# Polylactic Acid/Carbon Nanoparticle Composite Filaments for Sensing
## 3 Results
### Thermal Characterization  
The thermal characterization of the extruded filaments was performed by TGA and DSC analysis. Thermal degradation curves, and the corresponding derivatives (DTG), of PLA and composites are presented in Figure 6.  
Neat PLA displays one degradation step with first derivative peak maximum occurring at 394 \({}^{\circ}\)C. Two weight loss steps are observed for PLA/20 PPG: one with a first derivative peak maximum at 255 \({}^{\circ}\)C that may be attributed to PPG vaporization and the second near 396 \({}^{\circ}\)C, corresponding to PLA degradation (Figure 6, Table 4). Figure 5: Raman spectra of neat PLA, composites, and pristine MWCNT. Dashed lines indicate MWCNT’s D, G, and 2D band positions. The weight losses observed for the composites containing PPG in the temperature range of 200-350 \({}^{\circ}\)C are approximately 10 wt.% (Table 4), while the PLA/PPG composition shows a weight loss near 15 wt.% in the same temperature range. Considering this weight loss an indicator of the PPG content, it remains below the target composition of 20 wt.%, suggesting loss by vaporization during melt-extrusion. A decrease in T\({}_{\rm g}\) and melting temperature (T\({}_{\rm M}\)) is observed for PLA/20 PPG as compared to neat PLA, confirming the plasticizing effect of PPG (Figure 7). Composites without plasticizer show T\({}_{\rm M}\) and T\({}_{\rm g}\) similar to PLA (Table 5, Figure 7). The plasticizing effect of PPG on the temperature range of 200-350 \({}^{\circ}\)C is also observed for PLA/20 PPG. \begin{table}
\begin{tabular}{l c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multirow{2}{*}{**Weight Loss at 350 \({}^{\circ}\)C**} & \multicolumn{2}{c}{**Temperature of PPG**} & \multicolumn{2}{c}{**Temperature at Max.**} & \multirow{2}{*}{**Residue at 700 \({}^{\circ}\)C**} \\  & **(wt.\%)** & & & \\  & & **Peak, \({}^{\circ}\)C** & & \\ \hline PLA & 1.4 \(\pm\) 0.7 & - & 394.0 \(\pm\) 3.6 & 2.8 \(\pm\) 0.8 \\ PLA/20 PPG & 15.3 \(\pm\) 1.1 & 254.6 \(\pm\) 4.8 & 395.5 \(\pm\) 0.4 & 1.6 \(\pm\) 0.2 \\ PLA/2 MWCNT/20 PPG & 9.3 \(\pm\) 0.3 & 253.7 \(\pm\) 3.7 & 409.6 \(\pm\) 6.7 & 2.80 \(\pm\) 0.04 \\ PLA/2 MWCNT/20 PPG\_s & 11.0 \(\pm\) 0.4 & 246.9 \(\pm\) 7.6 & 396.0 \(\pm\) 0.3 & 5.4 \(\pm\) 0.2 \\ PLA/2 MWCNT\_s & 1.0 \(\pm\) 0.4 & - & 400.6 \(\pm\) 1.1 & 4.3 \(\pm\) 0.9 \\ PLA/3 MWCNT/20 PPG & 8.9 \(\pm\) 0.2 & 264.7 \(\pm\) 6.6 & 409.2 \(\pm\) 5.8 & 3.1 \(\pm\) 0.1 \\ PLA/3 MWCNT/20 PPG\_s & 11.6 \(\pm\) 0.2 & 244.4 \(\pm\) 2.0 & 395.3 \(\pm\) 0.4 & 6.1 \(\pm\) 0.3 \\ PLA/3 MWCNT\_s & 1.7 \(\pm\) 0.4 & - & 396.4 \(\pm\) 2.7 & 4.1 \(\pm\) 0.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Thermogravimetric analysis results for PLA, PLA with PPG, and its MWCNT composites. Figure 6: Thermal degradation of neat PLA and composites under N\({}_{2}\) atmosphere. Thermogravimetric analysis curves are displayed in the top row and the corresponding derivatives in the bottom row. PPG is also noticeable in MWCNT composites, which present lower \(\rm T_{g}\) and \(\rm T_{M}\) than the corresponding nonplasticized compositions. After melt processing into filament form, the neat PLA shows a pronounced cold-crystallization peak in the DSC crystallization scan (Figure 7), i.e., upon heating, the PLA filaments can still rearrange and crystallize further when temperature rises above \(\rm T_{g}\). Compositions containing PPG appear more stable upon heating, showing lower tendency for molecular rearrangement and depicting smaller cold crystallization enthalpy (\(\rm\Delta H_{cc}\)). This observation is quite relevant, as it may indicate higher dimensional stability of the filaments prepared with plasticizer, and their possible application at higher service  
Figure 7: Representative differential scanning calorimetry (DSC) scans of (**a**) neat PLA filaments and composite filaments with MWCNT, and (**b**) composite filaments with PPG. temperatures.