Kinetics of relaxation and crystallization of poly(ethylene terephthalate)

Abstract

We study the kinetics of structural relaxation and crystallization of poly(ethylene terephthalate) (PET). It is demonstrated that the activation energy depends strongly on fictive temperature T f. The dependence of the glass transition temperature T g on the heating rate q + is investigated for samples that were previously cooled down at a rate q −=12.5 K/min. Angell et al. [M. Tatsumisago, B. Halfpap, J. Green, S. Lindsay, A. Angell, Phys. Rev. B 45 (1992) 10091] compare the thermal properties of many glasses to their `fragility'. The dimensionless fragility is a measure of the dependence of the activation energy for spatial rearrangement on changes of structure. We define a parameter α, easily determinable from experimental, which is close to one for `strong' substances and much larger for `fragile' systems. According to the present measurements α is extremely large for polymers (i.e. this is a class of very fragile substances).