Self and collective dynamics of ordered star polymer solutions

We investigated the dynamics of 18-arm poly-isoprene 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 D_eff is well described by the term D₀/S(Q), with D₀ the diffusion coefficient at infinite dilution. At Q_m, 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 Q_m the time range of NSE was extended for the first time up to 350 nms using long wavelengths, λ = 19 Å, 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 < r² >^1/2 of star cores, which is related to the blob size ξ of the surrounding star polymer solution obtained by dynamic light scattering.