Relaxation Dynamics of Entangled and Unentangled Multiarm Polymer Solutions: Comparisons with Theory

Abstract

Linear viscoelastic properties of model branched A3−A−A3 1,4-polybutadiene melts and solutions are investigated experimentally, using small-amplitude oscillatory shear measurements, and theoretically, using a recent tube model theory for stress relaxation of entangled H-shaped polymers. This study provides experimental guidance on two key assumptions in the theory: namely, that stress relaxation of multiply branched, entangled polymers is hierarchical and that relaxed portions of a branched molecule accelerate stress relaxation of unrelaxed sections of the same molecule in qualitatively the same manner as a low molecular weight diluent. Experimental results for multiarm A3−A−A3 polymer melts and solutions covering a wide range of arm A and connector A molecular weights provide broad support for a hierarchical relaxation process in these materials. However, linear viscoelastic data for A3−A−A3 polymers strongly disagree with the second assumption. Specifically, we find that the relaxed arms provide a much greater than expected retardation of branch point motion even on time scales well beyond the arm relaxation time. Branch point diffusivities estimated from the experimental data are typically 1−2 orders of magnitude lower than predicted by theory. The theory also underpredicts the breadth and amplitude of the dynamic storage and loss moduli of multiarm melts and solutions, particularly for materials where the number of entanglements per arm is low.