Shape‐Memory Nanocomposites with Magnetically Adjustable Apparent Switching Temperatures

Thermosensitive shape-memory polymers (SMP) and composites thereof, which are capable of performing one (dual-shape), [ 1–8 ] two (triple-shape), [ 9–15 ] or multiple (multiple-shape) [ 16 , 17 ] shape changes at predefi ned switching bulk temperatures ( T sw,bulk ), have attracted tremendous scientifi c interest and are of technological signifi cance for a broad range of applications such as intelligent textiles, morphing structures, and smart medical devices. The shape-memory effect (SME) is typically induced by increasing the environmental temperature T env . Accordingly, the characteristic apparent switching temperature T sw,app is determined as T env at which the shape recovery occurs. [ 18–20 ] Recently, the noncontact stimulation of dualand tripleshape effects in alternating magnetic fi elds was reported for (nano)composites consisting of magnetic particles embedded in a SMP matrix, [ 21–27 ] which heat the material via hysteresis loss and/or superparamagnetism related processes. [ 28 ] The magnetically triggered SME is characterized by the switching magnetic fi eld strength H sw , which needs to be exceeded to induce the shape change. [ 29 ] Here, we explore whether T sw,app in dualshape nanocomposites or both T sw,app (A → B) and T sw,app (B → C) for triple-shape nanocomposites characterizing the subsequent shape changes from a fi rst temporary shape (A) to a second temporary shape (B) and from there to the original shape (C) [ 10 ] can be adjusted reversibly by application of a weak alternating magnetic fi eld with H < H sw . Our concept is based on the assumption that the heat energy required for increasing the nanocomposites equilibrium bulk temperature T bulk and exceeding T sw,bulk can be obtained by combining contributions from two sources: the heat energy resulting from the interaction with the magnetic fi eld and the heat fl ow from the environment. In this way T sw,app of a dual-shape nanocomposite with 5 wt% nanoparticle content could be decreased reversibly up to 13 ° C and for triple-shape nanocomposites with 5 wt% or 10 wt% nanoparticle content both T sw,app (A → B) and T sw,app (B → C) could be lowered by up to 9 ° C and 19 ° C, respectively. Additionally,