Thermoresponsive polymer-based magneto-rheological (MR) composites as a bridge between MR fluids and MR elastomers

A new class of magnetorheological (MR) material is reported that bridges the gap between conventional MR fluids and MR elastomers. The key point is the use of a thermoresponsive polymer-based suspending medium in the formulation of the MR material whose rheological properties can be externally controlled through changes in the temperature. Under appropriate conditions, it is even possible to induce a thermally driven liquid-to-solid transition in the carrier medium. As a result, when the suspending medium is in the liquid phase, the MR composite behaves as a conventional MR fluid exhibiting a large MR effect (approx. 1000%). In contrast, when the suspending medium is in the solid phase and exhibits an apparent yield stress, the whole material behaves as a MR elastomer where the iron microparticles are entrapped in a polymer network. Two different examples are reported in this communication; on the one hand a triblock copolymer solution is used as a suspending medium that evolves towards a lyotropic liquid crystalline cubic phase upon heating. On the other hand, a concentrated microgel dispersion is used that reaches maximum packing as the temperature decreases forming a repulsive colloidal glass. The rheological properties of the materials are evaluated under dynamic oscillatory and steady simple shear to ascertain the MR effect.