Antiretrogradation Functions of Carbon Nanotubes with Defect Graphene-like Structures for Rubber Materials in Their Composites
Carbon nanotubes (CNTs) have been expected to be effective as fillers in polymer matrices for their functionalization due to CNTsí outstanding electrical/mechanical properties and peculiar structures. However, because of the highly entangled agglomerates owing to strong van der Waals interaction, it is difficult to individually disperse CNTs in polymer matrices. In our previous study with respect to the fabrication of electrically conductive rubber-composite materials consisting of styrene-butadiene rubber (SBR) and CNTs, we demonstrated a novel mixing technique with the combination of ultrasonication and a rotation/revolution mixing without mechanical shear . As a result, it has been found that the percolation threshold of the composites produced with our novel mixing method was significantly low (< 1 phr, phr : parts per hundred rubber), compared to that (ca. 10 phr) of the composites prepared with a conventional Banbury mixer. In the present study, we have reported the antiretrogradation functions of multi-walled carbon nanotubes (MWNTs) incorporated into natural rubber (NR). FT-IR spectra revealed that the intensity of the peak corresponding to carbonyl group (-C=O) for the MWNT/NR composite remained unchanged even after heat treatment, which suggested that the degradation of NR was suppressed by the addition of MWNTs. However, the antiretrogradation ability of MWNTs was strongly dependent on the difference in structural characteristics of MWNTs, in which some MWNT species had no antiretrogradation ability. Furthermore, the inhibition effect of MWNTs on the generation of radical owing to mechanical stretching for the MWNT/NR composite films was measured by means of ESR equipped with our-own making tensile test machine. The ESR results implied that the rate of radical elimination was increased with the defect degree of grapheme-like structures in MWNTs. Consequently, the disordered structure of MWNTs was suggested to give rise to the antiretrogradation, since there exist considerably high charge densities around the defects such as pentagons, heptagons and holes involved in a high affinity for radicals (polymer radicals or peroxy radials) generated by the heat and mechanical-stretching treatments. The influence of the defects of MWNTs on percolation thresholds was also examined for the electrically conductive rubber-composite materials. The addition of MWNTs with a certain amount of the defects resulted in the percolation threshold with the lowest value, at the expense of the dispersion of MWNTs in rubber matrices. The foregoing results led us to the conclusion that a certain amount of defect of CNTs was significantly involved in the improvement of the properties of rubber/CNT composites.
 K. Tsuchiya et al., Compos. Sci. Technol. 71 (2011) 1098-1104