Tuning reentrant phase behavior of silica nanoparticles in polymer suspension via interplay of interactions

Nanoparticle-polymer composites offer versatile platforms for engineering tunable soft materials with tailored structural and functional properties. The anionic silica nanoparticles dispersed in polymer solutions exhibit a remarkable reentrant phase behavior, where interparticle interactions evolve from repulsive to attractive and back to repulsive (or less attractive) with increasing polymer concentration. This evolution of interaction is accompanied by a corresponding rise and subsequent fall in the hydrodynamic size and viscosity of the system, reflecting a transition from stable to aggregated state followed by restabilization. We show that the phase boundaries of this behavior can be effectively tuned by adjusting the nanoparticle concentration and/or ionic strength. Increasing nanoparticle concentration shifts the attractive regime to higher polymer concentrations, while elevated ionic strength broadens this regime. Such tuning of phase boundaries enables precise control and targeted attainment of stable or aggregated states in nanoparticle-polymer systems. The systems are examined by dynamic light scattering, viscosity, and small-angle neutron scattering. The results demonstrate that controlled modulation of interparticle interactions (electrostatic repulsion, depletion attraction and stabilization) through nanoparticle, polymer, and salt concentrations offers a powerful strategy for designing responsive nanoparticle-polymer complexes.