Dynamic mechanical and creep studies of PMMA in the α- and β-relaxation regions. Physical ageing effects and non-linear behaviour

Abstract

Dynamic mechanical results for PMMA illustrate that the secondary (β) relaxation interacts with both shear and hydrostatic stress fields and covers a very broad range of frequency which becomes narrower with increasing temperature. It is suggested that the β-process involves coupled local motions of side groups and main chain which are hindered by a broad distribution of potential barriers (activation energies) corresponding to a wide range of local environments. The variation of the tensile creep compliance over 14 decades of timescale for as-received PMMA demonstrates the dominating influence of the β-relaxation region on the room-temperature creep behaviour and the onset of the overlapping primary (α) region at around 10²s. A method is proposed for extracting the α-contribution to the net creep compliance and is applied to linear data obtained as a function of ageing time at room temperature and to nonlinear results at different stress levels for as-received material. The onset of marked non-linear creep behaviour is ascribed largely to stress-induced deageing which preferentially decreases the α-relaxation times and increases the merging of the α- and β-regions. The increased mobility of the α-process is discussed by means of an adaptation of Robertson's theory of plasticity. This suggests that the shear stress increases the fraction of high-energy conformations, and hence the structural temperature, in local environments having relatively low glass transition temperatures. The magnitude of this effect may be influenced by the hydrostatic stress component in accordance with the yield investigations of Duckett, Rabinowitz and Ward.