## 3 Results and Discussion
### Raman Analysis
The weak blueshift of the G band at 6 wt% and 9 wt% MWCNT loadings could be understood in terms of the strain effect caused by the interaction between carbon nanotubes and the PLA matrix. Figure 3: Normalized Raman spectra of nanocomposites with rising MWCNTs concentration and pristine nanotubes as a reference (**a**), enlargement between 1000 and 1500 cm\({}^{-1}\) (**b**). As the number of graphitic layers increases, the interaction between these layers becomes significant, reflecting in the appearance of a shoulder over the 2D band in the Raman spectra of GNP/PLA nanocomposites (Figure 4). This shoulder shifts the position of the 2D band to the higher frequencies of the spectrum. Concerning graphene samples, the small differences in the G band position up to 3 cm\({}^{-1}\) (1575-1578 cm\({}^{-1}\)) are suggested to be within the range of fluctuations caused by unintentional electron or hole doping effects [48]. It was found that the influence of the PLA structure on the Raman spectra of the nanocomposites decreases with rising carbon filler loading. The I\({}_{\rm 2D}\)/I\({}_{\rm G}\) ratio shows the graphene quality. Regarding high quality defect-free graphene, this ratio will be equal to 2 (single or few layers graphene). The defined I\({}_{\rm 2D}\)/I\({}_{\rm G}\) ratios of TNGNP and GNP/PLA nanocomposites are rather a hint for the presence of multilayer graphene in the composite structure (Table 2). Mapping Raman analysis was carried out on each sample, acquiring spectra at least on three surface sample positions, in order to have a better picture of nanocomposite homogeneity. Peaks ratio calculations disclosed that even the pristine GNP and MWCNT are not fully homogeneous. However, a high level of homogeneity occurred in the nanocomposites. That was confirmed by the low standard deviations in the I\({}_{\rm D}\)/I\({}_{\rm G}\) and I\({}_{\rm 2D}\)/I\({}_{\rm G}\) ratios for each sample, showing a high level of homogeneity in the nanocomposites structure (see Table 2). The fewer lattice defects led to a lower I\({}_{\rm D}\)/I\({}_{\rm G}\) value. An indication of the low amount of structural defects in MWCNT/PLA nanocomposites is the narrow I\({}_{\rm D}\)/I\({}_{\rm G}\) ratio, mostly concerning the higher MWCNTs loadings that could \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **I\({}_{\rm D}\)/I\({}_{\rm G}\)** & **I\({}_{\rm 2D}\)/I\({}_{\rm G}\)** \\ \hline TNGNP & 0.10 \(\pm\) 0.01 & 0.45 \(\pm\) 0.01 \\ MWCNT\_Nanocyl & 1.00 \(\pm\) 0.01 & 0.55 \(\pm\) 0.02 \\
1.5 MWCNT/PLA & 1.14 \(\pm\) 0.03 & 0.50 \(\pm\) 0.01 \\
3 MWCNT/PLA & 0.85 \(\pm\) 0.03 & 0.33 \(\pm\) 0.02 \\
6 MWCNT/PLA & 0.71 \(\pm\) 0.002 & 0.14 \(\pm\) 0.01 \\
9 MWCNT/PLA & 0.72 \(\pm\) 0.005 & 0.12 \(\pm\) 0.01 \\
1.5 GNP/PLA & 0.30 \(\pm\) 0.06 & 0.63 \(\pm\) 0.03 \\
3 GNP/PLA & 0.18 \(\pm\) 0.04 & 0.50 \(\pm\) 0.04 \\
6 GNP/PLA & 0.15 \(\pm\) 0.002 & 0.49 \(\pm\) 0.03 \\
9 GNP/PLA & 0.11\(\pm\)0.02 & 0.48\(\pm\)0.04 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Raman intensities ratios of monofiller nanocomposites. Figure 4: Normalized Raman spectra of nanocomposites with growing GNPs concentration and pristine graphene as reference. signify good dispersion of carbon nanotubes in the polymer matrix. The peak intensity ratios \(\mathrm{I_{D}}/\mathrm{I_{G}}\) of graphene nanocomposites are slightly higher than that of TNGNP, suggesting the formation of more basal plane and edge defects in the carbon filler present in GNP/PLA that could enhance the interface interactions in the composite. Nevertheless the low \(\mathrm{I_{D}}/\mathrm{I_{G}}\) values of GNP/PLA nanocomposites, really close to the \(\mathrm{I_{D}}/\mathrm{I_{G}}\) ratio of pure graphene nanoplatelets, can be an indication that the structure of GNP has been well preserved during melt extrusion processing. The standard deviations for all \(\mathrm{I_{D}}/\mathrm{I_{G}}\) and \(\mathrm{I_{2D}}/\mathrm{I_{G}}\) ratios have been taken into consideration when presenting the data in Table 2.