Nonlinear Shear Rheology of Star Polymer Blends across the Entanglement Threshold

This work addresses the rheological response of polymeric mixtures with complex architecture. We investigate the nonlinear start-up shear response of polystyrene blends composed of a high-molecular-weight four-arm star mixed with low-molecular-weight four- or eight-arm stars. The small stars are unentangled or marginally entangled, exhibiting Rouse-like dynamics. The volume fraction of the large star is selected in such a way that the linear viscoelastic response of the blends displays either unentangled or marginally entangled features. In both cases, the transient shear response reveals two distinct regimes. When the Weissenberg number based on the terminal relaxation time of the long star, WiDL, becomes larger than unity, a single stress overshoot is detected, arising from the stretching and orientation of the large star within a matrix of relaxed short stars. As the Weissenberg number of the short star, WiDS, also exceeds unity, two overshoots are observed, akin to the response of linear binary blends in solution, entangled comb polymers, or pom-poms. The first overshoot is attributed to the stretching and orientation of the short stars, whereas the second is attributed to those of the large star. Flow curve analysis demonstrates validation of the Cox–Merz rule, with some deviations arising from high-rate thinning dominated by the small stars at WiDL > 500. The analysis of the stress relaxation upon flow cessation reveals a faster stress decay upon increasing the Weissenberg number, because of the activation of fast Rouse modes by segmental stretch. Furthermore, a stress undershoot appears at intermediate rates. Its extent decreases as WiDS exceeds unity, likely due to the broadening of the tumbling time distribution induced by flow. Taken together, these results indicate that blending star polymers in the unentangled or marginally entangled regime represents a simple strategy to tailor the transient stress response (overshoot and undershoot) and an effective way to probe nonlinear polymer dynamics of branched polymers.