Dielectric depolarization method

While in lumped-circuit methods the dielectric response is measured in the frequency domain, following the ajpplication of a sinusoidal alternating electrical field, for frequencies below 10 Hz it is advantageous to cany out the measurements in the time domain because it is less time consuming. The polarization or depolarization current following the application of a step-like electrical field is measured as a function of time. 

MicroBrownian dynamics of microemulsions can be studied by various techniques including dynamic-mechanical, dielectric, ultrasonic and NMR relaxation, ESR, volume, enthalpy and specific heat relaxation, quasielastic light and neutron scadering, fluorescence-depolarization experiments, and many other methods (90, 102-107). The information thus acquired provides an opportunity to clarify... 

This book describes the applications of important new NMR spectroscopic methods to a variety of useful materials and compares them with results from other techniques such as adsorption, differential scanning calorimetry, thermally stimulated depolarization cmrent, dielectric relaxation spectroscopy, infrared spectroscopy, optical microscopy, and small-angle and wide-angle x-ray scattering. The text explores the application of NMR spectroscopy to examine interfacial phenomena in objects of increasing complexity, beginning with immodified and modified silica materials. It then describes properties of various mixed oxides with comparisons to individual oxides and also describes carbon materials such as graphite and carbon nanotubes. 

The consistency of these various methods may be evaluated from the results listed in Table I of data obtained with polystyrene in solvents of low viscosity by the sound absorption (SA) and NMR relaxation methods, for poly(p-chloro-styrene) by dielectric dispersion (DD), for polystyrene labelled with nitroxyl by the ESR method and for a styrene copolymer with a small concentration of 9-p-vinylphenyl-lO-phenylanthracene residues by depolarization of fluorescence (DF). It may be seen that sound absorption and dielectric dispersion yield similar transition frequencies in the range of 12-35 MHz at 10-25 C,... 

In series of publications [25,27,29,35-40] several methods were used for eharaeterization of the microphase structure of the semi-IPNs studied. Small-angle X-ray seattering (SAXS), differential scanning calorimetry (DSC) [27, 35-37], dynamic mechanical thermal analysis (DMTA) [27, 30-32], dielectric relaxation spectroseopy (DRS), and thermally stimulated depolarization currents (TSDC) [25, 39, 40] measurements have shown that pure PCN is characterized by a typical homogeneous structure, but for segmented LPU the microphase separation on the level of hard and soft domains due to their thermodynamic immiscibUity was denoted. As for semi-IPNs, the destruction of the microphase separated morphology of LPU was observed and the microphase separation between PCN and LPU phases, expected from the difference of solubility parameters, was not found. 

An alternative method to observe dielectric properties is termed thermal stimulated currents (TSC). This method involves polarization of a sample at high temperature (relative to Tg) and quenching to a temperature where depolarization is kineticaUy prevented in the time scale of the experiment. The temperature is then increased and the depolarization current is measured, yielding peak values associated with polymer transitions analogous to t", E" and tan S values obtained by conventional dielectric and dynamic mechanical measurements. The TSC spectra can reveal secondary relaxations, glass transitions and liquid or crystalline phase transitions and hquid crystalhne phase transitions. TSC has been applied to PBT/PC and PA6/ABS blends to study the intermixing of the components of the respective blends [58]. The TSC method is described in several references [59-61]. 

A useful and common way of describing the reorientation dynamics of molecules in the condensed phase is to use single molecule reorientation correlation functions. These will be described later when we discuss solute molecular reorientational dynamics. Indirect experimental probes of the reorientation dynamics of molecules in neat bulk liquids include techniques such as IR, Raman, and NMR spectroscopy. More direct probes involve a variety of time-resolved methods such as dielectric relaxation, time-resolved absorption and emission spectroscopy, and the optical Kerr effect. The basic idea of time-resolved spectroscopic techniques is that a short polarized laser pulse removes a subset of molecular orientations from the equifibrium orientational distribution. The relaxation of the perturbed distribution is monitored by the absorption of a second time-delayed pulse or by the time-dependent change in the fluorescence depolarization. 

Fig. 14. Schematic views of (a) manifestation of transitions in a DMA spectrum, and (b) frequency dependencies of transition temperatures in polymers, measured by various techniques I, DSC, mechanical relaxation, radiothermoluminescence, thermostimulated depolarization II, mechanical and dielectric relaxation III, dielectric relaxation, NMR IV, ESR (probe method), Mandelstam-Brillouin scattering, acoustic measurements (From Ref 140),...

More exotic —that is, so far, less frequently used—methods are also worth noting dielectric relaxation on which we have a whole chapter by Jozef Moscicki thermally stimulated depolarization " electro-optical behavior" (time dependence of transmitted light intensity under a low frequency electric field) thermo-optical analysis" " (temperature dependence of the transmission of light through birefringent... 

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