Sub-diffusive dynamics and two-step yielding in dense thermo-responsive microgel glasses

Abstract

We have prepared dense colloidal suspensions of thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) microgel particles with different volume fractions by subjecting the particles to different amounts of osmotic compression. Structural ordering and dynamics have been investigated using light scattering and yielding behaviour by oscillatory rheology. At room temperature, suspensions have found to exhibit a glassy state with the microgel particle size to be smaller than that determined under dilute conditions (i.e. deswelling of particles under osmotic compression). Quite interestingly, we observed (a) sub-diffusive mean square displacement (MSD) at short times and (b) two-step yielding (i.e. loss modulus, G, exhibiting two peaks as a function of shear strain amplitude, γo) in the glassy state. These findings are interpreted in terms of the overlap of dangling polymer chains between shells of the neighbouring PNIPAM microgel particles which are known to have a core-shell structure. At elevated temperatures, the two-step yielding behaviour of the glasses has been observed to change into a single-step yielding. With further increase in temperature, it has been found that PNIPAM microgel glasses melt into a liquid-like order and the behaviour of MSD changes from sub-diffusive to diffusive type. The results are discussed in light of those reported for attractive and repulsive colloidal glasses.