Carbon nanotubes are essentially one or more graphene sheets rolled seamlessly into cylindrical tubes, existing as single-walled or multi-walled variants depending on the number of concentric layers. Their intrinsic tensile strength reaches several gigapascals, their elastic modulus is extraordinarily high, their aspect ratio exceeds 1000, and their density is remarkably low. They are also biocompatible, easy to functionalize and capable of conducting electricity and heat along their axis. These attributes allow nanotubes to form percolating networks inside a rubber matrix at filler loadings far below those required by spherical fillers, which is precisely why they have attracted such intense scientific attention. The review documents how research in this field accelerated from 2004 onwards, reaching roughly 250 published articles in 2018 alone, a tenfold increase within a decade that reflects the growing industrial appetite for these multifunctional elastomeric composites.

The central technical challenge in translating nanotube properties into rubber performance is dispersion. Because of their enormous surface area and strong van der Waals attraction, nanotubes naturally cling together as bundles and ropes, forming agglomerates that act as stress concentration centres rather than reinforcement agents. Poor solubility in common solvents and weak interaction with the polymer matrix compound the problem. The review explains that surface functionalization is the primary strategy to overcome these barriers, broadly divided into covalent and non-covalent approaches, along with endohedral filling of the hollow tube interiors. Covalent functionalization forms chemical bonds between incoming groups and the carbon skeleton, dramatically improving wetting, adhesion and load transfer between filler and matrix. Acid oxidation using mixtures of nitric and sulfuric acid, particularly under ultrasonication, remains one of the most efficient routes, introducing carboxyl groups that can be further converted into amides, esters or thiols. However, covalent chemistry converts surface carbon atoms from sp2 to sp3 hybridization, disrupting the conjugated pi network and reducing electrical conductivity, a trade-off that must be carefully managed.

Non-covalent functionalization offers an elegant alternative that preserves the intrinsic attributes of the nanotubes. Here, functional molecules or polymers wrap physically around the tube surface through van der Waals forces, hydrogen bonding, electrostatic interactions, or pi-pi stacking with conjugated polymers or pyrene-bearing chains. Surfactants such as sodium dodecyl sulfate, sodium dodecylbenzene sulfonate and cetyltrimethylammonium bromide are widely used, and ionic liquids have emerged as particularly green dispersants. Fukushima and Aida pioneered the use of imidazolium-based ionic liquids, whose cationic part interacts with the pi-electronic surface of the nanotube through cation-pi interactions, achieving dispersion without organic solvents. Because non-covalent modification leaves the sp2 network intact, the resulting composites retain the electrical conductivity and mechanical stability that make nanotubes so valuable in the first place.

Fabrication methods for CNT/rubber nanocomposites are equally diverse, and the review provides a detailed technical account of each. Solution mixing disperses nanotubes in a suitable solvent, often assisted by ultrasonication to break agglomerates, before gradually introducing the mixture into the elastomer. Gharib and colleagues introduced an innovative ternary system using electron-deficient acceptors and the green biosolvent dihydrolevoglucosenone to disrupt inter-tube pi-pi interactions, achieving debundling, dispersion and stability while reducing energy input. Mechanical mixing, typically with a two-roll mill or twin-screw extruder, remains the industrially preferred route. Das and co-workers developed a novel approach in which nanotubes pre-dispersed in ethanol were mixed with a 50:50 blend of solution-styrene-butadiene rubber and butadiene rubber at elevated temperature; the ethanol vaporises during mixing, impregnating the nanotubes uniformly into the matrix and producing significantly enhanced properties at very low loadings. In-situ polymerization disperses nanotubes directly into the monomer before curing, allowing higher loadings and stronger covalent interaction with the rubber, while latex stage compounding mixes stabilized aqueous nanotube dispersions with rubber lattices, achieving percolation thresholds as low as 1.5 parts per hundred rubber in natural rubber systems.

The interface between nanotube and rubber is where the magic of reinforcement actually happens, and the review devotes substantial attention to its modification. The interphase, a distinct region with altered polymer chain mobility and chemistry compared to the bulk matrix, can extend up to approximately 500 nanometres depending on filler dimensions. Silane coupling agents such as bis(3-triethoxysilylpropyl) tetrasulfide form chemical bridges between modified nanotubes and the rubber, while plasma treatment, layer-by-layer assembly of oppositely charged polyelectrolytes and the inclusion of secondary nanoscale fillers all enhance compatibility. In natural rubber, interfacial modification boosts tensile strength, tear resistance and modulus; in nitrile rubber it improves oil resistance; and in styrene-butadiene rubber it enhances abrasion resistance, making these materials attractive for tire manufacturing. Beyond performance, optimized interfaces extend product lifespan and reduce material waste, aligning the technology with global sustainability demands.

Mechanical property enhancement in these nanocomposites follows well-defined reinforcement mechanisms. Load transfer from matrix to nanotube is the fundamental process, enabled by strong interfacial bonding and high aspect ratio. Below the percolation threshold, nanotubes remain isolated and reinforcement is modest; above it, a connected network forms and tensile strength and modulus rise sharply. Alignment of nanotubes through mechanical stirring, magnetic or electric fields creates efficient load-bearing pathways, and proper dispersion maximizes the interphase volume. The review consolidates striking experimental results: ball-milled nanotubes raised natural rubber tensile strength from 7.2 to 25.5 megapascals at 25 parts per hundred rubber, ionic-liquid-modified nanotubes lifted styrene-butadiene rubber tensile strength from 1.63 to 6.80 megapascals at only 10 parts per hundred, and polydopamine-coated multi-walled nanotubes increased natural rubber tensile strength to 28.6 megapascals while preserving elongation at break. Fatigue resistance also improves dramatically through load transfer, energy dissipation and crack bridging, critical for flexible electronics and structural components subjected to cyclic deformation.

Electrical properties represent perhaps the most transformative aspect of these composites. Quantum mechanical tunnelling of electrons between neighbouring nanotubes, separated by thin polymer layers, governs conductivity below percolation, while above the threshold the composite behaves as an Ohmic conductor. Matching solubility parameters between nanotube and rubber lowers the percolation threshold and raises conductivity, with fluorinated rubber showing particularly good compatibility. Remarkably, a volume fraction of just 0.13 of nanotubes can push dielectric permittivity to 9000, roughly 2400 times that of the pristine elastomer, driven by interfacial polarization. Modifications push performance further: CTAB-functionalized nanotubes in natural rubber produce flexible materials with high electromagnetic interference shielding stability, chitosan-modified nanotubes reinforce styrene-butadiene rubber with superior conductivity, and incorporating eutectic gallium-indium liquid metal improved conductivity by 328 percent at 10 volume percent without sacrificing mechanical properties.

Thermal stability gains are equally impressive and technically grounded. Nanotube surfaces scavenge radical species, restricting the chain reaction of polymer degradation. Shimizu and colleagues demonstrated that well-dispersed nanotubes at only 0.5 weight percent, achieved through wet jet milling, allowed silicone rubber composites to maintain rubbery elasticity after seven days at 280 degrees Celsius, whereas virgin silicone rubber became embrittled after just one day at that temperature. Acid-treated nanotubes impart high thermal stability to natural rubber through improved dispersion and interfacial interaction, and ionic-liquid-modified nanotubes enhanced polychloroprene rubber degradation resistance. Thermal conductivity also rises substantially with loading because of the high charge transport rate through nanotube channels, and applying an AC electric field to align nanotubes in natural rubber produced composites with higher thermal conductivity than randomly oriented systems, exploiting the anisotropy of heat conduction along the nanotube axis.

Applications of CNT/rubber nanocomposites now span an extraordinary range. Stretchable strain sensors based on polydimethylsiloxane or Ecoflex track human body movements for healthcare monitoring, with SEBS-based systems achieving sensing ranges near 540 percent and gauge factors above 5. Porous fluorine rubber-nanotube composites withstand punch pressures of minus 120 kilopascals for force detection, while isoprene rubber composites serve as flexible oil spillage sensors. In biomedicine, natural rubber reinforced with multi-walled nanotubes has been developed as a prosthetic foot material with water absorption near 0.1 percent and exceptional wear resistance, while carboxylated nitrile rubber nanocomposites exhibit shape memory behaviour useful in drug delivery, orthopaedics and regenerative medicine. Stretchable acrylate rubber electrodes for supercapacitors achieve 155 percent stretchability and 9.6 siemens per centimetre conductivity. Electromagnetic interference shielding represents another frontier, with styrene-butadiene rubber composites reaching 35.06 decibels at 18 gigahertz and natural rubber-chlorobutyl blends achieving 59 decibels, while natural rubber latex nanocomposites show promising gamma-ray attenuation. Tires incorporating nanotubes exhibit improved wear resistance, fuel economy and traction without notably altering elasticity, and aerospace sealing applications exploit their thermal and electrical stability. The review concludes that while homogeneous dispersion remains the key challenge, continued innovation in functionalization and fabrication methods is rapidly expanding the unexplored potential of these remarkable materials across electronics, sustainable biomedicine and shielding technologies.

Subject of Research: Carbon nanotube filled rubber nanocomposites

Article Title: Carbon nanotube filled rubber nanocomposites

Article References: K. C, N., Abraham, J., Thomas, M. G., Vahabi, H., Maria, H. J., & Thomas, S. (2024). Carbon nanotube filled rubber nanocomposites. Frontiers in Carbon, 3, Article 1339418. https://doi.org/10.3389/frcrb.2024.1339418

Image Credits: AI Generated

DOI: 10.3389/frcrb.2024.1339418

Keywords: carbon nanotubes, rubber nanocomposites, elastomers, functionalization, electrical properties, EMI shielding, biomedical applications, Frontiers in Carbon, Carbon, nanotube, filled, rubber