We present experiments and theory on the melt dynamics of monodisperse entangled polymers of H-shaped architecture. Frequency-dependent rheological data on a series of polyisoprene H-polymers are in good agreement with a tube model theory that combines path-length fluctuation (like that of star polymer melts) at high frequency, with reptation of the self-entangled “cross-bars” at low frequencies (like that of linear polymer melts). We account explicitly for mild polydispersity. Nonlinear step-strain and transient data in shear and extension confirm the presence of a relaxation time not seen in linear response, corresponding to the curvilinear stretch of the cross-bars. This time is very sensitive to strain due to the exponential dependence of the branch-point friction constants on the effective dangling path length. Strain-induced rearrangements of the branch points are confirmed by small-angle neutron scattering (SANS) on stretched and quenched partially deuterated samples. We develop an extension of melt-scattering theory to deal with the presence of deformed tube variables to interpret the SANS data.

Experiments on solution-cast blends of two anionically synthesized monodisperse star-shaped polyisoprene molecules of widely different molecular weight exhibit a very rich rheological behavior. The time-dependent moduli are exponentially dependent on the relative volume fraction of each species. This work models these new features by extending existing theories for monodisperse melt of star polymers to the blend of two monodisperse star polymers with different molecular weight, keeping the same chemistry. The theory is based on the tube model with constraint release for star polymers in both an approximate and then a more exact level. The latter, with its treatment of nonactivated as well as activated breathing modes, is able to account quantitatively for the huge range of blend rheologies. With no extra parameters, it is able to account qualitatively for relaxation times and entire relaxation functions that vary over many orders of magnitude on blending.

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