## 3 Results and Discussion
### Chemical Analysis of Green Composites  
ATR-FTIR was performed in order to evaluate the chemical interactions between PHBV, MFC, and the reactive compatibilizers, that is, ESAO and TGIC, and spectra of the films are gathered in Figure 5. One can observe in Figure 5a that all of the film samples showed the peaks of the characteristic FTIR spectrum of PHBV without noticeable changes [41], which can be due to the relatively low MFC contents. The strongest band was observed at 1718 cm\({}^{-1}\), which has been assigned to the stretching vibration of the carbonyl group (C=O) in PHA copolyesters and it corresponds to the intramolecular bonding of their crystalline state [43]. The complex and multiple peaks that were observed in the 1000-1200 cm\({}^{-1}\) region arise mainly from the C-O stretching vibration in the ester groups of polyesters [44]. Finally, the peaks centered at \(\sim\)2975 and \(\sim\)2935 cm\({}^{-1}\) have been ascribed to asymmetric stretching vibrations of methyl (-CH\({}_{3}\)) and methylene (-CH\({}_{2}\)-) groups, respectively, which also produced smaller symmetric stretching intensities at \(\sim\)2875 and \(\sim\)2850 cm\({}^{-1}\). Figure 5b compared the FTIR spectra of the uncompatiphilized PHBV film with 5 wt % MFC and the green composite films compatiphilized with ESAO and TGIC. It can be observed that the crystalline C-O stretching vibration was kept at 1718 cm\({}^{-1}\). However, the carbonyl peak was slightly broadened in the spectra corresponding to the compatiphilized films and also generated a shoulder at approximately 1685 cm\({}^{-1}\). This band change, which was more noticeable in the green composite sample that was processed with ESAO, has been ascribed to an alteration of the hydrogen bonding in the molecular arrangement of PHAs [34]. This suggests that a stronger intramolecular disruption of the biopolymer chains by the presence of cellulose was achieved in the green composites that were processed with the compatibilizers, indicating a higher interaction of MFC with PHBV. Other authors have reported shifts in this peak due to the reaction between the epoxy groups of chain extenders and the carboxyl groups (-COO) in polyesters [45]. For both of the compatibilizers, and particularly more noticeable in the case of ESAO, all of the peak intensities increased in the 1500-800 cm\({}^{-1}\) region. These intensity increases were seen to be more intense for the -O-C-O- stretching modes of the carboxyl group, particularly for the band arising at \(\sim\)1184 cm\({}^{-1}\) and other ester-related bands that were observed at \(\sim\)1055 cm\({}^{-1}\) and \(\sim\)1043 cm\({}^{-1}\)[46]. Therefore, these band signal increases suggest the formation of new ester groups in the compatiphilized composite films of PHBV. However, one should further consider that the bands in this region are also due to the C-H bending modes [47] and, thus, the band assignments in this region are not straightforward. Additionally, the intensity of the latter bands can also be increased due to their close proximity to the bands that were ascribed to the newly formed OH-related bonds, which correspond to the -OH side groups that were produced during esterification reaction. In relation to the chemical changes observed above, it has been described that in the case of ESAO, this compatibilizing agent presents multiple epoxide groups in the styrene chains of the oligomer that are capable of forming new ester groups with the terminal nucleophilic groups of the biopolymer, that is, with its alcohol groups (R-OH) and, fundamentally, acid groups (R-COOH) present at the termination of the polyester chains [48]. On the other hand, it has been shown that part of the other epoxide groups in the ESAO structure, which do not react with the biopolymer, can also react with the -OH groups that are present on the surface of the cellulose, or even with silanol groups (Si-OH) of certain minerals, giving rise to covalent chemical bonding or \"graffing\" that subsequently achieves a better compatibility of the biopolymer composite [34].