Abstract
We investigate relaxation dynamics in a series of six‐arm star/linear 1,4‐polybutadiene blends with mechanical rheometry measurements. Blend systems are formulated to systematically probe constraint release and arm relaxation dynamics. Zero shear viscosity and terminal relaxation times of star/linear polymer blends with fixed star arm molecular weights (M~a~) and compositions (ϕ~S~) are found to follow nonmonotonic dependencies on the linear polymer molecular weight (M~L~). At low values of ϕ~S~, at least two scaling regimes are apparent from the data (ξ~0~ ∼ M and ξ~0~ ∼ M), where ξ~0~ refers to the zero shear viscosity or terminal relaxation time of the blend. The two regimes are separated by a critical linear polymer molecular weight M__^*^__ that is more than 20 times larger than the critical molecular weight for entanglements. When the linear polymer contribution to blend properties is removed, a clear transition from dilution dynamics, ξ~0~ ∼ M, to Rouse‐like constraint‐release dynamics, ξ~0~ ∼ M, is apparent at low values of ϕ~S~. At higher ϕ~S~ values, a new activated constraint‐release dynamic regime is evident in which ξ~0~ ∼ M and ξ~0~ ∼ ϕ, where α changes continuously from approximately 2 to 0.5 as ϕ~S~ increases and β varies from 2.0 to 1.0 as M~L~ increases. The experimental results are compared with theoretical predictions based on a drag coupling model for entangled polymer liquids. All features observed experimentally are captured by this model, including the value of M__^*^__ for the transition from dilution to Rouse constraint‐release dynamics. Predictions of the drag coupling model are also compared with published data for the zero shear viscosity and terminal relaxation time in bidisperse linear polymer blends and pure entangled starlike molecules. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2501–2518, 2001