## 3 Results and Discussion
### Mechanical Properties of NR Composites Using Different Cross-Linking Agents  
#### 3.3.1 Tensile Properties  
The mechanical performance of NR is highly dependent on (i) the shape and dispersion of the filler throughout the material, (ii) the dispersion of the cross-linking agent in the polymer chains that promotes the cross-linking, and (iii) the nature and degree of the resulting cross-linking network. Composite plates were prepared and vulcanized by compression molding using the vulcanization temperatures and _t.v.,_ listed in Table 4, with  
Figure 4: SEM images of NR-DCP-SWCNTs composite at different magnifications. (**a**) Distribution of SWCNTs into NR matrix, (**b**) red arrows show individualized and dispersed SWCNTs, (**c**) the red circle shows well-embedded SWCNTs in the matrix. Figure 3: HR-TEM images of SWCNTs. (**a**) Bright-field TEM image showing the inner and outer diameter size of SWCNTs, (**b**) HR-TEM of dispersed SWCNTs. The inset in (**b**) shows the diameter size distribution of SWCNTs. the purpose to evaluate the effect of CB, SWCNTs, and the cross-linking reaction on the mechanical properties of the NR composites with different vulcanizing agents. Figure 5 illustrates the tensile stress-strain curves obtained from the NR composites vulcanized with sulfur, organic peroxides, and CB or SWCNTs as fillers. Table 5 summarizes the values of tensile strength, strain at break, and modulus (at 100 and 300% strain) measured for each NR formulation. As observed in Figure 5a, tensile stress-strain curves for the NR composites vulcanized with sulfur exhibit the highest values of tensile strength and strain at breakage (up to 800%) in comparison with the tensile curves of the NR composites vulcanized with organic peroxides (Figure 5b-d). When SWCNTs are added to NR formulations, significant increases in tensile strength, 71, 75, and 40%, are observed when comparing with the neat NR composites vulcanized with sulfur, T29, and DCP, respectively. However, in all the cases the reinforcing effect produced by SWCNTs leads to a decrease in the strain at breakage up to 37% (for example, in the NR-DCP-SWCNTs composite) besides changes in the behavior of the curves showing steeper slopes, which are clearly attributed to the stiffening of the NR matrix. Despite this, the increase in tensile strength suggests an adequate dispersion and distribution of SWCNTs and good interfacial adhesion between SWCNTs and the elastomer matrix, as shown in the SEM images [20]. As for the NR-CB composites, the presence of CB particles does not produce increases in tensile strength of the NR composites vulcanized with different agents; nevertheless, decreases in the strain at breakage and steeper slopes in the curves compared to the NR composites without fillers can be observed, although, to a lesser extent than with SWCNTs, it is also related to the increase in the mechanical stiffness of the NR composites, as described below. Figure 5: Stress–strain curves of the NR composites vulcanized with different cross-linking agents: (a) sulfur, (b) T29, (c) DCP, (d) T145 and incorporating CB or SWCNTs as fillers. Figure 6 shows the comparison between the relative increments in modulus at 100 and 300% strain for all the NR composites samples with different vulcanization agents and incorporating CB or SWCNTs. Relative increments in modulus for each set of samples were calculated based on the modulus value obtained in the neat NR composite (NR-S, NR-T29, NR-DCP, or NR-T145). Remarkable increases in modulus for the NR-SWCNTs composites are observed when compared with the neat NR or NR-CB composites. For the NR-S composites with SWCNTs, increments of 26% (at 100% strain) and 62% (at 300% strain) compared to the neat NR-S sample are found.