Glass Transition state diagram

Nikolaidis, A. and Labuza, T.E Glass transition state diagram of a baked cracker and its relationship to gluten, /. Food Sci., 61, 803, 1996. 

This determines the glass transition line in the softness-concentration state space [i.e., in the ( x, <1) plane)]. By observing this glass transition phase diagram, the value of p, whose glass transition volume fraction (J) coincides with the experimentally reported value of 0.644, it was concluded that... 

FIG. 31 Schematic diagram illustrating the transition between a supercooled liquid state (rubber) and an amorphous solid state (glass). The glass transition event is typically caused by a decrease in water content and/or temperature. The reversibility of the transition, as indicated by the dotted arrow, is material dependent (see text for further discussion of the reversibility of the transition). 

Glass transition determinations Decomposition reaction Reaction kinetics Phase diagrams Dehydration reactions Solid-state reactions Heats of absorption Heats of reaction Heats of polymerization Heats of sublimation Heats of transition Catalysis... 

Sa, M.M., Figueiredo, A.M., Sereno, A.M. Glass transitions and state diagrams for fresh and processed apple.Thermochim. Acta 329, 31—38,... 

The thermophysical properties, such as glass transition, specific heat, melting point, and the crystallization temperature of virgin polymers are by-and-large available in the literature. However, the thermal conductivity or diffusivity, especially in the molten state, is not readily available, and values reported may differ due to experimental difficulties. The density of the polymer, or more generally, the pressure-volume-temperature (PVT) diagram, is also not readily available and the data are not easily convertible to simple analytical form. Thus, simplification or approximations have to be made to obtain a solution to the problem at hand. 

Figure 8.5 Schematic diagram of state and phase transitions ofstarch note the effect of moisture content and time on the various states. Tg1 Tg2 and Tg3 represent the glass transition at different moisture content levels. A-L and V-structures denote short- and long-range order of amylose-lipid complexes, whereas d.h. order corresponds to short-range B-type structures.

The diagram enables us to make a definition of the term coke. After extensive pyrolysis a residue will be obtained which on cooling and reheating will not pass through the glass transition temperature at all, i.e., it changes its composition within the solid state. 

Roos (1995) has used a combined sorption isotherm and state diagram to obtain critical water activity and water content values that result in depressing Tg to below ambient temperature (Figure 1-25). This type of plot can be used to evaluate the stability of low-moisture foods under different storage conditions. When the Tg is decreased to below ambient temperature, molecules are mobilized because of plasticization and reaction rates increase because of increased diffusion, which in turn may lead to deterioration. Roos and Himberg (1994) and Roos et al. (1996) have described how glass transition temperatures influence nonenzymatic browning in model systems. This deteriorative reaction... 

Figure 1-25 Modified State Diagram Showing Relationship Between Glass Transition Temperature (Tg), Water Activity (GAB isotherm), and Water Content for an Extruded Snack Food Model. Crispness is lost as water plasticization depresses Tg to below 24X2. Plasticization is indicated with critical values for water activity and water content. Source Reprinted with permission from Y.H. Roos, Glass Transition-Related Physico-Chemical Changes in Foods, Food Technology, Vol. 49, No. 10, p. 99, 1995, Institute of Food Technologists.

In systems as described above, the rate of cooling must be significantly faster than the rate of nucleation in order for a glass to be formed. Effectively, the system must be cooled rapidly through the crystallization zone of the state diagram bounded by the solubility curve and the glass transition curve. Systems that crystallize more rapidly than they can be cooled will not form glasses. 

Figure 5.4. A schematic state diagram showing water plasticization at increasing water weight fraction towards glass transition of water at -135°C. Relaxation times decrease rapidly above the glass transition as a result of thermal or water plasticization. Maximally freeze-concentrated solutes show glass transition at Tg and onset of ice melting at TJ. Equilibrium melting is described by the T curve.

Figure 5.5. State diagram of sucrose with typical experimental data at high concentrations and in the maximally freeze-concentrated state. Dynamic mechanical and dielectric measurements show glass transition-related relaxations.

Figure 7.3 is another example of a typical state diagram, developed for maltose. Maltose solutions are in glassy state below Tg curve. T g (onset of glass transition) and (onset of ice melting) show constant values for the maximally freeze concentrated solutions, where maximum ice formation occurs between T and Tg, and T is at the end point region of Tg (Figure 7.3) (Roos and Karel 1991b). State diagram for sucrose (Figure 7.4) also shows similar characteristics (Roos and Karel 1991a).

Figure 7.3. State diagram for maltose (Reproduced with permission from Roos and Karel, 1991b, Water and molecular effects on glass transitions in amorphous carbohydrates and carbohydrate solutions, J. Food Sci. 56, pp. 1676-1681, Institute of Food Technologists.)...

The effect of water and molecular weight on the glass transition has been shown on the state diagram for maltodextrins (Figure 7.5). Although T g and are observed individually with maltose (Figure 7.3), the difference between T g and T is... 

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