  The time-dependent response of polystyrene and poly(methyl methacrylate) is
studied in isothermal long-term shear creep tests at small strains and various
temperatures in the vicinity of the glass transition point. A micromechanical
model is derived to describe the experimental results. Constitutive equations
are developed under the assumption that the behavior of amorphous polymers is
governed by two micro-mechanisms: rearrangement of cooperatively relaxing
regions (CRR) reflects the viscoelastic response, whereas displacement of CRRs
with respect to each other is responsible for the anelastic response. It is
demonstrated that some critical temperature exists slightly above the glass
transition temperature, where the dependences of adjustable parameters on
temperature are dramatically changed. The critical temperature is associated
with transition from dynamic heterogeneity in amorphous polymers to static
inhomogeneity.
