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• enthalpic relaxation  (2)
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• 1
Electronic Resource
New York, NY [u.a.] : Wiley-Blackwell
Polymer International 39 (1996), S. 221-229
ISSN: 0959-8103
Keywords: polyhydroxybutyrate ; copolymers ; embrittlement ; physical ageing ; enthalpic relaxation ; crystallisation ; DSC ; DMTA ; DETA ; Chemistry ; Polymer and Materials Science
Source: Wiley InterScience Backfile Collection 1832-2000
Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
Notes: The embrittlement of poly(hydroxybutyrate), and its copolymers with hydroxyvalerate, on storage at room temperature has been considered in the light of the relaxation of the interlamellar glassy regions and also the development of secondary crystallisation. From a study of the kinetics of the two processes, the first mechanism has been eliminated particularly since the glass transition, as measured by dielectric thermal analysis (DETA), was below room temperature.Control of the primary crystallisation of PHB was observed to reduce the extent of the development of secondary crystallisation and to eliminate the embrittlement on storage. DMTA and DETA were found to be useful techniques for measuring the mobility of the amorphous regions in partially crystalline samples, under conditions where DSC was unable to detect any glass transition.
Type of Medium: Electronic Resource
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• 2
Electronic Resource
Springer
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
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