Field Alignment and Localized Field Screening in Block Copolymer Films Using Sub-1T Magnetic Fields (Invited Talk)

Magnetic fields offer significant promise for directed self-assembly of block copolymers. Their space-pervasive nature enables facile processing in arbitrary geometries over large areas, and without the dielectric breakdown concerns which attend the use of electric fields. Magnetic field alignment is predicated on the presence of a sufficiently large magnetic susceptibility anisotropy in the system. Such anisotropy is commonly achieved by integrating liquid crystallinity into the block copolymer by covalent or supramolecular attachment of mesogens to the polymer backbone. Although alignment can be readily achieved, to date, the required field strengths are typically on the order of several tesla (> 3 T), mandating the use of superconducting magnets or highly complex resistive systems.

Here we report on the use of a simple mesogen blending approach which dramatically reduces the critical field strength required for producing well aligned block copolymer nanostructures during a given processing timeframe. Appropriately chosen mesogens effectively co-assemble with the existing mesophases formed by covalently attached moieties in two side-chain LC block copolymers. The resulting systems display rapid alignment upon cooling from disordered states in magnetic fields as low as 0.5 T. We show that the improved alignment response is principally due to enhanced mobility in the system due to the presence of non-bonded mesogens, rather than due to the increased grain sizes observed in the blended system. Sub-1 T field alignment permits two important developments. First, block copolymer alignment can be performed using low cost permanent magnets, and second, the applied fields can be effectively screened using nanoparticles of simple magnetic materials such as cobalt. We demonstrate field screening by nanoparticle chains as well as by arrays of magnetic nanostructures at interfaces with block copolymer films. The localized field screening by these magnetic materials results in localized variation of the orientational order of the block copolymer structure. These developments offer a new route to local control of block copolymer morphology in thin films.