Investigating Polymer Transformation during the Encapsulation of Metal Nanoparticles by Polystyrene-b-poly(acrylic acid) in Colloids.

The colloidal self-assembly method holds great potential for large-scale synthesis at low expense of energy as compared to methods that assemble molecules by manipulating building blocks one after another. The development of the colloidal method, however, requires careful and intelligent design of the single building blocks as numerous degrees of freedom like isotropic nanoparticles (NPs) generally form highly repetitive, lattice-like structures or random aggregates upon self-assembly because of their identical surfaces throughout. Specifically, it is an interesting direction that if one can precisely control the localization of surface functionalities (i.e., ligands or polymer shells) on the NPs, a plethora of self-assembled structures (e.g., chains, sheets, rings, twisted, and even staircase structures) would be possible. Despite numerous simulations and modeling for this type of NPs, just a handful literature studies reported the controlling synthesis of metal-polymer patchy NPs through polymer shell shrinking/transformation in colloids. However, there are no detailed control experiments showing the mechanism of this polymer shell shrinking or transformation phenomenon. With the absence of a fundamental understanding of the driving forces and interactions between metal NP surface ligands and the hydrophobic polymer shell domain, simple and efficient design and synthesis of unique metal-polymer hybrid nanostructures are still obscure. Here, we report a detailed mechanistic study on the polymer shell transformation by using different types of surface ligands in encapsulation of metal NPs by polymer shells. The polymer shell transformation dynamic is studied after postheating treatment. The polymer shell transformation/shrinking on the metal NP surface depends on its surface ligand size being applied in the encapsulation step (polymer-ligand hydrophobic interaction effect). Longer-chain ligands provide stronger interactions between NPs and the hydrophobic domain of the polymer shell, which inhibits the polymer shell transformation. In contrast, short-chain ligands lead to weaker interactions, which assist in the polymer shell transformation. By understanding the underlying mechanisms, many new types of NPs, such as metal-polymer core-shell NPs, metal-polymer Janus NPs, silica-metal-polymer hybrid NPs, and silica-metal-polymer flower-like NPs have been synthesized for the first time. A new bottom-up platform for the synthesis of anisotropic NPs with the ability to control the patches in a precise manner has been created, which will benefit both nanotechnology (such as self-assembly in the nanoscale) and applications such as selective detection of the underlying ligands on the metal surface by using a surface-enhanced Raman spectrum study.