November 14, 2023
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4:00 PM
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5:00 PM
Michelle Calabrese
Department of Chemical Engineering and Materials Science
University of Minnesota
Minneapolis, MN
A novel mechanism for altering block copolymer (BCP) self-assembly and phase behavior via magnetic fields
Block copolymers (BCPs) are attractive for developing novel 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 limited range of responsive chemistries. We recently discovered anomalous magnetic field-induced ordering in weakly diamagnetic, aqueous block polymers (BCPs) exposed to low intensity magnetic fields (B ≤ 0.5 T). Prior work on magnetic field-directed assembly in BCPs has focused on alignment of a structure or phase with inherent anisotropy which leads to anisotropy in the magnetic susceptibility, Δχ. However here, ordered phases are created by temporarily applying magnetic fields to low viscosity solutions of spherical BCP micelles with no inherent anisotropy, causing up to a six order of magnitude increase in the dynamic moduli.
Using a combination of magnetorheology (MR), small angle neutron and x-ray scattering (SANS/SAXS), and vibrational spectroscopy, we demonstrate that low intensity magnetic fields likely facilitate these phase transitions by altering polymer-solvent interactions and hydrogen bonding, which in turn modify amphiphile packing. We then show that this assembly strategy can be used to produce two types of ordered materials: those that remain composed of isotropic micelles (i.e. cubic phases), and those that transform into aligned phases like cylinders or lamellae. Here, the induced elastic modulus, G’B, is up to three orders of magnitude larger than the maximum modulus that results from an analogous thermal ordering transition at 0 T. Additionally, this anomalous assembly behavior is time-dependent, where the critical induction time tc and resulting phase can be controlled by altering amphiphile molecular weight and processing conditions like field strength, magnetization time, and temperature. Finally, in a promising sign for practical applications, this transition is robust to shear processing and cycling, where tc is largely independent of strain amplitude γ0 up to 100%. While applying large γ0 during magnetization reduces G’ and G’’, the moduli rapidly recover upon cycling, when γ0 is repeatedly reduced to 0.01% following high shear. This new assembly strategy enables the discovery of structures and d-spacings inaccessible via traditional self-assembly, thus providing a platform for developing materials with long-range order using mild conditions and little energy input from external fields.