Molecular weight polydispersity effects on the melt viscoelasticity of styrene-acrylonitrile random copolymers

A series of polydisperse SAN (styrene-co-acrylonitrile) random copolymers was studied by means of oscillatory rheometry in the rubbery plateau zone and in the terminal zone. The plateau modulus, the Newtonian viscosity, and the critical frequencies for the onset of non-Newtonian behavior were extracted from the experimental data. All these viscoelastic quantities consistently indicate that the tail of molecular weights below approximately M_e (the entanglement spacing) acts as a solvent for the rest of the polymer with M>M_e.