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

Experiments on solution-cast blends of two anionically synthesized monodisp erse star-shaped polyisoprene molecules of widely different molecular weigh t exhibit a very rich rheological behavior. The time-dependent moduli are e xponentially dependent on the relative volume fraction of each species. Thi s work models these new features by extending existing theories for monodis perse 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 a n approximate and then a more exact level. The latter, with its treatment o f nonactivated as well as activated breathing modes, is able to account qua ntitatively for the huge range of blend rheologies. With no extra parameter s, it is able to account qualitatively for relaxation times and entire rela xation functions that vary over many orders of magnitude on blending.