Properties of amorphous and crystallizable hydrocarbon polymers. II. Rheology of linear and star‐branched polybutadiene

Some results are reported on the linear viscoelastic properties of polybutadienes with narrow‐molecular‐weight distributions. The zero shear viscosity η0 varies as M3.4 in the linear samples, and viscosity enhancement is found in star‐branched samples with long arms, in good agreement with results reported earlier by Kraus and Gruver. The temperature coefficient of viscosity appears to be slightly larger in stars when the arms become long. The steady state recoverable compliance J is 2.1 × 10 −7 cm2/dyn in linear samples of high molecular weight, but it increases to values as much as 10 times larger in the stars. The plateau modulus G , obtained from a composite curve for the linear samples, is 1.32 × 107 dyn/cm2. The terminal relaxation spectrum of the stars is too broad to allow an evaluation of plateau modulus.