(76a) Accessing Non-Equilibrium Phases in Aqueous Block Copolymers (BCPs) Using Low Intensity Magnetic Fields

Magnetic fields can induce a variety of physical phenomena in diamagnetic block copolymers (BCPs), including formation of self-assembled nanostructures with long-range order. However, most prior work in this area involves the alignment of BCP chains or self-assembled domains, which typically relies on the application of high-intensity magnetic fields or the presence of anisotropic liquid crystalline species, aromatic groups, or semicrystalline grains to ensure sufficient magnetic anisotropy to drive domain alignment. In contrast, this work demonstrates a magnetically-induced disorder-to-order transition (DOT) in spherical BCP micelle solutions formed from ten distinct triblock poloxamers when exposed to low intensity magnetic fields (B≥ 0.1 T). This DOT corresponds to a many order of magnitude increase in the dynamic moduli measured via magnetorheology (MR). Subsequent small angle scattering (SAS) measurements confirm a field-induced DOT in 20% wt poloxamer solutions from random spherical micelles coexisting with unimers to ordered cubic, hexagonal, and other phases – achieving dynamic moduli up to three orders of magnitude larger than those resulting from thermally-induced transitions. This anomalous assembly phenomenon is time-dependent, where the critical response time is a function of polymer molecular weight, block ratio, field intensity, and magnetization temperature. The critical time increases for solutions with higher unimer content – whether due to amphiphile solubility, concentration, or magnetization temperature. This trend suggests that weak B-fields must remove unimers from solution and add them to micelles to induce ordering, indicative of field-altered polymer-solvent interactions. Additional MR and SAS performed at lower polymer contents reveal that B-fields can induce DOTs at polymer concentrations that do not exhibit thermally-induced DOTs – greatly expanding the concentration range over which ordered phases can be formed. Additionally, field-induced DOTs can achieve similar or greater moduli to thermally-induced DOTs, but in solutions with ~40% less polymer. This new assembly strategy enables discovery of structures and d-spacings previously inaccessible via traditional thermal processing routes, thus providing a platform for developing materials with precisely-controlled features at mild conditions.