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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.422

Solvent-based Fabrication Method for Magnetic, Shape-Memory Nanocomposite Foams Karola Luetzow1, Thomas Weigel1, Andreas Lendlein1,2 1 Institute of Biomaterial Science and Berlin-Brandenburg Centre for Regenerative Therapies, Helmholtz-Zentrum Geesthacht, Teltow, Germany 2 Institute of Chemistry, University of Potsdam, Potsdam, Germany

*Correspondence to: Prof. Andreas Lendlein [email protected]

ABSTRACT This paper presents shape-memory foams that can be temporarily fixed in their compressed state and be expanded on demand. Highly porous, nanocomposite foams were prepared from a solution of polyetherurethane with suspended nanoparticles (mean aggregate size 90 nm) which have an iron(III) oxide core with a silica shell. The polymer solution with suspended nanoparticles was cooled down to -20 °C in a two-stage process, which was followed by freeze-drying. The average pore size increases with decreasing concentration of nanoparticles from 158 µm to 230 µm while the foam porosity remained constant. After fixation of a temporary form of the nanocomposite foams, shape recovery can be triggered either by heat or by exposure to an alternating magnetic field. Compressed foams showed a recovery rate of up to 76 ± 4% in a thermochamber at 80 °C, and a slightly lower recovery rate of up to 65 ± 4% in a magnetic field.

INTRODUCTION Devices made from shape-memory polymers (SMP) can be transported or stored efficiently in a smaller shape and then later be transformed into their application relevant shape [1-5]. Fibers [6, 7], flat sheets which fold up into cubes[8] or compressed foams that then expand have been considered as shaped bodies [9]. The stimulus inducing the shape-memory effect (SME) can be heat or light [10-12] or even alternating magnetic fields [5, 6, 13-17]. Initiation of the SME by alternating magnetic fields has been reported for polymeric films with incorporated magnetic particles with diameters in the nano- to micrometer size range [6, 16, 18]. Various ferro- or ferromagnetic cores have been investigated, among them iron(III) oxide particles with a silica matrix

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embedded within a polymer, e.g. a poly(etherurethane) (PEU) [6]. PEU is a copolymer consisting of hard segments made from bis(p-cyclohexyl isocyanate)(H12MDI)/1,4butanediol and soft segments made from H12MDI /poly(tetramethylene glycol). The SME is based on the glass transition of the switching phase, which consists of a mixed phase. This glass transition has a rather broad range from 20 °C to 90 °C [6, 9]. Inductive heating using alternating magnetic-fields acts as a remote, contact-free stimulus, and can be applied in situations in which direct heating by a warm fluid or gas is not possible. SMP foams [19] have been prepared with va