(104b) Morphological Transitions in Block Copolymers Directed By Superparamagnetic Nanoparticles

Hatton, T. A., Massachusetts Institute of Technology
Olsen, B. D., Massachusetts Institute of Technology
Raman, V., Massachusetts Institute of Technology

This computational and theoretical study investigates the self-assembly of superparamagnetic nanoparticles and block copolymers under external magnetic fields. A variety of morphological transitions are observed based on the field orientation, nanoparticle loading, and selectivity of the nanoparticles for the blocks. For symmetric block copolymers, chaining of superparamagnetic nanoparticles under in-plane magnetic fields is shown to achieve long range orientational order of the block copolymer nanodomains and is found to be dependent on nanoparticle size, volume fraction and magnetization strength. A critical selectivity of the particles for one nanodomain is observed, above which strong alignment results and below which comparatively disordered structures are formed.   Higher magnetization strengths are found to reduce equilibrium defect densities in the nematic-isotropic ordering of lamellar thin films, as corroborated by scaling arguments. For asymmetric coil fractions forming hexagonal block copolymer nanostructure, the in-plane field induced chaining of the nanoparticles selective for the minority block, leads to the formation of stripe phases oriented parallel to the magnetic field. Furthermore, in-plane field induced chaining of nanoparticles selective for the majority block leads to alignment of hexagonal morphology with <100> direction oriented parallel to the external 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.   Honeycomb lattices are obtained using nanoparticles selective for majority block under out of plane magnetic fields for certain specific nanoparticle loadings. Commensurability of nanoparticle size and loadings with the block copolymer structure is critical in optimizing the ordering of the final composite.