VISCOELASTICITY OF LOW-MOLECULAR-WEIGHT POLYMERS AND THE TRANSITION TO THE ENTANGLED REGIME

The viscoelasticity of unentangled polystyrene melts has been investig ated in terms of terminals parameters: zero-shear viscosity, steady-st ate compliance and relaxation spectrum. The Rouse model applies well f or molecular weights lower than M-e the average molecular weight betwe en entanglements, providing that one takes into account the proper var iations of the radius of gyration. Moreover, local motions at the scal e of Kuhn segments have to be considered in order to describe correctl y the relaxation modes intermediate between the terminal zone and the glassy plateau. On the other hand, reptation models are commonly used for describing the entangled regime. We propose an expression of the s hear modulus G(t) which accounts not only for the terminal modes (rept ation, tube length fluctuations and tube renewal), but also for the re laxation modes responsible for the plateau zone and the transition of the glassy plateau. A crossover region between the unentangled and unt angled regimes is located around 2M(e). When the molecular weight incr eases, a shift transfer of Rouse modes towards reptation modes occurs. That leads to a continuity of the expression of the shear modulus ove r the entire range of molecular weights.