Carbon nanotubes (CNTs) are cylindrical allotropes of carbon, which can be classified into single-wall carbon nanotubes (SWNTs), double-wall carbon nanotubes (DWMTs) and multi-wall carbon nanotubes (MWNTs) according to the number of graphite cylinders. Given the superior physical and chemical properties of CNTs, many researchers have focused on finding the promising applications for them since their discovery in 1991. The use of CNTs as reinforcements for polymer composites is one of the suggested applications and has received considerable attention, in respect that they can greatly enhance mechanical, electrical, thermal and flame retardant properties of the polymer materials.
Recent research found that CNTs can also affect the thermal oxidative stability of polymers. However, the influence of wall number and surface functionalization of CNT on its antioxidant ability and its antioxidant mechanism in polymer matrix has not been well-documented.
In order to figure out the above issues, the antioxidant effect of SWNT, MWNT, and hydroxylated multi-walled carbon nanotube (MWNT-OH) on high density polyethylene (HDPE) are studied. Based on the measurement of the oxidation induction temperature (OIT) and oxidation induction time (OIt) of HDPE nanocomposites, it is found that the antioxidant efficiencies of the three CNTs are in the following order: MWNT-OH > MWNT > SWNT. The antioxidant ability and mechanism of CNTs are further illustrated by electron spin resonance (ESR) spectra and Raman spectra. It is observed that the antioxidant behaviors of CNTs in polyolefin follow free radical scavenging mechanism. The order of the defect concentration and the radical scavenging efficiency for CNTs are in good agreement with the order of their antioxidant efficiencies in HDPE. It suggests that the lattice defects in CNTs which caused acceptor-like localized states are responsible for their radical scavenging ability as well as antioxidant ability in HDPE.