Dynamic structure of interacting spherical polymer brushes

Multiarm star polymers represent a valuable model system for investigating the dynamics of tethered chains, spherical brushes, or grafted colloidal sp heres. Because of their topology, the multiarm stars exhibit a nonuniform m onomer density distribution leading to a core-shell morphology, which is re sponsible for their rich dynamic structure. When the stars interpenetrate, they exhibit liquidlike (macrocrystalline) order due to the enhanced osmoti c pressure which balances the entropic stretching of the near-core segments and the excluded volume effects. Using dynamic light scattering, we probe three relaxation modes in the semidilute regime: (i) the fast cooperative d iffusion, which is characteristic of their polymeric nature (entangled shel l arms); (ii) the self-diffusion of the stars (essentially cores), probed b ecause of finite functionality polydispersity, as confirmed by independent pulsed-field gradient NMR measurements; and (iii) the structural mode, whic h corresponds to rearrangements of the ordered stars. We develop a mean-fie ld scaling theory, which captures all features observed experimentally with good quantitative agreement. The two slow modes, ii and iii, are reminisce nt of the behavior of interacting hard colloidal spheres and are governed e ssentially by the same physics. We propose these model soft spheres as appr opriate vehicles for unifying the descriptions of the dynamics of polymers and soft colloidal dispersions.