Controlling phase behavior of diamagnetic, low viscosity polymer solutions using low intensity magnetic fields

Magnetic fields provide a route for controlling long-range order in diamagnetic materials like homopolymers and block copolymers (BCPs). Prior work has focused on field-driven phase alignment, where high-intensity fields (>5T) or anisotropic liquid crystalline species and/or aromatic groups have been required to ensure sufficient magnetic anisotropy to drive alignment. Our recent experiments with solutions of polyethylene oxide (PEO) and isotropic spherical micelles of PEO-containing BCPs, however, show anomalous responses to low-intensity fields (B>0.05 T) via a mechanism other than alignment. Linear viscoelastic magnetorheology shows a reversible three-to-six order increase in the dynamic moduli under weak fields. Magneto X-ray scattering reveals that a disorder-to-order structural transition causes the change in mechanical properties and that magnetized samples exhibit d-spacings similar to quiescent samples with twice the polymer content. This change in micelle packing parameter appears to be driven by a change in chain conformation induced by the magnetic field, which is stabilized by water reorganization and hydrogen bonding. Assembling materials via this mechanism provides a new approach to develop BCP materials with long-range order using low-intensity magnetic fields.