The concept of ‘ageing phenomena’ colloquially refers to a change of properties of materials over time, even without any apparent forces acting on them. Ageing arises when the relaxation processes of a system, brought out of equilibrium by a sufficiently rapid change of its thermodynamic state variables, are governed by large fluctuation effects which prevent a rapid return to the stationary state. This may happen quite independently of whether the equilibrium state of the system is itself at an equilibrium critical point or not. Since glassy systems furnish many paradigmatic examples of ageing behaviour, we shall begin with a phenomenological description of mechanical ageing in polymer glasses, use this as a motivation to formulate a generic scaling description (essentially adapted to the description of non-glassy systems) and review results for non-equilibrium exponents and scaling functions. Although we shall initially use experimental information from glassy systems to enter the subject, the focus of this volume will nevertheless be on the ageing in non-glassy systems, which we hope to be more accessible to theoretical analysis, especially from the perspective of understanding their dynamical symmetries.
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