Controlling the Orientation of Block Copolymer Thin Films using Thermally-Stable Gold Nanoparticles with Tuned Surface Chemistry

In this study, we developed a novel strategy to control the orientation of microdomains in block copolymer thin films by introducing either selective or neutral gold nanoparticles (Au NPs) that were thermally stable. The Au NPs were modified with thiol-terminated polymeric ligands, poly[(methyl methacrylate -r-styrene)-b-azidostyrene] (P[(MMA-r-S)-b-S-N-3]-SH), having different compositions of methyl methacrylate (MMA) and styrene in P(MMA-r-S) block to precisely tune the interfacial interaction between the Au NPs and block copolymer template, poly(styrene-b-methyl methacrylate) (PS-b-PMMA). These Au NPs have a cross-linked polymeric shell, via UV cross-linking of P(S - N-3) block, and thus were stable under thermal annealing at temperatures up to similar to 200 degrees C. The selective Au NPs, which had 80 mol % PMMA in the P(MMA-r-S) block, were located within the PMMA domain of the PS-b-PMMA block copolymer. In contrast, the neutral Au NPs, which had 20 mol % PMMA in the P(MMA-r-S) block, were localized at the interface between the PS and PMMA[A blocks of the PS-b-PMMA. When these Au NPs were incorporated into PS-b-PMMA thin films, these different locations of Au NPs resulted in a remarkable difference in orientation of the block domains. When the selective Au NPs were added and were located in the PMMA domains, the microdomains were oriented parallel to the substrate. In contrast, when the neutral Au NPs that localize at the block copolymer interfaces were added, they induced a transition in the orientation of microdomains from parallel to perpendicular to the substrate. The lateral and vertical location of the Au NPs in the film was investigated by top-view and cross-sectional transmission electron microscopy (TEM). Also, we employed self-consistent mean field theory (SCFT) simulations to explain our experimental results.