## 3 Results and Discussion
### Validation of the Bionanocomposites
#### 3.2.1 First Service Life: Mechanical Performance and Fracture Surface of Dog-Bone Specimens
This behavior is of great importance, as it can be considered that the nanoparticles did not impair the deformability in materials with PEG of high molar mass, but improved, in all cases, the tensile strain of the pure PLA. In terms of elastic modulus, the presence of the NFCs resulted in a linear increase of the material rigidity, especially relevant in the PLA-1500-based bionanocomposites. At this point, evaluating the microscopic appearance of the surface of the samples both after cryofracture and after the tensile test may give valuable information about the matrix-filler interaction and serve as the baseline for understanding the mechanical performance of these materials. Therefore, field emission scanning microscopy (FE-SEM) was considered, and the obtained micrographs are shown in Figure 7. The cryofractured specimens before the mechanical tests revealed a compact structure without pores or cracks, which suggested a suitable processing strategy for preparing the dog-bone specimens. The frozen samples' typical fragile fracture behavior could be perceived, especially in pure PLA. Although a fragile cryofracture was also found with samples containing PEG, the presence of the plasticizing agent slightly promoted more rugosity [82]. This roughness was more significant for the PLA-1500 than for the PLA-400-based materials, highlighting the contribution of the PEG with higher molar mass to the more substantial increase in the crystallinity of PLA, as revealed in previous sections [81]. In addition, no phase separation was observed, which correlated to good miscibility between the plasticizer and the polymer matrix. For the bionanocomposites containing NFC, the cryofracture surface generally increased in roughness and heterogeneity with the nanofiller percentage, which was particularly relevant for materials based on PLA-1500. The changes mentioned above in the PLA matrix's crystalline microstructure due to the nanofiller's presence may have influenced this behavior. After the tensile test, the fracture surface of the virgin PLA was almost flat and smooth, indicating a brittle fracture characteristic for this polymer. With the combination of PLA and PEG, signs of ductility could be perceived. However, non-relevant differences were appraised between the plasticized PLA in terms of the molar mass of PEG. The presence of thread-like structures corroborates the plasticizing effect correlated with the growth in the elongation at break. For the case of bionanocomposites containing NFC, the main observation was the presence of high porosity and voids, together with an irregular fracture surface. This was more visible as the NFC content increased and may be related to a partial matrix-filler interaction along with the possible agglomeration of the nanoparticles, especially for high NFC percentages (5 wt%). Furthermore, bigger pores were noted in the nanocomposites with PEG-400 compared with PEG-1500 nanocomposites, according to the slightly lower resistance to fracture and higher elongation at break displayed by the PEG-400-NFC compositions. Here, it is necessary to remark that both the mechanical and microscopic performances are strictly related to the crystalline morphology of the dog-bone specimens after injection molding, of which calorimetric thermograms of the first heating scan are shown in Figure S2. Therefore, the crystalline structure of such dog-bone specimens is of great importance, and consequently, the degree of crystallinity (\(X_{c0}\)) was calculated and is presented in Table 3. Higher crystallinity was found with the addition of PEG and low percentages of NFC (1 wt%). Nevertheless, the higher deformation at break in nanocomposites containing PEG suggested that its plasticizing effect in the amorphous regions prevailed over the generation of crystalline domains. The general higher \(X_{c0}\) of the PLA-1500-NFC nanocomposites may explain their lower deformation at break compared to PLA-400-NFC, together with the increase in the rigidity of these materials.