Polymer–multiwalled carbon nanotube composite films were fabricated using two types of polymer matrices, namely poly(vinyl alcohol), (PVA) and chlorinated polypropylene. In the first case, the PVA was observed to form a crystalline coating around the nanotubes, maximising interfacial stress transfer. In the second case the interface was engineered by covalently attaching chlorinated polypropylene chains to the nanotubes, again resulting in large stress transfer. Increases in Young's modulus, tensile strength, and toughness of × 3.7, × 4.3, and × 1.7, respectively were observed for the PVA‐based materials at less than 1 wt.‐% nanotubes. Similarily for the polypropylene‐based composites, increases in Young's modulus, tensile strength and toughness of × 3.1, × 3.9, and × 4.4, respectively, were observed at equivalent nanotube loading levels. In addition, a model to describe composite strength was derived. This model shows that the tensile strength increases in proportion to the thickness of the interface region. This suggests that composite strength can be optimized by maximising the thickness of the crystalline coating or the thickness of the interfacial volume partially occupied by functional groups.

Tensile tests were carried out on free-standing composite films of poly(vinyl alcohol) and six different types of carbon nanotubes for different nanotube loading levels. Significant increases in Young's modulus by up to a factor of 2 were observed in all cases. Theories such as the rule-of-mixtures or the Halpin-Tsai-theory could not explain the relative differences between composites made from different tube types. However, it is possible to show that the reinforcement scales linearly with the total nanotube surface area in the films, indicating that low diameter multiwall nanotubes are the best tube type for reinforcement. In addition, in all cases crystalline coatings around the nanotubes were detected by calorimetry, suggesting comparible polymer−nanotube interfaces. Thus, the reinforcement appears to be critically dependent on the polymer−nanotube interfacial interaction as previously suggested.

Poly(methyl methacrylate) (PMMA)‐functionalized multiwalled carbon nanotubes are prepared by in situ polymerization. Infrared absorbance studies reveal covalent bonding between polymer strands and the nanotubes. These treated nanotubes are blended with pure PMMA in solution before drop‐casting to form composite films. Increases in Young's modulus, breaking strength, ultimate tensile strength, and toughness of ×1.9, ×4.7, ×4.6, and ×13.7, respectively, are observed on the addition of less than 0.5 wt % of nanotubes. Effective reinforcement is only observed up to a nanotube content of approximately 0.1 vol %. Above this volume fraction, all mechanical parameters tend to fall off, probably due to nanotube aggregation. In addition, scanning electron microscopy (SEM) studies of composite fracture surfaces show a polymer layer coating the nanotubes after film breakage. The fact that the polymer and not the interface fails suggests that functionalization results in an extremely high polymer/nanotube interfacial shear strength.