Voids induce wide-range modulation of elasticity for magnetic elastomers

The relationship between the magnetorheological effect and the void ratio for a polyurethane magnetic elastomer with voids was investigated using a dynamic viscoelastic measurement under a magnetic field of 500 mT. The magnetic elastomer contains iron particles with a diameter of 235 μm at a concentration of 85 wt% (volume fraction: 0.43). The void ratio defined using the volume of vacancies in the non-filled volume of magnetic particles was increased by reducing the amount of polyurethane up to a maximum void ratio of 0.56. The storage modulus of the magnetic elastomer without voids was 1.5 × 10⁵ Pa at 0 mT and 3.1 × 10⁵ Pa at 500 mT, respectively; that is, no significant change in the modulus was observed. The storage modulus at 0 mT for the magnetic elastomer was independent of the void ratio, while the storage modulus at 500 mT increased in proportion to the void ratio. At a void ratio of 0.56, the storage modulus for the magnetic elastomer was 5.6 × 10⁵ Pa at 0 mT and 6.1 × 10⁶ Pa at 500 mT, respectively; that is, the magnetic elastomer demonstrated a significant change in the storage modulus on the order of MPa. This strongly indicates that production of voids enables movement of magnetic particles in the elastomer. Under both strains of 10^-4 and 1, a significant and reversible response of storage modulus was observed after the first application of magnetic field even though the magnetic field was applied for 20 cycles, meaning that the change in the modulus is perfectly reversible although the elastomer contains many voids. SEM/EDX observations revealed that the area composed of carbon decreased with the increasing void ratio while the area composed of iron remained unchanged.