## 7 Methods to Fabricate Polymer Foams and Aerogels  
Polymer foams are mostly fabricated with the addition of chemical blowing agents, which release gases into the bulk polymer matrices due to a reaction that is mostly exothermic in nature. If the entrapped gas attains roughly spherical and separate pores, it results in the formation of a rigid closed-cell foam. Meanwhile, if the pores are well-interconnected, it gives rise to flexible open-cell foam. The open-cell foams are mostly used as upholstery  
Figure 5: Variation of the electrical conductivity and the EMI SE value in solid and foamed HDPE-GnP composites. Reproduced with permission from [99]. Copyright 2018, American Chemical Society.  
and acoustic insulation, whereas closed-cell foams are widely utilized for thermal insulation. There are several industrial techniques for the production of thermoplastic foams, viz. batch foaming, foam extrusion and foam injection molding [102]. The selection of a particular foaming process depends on the final product and end application of the foam. Moreover, the use of chemical blowing agents may lead to contamination as undesired residues can remain in the matrix even after the foaming process. Hence, physical blowing agents are more desirable for creating porous networks. Currently, the main focus is on the usage of supercritical CO\({}_{2}\) due to its non-flammability, chemical inertness, efficient process control, relative ease of handling and good interaction with polymers compared to other inert gases [103]. Moreover, it expediently replaces conventional volatile solvents, viz. chlorofluorocarbons (CFCs), butane or pentane, which are infamous for their contribution to the depletion of the ozone layer. Therefore, next-generation polymer-based EMI shielding foams are preferably manufactured by the CO\({}_{2}\)-aided technique.  
Aerogels are a class of porous solids which possess extremely low density, high surface area and tunable porosities (90-99%) that can be prepared from various polymeric precursors by sol-gel method and subsequent freeze/supercritical drying to remove the solvents from the wet gels and replace them with air [104]. However, aerogels possess even higher porosity and lower densities than polymeric foams and are hence suitable for high-end technological applications such as sensors, energy storage, biomedical scaffolds and many more [105]. The higher production cost associated with aerogel fabrication is the main drawback that restricts its commercial application. Depending on the morphology, aerogels can be further classified as isotropic and anisotropic aerogels. A rapid immersion of the polymer precursor into liquid N\({}_{2}\) will arrest the molecular motions within the solution, thereby preserving its original structure. This results in the formation of isotropic aerogels. However, when we adopt slower freezing of the precursor solution on copper plates using liquid N\({}_{2}\), it leads to the formation of anisotropic aerogels. This technique is also known as unidirectional ice templating (IT) [104]. In this process, the ice crystals slowly start to grow in one direction, which results in the formation of 2D pores/cells that are aligned parallel to the freezing direction, which eventually results in a honeycomb-like structure in the aerogels. In this review, we primarily focus on the fabrication of various conductive polymer nanocomposite aerogels and foams that can deliver high EMI shielding performance.