On the stiffening effect of magneto-rheological elastomers

Magneto-rheological elastomers (MREs), a class of polymer-based composites with dispersed magnetic filler particles, are known for their tunable stiffness. The magnetic-field-induced stiffening effect, namely the magneto-rheological (MR) effect, is often attributed to the magnetic dipolar interaction. The direct attraction between two magnetic dipoles could increase the shear modulus of the material, but not the tensile modulus, while experiments showed increases in both. In this paper, we investigate the possible mechanisms of the MR effect using two different methods: a dipolar interaction model and a micromechanics model based on a continuum field theory. It is found that while the magnetic dipolar interaction is indeed the major cause of the MR effect, the specific microstructure of an MRE is the key to the modulus increase. An MRE of straight chains has a negative MR effect in tension, but one of wavy chains can have tensile modulus increase under a magnetic field. While the chain-chain magnetic interaction is shown to be insignificant, the filler concentration plays an important role on the strength of MR effect, just as observed in experiments.