## 3 Results and Discussion
### Validation of the Bionanocomposites
#### 3.2.1 First Service Life: Mechanical Performance and Fracture Surface of Dog-Bone Specimens
Moreover, interfacial defects and nanovoids in the confluence of the matrix and nanoparticles may endorse new free-volume sites for water penetration. Interestingly, the plasticized PLA-PEG and the PLA-PEG-NFC bionanocomposites started losing mass after the initial water absorption, especially at higher temperatures, representing a non-Fickian behavior. Therefore, the initial fast water absorption was followed by progressive mass loss, which resulted in a peaking phenomenon. The curves at 8 and 23 \({}^{\circ}\)C reach a steady equilibrium after the mass loss, while the curves of the temperatures of 58 and 70 \({}^{\circ}\)C continue to fall, even reaching a neat mass loss of 2% for the most extreme temperature. Although it was accentuated in the nanocomposites, highlighting the contribution of the nanoparticles, the occurrence of this behavior in the plasticized PLA-400 and PLA-1500 without NFC suggested that the presence of PEG played a crucial role in this subject. Incorporating hydrophilic plasticizers can contribute to increased swelling and later allow soluble parts to be released from the matrix. The release of PEG domains due to the kinetic motion of the plasticized PLA matrix at temperatures above their \(T_{g}\) is the most reasonable circumstance, as found by other authors in PLA-based systems when immersed in water [102; 103; 104]. Predictably, this phenomenon was more significant for the PEG with lower molar mass in the PLA-400 blend than for the longer PEG segments in the PLA-1500 compositions [103]. Complementarily, with water-soluble PEG, the dissolution of low-molar mass PLA segments generated during processing or hydrolysis during immersion together with nanoparticles may also have to be considered feasible [99; 101; 102]. Altogether, this observation suggests a limited degree of chemical crosslinking of PEG and PLA after the applied processing sequence, highlighting the establishment of physical interactions between the matrix and the plasticizer. Given the confluence of several factors on the water uptake of these materials, i.e., temperature and composition, a detailed investigation was carried out. For this purpose, the water mass absorbed at saturation (\(M_{s}\)), the diffusion coefficient (\(D\)), the solubility coefficient (\(S\)), and the permeability coefficient (\(P\)) were calculated. The obtained values for every material at each immersion temperature are summarized in Figure S6 of the Supplementary Materials. It should be noted that for virgin PLA, the \(M_{s}\) involved the mass at equilibrium, while for the blends and bionanocomposites, the maximum of the curve was taken as the value of \(M_{s}\). All of the \(M_{s}\), \(S\), \(D\), and \(P\) values were statistically analyzed to determine the global influence of the immersion temperature and the composition. Figures 9 and 10 represent the main effects plot (MEP) for temperature and composition, respectively. The means of the obtained values for each selected temperature and composition were calculated and visually compared. Figure 9 shows the temperature contribution to the water immersion performance, where a pairwise grouping for the \(M_{s}\) and \(S\) values can be observed. While the polymer chains maintain the morphology and structure determined by processing below the glass transition in the glassy state (8 and 23 \({}^{\circ}\)C), they gain mobility and start moving above the glass transition in the rubbery state (58 and 70 \({}^{\circ}\)C), doubling the values of \(M_{s}\) and \(S\). This movement allows the chains to rearrange themselves, resulting in more free volume in the matrix, more penetration sites for water molecules, and higher absorbed quantities of water. Other studies also found this temperature dependency for the water absorption of PLA-plasticized materials [99; 102; 105]. The diffusivity (\(D\)) and permeability (\(P\)) coefficients also show sensitivity toward the immersion temperature.