Interfacial mechanism, dielectric

In addition to the Debye model for dielectric bulk materials, other dielectric relaxations expressed according to Maxwell-Wagner or Schwartz "interfacial" mechanisms exist. For example, the Maxwell-Wagner "interfacial" polarization concept deals with processes at the interfaces between different components of an experimental system. Maxwell-Wagner polarization occurs... 

The dielectric constant is a measure of the ease with which charged species in a material can be displaced to form dipoles. There are four primary mechanisms of polarization in glasses (13) electronic, atomic, orientational, and interfacial polarization. Electronic polarization arises from the displacement of electron clouds and is important at optical (ultraviolet) frequencies. At optical frequencies, the dielectric constant of a glass is related to the refractive index k =. Atomic polarization occurs at infrared frequencies and involves the displacement of positive and negative ions. 

Additional drawbacks to the use of polyimide insulators for the fabrication of multilevel structures include self- or auto-adhesion. It has been demonstrated that the interfacial strength of polyimide layers sequentially cast and cured depends on the interdiffusion between layers, which in turn depends on the cure time and temperature for both the first layer (Tj) and the combined first and second layers (T2) [3]. In this work, it was shown that unusually high diffusion distances ( 200 nm) were required to achieve bulk strength [3]. For T2 > Tj, the adhesion decreased with increasing T. However, for T2 < Tj and Tj 400 °C, the adhesion between the layers was poor irrespective of T2. Consequently, it is of interest to combine the desirable characteristics of polyimide with other materials in such a way as to produce a low stress, low dielectric constant, self-adhering material with the desirable processabiHty and mechanical properties of polyimide. 

Dielectric relaxation study is a powerful technique for obtaining molecular dipolar relaxation as a function of temperature and frequency. By studying the relaxation spectra, the intermolecular cooperative motion and hindered dipolar rotation can be deduced. Due to the presence of an electric field, the composites undergo ionic, interfacial, and dipole polarization, and this polarization mechanism largely depends on the time scales and length scales. As a result, this technique allowed us to shed light on the dynamics of the macromolecular chains of the rubber matrix. The temperature as well as the frequency window can also be varied over a wide... 

Nearly every polymeric system absorbs some moisture under normal atmospheric conditions from the air. This can be a difficult to detect, very small amount as for polyethylene or a few percent as measured for nylons. The sensitivity for moisture increases if a polymer is used in a composite system i.e. as a polymeric matrix with filler particles or fibres dispersed in it. Hater absorption can occur then into the interfacial regions of filler/fibre and matrix [19]. Certain polymeric systems, like coatings and cable insulation, are for longer or shorter periods immersed in water during application. After water absorption, the dielectric constant of polymers will increase due to the relative high dielectric constant of water (80). The dielectric losses will also increase while the volume resistivity decreases due to absorbed moisture. Thus, the water sensitivity of a polymer is an important product parameter in connection with the polymer s electrical properties. The mechanical properties of polymers are like the electrical properties influenced by absorption of moisture. The water sensitivity of a polymer is therefore in Chapter 7 indicated as one of the key-parameters of a polymeric system. 

With their two-phase systems (e.g., polymeric matrix and inorganic filler), composites permit enhancement of mechanical or dielectric properties, owing to the high interfacial area between matrix and filler particles. The appeal of nanocomposites discussed in recent literature [Schaefer and Justice, 2007] explains the wide variety of used fillers, with dimensions ranging from 10 nm to about 1 fx.m. This range is well outside that suggested by the lUPAC nomenclature for nanoparticles (nanoparticles must have at least one dimension d<2nm mesoparticles with 2 50 nm), but to be consistent with the data cited in this chapter, they all will be termed nanoparticles, regardless of size. 

In addition to the bulk Tg, siower relaxation was assigned to polymer chains close to the polymer-filler interface, whose mobility was restricted by the physical interactions. The existence of an interfacial layer was proposed to explain the DSC results (showing a double step in heat capacity) and TSC/DDS measurements (distinguishing two well-defined dielectric relaxation processes). These results confirmed earlier studies by dynamic mechanical spectrometry, where a second tan 5 peak, observed at 50 to 100°C above the mechanical manifestation of Tg, was attributed to the glass transition of an interfacial polymer layer with restricted mobiUty [Tsagaropoulos and Eisenburg, 1995],... 

In order to understand this complex relaxation behavior of the microemulsions, it is necessary to analyze dielectric information obtained from the various sources of the polarization. For a system containing more than two different phases the interfacial polarization mechanism has to be taken into account. Since the microemulsion is ionic, the dielectric relaxation contributions are related to the movement of surfactant counterions relative to the negatively charged droplet interface. A reorientation of AOT molecules, and of free and bound water molecules, should also be mentioned in the list of polarization mechanisms. In order to ascertain which mechanism can provide the experimental increase in dielectric permittivity, let us discuss the different contributions. 

A very important part of emulsion study is the availability of methodologies to study emulsions. In the past ten years, both dielectric methods (1) and rheological methods (2) have been exploited to study formation mechanisms and the stability of emulsions formed from many different types of oils. Standard techniques, including NMR, chemical analysis techniques, microscopy, interfacial pressure, and interfacial tension, are also being applied to emulsions. These techniques have largely confirmed findings noted in the dielectric and rheological mechanisms. 

The volume is divided into three parts Part I. Metallization Techniques and Properties of Metal Deposits, Part II, Investigation of Interfacial Interactions," and Part III, "Plastic Surface Modification and Adhesion Aspects of Metallized Plastics. The topics covered include various metallization techniques for a variety of plastic substrates various properties of metal deposits metal diffusion during metallization of high-temperature polymers investigation of metal/polymer inlerfacial interactions using a variety of techniques, viz., ESCA, SIMS, HREELS, UV photoemission theoretical studies of metal/polymer interfaces computer simulation of dielectric relaxation at metal/insulalor interfaces surface modification of plastics by a host of techniques including wet chemical, plasma, ion bombardment and its influence on adhesion adhesion aspects of metallized plastics including the use of blister test to study dynamic fracture mechanism of thin metallized plastics. 

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