Tailoring the recovery force in magnetic shape-memory nanocomposites
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Tailoring the recovery force in magnetic shape-memory nanocomposites M. Behl, M.Y. Razzaq, and A. Lendlein Institute for Biomaterial Science and Berlin Brandenburg Center for Regenerative Therapies, Helmholtz-Zentrum Geesthacht, Kantstr. 55, 14513 Teltow, Germany ABSTRACT Composites from magnetic nanoparticles in a shape-memory polymer (SMP) matrix allow a remote actuation of the shape-memory effect by exposure to alternating magnetic fields. At the same time the incorporation of MNP may affect the thermal properties and the structural functions of the SMP. Here, we explored the adjustability of the recovery force as an important structural function in magnetic shape-memory nanocomposites (mSMC) by variation of the programing temperature (Tprog) and nanoparticle weight content. The nanocomposites were prepared by coextrusion of silica coated magnetite nanoparticles (mNP) with an amorphous polyether urethane (PEU) matrix. In tensile tests in which Tprog was varied between 25 and 70 °C and the particle content from 0 to 10 wt% it was found that the Young’s moduli (E) decreased with temperature and particle content. Cyclic, thermomechanical experiments with a recovery module under strain-control conditions were performed to monitor the effect of mNP and Tprog on the recovery force of the composites. During the strain-control recovery the maximum stress (ım,r) at a characteristic temperature (Tı,max) was recorded. By increasing the mNP content from 0 to 10 wt% in composites, ım,r of 1.9 MPa was decreased to 1.25 MPa at a Tprog = 25 °C. A similar decrease in ım,r for nanocomposites with different mNP content could be observed when Tprog was increased from 25 °C to 70 °C. It can be concluded that the lower the deformation temperature and the particle content the higher is the recovery force. INTRODUCTION The incorporation of magnetic (nano)particles in shape-memory (SMP) polymers as a matrix enabled nanocomposites whose shape-memory effect could be actuated remotelycontrolled on demand.[1-5] This principle could be extended to triple-shape nanocomposites.[6] However, the influence of the mNP on the mechanical properties is not well understood. In shape-memory polymers and composites an important parameter in addition to the maximum deformability is the recovery stress as this influences the recovery ratio Rr under constraint conditions such as in an implant situation. The recovery stress can be determined from cyclic, thermomechanical experiments consisting of a deformation followed by recovery module under strain-control conditions. [7] The recovery under constant strain conditions is quantified by a characteristic temperature Tı,max, at which the maximum recovery stress ım,r occurs. Recently polymers with a broad transition temperature (ǻTtrans) have enabled a temperature-memory effect (TME), which is the capability to remember the temperature Tprog, where the polymer was deformed recently. [8, 9] We have asked us whether such a matrix with a programmable temperature (Tprog) at which the deformation is applied can be ut
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