Dielectric techniques

We have employed these dielectric techniques to study the hydration characteristics of hydrocolloids widely used in food systems 153]. 

The object of the present work was to Investigate the kinetics of the various photo-decomposltlons of polystyrene, in particular the early events in these reactions, using dielectric techniques, and to compare the results with those previously obtained. The results will, in addition, provide information regarding the effects of such photo-degradations on the dielectric properties of the polymer. 

Experimentally, all the relaxation times are included between 100 sec and 5000 sec at the maximum of the elementary peaks (Tm). So, by analogy with dynamic electrical experiments, an equivalent frequency veq can be attributed to the TSC technique, i.e., 2nveq i(Tm) = 1 and veq 10 3 Hz. This low-equivalent frequency explains the high resolution of TSC and constitutes one of the most attractive features of the technique. Moreover, TSC permits the operator to perform a rapid characterization of materials and is an alternative approach to conventional dielectric techniques over this frequency range. 

Anisaldazine, which also exhibits negative dielectric anisotropy, was also investigated (1) using microwave dielectric techniques. The results in the presence of a d.c. electric field are very similar to those shown for p-azoxyanisole. 

Two processes at 310°K. (102 c.p.s.) and 350°K. (102 c.p.s.) have been found (68), using the dielectric technique, for atactic poly (vinyl alcohol) crystals, grown from 0.05% triethylene glycol solution while cooling from 180° to 165°C. These maxima have been previously attributed (77) to local mode and glass transformation motion, respectively. 

Pissis P. Water in polymers and biopolymers by dielectric techniques. In Electromagnetic Aquametry. Berlin Heidelberg Springer 2005. 

Each one of the previous issues can influence the performance of the processing technique and the resulting composite in a variety of ways. Insufficient cure may result in a low and a consequent creep under stress. Inadequate flow may result in high levels of porosity or large voids. Online sensor systems permit process control [19] and are a useful tool toward improved quality in composites manufacture. Dielectric techniques have achieved the greatest success as commercial in-process monitoring systems. Optical-fiber methods show promise for further development, most notably where distinct features in chemical spectra can be obtained. 

FIGURE 13.22 Comparison of the loss peaks for the glass transition in polyfethylene tereph-thalate) measured at different frequencies from both dynamic mechanical and dielectric techniques. shifts to higher temperatures as the frequency of the measurement increases. (From Wetton, R.E. et al.. Inti. Lab., March, 1986. Figure 7, p. 80. With permission from International Laboratory.)... 

References 4 and 5 give details on experimental procedures and results for individual polymers. The use of dielectric techniques to monitor orientational changes in an ethylene-carbon monoxide 1 1 alternating copolymer is described in reference 6. 

Phosphazene dendrimers having aP = SorP = 0 core possess high thermal stability up to about 380°C, which depends on the nature of end groups but little on the generation number. The molecular mobility around the glass transition of these phosphorus dendrimers has been investigated by dielectric techniques. ... 

Samouillan et al. (2011) studied the dielectric properties of elastin at different degrees of hydration and specifically at the limit of freezable water apparition. The quantification of freezable water was performed by DSC. Two dielectric techniques were used to explore the dipolar relaxations of hydrated elastin dynamic dielectric spectroscopy (DDS), performed isothermally with the frequency varying from 10 to 3 x 10 Hz, and the TSDC technique, an isochronal spectrometry running at variable temperature, analogous to a low-frequency spectroscopy (10 to 10 Hz). A complex relaxation map was evidenced by the two techniques. Assignments for the different processes can be proposed by the combination of DDS and TSDC experiments and the determination of the activation parameters of the relaxation times. As already observed for globular proteins, the concept of solvent-slaved protein motions was checked for the fibrillar hydrated elastin (Samouillan et al. 2011). 

Panagopoulon, A., Kyritsis, A., Sabater i Serra, R., Gomez Ribelles, J.L., Shinyashiki, N., and Pissis, P. 2011. Glass transition and dynamics in BSA-water mixtures over wide ranges of composition studied by thermal and dielectric techniques. Biochim. Biophys. Acta. Protein. Proteom. 1814 1984-1996. 

As the dielectric technique probes molecular mobility, through the reorientation of dipoles, the observed shift to higher temperatures means that the molecular dynamics associated with the glass transition, becomes more hindered in the copolymer. The... 

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