Investigation of the Dynamic Rheological Properties of a Polycarbonate Melt Presenting Solid-Like Characteristics and a Departure from Pure Liquid Newtonian Behavior at Long Relaxation Times

We analyzed the dynamic rheological properties of a polycarbonate melt, a typical amorphous polymer with no molecular particularities which would render the results unique to that polymer. We observed an increase of the viscosity at low omega, as omega -> 0 (instead of a constant Newtonian viscosity) and a solid-like behavior for G'(omega) and G '' (omega), which diverges from the classical rheological description accepted for a simple homopolymer melt. The phenomenon was reversible, yet appeared to vary somewhat with the sample preparation conditions (thermal history), as well as strain (testing conditions). We investigated and ruled out explanations based on a modification of the molecular structure during the test because of the reversibility of the appearance of the lower viscosity tail in sequential frequency sweeps (downsweeps followed by upsweeps and vice-versa). We offer an interpretation of this low omega 'shear-thinning tail' in terms of the Dual-Phase visco-elastic model of entanglements previously proposed by one of the authors which suggests that the phase-line diffusion mechanism (the sweep of the phase lines) occurring in the Newtonian region is influenced by 'b-grain glassification' occurring above T-g. Thus the viscosity increase at low omega is essentially due to a 'free volume' fluctuation decrease explaining the instability of the Newtonian state.