Dynamics and rheology of colloidal star polymers

Abstract

We attempt to elucidate the dynamics of multiarm star polymer solutions, which are representative of a large class of soft hairy colloids, over a wide range of concentrations. In addition to the usual β-relaxation (in-cage rattling) and α-relaxation (terminal cage escape), the relaxation of stars in the glassy state involves a mode associated with arm interpenetration. From linear and nonlinear rheological measurements, we establish a set of criteria for identifying the colloidal glass transition, including aging, yielding and elastic properties. Linear viscoelasticity and steady-shear measurements merge at low frequencies, indicating that terminal behavior is accessible, albeit at very long times. The transition from linear to nonlinear response is controlled by star elasticity and a balance between Brownian diffusion and flow advection. In the nonlinear regime, the flow curves collapse on a universal flow curve using a scaling that expresses a competition between solvent-mediated interactions and elastic forces. While applied to stars, our framework appears to be a generic tool for fingerprinting liquid-solid transitions in colloidal suspensions.