Hierarchical structuring in block copolymer nanocomposites through two phase-separation processes operating on different time scales

Tailoring the size and surface chemistry of nanoparticles allows one to control their position in a block copolymer, but this is usually limited to one-dimensional distribution across domains. Here, the hierarchical assembly of poly(ethylene oxide)-stabilized gold nanoparticles (Au-PEO) into hexagonally packed clusters inside mesostructured ultrathin films of polystyrene-block- poly(methyl methacrylate) (PS-b-PMMA) is described. A close examination of the structural evolution at different nanoparticle filling fractions and PEO ligand molecular weights suggests that the mechanism leading to this structure-within-structure is the existence of two phase separation processes operating on different time scales. The length of the PEO ligand is shown to influence not only the interparticle distances but also the phase separation processes. These conclusions are supported by novel mesoscopic simulations, which provide additional insight into the kinetic and thermodynamic factors that are responsible for this behavior. Combining block copolymers with nanoparticles that are highly incompatible with one block and only slightly incompatible with the other leads to hierarchical nanoparticle structures. While the block copolymer domains evolve, the nanoparticles segregate to the least incompatible domain. Then, they phase separate from it, forming hexagonally packed arrays within its confines, where interparticle distance is determined by the ligand length.