Effect of Added Salt and Hydrophobic Comonomer on the Synthesis and Adsorption Behavior of Cationic Sterically-Stabilized Nanoparticles

Cationic sterically stabilized diblock copolymer nanoparticles are prepared by reversible addition–fragmentation chain transfer (RAFT) aqueous emulsion polymerization, whereby the steric stabilizer and core-forming chains comprise [2-(methacryloyloxy)ethyl]trimethylammonium chloride (METAC) and benzyl methacrylate (BzMA) repeat units, respectively. The mean degree of polymerization (DP) of the core-forming PBzMA block is systematically varied to produce a series of spherical nanoparticles of increasing mean diameter. In a second series of nanoparticles, a hydrophobic comonomer (lauryl methacrylate, LMA) is statistically copolymerized with METAC prior to RAFT aqueous emulsion polymerization of BzMA, which produces cationic amphiphilic nanoparticles. In each case, the effect of added salt on the nanoparticle particle size distribution is examined. Furthermore, the effect of added salt on the adsorption of such nanoparticles at the planar silica/water interface is studied using a quartz crystal microbalance (QCM). The presence of salt leads to the formation of more well-defined nanoparticles because it screens the electrostatic repulsion between the cationic steric stabilizer chains. Added salt also enables stronger adsorption onto planar silica as it reduces the lateral repulsion between neighboring cationic nanoparticles. In the case of the cationic amphiphilic nanoparticles, incorporation of the hydrophobic LMA comonomer leads to even stronger adsorption and higher corresponding surface coverage in the presence of either 1 or 10 mM KCl. However, surface aggregates of the latter nanoparticles are observed by SEM in the presence of 100 mM KCl.