  Results of torsional oscillation tests are reported that were performed at
the temperature T=230C on melts of a hybrid nanocomposite consisting of
isotactic polypropylene reinforced with 5 wt.% of montmorillonite clay. Prior
to mechanical testing, specimens were annealed at temperatures ranging from 250
to 310C for various amounts of time (from 15 to 420 min). Thermal treatment
induced degradation of the matrix and a pronounced decrease in its molecular
weight. An integro-differential equation is derived for the evolution of
molecular weight based on the fragmentation-aggregation concept. This relation
involves two adjustable parameters that are found by fitting observations. With
reference to the theory of transient networks, constitutive equations are
developed for the viscoelastic response of nanocomposite melts. The
stress-strain relations are characterized by three material constants (the
shear modulus, the average energy for rearrangement of strands and the standard
deviation of activation energies) that are determined by matching the
dependencies of storage and loss moduli on frequency of oscillations. Good
agreement is demonstrated between the experimental data and the results of
numerical simulation. It is revealed that the average energy for separation of
strands from temporary junctions is independent of molecular weight, whereas
the elastic modulus and the standard deviation of activation energies linearly
increase with mass-average molecular weight.
