Effect of multiwalled carbon nanotube diameter on mechanical behavior and fracture toughness of epoxy nanocomposites

Epoxy nanocomposites reinforced by multiwalled carbon nanotubes (MWCNT) with different diameters were prepared. The effect of MWCNT diameter on mechanical and fracture properties of epoxy nanocomposites were investigated. The results show that the MWCNT diameter has an important influence on its dispersion performance. As the MWCNT diameter increases, the dispersion performance becomes better. The reinforcing effect of MWCNTs on epoxy resins (EP) is affected by the combination of factors including the diameter and the additive amount of MWCNTs as well as its dispersion. At low additive amount of MWCNTs (≤0.3 wt%), the reinforcing effect of MWCNTs with smaller diameter is better. However, as the additive amount of MWCNTs increases (≥0.5 wt%), the reinforcing effect of MWCNTs with larger diameters becomes more significant. MWCNTs with diameter of 25 nm (MWCNT-25) have the best reinforcing effect when the additive amount is 0.5 wt%, and the tensile strength and elastic modulus of the MWCNT-25/EP composites are increased by 11.5% and 8.3% than those of EP, respectively. The fracture toughness of the composites shows a tendency to increase first and then decrease with the increase of MWCNT diameter. The main reason of this tendency is that the main failure mode of MWCNTs gradually changes from fracture to pull-out with the increase of MWCNT diameter. The optimal MWCNT diameter is 25 nm, reaching the best toughening effect (K_IC and G_IC are increased by 38.8% and 80.9% than those of EP, respectively) at the additive amount of 0.7 wt%. (paper)