Rheology of Three-Arm Asymmetric Star Polymer Melts

We present experimental and theoretical results for the linear rheology of melts of entangled, three-arm asymmetric polyisoprene stars. Asymmetric three-arm stars, in which two arms have the same length and the third is shorter, cross over from starlike to linear-like stress relaxation as the length of the third arm varies. We combine recent theories of stress relaxation in symmetric stars and in linear melts to predict the dynamic modulus of the asymmetric stars. For stars with short arm molecular weights of a few entanglement lengths, our theory underestimates the effective drag caused by the short arm, even when polydispersity effects are included. This unexplained discrepancy does not appear in a recent comparison of a related theory with measurements on polyisoprene H-polymers.