Mellichamp Lecture - Michelle Calabrese - Davidson School of Chemical Engineering - Purdue University

Controlling polymer self-assembly and solvation using external fields

Solvent-mediated interactions play a central role in governing the structure, function, and assembly of countless biological, catalytic, and soft matter systems. In block copolymer (BCP) solutions, the relative solubility of each polymer block influences the propensity for self-assembly, the ordered phase that forms, and the associated domain spacing – in addition to parameters such as block chemistry, composition, and length. To harness this tunable self-assembly for developing novel materials, BCP ordering on long length scales must be well-controlled. Unfortunately, techniques to achieve long-range order, such as magnetic field alignment, are typically infeasible because BCPs are weakly diamagnetic and should respond minimally to magnetic fields. However, we recently discovered that magnetic fields unexpectedly induce micelle formation and ordering in disordered BCP solutions via a mechanism distinct from domain alignment. This process effectively converts a “non-magnetic” water-like fluid to a soft solid using only a weak magnet. Here, the formation of stable ordered phases (cubic, cylinder, networks) causes up to a six order-of-magnitude increase in viscosity and modulus.

While prior studies on “non-magnetic” soft materials have reported unusual field effects that are distinct from field-induced alignment, the molecular-scale mechanisms connecting the negligible magnetic susceptibility of these systems and their magnetically-induced property changes remain elusive. Using magnetorheology, neutron and x-ray scattering, and vibrational spectroscopy, we show that magnetic fields facilitate BCP micellization and ordering by altering polymer-solvent interactions and hydrogen bonding – which in turn modify amphiphile packing. We then use quasielastic neutron scattering to identify that magnetization unexpectedly induces large, long-lasting changes in polymer and water mobility. These data strongly suggest that magnetic fields improve corona chain hydration and extend the hydration shell – revealing new fundamental physics about solute-solvent interactions once thought to be unaffected by magnetic fields. By identifying the molecular-scale mechanisms by which magnetic fields alter solvent structure and interactions with macromolecules, this work highlights a low-cost, low-energy approach to control aqueous solubility. This approach can thus be leveraged not only to create new polymeric materials, but also as a route for managing how water interacts with solutes and substrates for enhanced catalytic and separation processes.

About Michelle

Michelle A Calabrese is an associate professor and McKnight Presidential Fellow in the Department of Chemical Engineering and Materials Science at the University of Minnesota. She received her BS in Chemical Engineering from the University of Pennsylvania in 2012, and a PhD in Chemical Engineering at the University of Delaware in 2017. Following her postdoc in chemical engineering at MIT, she joined the faculty at UMN in fall 2019. Her research group employs rheology, soft matter physics, and polymer and nanoparticle synthesis to address a range of fundamental and applied problems in polymer and soft materials engineering. Select recent recognitions include early career awards from the NSF, NIH and DOE, the AAAS Marion Mason Milligan Award, ACS PMSE Early Investigator, and the APS UKPPG/DPOLY Lecture Exchange