*3.4. Barrier properties*

Table 5 gathers the WVP and LP values of the electrospun PHBV/MCM-41 with eugenol films. It can be observed that the incorporation of low contents of MCM-41 with eugenol, that is, 2.5 and 5 wt.-%, induced an increase in the WVP values of the electrospun PHBV films while the water vapor barrier properties were improved for contents higher than 7.5 wt.-%. A similar effect was observed in the case of LP. The resultant increase in permeability observed at low filler loadings can be related to the plasticizing effect of eugenol on the PHBV matrix outweighing the barrier effect of the MCM-41 filler, as above discussed during the thermal analysis, with a subsequent increase in the matrix free volume. At higher contents, however, the barrier improvements can be ascribed to the presence of large quantities of mesoporous silica nanoparticles. Then, MCM-41 successfully acted as barrier elements forcing the permeant molecules to travel through a longer path to permeate across according to the early theory suggested by Nielsen [71]. It is also worthy to mention the change in the barrier trend observed for the film samples containing the highest filler contents, that is, 20 wt.-% MCM-41 with eugenol. This permeability change in trend can also be ascribed to the above-mentioned higher filler agglomeration resulting in a somewhat increased porosity as observed in the morphological analysis, leading to preferential paths for diffusion.

The here-prepared electrospun films of PHBV/MCM-41 with eugenol showed higher WVP values than PHBV films with 12 mol.-% HV prepared by solvent casting, that is, 1.27 × 10−<sup>14</sup> kg·m·m<sup>−</sup>2·Pa−1·s<sup>−</sup><sup>1</sup> [72] or PHB films obtained by compression-molded, that is, 1.7 × 10−<sup>15</sup> kg·m·m<sup>−</sup>2·Pa−1·s<sup>−</sup><sup>1</sup> [73], which can be related to the higher mol.-% HV fraction in the here-used copolyester. Therefore, it can be considered that intermediate contents of MCM-41 with eugenol inside the fibers promoted lower free volume available for diffusion. Similar results were reported for instance by Hashemi Tabatabaei et al. [74], who showed that the incorporation of 5 wt.-% mesoporous silica microparticles decreased the WVP value from 8.9 <sup>g</sup>·m·m<sup>−</sup>2·Pa−1·s<sup>−</sup><sup>1</sup> to 1.6 × 10−<sup>11</sup> g·m·<sup>m</sup> −2 ·Pa−1·s<sup>−</sup><sup>1</sup> in gelatin/k-carrageenan films. Also, Hassannia-Kolaee et al. [75] reported a reduction of up to approximately 33% in WVP for whey protein isolate (WPI)/pullulan (PUL) films containing 1, 3, and 5 wt.-% mesoporous silica nanoparticles prepared by a casting method. The barrier improvement achieved was attributed to the formation of hydrogen bonds between the polymer hydroxyl groups and the oxygen atoms of silica and also to the good dispersion of the nanoparticles in the polymer matrix. In relation to eugenol, some studies have also demonstrated that the direct incorporation of EOs, among them eugenol, in polymer films do not induce significant change in WVP, concluding that water permeability basically depends on the hydrophilic–hydrophobic ratio of the film constituents [76,77]. Other studies have reported that the addition of EOs can negatively increase the water permeability depending on the nature of the polymer matrix and the type and concentration of EO [78]. However, this impairment may be attributed to the difficulties encountered to integrate the hydrophobic EO in hydrophilic networks that might cause matrix disruptions and create void spaces

at the polymer–oil interface [79]. However, in the current study, the expected plasticizing effect of the hydrophobic eugenol within the PHBV matrix is seen detrimental for the barrier performance at lower silica loadings.

**Table 5.** Permeability values in terms of water vapor permeability (WVP) and D-limonene permeability (LP) for the electrospun films of poly(3-hydroxybutyrate-*co*-3-hydroxyvalerate) (PHBV) and PHBV/Mobil Composition of Matter (MCM)-41 with eugenol.

