2.3.3. Polyurethane Elastomer

Polyurethane (PU) elastomers have a grea<sup>t</sup> deal of structure and properties because they can be synthesized with a wide variety of materials. For this reason, it has been used in many applications such as shoes, tires, construction, sports, electricity, etc. For the synthesis of PU elastomers, diisocyanate such as aromatic or aliphatic, macrodiol such as polymeric diol, and small molecule diol or diamine as chain extenders are used to form a copolymer. Macrodiol sequence is a soft-segment and diisocyanate and chain extender sequence are a hard-segment to have the character of elastomer [75–78]. In order to provide mechanical properties such as the elastomer of the PU, the soft segmen<sup>t</sup> generally has a glass transition temperature substantially lower than the desired service temperature, and the hard segmen<sup>t</sup> has a glass transition temperature or melting temperature much higher than the desired service temperature.

These characteristics of the PU elastomer attract much attention as the matrix of MRE. This is because properties such as tensile strength, sti ffness, chemical resistance and friction coe fficient of the PU elastomer matrix can be controlled well by changing the type of polyol and diisocyanate according to the application of MRE. Wu et al. [79] manufactured isotropic MREs in an in-situ one-step

polycondensation procedure using CI as magnetic particles in the PU base. They improved the phase separation by using surface milling and ball milling of CI particles to increase the dispersibility of CI in the PU elastomer. On the other hand, Hu et al. [80] studied MREs with PU/Si-rubber hybrids and found that they had higher MR e ffects than pure Si-rubber or PU matrices.
