Glass transition enthalpy relaxation

Precise analysis of enthalpy relaxation is not possible owing to the nonequilibrium nature of glassy polymers above and below the glass transition. Enthalpy relaxation can be characterized under certain limiting assumptions. If the viscous or rubbery state of the polymer above is assumed to be an... 

Chung, H.-J., Lee, E.-J., and Lim, S.-T. (2002). Comparison in glass transition and enthalpy relaxation between native and gelatinized rice starches. Carbohydr. Polym. 48, 287-298. 

Chung, H. -J., Woo, K. -S., Lim, S. -T. (2004). Glass transition and enthalpy relaxation of eross-linked corn starches. Carbohydr. Polym., 55, 9-15. 

Privalko, V. P, Demchenko, S. S., and Lipatov, Y. S. (1986). Structure-dependent enthalpy relaxation at the glass transition of polystyrenes. Macromolecules 19(3), 901-904. 

It has been recently realized that the activation enthalpy of the Johari-Goldstein beta ((3) relation is cross-correlated with the glass transition temperature of the alpha (a) relaxation of various glass formers [168,169]. This indicates a deep link between the fragility index of the a and the [3 relaxation [170],... 

A melted sample of the terpolymer was cooled in the calorimeter at 320°/min to — 40°C and was allowed to remain at — 40°C for 16 hr after which the temperature was lowered to — 150°C. Upon reheating, the Tg of the annealed sample increased to — 31 °C moreover, an additional adsorption of thermal energy was superimposed upon the normal increase in Cp during the glass transition. The additional increase in enthalpy, 0.5 cal/g, is the result of enthalpy relaxation occurring during... 

Table 12.3. Glass transition temperatures Tg and the activation energies AFreiax of enthalpy relaxation of dry amorphous disaccharidesa...

Haida O, Matsuo T, Suga H, Seki S (1974) Calorimetric study of the glassy state, X. Enthalpy relaxation at the glass transition temperature of hexagonal ice. J Chem Thermodyn 6 815-825... 

Figure 5.3. Changes in volume, V, enthalpy, H, and entropy, S, around glass transition and melting. Amorphous materials can have an infinite number of glassy states which result in relaxations around glass transition.

Fig. 4 (A) DSC scan of the glass transition temperature of a miscible blend of a MK-0591 with 10% PVP, showing the onset, midpoint, and offset glass transition temperatures. (B) DSC scan of the glass transition of sucrose with the enthalpic relaxation endotherm and enthalpy (AHr). (From Ref...

In chapter 7, several aspects of conductivity and dielectric relaxation were discussed. Various other properties such as shear modulus, viscosity, refractive index, volume, enthalpy etc. also exhibit relaxational behaviour particularly in the glass transition region. In this chapter, few further aspects of relaxation are discussed. Relaxation of generalized stress or perturbation whether electrical, mechanical or any other form is typically non-exponential in nature. The associated property is a function of time. A variety of empirical functions, (/) t), have been used to describe the relaxation. Some of them have already been discussed in chapters 6 and 7. The most widely used function is the Kohlraush-Williams-Watts (KWW) function (Kohlraush, 1847 Williams and Watts, 1970 Williams et al., 1971). It is more commonly referred to as the stretched exponential function. The decay or relaxation of the quantity is given by,... 

Enthalpy relaxation is one of the most widely studied in the context of both non-linearity and non-exponentiality of the measured glass properties. A convenient technique for these studies is scanning calorimetry. In simple cooling and heating experiments, heat capacity curves exhibit normal increase with characteristic hump of Cp above the glass transition as represented in Figure 9.08(A). The fictive temperature,... 

The experimental glass transition is, therefore, associated with a relaxing property - the enthalpy. The enthalpy/specific heat modes, in principle, couple to all phenomena which occur in the glass transition region. Enthalpy relaxation can be studied by applying a sinusoidal temperature pulse, much like an alternating electric field, which is applied to study dielectric relaxation. The specific heat is therefore treated as a frequency (of thermal field) dependent property. The traditional adiabatic technique cannot be applied because the time required for heat diffusion across the sample has to be short compared to the measurement time and the former is determined by the thermal diffusivity, which is low for most solids and is of the order of 10 cm s. ... 

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