Mechanistic Insights into Diblock Copolymer Nanoparticle-Crystal Interactions Revealed via in Situ Atomic Force...

Recently, it has become clear that a range of nanoparticles can be occluded within single crystals to form nanocomposites. Calcite is a much-studied model, but even in this case we have yet to fully understand the details of the nanoscale interactions at the organic-inorganic interface that lead to occlusion. Here, a series of diblock copolymer nanoparticles with well-defined surface chemistries were visualized interacting with a growing calcite surface using in situ atomic force microscopy. These nanoparticles comprise a poly(benzyl methacrylate) (PBzMA) core-forming block and a non-ionic poly(glycerol monomethacrylate) (Ph-PGMA), a carboxylic acid-tipped poly(glycerol monomethacrylate) (HOOC-PGMA), or an anionic poly(methacrylic acid) (PMAA) stabilizer block. Our results reveal three modes of interaction between the nanoparticles and the calcite surface: (i) attachment followed by detachment, (ii) sticking to and "hovering" over the surface, allowing steps to pass beneath the immobilized nanoparticle, and (iii) incorporation of the nanoparticle by the growing crystals. By analyzing the relative contributions of these three types of interactions as a function of nanoparticle surface chemistry, we show that ∼85% of PMAA₈₅-PBzMA₁₀₀ nanoparticles either "hover" or become incorporated, compared to ∼50% of the HOOC-PGMA₇₁-PBzMA₁₀₀ nanoparticles. To explain this difference, we propose a two-state binding mechanism for the anionic PMAA₈₅-PBzMA₁₀₀ nanoparticles. The "hovering" nanoparticles possess highly extended polyelectrolytic stabilizer chains and such chains must adopt a more "collapsed" conformation prior to successful nanoparticle occlusion. This study provides a conceptual framework for understanding how sterically stabilized nanoparticles interact with growing crystals, and suggests design principles for improving occlusion efficiencies.