Structure and viscoelasticity of interacting spherical brushes

Star polymers with a high functionality of 128 are viewed as ideal spherica l brushes with soft colloidal character. encompassing both polymer-like sho rt range and colloid-like long range interactions. We have identified three mechanisms for relaxing the concentration and density fluctuations in semi dilute solution: cooperative diffusion, characteristic of polymers, and sel f-diffusion and structural relaxation, characteristics of colloidal hard sp heres. Upon heating these systems undergo a reversible liquid-solid transit ion, yielding weak gels. It is attributed to the trapping of the spheres in "cages" due to the swelling with temperature and hence stronger arm interp enetration. At the transition strong hysteresis phenomena are observed duri ng steady and transient shear measurements, and relate to the changing visc oelastic character of the fluid with temperature.