A method to extract the monomer friction coefficient from the linear viscoelastic behavior of linear, entangled polymer melts

The rheological properties of isotactic and atactic polypropylene melts are analyzed in order to obtain the equilibration time, tau(e), and monomer friction coefficient, zeta. A procedure is proposed to determine tau(e) from the zero shear rate viscosity, eta(0), using the magnitude of the molecular weight per entanglement, M-e, from the literature. This procedure can be applied to both mono- and polydisperse linear, entangled polymer melts. For different polymers this procedure gives very similar results compared to the description of the storage and loss modulus of nearly monodisperse linear, entangled polymer melts by molecular based theories, as well as with values of zeta reported in the literature for linear, non-entangled polymer melts. It is observed that for isotactic and atactic polypropylene M-e differs by a factor 1.25 depending on the approach taken. As a consequence, the magnitude of tau(e) and zeta differ by a factor of about 3.0 and 1.8, respectively. The knowledge of tau(e) (or zeta) is of importance in order to obtain a better understanding of flow induced crystallization experiments on iPP.