(374f) Phase Separation of Mixed Monolayers On Silica Nanoparticles Induced by Hydrogen Bonding

Controlling the self-assembly of mixed monolayers on silica nanoparticles is challenging due to irreversible ligand attachment. In the course of attempting to control the graft density of polystyrene brushes on silica nanoparticles, we synthesized both a surface atom transfer radical polymerization (ATRP) initiator along with an inert analog containing an amide group. The initiator and analog were reacted to the nanoparticles in methylisobutyl ketone to form a mixed monolayer from which polymers could grow. The inert analog reacted preferentially to the nanoparticle surface, which affected the polymerization of styrene with ATRP. In particular, analysis of the initiation efficiency, or the ratio between the ATRP and graft polymer surface coverage provides evidence for phase separation between the ATRP initiator and inert analog in the mixed monolayer. This is most likely due to hydrogen bonding primarily between the inert analog in solution and on the particle surface. Small-angle x-ray scattering (SAXS) and NOESY NMR were used to show that in solution, the analog and initiator form disc-shaped molecular aggregates about 1 nm thick and 1.6 nm in diameter through hydrogen bonding between amide groups. Based on kinetic deposition experiments and FTIR measurements, the molecular aggregates, composed primarily of the inert analog, adsorb strongly to the surface. This adsorption leads to phase separation between the inert analog the ATRP initiator in the monolayer as inferred from FTIR by tracking the non-linear shift in the methylene stretches of the carbons in the ligands, which was a function of the amount of the initiator and analog used to functionalize the nanoparticles. To our knowledge, this reaction mechanism in which the ligand solution behavior affects mixed monolayer structure on silica nanoparticles has not been previously observed.