## 3 Results and Discussion
### Raman Spectroscopy of NR-SWCNTs Composites  
Raman spectroscopy has been used as an efficient method to study the effect of SWCNTs on NR in composites [17; 18]. Figure 7 shows the Raman spectrum of the pristine SWCNTs employed as filler in the NR composites. As observed, the characteristic absorption bands of SWCNTs comprise the radial breathing mode (RBM) at low frequencies in the range 100-250 cm\({}^{-1}\), the tangential G-band at 1585 cm\({}^{-1}\), and several weak features such as the D-band at 1310 cm\({}^{-1}\) related to defects in the nanotube structure [42; 43] and the M-band near 1750 cm\({}^{-1}\) (an overtone of the out-of-plane infrared-active mode in graphite) [44]. The presence of such absorption bands matches the semiconductor nature of SWCNTs. Additional evidence of the semiconductor nature of the SWCNTs employed in the present work is the distortion of the line shape of the G-band (inset in Figure 7) attributed to the \(G^{+}\) and \(G^{-}\) features. The first feature (\(G^{+}\)) is associated with carbon atom vibrations along the nanotube axis (longitudinal optic phonon mode, which appears at 1585 cm\({}^{-1}\)) and the second \(G^{-}\) feature (at 1556 cm\({}^{-1}\)), which is sensitive to charge transfer from dopant additions to SWCNTs [43; 45]. The G'-band is considered a second-order feature and gives information about the electronic structure of the semiconductor or metallic nature of SWCNTs. Figure 7: Raman spectrum of pristine SWCNTs (semiconducting). The inset shows the deconvolution of the G-band into gaussian components performed to show the weak double resonance features on G-band associated with the M-band. Here, the blue solid line corresponds to experimental data, the dashed line to the curve fitting, and the red and green solid lines indicate the computed fitting curves. Figure 8 displays the G- and G'-band Raman spectra of the NR-SWCNTs composites formulated with different cross-linking agents compared with the Raman spectrum of the pristine SWCNTs. As shown in Figure 8a, the intensity of the G-band for pristine SWCNTs monotonically decreases when reacting with the different cross-linking promoters; such tendency seems to be more evident in the NR-SWCNTs composites vulcanized with organic peroxides. Another important observation is that the \(G^{-}\) feature of SWCNTs completely disappears once the vulcanization process with each cross-linker occurs, which can reflect the hardening of the C-C bonds as the electronic density decreases [46], probably due to the reaction between the free radicals formed during the vulcanization process with organic peroxides and the wall surface of the SWCNTs. Additional data to support such assertions can be obtained by analyzing the D-band (Figure 7); the absorption at 1310 cm\({}^{-1}\) corresponds to disordered graphite structures. The D-band is activated through the first-order excitation process of sp\({}^{2}\) carbons by the presence of in-plane-substituted heteroatoms, vacancies, and grain boundaries, and provides unique information on the chirality and diameter dependence [43]. This second-order feature can be used for materials characterization to probe and monitor structural modifications of the nanotube sidewalls that come from the introduction of defects and the attachment of different chemical species. It has been accepted that the sample purity or the modification degree of the nanotube sidewall can be obtained by calculating the D/G band intensity ratio (\(I_{D}/I_{G}\)) from the Raman spectra of SWCNT composites [47]; thus, the intensity ratios of the NR-SWCNTs composites with different vulcanization agents were calculated and listed in Table 7. For the vulcanized composites, increments in the \(I_{D}/I_{G}\) ratio due to increases in the D-band intensity are obtained. This result indicates a sidewall modification in the SWCNTs due to the reaction with the free radicals generated during the vulcanization process; such an interaction introduces defects and probably attaches reactive chemical species to the nanotube walls.