Temperature dielectric analysis

As applied to thermal analysis, dielectric analysis consists of the measurement of the capacitance (the ability to store electric charge) and conductance (the ability to transmit electrical charge) as functions of applied temperature. The measurements are ordinarily conducted over a range of frequencies to obtain full characterization of the system. The information deduced from such work pertains to mobility within the sample, and it has been extremely useful in the study of polymers. 

Morris et al. [1.126] proposed to use dielectric analysis (DEA) to predict the collapse temperature of two component systems. The background of DEA is explained and the take off frequency (TOF) is chosen as the best analytical method to identify the collapse temperature. Figure 1.55.5 shows the dielectric loss factor as a function of the frequency. 

Bidstrup, S.A. and Day, D.R. 1994. Assignment of the glass transition temperature using dielectric analysis A review. In Assignment of the Glass Transition (RJ. Seyler, ed.), pp. 108-119. American Society for Testing and Materials, Philadelphia, PA. 

Glass-Forming Liquids I. Temperature Derivative Analysis of Dielectric Relaxation Data. 

Dielectric analysis (electrothermal analysis, dielectric spectroscopy) is the measurement of dielectric properties as a function of frequency and temperature. It is increasingly finding use in characterising polymer structure and, in particular, the curing process. Its use in this respect has been considered in Chapter 6. 

Deng and Martin (1996) also showed the necessity of including a diffusional resistance in the rate equation for the cyclotrimerization of dicyanates, well before vitrification. They observed a significant decrease in the diffusion coefficient from conversions of about 0.40, using dynamic dielectric analysis. They could fit experimental kinetic data in the whole conversion range using Eq. (5.50). Experimental values of the decrease in the diffusion coefficient with conversion were used to estimate kd for different cure temperatures. 

Morris et al. [1.126] proposed the use of dielectric analysis (DEA) to predict the collapse temperature of two-component systems. The background of DEA is explained and the >take-off frequency< (TOF) is chosen as the best analytical method to identify the collapse temperature. Figure 1.55.5 shows the dielectric loss factor as a function of the frequency. The frequency at the minimum of this curve is called TOF by the authors. TOF varies with the temperature as shown in Figure 1.55.6. The extrapolated intersection of the two linear portions identifies the collapse temperature. The predicted Tc by TOF for 10% sucrose, 10% trehalose, 10% sorbitol and 11% Azactam solution deviates from observations with a freeze-drying microscope (Table 1 in [1.126]) to slightly lower temperatures, the differences being -3, -1.4, 2.2 and 0.7 °C. 

Abbreviations DEA, dielectric analysis >OC. degree of crystallinity DSC, di erential scanning calorimetry LM, local mobility (secondary relaxations) SR, structural relaxation 7g, determination of glass transition temperature TSDC. thermally stimulated depolarization current spectroscopy XRD, X-ray difTractometry. Source Adapted from Ref. 15. 

The low temperature properties of a dodecane-hexanol-K.oleate w/o microemulsion from 20°C to -190°C were studied vs. increasing water content (C,mass fraction) in the interval 0.021+-0.1+, by Differential Scanning Calorimetry and dielectric analysis (5 Hz-100 MHz). A differentiation between w/o dispersions is obtained depending on whether they possess a "free water" fraction. Polydispersity is evidenced by means of dielectric loss analysis. Hydration processes occurring, at constant surface tension, on the hydrophilic groups of the amphiphiles, at the expenses of the free water fraction of the droplets, are shown to develop "on ageing" of samples exhibiting a time dependent behavior. 

Previous findings on the actual microemulsion are given in references (9 17) where the system s phase map vs.concentration in the temperature interval (-20°C + 80°C), viscosity measurements,dielectric analysis of liquid samples against both concentration and frequency, the thermally stimulated dielectric polarization release (TSD), electro-optical phenomena, light scattering, Raman spectroscopy and sound propagation investigations are reported. 

Dielectric analysis. The real (e ) and the imaginary (e") part of the relative complex permittivity were determined,at a given fixed frequency, as a continuous function of decreasing temperature in the interval from 20°C down to -190°C and, at a constant temperature of 20°C, as a function of frequency in the range (5 Hz- 100 MHz). The dielectric analysis was applied to several samples within the aforementioned concentration interval following the procedure described in references (j>) (11-12). The low temperature measurements were perform ed by applying a rate of 0.18 °C/s (10.8 °C/min). 

Water absorbed in a polymer can exist in an unassociated state or as a separate phase (cluster). In this investigation the DSC technique of water cluster analysis was used in conjunction with coulometric water content measurements to characterize the water sorption behavior of polysulfone and poly(vinyl acetate) The polysulfone had to be saturated above its Tg (190°C) and quenched to 23°C for cluster formation to occur while cluster formation occurred isothermally at 23°C in the poly(vinyl acetate) Both polymers showed an enchancement of their low temperature 3-loss transitions in proportion to the amount of unclustered water present. Frozen clustered water produced an additional low-temperature dielectric loss maximum in PVAc and polysulfone common to polyethylene and polycarbonate as well. Dielectric data obtained on a thin film of water between polyethylene sheets was in quantitative agreement with the clustered water data. 

The application of dielectric constant techniques to thermophysical measurement of solids has been used for a number of years (114, 115). The early uses of the technique involved isothermal measurements employing bridge methods. Recently, techniques have been developed that permit the measurement of the dielectric constant of a solid as a function of temperature, in a manner similar to other TA techniques. Chiu (116) used the term dynamic electrothermal analysis (ETA) to describe the measurement of both the capacitance and the dissipation factor of polymeric samples. Nottenburget al (117) developed an automated technique that permitted the rapid determination of the dielectric properties of a substance over a wide range of temperature and frequencies. This technique, which was called dynamic dielectric analysis (DDA), was modified to measure concurrently the DTA curve of the sample as well (117, 118). This new technique was called dynamic dielectric analysis-differential thermal analysis, DDA-DTA,... 

The principal characteristics of the triboelectret state in polymers recorded experimentally are i) the efficient surface charge density (ESCD) value and ii) the thermally stimulated depolarization (TSD) current spectrum, i.e. the discharge current dependence of the electret on its heating temperature. The analysis of TSD spectra helped to estimate the parameters of the triboelectret state, including the homo- to heterocharge relation in a dielectric, activation energy of the charge relaxation processes, relaxation time and others. 

Stickel, F., Fischer, E. W., and Richer , R., Dynamics of glass-forming liquids 1. Temperature-derivative analysis of dielectric relaxation data, J. Chem. Phys., 102, 6251-6257 (1995). 

This study compared methacrylate and acrylate polymers to structural analogs with fluorinated ester groups. Two types of relaxations were characterized, the primary relaxation associated with the glass transition and secondary relaxations associated with side group motion and localized segmental motion. Dielectric analysis was used to characterize the response of dipoles to an electric field as a fimction of temperature. Mechanical properties were analyzed via dynamic mechanical analysis and stress relaxation measurements. Relaxation behavior was interpreted in terms of intermolecular and intramolecular mechanisms. 

Dielectric Analysis. Samples of 0.80 0.01 mm thickness were compression molded into disks one inch in diameter. Permittivity, e and the loss factor, e", were determined at temperatures fi om -150°C to 30°C above the glass transition temperature of the samples using a TA Instruments 2970 DBA. The frequency range scanned was from 10" to 10 Hz. 

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