## 8 Elastomer Nanocomposite Foams for Electromagnetic Interference Shielding  
Owing to their superior compressibility and flexibility, elastomer-based nanocomposite foams have been widely explored for their EMI shielding ability and microwave absorption performance. Amongst elastomers, silicon rubber, natural rubber and butadiene rubber have been converted into three-dimensional porous foams by using chemical/physical blowing agents or via the supercritical carbon dioxide (Sc-CO\({}_{2}\)) foaming technique. [PERSON] et al. [106] fabricated flexible and very lightweight methyl vinyl silicon rubber (VMQ)/multi-walled carbon nanotubes (MWCNTs)/ferriferous oxide (Fe\({}_{3}\)O\({}_{4}\)) nanocomposite foams using Sc-CO\({}_{2}\) foaming, as shown in Figure 6. The microwave-absorbing ability of these nanocomposite foams was found to be enhanced due to the formation of a porous structure, thereby considerably reducing the secondary EM pollution due to the reflection of microwaves. These flexible and compressible foams showcased low density (-0.48 g.cm\({}^{-3}\)), with an average EMI SE of 27.5 dB and an average absorption efficiency of 64% in the X-band region (8.2-12.4 GHz) [106]. It was reported that, at 1.78 vol.% filler loading, a maximum electrical conductivity of -14.6 S/m and specific EMI SE of -72 dB-g\({}^{-1}\) cm\({}^{3}\) were achieved by using this process. It is also worth mentioning that the foams exhibited excellent EMI shielding stability, with a 96.2% retention of EMI SE even after 1000 bending cycles. Backfilling an elastomer into a preformed three-dimensional (3D) conductive skeleton of nanofillers is yet another burgeoning approach for developing highly conductive elastomer nanocomposite foams. [PERSON] et al. [107] made use of this strategy to fabricate porous EMI shielding materials with enhanced microwave absorption properties. Herein, an elastomer matrix, i.e., polydimethylsiloxane (PDMS), was used to reinforce a highly conductive and robust three-dimensional bi-continuous skeleton of graphene/silver nanowires. Initially, GO suspension was added to AgNWs dispersion and homogenized to form stable hydrogels. This was followed by a thermal reduction of the hybrid hydrogel into rGO/AgNWs networks, which were eventually freeze-dried for 48 h to form 3D bi-continuous conductive structures. This porous network was then immersed in PDMS with a curing agent to form robust and compressible PDMS-reinforced GACs (PGACs), as shown in Figure 7A. The EMI shielding value of these hybrid foams reached 34.1 dB at 0.43 wt.% rGO and 0.33 wt.% AgNPs loading, and showcased superior conductivity of 10.6 S/cm and high specific EMI shielding effectiveness of 22.43 dB/unit filler wt.%/mm [107]. In addition, these monoliths demonstrated excellent compressibility, thermal stability and flame retardancy (see Figure 7B). In another work, [PERSON] and co-workers developed flexible and high-performance EMI shielding foams from PDMS/rGO/SWCNTs nanocomposites using a similar backfilling approach by exploiting a preformed rGO/SWCNTs aerogel as a 3D conductive skeleton. These nanocomposite foams were fabricated based on a two-step process using GO and pristine SWCNTs, as shown in Figure 8. Initially, a sol-gel self-assembly of reduced GO and SWCNTs was performed, and they were subsequently freeze-dried to create a robust 3D rGO/SWCNTs aerogel skeleton. The next step was penetration of PDMS into the formed porous network, followed by the curing process. The 3D bi-continuous carbonaceous network in the foams created a highly efficient pathway for the electron transport and demonstrated high electrical conductivity of 1.2 S/cm, with a maximum EMI SE of 31 dB in the X-band region at very low loading of 0.28 wt.% rGO and SWCNTs [108]. Figure 6: Schematic for the preparation of VMQ/MWCNTs/Fe3O4 nanocomposite foams. Reproduced with permission from [106]. Copyright 2019, Elsevier Ltd.  
Figure 8: (**a**) Schematic depicting the fabrication of rGO/SWCNTs/PDMS aerogel. (**b**) Digital photograph of the ultralight aerogel.