ISSN:
1572-8943
Keywords:
DSC
;
enthalpic relaxation
;
glass transition
;
physical ageing
Source:
Springer Online Journal Archives 1860-2000
Topics:
Chemistry and Pharmacology
Notes:
Abstract Enthalpic relaxation has been used to model the development of the glass transition in polymers, using kinetic parameters determined separately. For this purpose the Kohlrausch-Williams-Watt stretched exponential function, relating the extent of relaxation, Φ(t), to time t and an average relaxation time, τa, i.e. $$1 - \Phi \left( t \right) = \exp \left( { - t/ta} \right)^{\beta }$$ where β is inversely related to the breadth of the relaxation spectrum, has been adopted. The relaxation time dependence on temperature was taken to follow the modified Arrhenius relationship, $$\tau _a = A\exp \left[ {\frac{{X\Delta H}}{{RT}} + \frac{{\left( {1 - X} \right)\Delta H}}{{RT'}}} \right]$$ where T is the storage and T′ the fictive temperature, X is the structure factor and ΔH the activation enthalpy. Both have been found to describe the process of enthalpic relaxation in polymer glasses and a direct comparison has been made with the change in specific heat observed with different cooling rates in DSC experiments. The effect of variables, such as activation enthalpies, pre-exponential factors, and the non-linear factors such as X and β on the observed Tgs and the temperature range over which the transition occurred have been determined.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1023/A:1010120004208
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