3.3.1. Morphology

SEM micrographs of the electrospun fibers before the annealing treatment and their corresponding diameter distribution histograms are shown in Figure 1 (A1-D1, A2-D2, vertically). The electrospun mats presented a fibrous, bead-free morphology. The fibers were regular, smooth, and without aggregates, suggesting that a homogeneous solid dispersion of SE is achieved in the fibers (Figure 1, A1-D1, vertically). As shown in histograms (Figure 1, A2-D2, vertically), SE addition affected the fibers' diameter. It was observed that the increase in SE content led to reduced mean values of fiber diameter (Table 1). This effect is related to the changes in the solution properties induced by SE addition. Likewise, higher conductivity and lower viscosity and surface tension altered the polymer chain entanglements, generating an increase in the stretching forces in the jet which resulted in the decreased electrospun fiber diameters [6,40]. Similarly, decreased diameter of PCL fibers was reported when other active compounds, such as black pepper oleoresins [25], carvacrol [41], or cefazolin [42] were incorporated within the polymeric matrix.

**Figure 1.** Scanning electron microscopy (SEM) micrographs of the electrospun fibers (A1-D1, vertically), diameter distribution histograms (A2–D2, vertically) and cross-sections of the annealed films (A3–D3, vertically) of the plain poly(ε-caprolactone) PCL (A1–3) and the formulations containing 5% (B1–3), 10% (C1–3), and 20% (D1–3) of SE.

Figure 1 (A3–D3, vertically) presents the cross-sections of the cryo-fractured annealed films. As can be observed, the electrospun fibrous mats were packed into continuous films due to fibers' coalescence during the annealing treatment. Furthermore, different structures were revealed depending on the SE content. The plain PCL film exhibited a somewhat rougher appearance as compared to the samples containing SE at levels 5% and 10%, which appeared somewhat more compact, smooth, and homogeneous. Higher roughness for the PCL film may be due to the higher fiber diameter. On the other hand, the incorporation of 20% SE led to a more porous morphology. This may be due to the generation of volatiles arising from the higher SE concentration during the annealing process.

Film thickness was found to be lower for the samples with the higher SE content due to most likely lower fiber diameter as a result of a more efficient fiber packing after the annealing treatment (see Table 1).
