3.5.2. D-Limonene Permeability

To test the barrier performance for volatile compounds such as aromas, D-limonene is commonly used. Figure 7 shows the values of LP, where one can observe that the neat PHB film presented the lowest permeability for D-limonene with a value of 3.2 × 10−<sup>15</sup> kg·m·m<sup>−</sup>2·Pa−1·s<sup>−</sup>1. In the case of the nanocomposite films, the LP values increased from 4.7 × 10−<sup>15</sup> kg·m·m<sup>−</sup>2·Pa−1·s<sup>−</sup>1, for the PHB/PdNP film, to 9.6 × 10−<sup>15</sup> and 9.0 × 10−<sup>15</sup> kg·m·m<sup>−</sup>2·Pa−1·s<sup>−</sup>1, for the CTAB- and TEOS-containing PHB/PdNP films, respectively. As discussed above, the presence of the PdNPs and their agglomerates may result in the creation of preferential paths for sorption and diffusion of the aroma molecules hence resulting in a reduced barrier performance. The here-obtained results are showing opposite behavior as the ones reported earlier by Busolo and co-workers [37] who dispersed silver nanoparticles (nAg) in PLA, yielding nanocomposites with enhanced barrier properties. Similarly, Rhim et al. [38] reported agar/nAg composites where they confirmed a substantial improvement in barrier properties of the composite. In the case of the surfactants-containing PHB/PdNP films, it should be also taken into account that permeability of D-limonene in PHB is mainly controlled by a solubilization mechanism due to the capacity of PHAs to uptake large amounts of this organic compound [39]. This supports the fact that plasticized PHB materials present increased values of aroma permeability.

**Figure 7.** Values of D-limonene permeability (LP) of the electrospun poly(3-hydroxybutyrate) (PHB) and palladium nanoparticles (PdNPs) films with and without hexadecyltrimethylammonium bromide (CTAB) and tetraethyl orthosilicate (TEOS) surfactants. Different letters indicate significant differences among the samples (*p* < 0.05).

#### 3.5.3. Oxygen Scavenging Activity

The oxygen scavenging activity of the here-prepared electrospun fibers and films containing the PdNPs was determined by measuring the oxygen scavenging rate (OSR). In relation to the electrospun fibers, Figure 8 shows the decay or depletion of the oxygen concentration as a function of time, for a span time of 800 min, at both 50% and 100% RH. From observation of the graph it can be seen that, while the neat PHB fibers were unable to reduce the amount of oxygen in the cell, comparatively, the free PdNPs in powder form were able to reduce all available headspace oxygen in an extremely short time. The incorporation of the PdNPs into the PHB fibers by electrospinning generated mat samples with intermediate oxygen scavenging activity. However, as it can also be observed in the graph, the performance of the developed nanocomposite fibers was strongly dependent on the RH conditions. All electrospun mats presented a significantly lower oxygen scavenging activity at 50% than at 100% RH. For instance, at the end of the experiment carried out at 50% RH, oxygen depletion varied from almost 40%, for the PHB/PdNP/CTAB fibers, to only ca. 10%, for the PHB/PdNP/TEOS fibers. However, at 100% RH, the electrospun PHB/PdNP mat reached a reduction in the oxygen volume of approximately 85% while both surfactant-containing PHB/PdNP fibers were able to fully consume the whole amount of oxygen after 800 min. It is also worthy to mention that the depletion rate was faster in the case of the PHB/PdNP/CTAB mat, which further confirmed the higher dispersion achieved for the nanoparticles with this surfactant. In relation to the effect of humidity, it is known that moisture favors the catalytic activity of the PdNPs, which can be mainly related to the fact that water can be associatively adsorbed directly on the PdNP surface and thereby interact with the adsorbed hydrogen and oxygen [10]. A possible mechanism is that the adsorbed atomic oxygen and hydrogen forms an OH intermediate that reacts with an adsorbed hydrogen atom or another OH molecule. Another possible explanation is the reaction of adsorbed oxygen with gas-phase hydrogen or with some kind of dihydrogen species weakly adsorbed on the surface. Finally, a concerted reaction of two adsorbed hydrogen atoms and an adsorbed oxygen atom has been also considered [40].

**Figure 8.** Oxygen depletion of the electrospun poly(3-hydroxybutyrate) (PHB) and palladium nanoparticles (PdNPs) fibers with and without hexadecyltrimethylammonium bromide (CTAB) and tetraethyl orthosilicate (TEOS) surfactants. Values were measured at 50% and 100% relative humidity (RH).

In Figure 9, the oxygen volume depletion obtained with the electrospun films at 100% RH are shown. Comparison between Figures 8 and 9 revealed that the oxygen scavenging effect of the films was considerably lower than that of the same material in the fiber form. This reduction in the OSR is related to the higher surface-to-volume ratio of the electrospun fibers than the films, since the fibers mats present an extremely high porosity. In any case, all PHB/PdNP films still presented significant oxygen scavenging capacity and, among the samples tested, the CTAB-containing films showed the highest performance as expected in view of all the above observations. This result can be explained by the better dispersion of the PdNPs achieved in the PHB matrix using CTAB. In agreemen<sup>t</sup> with the data reported here, Ahalawat and co-workers [41] evaluated the simultaneous effects of cationic surfactants on the textural and structural properties of silica nanoparticles. It was observed that the silica nanoparticles displayed better dispersion and lower size than those prepared with other two cationic surfactants using an aqueous TEOS precursor solution with CTAB. In relation to this, it has also been reported that an SiOx matrix between a coating of palladium and the substrate presents higher values of OSR than the palladium coated directly onto PET films [42]. These findings were later confirmed on PLA [10].

**Figure 9.** Oxygen depletion of the electrospun poly(3-hydroxybutyrate) (PHB) and palladium nanoparticles (PdNPs) films with and without hexadecyltrimethylammonium bromide (CTAB) and tetraethyl orthosilicate (TEOS) surfactants. Values were measured at 100% relative humidity (RH).
