## 3 Results and Discussion
### Barrier Properties of Green Composites  
Table 6 shows the barrier properties against water and limonene vapors and oxygen of the PHBV films with MFC and compatibilizers. In the food packaging field, among other applications, the barrier performance is one the main parameters to consider, especially for the shelf-life extension of foodstuff [63]. Both water vapor and oxygen barrier properties are important in order to avoid physical and chemical deterioration, whereas limonene transport properties are measured to ascertain the aroma barrier. As can be seen in the table, all of the film samples showed significant differences (\(p<0.05\)) and the neat PHBV films presented values for WVP, LP, and OP of 1.84 \(\times\) 10\({}^{-15}\) kg\(\cdot\)m\(\cdot\)m\({}^{-2}\cdot\)Pa\({}^{-1}\cdot\)s\({}^{-1}\), 1.41 \(\times\) 10\({}^{-14}\) kg\(\cdot\)m\(\cdot\)m\({}^{-2}\cdot\)Pa\({}^{-1}\cdot\)s\({}^{-1}\), and 2.09 \(\times\) 10\({}^{-19}\) m\(\cdot\)m\({}^{-2}\cdot\)Pa\({}^{-1}\cdot\)s\({}^{-1}\), respectively. These permeability values are very similar to those that have been reported previously for films of PHBV with 2-3 mol% 3 HV, also obtained by thermo-compression [41]. In terms of water vapor, one can observe that the incorporation of both MFC and the reactive compatibilizers increased the permeability of PHBV. In this regard, one should consider that PHBV is a medium-to-high barrier material against water vapor, having a slightly better performance than petrochemical PET, that is, 3.01 \(\times\) 10\({}^{-15}\) kg\(\cdot\)m\(\cdot\)m\({}^{-2}\cdot\)Pa\({}^{-1}\cdot\)s\({}^{-1}\)[64] and relatively close to those of polyolefins, such as low-density polyethylene (LDPE) (1.20 \(\times\) 10\({}^{-15}\) kg\(\cdot\)m\(\cdot\)m\({}^{-2}\cdot\)Pa\({}^{-1}\cdot\)s\({}^{-1}\)) and polypropylene (PP) (0.73 \(\times\) 10\({}^{-15}\) kg\(\cdot\)m\(\cdot\)m\({}^{-2}\cdot\)Pa\({}^{-1}\cdot\)s\({}^{-1}\)) [65]. However, cellulose is a hydrophilic material due to its large number of free -OH groups and the presence of nanocelluloses in PHB-based films has been reported to cause a wettability increase [66]. In the case of the compatibilizers, the lower water-barrier performance can be ascribed to the lower crystallinity attained in these samples, as shown above during DSC analysis, due to development of a macromolecular structure that is composed of longer, branched, and/or cross-linked chains [34; 42]. However, it is also worth noting that the green composite film that was compatiplified with TGIC and DCP showed higher barrier performance to water vapor than the uncompatibilized green composite films, but it was still two times more permeable than neat PHBV. This result can be ascribed to the higher dispersion of MFC in this compatibilized sample, by which this presented lower nanofiber aggregates and displays a reduced number of water diffusion paths. In the case of limonene, as opposed to moisture, this is a strong plasticizing component for PHAs and, additionally, solubility plays a key role in permeability [67]. Thus, the presence of MFC contributed to improve the LP values of PHBV, particularly at a filler content of 5 wt %. However, similar to WVP, the use of the reactive compatibilizers in the unfilled PHBV sample films significantly increased (\(p<0.05\)) their permeability to aroma. Interestingly, the compatibilized green composite films, particularly those that were processed with TGIC and DCP, showed a significant improvement (\(p<0.05\)) in the aroma barrier. The latter