Reduction of monomeric friction coefficient for linear isotactic polypropylene melts in very fast uniaxial extensional flow

For the first time, the monomeric friction coefficient for fully aligned chains, zeta (aligned), was determined for three linear isotactic polypropylene melts (iPP) using a high-strain-rate limiting value of uniaxial extensional viscosity, eta (E,U,infinity), obtained from our recent experimental data [Drabek and Zatloukal, Phys. Fluids 32(8), 083110 (2020)] and expression relating eta (E,U,infinity) with zeta (aligned), which was derived for a fully stretched Fraenkel chain [Ianniruberto et al., Macromolecules 53(13), 5023-5033 (2020)]. It was found that the obtained zeta (aligned) value is lower by a factor of 2.9-5.0 (or even by a factor of 8.7-16.5 if the effect of polydispersity is included) compared to the equilibrium friction coefficient, zeta (eq), defined according to Doi and Edwards. This strongly supports recent arguments from rheological data and molecular simulations that a reduction in the friction coefficient must be considered in order to understand dynamics of polymer melts in very fast flows.