Effects of attractions on the dynamical arrest of soft colloids

We consider various scenarios leading to molecular control of the rheological state of concentrated solutions of soft colloids, based on the star polymer prototype system. At high values of star functionality, where a dynamically arrested state exists, we discuss the mechanisms that bring about a restoration of ergodicity upon addition of polymer chains, and the theoretical evidence for this effect which is supported by previously published experimental results. For intermediate functionalities, a systematic shift of the glass transition to higher star densities upon addition of additive-induced attractions is established. Finally, for low functionalities, we put forward evidence for a novel phenomenon that is specular to that appearing at high functionalities: a glassy state, which is absent for the pure star system, and that can be induced by the introduction of suitable additives. The latter phenomenon takes place when the effective star–star interaction is modified to display short-range attractions and longer-range repulsions.