Magnetic field-dependent rheological behavior of thermoresponsive poly(N-isopropylacrylamide) solutions

Abstract: Magnetic (B) fields are an intriguing route for manipulating soft materials. While most research on B field manipulation of diamagnetic polymers has focused on alignment of ordered structures or anisotropic domains, our recent work uncovered a previously unrecognized effect: B fields alter hydration and hydrogen bonding in thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) solutions. Despite the well-known thermoreversible coil-to-globule transition and hydrogel formation upon heating, the impact of magnetic fields on these structural and rheological transitions has been largely unexplored. In this study, we thoroughly examined the temperature-dependent magnetorheology of PNIPAM solutions, varying B field strength, polymer content, and molecular weight. Linear magnetorheology reveals that increasing the B field intensity decreases the dynamic moduli of the resulting physical hydrogel, across polymer concentrations (5–20% wt) and molecular weights (30–108 kDa), by up to an order of magnitude. Conversely, the gelation onset temperature does not change substantially. This weakening effect is more pronounced at longer magnetization times and slower temperature ramp rates. Nonlinear magnetorheology following hydrogel formation reveals a two-step yielding process characteristic of attractive-driven glasses, suggesting that magnetization decreases both the stress and length scales associated with mesoglobule cage breaking. We propose that B fields impact the hydrogel rheology by altering the mesoglobule size and water content. This work uncovers essential understanding of how B fields alter hydrogel formation in PNIPAM solutions, broadening the scope of magnetic field manipulation of diamagnetic polymer solutions. Graphical abstract: (Figure presented.)