Antioxidant Properties of Multiwall Carbon Nanotubes: First Measurements Using a Model Oxidative Reaction

Carbon nanotubes (CNTs) have electron affinities similar to those of fullerenes and they are therefore capable of acting as radical traps in free radical chain reactions, including polymerisation and the thermo-oxidative degradation of polymers. It is proposed that the CNTs used as integral part of polymer composites are able to interrupt chain propagation, leading to an antioxidant effect in these materials. This is because of their radical accepting capacity. To support this postulate, the antioxidative activity of commercial carbon nanotubes (denoted by CNT-MW(1b)) has been studied by means of a model reaction, namely the oxidation of cumene initiated (2,2'-azobisisobutyronitrile, AIBN) in the liquid phase. This reaction was designed to simulate the thermo-oxidative processes in carbon-chain polymers and it allows for the comparison and transfer of results to a polymer system. Measurements of oxidation rates showed that the inhibition of the model oxidative reaction in the presence of the original CNT-MW (1b) is very weak over the concentrations range up to 0.05 wt.%. Increasing the CNT-MW (1b) concentration above 0.05 wt. % facilitates catalytical action of the metal (Co) originally contained in the commercial CNT-MW (1b). In general, the rate of oxidation Wo ₂ depends on two competing reaction rates. These are the rate of inhibition W _inh.(CNT) caused by the structure of the CNT and the rate of initiation W _i(Co) due to the interaction ROOH + M (Co) i.e. Wo ₂ ~ W _{inh (CNT)} + W _i(Co) . Nevertheless, experimental explorations under the condition W _{inh (CNT)} >> W _i(Co) allow us to assess the inhibiting parameters of the CNT- MW. The effective rate constant for the addition of cumyl R· radicals to CNT-MW (1b) was determined to be k [CNT] _{(333–353 K)} = (4.0 ± 2.0)·10 ³ s ⁻¹ . This is an order of magnitude smaller than the constant for their addition to fullerene C ₆₀ . The model kinetic data obtained specify the limits of rational use of the carbon nanotubes in polymer composites.