Dielectric relaxation spectroscopy measurements

The polarization reversal measurement is a large signal method requiring full switching of the liquid crystal, and therefore cannot be expected to coincide with the much more precise results from the low signal dielectric relaxation spectroscopy measurements. Basically, a Yq> value from the polarization reversal technique involves spurious contributions of elastic effects due to the... 

Dielectric relaxation spectroscopy has been utilized as a very sensitive tool to detect photo-oxldatlon of PS (4,5) by the appearance of a relaxation process at 230 K when measured at 24 KHz. Our measurements (Figure 3) Inldlcate a similar relaxation process Is occurlng at 27O K when measured at 100 kHz. In addition a portion of a large relaxation process that appears to be at or below 115 K Is apparent In the ozone treated PS. The origin of this relaxation process Is presently unclear but may be due to a molecular charge transfer complex similar to that formed... 

Polystyrene and polybutadiene homopolymers as well as random and block copolymers of these mers have been studied via dielectric relaxation spectroscopy and tensile stress-strain measurements. The results suggest that some block copolymer systems studied have styrene rich surfaces which appear to partially crosslink upon initial exposure to ozone even though surface oxygen concentrations are not significantly affected. After continued exposure these samples appear to then undergo chain scission. Complex plane analysis implies that after degradation... 

The rates of change (slopes of the curves) of many important properties (such as the refractive index, surface tension, and gas permeabilities) as a function of temperature, the value of the dielectric constant, and many other optical and electrical properties, often change considerably at Tg. These changes enable the measurement of Tg by using techniques such as refractometry and dielectric relaxation spectroscopy. Refractometry provides results which are similar to those obtained from dilatometry, because of the correlation between the rates of change of the specific volume and of the refractive index with temperature. Dielectric relaxation spectroscopy is based on general physical principles which are similar to those in dynamic mechanical spectroscopy, the main difference being in its use of an electrical rather than a mechanical stimulus. 

Secondary relaxations are usually measured either by mechanical methods such as dynamic mechanical spectroscopy or (somewhat less often) by electrical methods such as dielectric relaxation spectroscopy [159], The existence of Tp is generally ascribed to the onset of a significant amount of some kind of motion of the polymer chains and/or the side groups attached to them, on a much smaller and more localized scale than the large-scale cooperative motions of chain segments associated with Ta. These motions are usually inferred from the results of measurements using methods such as nuclear magnetic resonance spectroscopy. See... 

In the previous section, drug stability was shown to depend on the physical state of water in excipients. Detailed information on the physical state of water can be obtained by measuring the dynamics or the mobility of water molecules. The effect of water mobility on drug stability has been studied by determining water mobility in mixtures of water and polymers used as pharmaceutical excipients. Methods used include the measurement of spin-lattice relaxation time and spin-spinrelaxation time by nuclear magnetic resonance (NMR) spectroscopy as well as of dielectric relaxation time by dielectric relaxation spectroscopy. 

In dielectric relaxation spectroscopy, both components of the permittivity are measured as a function of frequency (see the next section). Figure 2 shows the typical shape of both z logf) and s (log/)> where f=(ol2n is the frequency (in Hz). 

Dielectric relaxation spectroscopy (DER), e.g. [103-105], DER monitors the mobility of dipolar groups in the polymer and also of small dipolar molecules (e.g. water) that may be dissolved in the polymer system. Corresponding to mechanical measurements, the maxima of dissipated energy indicate phase transition processes. Dilatometry, pVT measurements, e.g. [50,106]. These measurements unequivocally show a first order transition by a step in V T) and a bend if there is a glass transition. The important partial derivatives isobaric expansivity and isothermal compressibility can be derived from the corresponding measurements. The method is, however, quite time consuming and not widely used. 

The dielectric relaxation spectroscopy can effectively measure the relaxation processes of dipoles in the polymers. Like the dynamic mechanical spectroscopy, the sinusoidal electric field is the imposing stimulation, and again in a complex form. 

Although the investigation in this paper mainly concentrated on the time evolution of the 001 reflection and the 4-point pattern in the SAXS region, the measurements of the time dependence of overall diffraction patterns in the WAXS region will be required in order to elucidate the microscopic mechanism of the structure formation in oriented PET. In particular, real-time measurement of the intensity distribution of the meridional reflections of higher order will reveal the detailed mechanism of the structure formation. Real-time relaxation measurements such as dielectric relaxation spectroscopy are also desired for this purpose. 

A number of studies have been carried out of the glass transition process using dielectric relaxation spectroscopy. With the advent of computer-assisted measurements the DRS technique has increased in popularity and is now routinely used for the study of molecular mobility in polymeric materials. Chemical bonds may possess dipole moments as a consequence of the dilferences in... 

The shift factor aj could be measured by dynamic mechanical or dielectric relaxation spectroscopy. 

The activation energy of secondary relaxation in polymers can be measured by means of dynamic mechanical or dielectric relaxation spectroscopy. Dynamic mechanical or dielectric relaxation spectra of polymers can be obtained as a function of temperature at different frequencies. As an example, the dielectric relaxation behavior of the secondary relaxation in some AB cross-linked polymers is shown in Figure 4.63. 

Table 142 Structural properties of ASD measurable by dielectric relaxation spectroscopy (DRS) and possible hurdles... 

Experimentally, (R(O R(0)> can be measured in dielectric relaxation spectroscopy for a polymer molecule that has a nonzero permanent dipole moment along the chain backbone. Other normal modes can also be observed. 

Dielectric relaxation spectroscopy Dielectric polarization Specific relaxation behavior of solid states in different ordering levels Polarizability Different measuring modes... 

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