RESUMO
The relationship between the particle mobility and magnetorheological effect was investigated for polyurethane magnetic elastomers containing carbonyl iron particles with various cross-linking densities or plasticizer concentrations. The storage modulus at 0 mT increased and the on-field modulus at 500 mT decreased with the cross-linking density. The critical magnetic field where the storage modulus starts to rise up increased with the cross-linking density, indicating that the movement of magnetic particles is depressed by the cross-linking points of the polyurethane network. Magnetic elastomers with various plasticizer concentrations revealed that the storage modulus at 0 mT decreased and the on-field modulus at 500 mT increased with the plasticizer concentration. The critical magnetic field decreased with increasing plasticizer concentration, indicating that a dense polyurethane network prevents magnetic particles from moving. It was found that the change in the modulus due to the magnetic field can be scaled by the storage modulus at 0 mT as well as the critical magnetic field. Thus, there is a certain correlation between the macroscopic modulus of elasticity (storage modulus at 0 mT) and the microscopic mobility of magnetic particles reflected in the critical magnetic field.
RESUMO
The relationship between the magnetorheology of bimodal magnetic elastomers with high concentrations (60 vol %) of plastic beads with diameters of 8 or 200 µm and the meso-structure of the particles was investigated. Dynamic viscoelasticity measurements revealed that the change in storage modulus of the bimodal elastomer with 200 µm beads was 2.8 × 105 Pa at a magnetic field of 370 mT. The change in the storage modulus for monomodal elastomer without beads was 4.9 × 104 Pa. The bimodal elastomer with 8 µm beads hardly responded to the magnetic field. In-situ observation for the particle morphology was performed using synchrotron X-ray CT. For the bimodal elastomer with 200 µm beads, a highly aligned structure of magnetic particles was observed in the gaps between the beads when the magnetic field was applied. On the other hand, for the bimodal elastomer with 8 µm beads, no chain structure of magnetic particles was observed. The orientation angle between the long axis of the aggregation of magnetic particles and the magnetic field direction was determined by an image analysis in three dimensions. The orientation angle varied from 56° to 11° for the bimodal elastomer with 200 µm beads and from 64° to 49° for that with 8 µm beads by applying the magnetic field. The orientation angle of the monomodal elastomer without beads changed from 63° to 21°. It was found that the addition of beads with a diameter of 200 µm linked the chains of magnetic particles, while beads with a diameter of 8 µm prevented the chain formation of the magnetic particles.
RESUMO
The magnetic response of dynamic modulus was investigated for polyurethane-based magnetic elastomers densely packed with magnetic particles with different diameters (7 µm and 235 µm). The density indicated that voids were created at volume fractions of magnetic particles above 0.47 (87 wt%) for 7 µm and 0.44 (85 wt%) for 235 µm. At volume fractions below these critical volume fractions, no apparent increase in the storage modulus was observed for the magnetic elastomers by applying a magnetic field of 500 mT. At above the critical volume fractions, dramatic increase in the storage modulus was observed; the maximum changes in the storage modulus were 8.0 MPa and 6.0 MPa, which corresponds to the relative changes in the modulus (ΔG/G0) of 74% and 11%, for magnetic elastomers of 7 µm and 235 µm, respectively. These results strongly indicate that the creation of a space enables the movement of magnetic particles in the elastomer resulting in the formation of chain structure. It was also found that the creation of a space in the matrix of elastomers can be detected by the critical strain, the amplitude of Payne effect, or the critical magnetic field since significant changes in these parameters were found at the critical volume fraction for both magnetic elastomers. SEM images displayed a clear difference in the creation process of voids that magnetic particles of 7 µm suddenly formed many macroscopic voids at the critical volume fraction, meanwhile, magnetic particles of 235 µm formed gaps around magnetic particles. The storage modulus for both magnetic elastomers changed perfectly in response to the magnetic field even after the on-off switching of the magnetic field with 20 cycles.
RESUMO
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.