Rheology and tube model theory of bimodal blends of star polymer melts

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.