Magnetic field-directed self-assembly of block copolymers (BCPs) provides a viable pathway to create large-scale, highly ordered nanostructures that are of interest in many applications. This approach typically relies on the presence of a liquid crystalline mesophase, generated by covalent or hydrogen-bond attachment of mesogens to the BCP, to provide the required magnetic anisotropy. To date, this anisotropy has been diamagnetic in origin but there is a strong desire to utilize paramagnetic species due to their larger magnetic susceptibilities by comparison with diamagnetic mesogens. Here, we show that 4-Hydroxy-TEMPO, a paramagnetic small molecule, can be effectively used to enable field-alignment of poly(styrene-block-4-vinylpyridine) (PS-b-P4VP) domains. Hydrogen bonding between the hydroxyl group and the pyridine unit drives sequestration of the small molecule to the P4VP domains and provides the required magnetic anisotropy for alignment. We explore the phase behavior and alignment kinetics of this system with in-situ small angle x-ray scattering (SAXS) at varying field strengths from 0 to 6 Tesla. As anticipated, slower cooling from the melt and higher field strength result in better aligned materials. We find that in lamellar systems, the lamellar normal aligns perpendicular to the field, resulting in a degenerate configuration. We are able to eliminate this degeneracy, markedly improve alignment, and create highly ordered structures by rotating the sample in the presence of the field while cooling from the isotopic state. Additionally, due to its water solubility, 4-Hydroxy-TEMPO can be selectively leached out of the system post-alignment to produce nanoporous materials. These results suggest that paramagnetic moieties capable of supramolecular attachment can be effectively leveraged in designing schemes for magnetic field directed self-assembly of block copolymers.

Magnetic field alignment of PS-b-P4VP block copolymers mediated by supramolecular complexation with a paramagnetic species

248th National Meeting (2014)

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