RHEOLOGICAL BEHAVIOR OF STAR-SHAPED POLYMERS

The rheological properties of star-shaped polyisoprenes having a wide range of arm numbers and arm molecular weights are reported. In contrast to linear polymers, stars have a broad relaxation spectrum and a viscosity that increases exponentially with arm molecular weight. A comparison of eight pairs of samples having 3 and 4 arms and identical arm molecular weights showed that the viscosity of 3-arm stars is approximately 20% lower. For higher degrees of functionality, 4 less-than-or-equal-to f less-than-or-equal-to 33, the effect of functionality saturates and the viscosity is determined by arm molecular weight only. The nonlinear properties of one sample were studied using step-strain tests and found to be essentially identical to those of linear polymers. The predictions of a molecular theory for star polymers based on an extension of the reptation model are reviewed and shown to be in good agreement with the experimental data.