Controlled nanostructure and high loading of single-walled carbon ...

Nanotechnology 18 (2007) 095708SWanget al(a)(b)Figure 9. SEM images of SWNTs/PC composite fracture surfaces: (a) 2 wt% SWNTs loading casting sample; (b) 20 wt% SWNT loadingbuckypaper composite sample.Table 1. Modulus of CNTs/PC composites.Tubes loading by weight Storage modulus ReferredApproaches (%) (GPa) literatureBuckypaper SWNT: 20 6.2 This studyimpregnationMelting mixing MWNT: 1.5–15 ∼1.0 [15]Solution casting SWNT: 0.05–0.25 1.6–2.1 [3]Functionalized 2.35–2.52 [16]SWNT: 0.5–2MWNT: 5–20 1.1–2.42 [17, 18]Figure 8 shows the storage modulus of a pristine polycarbonateis 1.8 GPa. Comparatively, the modulus of 2 wt% SWNT reinforcedpolycarbonate (produced by solution casting method)increased to 2.4 GPa, a factor of 1.3 increment, and the modulusof 20 wt% SWNT-buckypaper reinforced polycarbonatejumped to 6.2 GPa, a factor of 3.4 increment. Obviously,the controlled nanostructure and high loading contributed tothe improved mechanical performance in the thermoplasticnanocomposites. Table 1 lists the storage modulus of CNTsreinforced polycarbonate composites prepared from conventionalmethods in the reported literature. It can be seen that thebuckypaper/PC solution infiltration method achieved a highermechanical property.The fracture surfaces of the SWNT-buckypaper/PCcomposites with 20 wt% tube loading were characterizedwith SEM, as shown in figure 9. The dimpled morphologyin the fracture surface indicates that considerable level oflocalized plastic deformation occurred to prevent formationor coalescence of microvoids, which can potentially lead tocrack propagation. This type of dimple fracture suggests thatmechanical properties of SWNT-buckypaper/PC compositeswere enhanced due to the well-dispersed SWNT network.Theoretical modulus of PC/SWNTs composite wasestimated by the Cox–Krench model [11]. Computationresults showed that the elastic modulus of nanocompositeswas 3.7 GPa for 2 wt% tube loading and 64.25 GPa for20 wt% loading. These theoretical values are much higherthan the experimental measurements. Theoretical calculationmay overestimate the modulus by using individual nanotubemodulus rather than bundle modulus, which have a much lowermodulus [19]. It is very difficult to achieve the individual tubedispersion in the experiments. Another possible major reasonof this mismatch may arise from the weak interfacial bonding,a consequence of the smooth and chemically inactive surfaceof SWNTs. The weak interfacial bonding significantly reducedthe efficiency of the load transfer and resulted in low modulusin the experiments. In addition, the low experimental valuescould also stem from the short SWNT lengths, which were547 nm on average [20]. Therefore, even though this paperhas demonstrated an effective method to fabricate high-loadingthermoplastics nanocomposites with controlled nanostructure,much more efforts, such as nanotube functionalization, areneeded in order to realize the full potentials of carbonnanotubes in the composite applications.4. ConclusionsAn effective approach was presented to acquire good dispersionand high loading of nanotubes in the fabrication ofSWNTs/PC composites, which is very difficult using conventionalmanufacturing techniques. A controlled nanostructure ofSWNTs/PC composite can be achieved through impregnatingpreformed SWNT networks or buckypaper with PC solution.This method can also be extended to other suitable thermoplasticresins for high nanotube loading nanocomposites. Thecomparison between experimental value and theoretical predictionindicates that functionalization is necessary to improveload transfer efficiency. In addition, this method also makes itfeasible to fabricate aligned nanocomposites if buckypaper iscomposed of aligned nanotubes.6