# Polylactic Acid/Carbon Nanoparticle Composite Filaments for Sensing
## 3 Results  
### Morphology of Melt-Extruded Filaments  
OM images of the composite filament cross sections with different loads of MWCNT, with and without PPG, are displayed in Figure 2. OM analysis provides a 2D description of the MWCNT fraction that did not disperse in the polymer during melt mixing, remaining as agglomerates. The micrographs show that filaments with 3 wt.% MWCNT present a larger number of agglomerates compared to 2 wt.% MWCNT. They also illustrate the effect of the predispersion of MWCNT in PPG, resulting in improved dispersion of MWCNT with consequent decrease in the number of agglomerates present as well as formation of smaller agglomerates.  
Statistical analysis confirms that melt-extruded filaments of neat PLA and MWCNT present agglomerates with larger area (Table 3) with medians ranging from 18 to 23 \(\upmu\)m\({}^{2}\). Nanocomposites incorporating PPG present agglomerates with smaller median areas, within a narrow range from 6 to 7 \(\upmu\)m\({}^{2}\). OM images depict prominent agglomerates of MWCNT for composites without PPG and smaller MWCNT agglomerates for composites prepared with PPG.  
Figure 2: Optical microscopy (OM) (transmission mode) images of filament composites with different diameters, filler load, and PPG wt.%.  
Density plots (Figure 3) and the cumulative distribution (Figure S1) of agglomerate areas show that composites with PPG present smaller agglomerates with a narrower area range (75% of agglomerates \(<\) 10 \(\upmu\)m\({}^{2}\), Table 3), whereas composites without PPG present a wider dispersion in agglomerate areas. A Kruskal-Wallis statistic for alpha = 5% shows that both the addition of PPG (for H\({}_{(1)}\) = 5.39, \(p\)-value = 0.02) and MWCNT concentration (for H\({}_{(1)}\) = 7.79, \(p\)-value = 0.005) have a significant effect in the agglomerate area ratio. It may be concluded that (i) the agglomerate area ratio is significantly lower for composites containing PPG and that (ii) increasing MWCNT concentration increases the agglomerates area ratio. The latter conclusion is expected and reported in Table 3. Comparison of filaments with similar composition shows that the composites without PPG display the highest agglomerate area ratio values, with a larger area covered by MWCNT agglomerates.  
SEM images of the cross sections of melt-extruded composite filaments (Figure 4) depict a homogenous matrix, indicating good miscibility of PPG in PLA at the prepared compositions. The dispersed fraction of MWCNT is well distributed across the composite.  
Figure 3: Density plot of MWCNT agglomerate areas. Red lines indicate the quartiles.  
\begin{table}
\begin{tabular}{l c c c c c} \hline \hline \multicolumn{1}{c}{**Sample**} & \multicolumn{1}{c}{**Median (\(\upmu\)m\({}^{2}\))**} & \multicolumn{1}{c}{**Q\({}_{1}\) (0.25) (\(\upmu\)m\({}^{2}\))**} & \multicolumn{1}{c}{**Q\({}_{3}\) (0.75) (\(\upmu\)m\({}^{2}\))**} & \multicolumn{1}{c}{**Area Ratio (\%)**} & \multicolumn{1}{c}{**Number of Agglomerates (mm\({}^{-2}\))**} \\ \hline PLA/2 MWCNT\_s & 18.05 & 8.25 & 39.96 & 1.74 & 356 \\ PLA/3 MWCNT\_s & 23.30 & 10.83 & 53.60 & 5.83 & 789 \\ PLA/2 MWCNT/20 PPG\_s & 6.44 & 5.52 & 7.91 & 1.38 & 303 \\ PLA/3 MWCNT/20 PPG\_s & 6.91 & 5.73 & 9.42 & 1.93 & 648 \\ PLA/2 MWCNT/20 PPG & 6.91 & 5.02 & 8.80 & 0.37 & 178 \\ PLA/3 MWCNT/20 PPG & 6.82 & 5.68 & 9.09 & 1.88 & 336 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Quartiles (Median, Q\({}_{1}\), and Q\({}_{3}\)) of the areas measured per sample, area ratio, and count of agglomerates per mm\({}^{2}\).