Mixing and solvation transitions in aqueous mixtures of free polymers and microgels

Interest in liquid-liquid phase separation (LLPS) within complex materials has grown recently, driven by the discovery of LLPS in living cells where it has been found that phase separated liquid condensates of biomolecules play a vital role in numerous intracellular processes. Inspired by these discoveries in living cells, investigations have expanded beyond basic aqueous two-phase systems to studies of LLPS in more complex systems like crosslinked networks of solvated polymers. Inspired by these studies, we hypothesized that aqueous solutions of free polymers and microparticles made from crosslinked solvated polymers (microgels), should exhibit transitions in the intermixing of the two polymer species across the microgel surfaces. In this presentation, we will describe how solvated poly(N-isopropylacrylamide), or PNIPAM, does not penetrate into polyethylene glycol (PEG) microgels dispersed into an aqueous PNIPAM solution at low temperatures. At slightly elevated temperatures, a clear mixing transition occurs where PNIPAM enters the PEG microgels, depending on the PNIPAM molecular weight and the relative polymer concentrations. In addition to this mixing transition, we observe aggregation and precipitation of PNIPAM at even higher temperatures, corresponding to the upper-critical solvation temperature of PNIPAM in water. We demonstrate that this phase behavior can be leveraged in applications such as synthesizing PEG microgels with a well-defined PNIPAM coating. We will describe the rheological properties of these coated microgels over a range of temperatures