## 3 Results and Discussion
### Validation of the Bionanocomposites
#### 3.2.1 First Service Life: Mechanical Performance and Fracture Surface of Dog-Bone Specimens  
The mechanical validation of the dog-bone specimens was performed through tensile tests in a universal testing machine. The stress-strain diagrams, together with the values of the tensile strength (\(\sigma_{max}\)), the elastic modulus (\(E\)), and the elongation at break (\(\epsilon_{break}\)), are represented in Figure 6.  
For neat PLA, a \(\sigma_{max}\) of 59.3 MPa, an \(E\) of 3.51 GPa, and an \(\epsilon_{break}\) of 5.6% were measured, all in line with the results reported for this PLA grade, with low flexibility and deformability [88; 93]. The PEG-plasticized PLA exhibited a decrease in the \(\sigma_{max}\) with higher \(\epsilon_{break}\)[8; 81; 82]. The reduction of the \(\sigma_{max}\) was around 28% and 19% for the PLA-400 and PLA-1500, respectively. However, a relevant increase in the elongation and, therefore, fracture deformation was registered with the addition of PEG, especially for the PLA-400, with a growth in \(\epsilon_{break}\) of nearly 230%. The plasticized PLA-1500 also increased ductility, more restrainedly, with an almost 61% higher elongation. Although the overall deformability increased more the lower the molar mass of PEG, it is interesting to highlight the contribution of the PEG in the elastic region, as can be appreciated in the inset of Figure 6. The stiffness slightly decreased with the addition of PEG given by the lower slope of the elastic deformation region. It is well known that using plasticizing agents such as PEGendorses flexibility and favors elastic and plastic deformation while decreasing the stiffness and the maximum strength [20,88], as demonstrated in this work. In the bionanocomposites, the addition of NFC generally caused a moderate reduction of \(\sigma_{max}\) concerning their corresponding PLA-PEG counterparts, together with a decrease of the \(\varepsilon_{break}\) and a slight increase in the \(E\). Although the use of fillers in composite materials is usually accompanied by an improvement in the tensile strength, a minor reduction (\(<\)5%) was found in the current study, which may indicate the prevalence of the plasticizing effect of PEG, together with a feasible limited stress transfer across the matrix/filler interphase [27,86,87,94]. This phenomenon may provoke the generation of voids during the tensile test, leading to slightly lower strength values [26]. Complementarily, high NFC contents may be related to forming particle agglomerates with a low aspect ratio, which may affect the tensile strength [95]. Altogether, the variation in the \(\sigma_{max}\) was not censorious, and it may be sacrificed for controlling other parameters such as deformability or rigidity. Regarding the deformation at break, it was more evidence of the contribution of the NFC on the PLA-400-NFC bionanocomposites than those of PLA-1500-NFC. The addition of NFC inhibited the reduction in the stiffness, although the value of pure PLA was not reached. The perceived decrease in the \(\varepsilon_{break}\) by adding NFC was ascribed to the matrix/filler interaction along with the steric hindrance of the molecular movement of the polymer chains caused by the NFC particles and the changes in the crystalline microstructure that they promoted in the nanocomposites [27,94,95]. This effect was predominant for the samples containing PEG-400. Non-significant differences were noted in the deformation strain caused by the addition of NFC to the PLA-1500 compositions. In this case, the effect of  
Figure 6: Tensile-stress curves and elastic modulus (\(E\)), tensile strength (\(\sigma_{max}\)), elongation at break (\(\varepsilon_{break}\)) for the dog-bone specimens of PLA, plasticized PLA-PEG, and PLA-PEG-NFC bionanocomposites. A standard deviation between 1% and 6% was omitted for the sake of clarity. the PEG-1500 on the elongation at break prevailed above the contribution of the NFC.