Surface polymerization of iron particles for Magnetorheological Elastomers (MREs) and their potential application as sensors

Magnetorheological Elastomers (MREs) are intelligent materials which are fabricated from magnetic particles and a crosslinked polymer matrix. MRE have viscoelastic, magnetorestrictive, and magnetoresistive properties which makes it possible to be used for vibration isolator and sensing devices. This is possible because of the alignment of the magnetic particles in the polymer matrix. Chainlike magnetic particles are formed by the induced magnetic field. These chains are locked in place during chemical cross linking of the elastomer. In addition, changes in the magnetostrictive and magnetoresistive properties are controlled by varying the magnetic field. Chain-like magnetic particles provide rapid time response of milliseconds.;The magnetostrictive effect of MRE is caused by changing the distance between magnetic particles in the elastomer matrix under the influence of an external magnetic field. The magnetostrictive effect includes stiffness change and dimensional change in MRE. Under the influence of a magnetic field the MRE becomes stiffer due to decreased distance between the magnetic particles. This phenomenon is suitable for the vibration isolator application.;Another phenomenon, the magnetoresistive effect, is important for sensor applications. The magnetoresistive effect is due to the change in electric impedance of the material. The impedance consists of a real and imaginary part. The real impedance represents the material resistance and the imaginary part represents the capacitance. The applied external magnetic field onto the MRE only causes the magnetostrictive effect. The addition of an external force applied to the magnetized MRE will cause the deformation of the chain-like structure of magnetic particles. The electric properties of MRE will be changed by deformation of the chain-like structure.;Surface polymerization plays an important role in MR materials development. Atom transfer radical polymerization (ATRP) involves an organic halide initiator and metal halides, e.g.: cuprous halide, as a catalyst and ligand to improve the solubility of metal salt in the organic reaction system. The metal halides will react based on reversible redox process and the released electron will initiate the organic halide initiator during the initiation step. During the propagation stage, the active radical will initiate the monomer and the monomer will propagate becoming a living polymer. The polymer will be terminated and endcapped by halide group during the termination step.;In the present work, MREs are synthesized from polyurethane (PU) elastomer and silicone RTV (Room Vulcanizing Temperature) elastomer with iron particles. The iron particles concentration is varied from 10 wt.%, 30 wt.%, 50 wt.%, and 70 wt.%. The ATRP technique is used for surface polymerization of iron particles with fluorinated styrene as monomer. As a comparison, the ordinary living polymerization is also used for surface polymerization of iron particles. The mechanical properties and electrical properties of MREs, thermal properties of surface coating polymer, and the alignment of iron particles within MRE have been characterized.