Self-Assembly of Magnetic Nanoparticles at the Surface and Within Block Copolymer Films

We investigate the self-assembly of magnetic Fe$_{3}$O$_{4}$ nanoparticles in thin films of a symmetric block copolymer of poly(styrene-$b$-methyl methacrylate), PS-$b$-PMMA (75 kg/mol). The Fe$_{3}$O$_{4}$ nanoparticles (4nm) are grafted by poly(methyl methacrylate) (PMMA) (2.7 kg/mol) brushes to improve their compatibility. The weight percent of Fe$_{3}$O$_{4 }$in PS-$b$-PMMA is 1, 4 and 10. The Fe$_{3}$O$_{4 }$reside at the intermaterial dividing surface and also form small disk-like aggregates within the PMMA phase. The addition of Fe$_{3}$O$_{4}$ slows down the transition from perpendicular to parallel lamellae morphology at the surface and slowing down increases as weight percent Fe$_{3}$O$_{4 }$increases. Using cross-sectional TEM, nanoparticles are found to be rejected from the parallel lamellae and gather preferentially within the perpendicular lamellae. These studies demonstrate that the Fe$_{3}$O$_{4}$ particles influence thin film morphology and visa versa. Because of widespread interest in nanodevices, this study shows that arrays of functional nanoparticles can be formed using block copolymer templates.