Phenomenology of Self-Diffusion in Star-Branched Polyisoprenes in Solution

We have measured polymer and solvent self-diffusion at 50°C in equiarmed star-branched polyisoprenes of low molecular weights in solution with C₆F₅Cl and CCl₄, using pulsed-gradient spin-echo methods supplemented by NMR T₂ relaxation. In solutions and melts of stars with M(arm) ≃ 10⁴ and f ≤ 18, entanglements are less pronounced than in linear polymers of equal molecular weight. Polymer diffusion depends differently on M(arm) and f, the f dependence becoming weak at high f at all solvent concentrations. In dilute solution, diffusion behavior shows that these stars are relatively highly expanded; at high polymer concentrations, segmental diffusion mechanisms begin to contribute. Free volume theories explain solvent but not polymer diffusion.