Dielectric relaxation and Poole–Frenkel conduction in poly(vinyl chloride) blends with bisphenol A/Egyptian corncob resin

The dielectric constant (ε′), dielectric loss index (ε″), direct-current conductivity, and current–voltage (I–V) characteristics of pure poly(vinyl chloride) (PVC) and blends of PVC and bisphenol A/Egyptian corncobs (BCC) were investigated at different temperatures. The relaxation processes for PVC and its blends revealed that PVC and BCC had an incompatible phase. PVC blends with 5 wt % BCC exhibited a peculiar I–V behavior. Both ε′ and ε″ were used to study miscibility and phase behavior in blends of PVC. The activation energies of all PVC samples were calculated. At higher voltages, the conduction mechanism could be identified as the Poole–Frenkel type. In addition, the ionic groups of BCC could enhance the PVC conductivity. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Dielectric relaxation of poly(tetrafluoroethylene–perfluorovinyl ether) films

Dielectric constant and dielectric loss have been studied for poly(tetrafluoroethylene–perfluorovinyl ether) (PFA) films over a wide temperature range in the frequency range 0.1–100 kHz. Two relaxation peaks were observed, one at room temperature (α_a-relaxation) and the other in the range 170–140 K (β-relaxation), with activation energies of 143·2 and 16·4 kcal/mol, respectively. The β-absorption is attributed to the short segmental local mode motion of the main chains. The α_a-relaxation can be interpreted as due to large-scale conformational rearrangement. The Cole–Cole diagrams are given at different temperatures and the distribution parameters (ϵ₀–ϵ_∞) and (1–α) of the relaxation times were calculated. The X-ray diffraction pattern of PFA shows both a diffuse halo and sharp reflections, characteristic of amorphous and crystalline phases of conventional semicrystalline polymers. Also, no evidence of crystallinity in the films due to thermal treatment during dielectric measurements was observed. IR spectra revealed the absence of any new peaks after the heat treatment.     

Viscosity-temperature relationship of dilute solution of poly(vinyl chloride) in cyclohexanone and in its blends with xylene

Absolute viscosities of dilute solutions of poly(vinyl chloride) (PVC) in cyclohexanone and in its blends with xylene were determined at temperatures ranging from 5 to 50°C and in THF at 5–30°C. The simple Arrhenius reaction kinetics equation, used by Anrade to analyze the viscosity data of liquids and later by Moore for dilute polymer solutions, was used to estimate the thermodynamic parameters for the flow of PVC solutions and the quality of solvents. This relationship was also used to calculate thermodynamic parameters for PVC in binary solvent systems. The data of the activation energy of the viscous flow and preexponential factor were further analyzed by using the empirical relationships suggested by Moore for flexible polymer chains. The parameters obtained from these relationships are used to estimate and compare the solvency power of solvents and solvent blends. A blend of 1 : 1 cyclohexanone and xylene is found to have better dissolution characteristics for PVC than have cyclohexanone and other blends. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 2749–2760, 1997     

Dielectric properties of low molecular weight poly(ethylene glycol)s

This paper reports the measured values of dielectric permittivity ε′ and dielectric loss ε″ of ethylene glycol, diethylene glycol and poly(ethylene glycol)s of average molecular weight 200, 300, 400 and 600 g mol⁻¹ in the pure liquid state. The measurements have been carried out in the frequency range 200 MHz to 20 GHz at four different temperatures of 25, 35, 45 and 55 °C. The complex plane plots (ε″ versus ε′) of these molecules are Cole–Cole arcs. The static dielectric constant ε₀, high‐frequency limiting dielectric constant ε_∞, average relaxation time τ₀ and distribution parameter α have been determined from these plots. The value of the Kirkwood correlation factor g and the dielectric rate free energy of activation ΔF have also been evaluated. The dependence of relaxation time on molecular size and viscosity has been discussed. A comparison has also been made with the dielectric behaviour of these molecules in dilute solutions of non‐polar solvents, which were carried out earlier in this laboratory. The influences of intermolecular hydrogen bonding and molecular chain coiling on the dielectric relaxation of these molecules have been recognized. © 2000 Society of Chemical Industry     

Effects of the composition ratio,eosin addition,and γ irradiation on the dielectric properties of poly(vinyl alcohol)/glycogen blends

The dielectric relaxation spectra of various poly(vinyl alcohol) (PVA)/glycogen blends and irradiated blend samples with 70 wt % PVA content that were undoped and doped with eosin were measured in extended temperature (30–160°C) and frequency (1 kHz to 1 MHz) ranges. Dielectric relaxation spectroscopy separates different molecular groups of a repeating unit of a polymer with respect to the rate of its orientation dynamics. In the high‐temperature range (>100°C), the σ relaxation, which is associated with the hopping motion of ions in the disordered structure of the biopolymeric material, can be measured. The electric dipole moment and the activation energy of the glass‐transition temperature relaxation process were calculated. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Dielectric investigation of isotropic and anisotropic solutions of hydroxypropyl cellulose in dioxan

The dielectric behaviour of hydroxypropyl cellulose in dioxan has been studied at 10–50°C over a range of concentration of 10–55 wt% to include the isotropic and anisotropic phases. The study showed that the loss maximum ε″_max magnitude of polarization ε₀ ? ε_∞ relaxation time 1/2πf_m degree of broadening of the absorption curves 1–h or α, and the mean-square dipole moment 〈gμ²〉, steadily increase with concentration up to 42 wt%, above which a rapid decrease takes place. This indicates that the isotropic solution transforms to an anisotropic solution with a smaller mean dipole moment. The critical concentration is realized to be temperature invariant. This was evidenced by measuring the refractive index of solutions covering the same concentration and temperature ranges.     

Dielectric investigation of isotropic and anisotropic solutions of cellulose acetate in dioxane

The dielectric and optical characteristics of a sample of cellulose acetate (DS = 2.45) in dioxane solutions were studied at 10–50°C of concentration 10–50 wt% to include both isotropic and anisotropic phases. The study showed that the loss maximum, ε_max″, magnitude of polarization, (ε₀ ? ε_∞), static dielectric constant, ε₀, time of relation, (2πf_m)^?1, and refractive index, n_D, steadily increase with concentration up to the critical concentration (41 wt%) and then decrease. The mean-square dipole moment, 〈gμ²〉, decreases steadily up to the critical concentration then remains nearly constant, indicating that the isotropic solution changes to anisotropic, with smaller mean-square dipole moment. Comparison between the results of cellulose acetate (CA) and those of hydroxypropyl cellulose (HPC) reveals that, at the critical concentration in dioxane, the cholesteric structure of HPC possesses a greater mean-square dipole moment with higher temperature coefficient than does CA. The activation energy of the relaxation process for hydroxypropyl cellulose is higher, indicating a greater intrachain interaction compared with cellulose acetate.     

Dielectric dispersion studies of poly(vinyl alcohol) in aqueous solutions

The dielectric constant ε′ and loss factor ε″ of deionized water and poly(vinyl alcohol) in aqueous solutions are measured in the frequency region 200 MHz to 20 GHz at four different temperatures (25, 35, 45 and 55 °C). Complex plane plots (ie ε″ vs ε′) are drawn to obtain the static dielectric constant ε₀, high frequency dielectric constant ε_∞, distribution parameter α and average relaxation time τ₀. The variations of dielectric constants with increasing solvent concentration and temperature are discussed in terms of solute–solvent and solute–solute interactions. The average relaxation time τ₀ of poly(vinyl alcohol) aqueous solutions is found to the very short. It is also observed that the relaxation time is almost independent of the viscosity of the solution. The effect of water concentration on macromolecular size, shape and flexibility of the molecular chain are discussed using the observed values of dielectric relaxation times at different temperatures. The possibility of multiple dielectric dispersion is also discussed with concentration variation. © 2000 Society of Chemical Industry     

Influence of diamine structure on the low temperature dielectric relaxation of some poly(ether imide)s

Broadband dielectric relaxation spectra are reported on a range of poly(ether imide) polymers in which the chemical structure of the diamine used to create the polymer is systematically varied with the anhydride structure based on 2,2‐bis‐[4‐(3′,4′‐dicarboxyphenoxy)phenyl]hexafluoroisopropylidine dianhydride. In all the polymers examined, a dipole relaxation was observed below room temperature. The magnitude and activation energy associated with the relaxation process varied with the chemical structure reflecting the effects of steric hindrance on the conformational change associated with the N? C and C? O? C linkages. Values of the activation energies varied between 29 and 34 kJ/mol^?1, and are consistent with the observed relaxation being associated with constrained local oscillatory motions of small elements of the polymer backbone. The glass transition temperatures of these polymers are in the range 195–243°C and are associated with the large scale motion of the polymer backbone. Changes in the backbone structure influence the extent of inter chain–chain interaction and are reflected in the amplitude of the relaxation process and the high frequency limiting dielectric permittivity ε_∞ values which are important when these polymers are used in thin film electronic applications. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41684.     

Dielectric relaxations in poly [2,2-propane-bis-(4-phenyl thiocarbonate)]

The dielectric relaxation properties of poly[2,2-propane-bis-(4-phenyl thiocarbonate)] (PTC) have been studied. The existence of crystallinity, which can be eliminated by quenching, is detected. The degree of crystallinity of polymer samples was determined by differential scanning calorimetry in order to investigate the effect of this factor on the dielectric behaviour of this polymer. The thermal degradation of the samples was studied by thermogravimetry. The degradation of the polymer begins before the glass transition temperature T_g. The dielectric spectrum is complex showing several relaxation phenomena. With increasing temperature a γ relaxation can be observed at - 100°C (5 kHz). The activation energy obtained from an Arrhenius plot (lnfvs T^?1) is 6 kcal mol^?1. At 160°C the α relaxation which is associated with the glass transition temperature T_g is detected. The dielectric behaviour of this poly(thiocarbonate) is compared with the corresponding poly(carbonate).     

Dielectric properties of copolymers based on cyano monomers and methyl α‐acetoxyacrylate

With the aim of developing dielectric polymers containing CN groups with strong dipole moment, alternating and statistical copolymers of the cyano monomers vinylidene cyanide (VCN), acrylonitrile and methacrylonitrile with methyl α‐acetoxyacrylate (MAA) were synthesized and characterized. The copolymer's composition and microstructure were analysed by NMR spectroscopy, SEC and elemental analysis. The reactivity ratios calculated from the Q–e Alfrey–Price parameters for these copolymers indicated the alternating and statistical structures confirmed by NMR analysis. The copolymers have glass transition temperatures T_g in the range 83–146 °C and are stable up to 230 °C. The thermal stability of the copolymers depends on the nature of the cyano monomers. Their molecular dynamics were investigated by dielectric relaxation spectroscopy. We revealed a weak relaxation β at sub‐T_g temperature for poly(VCN‐co‐MAA) usually originating from molecular motions that are restricted to the scale of a few bond lengths. Strong α‐relaxation processes occurred above T_g for these copolymers. This primary relaxation was associated with cooperative movements of the polar groups (CN) at the time of mobility of the principal chains. The activation energy of the α‐relaxation process was also calculated. The values of the dielectric increment Δε for these copolymers were determined by Cole–Cole plots and indicated that the copolymers exhibit interesting dielectric properties compared with similar cyano materials. The polarity–permittivity relationship was also established. © 2012 Society of Chemical Industry     

Solvent effects on microwave dielectric relaxation in poly(ethylene glycols)

This paper reports the results of a systematic study of microwave dielectric relaxation times of poly(ethylene glycols), average molecular weight 200–9000, in dilute solutions of benzene at 9·83GHz. These results are compared with the values of relaxation times obtained earlier in carbon tetrachloride solutions. This shows that the average relaxation times τ₀ and the relaxation time corresponding to segmental reorientation τ₁ are influenced by the solvent environment. The variation in chain flexibility in these polymers with the increase in degree of polymerization and formation of intra- and inter-molecular hydrogen bonding in benzene and carbon tetrachloride solutions is discussed with the help of relaxation data. The relaxation time τ₂ corresponding to group rotations has been determined. It is found that the τ₂ value is independent of solvent environment and degree of polymerization, and may be attributed to the rotation of chain −OH end-groups around the C−O bonds in dynamic equilibrium, with the formation of a five-membered ring due to intra-molecular hydrogen bonding at the end of the chain. © 1998 SCI.     

Dielectric relaxation and the molecular motion of poly(vinylidene fluoride) crystal form II under high pressure

The α dielectric relaxation of poly(vinylidene fluoride) crystal form II is studied under pressures up to 7 kbar at temperatures from 100° to 200°C. The isotherm for variation of the dielectric increment with pressure shows a maximum. This behaviour is examined on the basis of models of molecular motion for the α relaxation previously proposed; longitudinal disorder exists in the crystalline chains. The calculations reproduce the experimental results except for the pressure coefficient of the dielectric increment. The metastable conformation exists together with the most stable conformation in one chain, and dipole reversal parallel to the molecular axis occurs throughout the whole chain.     

Dielectric relaxation and thermodynamic properties of polyvinylpyrrolidone using time domain reflectometry

Temperature‐dependent values of dielectric permittivity ε′ and dielectric loss ε″ of polyvinylpyrrolidone (PVP, commercialized as PVP K‐60) solution of average molecular weight 160 000 g mol^?1 were measured. The measurements were carried out in the frequency range 10 MHz to 20 GHz using time domain reflectometry at temperatures from 25 to 0 °C. The dielectric spectra can be described by the Davidson‐Cole model. Dielectric parameters such as the static dielectric constant ε₀, the high frequency limiting dielectric constant ε_∞, the relaxation time τ₀ and the distribution parameter β and thermodynamic parameters such as the free energy of activation ΔF_τ, the enthalpy of activation ΔH_τ and the entropy of activation ΔS_τ were determined. The average free energy of activation was found to be in the range 12.55–14.65 KJ mol^?1 and the enthalpy of activation was found to be 6.86 KJ mol^?1. Entropies of activation were found to be positive at all the measured temperature values and these large positive values of entropies reveal a less ordered structure of the PVP solution. The Kirkwood correlation factor g and the dipole moment µ were also determined for PVP solution. The results were compared with the results of the PVP‐water system studied previously. Copyright © 2011 Society of Chemical Industry     

Microwave dielectric relaxation study of poly(propylene glycol) in dilute solution

Dielectric relaxation studies of poly(propylene glycol), average molecular weight 2000 g mol⁻¹, in dilute solution of cyclohexane, decaline, benzene and carbon tetrachloride have been carried out at 10.10 GHz and 35 °C. Average relaxation time τ₀, and relaxation times corresponding to segmental motion τ₁, group rotations τ₂ and dipole moment µ have been determined. It is found that τ₀, τ₁ and µ are influenced by the solvent environment while the τ₂ value is solvent‐independent. A comparison has been made with the dielectric behaviour of poly(ethylene glycol), average molecular weight 1500 g mol⁻¹, in dilute solutions of benzene and carbon tetrachloride because both systems have an equal number of monomer units. The effect of methyl side‐groups on dielectric relaxation in poly(propylene glycol) molecules is discussed. The Kirkwood correlation factor is also evaluated in dilute solutions with concentration variation and it is found that these molecules exist in cluster form due to intermolecular hydrogen bonding. © 2000 Society of Chemical Industry     

Effect of Wood Flour Content on the Thermal,Mechanical and Dielectric Properties of Poly(vinyl chloride)

Several composite formulations of poly(vinyl chloride)/olive wood flour (PVC/WF) were manufactured by dry‐blending PVC, wood flour, plasticizer and other processing additives in a high‐intensity mixer. The dry‐blended compounds were calendered into film samples (T = 180°C, calendered time = 8 min). The films obtained are cut into normalized samples for thermal, mechanical, and dielectric characterization. The results obtained show that stress as well as strain at break decrease sharply as the wood flour content increases. On the other hand, this filler content has little influence on the glass transition temperature. It decreases the temperature of decomposition setting and retards the PVC thermal decomposition. It increases permittivity as well as dielectric losses. The thermal stability, as measured by thermogravimetry (TGA) and differential scanning calorimetry (DSC) methods, is good enough to permit processing of these types of PVC compounds using conventional processing techniques and temperatures under 210°C.     

Effect of Various Temperature Environments on the Performance of PVC/ATH Composites as Insulating Material

Insulating materials should be heat and fire resistant within the specified limits without damaging other properties. In the present work, different samples of polyvinyl chloride (PVC) and PVC/alumina trihydrate (ATH) composites have been prepared and exposed to various degrees of temperatures, which represent the variation of the temperature in the surrounding environment for outdoor purposes. The permittivity () and dielectric loss (ε″) over the frequency range from 100 Hz to 100 kHz and also the electrical resistivity (ρ) have been investigated. The mechanical properties and the fire-retardant efficiency of the PVC and PVC/ATH composites have also been studied. It was found that the permittivity and dielectric loss (ε″) increase slightly by increasing the ATH content up to 25 phr, after which an abrupt increase was noticed. The electrical resistivity (ρ) was found to be 8.3×10¹⁰ and 3.1×10¹⁰ Ω · cm for the samples containing 25 phr ATH filler at 25 and 70°C, respectively. The effect of various temperatures (5, 25, and 70°C) on the mechanical properties has been evaluated for the different samples. Increasing the ATH content to 45 phr improves the compressive strength at room temperature and at 5°C.

The investigations demonstrate that the rate of burning time and the density of smoke of PVC/ATH composites decrease as the concentration of ATH increase.     

Studies of dielectric characteristics and surface energies of spin‐coated polyimide films

A two‐step method, that is, polyamic acid formation with subsequent curing, was used to synthesize six kinds of polyimides. Dielectric constants and surface energies were investigated to determine the nature of the fluorinated and nonfluorinated polyimides. The dielectric constant decreased from 3.3 (at 100 kHz) for PMDA/ODA to 2.6 (at 100 kHz) for 6FDA/4,4′‐6F when the fluorine content increased from 0 to 30.7 wt %. Simultaneously, the water contact angle increased from 65° for PMDA/ODA to 78° for 6FDA/4,4′‐6F. Experimental results indicated that fluorinated polyimides contained a lower dielectric constant with improved water resistance. The surface energy values obtained from experiments agreed well with Holmes' correlation between surface energy and dielectric constant. The surface energies and dielectric constants were significantly affected by the polymer backbone structures, especially by the fluorination effect. Therefore, by choosing the appropriate monomers, polyimides of low dielectric constants with hydrophobic surfaces could be obtained. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1642–1652, 2001     

Effects of solvents on electrospun polymeric fibers: preliminary study on polystyrene

Six solvents [acetic acid, acetonitrile, m‐cresol, toluene, tetrahydrofuran (THF) and dimethylformamide (DMF)] with different properties (eg density, boiling point, solubility parameter, dipole moment and dielectric constant) were used to prepare electrospun polystyrene (PS) fibers. Fiber diameters were found to decrease with increasing density and boiling point of the solvents. A large difference between the solubility parameters of PS and the solvent was responsible for the bead‐on‐string morphology observed. Productivity of the fibers (the numbers of fiber webs per unit area per unit time) increased with increasing dielectric constant and dipole moment of the solvents. Among the solvents studied, DMF was the best solvent that provided PS fibers with highest productivity and optimal morphological characteristics. The beadless, well‐aligned PS fibers with a diameter of ca 0.7 µm were produced from the solution of 10 % (w/v) of PS in DMF at an applied electrostatic field of 15 kV/10 cm, a nitrogen flow rate of 101 ml min^?1 and a rotational speed of the collector of 1500 rev min^?1. Copyright © 2004 Society of Chemical Industry     

The dielectric behavior of the natural resin manila copal

The dielectric relaxation data of the natural resin Manila copal, obtained by Bhattacharya,¹ has been analyzed by the Cole–Cole method at temperatures from 30° to 150°C. Two distinct relaxation processes were found for temperatures of 70°, 80°, and 90°C as opposed to only one as observed by Bhattacharya.¹ The relaxation at 100°C and at higher temperatures could be represented by the typical Cole–Cole patterns. The variation by temperature of the static dielectric constant (ε₀) exhibited three distinct slopes, which indicated the different phases of the resin during thermal transformation. Two “transformation points”, around 70° and 100°C, were found instead of only one at 105°C as observed by Bhattacharya.¹ The static dielectric constant, the dielectric increment, the Cole–Cole distribution parameter, and the relaxation time decreased markedly owing to the increase of temperature from 100° to 150°C. These indicated some changes in conformation of the resin molecules during transition from the solid to the liquid state.