Branch-Point Motion in Asymmetric Star Polymers

Abstract

Linear viscoelastic behavior of a series of 1,4-polyisoprene asymmetric star polymers was investigated experimentally and theoretically in order to determine how branch point motion affects terminal relaxation dynamics. For the systems studied, the branch point is connected to a moderately entangled short arm (MB/Me= 7) and two longer arms with varying numbers of entanglements (MA/Me = 7−43). The measured loss modulus manifests a clear transition from starlike to linear-like dynamics as the molecular asymmetry increases. We show that this behavior can be predicted using a tube-model for branched molecules (Frischknecht et al. Macromolecules 2002, 35, 4801) with a self-consistently determined branch-point diffusivity, Dbp = (pa0)2/2τa,B. Here a0 is the effective bare tube diameter, τa,B is the arm retraction time, and p2 = 1/[(2NAR)b2/a02] is an effective drag produced by unrelaxed backbone segments. We also compare relaxation moduli predicted by the tube model with experimental data and find good agreement over a range of short- and long-arm entanglement densities.