–insertion polymerization. Blend membranes of poly(styrene-block-methyl methacrylate) and polyethylene-block-poly(ethylene glycol)-block-polcaprolactone were used as matrix. Gold/polystyrene nanoparticles were used as nanofiller in polyethylene-block-poly(ethylene glycol)-block-polcaprolactone/poly(styrene-block-methyl methacrylate)/gold/polystyrene nanoparticles membranes. The double gyroid pattern was depicted by blend chains in polyethylene-block-poly(ethylene glycol)-block-polcaprolactone/poly(styrene-block-methyl methacrylate)/gold/polystyrene nanoparticles. An improvement of 22% in tensile strength and 54% in tensile modulus was observed with 1 wt% nanoparticle addition. Polyethylene-block-poly(ethylene glycol)-block-polcaprolactone/poly(styrene-block-methyl methacrylate)/gold/polystyrene nanoparticles 1 showed water flux of 45.2 mL cm^?2 min^?1 and salt rejection ratio of 17.5%. Efficiency of gold nanoparticles–polystyrene nanoparticles reinforced membranes in removal of heavy metal ions was 100%.     

Achievement of quasi-nanostructured polymer blends by solid-state shear pulverization and compatibilization by gradient copolymer addition

Nanoblends, in which dispersed-phase domains exhibit length scales of order 100 nm or less, are made using a continuous, industrially scalable, mechanical process called solid-state shear pulverization (SSSP). An 80/20 wt% polystyrene (PS)/poly(methyl methacrylate) (PMMA) blend processed by SSSP and consolidated by platen pressing, without melt processing, exhibits a quasi-nanostructured morphology with many irregular, minor-phase domain sizes of ∼100 nm or less. After short-residence-time single-screw extrusion, the pulverized blend exhibits spherical dispersed-phase domains with a number-average diameter of 155 nm. Thus, SSSP followed by certain melt-processing operations can yield nanoblends. However, the pulverized blend exhibits significant coarsening of the dispersed-phase domains during long-term, high-temperature static annealing, indicating that SSSP followed by other melt processes may yield microstructured blends. In order to suppress coarsening, a styrene (S)/methyl methacrylate (MMA) gradient copolymer is synthesized by controlled radical polymerization. When 5 wt% S/MMA gradient copolymer is added to the PS/PMMA blend during SSSP, the resulting blend exhibits a nanostructure nearly identical to that of the blend without gradient copolymer, and coarsening is nearly totally suppressed during long-term, high-temperature static annealing. Thus, SSSP with gradient copolymer addition can yield compatibilized nanoblends. Morphologies obtained in the pulverized PS/PMMA nanoblend are compared with those in blends of PS/poly(n-butyl methacrylate) and PS/high-density polyethylene made using identical SSSP conditions, providing for commentary on the ability of SSSP to produce nanostructured blends as a function of blend components.     

Dielectric Characteristics of Na2O · 3SiO2 Glasses with High Water Contents

Dielectric characteristics of Na₂O·3SiO₂ glasses with water contents up to ∼12 wt% were found to be drastically affected by incorporated water. The high-frequency dielectric constant increased with water content, while both the static dielectric constant and the low-frequency dielectric relaxation strength showed a pronounced minimum at a water content of ∼3 wt%.     

Thin Film Nanocomposites Based on SBM Triblock Copolymer and Silver Nanoparticles: Morphological and Dielectric Analysis

Hybrid organic/inorganic thin film nanocomposites based on poly(styrene)‐b‐poly(butadiene)‐b‐poly(methyl methacrylate) triblock copolymer and silver nanoparticles are prepared and characterized. In order to improve the compatibility of nanoparticles with the polymeric matrix, their surface is modified with dodecanethiol surfactant, which enables a good dispersion of nanoparticles through the triblock copolymer, without the formation of aggregates. By atomic force microscopy (AFM), the dispersion level of nanoparticles is analyzed, together with their effect on the thin film surface morphology, for nanocomposites up to 15 wt% of nanoparticles. Dielectric properties of nanocomposites are studied by dielectric relaxation spectroscopy (DRS), analyzing the effect of nanoparticles on dielectric properties. Even if conductivity and permittivity of composites increase with nanoparticle content, percolation threshold is found to be at around 15% in volume. Morphologically analyzed nanocomposites are, in this way, below the threshold.     

Effects of Pore Morphology and Grain Size on the Dielectric Properties and Tetragonal–Cubic Phase Transition of High-Purity Barium Titanate

The effects of pore morphology and grain size on the dielectric behavior of high-purity stoichiometric BaTiO₃ have been intensively investigated. It was found that the dielectric constant was influenced not only by grain size but also by pore morphology. Dielectric constants below the Curie temperature could be evaluated by the Maxwell relationship for specimens with fractional density >90%ρ_t and be estimated by the modified Niesel's equation, but depolarization might be involved for specimens with fractional density <90%ρ_t. Dielectric Behavior above the Curie temperature followed the Curie–Weiss low. The Curie constants could be separated into two regions depending on the pore morphology, decreasing linearly with increasing porosity at different rates. The results suggest that the tetragonal–cubic phase transition temperature of specimens with fractional density <90%ρ_t is affected by depolarization due to the presence of continous channel pores. The dissipation factor was increased with increasing porosity due to the adsorption of water. In this study, a high-density (<99%ρ_t), uniform, and fine-grained (∼1.2 μm) microstructure of high-purity stoichimetric barium titanate has been produced by using wet processing ad pressureless sintering, in which a high dielectric constant (>6100 at 25°C and 1 kHz) and a low dissipation factor (<0.025) could be achieved.     

Physical Properties of Triglycerides IV. Dielectric Constant

Dielectric constants at 20° and at 40° C of a number of triglycerides in the liquid state have been measured. A molar additive function of the dielectric constant, based on a relation derived by J. van Elk, was used in combination with a previously derived equation for triglycerides to give an equation which relates the dielectric constant of a triglyceride or triglyceride mixture to its refractive index, density, and iodine value. Quantification of this relation was based on prime data obtained from these model triglycerides and from previously published data on fatty acid methyl esters.