Mechanical and dielectric properties of bulky side chain poly(methacrylates). Analysis of the low frequency phenomena. 1: Poly(5-indanyl methacrylate)

This paper composes the dynamic mechanical and dielectric relaxation properties for Poly (5-indanyl methacrylate) (P51M), a polymer with a bulky side chain. Measurements were carried out from about ?100°C to near 250°C. A secondary loss peak was observed in dielectric measurements at room temperature (20°C at 0.1 Hz.), whereas nothing was resolved in the mechanical spectrum. A prominent α relaxation associated with the glass transition was also observed near 107°C at 0.1 Hz. in the dielectric and the mechanical spectra. Dipolar dielectric loss overlapped with conductivity at high temperatures and low frequencies. A new method to split the conductive, interfacial, and dipolar contributions to the spectrum is proposed.     

Dielectric and dynamic mechanical relaxation studies of styrene-acrylonitrile copolymers

Dynamic mechanical measurements (～1 Hz) and dielectric measurements (10^?4 to 10⁵ Hz) are presented for styrene-acrylonitrile copolymers containing 25 and 32.5% acrylonitrile. Two dielectric (α, β) and one mechanical (α) processes were observed and their mechanisms are discussed. It is shown that the Montrose-Litovitz fluctuation theory and Phillips-Barlow-Lamb defect-diffusion theory give reasonable representations of the dielectric α relaxation process.     

Relaxations in double base propellants

Double base propellants are composed of nitrocellulose(NC) plasticized with various amounts of nitroglycerine(NG). A series of propellants was made with NG contents of 10%, 35% and 60% by incorporating with solvents, extruding into cords and then drying. A study of relaxation behaviour of these propellants was made using dynamic mechanical analysis, thermomechanical analysis and differential scanning calorimetry (d.s.c.). Three relaxations were observed, one at about ?80°C designated γ, one in the temperature range ?10° to ?60°C designated β, and one above 35°C designated α. The γ relaxation is attributed to motion of the pendant nitrate side groups of NC. The β relaxation is attributed to motion of NC molecule side groups and associated NG molecules. It causes a peak in mechanical loss, and changes in modulus, thermal expansion and heat capacity. The β relaxation has previously been reported as the glass transition, but there is some doubt and this may be incorrect. The α relaxation occurs in the same temperature range as the glass transition reported in other plasticized cellulosic polymers, but it is unusual in that it involves a large change in modulus and mechanical loss, but no change in thermal expansion or heat capacity.     

The Electrical Behavior of Mica-Polystyrene Composite

Abstract

The electrical properties of mica-polystyrene composites were studied using the impedance measurements technique. The study was carried out as a function of frequency and mica concentration. It was observed that the AC conductivity and the dielectric constant are increased with increasing of the mica content in the composite. The observed electrical results fit approximately the reported empirical equations concerning the AC conductivity and dielectric behavior of polymer composites. Relaxation processes were observed to take place for composites having high mica concentration. The observed relaxation and polarization effects of the composite are mainly attributed to the dielectric behavior of the mica filler. However, the results were explained on the basis of the interfacial (space-charge) polarization, dipolar polarization and decrease of the hindrance produced by the polymer matrix.     

Dielectric Relaxation Spectroscopy of Poly (Vinyl Chloride-co-Vinyl Acetate-co-2-Hydroxypropyl Acrylate)/Poly (Acrylonitrile-Butadiene-Styrene) Polymer Blend

The dielectric techniques used to investigate the relaxation behavior of poly (vinyl chloride-co-vinyl acetate-co-2-hydroxypropyl acrylate) (PVVH), poly (acrylonitrile-butadiene-styrene) (ABS), and its polyblends include broadband AC dielectric relaxation spectroscopy (DRS) in the frequency range from 10^?2 to 10⁵ Hz, and thermally stimulated depolarization current (TSDC) technique in the temperature range from 300 K to 413 K. It was observed that PVVH is characterized by a dipolar relaxation peak around 347 K and a space charge peak in the temperature range 353–383 K, whereas pure ABS is characterized by a dipolar relaxation peak at 389 K. On the other hand, polyblend samples are found to be characterized by two different relaxation peaks at 359 K and 387 K, respectively. The dielectric properties of pure materials and their polyblend are investigated. All samples are characterized by high dielectric constant (ε′) at very low and high temperature. The dielectric loss shows a single peak for pure materials, whereas polyblends show a broad peak at low frequency which could be attributed to the Maxwell-interfacial polarization (MWS) and another peak at high frequency which could be attributed to the dipolar relaxation. The temperature dependence of AC conductivity was investigated for all samples. The values of the exponent n suggest that the hopping mechanism dominates at lower temperatures.     

Real-time dielectric studies of network formation in thermally activated epoxy-amine and isocyanate-triol systems

Dielectric measurements are reported on the changes that occur in the nature of the dipole relaxation processes during cure of an epoxy-amine and a diisocyanate-triol system. The epoxy resin forms a vitrified solid in the final cured state, whereas the urethane retains its elastomeric properties. The initial behaviour for the epoxy resin is dominated by ionic conduction processes. Subtraction of the conductivity contribution reveals a dipolar relaxation process, which is analysed using the Havriliak-Negami (HN) equation. The characteristic exponent 1 - n of the HN equation changes in a similar manner to that found with other epoxy-amine systems. However, the dipolar process in the urethane occurs at very high frequency and a different form of the 1 - n dependence is observed. Sensitivity of the dielectric method for the detection of vitrification and gelation is critically assessed. If the vitrification is allowed to occur, then detection of a gel point may not necessarily be inferred from the dielectric data.     

Structural,dielectric, and magnetic properties of reduced graphene oxide-wrapped MnFe2O4 composites synthesized by in-situ sol-gel autocombustion method

Herein, the reduced graphene oxide (rGO) wrapped MnFe₂O₄ (MFO@rGO) composites with different rGO content of 10, 20, 30, and 40 wt% have been synthesized by a one-step in-situ sol-gel autocombustion method. The synthesized composites have been tested for their structural, electrical, dielectric, and magnetic characteristics. The composites are characterized by using standard techniques (XRD, HR-TEM, FTIR, and Raman spectroscopy). The composite having 20 wt% of rGO exhibits the highest value of dielectric constant (ε′～1.32 × 10⁴ at 100 Hz, ε′～143 at 1 MHz) and dc conductivity (σ_dc = 4.31 × 10^?6 Ω^?1-cm^?1) among the investigated composites. The dipole polarization contribution to the dielectric relaxation behavior of MFO@rGO composites is observed, which arises from the increased vacancy defect dipoles in rGO sheets. The impedance studies show the existence of two different time relaxation phenomena in MFO@rGO composites. The magnetic measurements reveal the superparamagnetic behavior of composites. The saturation magnetization of composites decreases first with the increase in rGO content up to 20 wt% and then increases with a further increase in rGO content. The findings of this research make these composites a potential candidate for various applications such as EMI shielding and energy storage devices.     

Influence of ethoxylate chain length for synperonic NPX surfactants on the film formation behaviour of methylmethacrylate-2-ethylhexyl acrylate copolymer latexes: II. A dielectric investigation

A real time dielectric relaxation investigation covering the frequency range from 10⁻² to ∼10⁵ Hz, on the film forming process in methylmethacrylate-2-ethylhexyl acrylate latex copolymers stabilised with nonylphenol ethoxylate is presented. The three systems investigated have essentially identical latex compositions and the only difference between these materials is the length of the ethoxylate chain in the nonylphenol ethoxylate stabiliser. The dielectric relaxation measurements allow changes in the dipolar nature of the medium to be monitored as the process of film formation and coalescence occur. Combining the dielectric data with other observations of the coalescence process reported in part 1 has allowed identification of various stages in the film formation process. It is evident that the formation of a micro-crystalline phase between the emulsion particles by the longer chain ethoxylate molecules, inhibits the coalescence process. Moreover, the stabiliser can also be seen to play a role in determining the dynamics of the molecules in the latex and a critical role in the overall coalescence process. A schematic model describing the various stages of coalescence is presented.     

Dielectric dispersion near the morphotropic phase boundary of 0.64PMN-0.36PT ceramics

A near morphotropic phase boundary (MPB) composition of PMN-PT ceramic, 0.64Pb(Mg_1/3Nb_2/3)O₃-0.36PbTiO₃, has been synthesized for various piezoelectric, ferroelectric and dielectric applications. The relaxation mechanisms and dielectric characteristics of this solid solution have been investigated with dielectric spectroscopy measurements in the frequency range of 20 Hz–2 MHz. The dielectric properties have shown distinct and prominent Debye type relaxation at the temperatures corresponding to the ferroelectric phase. The peak in dielectric loss parameter has been found to be dominated by the dc conductivity in paraelectric phase. The experimentally obtained values of various parameters have been found in agreement with the values obtained by fitting of the experimental data in Debye model. An excellent agreement of the results with Debye distribution of relaxation times was obtained with the distribution parameter α showing minima around the ferro-to para-electric phase transition. Different activation energies of the relaxation time have been observed in the ferroelectric and paraelectric phases. An average relaxation time has been found to decrease from ～10^?3 s to ～10^?6 s with the increasing temperatures.     

Morphological,structural, dielectric and electrical properties of PEO–ZnO nanodielectric films

The morphological, structural, dielectric and electrical properties of aqueous solution-cast prepared poly(ethylene oxide)–zinc oxide (PEO–ZnO) nanocomposite films have been investigated as a function of ZnO nanoparticle concentrations up to 5 wt%. Scanning electron microscopy (SEM) images of these films show that the morphology of pristine PEO aggregated spherulites changes into fluffy, voluminous and highly porous with dispersion of ZnO nanoparticles into the PEO matrix. X-ray diffraction (XRD) study confirms that the crystalline phase of PEO greatly reduces at 1 wt% ZnO, and it again increases gradually with further increase of ZnO concentration. The dielectric relaxation spectroscopy (DRS) over the frequency range 20 Hz–1 MHz reveals that the real part of complex dielectric permittivity at audio frequencies decreases non-linearly whereas it remains almost constant at radio frequencies for these polymeric nanocomposites. Dispersion of nanosize ZnO particles into the PEO matrix reduces the values of dielectric permittivity which also exhibits a correlation with the dispersivity of ZnO nanoparticles. The relaxation peaks observed in the dielectric loss tangent and electric modulus spectra reveal that the electrostatic interactions of nanoscale ZnO particles with the ethylene oxide functional dipolar group of PEO monomer units decrease the local chain segmental dynamics of the polymer. Real part of ac conductivity spectra of these films have been analyzed by power law fit over the audio and radio frequency regions, respectively, and the obtained dc conductivity values for these regions differ by more than two orders of magnitude. The temperature dependent relaxation time and dc conductivity values of the nanodielectric material obey the Arrhenius relation of activation energies and confirm a correlation between dc conductivity and PEO chain segmental motion which is exactly identical to the characteristics of solid polymer electrolytes. Results imply that these nanocomposite materials can serve as low permittivity flexible nanodielectric for radio frequency microelectronic devices and also as electrical insulator for audio frequency operating conventional devices in addition to their suitability in preparation of solid polymer electrolytes.     

Dielectric relaxation in chlorinated polyethylene–polypropylene copolymers

Dielectric relaxation measurements were carried out on eight chlorinated polyethylene–polypropylene (PEPP) copolymers in the range of temperatures covering the main dielectric absorption. Chlorination of PEPP is expected to change the dynamic dielectric properties gradually with increasing amount of chlorine in the polymer chains. Thus, in the present study, increasing degrees of chlorination give a clear shift of the glass transition temperature towards higher values, except in the range between 40 and 51% chlorine, where an anomalous behaviour was observed. The same tendency is also observed in the relaxation strength (Δϵ). The value of Δϵ has been estimated by using a nonlinear squares regression program (LEVM6) to calculate the parameters of the Havriliak–Negami empirical equation. It appears reasonable to assume that the anomalous behaviour observed can be attributed to a compensation of the dipolar moments of chlorine groups in the macromolecules.     

Dependence of the electrical conductivity and the low-frequency dielectric constant upon pressure in porous media containing a small quantity of humidity

The interaction between the matrix of a porous material and a small quantity of a liquid located in its porous space induces intense polarisation phenomena and enhances the charge transport. The temperature dependence of the corresponding relaxation time was reported recently by employing the high-resolution thermally stimulated depolarisation current spectroscopy. The present work includes d.c. conductivity and a.c. capacitance measurements under pressure up to 0.3 GPa in two porous silicate materials: sandstone with 0.26 wt.% and pelite with 0.07 wt.%. The water was adsorbed from the atmospheric humidity. Electrical impedance measurements are used to monitor the effects of confining pressure on the matrix-liquid interaction. Positive activation volumes are computed from the observed variations in the d.c. conductivity of the samples with pressure. The pressure derivatives of the dielectric constant at 10⁴ Hz were also obtained. The effect of pressure on the effective relaxation time that determines the humidity-related polarisation processes was estimated.     

Effect of Surface Moisture on the Polarization of Individual Glass Spheres

When glass particles adsorb water from humid environments their low frequency dielectric properties change drastically. To investigate the polarization mechanism, variable frequency measurements upon individual glass particles (～ 50 μm diameter) suspended in silicone oil were performed with a computer-controlled dielectrophoretic levitator. The electrical properties of individual particles have been measured in the frequency range between 10 Hz and 100 kHz. A relaxation in the induced dipole moment of moistened glass particles suspended in silicone oil is clearly evident in measured spectra. Surface moisture causes the characteristic relaxation times of soda-lime glass particles to change by up to 5 orders of magnitude. The relaxation spectra are satisfactorily modeled by the Debye equation and a model based on ohmic surface conduction appears to explain the phenomenon. This research has potential implications with respect to the controversy surrounding the influence of moisture on the performance of electrorheological fluids.     

Effect of copolymerization with methacrylonitrile on the dielectric relaxation of polyacrylonitrile

Dielectric relaxation in the glass transition region of polyacrylonitrile (PAN), polymethacrylonitrile (PMAN), and their random copolymers containing 6–46 wt % MAN has been studied in the frequency range 10²–10⁵ Hz and temperature range 40–160°C. Changes in the dielectric constant increment, relaxation temperature, and the loss peak amplitude with changing MAN content in the copolymers were observed. Dielectric constant increment and loss peak amplitude showed maxima around 6–12 wt % MAN content while the relaxation temperature increased continuously over the entire range. The results are discussed in terms of the possible structural differences arising from the effect of the introduction of MAN units on the dipolar interactions between the nitrile groups.     

High‐k dielectric composites of poly(2‐cyanoethyl vinyl ether) and barium titanate for flexible electronics

High‐k dielectric composite material for electronic applications was obtained by mixing a polymer with high dielectric constant, poly(2‐cyanoethyl vinyl ether) (CEPVA), and highly crystalline barium titanate (BT). Barium titanate nanoparticles of a size in the range 40–90 nm were prepared by the solvothermal method. By optimizing the reaction conditions, the formation of carbonate impurities and the agglomeration of formed nanoparticles were significantly reduced compared to state‐of‐the‐art procedures. Dielectric spectroscopy was measured in the range of 0.01 Hz to 10 MHz and showed the dielectric constant to be ?′ ～ 35–40 with only 30 vol % content of BT in the composite. Extrapolating to 100% BT nanoparticle concentration and using the Lichtenecker model, the dielectric constant ?′ = 365 ± 27 at 10 kHz was obtained. The relaxation and electrical properties were investigated in depth, and a new relaxation phenomenon was revealed. CEPVA/BT composite is considered suitable for electronic applications, in which high ?′ together with a good mechanical flexibility are required, such as organic field effect transistors. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 45236.     

Structure and properties of ethylene—methacrylic acid copolymers and their sodium salts: Dielectric and proton magnetic relaxation studies

Dielectric and proton magnetic relaxation data have been obtained for an ethylene-methacrylic acid copolymer (containing about 4 mole% methacrylic acid units) and its 53% ionised sodium salt. The degrees of crystallinity and percentage ionisation of the samples investigated were estimated by infra-red methods. The dielectric results were obtained principally in the frequency range 100 Hz to 10 kHz and at temperatures ranging from 80° to – 120°. A few results are also reported at frequencies down to 0·1 Hz and up to 100 MHz. For the acid copolymer, two dielectric loss regions are observed and these are correlated with the reported mechanical β′- and γa-processes respectively. The partly ionised copolymer exhibits three dielectric relaxation regions which correlate with the mechanical α-, β- and γa-relaxations respectively. In addition, a dielectric peak appears at about —40° in the presence of absorbed water, a result similar to that found in the polyamides. The proton magnetic relaxation results were obtained by pulse methods which yielded the spin-lattice relaxation times T₁ (at 30 MHz) and T_1p (at kilohertz frequencies) as a function of temperature from ?180° to 100°. Two components were generally observed for T_1p. For the acid copolymer the β′- and γ-processes have been observed from these results, as well as a lower-temperature (δ) process which has not been detected by the mechanical or dielectric methods. For the sodium salt the γ- and δ-processes are also found, in addition to a high-temperature process in the region of the merged α- and β-processes. The present data are consistent with previous assignments for the β′-, α-, β- and γ-processes. The ‘water’ relaxation appears to involve some rotation of water molecules, or of ionic segments to which water molecules are attached, in the proposed ionic domains. The δ-process is ascribed to the rotations of methyl groups present in the methacrylic acid units.     

Environmental effects and relaxation in natural rubber

The change in attenuation of longitudinal ultrasonic waves has been measured as a function of frequency and temperature for natural rubber samples loaded with intermediate super abrasion furnace (ISAF) in increasing quantities and vulcanized with N-oxidiethylene benzotriazol sulfunamid (OBTS) after being subjected to natural aging for 5 years. Three relaxation processes, namely α, β, and γ, were noticed. The activation energy for each process was calculated and the results are discussed. Also, the dielectric constant ε′ and dielectric loss ε″ have been investigated for those samples after being immersed in various water media. The measurements were carried out in the frequency range from 100 Hz up to 100 kHz and at room temperature (～ 28°C). The results are compared with each other and the differences are interpreted.     

The electrical conductivity of polypropylene and nickel-coated carbon fiber composite

The effect of filler content, temperature, and frequency of the applied electric field on DC and AC electrical conductivities of composites of nickel-coated carbon fibers and polypropylene are considered. The impedance behavior and the dielectric properties of these composites were studied in the low frequency range 10 Hz–30 kHz. It was found that the volume electrical resistivity shows filler content and temperature dependence. The calculated activation energy of the thermal rate-process decreases with the filler concentration, while the shielding effectiveness increases. The overall observed permittivity of the composites increases with the filler concentration, and the dielectric behavior is discussed in terms of the space charge, electronic and interfacial polarization within the covered frequency range. It was observed that the AC conductivity is nearly independent of the frequency below 100 Hz and increases with frequency above this range. Finally, it was concluded that addition of nickel-coated carbon fibers could alter the electrical conduction mechanism and the polarization process of the polymeric matrix.     

Phase transitions and mechanical properties of nitrocellulose plasticized by glycidyl azide polymer and nitroglycerine

Films of nitrocellulose (NC), glycidyl azide polymer (GAP), and nitroglycerine (NG) have been evaluated using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy, dynamic mechanical analysis (DMA), and tensile testing. The SEM micrographs demonstrate that, even at low GAP concentration, a portion of GAP will coalesce into spherical domains due to a saturation effect. This is related to the inability of higher molecular weight GAP to effectively situate itself between NC polymer chains. The addition of a small fraction of lower molecular weight NG completely changes this behavior. DMA confirms that two transitions are present and can be attributed to a plasticizer rich phase (β), a polymer rich phase (α) and that NC plasticized with GAP is in accordance with the Gordon-Taylor equation. Tensile results show that the addition of a small fraction of NG to a NC/GAP based-formulation increases elongation at break to values similar to that of the NC/NG base formulation. The combination of these two plasticizers, GAP and NG, allows for the plasticization of NC at significantly lower environmental and human toxicity levels.     

Dielectric investigation of the molecular dynamics of blends (II): polymers with dissimilar molecular architecture (PS/TMPC blend)

The molecular dynamics of polystyrene/tetramethyl polycarbonate (PS/TMPC) blends were investigated using dielectric spectroscopy in the frequency and temperature ranges 10^?2?10⁷ Hz and 50–220°C, respectively. Blends with different compositions, namely 12.5, 25, 50, 75 and 87.5 wt% PS/TMPC were found to be compatible over the entire composition range and showed one glass relaxation process corresponding with the transition observed by calorimetric measurements. It was found that neither the kinetics or the distribution of relaxation times of the local process observed in pure TMPC were affected by blending. This fact implies that the local environment of the segments is not changed upon blending. Furthermore, these blends showed that the blending of TMPC with weakly polar polystyrene does not cause any change in the dipolar interaction of the TMPC segments, so that the relaxation strength varies linearly with composition. It is concluded that the polymeric chains of the TMPC and PS are not mixed at a segmental level, but at a higher structural level lying somewhere between segmental and molecular. This conclusion is in good agreement with that obtained in the case of blends composed of polymers with similar molecular architecture.