## 3 Results and Discussion
### Microstructural Analysis and Processability of the Materials  
It is well known that the thermo-mechanical behavior of particulate nanocomposite is strongly influenced by the aggregation state of the nanofiller within the polymer matrix. Therefore, a detailed morphological analysis could give important information about the influence of the nanofiller content on the silica dispersion behavior in PLA. In Figure 1, FESEM images of the fracture surface of PLA-based nanocomposites are reported. PLA-Ar805-1 and PLA-Ar805-2 samples were characterized by the presence of iso-dimensional-funed silica aggregates having a mean dimension of less than 100 nm. This confirms that a good silica dispersion was obtained during the melt compounding process. As reported in our previous paper on the mechanical behavior of PLA nanocomposites [46], the presence of the organosilane on the surface of the nanoparticles leads to the formation of silica aggregates with lower size, and this microstructural feature positively affects the fracture behavior of the material. This interpretation is supported by the conclusions reported in a paper of [PERSON] et al. [61], in which the influence of the surface treatment on mechanical behavior of fumed silica/epoxy resin nanocomposites was investigated. Additionally, in that work, the improvement of the dispersibility of functionalized silica nanoparticles was attributed to a weakening of the inter-particle attractive forces due to the presence of a surface compatibilizer. As the filler concentration increased, a higher surface corrugation and a marked aggregation tendency could be detected. Such aggregates explain the detrimental effects on the tensile behavior of the composites observed at elevated filler loadings in our previous paper on these systems [46].  
An evaluation of the processability of the prepared samples is of fundamental importance from a technological point of view. Therefore, melt flow index (MFI) tests were performed. In Table 1, melt flow index (MFI) values of PLA-based nanocomposites at different temperatures, with an applied weight of 2.16 kg, are reported. It is interesting to note how the processability of the material was not affected until there was a silica amount of 2 vol%, regardless of the testing temperature. MFI values of the tested materials started to considerably decrease for silica contents higher than 4 vol%. For instance, MFI values at 170 \({}^{\circ}\)C passed from 5.7 to 2.3 g/10 min with a filler concentration of 8 vol%. As reported in rheological studies on PLA-grafted silica nanocomposites [37], the observed viscosity increase could be attributed to the formation of physical crosslinking points and a percolative network. In these conditions, the flow of the molten polymer was hindered by the silica network within the material, with a consequent melt viscosity increase. On the other hand, the fluidity of the material increased with the temperature, and the activation energy of the process, calculated according to Arrhenius plot [62], showed only minor variation with the composition, ranging from 89 for the PLA matrix to 82-94 kJ/mol for fumed silica nanocomposites, as shown in Table 1. These results were lower than those determined for PLA copolymer of a higher molecular weight (208 kJ/mol), as direct dependence on polymer chain length and mobility. From these findings, it is possible to conclude that PLA-FS nanocomposites could be rather easily processed by properly adjusting the temperature with the filler content. For instance, nanocomposite at 24 vol% exhibited at 210 \({}^{\circ}\)C the same melt flow of PLA matrix at 170 \({}^{\circ}\)C.