Carbon nanotubes (CNTs) possess exceptional mechanical, electrical, and thermal properties, making them ideal reinforcements for polymer composites. However, their inherent tendency to agglomerate due to strong van der Waals forces limits their effectiveness. Functionalization—either covalent or non-covalent—addresses this challenge by improving CNT dispersion within polymer matrices. This article explores covalent and non-covalent functionalization methods, dispersion techniques, and their impact on the mechanical and electrical properties of CNT-polymer composites.

Covalent functionalization involves chemically modifying the CNT surface by attaching functional groups, which enhances compatibility with the polymer matrix. One of the most common covalent methods is carboxylation, where carboxylic acid groups (-COOH) are introduced onto the CNT surface through oxidative treatments using strong acids like nitric acid or sulfuric acid. This process not only improves dispersion but also provides reactive sites for further chemical grafting. For example, carboxylated CNTs can form ester or amide bonds with polymers containing hydroxyl or amine groups, respectively, leading to stronger interfacial adhesion. However, covalent functionalization can disrupt the sp² hybridization of CNTs, potentially reducing their intrinsic electrical conductivity. Despite this trade-off, studies show that carboxylation can enhance the tensile strength of epoxy-CNT composites by up to 40% at optimal loading levels, typically between 0.5% and 2% by weight.

Non-covalent functionalization relies on physical interactions such as π-π stacking, van der Waals forces, or electrostatic attraction to modify CNT surfaces without altering their chemical structure. Surfactants like sodium dodecyl sulfate (SDS) or polymers such as polyvinylpyrrolidone (PVP) adsorb onto CNTs, creating steric or electrostatic repulsion that prevents agglomeration. This method preserves the CNTs' electronic properties while improving dispersion. For instance, non-covalent functionalization with PVP has been shown to maintain the electrical conductivity of CNT-polyethylene composites, with percolation thresholds as low as 0.1% by weight. However, non-covalent interactions are generally weaker than covalent bonds, which may limit stress transfer efficiency in mechanically loaded composites.

Effective dispersion of CNTs in polymer matrices is critical for maximizing composite performance. Common techniques include sonication, shear mixing, and calendering. Sonication uses ultrasonic energy to break apart CNT bundles, while shear mixing applies high mechanical shear forces to separate individual tubes. Calendering, a roll-based process, further enhances dispersion by subjecting the composite to intense compressive and shear stresses. Studies indicate that combining sonication with calendering can reduce CNT agglomerate size from several micrometers to below 100 nm, significantly improving mechanical reinforcement. For example, polypropylene-CNT composites processed with dual sonication and calendering exhibit a 50% increase in Young's modulus compared to those prepared with sonication alone.

The mechanical properties of CNT-polymer composites benefit from both functionalization and dispersion. Covalent functionalization strengthens the CNT-polymer interface through chemical bonding, enhancing load transfer. Tensile tests reveal that epoxy composites with carboxylated CNTs achieve a 30-50% improvement in modulus and strength compared to untreated CNTs. Non-covalent functionalization, while less effective in load transfer, still improves toughness by promoting uniform CNT distribution. For instance, polyamide-6 composites with surfactant-wrapped CNTs show a 20% increase in impact resistance due to reduced stress concentrations around well-dispersed nanotubes.

Electrical conductivity in CNT-polymer composites depends on the formation of a percolating network, where CNTs are sufficiently close to allow electron tunneling or direct contact. Functionalization influences this network by altering CNT spacing and interfacial resistance. Covalent functionalization typically raises the percolation threshold due to disrupted electron transport along the CNT surface. In contrast, non-covalent methods maintain high conductivity by preserving the CNT structure. For example, polystyrene composites with non-covalently functionalized CNTs achieve conductivity of 1 S/m at 1% loading, while covalently functionalized counterparts require 3% loading for similar performance.

Thermal properties also improve with CNT incorporation. Functionalized CNTs enhance interfacial thermal conductance by reducing phonon scattering at the CNT-polymer boundary. Measurements show that polyimide composites with carboxylated CNTs exhibit a 70% increase in thermal conductivity at 5% loading, attributed to improved phonon transfer across chemically bonded interfaces.

Challenges remain in optimizing functionalization for specific applications. Excessive covalent modification can degrade CNT properties, while insufficient non-covalent stabilization may lead to re-agglomeration during processing. Future research focuses on balancing functionalization intensity to achieve optimal dispersion without compromising intrinsic CNT advantages.

In summary, covalent and non-covalent CNT functionalization techniques play pivotal roles in developing high-performance polymer composites. Covalent methods enhance interfacial adhesion and mechanical properties, while non-covalent approaches preserve electrical conductivity. Combined with advanced dispersion techniques, these strategies enable tailored composite designs for applications requiring superior strength, conductivity, or thermal management. The continued refinement of functionalization protocols promises further advancements in CNT-polymer composite technology.