  Ianniruberto and Marrucci developed a theory whereby entangled branched
polymers behave like linear ones in fast elongational flows. In order to test
such prediction, Huang et al. performed elongational measurements on star
polymer melts, indeed revealing that, in fast flows, the elongational viscosity
is insensitive to the molecular structure, provided the molecular weight of the
backbone is the same. Inspired by these studies, we here report on results
obtained with multi-chain slip-link simulations for symmetric and asymmetric
star polymer melts, as well as calculations of the Rouse time of the examined
branched structures. The simulations semi-quantitatively reproduce the
experimental data if the Kuhn-segment orientation-induced reduction of friction
(SORF) is accounted for. The observed insensitivity of the nonlinear
elongational viscosity to the molecular structure for the same span molar mass
may be due to several factors. In the symmetric case, the calculated Rouse time
of the star marginally differs from that of the linear molecule, so that
possible differences in the observed stress fall within the experimental
uncertainty. Secondly, it is possible that the flow-induced formation of hooked
star pairs makes the effective Rouse time of the aggregate even closer to that
of the linear polymer because the friction center moves towards the branchpoint
of the star molecule. In the asymmetric case, it is shown that the stress
contributed by the short arms is negligible with respect to that of the long
ones. However, such stress-reduction is balanced by a dilution effect whereby
the unstretched arms reduce SORF as they decrease the Kuhn-segment order
parameter of the system. As a result of that dilution, the stress contributed
by the backbone is larger. The two effects compensate one another so that the
overall stress is virtually the same as the other architectures.
