The physical ageing response of an epoxy glass subjected to large stresses

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Physical ageing studies were made using model epoxy network glasses. Non-linear viscoelastic responses were measured after quenching the glasses from above T g to the temperature of interest. The physical ageing responses at different magnitudes of applied load, for different duration times of the load application and at different temperatures were studied. The creep compliance curves at different ageing times were able to be superimposed to form a single master curve, demonstrating the validity of a time-ageing time superposition principle for this epoxy network. Similar to many other physical ageing studies, we observed that the double-logarithmic shift rate μ decreases as the stress amplitude increases. However, this study differs from others in that the glasses were aged close to T g and it was possible to age them into structural equilibrium, i.e. the glasses cease to age. Thus, t∗, the time required to reach structural equilibrium, was obtained. Results showed that t∗ did not change with the magnitude of applied stress; therefore, it will be argued that ageing is not ‘erased’ by large mechanical stimuli. Furthermore, the creep response after reaching equilibrium for the glass that had been subjected to the repeated stresses as it aged into equilibrium was compared with that of the same glass that was aged thermally into equilibrium without any stress application. There was no significant difference between the responses in these two conditions. This demonstrates that, in spite of the mechanical stimulus, the response of the glass in the equilibrium state has not changed. All these results support the argument that the volume recovery that occurs during ageing impacts the small-deformation response differently than it does the large-deformation response.