Comprehensive modelling of the experimental temperature and stress response of time-dependent materials

The stress- and temperature-dependent non-linear creep behaviour of polymers and other materials is quantitatively modelled as a superposition of a wide range of activated motions at the molecular scale, covering wide but well-defined space and time scales. The resulting time, temperature and stress dependences are coupled (not fully separable) except at low stress values, where linear viscoelastic behaviour is directly obtained, improving on previous (approximate or questionable) time-temperature and time-stress superposition or equivalence relationships. The behaviour has been shown to have a strong cooperative nature, which may be interpreted in terms of varying clusters of identifiable structural elements within the material. Reasonably good descriptions of the experimental creep behaviour of both amorphous and semicrystalline polymers have already been achieved, but this work concentrates on the behaviour of an amorphous polycarbonate (PC), including the physical characterization and modelling of its retardation time spectrum.