## 3 Results and Discussion
### Performance of Tensile Properties  
The mechanical properties of polymer composites are highly correlated with the intrinsic properties, amount, dispersion of the fillers, the properties of the polymer matrix, and the interaction between fillers and polymer matrix. Figure 5 and Table 3 indicate the significant influence of the loading level of GNs over the mechanical properties of PP/GNs conductive composites and show that it is inversely proportional to the tensile strength but proportional to the tensile modulus. Compared to the mechanical properties of pure PP, the PP/GNs conductive composites containing 20 wt% of GNs have a 34% lower tensile strength (from 33.74 MPa to 22.28 MPa) and a 150% greater tensile modulus (from 1621 MPa _vs._ 4037 MPa). Figure 4: Optical microscopic images (500\(\times\)) of PP/CF composites at (**a**) 200 \({}^{\circ}\)C and (**b–d**) near 130 \({}^{\circ}\)C.  
Figure 3: Optical microscopic images (500\(\times\)) of PP/GNs composites at (**a**) 200 \({}^{\circ}\)C and (**b–d**) near 130 \({}^{\circ}\)C.  
In the tensile test, the fracture mode of PP is ductile fracture, which is quite different from that of PP/GNs conductive composites, which have an obvious yield phenomenon before fracture. Furthermore, when containing more content of GNs, PP/GNs conductive composites exhibit a classical brittle fracture, namely they do not exhibit a significant yield phenomenon before fracture. Additionally, their elongation at break is considerably low [30,31] as indicated in Figure 7a. GNs create a discontinuity of PP in PP/GNs conductive composites, and at the same time create a stress concentration phenomenon. When the composites are posed with load, the initial cracking occurs and then becomes worse where the stress concentration occurs, and eventually causes structural failure. Although serving as the dispersed phase, GNs are actually isolated from the PP matrix in the PP continuous phase, and resemble a sea-island structure. An increasing content of GNs inevitably decreases the force bearing area of matrix, thereby decreasing the tensile strength of the composites in comparison to that of pure PP. In addition, the conductivity of GNs stems from their own sp\({}^{2}\) hybridization structure; therefore, adding functional groups to GNs certainly destroys such a structure and then decreases GNs' initial conductivity. This study does not functionally modify GNs as it aims to develop polymer conductive composites with better conductivity. The modulus of the polymer composites can be augmented by adding fillers that have a high modulus to polymers that have a lower modulus, regardless of the interaction between them. Such a result is ascribed to a complex explanation, which is beyond any complete theories but it can be simply put that the fillers possess a restrictive effect over the polymers by restricting the motion and deformation of their molecular chains. Figure 6 and Table 3 show that the increasing content of CF significantly influences the mechanical properties of the PP/CF conductive composites with their tensile strength first decreasing then increasing and the tensile modulus constantly increasing. Compared to the tensile strength (33.74 MPa) and modulus (1621 MPa) of pure PP, the combination of 20 wt% of CF causes the tensile strength of the PP/CF conductive composites to decrease by 8.3% to 30.94 MPa and increases their tensile modulus by 159% to 4207 MPa. The combination of CF has a similar augmentation of the modulus to the combination of GNs. The PP/CF conductive composites do not exhibit an obvious yield phenomenon before fracture (_i.e._, classical brittle fracture), and also have a remarkably low elongation at break [30,31], as indicated in Figure 7b. However, this elongation is still relatively greater than that of PP/GNs conductive composites. The increase in tensile modulus and the decrease in tensile strength of PP/CF conductive composites are caused by the same factors as that for PP/GNs conductive composites.