(613e) Anomalous Phase Formation in Spherical Block Copolymer Micelles in Low Intensity Magnetic Fields | AIChE

(613e) Anomalous Phase Formation in Spherical Block Copolymer Micelles in Low Intensity Magnetic Fields


Calabrese, M. - Presenter, University of Minnesota
Kresge, G., University of Minnesota twin cities
Suresh, K., University of Illinois Chicago
Block copolymers (BCPs) are attractive for use in developing advanced materials due to their tunable properties and self-assembly via block chemistry, composition, and length. However, practical methods for processing BCPs into advanced materials with long-range order remain difficult, where techniques like magnetic field alignment are typically infeasible because of large required field strengths and the limited range of responsive chemistries. We recently discovered magnetic field-induced phase formation in weakly diamagnetic, coiled BCPs that cannot be attributed to traditional mechanisms of phase alignment. Here, stable ordered phases are created by temporarily applying weak magnetic fields (0.05-0.5 T) to low viscosity solutions of spherical BCP micelles, causing up to a six order of magnitude increase in the modulus that is stable upon field removal. 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, and has focused on alignment of a structure or phase with inherent anisotropy, which causes anisotropy in the magnetic susceptibility. 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. While this technique produces both cubic and aligned phases, the absence of structural anisotropy in the micelle building blocks eliminates phase alignment as the primary driver of magnetic this field­induced phase creation.

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 magnetorheology (MR), small angle x-ray/neutron scattering, and Fourier transform infrared spectroscopy (FTIR) we show that in low intensity magnetic fields, the amphiphile chain conformation and polymer-solvent interactions are altered, facilitating structural transitions. While MR is performed at temperatures far below zero-field thermal phase transitions, the induced elastic modulus, G’B, is up to 3 orders 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. This new assembly strategy enables discovery of structures and d­spacings inaccessible via traditional self­assembly, thus providing a platform for developing materials with precisely­controlled dimensions and grain size at mild conditions and with minimal input from external fields.