Influence of Arm Numbers on the Extensional Rheology of Entangled Star Polymer Melts

In previous studies, stress overshoot in transient extensional flow was observed in entangled 8-arm star polystyrene (PS) melts ( ACS Macro Lett. 2024 13, 812−817, DOI: 10.1021/acsmacrolett.4c00182) but not in linear and 3-arm star PS melts. In this work, we show that the functionality (i.e., arm numbers) of star PS melts, f, does not influence their nonlinear rheological behavior in extensional flow when f ≥ 4. The entangled 4-, 32-, and 64-arm star PS melts all show stress overshoot in transient extensional flow with a similar degree of strain hardening. Stress relaxation measurements following startup extensional flow on the 32- and 64-arm stars also reveal identical relaxation behavior, but compared to the 3-arm star, their stress decays more pronouncedly at short time (close to the Rouse time), indicating an excessive arm retraction. The transition from linear-like to star-like nonlinear behavior is probably at f = 4.