Entanglement friction and dynamics in blends of starlike and linear polymer molecules

We investigate relaxation dynamics in a series of six-arm star/linear 1,4-p olybutadiene blends with mechanical rheometry measurements. Blend systems a re formulated to systematically probe constraint release and arm relaxation dynamics. Zero shear viscosity and terminal relaxation times of star/linea r polymer blends with fixed star arm molecular weights (M-a) and compositio ns (phi (S)) are found to follow nonmonotonic dependencies on the linear po lymer molecular weight (M-L). At low values of phi (S), at least two scalin g regimes are apparent from the data (xi (0) similar to M-L(2) and xi (0) s imilar to M-L(3)) where o refers to the zero shear viscosity or terminal re laxation time of the blend. The two regimes are separated by a critical lin ear polymer molecular weight M-C** that is more than 20 times larger than t he critical molecular weight for entanglements. When the linear polymer con tribution to blend properties is removed, a clear transition from dilution dynamics, xi (0) similar to M-L(0), to Rouse-like constraint-release dynami cs, xi (0) similar to M-L(2.4 +/-0.4), apparent at low values of phi (S). A t higher phi (S) values, a new activated constraint-release dynamic regime is evident in which xi (0) similar to M-L(alpha) and xi (0) similar to phi (beta)(s), where a changes continuously from approximately 2 to 0.5 as phi (S) increases and beta varies from 2.0 to 1.0 as M, increases. The experime ntal results are compared with theoretical predictions based on a drag coup ling model for entangled polymer liquids. All features observed experimenta lly are captured by this model, including the value of M-C** for the transi tion from dilution to Rouse constraint-release dynamics. Predictions of the drag coupling model are also compared with published data for the zero she ar viscosity and terminal relaxation time in bidisperse linear polymer blen ds and pure entangled starlike molecules. (C) 2001 John Wiley & Sons, Inc.