Solution Rheology of Poly(acrylic acid)-Grafted Silica Nanoparticles

Network formation of polymer-grafted nanoparticles in aqueous solutions is an unexplored area in polymer science. In this study, nanoparticles grafted with poly­(acrylic acid) (PAA) chains with different grafting densities and similar graft chain lengths at semidilute concentrations are investigated using small-angle neutron scattering (SANS) and rheology experiments. We found that at a low graft density and fully ionized state, stretching of grafted chains accommodates a thick lubricating water layer, which lowers the viscosity. At low graft density and 50% ionization, grafts are in extended conformations as observed from the broad PAA volume fraction profile around the particles in SANS data. This is attributed to the hydrogen bonding between ionized and unionized carboxylic acid groups. The highest viscosity measured at this pH confirms the intraparticle bonding between the grafted chains. Viscosity adjustment with the addition of short poly­(N-vinylpyrrolidone) chains suggests that hydrogen bonding is possible within the grafted chains of individual particles at low graft density, whereas interchain networking occurs at high graft density. Molecular dynamics simulation results of model systems confirmed the interparticle bridging at high grafting density. These results demonstrate that hydrogen bonding between polymer-grafted nanoparticles using small molecules can be used to modulate the viscosity of the physical networks in solution.