Arm-length dependence of stress relaxation in star polymer melts

We apply our recent microscopic theory of stress relaxation in star polymer melts to dynamic rheology data of Fetters et al. on melts of monodisperse four-arm polyisoprene stars, with arm molecular weights in the range 17 000 to 105 000 (M/M-e from 3.4 to 21.0, with M-e = 5000). The agreement between theory and experiment is excellent for all frequencies and arm lengths, with physically reasonable values of plateau modulus and friction factor and no other adjustable parameters. These data obey time-temperature superposition, with no evidence for anomalous arm-length dependence of activation energies as seen in polyethylene-branched polymers.