Rheological studies on homogeneous and inhomogeneous core dense colloidal PNIPAM microgels.

Aqueous suspensions of thermo responsive poly (N-isopropyl acrylamide) (PNIPAM) microgel particles exhibit volume phase transition (VPT) around 34°C. PNIPAM microgel spheres are known to be core-shell type having a dense core and a fuzzy shell of dangling polymer chains. Recently (Joshi and Tata Colloid and Polymer Sci. 295, 1671 (2017)) it was shown that under dense conditions i.e. volume fraction, ϕ > 0.64 PNIPAM microgel suspensions exhibit glassy state and undergo two step yielding upon application of shear. Two-step yielding is observed only in colloidal systems with attractive interactions. Observation of two-step yielding is unusual in a PNIPAM microgel glasses as PNIPAM microgel particles are known to be repulsive soft-spheres. Tata and Joshi argued that two-step yielding in dense PNIPAM microgel occur due to the breaking of entanglements of dangling polymer chains between shells of microgels and breaking of the near neighbor cages, respectively. Motivated by these studies and to verify entanglements have really a role in yielding, we synthesized two types of PNIPAM microgel spheres one having only a homogeneous core (referred to as HC) and the other with usual core-shell type (referred to as CS). We report here the results obtained by performing non-linear rheological measurements on dense suspensions of HC and CS type PNIPAM microgel spheres. We observed two steps yielding in dense CS PNIPAM microgels suspension in its glassy state, but not with HC-PNIPAM spheres. Apart from the above observation, we report here the detailed non-linear rheological study in dense colloidal glasses of HC and CS type PNIPAM microgel particles. [ABSTRACT FROM AUTHOR]