## 3 Results
### Mechanical Strength of PS-CNWs Nanocomposites  
The mechanical properties of the pristine PS and the corresponding nanocomposites were evaluated with tensile measurements and representative stress-strain curves are presented schematically in Figure 9, while the average values of modulus of elasticity (E), tensile strength (\(\sigma_{\text{TS}}\)) and strain at break % (\(\varepsilon_{\text{b}}\)) are depicted in Table 1.  
The representative curves (Figure 9) for the non-modified polymer, as well as for its nanocomposites, are typical of brittle materials, since almost no plastic deformation is observed in all cases. According to these curves, the general trend is an increase in the stiffness and strength and a respective reduction of the strain at the break with nano-reinforcement. As observed, specimens with 1 wt% and 3 wt% CNW behave similarly, while a further increase in CNW content results in lower mechanical enhancement. Based on the values that are presented in Table 1, an up to 13% increase is observed in the stiffness of the samples, the strain at break is reduced by up to approximately 17%, while the tensile strength is fluctuating by the CNWs addition into the polymer.  
These trends are better illustrated in the graphs of Figure 10a-c that present the modulus of elasticity (Figure 10a), the tensile strength (Figure 10b), and the elongation at break (Figure 10c) as a function of the CNW weight content. Based on the three graphs, it can be deduced that the best results are obtained after the addition of 3 wt% nanowires.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Concentration** & **Stiffness** & **Tensile Strength** & **Distortion to** \\  & **of CNWs (\%)** & **(MPa)** & **(MPa)** & **Breakage (\%)** \\ \hline PS & 0 & 3349 \(\pm\) 107 & 79.87 \(\pm\) 11.02 & 2.86 \(\pm\) 0.19 \\ PS-CNWs-1\% & 1 & 3557 \(\pm\) 278 & 81.90 \(\pm\) 6.23 & 2.56 \(\pm\) 0.21 \\ PS-CNWs-3\% & 3 & 3792 \(\pm\) 394 & 83.63 \(\pm\) 5.04 & 2.64 \(\pm\) 0.31 \\ PS-CNWs-5\% & 5 & 3568 \(\pm\) 669 & 76.00 \(\pm\) 15.77 & 2.38 \(\pm\) 0.62 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Stiffness, strength and deformation in fracture of nanocomposites PS-CNWs with different rates of nano-additive, as well as the pristine PS, are presented the mean values and the standard deviations.  
Figure 9: Stress–strain diagrams for the nanocomposite samples as well as for the pristine polymer.  
Figure 10: Diagrams of (**a**) modulus of elasticity (E), (**b**) tensile strength (\(\sigma_{\rm TS}\)) and (**c**) strain at break (\(\epsilon_{\rm b}\)) as a function of the weight fraction of the nano-additive into the samples.  
As discussed in other polymer/nanoparticle systems, the addition of rigid nanoparticles seems to positively affect the modulus of elasticity (stiffness), whereas the values of strength and strain at break depend much more on the effect of nano-addition on the morphology of the sample (change in crystallinity, the orientation of the amorphous phase, etc.) and less on the mechanical properties of the nanoparticles. From the above, we can conclude that the addition of 3% CNWs to the nanocomposite material imparts the maximum increase in stiffness, which is associated with satisfactory dispersion and lack of large aggregates. For higher rates of nano-reinforcement, there is much more difficulty in achieving satisfactory dispersion, which is also confirmed by the large range of values displayed at a 5% addition (high standard deviation). The slight change in the strength of nanocomposite materials can be justified by two competing phenomena, the phenomenon of strength enhancement due to the addition of CNWs and the change in the morphology, where the nanowires lead to the loss of the crystalline structure of the polystyrene chains, degrading the strength.