Positron annihilation studies of ageing in polystyrene

Structural relaxation processes are reported in polystyrene as a function of ageing time using positron annihilation lifetime and Doppler broadening spectroscopy. Changes in the intensity, lifetimes and line shapes with increasing temperature for ortho positronium annihilation show an internal consistency between the two experiments and may be described in terms of a simple logarithmic decay process. The isochronous and isothermal ageing data show internal consistency and are best described by either a Narayanaswamy distribution or a double exponential fit of the data. The possible effects of charging during the isothermal decay measurements are explored and found to influence the short term decay behaviour. Measurements of the liquid phase are extrapolated to lower temperatures to allow estimation of the equilibrium lifetimes in the glassy phase. Changes in the lifetimes and the long time component of the intensity are interpreted as being indicative of the structural relaxation process; they are not influenced by the effects of charging and exhibit a good correlation with physical ageing behaviour as determined by other techniques. © 1998 SCI.