Aging physical

When the heat capacities of the glass and the liquid are defined by linear functions of temperature, the value of Tj can be calculated, from numerical integration of the DSC curves to obtain HCT ) - H(T). 

The annealing of the glass at the aging temperature T results in a relaxation of the enthalpy towards the equilibrium value H(°o). This can be described by a relaxation function 0(t), which is defined by  

0(0 can also be expressed in terms of a semi-empirical function introduced originally by Kohlrausch [1847] and revived and redeveloped by Williams and Watts, [1970], abbreviated as the KWW equation  

The relaxation time x can be expressed in terms of the fictive temperature T following Narayanaswamy, [1971] and Moynihan et al., [1976] who used form  

The parameters in Eq 14.8 can be estimated by assuming Ah has a fixed value and allowing In A, X and p to vary until the best fit to the experimental, normalized data is obtained. A more informative assessment can be made by comparing AH(t, T ) values from the model with experimental data. 

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The three polymers studied display remarkable physical aging effects [24], with a strong increase of modulus and changes in the location and intensity of the glass transition. This phenomenon is exemplified in Fig. 17 where the changes of dynamic moduli in a PDEB sample aged for 14 months are very apparent. 

Struik LCE (1978) Physical aging in amorphous polymers and other materials, Elsevier, Amsterdam... 

Struik, L. C. E. (1978), Physical Ageing in Amorphous Polymers and Other Materials, Elsevier, New York. 

The effect of physical aging on the crystallization state and water vapor sorption behavior of amorphous non-solvated trehalose was studied [91]. It was found that annealing the amorphous substance at temperatures below the glass transition temperature caused nucleation in the sample that served to decrease the onset temperature of crystallization upon subsequent heating. Physical aging caused a decrease in the rate and extent of water vapor adsorption at low relative humidities, but water sorption could serve to remove the effects of physical aging due to a volume expansion that took place in conjunction with the adsorption process. 

Wungtanagom, R. and Schmidt, S.J. 2001. Thermodynamic properties and kinetics of the physical aging of amorphous glucose, fructose, and their mixture. J. Therm. Anal. Calorim. 65, 9-35. 

Physical aging, defined, 10 424 Physical analysis, of wine, 26 324 Physical bonding processes, 17 496 Physical-chemical waste treatment,... 

A minimum storage time is advisable between moulding and measurement to reduce the effects of physical ageing. Test pieces should, so far as is feasible within the limitations of the test programme, be identical in their source and conditioning. Where this is not possible, then pieces from different sources should be identified but then distributed proportionally, not randomly, among the exposed and control samples. 

The creep tests will have been performed at identical stresses and the shifting procedure excludes corrections for physical ageing and other effects. 

At lower stresses where the compliance is independent of stress, this will allow the derivation of a creep curve. At higher stresses the compliance is dependent on stress. This has been explained by stress de-ageing, in which the effects of physical ageing can in part be undone by the application of medium level mechanical stresses. Thus A becomes a function of stress C ... 

However, it is expected the reaction rates below Tg may be affected also by volume relaxation (physical aging) which was not taken into account and which will result in the dependence of k-p not only on T and a but also on time t. If we take the positive deviations of experiments in Figure 14 as a measure of the volume relaxation effect then the physical aging increases the apparent mobility although it leads also to a denser (and less mobile) state. 

A unified approach to the glass transition, viscoelastic response and yield behavior of crosslinking systems is presented by extending our statistical mechanical theory of physical aging. We have (1) explained the transition of a WLF dependence to an Arrhenius temperature dependence of the relaxation time in the vicinity of Tg, (2) derived the empirical Nielson equation for Tg, and (3) determined the Chasset and Thirion exponent (m) as a function of cross-link density instead of as a constant reported by others. In addition, the effect of crosslinks on yield stress is analyzed and compared with other kinetic effects — physical aging and strain rate. 

The nonequilibrium glassy state, 5(t) = f(t) -f, is determined by solving the kinetic equations which describe the local motion of holes in response to molecular fluctuations during vitrification and physical aging. The solution is (11)... 

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