Multi-walled carbon nanotubes acting as free radical scavengers in gamma-irradiated ultrahigh molecular weight polyethylene composites

Multi-walled carbon nanotubes (MWCNTs) were incorporated in ultrahigh molecular weight polyethylene (UHMWPE), which is a polymer used in industrial and orthopedic applications. The composites were prepared by ball milling and thermo-compression processes at concentrations up to 3 wt.% and subsequently gamma irradiated at 90 kGy. Electrical conductivity measurements showed a low percolation threshold of 0.5 wt.%. Electron spin resonance detection of the radiation-induced radicals proved the radical scavenger behavior of MWCNTs: when the nanotube concentration increased, the number of radicals generated by the gamma irradiation process decreased. Allyl radicals seem to be the radicals most affected by the presence of nanotubes in this polymeric matrix. Fourier transformed infrared spectroscopy measurements and an accelerated ageing protocol were performed to ascertain the influence of the irradiation on the oxidation index. The results pointed to the positive contribution of the MWCNTs in increasing the oxidative stability of the composite compared to pure UHMWPE. Crosslinking density induced by gamma irradiation was obtained by swelling measurements. The findings showed that, despite the radical scavenger performance, MWCNTs are capable of maintaining the efficiency of the crosslinking density, unlike the other antioxidants, which inhibit radiation crosslinking.