The enthalpic ageing behaviour of poly(methyl methacrylate) was studie
d by differential scanning calorimetry for several ageing temperatures
, and ageing times up to 10(4) min. The data are interpreted in terms
of both the phenomenological multiparameter model and the Cowie-Fergus
on model. The phenomenological multiparameter model was found not to p
rovide an adequate description of the enthalpic ageing process(es) occ
urring in poly(methyl methacrylate). Problems encountered in defining
the equilibrium glassy state by extrapolating from liquid enthalpy dat
a are also discussed.