Physical aging and tacticity effects on the a relaxation mode of amorphous polymers by thermally stimulated techniques

Samples of amorphous poly (ethylene terephthalate) (PET) vitrified under different cooling rates and a series of samples of poly (methyl methacrylate) (PMMA) of various tacticities have been characterized by differential scanning calorimetry (DSC), thermally stimulated current (TSC) and thermally stimulated creep (TSCr). The alpha relaxation and retardation modes, associated with the glass transition, have been analyzed. The dielectric relaxation mode observed in PET has relaxation times following a compensation law with compensation parameters independent of physical aging. Contrarily, the maximum value of the activation enthalpy increases upon aging. The anelastic retardation mode observed in PMMA is also constituted of retardation times following a compensation law. In that case, the T-c - T-g lag varies according to the chain conformation. It is also interesting to note that the maximum of the activation enthalpy increases with the level of local interactions. This evolution is due to the increasing of the domain sizes in which molecular mobility takes place. (C) 1998 Elsevier Science B.V. All rights reserved.