# Polylactic Acid/Carbon Nanoparticle Composite Filaments for Sensing
## 3 Results
### Conductivity and Sensing Ability of Composite Filaments  
The results obtained for the DC volume conductivity (\(\sigma_{\text{DC}}\)) of the PLA composite filaments are presented in Figure 9. Measurements were performed on the small-diameter (-0.5 mm) composite filaments with and without PPG, and on the large-diameter filaments (-1.5 mm) with PPG, to monitor possible effects that may be induced during filament collection, which may affect the MWCNT distribution in the composite. All composite filaments produced without plasticizer show \(\sigma_{\text{DC}}\) in the order of \(10^{-7}\) S-cm\({}^{-1}\). The composite filaments with 3 wt.% MWCNT and PPG presented a \(\sigma_{\text{DC}}\) in the order of \(10^{-3}\) S-cm\({}^{-1}\) for both small and large diameter, and similar results were obtained for large-diameter 2 wt.% MWCNT/PPG filaments. The plasticizer played a major \begin{table}
\begin{tabular}{l c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{2}{c}{**At 0 \({}^{\circ}\)C**} & \multicolumn{2}{c}{**Temperature of tan \(\delta\)**} & \multicolumn{1}{c}{**Temperature of E\({}^{\omega}\)**} \\ \cline{2-5}  & **E\({}^{\prime}\) (GPa)** & **E\({}^{\prime\prime}\) (MPa)** & **Peak (\({}^{\circ}\)C)** & **Maximum (\({}^{\circ}\)C)** \\ \hline PLA & 4.0 & 24.7 & 66.4 & 60.9 \\ PLA/20 PPG & 1.7 & 92.9 & 52.5 & 33.8 \\ PLA/2 MWCNT\_s & 5.1 & 33.2 & 71.5 & 61.4 \\ PLA/2 MWCNT/20 PPG\_s & 3.4 & 121.3 & 57.7 & 38.8 \\ PLA/3 MWCNT\_s & 6.7 & 41.4 & 73.7 & 63.0 \\ PLA/3 MWCNT/20 PPG\_s & 2.7 & 110.6 & 57.1 & 33.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: E\({}^{\prime}\), E\({}^{\prime\prime}\), and tan \(\delta\) peak for neat PLA and composites. Figure 9: Direct current (DC) conductivities of composite filaments. Error bars denote maximum and minimum \(\sigma_{\text{DC}}\). role aiding the dispersion of MWCNT, as observed by OM, and the enhanced MWCNT dispersion allowed the formation of electrically conductive filaments. However, the small-diameter filaments with 2 wt.% MWCNT/PPG yielded lower \(\sigma_{\text{DC}}\), which may be due to the filament preparation procedure, namely the pulling action imposed on the melt during filament forming. This slight pulling of the polymer melt may induce an orientation effect, leading to a decrease of the electrical conductivity [43]. This marginal pulling effect was consequent for the electrical conductivity of the composites with lower MWCNT content but not for the filaments with higher MWCNT concentration. The latter presented similar average electrical conductivity for large- and small-diameter filaments, despite increasing its scatter along the filament's length. The cyclic sensing response of PLA composite filaments was investigated using saturated acetone vapor. As observed in Figure 10, small- and large-diameter samples react to the immersion in acetone vapor by an increase in the relative resistance R\({}_{\text{rel}}\) and to the rinsing with dry air with a resistance decrease. Thereby, large differences are observed depending on the concentration of MWCNTs and the addition of PPG. The small-diameter composite filaments with 3 wt.% MWCNT without PPG shows the lowest response with R\({}_{\text{rel}}\) values of approximately 15% after the 150 s immersion time. Addition of PPG significantly increases R\({}_{\text{rel}}\) values up to about 150% in the large-diameter filaments and up to 3700% in the small-diameter filaments. Filaments with 2 wt.% MWCNT and PPG yielded lower signal than filaments with 3 wt.% MWCNT, with maximal R\({}_{\text{rel}}\) near 125% for the thick and near 250% for the thin filaments. Thereby, all the large-diameter filaments present lower signal noise but also a much lower reversibility in the drying process, retaining high R\({}_{\text{rel}}\) values even after 100 s exposure to dry air. The smaller-diameter filaments show higher signal noise, despite higher reversibility within the drying period with a small shift of the starting R\({}_{\text{rel}}\) values after each cycle.