Plastic deformation and physical ageing in PMMA

Ageing and rejuvenation of PMMA are investigated just above Tβ and just below Tg by means of tensile creep experiments, in the non-linear range, i.e. for high stress levels. The molecular mobility is probed using torsional microcreep. Rejuvenation is induced by tensile Struikʹs ‘type II’ experiments, i.e. long-term ones. It is shown that, at low temperature, creep induces rejuvenation due to anelastic (or reversible) deformation, and so does recovery after the load removal. When the creep (or recovery) rate becomes very low, ageing proceeds at the same rate as natural ageing does. At high temperature, plasticity occurs and represents the main cause for rejuvenation that is no more reversible. In this case, plasticity seems to have a very significant influence on the thermodynamic equilibrium state reached after full ageing. These observations are finally interpreted in terms of microstructural mechanisms of deformation.