## 3 Results and Discussion
### Thermal Properties of Green Composites

Table 2: Color parameters (\(L^{\star}\), \(a^{\star}\), \(b^{\star}\)) hue angle (\(h_{ab^{\star}}\)), color saturation or chroma (\(C_{ab^{\star}}\)), and color difference (\(\Delta E_{ab^{\star}}\)) 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). energy barrier during crystal formation. Thus, for these two film samples, it seems that the cellulose microstructures could serve as nucleating agents of the biopolymer crystals, favoring heterogeneous crystallization. In the rest of the samples, the T\({}_{\rm c}\) values were found to be very similar to that of PHBV, although somewhat in a lower range, that is, between 73 and 78 \({}^{\circ}\)C. This effect was significantly noticeable (\(p<0.05\)) for the green composite sample filled with 10 wt % MFC and in the unfilled PHBV film samples that were compatibilized with TGIC and DCP and, more remarkably, ESAO. In particular, the latter sample reached the lowest T\({}_{\rm c}\) value, that is, -73 \({}^{\circ}\)C. For the green composite films, this result suggests that the cellulose microparticles tended to agglomerate at the highest content, impairing the biopolymer chain arrangement during crystallization. In the case of the compatibizing agents, these could facilitate the formation of longer biopolymer chains by a chain-extension effect or, even, a process of branching and cross-linking [42], which resulted in a macromolecular structure with chains of impaired movement. However, in the case of the compatibilized composite film samples, as a consequence of the aforementioned grafting process of the MFCs onto the macromolecular structure of the PHBV described during the FTIR analysis and further supported by the FESEM micrographs, the dispersion of the cellulose microstructures was enlarged and it could favor the formation of crystals. This effect was more noticeable in the case of the green composite film that was processed with TGIC and DCP, suggesting a higher compatibizing effect. On the other hand, during the second heating of the samples, represented in Figure 9b, it can be observed that the biopolymer presented a second order change in the \(-5\) to 5 \({}^{\circ}\)C range, which corresponds to the glass transition of PHBV. In the case of the neat PHBV, the T\({}_{\rm g}\) value was observed at approximately \(-1.7\)\({}^{\circ}\)C, having no significant differences (\(p<0.05\)) among the samples. Glass transition is a region where the biopolymer passes from a glassy state, without molecular mobility, towards a rubbery state, with greater molecular mobility and, therefore, greater toughness. Finally, it can be observed that the PHBV crystals formed during cooling melted at a temperature close to 171 \({}^{\circ}\)C. In general, all of the PHBV samples with MFC and/or compatibilizers presented values that were very similar to those of PHBV, although somewhat lower, in the range of 166-170 \({}^{\circ}\)C. This reduction in the melting point values can be related to certain crystal imperfections or a lower lamellar thickness growth of the PHBV crystals [56]. This observation suggests that both the presence of MFC and the effect of the compatibilizers on the molecular structure of the biopolymer affected the growth of the PHBV crystals. In terms of crystallinity, small differences were observed in most cases, but still significant (\(p<0.05\)), being more noticeable in the green composite filled with 5 wt % MFC and the unfilled PHBV film samples that were processed with the compatibilizers. Thus, the percentage of crystallinity, that is \(\chi_{\rm c}\), reached a value of approximately 65% in the neat PHBV sample. The presence of the cellulose microstructures