Paper Number
SF18
Session
Surfactants, Foams and Emulsions
Title
Magneto-rheology of magnetically-induced phase formation in block copolymer micelle solutions
Presentation Date and Time
October 11, 2022 (Tuesday) 3:45
Track / Room
Track 7 / Ontario
Authors
- Kresge, Grace (University of Minnesota, Chemical Engineering and Materials Science)
- Calabrese, Michelle A. (University of Minnesota, Chemical Engineering and Materials Science)
Author and Affiliation Lines
Grace Kresge and Michelle A. Calabrese
Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55414
Speaker / Presenter
Kresge, Grace
Keywords
gels; polymer solutions; rheometry techniques; surfactants
Text of Abstract
Magnetic fields induce a rich variety of physical phenomena in diamagnetic block copolymers (BCPs), including the formation and enhancement of various nanostructures. Prior work in the directed assembly of BCPs via magnetic fields has primarily relied on the alignment of BCP chains or phases in bulk. In contrast, in this work, BCP micelle solutions exposed to low intensity magnetic fields (B = 0.05 T) demonstrate a magnetically-induced disorder-to-order sol-gel transition. This ordering transition corresponds to a three-to-six order of magnitude increase in dynamic moduli, measured via magneto-rheology. Subsequent small angle X-ray scattering measurements confirm a field-induced ordering transition from random spherical micelles to ordered cubic and hexagonal packed cylinder phases which are reversible after long relaxation times. The phases formed are dependent on magnetization time, polymer molecular weight, block ratio, and field intensity. The stability of these structures was probed via a series of relaxation and shear recovery experiments, which demonstrates that the gel modulus is remarkably robust. Fourier transform infrared spectroscopy on magnetized and unmagnetized samples suggest that weak magnetic fields alter polymer-solvent interactions and hydrogen bonding to facilitate structural transitions. In particular, the dehydration of micelle coronas due to increased PEO interactions yields phases only observed in 10-20wt% more concentrated polymer solutions as well as d-spacing and micelle sizes smaller than those observed in zero-field controls. After tunable assembly via magnetic fields, these soft solids can be crosslinked to preserve structure and functionality. This new BCP processing strategy enables discovery of structures and d-spacings inaccessible via traditional thermal processing routes, thus providing a platform for developing materials with precisely-controlled features at mild conditions.