The
ageing kinetics of amorphous atactic (a-PS), isotactic (i-PS),
and syndiotactic (s-PS) polystyrene were studied by means of flash-differential
scanning calorimetry. The specimens were aged for up to 2 h at six
different ageing temperatures: the optimum ageing temperature, that
is, the temperature at which the enthalpy overshoot at the glass transition
is maximal for the given elapsed time, and five ageing temperatures
ranging from 20 to 80 K below the optimum ageing temperature. A logarithmic
increase of the enthalpy overshoot with ageing time is observed for
specimens at their optimum ageing temperatures. For temperatures significantly
lower than the optimum, there is a range where the enthalpy overshoot
is constant, but for higher temperatures (still below the optimum),
a logarithmic increase is also observed. Moreover, the ageing kinetics
appear to depend on tacticity, with s-PS and i-PS exhibiting the slowest
and fastest ageing kinetics, respectively, and a-PS exhibiting ageing
kinetics between these two extremes.