# Shape Memory Polyurethane and its Composites for Various Applications
## 2 Shape Memory Polyurethane in Different Applications
### Electromagnetic Interference Shielding
In the case of SMP, direct incorporation of a high aspect ratio nanoparticles may lead to the decline in shape memory properties. It is highly required to enhance the shielding effect without compromising shape memory properties. In order to improve the shielding effectiveness and interaction of electromagnetic wave with nanoparticles, [PERSON] et al. [20] successfully developed a new method for doping nanoparticles/MWCNTs and/or graphene oxide into a polymer matrix. They prepared the porous polyurethane membrane by way of the phase inversion method using polyethylene glycol (PEG) as pore forming agent. The developed membrane had 13 \(\upmu\)m pore size which facilitated the vacuum filtration of nanoparticles solution through the porous structure leading to the homogeneous coating of nanoparticles on the pores of membranes. They grafted magnetic Fe\({}_{3}\)O\({}_{4}\) nanoparticles with graphene oxide using the solvothermal method and coated the porous membrane with same using filtration technique. It is understood that the addition of MWCNTs or any conducting fillers in SMP at less than the threshold amount--called the percolation concentration--majorly shields reflection whereas the absorption of EMI wave manifest beyond the percolation concentration. Due to the higher concentration of conducting fillers, an interconnected network forms in the bulk of SMP which leads to improved charge transport and also can be responsible for multiple scattering, ultimately reducing the intensity of the EMI wave [21]. They found that the SMP coated with MWCNTs showed shielding effectiveness of \(-\)10 dB whereas SMP coated with Fe\({}_{3}\)O\({}_{4}\) grafted graphene exhibited the effectiveness of \(-\)4 dB at 18 GHz for 100 \(\upmu\)m thickness. It may happen due to the formation of the percolation network. An electromagnetic (EM) wave is scattered multiple times in the bulk resulting in absorption of the EM wave. In addition, magnetic nanoparticles interact with the magnetic component of the EM wave resulting in the absorption of the wave. Overall, they come up with an idea to stack the components together and sandwich the MWCNTs composite between membrane containing Fe\({}_{3}\)O\({}_{4}\) nanoparticles with graphene. They summarize that the multilayer stacked membrane showed a \(-\)32 dB shielding value manifesting in 99.9% attenuation of an EM wave with 400 \(\upmu\)m thickness. The schematic of this multilayer membrane stacking (absorber-reflector-absorber) is shown in Figure 1. They also concluded that the composite had excellent heat dissipation ability which confirms the material's capability to absorb EM waves effectively without affecting shape memory properties. Similarly, [PERSON] et al. [22] proposed a novel facile method for the development of a sponge filled with graphite for electromagnetic wave shielding. They focused on preparing a high carbon loaded sponge with high porosity which can be utilized for the repeated reflection and interference of an EM wave without affecting the shape memory property. As proposed in Figure 2b, they prepared a graphite coated melamine sponge by way of the dip coating drying method. They found that that the electrical conductivity of the sponge was enhanced to 0.01 S/cm with an increase in the graphite content in the bulk. The shape memory activity was intact to more than 90% for 18% graphite content with the electromagnetic shielding effectiveness of 30 dB. It indicates that the developed material achieved a shielding percentage over 99%. They also proposed the mechanism for the microwave shielding shown in Figure 2a. The shielding can be achieved by the multiple internal reflection which reduces the intensity of the EM wave which is attributed as absorption of the wave. Figure 1: Illustration of the mechanism for electromagnetic wave attenuation in a multilayer sandwich structure [20]. Figure 2: (**a**) Multi-reflection mechanism for microwave shielding, (**b**) fabrication method for shape memory graphite filled polyurethane sponge [22].