Large amplitude oscillatory shear studies on dense PNIPAM microgel colloidal glasses

Abstract

We report here the yielding behavior and large amplitude oscillatory shear (LAOS) response of dense colloidal suspensions of two types of thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) microgel spherical particles: (i) having core–shell (CS-PNIPAM) structure with dangling polymer chains and (ii) having only a homogeneous cross-linked core (HC-PNIPAM). By performing systematic rheological studies on the glassy state of these two types of colloidal particles, we show that dangling polymer chains that exist on CS-PNIPAM particles exhibit two-step yielding analogous to that reported in soft-sphere colloidal glasses with attractive interactions, whereas the dense suspensions of HC-PNIPAM spheres, in which dangling polymer chains are absent, showed single-step yielding similar to that of hard-sphere colloidal glasses. Dynamic light scattering measurements on CS-PNIPAM clearly show that dangling polymer chains, which are absent in HC-PNIPAM particles, get entangled under dense conditions and are responsible for (i) observed sub-diffusive behavior in mean squared displacement at small time and (ii) two-step yielding behavior in dense colloidal glass of CS-PNIPAM spheres. Further, we did LAOS measurements to gain further insight on the role of dangling polymer chains and their entanglements on the yielding behavior of the glassy state. We determined the energy associated with the yielding in the glassy state of both types of microgel particles and interpret its role in disentangling the dangling polymer chains between the CS-PNIPAM particles in their dense state as well as its contribution to the cage breaking under shear in the glassy state.