Nanoparticle assembly under block copolymer confinement: The effect of nanoparticle size and confinement strength.

We investigate the self-assembly of cylinder-forming polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymers (BCP) mixed with metal nanoparticles (NP) coated with short-chain polystyrene (PS) ligands. The NP formed hierarchical superstructures under confinement of cylindrical PS domains of PS-b-P4VP BCP. The complexity of NP superstructures was found to depend on the ratio between PS cylindrical domain size and NP size (DC/DNP). As the DC/DNP ratio increased, the number of NP layers normal to the cylinder axis also increased. However, the packing density of the NP decreased at higher DC/DNP. Furthermore, the morphology of the structures obtained during different solvent casting conditions revealed that the initial clustering of NP and micellization around these clusters act as a precursor for the subsequent formation of closely packed structures of NP in cylinders. The experimental results were further supported by modeling results obtained from molecular dynamics (MD) simulation. Based on MD simulations, we constructed structural phase diagram of nanoparticle assemblies in the presence of asymmetric diblock copolymers comprising short NP-attractive blocks. The MD simulation results indicate that NP undergo transition from spherical to cylindrical assemblies depending on the NP size, the overall concentration of components and the degree of affinity of the minor block to NP.