Thermal analysis dielectric

An isothermal DSC curve shows at a glance whether a reaction proceeds normally. In other words, the rate of reaction and thus, the heat flow, reaches maximum upon the reaction mixture s attainment of the reaction temperature. To locate a suitable isothermal reaction temperature, a dynamic experiment is carried out at 10 C/min. The optimum isothermal temperature will lie between the start of reaction (at 20% of the peak height) and the peak maximum temperature. An epoxy resin used for powder coating gives values of 180 °C to 200 °C for example. Conversely, an autocatalytic reaction shows an increasing reaction rate after an induction period. 

To determine the extent of reaction as a function of reaction time, it is assumed that the area under the curve increases proportionally to the conversion, i.e., the conversion at a time, t, is equal to the partial area at the time, t, divided by the total area. The graph of the extent of reaction versus reaction time is constructed by taking for example, five calculated values. 

The technique has been applied to unsaturated polyesters [4, 5] and polyallyl azides [6], and unsaturated polyester amides [7]. 

Patel and Panchal [7] monitored the curing of benzoyl peroxide catalysed polyunsaturated polyester amide resins by DSC. 

Sickfeld and Heinze [9] and Sourour and Kamal [10] carried out a kinetic study of isothermal cure of epoxy resins using DSC. 

Dielectric thermal analysis (DETA) has been used in measnrement of Tg of neoprene, styrene-butadiene, polyisoprene, polybutadiene, polychloroprene, nitrile, ethylene-propylene-diene and butyl rubbers [4]. 

The ultra-sensitivity of this technique makes it possible to detect transitions that are not seen by other techniques. Its ability to measure bulk or surface properties of materials in solid, paste, or liquid form makes DETA versatile and very useful. 

The a transition, which involves motion in long segments of the main polymer chain, is related to the Tg. The P transition involves rotation of short-chain ester side groups in PMMA and therefore occurs below the Tg. The frequency dependency of the p-Tg can be used to calculate the activation energy for the molecular motion, which provides important information for characterising the structure and predicting the performance of polymeric materials. In a dielectric experiment, the calculated activation energy for the P transitions in PMMA was 17.7 kcal/mol. This correlates well with the values calculated from DMA and creep experiments. 

---

An alternative method of studying the molecular motions of a polymeric chain is to measure the complex permitivity of the sample, mounted as dielectric of a capacitor and subjected to a sinusoidal voltage, which produces polarization of the sample macromolecules. The storage and loss factor of the complex permitivity are related to the dipolar orientations and the corresponding motional processes. The application of the dielectric thermal analysis (DETA) is obviously limited to macromolecules possessing heteroatomic dipoles but, on the other hand, it allows a range of frequency measurement much wider than DMTA and its theoretical foundations are better established. 

The relaxation methods employed are Dynamic Mechanical Thermal Analysis (DMTA) and Dielectric Thermal Analysis (DETA). Generally in both cases a single excitation frequency is used and the temperature is varied,... 

In an analogous manner to DMA, dielectric thermal analysis (DETA) represents a technique to apply an alternating electric field across the tested sample, which contributes to a polarization of the material with consequent current flow. DETA enables measurement of dielectric properties, which can further be related to material properties and thermal transitions as described below. 

Figure 18.26. Dielectric thermal analysis scans of polycarbonate at 1Hz and at a heating rate of 3°Cmin 1. Reproduced from Foreman et al. (1995), by permission of TA Instruments, Inc.

Figure 18.27. Dielectric thermal analysis online monitoring of a resin transfer molding process at 1000 Hz. Reproduced from Mcllhagger et al. (2000), by permission of Elsevier, Ltd.

Thermal analysis is a group of techniques in which a physical property of a substance is measured as a function of temperature when the sample is subjected to a controlled temperature program. Single techniques, such as thermogravimetry (TG), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), dielectric thermal analysis, etc., provide important information on the thermal behaviour of materials. However, for polymer characterisation, for instance in case of degradation, further analysis is required, particularly because all of the techniques listed above mainly describe materials only from a physical point of view. A hyphenated thermal analyser is a powerful tool to yield the much-needed additional chemical information. In this paper we will concentrate on simultaneous thermogravimetric techniques. 

Differential scanning calorimetry (DSC) Modulated or stepwise DSC Dielectric thermal analysis (DETA)... 

The relaxation methods employed are Dynamic Mechanical Thermal Analysis (DMTA) and Dielectric Thermal Analysis (DETA). Generally in both cases a single excitation frequency is used and the temperature is varied, typically over a range between — 100 °C and +200 °C. Changes in molecular motion, and hence 7, are detected by both techniques, but in the case of DETA the process has to involve movement of dipoles or fully developed electrical charges on the polymer in order to be detected. Thus the two techniques can be used to complement each other, since transitions can be detected on DMTA and assigned as due to dipoles according to whether or not they also occur with DETA. 

Dielectric thermal analysis Change in permittivity Phase changes, changes in polymers... 

Electrical Properties Dielectric Thermal Analysis Thermally Stimulated Current DEA TSC Dielectric Constant/ Dielectric Loss measured Current... 

Figure 9 Schematic diagram of a dielectric thermal analysis instrument. Inset shows a single-surface interdigitated electrode...

---