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Final Activity Report Summary - UNIMAGEL (Uniaxal magnetic gels: Kinetics of formation of field sensitive and field responsive hybrid materials and their mechanical anisotropy)

This project was dedicated to the study of uniaxial magnetic gels, which are promising candidates for bio-mimetic materials, e.g., artificial muscles, and/or for actuators. Uniaxial magnetic gels consist of a polymeric network with embedded magnetic particles aligned along a common direction. Macroscopically, these hybrid materials integrate the properties of their building parts, resulting in elastic and magnetic field responsive materials. The special emphasis of this project was on the, before merely unstudied, kinetics of formation of uniaxial magnetic gels and on their anisotropic mechanical, optical, and magnetic properties.

The set of experimental techniques available in the host group offered unique possibilities for this study, combining mechanical (piezo-rheology) and optical (microscopy) techniques on the same sample. Piezo-rheology is ideal for the study of fragile materials, like forming gels, as it allows for very small applied strains. To study the optical and mechanical properties of the samples in a magnetic field, we added a light scattering set-up on the piezo-rheometer. In the chosen model system (magnetite particles covered with a thin silica layer embedded in PDMS) the magnetic interaction is strong enough to overcome thermal motion but weak enough so that the system is stable over longer periods.

We have shown that small particle concentrations (less than 3.5 wt %) are sufficient to induce strong anisotropies in the mechanical, optical, and magnetic properties of the hybrid material. The superstructure of the magnetic particles picks up easily an anisotropy induced by an applied magnetic field. This internal anisotropy is transferred to the macroscopic properties of the hybrid material in a very efficient way; its manifestations can be observed in both liquid (polymer melts) and elastic matrices. We have shown that the anisotropy of the polymer gel is the result of a two-step formation process: First, the superstructure of the magnetic particles is turned anisotropic. Then, the forming polymer gel integrates this anisotropy into its properties. Macroscopically, uniaxial magnetic gels react to magnetic fields by aligning their internal anisotropy axis with the applied magnetic field, i.e., they are field responsive materials.

This project was intrinsically interdisciplinary; it included, e.g., the physico-chemical preparation of the sample, the experimental study of the physical properties of the uniaxial magnetic gel, and the theoretical interpretation of the results.

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