## 3 Results and Discussion
### Raman Analysis  
Raman spectroscopy has become a standard characterization tool for carbon nanostructures [36; 37; 38; 39]. Tensile, compressive and shear stress applied to carbon nanotubes induce strain, leading to changes in the C-C bond vibrations, which strongly affect the Raman spectrum [40]. Stress induced changes in peak positions and intensities have been observed for individual carbon nanotubes [41], SWCNT bundles [42] and nanotube-polymer composites [43]. Normalized Raman spectra of the nanocomposites with increasing MWCNTs and GNPs concentration including pristine nanofiller as a reference are depicted in Figures 3 and 4. In all single spectra, the main Raman bands--G, D and 2D, or also denoted as G\({}^{\prime}\), band have been registered. The 1582 cm\({}^{-1}\) band of graphene is known as the G band or tangential band and originates from in-plane vibration of sp\({}^{2}\) carbon atoms. The band at about 2700 cm\({}^{-1}\), which is recognized as the G\({}^{\prime}\) or 2D band, is much more intense than the G band in graphene compared to graphite or to multilayer graphene. This band is a second-order overtone mode and does not represent a disorder-induced mode. The 2D band has frequently been used to study MWCNT-polymer interfacial stress transfer. The 2D peak is an overtone of the D band, resulting from a two-phonon double-resonant Raman process [44]. The D band at around 1350 cm\({}^{-1}\) is often referred as disorder band or defect band and its intensity relative to the G  
Figure 1: IR spectrum of GNPs. Figure 2: IR spectrum of multi walled carbon nanotubes. band intensity is often used as a measure of carbon nanotubes quality. All Raman results concerning monofiler PLA-based nanocomposites are summarized in Figures 3 and 4, and Table 2.  
The presence of MWCNTs in the composites is supported by the characteristic D, G, and 2D modes of MWCNTs (Figure 3). D, G, and 2D modes of the MWCNT/PLA composites exhibit positional upshift, more pronounced in the 2D band, as compared to the pure MWCNTs itself. Shifts to higher-frequencies, especially for the G band, have been reported in MWCNTs composites [45]. This blue shift has been attributed to a disentanglement and dispersion of the CNT bundles in the polymer matrix, decreasing nanotube-nanotube interactions. The application of a deformation to CNTs results in a change in the C-C bond vibrations, leading to a change in the vibrational frequencies of the normal modes and thus to Raman band shifts. Strain-induced frequency changes have been already reported for MWCNTs composites [43]. In terms of the relative Raman intensity, evaluated against the G mode intensity, and the full width at half maximum intensity (FWHM), we can observe different distinct behaviors: the D intensity decreased substantially upon rising carbon nanotube content in the composite, whereas the 2D mode exhibits a cutback of the relative intensity as well as a significant reduction of FWHM. These changes can be due to the interaction between PLA and carbon nanotubes. It is worth noticing, that the resonances of pure PLA, due to C-CH\({}_{3}\) stretching at about 1042 cm\({}^{-1}\) and -CH\({}_{3}\) rocking mode at about 1127.4 cm\({}^{-1}\), were red-shifted by 2.1, 1.3, and 1.8 cm\({}^{-1}\) in the Raman spectrum of 3 wt% MWCNT/PLA composite regarding the spectrum of 1.5 wt% MWCNT/PLA composite, indicating that the interaction of MWCNTs with the polymer occurs through C-CH3 groups [46; 47], see Figure 3b. As already mentioned, the 2D band of the nanocomposites in Figure 3 is barely shifted to higher Raman frequencies, compared with the 2D band of pure carbon nanotubes. A possible reason for this could be as well the relatively low filler concentration in the polymer matrix compared to the neat MWCNTs [23]. When the carbon nanotubes are multi walled, their diameter becomes larger, so the lattice strain is reduced, leading to a decline in the number of sp\({}^{3}\) hybridized carbon, which means the tubular structure is not so stable [28].