Dielectric analysis of cellulose and its vinylic copolymers

The real and imaginary parts of the complex dielectric permittivity, ε′ and ε″, for some vinylic copolymers of cellulose [prepared with vinyl acetate (VA) and methyl acrylate (MA) and Ce (IV) ions as initiator ] and for cellulose were measured over a frequency band of 0.1–10² kHz and a temperature range from ?40 to 100°C. In vinylic copolymers of cellulose, we observed one dielectric relaxation attributed to the α-relaxation of the vinylic side chain grafted on cellulose. In cellulose dielectric spectra, this relaxation did not appear, but we detected one relaxation that may correspond to the β-relaxation. For these vinylic copolymers of cellulose, the ε″ against ε′ plot gives a skewed are that closely resembles that of the Davidson–Cole model, with a broader distribution for high frequencies that shows the overlap of several relaxations in the process considered. Some differences observed between the vinylic copolymers of cellulose may be due to the composition and the length of the vinylic side chains and to the frequency of grafting on the cellulose.     

Dielectric constant and loss investigations of pristine,iodine‐doped and annealed polyetherimide film

Temperature‐ and frequency‐dependent dielectric behaviour has been investigated for pristine, iodine‐doped and annealed polyetherimide (PEI) in the temperature range 303–523 K at various frequencies (120 Hz, 1 kHz, 10 kHz and 100 kHz). In pristine PEI, the rapid decrease in the dielectric constant (ε′) in the temperature range 303–373 K is governed by the Kirkwood model whereas the gradual decrease in ε′ in the temperature range 373–473 K is governed by β‐relaxation (dipolar) and αβ‐relaxation (hybrid) processes. The occurrence of these relaxations is confirmed by the appearance of tan δ loss peaks at ca 403 and 443 K, respectively, in ε″–T curves. The temperature‐independence of ε′ in the temperature range 473–523 K is mainly governed by the α‐relaxation process associated with large segmental groups. The tan δ loss peak appearing at 503 K confirms this relaxation process. In iodine‐doped samples, an overall increase in ε′ is attributed to the formation of charge‐transfer complexes in the polymer structure. The dominance of a new relaxation process due to interaction of iodine with ether linkages neutralizes the 1/T rule in the low‐temperature region. A significant decrease in ε′ in annealed samples below 393 K is due to the suppression of the dipolar relaxation process. The enhancement in ε′ above 393 K is due to the dominance of the α‐relaxation process. The shift in the high‐temperature tan δ loss peak towards higher temperature with increasing frequency shows the distributive nature of relaxation time for this relaxation process. Copyright © 2011 Society of Chemical Industry     

Vitrification,devitrification, and dielectric relaxations during the non‐isothermal curing of diepoxy‐cycloaliphatic diamine

The curing of an epoxy resin based on diglycidyl ether of bisphenol A (DGEBA) with a diamine based on 4,4′‐diamino‐3,3′‐dimethyldicyclohexylmethane (3DCM) was analyzed by dielectric relaxation spectroscopy (DRS) between ?100 and 220°C, at heating rates ranging from 0.1 to 2 K min^?1. The permittivity, ε′, and the loss factor, ε″, were measured by DRS in the frequency range between 1 and 100 kHz. The dielectric relaxations were correlated with the relaxations observed previously by temperature modulated differential scanning calorimetry (TMDSC) at the same heating rates and in modulation conditions of amplitude 0.2 K and a period of 60 s, which is equivalent to a measuring frequency of 16.7 mHz. The dielectric measurements showed three frequency‐dependent dipolar relaxations and one ionic relaxation, which was independent of the frequency. The dipolar relaxations were associated with the glass transition of the unreacted system and the vitrification and the devitrification processes of the system during the crosslinking reaction, and the ionic relaxation was associated with the beginning of the crosslinking reaction. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 558–563, 2006     

Effect of Substituents on Dielectric β-Relaxation in Cellulose

The dielectric characteristics for some cellulose derivatives, namely chlorodeoxycellulose (Cell-Cl; degree of substitution of chlorine, DS_Cl=0·87), bromodeoxycellulose (Cell-Br; DS_Br=0·92) and thiocyanatodeoxycellulose (Cell-SCN; DS_SCN=0·88), all substituted only at C-6, together with those of regenerated cellulose, have been investigated in the temperature range -60 to 120°C, and in the frequency range 0·2–100kHz. Only one relaxation process, designated as β, was identified within the frequency and temperature ranges studied. The activation energy of this relaxation increases in the order Cell-Cl<Cell-Br<Cell-SCN, suggesting that the bulkiness of the substituent was the determining factor of the activation energy. The characteristic dielectric parameters, namely polarization magnitude (Δε) and shape parameter (α or β-), were obtained by the analysis of absorption bands and are discussed in relation to the substituent effect. © of SCI.     

Dielectric behaviour of pure and nickel-doped cellulose acetate films

Pure and Ni (0·05%, 0·1% and 0·15%)-doped cellulose acetate (CA) films were formed on well-cleaned glass substrates using a solution growth technique. Metal–insulator–metal (MIM) sandwich structures of thin film capacitors were fabricated. Dielectric studies were carried out in the frequency range 1kHz to 10MHz in the temperature range 303–450K. The variation of capacitance and dielectric loss with temperature were studied at various frequencies. The variation of dielectric constants (ε′ and ε″) with frequency and temperature is discussed. An increase in capacitance with temperature was observed and this may be due to the chaotic oscillations of molecules in the polymer matrix. β (at lower temperature) and α (at higher temperature) relaxation loss peaks were observed. Doping results in the formation of charge transfer complexes/molecular aggregates in the polymer, which cause the shift of C_max value and the β and α relaxation loss peaks towards higher temperature. An appreciable dispersion of tan δ at lower frequency was noticed in all the samples studied. © 1998 SCI.     

Dielectric study of β-relaxation in some cellulosic substances

6-(2-Cyanoethylamino)-6-deoxycellulose (Cell-CEA) as a novel cellulose derivative was prepared from 6-bromo-6-deoxycellulose. The influence of the reaction conditions on the degree of substitution was studied in detail. The dielectric characteristics of a Cell-CEA sample having degree of substitution of 0.86, together with those of microcrystalline and regenerated celluloses, have been investigated in the temperature range from −60 to 120°C, and frequency range 30 Hz to 100kHz. One relaxation process, designated as β, was recognised, which was attributed to side-group motions, probably −CH₂X (X = OH or NHCH₂CH₂CN). The higher activation energy of this process and the shift to higher temperature in Cell-CEA, compared with microcrystalline and regenerated celluloses, reflects the enhanced steric constraints involved in the motion of the bulkier, −CH₂NHCH₂CH₂CN, side groups. The results obtained are discussed in terms of the dielectric parameters ϵ″_max′ β and Δϵ.     

Phase transition studies of polymer–liquid crystal composite using dielectric and optical transmittance techniques

Dielectric properties of polymer–liquid crystal composite comprised of the polymer poly(methyl methacrylate) and low‐molecular mass liquid crystal, cholesteryl palmitate are investigated in the frequency range from 1 kHz to 10 MHz and in the temperature range of 27–120°C. The dielectric permittivity and dielectric loss shows substantial changes with the addition of polymer. The significant feature is that the pure liquid crystal exhibits broad absorption, showing dielectric relaxation in the range of 10 – 100 kHz whereas the composite shows relaxation peaks at 10 MHz. The percentage optical transmittance of pure liquid crystal and composite have also been measured and compared to study its phase transition behavior. The smectic phase observed in the pure liquid crystal is suppressed on formation of polymer composites. The mass spectrometry measurement does not show any abduct formation between the pure liquid crystal and the composite. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

The variation of the dielectric constant and loss index with temperature and draw ratio in α‐PVDF

In this study, the effect of measurement temperature and uniaxial drawing on the real (dielectric constant, ε′) and imaginary (loss index, ε″) parts of the complex dielectric constant of α‐crystalline phase poly(vinylidene fluoride) (PVDF) was investigated. The samples having different draw ratios (λ) were obtained by drawing the PVDF film at constant speed and temperature. The dielectric measurements were performed in the frequency range of 100 Hz–1 MHz and in the temperature range of 80–400 K. Although ε′ and ε″ were not affected by the orientation process during the β‐relaxation transition, it was observed that there were systematical variations for the α‐relaxation transition. ε′ and ε″ showed different behaviors depending on the draw ratio at different temperatures. Especially, ε″ was more affected by the orientation process at 380 K. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Dielectric analysis of α‐relaxation process of chlorinated poly(vinyl chloride) stabilized with nitro‐phenyl maleimide

Nitro‐phenyl maleimide (NPM), is the organic stabilizer for poly (vinyl chloride) (PVC), has been investigated as thermal plasticization for rigid chlorinated poly (vinyl chloride) (CPVC). Dielectric relaxation of CPVC stabilized with 10 wt% of NPM has been studied in temperature and frequency ranges of 300–450 K and 10 kHz–1 MHz, respectively. An analysis of the dielectric constant, ε′ and dielectric loss index, ε″, was performed assuming a plasticization effect of NPM molecules. The plasticization effect of NPM molecules was confirmed by the behavior of the dielectric modulus M′ and M″ spectra. A clear dielectric α‐relaxation process has been obtained in the studied temperature range. The results showed that NPM reduce the glass transition temperature, T_g, of CPVC by about 20 K. This effect has been assigned to the plasticization effect of NPM. At lower temperatures, dielectric modulus spectra reveal that there is a role of the effect of the electrode polarization in the relaxation process. The behavior AC conductivity, σ_ac, indicated that the conduction mechanism in all CPVC samples is hopping type conduction. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Effect of heat treatment on dielectric relaxation of polyacrylonitrile: Reversible thermally induced structural change

The effect of heat treatment on the dielectric relaxation of polyacrylonitrile (PAN) was studied. Dielectric measurements, in the frequency range 0.1–100 kHz and temperature range 30–170°C, on PAN heat treated at two temperatures (120 and 160°C) and the untreated PAN are reported. The loss peak height, the dielectric relaxation strength, loss peak width, and the loss peak temperature were found to vary systematically with the temperature of the heat treatment. The changes in dielectric relaxation produced by heat treatment at the higher temperature (i.e., 160°C) were considerably less than those produced by heat treatment at 120°C. The results are discussed and attributed to the variation of the degree of bound nitrile groups (through their dipole–dipole interaction) with heat treatment temperature. Heat treatment of the higher temperature (160°C) results in lower degree of bound nitrile groups in the structure than heat treatment at the lower temperature (120°C).     

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.     

Investigation on gradient dielectric characteristics of bamboo (Dentroclamus strictus)

Dependence of dielectric constant (ε′) and dielectric dissipation factor (tan δ) on distance from outermost skin to the center of bamboo has been determined. Dielectric measurements have been done in the temperature range of 24–120°C and in the frequency range, 4–100 kHz. Gradient behavior in ε′ and tan δ has been found in bamboo. It has also been observed that ε′ and tan δ increase with increasing temperature and decrease with increasing frequency. Relaxation times have been calculated for the four samples at 80, 90, and 100°C temperatures, which show that relaxation time decreases with the increase of temperature because of the increased molecular mobility. A continuous increase in the hardness from center (48) to outer surface (70) and density from 0.45 to 0.80 g/cc has been observed. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 380–386, 2006     

The effect of moisture on the dielectric behavior of poly(vinyl acetate)–natural rubber blend

Dynamic dielectric analysis was used to analyze the miscibility of poly(vinyl acetate) and natural rubber and the effect of moisture on the dielectric properties of the polymer blend. Two separate dielectric relaxations were observed within the temperature range studied. The high-temperature relaxation was a result of the glass transition of poly(vinyl acetate), whereas the low-temperature relaxation was attributed to that of natural rubber. The peak temperatures of ε″ of the high-temperature relaxation decreased with increasing aging time, while the magnitudes of the ε″ peaks increased with increasing days of aging. The peak temperatures of ε″ of the low-temperature relaxation stayed constant, while the magnitudes of the ε″ peaks increased with increasing days of aging. This indicates that both polymers are responsible for the sorption of moisture; however, water is an effective plasticizer only for poly(vinyl acetate). Anomalous increases in the magnitudes of ε″, especially in the low frequency sides, of the Argand diagrams were explained by Maxwell–Wagner–Sillar (MWS) polarization at component interfaces. The oscillator strength, relaxation time, and the two distribution parameters, A and B, calculated from the skewed circular are equation, were affected with days of aging.     

(1-x)NBT-xBaTiO_3系陶瓷材料的弛豫特性的研究

测量了各样品在1kHz、10kHz、100kHz频率下从室温至500℃的介电温谱,表明材料为弛豫铁电体,Ba2+的掺入使其弛豫特性有所降低。当x≥0.10时,材料在低温下具有正常铁电体的特征,高温下具有弛豫铁电体的特征。两个介电反常峰的存在表明在此温度区间材料发生了两次相变。     

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     

Physicochemical properties of hydrolyzed and blended chitin

The molecular structures of chitin and chitin hydrolyzed with sodium hydroxide for different time intervals at 160°C were followed using infrared spectroscopy in the range 200–4000 cm^?1. The frequency and intensity of active groups NHCOCH₃, NH₂, OH, and OCH₃ in chitin, chitosan, cellulose, and lignin, respectively, were calculated and correlated with molecular structural changes. The dielectric constant ?′, dielectric loss ?″, and dissipation factor tan δ for the investigated samples were measured in the frequency range 0.1–100 kHz and interpreted in terms of the molecular structure elucidated from the infrared spectroscopic studies. Also, the effect of blending of cellulose and lignin with chitin on their dielectric properties was investigated. It was found that hydrolysis of chitin improved its insulating properties.     

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     

(1-x)NBT-xBaTiO3系陶瓷材料弛豫特性的研究

测量了各样品在1kHz、10kHz、100kHz频率下从室温至500℃的介电温谱,表明材料为弛豫铁电体,Ba2+的掺入使其弛豫特性有所降低。当x≥0.10时,材料在低温下具有正常铁电体的特征,高温下具有弛豫铁电体的特征。两个介电反常峰的存在表明在此温度区间材料发生了两次相变。     

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.     

Improved dielectric properties in A′‐site nickel‐doped CaCu3Ti4O12 ceramics

The improved dielectric properties and voltage‐current nonlinearity of nickel‐doped CaCu₃Ti₄O₁₂ (CCNTO) ceramics prepared by solid‐state reaction were investigated. The approach of A′‐site Ni doping resulted in improved dielectric properties in the CaCu₃Ti₄O₁₂ (CCTO) system, with a dielectric constant ε′≈1.51×10⁵ and dielectric loss tanδ≈0.051 found for the sample with a Ni doping of 20% (CCNTO20) at room temperature and 1 kHz. The X‐ray photoelectron spectroscopy (XPS) analysis of the CCTO and the specimen with a Ni doping of 25% (CCNTO25) verified the co‐existence of Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺. A steady increase in ε′(f) and a slight increase in α observed upon initial Ni doping were ascribed to a more Cu‐rich phase in the intergranular phase caused by the Ni substitution in the grains. The low‐frequency relaxation leading to a distinct enhancement in ε′(f) beginning with CCNTO25 was confirmed to be a Maxwell‐Wagner‐type relaxation strongly affected by the Ni‐related phase with the formation of a core‐shell structure. The decrease of the dielectric loss was associated with the promoted densification of CCNTO and the increase of Cu vacancies, due to Ni doping on the Cu sites. In addition, the Ni dopant had a certain effect on tuning the current‐voltage characteristics of the CCTO ceramics. The present A′‐site Ni doping experiments demonstrate the extrinsic effect underlying the giant dielectric constant and provides a promising approach for developing practical applications.