Structure dielectric analysis

Because of the structural variety possible with polyimides, many smdies have sought to understand their structure-property relationships, often focusing on one specific target property. Such studies have included those aimed at understanding thermal expansion behavior, optical properties electronic structure, dielectric constant and loss, PTIR analysis, adhesion, water absorption, and molecular ordering, as well as others. The references cited... 

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. 

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 physical structure of creams has been investigated by a variety of techniques, e.g., DSC, TGA, microscopy, reflectance measurement, rheology, Raman spectroscopy and dielectric analysis (see references in Peramal et al. 1997). Focussing on TGA and rheology, Peramal et al. (1997) found that when aqueous British Pharmacopoeia (BP) creams were analyzed by... 

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. 

The alpha transition, which involves motion in long segments of the main polymer chain, is related to the T. The beta transition involves rotation of short-chain ester side groups and therefore occurs below the Tg. The frequency dependency of the beta T 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 analysis experiment, the calculated activation energy for the beta transitions in PMMA was 17.7 kcal/mol. This correlates well with the values calculated from DMA and creep experiments. 

Influence of thermal history on Nylon 6,6. Dielectric analysis is an effective tool for characterising the influence of thermal history on the molecular structure of a material and, in turn, on molecular relaxation. 

Sato, T., Chiba, A., and Nozaki, R. (2002). Composition-dependent dynamical structures of monohydric alcohol-water mixtures studied by microwave dielectric analysis. J. Mol. Liq., 96-97, 327-339. 

Figure 2. The geometry shown in Figure 1 is refined for dielectric analysis by fixing a microsphere concentric with the macrosphere and contains the elements of molecular structure.

Dielectric analysis has also been applied to study polymer thermal ageing [894], e.g. of adhesive bonded structures [895]. Dielectric loss is a means for detecting early steps in polymer degradation by oxidation, although nowadays CL is a strong competitor. The dielectric behaviour of post-irradiation oxidised PP/HALS was also reported [796]. Dielectric analysis has not been extended to many elastomer applications. Dissociation and mobility of salt complexes as heat stabilisers in PA4.6 have been investigated by means of DIES the technique was also used to study thiourea-doped PVAL [903]. 

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