Physical Ageing in Polymethylmethacrylate — A Dielectric Investigation

Abstract

Polymethylmethacrylate aged for various times at temperatures below the glass transition temperature, Tg, were studied using dielectric spectroscopy. The study uses dipole relaxation as a probe for the physical ageing process. Isothermal measurements indicate that the dominant changes occurring in the dielectric loss spectrum at temperatures between the Tg and the side chain (β) relaxation process are associated with changes in the dipole relaxation distribution and the ionic charge mobility. Differences between the spectra for aged and unaged samples are used to assess the nature of the changes occurring at a molecular level. The rate of change of dielectric loss was estimated as a function of the ageing temperature and the observed behaviour was found to be similar in form to that observed from creep and structural relaxation.