Viscoelasticity and shear melting of colloidal star polymer glasses.

Dispersions of multiarm star polymers in an athermal solvent are studied as a model system to explore the effects of soft colloidal interactions on the dynamics of colloidal glasses. Linear viscoelastic measurements in the glassy state are congruent with Mode Coupling Theory predictions for hard sphere glasses at moderate frequencies, indicating similarities in the relaxation processes of hard and soft colloidal glasses near equilibrium. On the other hand, distinct features of the star relaxation (related to arm disengagement) are observed to affect the nonlinear behavior associated with shear melting of the glass, which exhibits transitions previously unreported for hard sphere systems. Whereas a single maximum in G″(γ) is evident under large amplitude oscillatory shear at frequencies near the beta relaxation, secondary transitions between yielding and the onset of macroscopic flow are observed at higher and lower frequencies. The latter are annealed out upon complete fluidization of the star polymer suspension. The overall nonlinear behavior of the soft glassy suspension is interpreted in terms of two primary mechanisms: (i) deformation and yielding of the cage-like glassy microstructure, as in the hard sphere case, and (ii) local stretching and disengagement of interpenetrating arms of the colloidal star polymers. [ABSTRACT FROM AUTHOR]