Structural relaxation of supercooled liquids based on soft particles: Avoiding the glass transition at high concentrations

Abstract

Using 3D dynamic and static light scattering (DLS and SLS) and small-angle neutron scattering (SANS), we investigate the intra-particle structure of neutral and partially ionized (N-isopropylacrylamide)-co-(acrylic acid) (NIPAM-co-AAc) microgels. In the case of neutral microgels, we find that the collapse of polyNIPAM above the LCST is partially frustrated, due to breakup of the NIPAM sequences by large amounts of copolymerized AAc. In the case of the partially ionized microgels, we find that at high temperatures, the combination of poor solvent conditions and weakly-charged polymer results in phase separation between the charged and uncharged regions of the network. We also investigate the structural relaxation of dense suspensions of the microgels in conditions of good solvent and high charge using DLS, SLS, and oscillatory and steady-state rheology. We find that even at very high particle concentrations, the suspensions remain in the supercooled-liquid regime, and never enter the glassy state. This is caused primarily by a combination of deswelling due to increasing ionic osmotic pressure and the inherent softness of the microgel particles. This represents a key difference between dense suspensions of soft and hard colloidal particles, and between atomic and colloidal glassformers in general.