Impedance spectroscopy of reactive polymers. 2. Multifunctional epoxy/amine formulations

Dielectric measurements were utilized to follow the advancement of cure in a bifunctional and a tetrafunctional epoxy/amine formulation. In deferance to earlier dielectric studies of cure, complex impedance was measured and used to calculate ionic resistivity. By using complex impedance we were able to separate, according to their frequency dependence, the contributions to overall polarization from electrode blocking layers, mobile charge carriers, 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, it measures dielectric response in real time, and is conducive to the development of phenomenological models based on equivalent circuits. Values of ionic resistivity measured at different cure time and temperature were used to quantify the progress of cure. Excellent agreement was reported between the calculated values of normalized degree of cure obtained by dielectric and calorimetric measurements. It was suggested that apart from the extrinsic conductivity by ionic impurities, an intrinsic mechanism which involves the reactive molecules contributes to the overall ionic conductivity. © 1995 John Wiley & Sons, Inc.     

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     

Impedance spectroscopy of reactive polymers. 4. An improved experimental procedure for measurement of effective resistivity

Impedance spectroscopy of reactive systems, polymer-forming and non-polymer-forming alike, has been under active investigation in our laboratory for several years. In this article we describe an improved experimental procedure for the determination of fundamental dielectric properties during reaction. A series of experimental steps was established and is recommended in order to generate reproducible and reliable results. © 1996 John Wiley & Sons, Inc.     

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.     

Impedance spectroscopy of lithium-carbon electrodes

The methods of coulometric titration and electrode impedance spectroscopy are used in studying the behavior of carbon film electrodes free of binding and conducting additives in the course of reversible lithium intercalation from nonaqueous electrolytes. The electrodes with the high and low degrees of graphitization are studied. The measurements are performed in the frequency range from 10⁵ to 10^?2 Hz with the lithium concentration in intercalate varied from 0.025 mol/cm³ (corresponds to LiC₆) to a state free of lithium. The factors responsible for the hysteresis in charge-discharge curves, the versions of equivalent circuits (EC) suitable for modeling the impedance spectra of Li _x C₆ electrodes, the dependence of EC parameters and the lithium diffusion coefficient on the concentration are discussed. It is shown that all experimental impedance spectra can be adequately modeled by a common general EC. The concentration dependences are consistent with the earlier data of pulse methods. The diffusion coefficient varies approximately from 10^?12 to 10^?13 cm²/s.     

Impedance spectroscopy of lithium-tin film electrodes

A method of electrochemical impedance spectroscopy was used to study the reversible lithium intercalation from nonaqueous electrolyte into tin films with the thickness of 0.1–1 μm. The impedance spectra of lithium-tin (Li _x Sn) electrodes have a complicated shape depending on the electrode state and prehistory; they reflect the occurrence of several consecutive and parallel processes, including the lithium migration, diffusion, and accumulation. The formation of a solid-electrolyte layer on the surface at Li intercalation into Sn is observed. Equivalent circuits are proposed that adequately model the experimental data on the Li _x Sn electrodes both freshly prepared and after prolonged cycling. Problems associated with the choice of equivalent circuits and determination of their parameters, the accuracy of the diffusion coefficient determination, the trends in the parameters’ variation with electrode potential (composition) are discussed.     

Impedance laser spectroscopy in a small RF-excited neon discharge

Laser impedance spectroscopy in a miniature neon RF-discharge operating at 0.27 bar was performed. The dominant role of the metastable atoms to the size and the sign of the RF-signal was investigated. Excellent linearity of the RF-signal but poor detection sensitivity of excited metastable neon atoms were found at low laser powers.     

On the Integration of Dielectrometry into Electrochemical Impedance Spectroscopy to Obtain Characteristic Properties of a Dielectric Thin Film

We demonstrate a novel impedimetric approach providing unprecedented insight into characteristic properties of dielectric thin films covering electrode surfaces. The concept is based on the joint interpretation of electrochemical impedance spectroscopy (EIS) together with dielectrometry (DEM) whose informative value is mutually interconnected. The advantage lies in the synergistic compensation of individual shortcomings adversely affecting conventional impedimetric analysis strategies relying exclusively on either DEM or the traditional EIS approach, which in turn allows a reliable determination of thickness and permittivity values. The versatility of the method proposed is showcased by an in-situ growth-monitoring of a nanoporous, crystalline thin film (HKUST-1) on an interdigitated electrode geometry.     

微波介质材料BaEu2Ti4O12的交流阻抗谱表征

采用交流阻抗谱技术对微波介质材料BaEu2Ti4O12进行了表征。分析了lgε/lgf, Z″/Z′和Y′/lgf等图谱的特征与材料性能的关系.研究结果表明：1）阻抗谱测试得到的介电常数与所发表的微波测试结果有很好的一致性,高频下得到的介电常数实际上是晶粒和晶界的净结果,高质量烧结的材料应具有薄的晶界，因而应表现出较大的介电常数; 2）如果将材料从高温下缓慢降温或在O2气氛中退火，材料的导电性都会下降，其原因可以理解为这样处理的材料氧空位浓度降低，进而可以推测这有利于降低材料的介电损失.     

Impedance Spectroscopic Measurements of Pure Gas Adsorption Equilibria on Zeolites

Physisorption equilibria of gases on inert porous solids like activated carbon or molecular sieves can be characterized by measuring the (frequency dependent) capacitance of a capacitor filled with a sample adsorbent. This quantity strongly depends not only on the physico-chemical structure of the empty adsorbent in vacuum, but also on the permanent or induced dipole moments of the molecules adsorbed and of the fluid phase. Consequently, it should be possible to determine the excess mass being adsorbed on the internal surface of a highly porous solid by measurements of the dielectric constant. The aim of this work is to show for various pure gases and adsorbents that the change of the capacitance of an adsorption system depends on the adsorbed mass. Therefore, this effect can be used to characterize porous solids and their adsorbates and, for example, to check the state or the quality of industrial adsorbents during a process on site if calibration measurements have been taken (Staudt et al., 1994, 1998).     

Practical Insights into the Impedance Response of Interdigitated Electrodes: Extraction of Relative Static Permittivity and Electrolytic Conductivity

This work aims to provide a detailed understanding of the challenges related to the computation of the relative static permittivity and electrolytic conductivity of a sample medium from its impedance response recorded with interdigitated electrode (IDE) geometries. Within the scope of the study, impedance data has been measured and evaluated for a total of nine sample media using two distinct IDE geometries. Particular emphasis is laid upon the compensation of parasitic influences affecting the impedance response. With the raw data supporting this study fully disclosed, the reader is offered the opportunity to comprehensively retrace the evaluation procedure proposed in the text.     

Electrochemical impedance spectroscopy for characterization of coatings with intrinsically conducting polymers

The method of electrochemical impedance spectroscopy (EIS) was applied to investigate the behaviour of a thin intrinsically conducting polymer film (ICP) deposited on a metal substrate. Especially the conductivity, the redox properties, the anion release properties, and the corrosion protection of a coating with and without ICP film on an iron or steel substrate were studied. Combined with other electrochemical methods, the reactions taking place at an injured surface area of the coated iron were studied. The corrosion protection mechanism of polythiophene could be explained.     

玻碳电极界面的阻抗谱数学表达及定量分析

玻碳电极（GCE）是各类电化学传感器常用的基础电极,其界面特征直接影响检测性能。本文详细考察了电极体系的电化学过程,针对GCE传感界面,探讨了一个等效电路中电解质电阻、电荷输运电阻、扩散阻抗、电化学（氧化/还原）反应阻抗、表面吸附阻抗和双电层电容等电学元件的物理意义,并给出了对应的数学模型。通过改变模型中5个参数值,模拟了不同状态下的阻抗谱,分析了电极系统各参数对GCE阻抗谱的贡献规律。最后,采用该数学模型对裸GCE和修饰GCE在铁氰化钾溶液中的阻抗谱进行分析,拟合结果与实验数据吻合度高;基于拟合获得参数,定量对比分析了修饰前后电极表面的特征变化。     

Dielectric in situ sensor monitoring of phase separation and changes in the state of each phase

Frequency dependent dielectric measurements have been used to monitor and characterize the phase separation process and changes in state of each phase. The measurements are made in situ using a micro planar sensor. They can be made both in the laboratory as well as in an industrial production or use environment. Two examples are presented. The first is monitoring the onset of phase separation, the buildup in Tg and change in composition of each phase during “reactive processing” of a high performance thermoplastic (TP) PPI, thermoset precursors (TS) DGEBA-MCDEA intially homogeneous blend. The second example involves monitoring the stability, onset of phase separation, as a function of temperature on a mineral oil, stearyl alcohol, water, sufactant emulsion used in the cosmetic industry.     

Temperature modulated calorimetry and dielectric spectroscopy in the glass transition region of polymers

The results from temperature modulated DSC in the glass transition region of amorphous and semicrystalline polymers are described with the linear response approach. The real and the imaginary part of the complex heat capacity are discussed. The findings are compared with those of dielectric spectroscopy. The frequency dependent glass transition temperature can be fitted with a VFT-equation. The transition frequencies are decreased by 0.5 to 1 orders of magnitude compared to dielectric measurements. Cooling rates from standard DSC are transformed into frequencies. The glass transition temperatures are also approximated by the VFT-fit from the temperature modulated measurements. The differences in the shape of the curves from amorphous and semicrystalline samples are discussed.Dedicated to Professor Bernhard Wunderlich on the occasion of his 65th birthday     

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.     

Synthesis and characterization of arylene ether imide reactive oligomers and polymers containing diaryl alkyl phosphine oxide groups

The diamine monomer bis(m-aminophenyl) methyl phosphine oxide (DAMPO) was synthesized via nitration and reduction of diphenyl methyl phosphine oxide. Rigorous purification of this monomer enabled its utilization in the synthesis of high molecular weight poly(ether imide)s. Both thermoplastic materials and thermosetting systems, endcapped with either phthalic or phenylethynylphthalic anhydride, respectively, have been produced. Major emphasis has been placed on polyimides derived from 2,2′-bis(4-(3,4-dicarboxyphenoxy) phenyl) propane dian- hydride, also known as bisphenol-A dianhydride, or BPADA. High molecular weight homo- and copolyimides based on BPADA/DAMPO had glass transition temperature values in the range of 215–223°C, and were totally amorphous. They displayed higher modulus and tensile strength values than the polyetherimide control based on meta-phenylene diamine and also generated high TGA char yields in air. Phenylethynyl crosslinkable materials were effectively cured at 380°C to produce networks that are ductile, very solvent resistant and also generate high char yields, which suggest their possible utilization in fire resistant matrix systems. © 1998 John Wiley & Sons, Ltd.     

Impedance of faradeic process with partial charge transfer

The electrochemical impedance of charge-transfer reaction involving the stages of adsorption and desorption of intermediate partially charged substance is analyzed theoretically in the frequency-potential plane in order to study the possibility for measuring the partial charge transfer. The simplest case of adsorption two-electron transfer, namely, the Langmuir adsorption with liner dependence of reaction rate on the coverage, is considered. It is assumed that the partial charge transfer is independent of the electrode potential.     

交流阻抗法在细胞分析中应用的研究进展

交流阻抗法作为一种新型的无标记、全程动态、实时分析方法已在细胞研究中得到了广泛应用。本文综述了基于交流阻抗法进行细胞分析的研究新进展,重点对非法拉第阻抗谱法和法拉第阻抗谱法用于细胞分析的原理及应用进行了总结,主要包括交流阻抗法在细胞形态、细胞生长、细胞增殖、细胞凋亡以及作用于细胞的药效和毒性研究中的应用,并对其发展趋势进行了展望。