# Preparation of Piezo-Resistive Materials by Combination of PP, SEBS and Graphene
## 3 Results and Discussion
### Resistivity Measurements  
The conductivity of the nanocomposite films was measured by the 4-point probe method and the values for each sample are listed in Table 3. The highest conductivity values were obtained for the sample prepared with the G filler, reaching 5.2 \(\times\)\(10^{-3}\) S/cm and 6.8 \(\times\)\(10^{-3}\) S/cm for R1 and R2, respectively. The samples prepared with the GPP filler are slightly less conductive with respect to the graphene counterpart. This is expected as a consequence of the grafting of an insulating moiety and the introduction of new defects to the sp\({}^{2}\) network of graphene. A substantial reduction of conductivities has been previously measured for GPP pellets when compared to the pristine G [10]. Additionally, the constant conductivity value of iPP/SEBS/GPP for both routes can be attributed to the good polymer/filler interface established, independent of the formulation route, as demonstrated by SEM. On the other hand, it is worth noting that the conductivity values obtained for all samples in this study were higher than values previously reported for nanocomposites with comparable filler contents: PP/SEBS/G-nanoplatelet (GN) (\(\sim\)\(10^{-5}\) S/cm, 5 wt.% GN) [12] prepared through melt compounding, iPP/G (8.7 \(\times\)\(10^{-3}\) S/cm, 6.1 wt.% G), or iPP/GPP (1.2 \(\times\)\(10^{-3}\) S/cm, 4.6 wt.% G) [4] prepared via SoM and melt compounding.  
Furthermore, the electrical resistance of these samples was observed to vary upon bending the composite films (Figure 5). For example, the resistance for a iPP/SEBS/G film (thickness of 0.4 mm) increased with increasing degree of flexion, and returned to the original value upon removal of the external stress, even after repeated cycles. This observation can be explained by pulling apart  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Method** & **Sample** & **E (MPa)** & \(\epsilon_{\text{b}}\) **(\%)** & \(\sigma\) **(MPa)** & **T (MJ-m\({}^{-3}\))** &
\begin{tabular}{c} **SEBS Mean** \\ **Area (\(\mu\)m\({}^{2}\))** \\ \end{tabular} \\ \hline \multirow{4}{*}{R1} & iPP \({}^{*}\) & 660 \(\pm\) 30 & 11.5 \(\pm\) 1.0 & 24.0 \(\pm\) 7.0 & 1.3 \(\pm\) 0.2 & \\ \cline{2-6}  & iPP/SEBS & 652 \(\pm\) 31 & 450.1 \(\pm\) 28.0 & 21.7 \(\pm\) 1.7 & 97.3 \(\pm\) 6.4 & 1.20 \(\pm\) 0.20 \\  & iPP/SEBS/G & 693 \(\pm\) 7 & 5.3 \(\pm\) 1.3 & 23.1 \(\pm\) 1.5 & 0.9 \(\pm\) 0.1 & 0.20 \(\pm\) 0.03 \\  & iPP/SEBS/GPP & 701 \(\pm\) 22 & 4.2 \(\pm\) 0.5 & 20.9 \(\pm\) 1.0 & 0.6 \(\pm\) 0.1 & 0.25 \(\pm\) 0.04 \\ \hline \multirow{4}{*}{R2} & iPP/SEBS & 622 \(\pm\) 27 & 55.0 \(\pm\) 8.0 & 21.0 \(\pm\) 0.4 & 11.1 \(\pm\) 1.1 & 1.50 \(\pm\) 0.30 \\  & iPP/SEBS/G & 690 \(\pm\) 9 & 17.2 \(\pm\) 2.5 & 26.6 \(\pm\) 0.6 & 4.0 \(\pm\) 0.5 & 0.20 \(\pm\) 0.03 \\ \cline{1-1}  & iPP/SEBS/GPP & 704 \(\pm\) 22 & 10.0 \(\pm\) 2.3 & 23.6 \(\pm\) 0.8 & 1.9 \(\pm\) 0.4 & 0.31 \(\pm\) 0.05 \\ \hline \hline \end{tabular}  
* E: Young’s modulus, \(\epsilon_{\text{b}}\): Elongation at break, \(\sigma\) tensile strength, T: Toughens. \({}^{*}\) Data from previous studies [10].  
\end{table}
Table 2: Mechanical properties of iPP and composites (iPP/SEBS/G and iPP/SEBS/GPP) determined from tensile tests and SEM area analysis.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **SoM** & **SoM/SSM** \\ \hline iPP/SEBS/G & 5.2 \(\times\)\(10^{-3}\) & 6.8 \(\times\)\(10^{-3}\) \\ iPP/SEBS/GPP & 1.9 \(\times\)\(10^{-3}\) & 1.9 \(\times\)\(10^{-3}\) \\ \hline \hline \end{tabular}  
\end{table}
Table 3: Electrical conductivities (in S/cm) of iPP/SEBS/G and iPP/SEBS/GPP calculated from the 4-point probe tests.  
the graphene sheets while applying the external force, thus weakening their contacts and thereby increasing the resistance.