Weak magnetic fields induce anomalous disorder-to-order transitions in spherical block copolymer micelle solutions

While block copolymers (BCPs) are promising materials due to their tunable structure and functionality, practical methods for processing BCPs with controlled grain size and orientation remain challenging. We recently discovered anomalous magnetic field-induced disorder-to-order transitions in weakly diamagnetic, coil-coil BCPs that cannot be explained by traditional mechanisms of domain alignment. While prior work on field-directed BCP assembly focused on alignment of a structure with inherent anisotropy, here stable ordered phases rapidly form during magnetorheology measurements (B ≤ 0.5 T) on low viscosity solutions of geometrically-isotropic, disordered BCP micelles; this phenomena is accompanied by an up to six order-of-magnitude increase in modulus. Via small angle neutron and X-ray scattering, we demonstrate that weak magnetic fields induce formation of highly ordered phases typically observed in analogous non-magnetized solutions at higher polymer concentrations. At early magnetization times, BCPs form face-centered (fcc) and body-centered (bcc) cubic packings composed of spheres with a lower aggregation number than those that form at zero-field conditions. With increasing magnetization time, new order-to-order transitions are identified, including transitions between cubic packings and hexagonally-packed cylinders (fcc->bcc->cyl). Fourier transform infrared spectroscopy suggests that the increase in per-chain interfacial area resulting during these ordering transitions is likely driven by both changes in corona-solvent interactions and in chain conformation induced by the applied field. Fully understanding this anomalous assembly phenomena will afford access to BCP structures and associated lengthscales inaccessible via traditional routes, and will provide a platform for developing well-ordered BCP materials under mild processing conditions.