Structure and viscoelasticity of interacting spherical brushes

Star polymers with a high functionality of 128 are viewed as ideal spherical brushes with soft colloidal character. encompassing both polymer-like short range and colloid-like long range interactions. We have identified three mechanisms for relaxing the concentration and density fluctuations in semidilute solution: cooperative diffusion, characteristic of polymers, and self-diffusion and structural relaxation, characteristics of colloidal hard spheres. Upon heating these systems undergo a reversible liquid-solid transition, yielding weak gels. It is attributed to the trapping of the spheres in "cages" due to the swelling with temperature and hence stronger arm interpenetration. At the transition strong hysteresis phenomena are observed during steady and transient shear measurements, and relate to the changing viscoelastic character of the fluid with temperature.