  The rate of physical aging of glassy polymethylmethacrylate (PMMA), followed
from the change in the secondary relaxation with aging, is found to be
independent of the density, the latter controlled by the pressure during glass
formation. Thus, the aging behavior of the secondary relaxation is the same
whether the glass is more compacted or less dense than the corresponding
equilibrium liquid. This equivalence in aging of glasses formed under different
pressures indicates that local packing is the dominant variable governing the
glassy dynamics. One consequence is that pressure densification yields a
reduction in the glass transition temperature. The fact that pressure
densification yields different glass structures is at odds with a model for
non-associated materials having dynamic properties exhibited by PMMA, such as
density scaling of the relaxation time and isochronal superposition of the
relaxation dispersion.
