(267i) Anomalous Phase Formation in Self-Assembled Polymers Under Low Intensity Magnetic Fields

While block polymers (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 assembly in weakly diamagnetic, aqueous block polymers (BCPs) exposed to low intensity magnetic fields. Prior work on field-directed assembly of BCPs has required large field strengths (B > 5 T), use of rod-like blocks, substantial chain anisotropy, or combinations thereof to achieve field-induced responses. However here, a thermally-reversible three-to-six order of magnitude increase in the suspension modulus is observed when low intensity fields (0.05 ≤ B ≤ 0.5 T) are applied to a number of aqueous polymers and oligomers, across a range of concentrations (5-60% wt). This anomalous behavior is reproducible across sample preparations, polymer batch, and supplier. Magnetic susceptibility and inductively coupled plasma measurements illustrate that the behavior is not due to impurities; in situ small angle x-ray and neutron scattering (SAXS/SANS) confirm that the response is not an artifact of the measuring device, confinement, or sample drying.

The anomalous rheological response results from field-induced microstructure changes, which occur at field strengths far below that expected based on the polymer magnetic susceptibility anisotropy, Δχ. Using a combination of magneto-rheology (MR), SANS, and SAXS, three distinct field-induced behaviors have been identified, which may occur independently or in tandem: phase formation typically associated with a lyotropic or thermotropic phase transition, and grain growth and orientation. MR is performed at temperatures far below (>10 ºC) quiescent thermotropic phase transitions, and the induced elastic modulus, G’_B, is up to an order of magnitude larger than results from temperature ramps at 0 T. Distinct time- and field intensity-dependent rheological features during magnetization suggest that phase selection and access to metastable states can be controlled by altering field strength and magnetization time. Understanding these directed-assembly mechanisms is of significant scientific interest for its potential to enhance assembly with minimal input from external fields, and the potential to discover new structures not accessible through traditional self-assembly routes.