*3.2. Morphology*

The morphology of the electrospun ultrathin neat PHBV fibers and the PHBV fibers containing the OEO, RE, and GTE was analyzed by SEM and the images are shown in Figure 1. The neat PHBV fibers, without EOs and NEs, were relatively uniform and presented a mean diameter of approximately 1 μm, as seen in Figure 1A. The morphology of the here-obtained electrospun ultrathin PHBV fibers was similar to those fibers reported by Melendez-Rodriguez et al. [39], showing diameters of ~1.32 μm. The PHBV fibers containing 10 wt% OEO, RE, and GTE are presented in Figure 1B–D, respectively. The diameters of the fibers were relatively similar, with a mean size of approximately 0.8 μm. The reduction achieved in the fiber diameter could be related to the slightly lower viscosities observed for the PHBV solutions containing the active substances. It was also evident that all the electrospun fibers were uniform and smooth, without any superficial and structural defects, which indicated that the addition of both EOs and NEs did not alter the fiber formation during electrospinning.

Figure 2 shows the SEM images of the electrospun materials, after annealing at 125 ◦C, in their cross-section and top views. In all cases, one can observe that the thermal post-treatment on the electrospun mats resulted in the formation of a continuous film. Figure 2A corresponds to the cross-section of the neat PHBV film, that is, without OEO and NEs, which presented an average thickness of ~80 μm. In Figure 2B, one can observe that the film sample also exhibited a homogeneous surface without cracks and/or pores. Similar morphologies were reported, for instance, by Cherpinski et al. [38] for electrospun PHB fibers thermally post-treated at 160 ◦C. The particular change from fiber-based to film-like morphology was ascribed to a process of fibers coalescence during annealing. Figure 2C,E,G show the cross-sections of the electrospun PHBV films containing OEO, RE, and GTE, respectively. The thicknesses of all the film samples were kept at ~80 μm. The presence of a certain number of pores can be related to the partial evaporation of the oily materials enclosed in the PHBV film during the thermal post-treatment. Similar voids were observed in the electrospun PHBV films derived from biowaste by Melendez-Rodriguez et al. [39], when temperatures close to *Tm* were applied, which was ascribed to the partial material melting and/or degradation. However, in Figure 2D,F,H, showing the top view of the film samples containing the active substances, it can be seen that the PHBV films still showed a smooth and homogeneous surface without pores and cracks. Therefore, despite the fact that the active substances were partially released during the film-forming process, a good compatibility and then a high solubility of OEO and the NEs with the PHBV matrix was attained.

**Figure 1.** Scanning electron microscopy (SEM) micrographs of the electrospun fibers of: (**A**) Neat poly(3-hydroxybutyrate-*co*-3-hydroxyvalerate) (PHBV); (**B**) Oregano essential oil (OEO)-containing PHBV; (**C**) Rosemary extract (RE)-containing PHBV; (**D**) Green tea tree extract (GTE)-containing PHBV. Scale markers of 50 μm.

**Figure 2.** Scanning electron microscopy (SEM) micrographs of the electrospun films in their cross-section (left column) and top view (right column) of: (**A**,**B**) Neat poly(3-hydroxybutyrate-*co*-3- hydroxyvalerate) (PHBV); (**C**,**D**) Oregano essential oil (OEO)-containing PHBV; (**E**,**F**) Rosemary extract (RE)-containing PHBV; (**G**,**H**) Green tea tree extract (GTE)-containing PHBV. Scale markers of 50 μm.
