Self-organization of homopolymer brush- and mixed homopolymer brush-grafted silica nanoparticles in block copolymers and polymer blends

Abstract

In this report, hierarchical self-organization of poly(tert-butyl acrylate) (PtBA) brush- and mixed PtBA/polystyrene (PS) brush-grafted silica nanoparticles (NPs) in PtBA-b-PS block copolymer and PtBA/PS polymer blend matrices having different molecular weights was investigated using transmission electron microscopy (TEM). Driven by the entropic interactions between the brushes and the polymer matrices (i.e., wet- and dry-brush effects), various morphologies were observed. First, PtBA brush-grafted 67 nm silica NPs could induce an onion-like structure in a low molecular weight block copolymer matrix because of the wet-brush effect, low particle curvature, and low bending rigidity of the block copolymer lamellae. On the contrary, PtBA brush-grafted 23 nm silica NPs could not induce the onion-like structure, primarily owing to the high particle curvature and high bending penalty of the block copolymer lamellae. Second, the attempt to use ordered block copolymer lamellae to direct microphase separation of mixed homopolymer brushes with a goal of achieving ordered nanopatterns was unsuccessful, likely because the mixed PtBA/PS brushes on NPs underwent microphase separation prior to the ordering of block copolymers during the solution-casting process. Third, in polymer blends, the wet- and dry-brush entropic effects played an important role. In the wet-brush regime, the hairy NPs were uniformly distributed in the preferred polymer phase. In the dry-brush regime, the hairy NPs were driven to the interfaces of immiscible PtBA/PS domains. The knowledge gained on hierarchical self-organization of homopolymer brush- and mixed homopolymer brush-grafted NPs in block copolymer and polymer blend matrices will help us design better polymer nanocomposites with precisely controlled locations of NPs.