High-performance nanotube-reinforced plastics: Understanding the mechanism of strength increase

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 x 3.7, x 4.3, and x 1.7, respectively were observed for the PVA-based materials at less than I wt.-% nanotubes. Similarily for the polypropylene-based composites, increases in Young's modulus, tensile strength and toughness of x 3.1, x 3.9, and x 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.