## 3 Results and Discussion
### Characteristics of the Fe\({}_{3}\)O\({}_{4}\)/SBS/LLDPE Composites
#### 3.2.4 Magnetically Induced Shape Recovery of Fe\({}_{3}\)O\({}_{4}\)/SBS/LLDPE Composites  
The effect of the magnetic field intensity on the shape memory of the Fe\({}_{3}\)O\({}_{4}\)/SBS/LLDPE composites was analyzed to choose the optimal magnetic field intensity. The SBS/LLDPE composite with 13.5 wt % Fe\({}_{3}\)O\({}_{4}\) nanoparticles was chosen to test the shape memory behavior of the composite in  
Figure 5: Differential scanning calorimetry (DSC) curves of the Fe\({}_{3}\)O\({}_{4}\)/SBS/LLDPE composites.  
Figure 6: Storage modulus of the Fe\({}_{3}\)O\({}_{4}\)/SBS/LLDPE composites.  
different magnetic field intensities (\(H\) = 7.07, 14.14, 21.21, 28.28, and 35.36 kA\(\cdot\)m\({}^{-1}\)). Figure 7 indicates that in a higher-intensity magnetic field, the shape recovery time decreases drastically. However, the shape recovery time show balance when the magnetic field intensity was more than 21.21 kA\(\cdot\)m\({}^{-1}\). Thus, in this article, we studied the shape memory properties of the samples under a fixed alternating magnetic field (\(f\) = 60 kHz, \(H\) = 21.21 kA\(\cdot\)m\({}^{-1}\)).  
Because of the hysteresis loss and eddy current losses in an alternating magnetic field, embedded magnetic particles in a polymer matrix will generate heat which can induce the shape memory effect [25]. A shape recovery process of 13.5 wt % Fe\({}_{3}\)O\({}_{4}\)/SBS/LLDPE composites is illustrated in Figure 8. The folded sample is shown as well. In the first 20 s, the sample recovery speed is low, but the shape recovery speed is very fast after 20 s. The reason is that in the first 20 s, the heat produced by Fe\({}_{3}\)O\({}_{4}\) nanoparticles is not enough for the temperature of the sample to reach the shape transition temperature. Thus, the internal frozen stress cannot be released to drive the shape deformation. However, when the temperature is near or higher than the shape transition temperature, the sample quickly reaches its original shape.  
Figure 8: Magnetically induced shape recovery process of SBS/LLDPE composites with 13.5 wt % Fe\({}_{3}\)O\({}_{4}\) in an alternating field (\(f\) = 60 kHz, \(H\) = 21.21 kA\(\cdot\)m\({}^{-1}\)).  
Figure 7: Relation between recovery time and magnetic field intensity of the 13.5 wt % Fe\({}_{3}\)O\({}_{4}\)/SBS/LLDPE composite.  
The shape memory properties of Fe\({}_{3}\)O\({}_{4}\)/SBS/LLDPE composites are shown in Table 1. It can be found that with increasing the content of Fe\({}_{3}\)O\({}_{4}\) nanoparticles, the shape retention rate and shape recovery rate of the composites increase significantly. In addition, the shape recovery time decreases with the increase of the Fe\({}_{3}\)O\({}_{4}\) nanoparticle content. The reason for this phenomenon is that with the increasing content of Fe\({}_{3}\)O\({}_{4}\) nanoparticles, the composites can produce more heat to let the temperature reach or near the shape transition temperature where the magnetic field response time and recovery time decrease while the shape recovery rate increases. So the recovery rate of the samples increment with Fe3O4 nanoparticles content increasing [26]. It can be concluded that more nanoparticles in SBS/LLDPE blends can lead to a shorter magnetic field response time and less stress relaxation so that samples can attain a higher shape recovery rate. Furthermore, a larger Fe\({}_{3}\)O\({}_{4}\) nanoparticle content is a benefit for fixing the deformation of the composites when the temperature is near or higher than the shape transition temperature; thus the shape retention rate increases with the increase of the Fe\({}_{3}\)O\({}_{4}\) nanoparticles.