Magnetic Field Induced Morphological Transitions in Block Copolymer/Superparamagnetic Nanoparticle Composites

This two-dimensional computational study investigates the effect of external magnetic fields on thin film nanocomposites comprised of superparamagmetic nanoparticles dispersed within block copolymer melts, which display a variety of morphological transitions based on the field orientation, nanoparticle loading, and selectivity of the nanoparticles for the blocks. In-plane magnetic fields lead to chaining of the nanoparticles; when selective for the minority block in a hexagonal block copolymer nanostructure, this chaining results in the formation of stripe phases oriented parallel to the magnetic field. When selective for the majority block of the hexagonal structure, nanoparticle chains of sufficient persistence length drive the orientation of the hexagonal morphology with the (100) direction oriented parallel to the magnetic field. Out-of-plane magnetic fields induce repulsive dipolar interactions between the nanoparticles that annihilate the defects in the hexagonal morphology of the block copolymer when the nanoparticle is selective for the minority block. When the nanoparticles are selective for the majority block and the field is oriented out of plane, repulsive dipolar interactions lead to the formation of honeycomb lattices. In all cases, the nanoparticle size and volume fraction must be chosen to maximize the commensurability with the block copolymer structure to optimize the ordering of the final composite.