Self-Diffusion of Linear and 4-and 18-armed Star Polyisoprenes in TetraChloromethane Solution

We have measured the diffusion of polyisoprene molecules, linear as well as 4-and 18-armed star branched, at 50 °C in dilute and semidilute CCl₄ solution over 4 orders of magnitude in molecular weight, using pulsed-gradient spin-echo NMR techniques. Flory's theory of dilute solutions (good solvent) and the Pyun-Fixman model account for the molecular weight and concentration dependences of the diffusivities for each architecture. Dilute diffusivity of the stars is significantly faster than that of linear molecules of equal molecular weight; the resulting hydrodynamic ratios h are independent of molecular weight and in good agreement with the Stockmayer-Fixman calculation. Certain predictions of scaling theory are confirmed.