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     

Alpha Relaxation Study of Poly(Vinyl Chloride Co-Vinylacetate-Co-2-Hydroxypropyl Acrylate)

By using thermally stimulated depolarization current (TSDC) technique, and coupling it with the thermal sampling (TS) method, the relaxation behavior of Poly(vinyl chloride-co-vinylacetate-co-2-hydroxypropyl acrylate) (PVVH) has been investigated in the vicinity of its glass transition temperature, T_g. The global TSDC spectra of amorphous PVVH with varying the poling field and poling temperature revealed the presence of two TSDC peaks. The first peak at about 347 K and is ascribed to the glass transition temperature and can be referred as α-relaxation. The second one is obtained in the temperature range 353–383 K and is attributed to the space charge relaxation and can be referred as ρ-relaxation. Fine structure of α-relaxation peak was obtained using the thermal sampling method. All the molecular parameters, such as activation energy (E_a) and preexponential factor (τ₀), have been estimated. In addition, the compensation law was found to be valid; and the compensation parameters such as compensation temperature (T_c) and compensation time (τ_c) have been determined.     

Dielectric Relaxation and Electrical Conductivity Study in Thiourea- Doped Poly (Vinyl Alcohol)

Abstract

FT-IR, UV-spectroscopy and X-ray diffraction of thiourea-Poly (vinyl alcohol) (PVA) complexes have been investigated. It was observed that thiourea affected the structure of PVA. X-ray study revealed that as the thiourea salt increases the crystallinity of PVA increases. The dielectric properties of thiourea-PVA complexes were investigated in the frequency range 0.2 to 100 kHz and the temperature range 293 to 433 K. All samples are characterized by high values of dielectric permittivity at low frequencies and high temperatures. The results are discussed in terms of interfacial Maxwell-Wagner-Sillars (MWS) polarization and space-charge polarization. Different relaxation modes, i.e., β and α-relaxations have been observed in pure PVA. AC conductivity measurements of samples showed that the hopping conduction mechanism dominates at low temperature region, from room temperature to the glass transition temperature.     

DIELECTRIC RELAXATION OF POLY(VINYL CHLORIDE) STABILIZED WITH DIBUTYLTIN LAURATE-MALEATE

The dipolar relaxation mechanisms of polyvinyl chloride stabilized with dibutyltin laurate-maleate have been studied by dielectric relaxation spectroscopy. The study was carried out in the frequency range 1 k–1 MHz at fixed temperatures above and below the glass transition temperature. The broadening of the loss peaks was considered because of the multiplicity of molecular conformations, which stablizer molecules can form. The results show a distinct broad relaxation process with activation energy of 0.5 eV. The dielectric strength increases with temperature and the origin of the relaxation was ascribed to the reorientation of the polar C–Cl and C=O groups.     

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.     

Dielectric Properties of Pure and Gd‐Doped HfO2 Ceramics

The pure, 2 at.%, and 20 at.% Gd‐doped HfO₂ ceramics were prepared by the standard solid‐state reaction technique. Dielectric properties of these ceramics were investigated in the temperature range 300–1050 K and frequency range 20–5 × 10⁶ Hz. Our results revealed an intrinsic dielectric constant around 20 in the temperature below 450 K for all tested ceramics. Two oxygen‐vacancy‐related relaxations R1 and R2 were observed at temperatures higher than 450 K, which were identified to be a dipolar relaxation due to grain response and a Maxwell–Wagner relaxation due to grain‐boundary response, respectively. The dielectric properties of the pure and slightly doped (2 at.%,) samples are dominated by the grain‐boundary response, which results in a colossal dielectric behavior similar to that found in CaCu₃Ti₄O₁₂. The doping level of 20 at.% leads to the structural transformation from monoclinic phase to cubic phase. The dielectric properties of the heavily doped HfO₂ are dominated by the grain response without any colossal dielectric behavior.     

Giant permittivity up to 100 MHz in La and Nb co-doped rutile TiO2 ceramics

Dielectric spectroscopy was carried out for reduced and stoichiometric La_0.0025Nb_0.0025Ti_0.995O₂ ceramics synthesized by sintering in different atmospheres. A giant permittivity (~1 × 10⁴) was obtained at a frequency of 100 MHz and temperature range from 170 to 350 K. Three dielectric relaxation mechanisms were observed within the temperature range of 10-300 K via dielectric spectroscopy. A low temperature dipole relaxation peak (in the temperature range of 10-30 K) in the spectra was identified to be associated with the giant permittivity specifically measured at 100 MHz. The origin of such giant permittivity was attributed to dipole orientation polarization. Hopping polaron and interfacial effect contributed to giant permittivity. After annealing treatment, all the relaxation contributions were weakened. Low dielectric loss was attributed to high resistance of grain and grain boundaries. Annealing in ambient conditions led to decreased relaxation times which gives the signature of decreased concentration of oxygen vacancies and Ti³⁺. Dipoles which were related to oxygen vacancies and Ti³⁺, resulted in giant permittivity up to 100 MHz.     

Dielectric behaviour and relaxation in poly(propylene glycol)–water mixtures studied by time domain reflectometry

Dielectric behaviour of poly(propylene glycol) (PPG) of number‐average molecular weight 2000 g mol^?1 and binary mixtures of PPG with water (PPG–W) of various concentrations were carried out in the frequency range 10 MHz to 4 GHz at 25 °C. The dielectric dispersion and absorption curves related to the orientational motion of these molecules in the binary mixtures are described by a single relaxation time using Debye's model. The values of static dielectric constant ε_o, high frequency limiting dielectric constant ε_∞, and dielectric relaxation time τ_o were determined for PPG and PPG–W mixtures. The values of the dielectric parameters were used to explore the nature of homogeneous and heterogeneous dynamic networks formed through hydrogen bonding in the binary mixtures of PPG and water molecules with concentration variation. The dielectric studies of PPG molecules were also carried out in the same frequency range at four temperatures, namely 25, 35, 45 and 55 °C. The temperature‐dependent relaxation times were used to evaluate the thermodynamical parameters for the dielectric relaxation processes. The dielectric relaxation free energy of activation ΔF_τ for PPG molecules was found in the range ～4.5 to 4.7 kcal mol^?1, which corresponds to the activation energy needed for the breakage of hydrogen bonds. Furthermore, the large negative value of the entropy ΔS_τ of PPG molecules confirms that the configuration involved in dipolar orientation has an activated state, which is more ordered than in the normal state. Copyright © 2004 Society of Chemical Industry     

Dielectric studies of nitrocellulose-nitroglycerine mixtures

Dielectric measurements are reported over a frequency range from 10 Hz to 10⁷ Hz and over a temperature range from ～220–280 K for nitrocellulose (NC) (12.2% nitrated) with varying levels of nitroglycerine (NG), up to 60 wt% NG. A dipolar relaxation is observed and its variation with concentration of NG, temperature, with the presence of stabilizer and method used in preparation of samples are reported. It is observed that the amplitude of the dipolar relaxation increases slowly with the addition of NG up to ～30%, whereupon it increases markedly with further addition of NG. The activation energy for the relaxation process is similarly observed to be sensitive to the level of NG in the system and depends critically on the concentration up to 30%, whereupon it becomes independent of concentration. Comparison of the dielectric data reported here with absorption measurements and nuclear magnetic relaxation studies indicates that these observations can be interpreted in terms of a critical adsorption model. At low concentrations, the NG is adsorbed onto the surface of the microcrystallite structures present in the NC system. Monolayer coverage is believed to occur at ～30% and the marked changes in the dielectric behaviour are consistent with the formation of a multilayer structure.     

Dielectric Relaxation and Spectroscopic Investigation of Polyhydroxybutyrate PHB Blended with Polyvinyl Acetate PVAc and Poly(Vinylacetate-Co-vinyl Alcohol) PACA

Using frequency response analyzer covering a frequency range from 10²–10⁶ Hz in a wide range of temperature, the dielectric behavior of the investigated systems was studied. In order to investigate the relaxation mechanisms of such systems, the dielectric loss data on the frequency domain were analyzed using Havriliak-Nagami and/or Fröhlich functions in addition to the conductivity term. These mechanisms are discussed in terms of the orientation of the main chain and its related motions. The relaxation times related to both mechanisms noticed for PHB/PVAc blend with composition 50% are found to be higher than those for other compositions. This could be attributed to the interaction expected through hydrogen bond formation. This result is supported by the data given by FTIR spectroscopy as the carbonyl region at 1750 cm^–1 showed a more broad band spectrum when compared with those for the other compositions.     

Structure,electrical, dielectric,and optical investigation on polyvinyl alcohol/metal chloride nanocomposites

Different concentrations of metal chlorides/polyvinyl alcohol nanocomposites have been prepared by the typical solvent cast technique. The prepared samples were investigated by different techniques such as X‐ray diffraction, differential scanning calorimetry, and scanning electron microscope. DC and AC conductivities are examined at different temperatures and frequencies. An activation process was found in the DC conductivity versus temperature relation and the activation energy was calculated. The AC conductivity obeyed the ω^S power law. The behavior of S with temperature was studied. Various dielectric parameters such as dielectric constant (ε′), dielectric loss (ε″) and loss tangent (tan δ) have been determined in the temperature range 303–443 K at different frequencies. The dielectric parameters were found to decrease with increasing frequency. The study of dielectric relaxation as a function of temperature at constant frequency shows two relaxation mechanisms. The optical band gaps and band tails were estimated from the measured absorption spectra. The applied photon energy found to affect the observed optical band gaps. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Transient currents in poly(vinyl alcohol)–poly(vinyl pyrrolidone) polymer blend films

Transient currents (charging and discharging currents) in poly(vinyl alcohol) (PVA)–poly(vinyl pyrrolidone) (PVP) polymer blend films were measured over the temperature range 30–150°C at field strengths of 2.32–23.2 × 10⁶ Vm^?1. Polymer films were prepared by the isothermal immersion technique. Activation energies were evaluated from quasi-steady-state currents. A single relaxation peak was observed both from isochronal currents and low frequency dielectric relaxation. Activation energies evaluated from these two methods are found to be in fairly good agreement. The polarization is considered to be due to space charge origin along with some contribution from dipolar groups. The maximum loss was observed in Sample I (PVA: PVP = 25:75), suggesting maximum heterogeneity in this blend ratio.     

Ionic conductivity,dielectric behavior,and HATR–FTIR analysis onto poly(methyl methacrylate)–poly(vinyl chloride) binary solid polymer blend electrolytes

Solid polymer electrolytes comprising blends of poly(vinyl chloride) (PVC) and poly(methyl methacrylate) (PMMA) as host polymers and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) as dopant salt were prepared by solution‐casting technique. The ionic conductivity and dielectric behavior were investigated by using AC‐impedance spectroscopy in the temperature range of 298–353 K. The highest ionic conductivity of (1.11 ± 0.09)×10^?6 S cm^?1 is obtained at room temperature. The temperature dependence of ionic conductivity plots showed that these polymer blend electrolytes obey Arrhenius behavior. Conductivity–frequency dependence, dielectric relaxation, and dielectric moduli formalism were also further discussed. Apart from that, the structural characteristic of the polymer blend electrolytes was characterized by means of horizontal attenuated total reflectance–Fourier transform infrared (HATR–FTIR) spectroscopy. HATR–FTIR spectra divulged the interaction between PMMA, PVC, and LiTFSI. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Studies on dielectric relaxation and ac conductivity of cellulose acetate hydrogen phthalate–poly (vinyl pyrrolidone) blend

The dielectric constant, dielectric loss, and ac conductivity of polyblends of cellulose acetate hydrogen phthalate (CAP) and poly (vinyl pyrrolidone) (PVP) of different compositions were measured in the temperature range of 300–430 K and in the frequency range of 50 Hz–100 kHz. In the blends, the dielectric constant as well as the dielectric loss as a function of the temperature display a single peak corresponding to the glass transition temperature (T_g) in the region between the T_g values of the pure polymers. The T_g values observed agree well with those values obtained from DSC. Dielectric studies show that CAP forms a miscible blend with PVP. Ac conductivity values were calculated from the dielectric data and the conduction mechanism is discussed. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1702–1708, 2002     

On the microhardness testing of electrically stressed poly(phenylene oxide): Polystyrene blends

Microhardness measurements have been performed on untreated (virgin) and electrically stressed, solvent‐cast laboratory‐prepared samples of pure poly(phenylene oxide) (PPO), pure polystyrene (PS), and PPO : PS polyblends with different weight proportions. Results of such measurement on untreated polyblend sample show that microhardness (H_v) increases with increase in the content of PS up to 10 wt %, which attributed to the existence of homogeneous phase morphology. However, this feature is not observable in samples containing higher content of PS. Electrical stress is found to modify considerably the mechanical property of polymer. The effect of electric field on the microhardness of such samples (PPO : PS :: 90 : 10) has been characterized by the existence of a peak. Trapping of charge carriers in electrically stressed samples imparts hardening to the polyblend up to an applied step field of 190 kV/cm. However, the excessive charging beyond this step field value destroys this characteristic. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

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.     

Thermal,conductivity, and dielectric properties of poly(2-ethyl-2-oxazoline)-polyvinylpyrrolidone blends

Bio-compatible polymer blends of poly(2-ethyl-2-oxazoline) [PEOX] and polyvinylpyrro-lidone [PVP] were prepared at various compositions (80:20, 60:40, 40:60, and 20:80 wt%). These polymer blends were characterized by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The thermal stability of the blends was evaluated by thermogravimetry. The kinetic parameters such as activation enthalpy, ?H, activation entropy, ?S, and free energy of activation, ?G, were calculated using kinetic model given by Broido for all the blends. The thermal studies show that PEOX: PVP (20:80) blend has good thermal stability compared to other blends. The results show that thermal stability and decomposition temperature of PEOX was considerably improved by the addition of PVP. The electrical and dielectric properties of PEOX:PVP (80:20) blend were measured in the temperature range of 313 K - 353 K using an LCR meter for the frequency range 100 Hz - 8 MHz. The dielectric studies shows that dielectric constant, dielectric loss and electric modulus decreases with frequency and increases with temperature, whereas AC conductivity increases with frequency and temperature.     

Dielectric analysis of chlorinated polyvinyl chloride stabilized with di‐n‐octyltin maleate

Dielectric relaxation of chlorinated polyvinyl chloride stabilized with di‐n‐octyltin maleate has been studied in the frequency range 1 kHz–1 MHz at various temperatures between 300 and 450 K. A clear dielectric relaxation band (α‐type) associated with dipolar polarization has been observed in the studied range. The stabilizer molecules increase the segmental mobility of the polymer, and specific interaction occurs between both the C? Cl and C?O polar groups. The temperature dependence of the relaxation process was analyzed by combining the critical free volume expression by Cohen and Turnbull with the Williams‐Landel‐Ferry (WLF) model of the thermal expansion of free and occupied volumes. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

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     

Frequency and Temperature Dependence of Dielectric Properties of Au/Polyvinyl Alcohol (Co,Ni-Doped)/n-Si Schottky Diodes

The dielectric properties and AC conductivity of Au/polyvinyl alcohol (Co, Ni-doped)/n-Si Schottky diodes (SDs) were investigated in the frequency range 1 kHz–1 MHz and in the temperature range 80–400 K. The frequency and temperature dependence of dielectric constant (?′), dielectric loss (?″), loss tangent (tan δ), AC electrical conductivity (σ _ac ) and the real and imaginary parts of the electric modulus (M′ and M″) were found to be a strong function of frequency and temperature. The values of ?′, ?″ and tan δ decrease with increasing frequency, while they increase with increasing temperature, especially above 275 K. The values of σ _ac increase with both increasing frequency and temperature. Such temperature-related behavior of σ _ac can be attributed to the high mobility of free charges at high temperature. Electric modulus formalism was also analyzed to obtain experimental dielectric data. The values of M′ and M″ increase with increasing frequency, while they decrease with increasing temperature. The interfacial polarization, which more easily occurs at low frequencies and high temperatures, consequently contributes to the improvement of the dielectric properties of SDs.