Impedance spectroscopy of reactive polymers. 1

Dielectric measurements were utilized to follow the advancement of cure in an epoxy/amine formulation. In contrast to earlier studies, complex impedance was measured during cure and used to calculate ionic resistivity. By using complex impedance were able to separate according to their frequency dependence the contributions to overall polarization from electrode blocking layers, migrating charges, and dipole relaxations. At any stage of cure, there is a unique frequency at which ionic resistivity can be singularly measured. Our approach does not involve trial-and-error frequency search and is conducive to the development of phenomenological models based on equivalent circuits. Excellent agreement was reported between the calculated values of normalized degree of cure obtained by dielectric and calorimetric measurements. © 1994 John Wiley & Sons, Inc.     

Dielectric Dispersion and Electric Relaxation Processes Induced by Ionic Conduction in Formamide, 2-Aminoethanol and Their Binary Mixtures

The complex dielectric permittivity, ionic conductivity, electric modulus and impedance spectra of the dipolar molecules formamide (FA), 2-aminoethanol (AE) and their binary mixtures were investigated in the frequency range from 20 Hz to 1 MHz at 303.15 K. Debye-type distributions of the frequency dependent electric modulus and complex impedance were found, corresponding to an ionic conduction relaxation process in the upper frequency regime of the spectra, whereas a spike in the impedance spectra at low frequencies confirms the contribution of an electrode polarization (EP) relaxation process induced by ionic conduction. Due to the high static permittivity of FA, its ionic conductivity was found more than one order of magnitude higher than that of the AE, which is also shown by the comparative values of their EP and ionic conductivity relaxation times. The dependences of dc ionic conductivity values of the binary mixtures on their relaxation times and static permittivity were explored. The concentration dependent static permittivity and the relaxation times led us to infer the formation of a 1:1 H-bonded stable complex between FA and AE molecules with reduction in the number of effective parallel-aligned dipoles.     

Impedance spectroscopy of reactive polymers. 5. Impedance as a measure of chemical and physical changes in glass formers

A study was conducted aimed at establishing the nature of chemical and physical phenomena in polymeric and nonpolymeric glass formers that can be observed by impedance measurements. Various systems were investigated that undergo a temporal evolution of structure as a result of chemical reactions and physical processes such as crystallization, vitrification, or phase separation. Distinct and systematic changes in impedance during crystallization and vitrification confirmed that these events could be monitored by impedance spectroscopy. Of particular interest was the potential use of impedance measurements in detecting gelation in crosslinking polymers. It was shown that the experimentally observed “knee” in imaginary impedance during reaction shifts with frequency and, hence, cannot be used to measure gelation. But a new insight at the molecular level was obtained by employing a novel experimental approach based on simultaneous dielectric-infrared measurements. Evidence was generated to support the formation of a hydrogen-bonded complex in the vicinity of gel point in polymer networks, which affords a vehicle for the migration of intrinsic charges and provides a contribution to the overall conductivity. This finding should be explored further because it suggests the possibility of correlating dielectric response with gelation. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 641–653, 1998     

Relaxations in thermosets. XVI. Dielectric studies of negative feedback during curing of an epoxide-ethylene-diamine thermoset

The complex dielectric permittivity of thermosets of diglycidly ether of bisphenol-A cured with ethylene diamine has been studied during their isothermal curing at several temperatures. As cross-linking progresses, the dc conductivity decreases. At the beginning of the cure the dc conductivity can be fitted to both the scaling laws with a critical exponent of about 4 and an equation indicating approach toward a singularity. In the later stage of the cure, the change in permittivity corresponds to dipolar relaxation of an infinitely connected network, and the Argand diagram for the complex permittivity measured at a fixed frequency obtained as the curing process proceeds at 305 K is similar to that for the complex permittivity as frequency is varied for a time-invariant system which obeys a stretched exponential relaxation function with the curing parameter or exponent, γ = 0.29. Increase in the temperature of isothermal curing lowers both γ and the net decrease in the equilibrium permittivity on curing. A plot of the calculated relaxation time with curing time is sigmoidal and shifts to shorter times on increasing the curing temperature. Measurement of the dielectric properties during the cure but for different frequencies show that the various parameters for the curing kinetics are independent of the frequency of measurement. These observations confirm the development of our concepts of thermoset curing in terms of a phenomenon of negative feedback between molecular diffusion and chemical reactions.     

高聚物固体电解质聚氧化乙烯－溴化铜复阻抗谱及介电常数的静水压效应

采用分子量５００万的聚氧化乙烯和无水溴化铜，通过混溶蒸发法制备成高聚物固体电解质Ｐ（ＥＯ）ｎ－ＣｕＢｒ２薄膜，并在０．１～３００ＭＰａ范围不同的流体静水压下详细测量其复平面阻抗谱，分别得到在不同压力下离子电导率和介电常数与测量频率的关系．进一步解谱准确地求出Ｐ（ＥＯ）ｎ－ＣｕＢｒ２（ｎ＝１２、１６）薄膜离子电导率和介电常数的静水压效应，并结合Ｘ－光物相分析，根据离子迁移通道的物理图象和高聚物的极化机构进行了初步的讨论．添加２０％的增塑剂碳酸丙烯酯，较大改进了压力下的导电性．１２０～３００ＭＰａ的离子电导率提高一个数量级     

Impedance spectroscopy of reactive polymers. 3. Correlations between dielectric,spectroscopic, and rheological properties during cure of a trifunctional epoxy resin

An investigation was carried out of correlations between dielectric, vibrational spectroscopic, and rheological properties during cure of a thermoset formulation composed of trifunctional epoxy resin and tetrafunctional amine. Experimental techniques utilized include impedance spectroscopy, near-infrared spectroscopy, steady shear, and dynamic mechanical measurements. Reaction kinetics obtained from dielectric and spectroscopic results were in excellent agreement. Gelation and vitrification times determined by dielectric and rheological measurements were also found to agree very well, despite the empirical nature of such correlations. A characteristic pattern in plots of imaginary impedance as a function of reaction time was reported for the first time in the open literature, and it was suggested that it could be used to identify gelation and vitrification during the network formation. A realization of the full potential of dielectric impedance spectroscopy in monitoring the progress of chemophysical changes in reactive polymers, however, hinges upon a development of fundamental scientific correlations between dielectric and chemorheological phenomena during cure.     

Dielectric relaxation processes and ionic conduction behaviour in poly(ethylene oxide)–montmorillonite clay nanocomposite aqueous colloidal suspensions

The relative complex dielectric function, electric modulus, alternating current (ac) electrical conductivity and complex impedance spectra of poly(ethylene oxide) (PEO)–montmorillonite (MMT) clay aqueous colloidal suspension (hydrocolloids) were investigated over the frequency range 20 Hz to 1 MHz at 27 °C. The relaxation time corresponding to electrode polarisation and Maxwell–Wagner polarisation processes (ionic conduction) were determined from these plots. The direct current (dc) electrical conductivity is evaluated from the fitting of real part ac conductivity data to the Jonscher power law. A correlation of increase in dc conductivity and decrease of ionic conduction relaxation time with increase of clay concentration is discussed considering intercalation of PEO chains and its dynamics and exfoliation of MMT clay nanoplatelets in these complex fluids. The formation of PEO–MMT clay supramolecular lamellar nanostructures with increase in continuity of lamellae arrangements were explored for the structural conformation of these nanocomposite novel materials.     

Investigation of ion-selective electrodes with neutral ionophores and ionic sites by EIS. I. Theory

Models of an ion selective electrode involving an ionophore and mobile sites in a membrane are proposed. The first model, called the phase boundary potential model, supposed thermodynamic equilibrium; it allows the concentrations of the various species to be calculated. Then, a kinetic model, which takes into account the ionic transfer at the membrane|solution interfaces, was derived. The impedance of the membrane was calculated. It shows that a membrane with nernstian behavior shows only one capacitive loop in the impedance diagram, which is related to the conductivity and dielectric properties of the material of the membrane. Non-nernstian behavior is related to slow ionic transfer at the membrane|solution interfaces or/and transport limitation of the species in the membrane. Finite rate constants of the ionic transfer lead to a capacitive loop in the middle frequency range, whereas finite rate transport leads to a diffusional impedance in the low frequency range.     

In Situ Monitoring of Polymer Processing Properties

Frequency-dependent dielectric measurements, often called frequency-dependent electromagnetic sensing (FDEMS), made over many frequency ranges, Hz–MHz, provide a sensitive, convenient automated means for monitoring in situ the processing properties of thermosets and thermoplastics. Using a planar wafer-thin sensor, measurements can be made in situ in almost any environment. Through the frequency dependence of the impedance, this sensing technique is able to monitor chemical and physical changes throughout the entire cure process. In this presentation we discuss how to use the frequency dependence in the Hz–MHz range to separate and determine parameters governing ionic and dipolar mobility is discussed. In turn, the relationship between these two molecular probes, ionic translational mobility and dipolar rotational mobility, and macroscopic processing properties is also discussed. The cure of a high-temperature polyimide (PMR-15) and the cure of several epoxy resins were monitored using dielectric sensing. The ionic and dipolar mobilities are quantitatively related to the viscosity, degree of cure and the build up in glass transition temperature. The ability of the change in ionic mobility to detect the gel point through the determination of an inflection point and through the use of a power law (1−t/t_gel)^x is discussed. Finally the application of in situ sensing to monitor processing properties and to control the cure process intelligently in a complex part in an autoclave is described. © 1997 John Wiley & Sons, Ltd.     

Origin of temperature dependent conductivity of alpha-polyvinylidene fluoride

The conductivity of alpha-polyvinylidene fluoride is obtained from dielectric measurements performed in the frequency domain at several temperatures. At temperatures above the glass-transition, the conductivity can be interpreted as an ionic conductivity, which confirms earlier results reported in the literature. Our investigation shows that the observed ionic conductivity is closely related to the amorphous phase of the polymer.     

Polyurethane anionomers synthesised with aromatic, aliphatic or cycloaliphatic diisocyanates, polyoxyethylene glycol and 2,2-bis-(hydroxymethyl)propionic acid. Part 3. Electrical properties of polyurethane coatings

The following electrical properties were found for polymer coatings obtained from polyurethane anionomers synthesised with the use of various diisocyanates: volume resistivity, permittivity and dielectric dissipation factor. The effects were discussed from the molecular structures and phase structures of those anionomers on the value of their ionic conductivity and polarizability. The anionomer prepared from the aliphatic diisocyanate was found to offer ionic conductivity; hence, that material can be considered to be a solid electrolyte, which exhibits a considerable susceptibility to structural modifications in the alternating electric field.     

Role of the diffuse layer of the ionic charge on the impedance spectroscopy of a cell of liquid

We investigate the role of the diffuse layer of the ionic cloud on impedance spectroscopy measurements. The analysis is performed assuming that the ions have equal mobility, the electrodes are perfectly blocking and adsorption phenomenon can be neglected. We find that the dielectric permittivity, in the limit of high frequency ω, tends to the dielectric permittivity of the pure liquid as ω^-3/2. The relationship between the detected equivalent permittivity and conductivity of the cell with the real and imaginary part of the complex dielectric constant is discussed. We show also that the presence of the ions is responsible for a distribution of relaxation times. An application to nematic liquid crystals is presented.     

Dielectric spectroscopy in a micromachined flow cytometer: theoretical and practical considerations

We propose a model to determine the influence of different cell properties, such as size, membrane capacitance and cytoplasm conductivity, on the impedance spectrum as measured in a microfabricated cytometer. A dielectric sphere of equivalent complex permittivity is used as a simplified model to describe a biological cell. The measurement takes place between a pair of facing microelectrodes in a microchannel filled with a saline solution. The model incorporates various cell parameters, such as dielectric properties, size and position in the channel. A 3D finite element model is used to evaluate the magnitude of the electric field in the channel and the resultant changes in charge densities at the measurement electrode boundaries as a cell flows past. The charge density is integrated on the electrode surface to determine the displacement current and the channel impedance for the computed frequency range. The complete impedance model combines the finite element model, the electrode-electrolyte interface impedance and stray impedance, which are measured from a real device. The modeled dielectric complex spectra for various cell parameters are discussed and a measurement strategy for cell discrimination with such a system is proposed. We finally discuss the amount of noise and measurement fluctuations of the sensor.     

Curing of an epoxy resin modified with poly(methylmethacrylate) monitored by simultaneous dielectric/near infrared spectroscopies

Simultaneous dielectric and near infrared measurements have been performed in “real-time” to follow polymerisation reactions on blends of a diglycidyl ether of bisphenol-A epoxy resin with 4,4^′-diaminodiphenylmethane hardener and different amounts of poly(methylmethacrylate) as modifier. The effect of the modifier amount on the polymerisation reactions has been studied, as well as that of the curing temperature. Epoxy and amine conversions have been followed by near infrared spectroscopy (NIR), while changes in molecular mobility in the reaction mixture have been analysed by dielectric relaxation spectroscopy (DRS). Evolutions of ionic conductivity and α-relaxation have been analysed and vitrification times have been obtained. The relaxational behaviour has been analysed through curing in the frequency domain, being the change of the main relaxation indicative of the cure reaction advancement. DRS data are also presented as complex impedance Z(ω). Vitrification times, obtained by dielectrometry have been compared with those obtained by rheological measurements and gelation times obtained by NIR have been compared with those obtained by solvent extraction.     

Transition from dilute to concentrated electrokinetic behavior in the dielectric spectra of a colloidal suspension

Dielectric spectroscopy is used to measure the complex permittivity of 200 and 100 nm diameter polystyrene latex suspended in potassium chloride (KCl) solutions over the frequency range 10(4)-10(7) Hz as a function of particle volume fraction (?) and ionic strength. Dilute suspension dielectric spectra are in excellent agreement with electrokinetic theory. A volume fraction dependence of the dielectric increment is observed for low electrolyte concentrations (0.01, 0.05, and 0.1 mM) above ? ≈ 0.02. This deviation from the dilute theory occurs at a critical frequency ω* that is a function of volume fraction, particle size, and ionic strength. The dielectric increment of suspensions at the highest salt concentration (1 mM) shows no volume fraction dependence up to ? = 0.09. Values of ω* are collapsed onto a master curve that accounts for the length and time scales of ion migration between neighboring particles. The measured conductivity increment is independent of volume fraction and agrees with theory after accounting for added counterions and nonspecific adsorption.     

Mixed protonic-electronic conduction and dielectric response in layered vanadyl phosphate nanocomposites

The layered vanadyl phosphate, VOPO(4)2H(2)O, is employed to prepare nanosized conducting polypyrrole by redox intercalation method. Transport and dielectric properties of various compositions have been investigated by impedance technique over a temperature range of 300-120 K. Grain boundary conductivity is larger than the bulk conductivity. The conductivity reveals a discontinuity at about 212-235 K. The conductivity is predominantly ionic at high temperature and electronic at low temperature. The dielectric spectra reveal a peak in the frequency range up to 2 MHz for higher concentration of intercalated polypyrrole. The activation energy of conductivity relaxation is different from that of total conductivity derived from the impedance plot.     

Electrochemical characterization of a bioceramic material: The shell of the Eastern oyster Crassostrea virginica

The shell of the Eastern oyster (Crassostrea virginica) is composed of multiple incongruent mineralized layers. This bioceramic composite material was investigated to determine the effects of shell thickness, orientation and layer composition on its electrochemical behavior using electrochemical impedance spectroscopy, potentiodynamic polarization and scanning electron microscopy-energy dispersive spectroscopy. SEM-EDS analysis of the oyster shell revealed that the multilayered biocomposite material is composed of calcium carbonate (CaCO(3)). EIS measurements in 3.5wt.% NaCl indicated that the impedance of the whole oyster shell in the low frequency region exhibited high impedance values which exhibited a decreasing trend with increasing immersion time. In terms of overall shell thickness, limiting currents measured by potentiodynamic techniques through the shell were observed to increase when the outer layers of the shell were sequentially removed by grinding, thus decreasing the shell thickness. These limiting current values remained relatively constant when the inner layers of the shell were removed. The impedance values of the oyster shell material as measured by EIS were shown to decrease with decreasing shell thickness. These findings suggest that the prismatic (outermost) shell layer in combination with the soluble organic matrix between all shell layers may influence the ionic conductivity through the oyster shell.     

Electrical and optical characterization of PMMA doped with Y0.0025Si0.025Ba0.9725 (Ti(0.9)Sn0.1)O3 ceramic

<正>Poly(methylmetacrylate)(PMMA)/Y_(0.0025)Si_(0.025)Ba_(0.9725)(Ti_((0.9))Sn_(0.1))O_3(YBTS) composites were prepared at different weight ratios of YBTS(0 wt%,5 wt%,10 wt%and 20 wt%YBTS) in order to investigate effect of YBTS additions on the electrical and optical properties of PMMA host.The electrical properties(impedance,dielectric constant dielectric loss and AC conductivity) were studied in the frequency range 10 kHz-1 MHz and in temperature range 20-90℃.Upon increasing the contents of YBTS ceramic,we observed a reduction in the impedance and an increase in the dielectric constant,dielectric loss and AC conductivity of PMMA host.We also found that the relaxation process at high YBTS contents was due to relaxation in the ionic conductivity.The absorption coefficient(α) has been determined in the wavelength range 230-800 nm at room temperature for all YBTS-PMMA composites.Moreover,the addition of YBTS ceramic highly enhances the UV absorption of PMMA host especially below 300 nm.Addition of 20 wt%YBTS ceramic to PMMA host decreases the optical energy gap from 5 eV to 3.41 eV.Correlations between electrical,optical and SEM results are reported.     

聚醚聚氨酯/低聚醚硫酸盐电解质的导电性能

聚合物固体电解质;聚二氧戊环;聚醚聚氨酯/低聚醚硫酸盐电解质的导电性能;离子电导率;     

Relaxations in thermosets. X. Analysis of dipolar relaxations in DGEBA-based thermosets during isothermal cure

Dielectric properties measured during isothermal curing of DGEBA-based thermosets using a mixture of aromatic amines as curing agent are analyzed. The evolution of the dielectric features of thermosets during curing and after a time when their dc conductivity has reached a negligibly small value are phenomenologically similar to the dielectric features of physically and chemically stable dipolar liquids and solids observed with increasing frequency or decreasing temperature. This equivalence is a consequence of the invariance of the dynamic behavior of dielectric susceptibility with respect to either the frequency of measurement or the relaxation time of the substance and demonstrates that crosslinking of a thermoset causes its relaxation time to increase monotonically. It is shown that the stretched exponential relaxation function formalism satisfactorily describes the dielectric results and that the value of its distribution parameter initially decreases and, after gelation, reaches a constant value, which we denote γ, in the latter part of the cure. The value of the curing parameter, γ, which lies between 0.2 and 0.4, monotonically decreases with increasing curing temperature and tends to a limiting value characteristic of a thermoset at higher temperatures. This is in contrast with the increase found in the corresponding representation in the Kohlrausch-Williams-Watts parameter β with increasing temperature. The curing time dependence of the dipolar relaxation time ι has been determined and found to have the shape of an elongated S, with a well-defined point of inflexion, as ι increases during the cure, from a value characteristic of a liquid to an ultimate value characteristic of a glass.