Glass transition processes

The quantitative analysis of the dielectric spectra of the glass-transition process was carried out by fitting the isothermal dielectric loss data according to the HN law (21). It was found from the fitting that (5 1 for the glass-transition... 

Fig. 1.5b DSC plot illustrating the glass transition process for a glassy polymer which does not crystallize and is being slowly heated from below Tg.

Differential scanning calorimetry (DSC) can be used to determine experimentally the glass transition temperature. The glass transition process is illustrated in Fig. 1.5b for a glassy polymer which does not crystallize and is being slowly heated from a temperature below Tg. Here, the drop which is marked Tg at its midpoint, represents the increase in energy which is supplied to the sample to maintain it at the same temperature as the reference material. This is necessary due to the relatively rapid increase in the heat capacity of the sample as its temperature is increases pass Tg. The addition of heat energy corresponds to the endothermal direction. 

It is possible to re culate the entire relaxation hmction, including the A, B, and glass transition processes in the same way as we did for linear polymers, with yet another remark it can be shown by theoretical arguments that the equilibrium relaxation time of the entire branch along its own contour, which is the equivalent of the B process of a linear chain, is 4 times the value of xb of a linear chain of same length ... 

The glass transition processes in foods may result from a rapid removal of water from solids. Based on that, e.g., the Tg values of anhydrous polysaccharides are high, and the food materials may decompose at temperatures below Tg (Kokini et al 1994 Roos and Karel, 1991b). The glass temperature transition affects viscosity, stickiness, crispness, collapse, crystallization, and ice formation, and can strongly influence deteriorative reaction rates. This provides a new theoretical and experimental framework for the study of food systems to unify structural and functional aspects of foods, described in terms of water dynamics and glass dynamics. 

A microscopic approach to the glass transition process was developed by Gotze... 

For the above reasons, theoretical interpretation of the glass transition process of polymers was developed witWn the framework of two directions - kinetic and thermodynamic (the last one simplified it and considered it as an equilibrium process). Simple empirical expressions were developed. 

In the above description of local motions, characterizes the segmental modes. In order to know whether these segmental motions observed by NMR in bulk at temperatures well above the glass-transition temperature belong to the glass-transition processes, it is of interest to compare the variations of Ti as a function of temperature with the predictions of the Williams-Landel-Ferry (WLF) equation [19]. The WLF equation describes the frequency dependence of the motional processes associated with the glass-transition phenomena. It can be written as [20]... 

Fig. 7.10. Temperature variation of relaxation time for the glass transition processes...

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