Graphene–Polymer Nanocomposite Foams for Electromagnetic Interference Shielding

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1		Jiaotong Sun	--	2935	2023-10-18 09:36:39	\|
2	Reference format revised.	Lindsay Dong	Meta information modification	2935	2023-10-19 02:33:40	\|

This entry is adapted from the peer-reviewed paper 10.3390/polym15153235

Graphene, a unique carbon nanomaterial with a two-dimensional structure and exceptional electrical and mechanical properties, offers advantages such as flexibility, light weight, good chemical stability, and high electromagnetic shielding efficiency. Consequently, it has emerged as an ideal filler in electromagnetic shielding composites, garnering significant attention. In order to meet the requirements of high efficiency and low weight for electromagnetic shielding materials, researchers have explored the use of graphene–polymer nanocomposite foams with a cellular structure.

graphene polymer composites foam electromagnetic interference shielding

1. Introduction

With the increasing convenience and rapid connectivity provided by wireless telecommunication networks, the use of numerous telecommunication devices has resulted in an unprecedented level of electromagnetic interference (EMI). Electromagnetic radiation in the transmission process interferes with other electronic devices via electromagnetic induction, causing the disturbance or malfunction of appliances. Moreover, the growing electromagnetic pollution poses potential risks to human health ^[1]^[2]. To tackle this issue, electromagnetic shielding materials are commonly employed to prevent unwanted electromagnetic radiation. These shields usually possess mobile charge carriers to reflect electromagnetic waves, which is the primary route to decrease EMI. Metals with high electrical conductivities are commonly effective EMI shielding materials; however, they are usually dense, prone to corrosion, and difficult to process. Consequently, there is a challenge in developing lightweight, corrosion-resistant, flexible, easy-to-handle, and efficient electromagnetic shielding materials as alternatives to metallic shields.

Polymer nanocomposites attract great attention since they combine the advantages of both polymers and nanoparticles ^[3]^[4]. Various nanoparticles with different functions can be dispersed into the polymeric matrix by simple solution/melt blending. Electrically conductive nanofillers such as carbon nanomaterials, nanostructured metals/metal oxides, and 2D transition metal carbides have been used to prepare conductive polymer nanocomposites for effective EMI shielding ^[5]^[6]^[7]^[8]^[9]^[10]^[11]. In particular, graphene has been widely investigated because of its superior electrical conductivity, extraordinary mechanical properties, and high specific surface area. Its intrinsic conductivity, which is higher than 107 S m⁻¹, makes it an ideal nanofiller in polymer nanocomposites for electromagnetic shielding applications ^[12]^[13]. Therefore, graphene–polymer nanocomposite films with sufficient electrical conductivity are used for efficient EMI shielding ^[8]^[14]^[15]^[16]^[17]^[18].

When electromagnetic waves contact the surface of the electromagnetic shielding material, electromagnetic radiation will be attenuated in three ways: reflection, the absorption of electromagnetic waves by the shield material, and multiple reflection attenuation inside the shield. Good EMI shielding materials require high electrical conductivity in order to effectively reflect electromagnetic radiation. This is because the shielding material reflects radiation through direct interaction with electromagnetic fields, facilitated by charge carriers. On the other hand, absorption occurs when the radiation interacts with electric or magnetic dipoles in the shielding material. Additionally, scattering at the interface between materials plays a crucial role in the multiple reflection mechanism ^[1]^[19]. Therefore, creating heterogeneous structures within the materials can enhance the overall electromagnetic shielding effectiveness of the shield. It is confirmed that the foaming of graphene films improves its EMI shielding efficiency (EMI SE) due to multiple internal reflections by microcellular structures ^[20]^[21]. Thus, cellular structures are introduced into graphene–polymer nanocomposites ^[22]. Foam structures can usually enrich the graphene within or on the cell walls and form electrically conductive networks above the percolation threshold. The resultant graphene–polymer nanocomposite foams have abundant micropores surrounded by a conducting frame, which not only consumes less material but also can achieve a higher EMI shielding performance. Because the porous architectures of the foams can reflect and scatter the incident electromagnetic waves many times between the cell walls, open or closed cells can eventually absorb microwaves after multiple reflections. Meanwhile, since strong EM reflection still causes environmental hazards, it is more environmentally friendly to absorb rather than reflect microwaves.

2. Methods for Preparation of Graphene–Polymer Nanocomposite Foams

There are three primary pathways for preparing nanocomposite foams consisting of graphene and a polymer ^[23]. The internal structures of these foams can be categorized into three types: (a) polymer foams that are coated with graphene nanosheets; (b) graphene foams that are coated with polymer layers; (c) graphene dispersed within the skeleton of the polymeric foam. The first approach involves using a pre-formed polymeric foam as a template to facilitate the formation of a continuous coating of graphene nanosheets on the foam surface. The second pathway, on the other hand, entails covering a pre-prepared graphene-based foam with a uniform polymeric coating. Lastly, an electrically conductive graphene network can be developed within the polymeric skeleton above the percolation threshold, creating a genuine polymer nanocomposite.

2.1. Coating Graphene onto Polymeric Foams

The widely used dip-coating method can be facilely implemented to prepare graphene–polymer nanocomposite foams by using polymeric foams as a template ^[24]^[25]^[26]^[27]^[28]. The coating of graphene is accomplished by immersing the polymeric foam in an aqueous graphene oxide (GO) nanosheet dispersion and then reducing GO into graphene. Jiang’s group used a commercial polyurethane (PU) foam to fabricate polymer-based graphene foams ^[24]. The process involved immersing the polyurethane (PU) foam into an aqueous solution of graphene oxide (GO) and subsequently chemically reducing it using hydrazine. This resulted in the assembly of hydrophobic graphene nanosheets on the surface of the PU skeleton. The researchers observed a color change in the graphene foams after the pyrolysis of PU, indicating the successful formation of a continuous layer of graphene in a straightforward manner. Xia et al. further modified the process to obtain a PU foam coated with reduced graphene oxide (RGO) ^[25]. They found that the choice of solvent in the GO dispersion greatly affected the formation of a complete and continuous layer of RGO on the polymer foam surface. To improve the wetting properties of the polymer, ethanol was added to reduce the surface tension of water in the aqueous GO dispersion. This facilitated the assembly of GO on the hydrophobic PU sponge. Finally, the coated GO was reduced through a solvothermal method.

2.2. Covering Graphene-Based Foams with a Polymer Coating

The second route to graphene–polymer nanocomposite foams is covering the graphene foams with a polymer layer. Graphene foams are typically made by the chemical vapor deposition (CVD) method using a nickel foam (Ni foam) as a template ^[29]. Since only a few layers of graphene nanosheets are usually produced on the Ni foam surface, the Ni foam should be conserved as a scaffold before further surface modification. The freestanding graphene foams can be obtained by self-assembly and/or the freeze-drying method after the thermal or chemical reduction of GO ^[21]^[22]^[30]. In comparison with a self-assembled graphene foam, the CVD graphene foam has higher electrical conductivity due to its fewer defect sites and the higher quality of its interconnected structure. Meanwhile, the conductivity can be adjusted by changing the layers of grown graphene ^[29].

For the CVD graphene foam, the Ni foam scaffold is typically treated with a HCl solution after applying a thin polymer layer onto the graphene film. The resulting composite foams made of graphene and polymers exhibit remarkably low densities, below 0.1 g cm⁻¹. Coating the graphene foam with a polymeric layer is a straightforward process that involves immersing it in a low-concentration polymer solution ^[29]^[31]^[32]^[33]. However, in order to maintain the porosity of the graphene foam, it is important not to completely fill the pores with the coating polymer.

2.3. Dispersion of Graphene within Skeleton of Polymer Foams

Graphene–polymer nanocomposite foams can also be easily fabricated via a two-step procedure: mixing graphene and polymers to synthesize the nanocomposite and then foaming. The nanocomposites can be obtained by solution mixing or the melt blending of graphene and the polymer, and then the nanocomposite foams are produced by utilizing foaming agents for large-scale production. There are usually two main types of foaming agents: chemical blowing agents and physical blowing agents. Chemical blowing agents include reactive chemicals that produce gases at the decomposition temperature, while physical foaming agents change state from liquid to gas in the foaming process. The most commonly used physical foaming agent is supercritical carbon dioxide (CO₂) for foam preparation ^[34]. Briefly, under defined pressure, the polymer nanocomposite sample is first saturated with supercritical CO₂ at a certain temperature. When the pressure is released, CO₂ gas bubbles appear and grow, leading to foam formation with the necessary equipment.

Foams with high porosity and a large cell size can be achieved by adjusting certain parameters. These parameters include the dissolution temperature of supercritical CO₂, the duration of heat treatment, and the rate of pressure release. By manipulating these factors, the size and density of the foam cells can be controlled. More detailed instructions on the foaming process for synthesizing polymer nanocomposite foams can be found in other reviews ^[35]^[36]^[37]. It is important to note that the growth of foam cells affects the spatial arrangement of graphene nanosheets, which in turn influences the interconnection of the nanosheets within the cell walls.

In addition to supercritical CO₂, chemical foaming agents have also been utilized for the preparation of graphene–polymer nanocomposite foams. For example, in the production of graphene–PU foams, distilled water is used to generate CO₂ gas through a reaction with isocyanates ^[38].

In addition to employing blowing agents, phase inversion and leaching are also utilized as methods to fabricate porous polymeric materials. The phase inversion process was first reported to form a polysulfone foam via coagulation of the polymer solution in N-methyl-2-pyrrolidone with water vapor ^[39]. The resultant foam has uniform and closed pores, whose sizes are jointly determined by the solution concentration and the relative humidity. When the latter two parameters both decrease, the pore sizes increase. Zhen et al. employed the method of water-vapor-induced phase separation (WVIPS) to prepare a graphene–polyetherimide (PEI) nanocomposite foam ^[40]^[41].

3. Typical Graphene–Polymer Nanocomposite Foams for EMI Shielding

3.1. Graphene–PMMA Nanocomposite Foams

Zhang and coworkers dispersed graphene sheets into a PMMA matrix to fabricate their nanocomposite foams. Microcellular cells with a size dispersity from 1 to 10 μm were made by using subcritical CO₂ as a foaming agent ^[42]. The added graphene sheets make the graphene–PMMA foams electrically conductive above the percolation threshold, which is about 0.5 vol% graphene sheets in PMMA foam. Since electrical conductivity greatly affects electromagnetic reflection, their EMI SE is necessarily improved when the content of graphene sheets increases from 0 vol% to 0.6 vol% and further to 1.8 vol%, microwave reflection (SE_R) contributes less than microwave absorption (SE_A) to the total shielding efficiency (SE_total).

3.2. Graphene–PSt Nanocomposite Foams

As mentioned before, Yan et al. proposed and implemented a novel method combining compression molding under high pressure and salt leaching to fabricate porous graphene–PSt (GPS) composites ^[43]. Two products with different porosities and densities were obtained by changing the CaCO₃ loading. They are labeled GPS045 and GPS027, representing their densities of 0.45 g cm⁻³ and 0.27 g cm⁻³. Correspondingly, their porosities are 60% and 76%. Since the same content (30 wt%) of graphene was loaded, the conductivity of GPS045 (1.25 S m⁻¹) was higher than that of GPS027 (0.22 S m⁻¹).

3.3. Graphene–Polyvinylidene Fluoride (PVDF) Nanocomposite Foams

Eswaraiah et al. first functionalized graphene with a H₂SO₄ and HNO₃ mixture and then mixed functionalized graphene (f-G) with PVDF and the foaming agent (2, 2′-azobisisobutyronitrile) in dimethyl formamide (DMF). The composite film was casted in a Petri dish and dried in the oven. The f-G/PVDF foam with cell sizes of 0.5–2 μm was prepared by hot pressing due to the decomposition of the foaming agent ^[44]. The effect of f-G content on the conductivity and EMI shielding effectiveness (EMI SE) of the composite foam was investigated. With the increasing mass fraction of f-G, the conductivity increases sharply from insulating 10⁻¹⁶ S m⁻¹ for neat PVDF to conducting 10.16 S m⁻¹ for the PVDF composite foam with 2 wt% f-G. The f-G fillers form conductive paths throughout the PVDF matrix above the percolation threshold (p_c = 0.5 wt%). The highest EMI SE was 28 dB in the X-band region for the foam composite with 7 wt% f-G. In contrast to the former graphene–PMMA and graphene–PSt composite foams, the f-G/PVDF foam showed a reflection-dominant shielding mechanism. Its reflectivity and absorptivity were 78% and 21%, respectively, and this result is consistent with that of the pure graphene film. Therefore, it is possible that the smaller and fewer pores in these composite foams could not play the same role as those in the former graphene–PMMA and graphene–PSt composite foams.

3.4. Graphene–PEI Nanocomposite Foams

Zhai et al. employed the WVIPS method to facilely prepare graphene–PEI nanocomposite foams ^[40]. The homogenized graphene–PEI nanocomposite in DMF was exposed to preset humidity and temperature, and then a 2.3 mm thick foam sheet with uniform cells was obtained. When the graphene content was below 3 wt%, the foam cells exhibited a diameter of around 16 μm. With an increase in graphene loading, the cell size gradually decreased due to higher viscosity in the graphene–PEI dispersion and the increased hindrance of cell coalescence caused by the presence of more graphene. Even at 10% graphene loading, the cell size was reduced to 9.0 μm, while all foam samples maintained a consistent density of 0.3 g/cm³. Transitioning from the solid nanocomposite structure to the foam structure led to a decrease in the percolation threshold from 0.21 vol% to 0.18 vol%. This could be attributed to the growth of cells that facilitated the flow of graphene sheets, resulting in the orientation and accumulation of graphene along the cell walls. At the same loading of graphene (10 wt%), the electrical conductivity was 4.8 × 10⁻⁶ S/cm for graphene–PEI nanocomposite solids, while for the foam counterpart, it was 2.2 × 10⁻⁵ S/cm. Additionally, foaming significantly increased the EMI SSE from 17 to 44 dB/(g/cm³). These graphene–PEI nanocomposite foams also demonstrated a high Young’s modulus and extremely low thermal conductivity.

3.5. Graphene–Polyimide (PI) Nanocomposite Foams

The heatproof aromatic polyimide (PI) was also used to prepare a graphene composite foam in three steps ^[45]. The polycondensation of 4,4′-diaminopheyl ether and pyromellitic dianhydride in the presence of reduced GO (rGO) gave birth to poly(amic acid) (PAA)-modified rGO. Then, the rGO/PAA composite foams were prepared by casting the rGO/PAA solution and soaking it in a nonsolvent (alcohol/water mixture) bath. The rGO/PI composite foams were obtained after thermal imidization and tested for EMI shielding. The lightweight rGO/PI composite foam with 16 wt% rGO showed an EMI SE of 17–21 dB at 8–12 GHz with a thickness of 0.8 mm. The thermostability and mechanical properties of the foam remained good in comparison with those of the PI solid, which helped pave the way for its practical application in the electronics industry.

In contrast to the previous approach of directly using reduced graphene oxide (rGO), Yang et al. implemented a different method to create lightweight rGO/PI composite foams ^[46]. They initiated the process by preparing a poly(amic acid) (PAA) solution using N,N-dimethylacetamide (DMAc) with dispersed graphene oxide (GO). They then induced phase separation by utilizing dibutyl phthalate as a nonsolvent to produce porous composite films. Subsequently, the porous films were subjected to heat treatment to simultaneously convert PAA and GO into polyimide (PI) and rGO through thermal imidization and thermal reduction.

3.6. Graphene–PU Nanocomposite Foams

Graphene–PU foams have been made by different groups using different methods due to the unique properties of the PU matrix. Gudarzi and coworkers fabricated multifunctional graphene–PU composite foams via in situ polymerization with reduced ultralarge graphene oxide (rUL-GO) ^[38]. The addition of 1 wt% rUL-GO turned the PU insulator into a composite conductor with an electrical conductivity of 4.04 S m⁻¹. A low percolation threshold and an EMI SSE of 253 dB (g⁻¹ cm⁻³) at 8–12 GHz were achieved due to the method of foam preparation and a uniform dispersion, together with a high aspect ratio for rUL-GO. The introduced rUL-GO improved the mechanical properties of the PU matrix without a decline in flexibility.

3.7. Graphene–PDMS Nanocomposite Foam

Chen et al. introduced a new method to overcome the limitations of chemical-derived graphene, which often exhibits poor electrical conductivity and high inter-sheet contact resistance, in 2013. They successfully grew graphene on a nickel foam by using the chemical vapor deposition (CVD) of methane at 1000 °C ^[31]. To enhance the quality and electrical properties of graphene, a thin layer of PDMS (polydimethylsiloxane) was applied to its surface through a dip-coating process. Subsequently, HCl was used to etch away the nickel substrate, resulting in the fabrication of a graphene–PDMS nanocomposite foam.

3.8. Graphene–Poly(3,4-ethylenedioxythiophene):Poly(Styrene Sulfonate) (PEDOT:PSS) Nanocomposite Foams

Graphene–poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) foam composites were prepared by drop coating PEDOT:PSS on freestanding graphene foams (GFs) ^[47]. GFs were also fabricated by the CVD process on a Ni foam and then Ni etching. To improve their wettability and enhance the interfacial bonds with PEDOT:PSS, the freestanding GFs were first functionalized with 4-dodecylbenzenesulfonic acid. The graphene–PEDOT:PSS composite foams showed a low density of 18.2 × 10⁻³ g/cm³ and a high porosity of 98.8%. The electrical conductivity was enhanced from 11.8 to 43.2 S/cm after the incorporation of the conductive PEDOT:PSS. The composites had an incredible EMI SE of 91.9 dB and a high SSE of 3124 dB·cm³/g because of their high electrical conductivity, porous structure, and effective charge delocalization.

3.9. Graphene–Poly(arylene ether nitrile) (PEN) Nanocomposite Foams

Zhang and coworkers recently reported a porous absorption-dominated EMI shielding material composed of poly(arylene ether nitrile) (PEN), graphene–carbon nanotubes, and Fe₃O₄ particles ^[48]. The composite foam was obtained by soaking the casting film of PEN/iron ions/graphene/carbon nanotubes in DMAc/NH₄ · H₂O via nonsolvent-induced phase separation (NIPS). The Fe₃O₄ particles were grown in situ in NH₄ · H₂O by the co-precipitation method. The incorporated graphene–carbon nanotubes enhanced the conductive loss of incident microwaves, while the magnetic particles contributed to the dielectric loss and magnetic loss. When the content of Fe₃O₄ was 3.55 wt%, the composite foam had the highest EMI SE of 38 dB and the highest absorption ratio of 94%. The PEN matrix also rendered good thermostability for the composite foam, which paved the way for its practical application in EMI shielding.

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