## 3 Results and Discussion
### Thermal Properties of Green Composites
Furthermore, the TGA curves of the different PHBV films with MFC and/or the compatibilizing agents are shown in Figure 10. In these curves one can see that the biopolymer was stable up to temperatures close to 280-290 \({}^{\circ}\)C. In particular, thermal degradation occurred through a single and rapid degradation step that took place in the thermal range from 290 \({}^{\circ}\)C to 310 \({}^{\circ}\)C. As shown in Table 4, where the thermal stability data that was obtained from the TGA curves gathers, the PHBV sample showed T\({}_{\rm onset}\) and T\({}_{\rm deg}\) values of 277.5 \({}^{\circ}\)C and 296.3 \({}^{\circ}\)C, respectively. It has been described in the literature that the thermal degradation of PHAs generally follows a random chain scission model of the ester bond, involving a cis-type elimination reaction of -CH and a six-membered ring transition  
Figure 9: Differential scanning calorimetry (DSC) thermograms of the films of poly(3-hydroxybutyrate-\(co\)-3-hydroxyvalerate) (PHBV) with microfibrillated cellulose (MFC) and compatibilized with epoxy-based styrene-acrylic oligomer (ESAO) and with triglycerides isocyanurate (TGIC) and dicumyl peroxide (DCP) as follows: (**a**) Cooling; (**b**) Second heating. to form crotonic acid and its oligomers [57]. It can be observed from the TGA curves of the respective PHBV films with the compatibilizing agents that they exhibited a significant drop in thermal resistance, with T\({}_{\rm onset}\) values of 227.1 \({}^{\circ}\)C and 256.3 \({}^{\circ}\)C for the PHBV film samples that were compatibilized with ESAO and with TGIC and DCP, respectively. In the case of T\({}_{\rm deg}\), these samples showed values in the range of 255-285 \({}^{\circ}\)C. These results thus indicate that the presence of the reactive compatibilizers resulted in the formation of macromolecular structures of lower thermal resistance. In this regard, it should also be noted that traces of these chemical agents, which could remain unreacted in excess, can catalyze the thermal degradation of PHBV. Regarding the PHBV/MFC composites and their respective composites compatibilized with ESAO and TGIC with DCP, it can be seen that the thermal stability of these films was similar to, or even in some cases higher than, that of the neat copolyester. The highest thermal properties were obtained for the PHBV composite with 10 wt % of MFC, reaching T\({}_{\rm onset}\) and T\({}_{\rm deg}\) values of 288.8 \({}^{\circ}\)C and 305.9 \({}^{\circ}\)C, respectively. This result is quite positive when comparing the present green composites to others based on lignocellulosic materials since these tend to show significantly lower thermal stability than that of unfilled biopolymers and usually induce thermal decomposition [31]. This difference can be ascribed to the fact that MFC is based on pure alpha (\(\alpha\))-cellulose, without hemicellulose, and then with a higher thermal stability. Furthermore, it is also noteworthy that the presence of the compatibilizers in the PHBV/MFC composite samples did not contribute to a significant improvement of the thermal stability properties, presenting very similar values to that of the uncompatibilized PHBV composite with a 5 wt % MFC, but still significantly higher (\(p<0.05\)) than that of the unfilled PHBV processed with the reactive agents. Therefore, this suggests that during the melt-mixing process of the green composites, by which the chemical anchoring of the cellulose microstructures on the PHBV structure was achieved, the reactive agents were successfully consumed, whereas these did not react completely in the case of the biopolymer samples without MFC. Finally, all of the PHBV films showed residual mass values of around 2%, with no significant differences (\(p>0.05\)) among the film samples.