  Local segmental relaxation properties of poly(methylmethacrylate) (PMMA) of
varying molecular weight are measured by dielectric spectroscopy, and analyzed
in combination the equation of state obtained from PVT measurements. The usual
variation of glass transition temperature and fragility with molecular weight
are observed. We also find, in accord with the general properties of
glass-forming materials, that a single molecular weight dependent scaling
exponent, g, is sufficient to define the mean segmental relaxation time,
tau_alpha, and its distribution. This exponent can be connected to the
Gruneisen parameter and related thermodynamic quantities, thus demonstrating
the interrelationship between dynamics and thermodynamics in PMMA. Changes in
the relaxation properties ("dynamic crossover") are observed as a function of
both temperature and pressure, with tau_alpha serving as the control parameter
for the crossover. At longer tau_alpha another change in the dynamics is
apparent, associated with a decoupling of the local segmental process from
ionic conductivity.
