Effect of number of arms on diffusion of star polymers

UNLIKE small-molecule liquids, polymer melts can display complex and often highly non-newtonian flow behaviour, which reflects the dynamics of the entangled polymer chains. Star-branched polymers, which have f arms each containing N_a monomeric units, joined covalently at a common branch point, provide good model systems for understanding the slow dynamics of branched polymers. These dynamics may be probed by measurement of the diffusion coefficient, D. It is well established_1-3 that for three-armed star polymers, Dα exp(-αN_a), where α is a constant. This result is consistent with early theories^4,5, which assumed that to diffuse, all but two arms of the star must retract simultaneously to the branch point; these theories predict Dα exp(-α(f-2)N_a). Here we present experimental measurements of D for polymers with f = 3-12, which indicate a much weaker decrease with f: Dα exp (-0.41f). This result is consistent with more recent theories₆ which predict that a star can diffuse by retracting just one arm at a time.