## 3 Results and Discussion
### Structure and Morphology
The 2D band of multilayer graphene can be fitted with multiple peaks due to the splitting of the electronic band structure of the multilayer material [31; 32]. Figure 3 presents the Raman spectra of neat PLA as well as its composites. Obtained PLA spectrum (Figure 3a) presented very strong bands at around 2900-2990 cm\({}^{-1}\) (assigned to the CH asymmetric and symmetric stretching region) and strong bands at about 1750 cm\({}^{-1}\) (assigned to the C=O stretching region), 1450 cm\({}^{-1}\) (assigned to CH\({}_{3}\)), 1370 cm\({}^{-1}\) (assigned to CH deformation and asymmetric bands) and 950 cm\({}^{-1}\) (helical backbone vibration with CH\({}_{3}\) rocking modes) [33]. It was noted that for both GNP concentrations, bands 2900-2990 cm\({}^{-1}\) decreased with increasing filler content. Sample PLA/1.5 wt.% GNP (Figure 3b) presented more intense D band (at 1356 cm\({}^{-1}\)) than sample PLA/6 wt.% GNP (with D band at 1348 cm\({}^{-1}\), Figure 3c) which can indicate more structural defects in the graphene lattice, probably due to heat/sheer damage during extrusion. The reduction of structural defects with increasing GNP content (PLA/6 wt.% GNP) could be the result of a lubrication process during the melting process. The small and broad 2D bands for PLA/1.5 wt.% GNP (at 2715 cm\({}^{-1}\), Figure 3b) and PLA/6 wt.% GNP (at 2719 cm\({}^{-1}\), Figure 3c) are in accordance to graphite profile. The I\({}_{\text{D}}\)/I\({}_{\text{G}}\) and I\({}_{\text{2D}}\)/I\({}_{\text{G}}\) ratios for PLA/1.5 wt.% GNP (0.66 and 0.37, respectively) revealed a high degree of structural disorder. In the case of PLA/6 wt.% GNP the I\({}_{\text{D}}\)/I\({}_{\text{G}}\) and I\({}_{\text{2D}}\)/I\({}_{\text{G}}\) ratios (0.09 and 0.33, respectively) indicated more surface integrity. Also, the Full Width Half Maximum (FWHM) value must be considered (Table 2). This is a measure of structural disorder, producing a quantitative guide to distinguish up to 5 layers [34]. The PLA/1.5 wt.% GNP nanocomposite showed the FWHM of 96 cm\({}^{-1}\), which represents a characteristic of multilayered GNP (more than 10 layers). The nanocomposite that presented the lowest FWHM value was PLA/6 wt.% GNP, 79 cm\({}^{-1}\), and this is also attributed to multi-layered platelets. Figure 3: _Cont._  
In the case of PLA/MWCNT composites, the diameter/number of walls of nanotubes, the presence of disorder in sp\({}^{2}\)-hybridized carbon lattice and the effect of nanotube-nanotube interactions on the vibrational modes have been studied using Raman spectroscopy [35; 36; 37]. Herein, broader D and 2D bands represent a higher number of walls, thus higher disorder and larger diameters. According to the literature, the intense G band relates to better nanotube dispersion in the polymer matrix [38]. At both concentrations the G band is very intensive indicating good MWCNTs dispersion and confirming results obtained by SEM (Figure 1a,b). Figure 4a-c presents the Raman mapping results of PLA/1.5 wt.% GNP/1.5 wt.% MWCNT composites. The D band mapping displays many regions with low filler density (black regions), while the G and 2D band mapping presented more homogeneous and well-dispersed distribution. Since the highest contribution for the D band is given by the CNT, the darker regions could represent a region with less presence of CNT bundles. \begin{table}
\begin{tabular}{c c} \hline
**Composition Code** & **FWHM (cm\({}^{-1}\))** \\ \hline PLA/1.5\% GNP & 96 \\ PLA/6\% GNP & 79 \\ PLA/1.5\% MWCNT & 96 \\ PLA/6\% MWCNT & 90 \\ \hline \end{tabular}
\end{table} 
Table 2: Full Width Half Maximum (FWHM) value of mono-filler composites. Figure 4: _Cont._  
Figure 3: Raman spectroscopy results obtained for **(a)** PLA, **(b)** PLA/1.5 wt.% GNP, **(c)** PLA/6 wt.% GNP, **(d)** PLA/1.5 wt.% MWCNT and **(e)** PLA/6 wt.% MWCNT composites.