Comparison of shear and extensional rheology between entangled star and linear polystyrenes with the same span molecular weight

Nonlinear shear and extensional rheology of entangled star polystyrenes (PS) are compared with those of linear PS with the same span molecular weight to investigate their different dynamics. During transient startup shear flow, 8 or 10-arm star PS exhibits a more pronounced stress decay following the stress overshoot and a larger peak strain compared to linear PS, while 3-arm star PS displays a nonlinear shear response nearly identical to that of linear PS. No significant differences are observed between the star and linear PS in stress relaxation after step strain; all samples exhibit similar nonlinear damping behavior, which aligns well with the Doi-Edwards prediction. In extensional flow, the 8- or 10-arm star PS shows more strain hardening but approaches a similar steady stress at large strain compared to the linear PS, while the 3-arm star and linear PS behave almost same. This difference is attributed to heterogeneous entanglement density of multi-arm star polymers. These findings provide valuable guidance for the molecular design of star-shaped polymers with tailored nonlinear rheological properties.