Self and collective dynamics of ordered star polymer solutions

We investigated the dynamics of 18-arm polyisoprene star polymer solutions well above their overlap concentration c*. Combining neutron spin echo spectroscopy (NSE) and selective H/D labelling, we were able to separate inter- (collective) and intra-star (self ) dynamics. Only at low Q-vectors do self and collective dynamics become discernible. Here, collective dynamics are found to be consistent with a colloidal approach resulting from star–star interactions. The collective short time diffusion coefficient Deff is well described by the term D0/S(Q), with D0 the diffusion coefficient at infinite dilution. At Qm, the peak position in the structure factor S(Q), no difference is observable between collective and self dynamics. For covering the slowed-down dynamics at Qm the time range of NSE was extended for the first time up to 350 ns using long wavelengths, λ=19 A, at IN15 (ILL, Grenoble). We found that S(Q,t)/S(Q,0) relaxes into a concentration-dependent plateau. The plateau height gives the mean-square displacement 1/2 of star cores, which is related to the blob size ξ of the surrounding star polymer solution obtained by dynamic light scattering.