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The process of programming and recovering of a magnetosensitive shape memory sample is shown in Scheme 1.[8] The permanent shape of the sample is determined by a conventional thermosetting of the composite. Then, the sample is deformed (generally at T > Ttrans) and fixed in the desired temporary shape by decreasing the temperature, leading to the formation of a glassy phase or crystallites acting as physical crosslinks. In thermosensitive shape memory polymers, the permanent shape is recovered by heating up the sample again across the transition temperature Ttrans, leading to a relaxation of the polymer segments. The transition temperature is determined in the present case by the melting temperature of the oligo(-caprolactone) switching segments. Using the magnetic heatability of the presented nanocomposites, the shape transition can alternatively be activated electromagnetically. Scheme 1. Schematic representation of the electromagnetically induced shape memory effect in SMP composites. By a programming process, the permanent shape (top) is transformed into a second, temporary shape stabilized by a crystalline phase of oligo(-caprolactone) segments (middle). Local heat development by induction heating of magnetic nanoparticles in an HF electromagnetic field leads to a selective temperature increase inside the matrix. By reaching the transition temperature Ttrans, the permanent shape is recovered (bottom). [Normal View 33K | Magnified View 49K] The synthetic pathway involves the surface-functionalization of freshly prepared Fe3O4 nanoparticles with a shell of oligo(-caprolactone) grafts by a surface-initiated polymerization, as previously described.[14] This surface manipulation aims to achieve compatibility of the nanoparticles with the later polymeric matrix. We implemented polymer network composites with different contents of Fe3O4 and BA, crosslinked with oligo(-caprolactone) dimethacrylate (see Table 1). All the materials are based on the same charge of oligo(-caprolactone)-grafted Fe3O4 nanoparticles with a magnetite content of 40 wt.-%, and on the same charge of oligo(-caprolactone) dimethacrylate ( = 10 000 g ?nbsp;mol-1). The two components were mixed with BA and 2,2-azoisobutyronitrile (AIBN) as radical initiator and cured (see Experimental Part).[4] Table 1. Properties of investigated magnetosensitive shape memory composites and comparison to similar thermosensitive SMP networks without magnetic nanoparticles (cited from ref.[8]). ?/I >M: Fe3O4 mass content obtained by VSM; dc: magnetic core diameter by VSM; G: gel content; Q: swelling degree in CHCl3; Tm: peak melting temperature by DSC; Hm: enthalpy of fusion by DSC; E: elastic modulus in tensile experiments; max: tensile strength; R: elongation at break. The composites obtained after the crosslinking process are black, optically isotropic films swellable in chloroform (CHCl3) and toluene. They were purified from soluble parts by several swelling cycles, showing high gel contents around 80 wt.-%. The observed swelling degrees increase with rising Fe3O4 content and are significantly higher than those of similar networks with comparable composition in terms of BA content, but synthesized without magnetite (results cited from a previous paper[8]) (see Table 1). The higher swellability of Fe3O4 containing composites is likely due to a lower crosslink density that may be explained by the influence of the particles on the crosslinking process and the surface-grafted oligo(-caprolactone) segments acting like dangling chains in the final network architecture. However, it must be pointed out that the composites are crosslinked by a thermoset process, using AIBN as the initiator, while networks without Fe3O4 are photoset with no initiator added. The photoset process was not successful for nanocomposites, possibly due to the intensive light absorption of the black Fe3O4 particles. Investigation of the materials' nanostructure by transition electron microscopy (TEM) shows a homogeneous distribution of the inorganic particles in the matrix (Figure 1). This clearly indicates an efficient dispersion stability of the oligo(-caprolactone)-coated particles in the precomposite mixture. |
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yangyasheng
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