Nanoparticle-polymer coupling in magnetic gels studied by means of computer simulations and experiments.

Magnetic gels are soft hydrogels with incorporated magnetic nanoparticles, combining viscoelastic properties with responsiveness to magnetic fields. In many experimentally relevant systems, the nanoparticles are not covalently attached to the polymer network but are instead physically trapped within its meshes. Despite this weak mechanical coupling, experiments reveal signatures of hindered rotational dynamics. Here, we investigate the microscopic origin of this behavior by combining experiments on polyacrylamide hydrogels loaded with cobalt ferrite nanoparticles with simulations that explicitly account for polymers, nanoparticles, and hydrodynamic interactions. We probe the nanoparticle-polymer coupling by exploring the systems' magnetic AC susceptibility-a quantity that is accessible both by experiment and in simulations, and that is sensitive to rotational dynamics. Experimentally, we observe a reduced low-frequency susceptibility, indicating partial orientational blocking of the nanoparticles, even in the absence of covalent bonding. Simulations show that this behavior cannot be explained by hydrodynamic coupling or isotropic van der Waals-like interactions alone. Instead, our results demonstrate that a degree of preferential attachment of polymers to spots on the nanoparticle surface-arising from chemical or topographical heterogeneity-is essential to reproduce the experimentally observed response.