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The uniaxial extensional-flow data of Huang and co-workers [ACS Macro Lett. 2013, 2, 741; J. Rheol. 2016, 60, 465] on several polystyrene systems, both melts and solutions, are here successfully compared with the predictions of a recent model [Ianniruberto, Macromolecules 2015, 48, 6306] that accounts for flow-induced friction-coefficient reduction. The model is here effectively simplified without loss of accuracy. Comparison with the solution data of Huang et al. [ACS Macro Lett. 2013, 2, 741], only differing for the size of the oligomeric solvent (from nearly one Kuhn length up to a few ones), reveals that friction reduction must include oligomer-size-dependent orientational coupling between the oligomeric solvent and the polymer molecules, such interactions vanishing for the smallest oligomer. The rheological "measure" of this nematic-like interaction as a function of the oligomer size (for small oligomers) is the novel result of the present paper, together with a further confirmation of the importance of friction coefficient reduction in strongly aligning flows. The model is also used to describe the stress relaxation data following fast uniaxial extension recently reported by Huang and Rasmussen [J. Rheol. 2016, 60, 465], confirming that the short-time relaxation process occurs at a faster rate because of the friction coefficient reduction induced by the previous flow. Those data also reveal the key role of convective constraint release induced by chain retraction, similarly to what happens in relaxation experiments following a large step strain. [ABSTRACT FROM AUTHOR]

The classical Doi–Edwards theory for the steady-statestressof entangled linear polymers in uniaxial elongational flow is hereextended to entangled branched polymers, specifically to stars andpompoms, both symmetric and asymmetric. The arm withdrawal idea advancedlong ago by McLeish and Larson is also fully exploited. Non-Gaussiancorrections are considered. Aside from quantitative details, the theoryfor branched polymers predicts that they behave similarly to linearones in the steady state (as opposed to the startup stage) of fastelongational flows. The theoretical predictions are then successfullycompared, both qualitatively and quantitatively, to data collectedby Hassager and co-workers on polystyrene (PS) melts of a pompom andof an asymmetric star. These are, to the authors’ knowledge,the only published data for branched polymers of known architecturethat, during startup of the elongational flow, extend in time longenough to allow determination of steady-state values of the elongationalstress. [ABSTRACT FROM AUTHOR]