Elastomer is a type of polymer that can approximately recover its original state immediately after being deformed. It is extensively employed in fields such as general production, advanced technologies, and defense. Elastomers, especially rubber, have low strengths and moduli, and nanofillers can be added to increase their strengths [1]. Nanofillers are compact but can have large surface areas. In addition, they are light and can achieve favorable mechanical properties, thermal stability, and dimensional stability with low filler content compared with microfillers [2]. With the rapid development of science and technology, functional elastomers are being increasingly employed in various fields, and nanofillers can endow elastomers with functionality, improve the transmission of electrons and phonons, and restrict quality transfer [3]. Various nanofillers are being applied to elastomers, such as carbon materials, including conductive carbon black, carbon nanotubes (CNTs), graphene, nanosilica, metal oxides, hexagonal boron nitride, layered double hydroxide, transition metal dichalcogenide, and 2D transition metal carbon/nitrogen/carbon nitride (MXene) ^[2][3][4][5][2,3,4,5]. Elastomers have shown diversity after composition with one or more nanomaterials ^[6][7][8][6,7,8].

MXene has attracted research interests in various fields since its introduction in 2011 [9]. The chemical formula of its family of 2D layered materials is M_n+1X_n, where M represents early transition metals, X can be C and/or N, and n = 1–4 [10]. MXene is obtained by removing layers A from three-dimensional (3D) layered ceramic materials of MAX phase by chemical etching. The corresponding chemical formula is M_n+1AX_n, where A is a main group element, such as Al and Si ^{[11][12][13][14]}[11,12,13,14]. Common MXenes include Ti₃C₂T_x and Ti₂CT_x, where T_x denotes surface functional groups, such as -OH, -F, and =O. Given the electronegative layered structure and surface functional group -OH, it can easily be dispersed in water and is more convenient for surface modification and assembly ^[15][16][17][15,16,17] compared with graphene-based 2D nanomaterials. The conductivity of MXene is comparable to that of graphene. When the MXene surface contains more groups, it can exhibit semiconductor properties, and when fewer surface functional groups are available, MXene is relatively pure, exhibiting metal conductivity [18]. For example, the conductivity of MXene can reach up to 20,000 S·cm⁻¹ after obtaining a thin film using the common Ti₃C₂T_x MXene aqueous dispersion. The maximum starting temperature of oxidation for Ti₃C₂T_x monolayer nanosheets is approximately 450 °C in air, with complete oxidation at 900 °C [19]. In addition, MXene has a high specific surface area and mechanical flexibility. Owing to its diverse characteristics, MXene can be suitably applied in electromagnetic interference (EMI) shielding and microwave absorption ^[20][21][20,21], sensors [22], energy storage [23], flame retardants [24], catalysis [25], adsorption [26], biomedicine [27], and other areas.

For latex blending, water dispersion or powder of MXene is mixed with latex and stirred to uniform dispersion. Then, a large amount of water is separated by filtration or flocculation, obtaining MXene/elastomer nanocomposites after drying. The superb hydrophilicity of MXene allows dispersion into latex using water as the solvent and interaction with the matrix through hydrogen bonds or electrostatic forces. Thus, uniform dispersion in the matrix, low filler content, and excellent comprehensive performance are achieved. Owing to the lamellar structure of MXene and the presence of -OH on the surface, lamellar stacking easily occurs after adding MXene. This problem can be mitigated and even suppressed by prior MXene surface modification or freeze-drying after blending, thereby improving the dispersion and compatibility of MXene in the matrix.

Luo et al. [20] mixed and stirred a Ti₃C₂T_x suspension with NR latex and prepared a Ti₃C₂T_x/NR film by vacuum filtration to prepare a cross-linked Ti₃C₂T_x/NR film with dicumyl peroxide as the cross-linking agent. The electrostatic repulsion between the negatively charged MXene and NR latex ensures that the MXene layer independently forms an interconnected 3D network structure between NR particles, thus achieving excellent dispersion, and electrons can transfer efficiently and rapidly throughout the whole structure. This stable network endows nanocomposites with excellent electrical conductivity and extraordinary tensile properties. Yang et al. ^[28][45] also prepared MXene/NR composite films by vacuum filtration. Introducing NR provides hydrophobicity to nanocomposites. Hence, the oxidation of MXene can be inhibited by encapsulation in NR, preventing easy MXene oxidation in the presence of water.

Li et al. ^[29][47] dispersed Ti₃C₂T_x into styrene–butadiene rubber (SBR) latex and prepared Ti₃C₂T_x/SBR composites by ordinary drying and freeze-drying. When the content of Ti₃C₂T_x was only two parts per hundred rubber (phr), the aggregation of Ti₃C₂T_x started to appear in the nanocomposites prepared by the ordinary drying method. Compared with ordinary drying method, freeze- drying method resulted in the dispersion of Ti₃C₂T_x thin sheets as monolayers in the nanocomposites with Ti₃C₂T_x of 2 and 4 phr. This is because rapid freezing of a Ti₃C₂T_x/SBR aqueous solution limits the restacking between Ti₃C₂T_x sheets, owing to hydrogen bonding and van der Waals forces. Consequently, Ti₃C₂T_x can be uniformly dispersed in the matrix. Aakyiir et al. ^[30][48] mixed a Ti₃C₂T_x suspension with butadiene–acrylonitrile rubber (NBR) latex and prepared MXene/NBR composites through flocculation. Infrared spectroscopy revealed that hydrogen bonds formed instantaneously between -OH on the surface of Ti₃C₂T_x and unsaturated carbon–carbon double bonds of NBR after mixing the two materials, indicating that Ti₃C₂T_x can be well-dispersed in the matrix.

For solution blending, an elastomer matrix is dissolved in an organic solvent, such as toluene, tetrahydrofuran, and N, N-dimethylformamide. Then, MXene in the solvent is ultrasonically dispersed and mixed with the matrix solution. Finally, MXene/elastomer nanocomposites are prepared by flocculation or casting. This method is simple and easy to perform, being suitable for some elastomers without latex. Lu et al. ^[31][49] adopted the solution blending method to prepare MXene/ethylene propylene diene monomer rubber (EPDM) composites using toluene as the solvent. This composite exhibited a low percolation threshold of 2.7 wt%. For a MXene content of 6%, the electrical conductivity of MXene/EPDM reached 106 S·m⁻¹, the EMI shielding effectiveness (SE) in the X-band reached 48 dB, and the thermal conductivity reached 1.57 W·m⁻¹·K⁻¹.

Many polar groups on the surface of MXene result in poor dispersion in weak polar or non-polar solvents. Consequently, composites prepared by solution blending show low performance. MXene is usually modified by approaches such as adding silane coupling agents, polyphenols, quaternary ammonium salts, surfactants, etc. ^[32][50], to enhance compatibility in solvents and elastomer matrices as well as dispersion. Qu et al. ^[33][46] modified polydopamine (PDA) on the surface of MXene nanosheets and then grafted KH550 to obtain MXene-PDA-KH550. Then, they prepared NBR-modified MXene composites by mixing NBR with modified MXene by solution blending, using tetrahydrofuran as the solvent. After modification with PDA and KH550, MXene was more easily combined with the NBR segment owing to the -NH₂ group on the surface, thus improving compatibility. Segment motion consumed more energy owing to the increase in internal friction between the modified MXene and NBR segments, and the loss factor (tanδ) increased to approximately 1.0, leading to an effective damping temperature range of approximately 40 °C, which was higher than that of 25 °C for NBR, indicating good damping performance.

For the backfill matrix method, a 3D MXene structure should be first constructed to form a conductive network. The network is then immersed in latex or a solution of elastomer, or the latex/solution is directly cast into a mold with the 3D MXene network to backfill the elastomer. The MXene/elastomer nanocomposite with a 3D network is prepared by drying, curing, and other steps. Owing to the good electrical conductivity and porosity of the prebuilt 3D MXene network, MXene/elastomer nanocomposites prepared by the backfill matrix method are commonly applied to EMI shielding, sensing, and other areas.

MXene dispersions can be prepared as light and low-density aerogels by freeze-drying and other methods, but the gelling ability of MXene is weak. Thus, graphene oxide ^[34][51], polyvinyl alcohol ^[35][52], alginate, or other solutions mixed with MXene are usually required to prepare porous aerogels to form a stable network through hydrogen bonding and electrostatic, gel, and ionic interactions ^[36][53]. Alternatively, MXene can be directly adsorbed on skeletons, such as cellulose scaffolds or nickel foams, to form a 3D network. Finally, the porous framework is impregnated into elastomers or latexes (e.g., PDMS) to increase the stability and durability of the structure.

MXene/elastomer nanocomposites can also be prepared by methods such as mechanical blending, melt blending ^[37][54], spin coating ^[38][55], and dip coating ^[39][40][56,57]. Mechanical blending is the most common blending method used to prepare composite materials. However, given the mechanical flexibility of MXene, the sheets are easily stacked through hydrogen bonds after drying. Compared with other nanofillers, MXene hardly blends with elastomers mechanically. Moreover, the intense heat generated by mechanical shearing force or heating conditions oxidizes MXene sheets.