# Shape Memory Effect in Micro-Sized Shape Memory Polymer Composite Chains
## 6 Shape Recovery Properties of Shape Memory Polymer/Nickel (SMP/Ni) Vertical Chain Arrays  
As illustrated in Figure 8, we heat initially vertical chains (Figure 8a) to 120 \({}^{\circ}\)C (well above \(T_{g}\)\(T_{g}\) = 43 \({}^{\circ}\)C-46 \({}^{\circ}\)C) for 20 min. After that, a piece of aluminum plate is placed atop the sample. Hence, the chain is flattened (Figure 8b). Subsequently, the SMP/Ni vertical chains are gradually cooled in the bending and flattened configuration while keeping the aluminum plate there. After the removal of the aluminum plate, the nickel protrusive chains are still in the bend-shape as shown in Figure 8c. Upon heating to 120 \({}^{\circ}\)C, if the chains can recover its initial shape (Figure 8d), the shape memory effect of SMP is demonstrated at microscale and nanoscale. The shape memory effect of vertical chains atop different samples with different nickel contents was tested following the procedure described above. Figure 9a shows one typical flattened chain of two samples with 2 vol% nickel. It is obvious that thin chains are easier to be flattened as compared with the thick chains. Hence, the chains obtained in samples with lower nickel content are preferred to demonstrate the SME. Furthermore, the local buckling phenomenon of SMP matrix can be found in the inner surface of the bending chain, which implies that the local compression strain is high. Figure 9b is the zoom-in view of chains of 2 vol% nickel content. Given that the size of nickel powders is around 3 to 7 \(\upmu\)m, it reveals that the dimension of SMP (in particular, the thickness) is around 1 \(\upmu\)m or less (i.e., at submicron scale), which is also indicated in Figure 7a. Hence, if the chains, although they are composite in nature, are demonstrated as with the SEM, we can conclude that the SMP/Ni chains structure does have the shape memory effect at around 1 micron and even submicron scale. Figure 10 reveals the patterns of several SMP/Ni vertical raised chains in shape memory recycle. In the original shape Figure 10a, the SMP/Ni protrusive chains are strictly straight along with the direction of magnetic field. After heated above the \(T_{g}\), the SMP matrix in the SMP/Ni chain became soft, and then the SMP/Ni protrusive chains were forced to be bent to approximate 180\({}^{\circ}\) to the flatten shape with a flat board. When cooled below \(T_{g}\), after releasing the external constraint, this flatted pre-deformed configuration were obtained as shown in Figure 10b. The bending areas of vertical chains are mainly located at the root. When re-heating the SMP/Ni vertical chains, they gradually recovered toward the original vertical configuration. The macroscopic shape memory effect of pure SMP is the same as that at microscale, with shape recovery ratio 95-100% above \(T_{g}\). However, with bending in solid and hard nickel particles, the shape recovery ratio of SMP/Ni vertical chains will reduce. The biggest unrecoverable angle in the recovered shape of SMP/Ni vertical chains is approximate 30\({}^{\circ}\) compared with the original straight shape, namely shape recovery ratio around 83%. Figure 8: Illustration of a thermomechanical cycle to evaluate the shape memory effect of a vertical protrusive chain. (**a**) Initial shape; (**b**) pre-deformed shape; (**c**) flattened shape; (**d**) recovered shape. **7. Formation Mechanism and Dimensions Control Method of Shape Memory Polymer/Nickel (SMP/Ni) Vertical Chain Arrays**  
Considering the formation mechanism for nickel chains aligned along the magnetic field direction, the general explanation could be referenced to magnetomechanical coupling theory of the magnetorheological fluids [26]. Without loading a magnetic field, there are no permanent dipole moments in nickel particles; hence, all the nickel particles are in Brown motion and distributed randomly in the SMP/DMF solution prior to curing. Upon loading a permanent static magnetic field \(\mathbf{H}\), the permanent dipole moments in all ferromagnetic particles were induced and interacted.