Often it is not feasible to graft hydrophilic polymer stabilizers covalently to polymer colloids as reactive binding sites are either not available or would interfere with the functional properties. Here, we present facile, general and scalable physical protocols for permanent noncovalent grafting of polyelectrolytes to hydrophobic polymeric nanoparticles with chemically inert surfaces using polyurea and polyurethane polymer nanocapsules as examples. The polyelectrolytes are adsorbed and physically entangled within a partially liquefied polymer surface upon adding a small amount of organic solvent as a swelling agent. Hydrophilic polyelectrolyte homopolymers and random copolymers were grafted upon overcoming unfavorable adsorption by adding salts and/or applying strong shear with sonication, with toluene as swelling agent. This grafting is reported either in an aqueous solution or by phase transfer from an oil phase to an aqueous phase in an oil/water emulsion. The resulting poly(2-acrylamido-2-methylpropanesulfonate, AMPS)-grafted nanocapsules exhibit long term (>1 month) colloidal stability in high salinity American Petroleum Institute (API) brine (8 wt.% NaCl and 2 wt.% CaCl2) at ambient and elevated temperature (50℃). Moreover, the mobility of the poly(AMPS)-grafted capsules in brine-saturated quartz sand is improved markedly compared to bare hydrophobic nanocapsules, given the electrosteric repulsion of the poly(AMPS) with the anionic quartz surface (mass breakthrough increased from 2% to 80.7%). These approaches are simple and generalizable to a wide range of polymer colloids where covalent grafting is either undesirable or impractical.
