Effect of Particles Orientation on Dynamic-Mechanical Response and Switching-Ability of Magneto-Sensitive Elastomers

Background: Smart materials are able to ensure adaptability to changing environmental conditions by varying in real time their dynamical and rheological properties. Magneto-Sensitive Elastomers (MSE) are a special class of such smart materials that can be switched by a magnetic field. They consist of an insulating polymer matrix in which magnetic filler particles are embedded. The presence of an external magnetic field yields to a coupling between the dynamical-mechanical and the magneto-rheological behavior of the MSE. This enables to actively adapt their responses to surrounding conditions. Methods: The magneto-rheological responses of the MSE were investigated with a plate-plate rheometer equipped with a magnetic measuring cell. Both melts and cross-linked rubbers were characterized in the presence of a magnetic field. The measurements were performed by applying a small shear strain of 0.1% and a frequency of 10 Hz. The direction of shearing was perpendicular to the particle orientation and field direction. Results: It is demonstrated how the magnetic filler content and particle orientation influence the properties of the MSE. Dynamical-mechanical and magnetorheological measurements show that the orientation of the magnetic filler particles, achieved due to curing in an external magnetic field, has a strong influence on the stiffness and the magneto-sensitivity of the samples. The magneto-rheological effect increases with filler content but reaches a maximum at about 20 vol.% of magnetic filler particles. Conclusion: Magneto-rheological investigations of the non-cross-linked samples indicate that the magnetic filler particles align themselves into strings once the external magnetic field is turned on. They owe this behavior due to strong magnetic dipole-dipole interactions between adjacent particles. This string structure delivers a pronounced contribution to the shear moduli and magneto-sensitivity of the samples.