## 3 Results and Discussion
### Thermal Properties  
#### 3.2.1 Thermogravimetric Analysis (TGA)  
In addition, the thermal degradation behavior of the prepared materials could be of some technological importance. In the present paper, thermogravimetric analysis was thus performed under a nitrogen atmosphere on the prepared materials. In Figure 2, TGA thermograms of neat PLA and relative nanocomposites are reported, while in Table 2 the most important results are summarized. Nanosilica introduction determined a slight increase of the temperature associated with a mass loss of 5% (T\({}_{\%}\)), i.e., only 3 \({}^{\circ}\)C with a silica loading of 24 vol%. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Temperature and** & **PLA** & **PLA-Ar805-2** & **PLA-Ar805-4** & **PLA-Ar805-8** & **PLA-Ar805-24** \\
**Kinetics** & **PLA-Ar805-2** & **PLA-Ar805-4** & **PLA-Ar805-8** & **PLA-Ar805-24** \\ \hline
170 \({}^{\circ}\)C & \(5.7\pm 0.2\) & \(5.2\pm 0.5\) & \(3.7\pm 0.2\) & \(2.3\pm 0.2\) & \(0.8\pm 0.1\) \\
190 \({}^{\circ}\)C & \(16.4\pm 1.3\) & \(14.1\pm 1.2\) & \(10.4\pm 0.3\) & \(6.6\pm 0.6\) & \(2.2\pm 0.5\) \\
210 \({}^{\circ}\)C & \(38.7\pm 2.5\) & \(37.3\pm 1.5\) & \(25.4\pm 0.8\) & \(18.9\pm 1.0\) & \(5.2\pm 1.2\) \\ Activation energy \({}^{1}\) & \(85.3\pm 2.9\) & \(82.4\pm 4.6\) & \(84.8\pm 1.5\) & \(94.8\pm 1.4\) & \(81.9\pm 1.0\) \\ (k/mol) & & & & & \\ \hline \end{tabular} \({}^{1}\) Calculated from Arrhenius plot (see Supplementary Figure S1).  
\end{table} 
Table 1: Melt flow index (at 2.16 kg) and activation energy of PLA-based nanocomposites at different temperatures. Figure 1: FESEM images of the fracture surface of polylactic acid (PLA)-based nanocomposites with 1, 2, 8, and 16 vol% of fumed silica. Interestingly, the decomposition temperature (T\({}_{\rm d}\)) was shifted towards higher temperatures, even at elevated filler amounts. For instance, a T\({}_{\rm d}\) shift of 10 \({}^{\circ}\)C was registered with a silica amount of 8 vol%. According to the conclusions reported in our previous paper on the thermal stability of high-density polyethylene-fumed silica nanocomposites, the thermal degradation stabilization observed for nanosilica filled samples could be explained in terms of their ablative behavior [29]. In fact, during the thermal degradation of the specimen, silica aggregates tended to agglomerate on the surface of the molten polymer, creating a physical barrier that protected the rest of the polymer and hindered the volatilization of the oligomers generated during the combustion process. The ability to form this protective shield depended on the capability of silica aggregates to form a continuous barrier, as shown also by [PERSON] et al. [44], in PLA nanocomposites up to 10% of silica. According to the FESEM images reported in Figure 1, the relatively fine dispersion of silica aggregates at all the considered concentrations permitted the creation of an efficient barrier even at limited filler contents. Increasing the silica concentration, the mean distance between the aggregates was considerably reduced, and the formation of a thicker and stronger protective shield was therefore favored. Interestingly, the residue left after the combustion of neat PLA in the air was nearly zero, as all the organic components decomposed into gaseous products, while the remaining mass at 700 \({}^{\circ}\)C (m\(\gamma_{700}\)) of the nanofilled samples corresponded to the actual nanosilica mass percentage (i.e., very close to the theoretical one). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **T\({}_{\rm 5\%}\)** & **T\({}_{\rm d}\)** & **m\({}_{\rm 700}\)** \\  & (\({}^{\circ}\)C) & (\({}^{\circ}\)C) & (\%) \\ \hline PLA & 350.0 & 384.0 & 0.1 \\ PLA-Ar805-2 & 350.3 & 386.1 & 2.7 \\ PLA-Ar805-4 & 351.0 & 393.2 & 5.5 \\ PLA-Ar805-8 & 349.0 & 394.3 & 10.2 \\ PLA-Ar805-16 & 352.3 & 395.3 & 21.3 \\ PLA-Ar805-24 & 353.0 & 396.4 & 30.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Results of TGA analysis on PLA-based nanocomposites.