Synthesis and suspension rheology of titania nanoparticles grafted with zwitterionic polymer brushes.
Titania nanoparticles were modified by free-radical graft polymerization of 2-methacryloyloxyethyl phosphorylcholine (MPC) at the particle surface, resulting in the formation of a 1-2 nm thick polymer brush. The zwitterionic nature of the polymer layer suggests that the suspension stability is a delicate function of pH, as well as volume fraction, salt concentration and the presence of charged or un-charged additives which may act as depletants or to screen charge interactions in the system. In this context, we characterized the suspension rheology as a function of volume fraction, pH, ionic strength and the concentration of surfactants in the suspension. Near-neutral pH, the brush layer is effective in stabilizing particles against aggregation with Newtonian behavior observed for volume fractions approaching 14%. Flocculation of particles and an onset of shear-thinning behavior was observed on decreasing pH from near-neutral. Conversely, suspension stability was maintained on increasing pH from near-neutral. Likewise, flocculation could be quickly induced by the addition of salt and cationic surfactant in small amounts, but the suspensions displayed greater stability to anionic and non-ionic surfactant additives. These results have important implications for the successful formulation of complex fluids employing zwitterionic colloids.
Department of Chemical and Environmental Engineering, Yale University, New Haven, CT 06511, United States. zhen.shao@yale.edu
Zhen
Z
Shao
Youngjun
Y
Yang
Hyunsuk
H
Lee
Jin Woong
JW
Kim
Chinedum O
CO
Osuji