## 3 Results and Discussion
### Mechanical Properties of Green Composites  
Figure 11 shows the mechanical tensile stress (\(\sigma\)) versus strain (\(\varepsilon\)) curves, obtained at room temperature, of the PHBV films with MFCs and/or the compatibilizing agents. Table 5 summarizes the values of E, \(\sigma_{y}\), and \(\varepsilon_{\rm b}\). As can be seen from the mechanical curve of the PHBV sample, the neat biopolymer film showed a mechanical behavior typical of a brittle material, with an E value of approximately 2.6 GPa and \(\sigma_{y}\) and \(\varepsilon_{\rm b}\) values of 27.4 MPa and 1.4%. This mechanical curve showed nearly no plastic deformation since, after elastic deformation, the film broke with little or no deformation upon exceeding the yield point. The high brittleness of the PHB-based materials, as well as those of PHBV copolymers with low 3 HV contents, that is, below 20 mol%, is due to their high crystallinity, which takes place mainly due to a secondary crystallization, and/or the physical aging of the amorphous region. In the first case, during the crystallization of PHB, the formation of large spherulites occurs due to the low nucleation density, together with the high degree of crystallinity and the slow crystallization rate [58]. Inside the spherulites, due to the difference between the radial and circumferential coefficient of thermal expansion, circular and radial cracks can form [59], which can act as stress concentration points and promote brittleness of the PHBV [60]. In terms of the physical aging process of PHB, produced during storage at room temperature, it implies a progressive restriction in the movement of interlamellar amorphous chains [61]. As a result of this, an increase of the rigid amorphous fraction is habitually observed during two weeks of storage at room temperature, that is, 25 \({}^{\circ}\)C [62]. In this regard, it should be noted that all of the PHBV films were allowed to age for at least 15 days, so this embrittlement effect was already reflected in the mechanical results of the biopolymer films. As can be observed in Table 5, the incorporation of MFC resulted in a progressive increase in the values of E. In particular, the film samples of PHBV filled with 2.5, 5, and 10 wt % of MFC presented values of 2685 MPa, 2747 MPa, and 2873 MPa, respectively. However, because of their low compatibility with the PHBV matrix, as evidenced earlier during the morphological analysis of the film composites, both \(\sigma_{y}\) and \(\varepsilon_{\rm b}\) were reduced to values within the range of 20-23 MPa and 0.8-1.1%. In the case of PHBV films without MFC, but processed with the compatibilizers, their tensile mechanical properties were similar to those of PHBV, with a slight but significant increase (\(p<0.05\)) in E, having values of approximately 2.7 GPa. In the case of the \(\sigma_{y}\) and \(\varepsilon_{\rm b}\) parameters, the values were similar to those of the PHBV film, in the range of 26-28 MPa and 1.2-1.4%, with no significant differences between them (\(p>0.05\)). However, a significant improvement (\(p<0.05\)) in terms of both mechanical strength and ductility was achieved in the film samples of PHBV/MFC that were compatibilized with ESAO and, more remarkable, with TGIC and DCP. In particular, for these composite films, the E values increased to 2863 MPa and 3071 MPa,