Study of effective particle shape-dependent magnetization behavior of soft magnetic polymeric composites

This reports an investigation of the effect of magnetic particles with same chemical composition but different aspect ratios on the effective magnetization response of magnetorheological elastomers (MREs). MREs are composites that consist of magnetically permeable particles dispersed in a non-magnetic polymeric matrix. These materials are known for the tunability of their magnetoelastic properties. When subjected to an externally applied magnetic field, changes occur in their mechanical properties such as stiffness; this is the so-called magnetorheological (MR) effect. This is usually attributed to the magnetic interactions between the magnetic filler particles. Several factors significantly influence the MR effect. These are the polymer matrix, volume fraction, size, and shape of the magnetic particles. In this study, based on continuum formulation theory, microscale modeling using a finite element analysis (FEA) was used to determine the effect of the latter on the macroscopic magnetization of MREs. Using Jiles-Atherton hysteresis model parameters, the hysteresis loops of MRE were numerically resolved in the FEA software COMSOL Multiphysics. The model calculations were performed for randomly oriented (unaligned) and aligned microstructures with constant particle-volume fraction (φ= ∼20%) and varying particle-aspect ratios (AR=1, 2.5, 5 and 7.5). A computational homogenization scheme was used to relate the microscopic behavior to the macroscopic properties of these composites. From the analysis, it was found that for unaligned MRE the effective magnetization increased with increase in the particle aspect ratio, particularly in the linear region, while the saturation magnetization is seen to be independent of the particle shape. The effect is much more noticeable for particles aligned with the applied field, while for particles aligned perpendicular to the applied field an opposite effect is seen, where increasing aspect ratio results in decreased magnetization relative to the applied field.