*3.4. Thermal Stability*

Figure 4 includes the weight loss curves of the free active substances and of the electrospun PHBV films obtained by TGA. The curves for the neat OEO, RE, and GTE are shown in Figure 4A, while the values of the onset degradation temperature, that is, the temperature at 5% weight loss (*T5%*), degradation temperature ( *Tdeg*), and residual mass at 700 ◦C are gathered in Table 3. One can observe that OEO presented the lowest thermal stability, showing values of *T5%* and *Tdeg* of 101.5 ◦C and 178.4 ◦C, respectively, with a respective weight loss of 74.16% at *Tdeg*, corresponding to the volatilization and/or degradation of low-M W volatile compounds present in the OEO (e.g., carvacrol, thymol, and pinene). In this sense, other authors have also reported that the EOs and NEs of oregano are among the most thermally unstable active substances. For instance, Barbieri et al. [53] reported that 96–97% of the OEO's weight was lost between 200 ◦C and 216 ◦C, attributed to its volatilization. In another work, Yang et al. [54] determined a significant degradation of all terpenes extracted oregano leaves in the 200–250 ◦C range. Similarly, Gibara Guimarães et al. [55] reported the fully thermal decomposition of carvacrol, the most representative active compound of oregano, at 168 ◦C. Opposite to OEO, both RE and GTE showed a high thermal stability with a similar mass loss profile. In particular, both active substances presented *T5%* values over 350 ◦C, with *Tdeg* values of 412.7 ◦C (52.45%) and 411.5 ◦C (49.89%) for RE and and GTE, respectively. Similar results, though slighlty lower, were reported for RE by Piñeros-Hernandez et al. [56], showing a significant mass loss at 300 ◦C, corresponding to the decomposition of phenolic diterpenes, that is, carnosic acid, carnosol, and rosmarinic acid. Likewise, Cordeiro et al. [57] obtained a mass loss as low as 6% up to 190 ◦C. In the case of GTE, <sup>L</sup>ópez de Dicastillo et al. [58] determined that it remained stable up to the range of 200–400 ◦C, where the thermal degradation of partially glycosylated catechins occurs. Furthermore, all active substances produced a residual mass below 1%.

In Figure 4B, the weight loss curves of the electrospun PHBV films containing OEO, RE, and GTE are gathered. The neat PHBV film was thermally stable up to 251.5 ◦C, showing a *Tdeg* value of 278.7 ◦C (47.74%) and a residual mass of 2.10%. While the incorporation of RE and GTE slightly reduced the thermal stability by 5–10 ◦C, the presence of OEO considerably reduced the onset of degradation, showing a *T5%* value of 197.5 ◦C. It is also worthy to mention, however, that all active substances increased the thermally decomposed mass at *Tdeg*, that is, the weight values decreased to the 60–70 % range. Therefore, the here-produced active PHBV films were stable up to 200 ◦C, which certainly opened up their application as an active food packaging interlayer and/or coating.

**Figure 4.** Weight loss as a function of temperature for: ( **A**) Oregano essential oil (OEO), rosemary extract (RE), and green tea tree extract (GTE); (**B**) Electrospun films of neat poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and PHBV containing OEO, RE, and GTE.


**Table 3.** Thermal properties of oregano essential oil (OEO), rosemary extract (RE), and green tea tree extract (GTE) and of the electrospun films of poly(3-hydroxybutyrate-*co*-3-hydroxyvalerate) (PHBV) containing OEO, RE, and GTE in terms of temperature at 5 % weight loss (*T5%*), degradation temperature (*Tdeg*), and residual mass at 700 ◦C.

#### *3.5. Water Contact Angle*

The water contact angle refers to the degree of affinity of water with a surface, which defines the degree of hydrophilicity/hydrophobicity of a given polymer material [59]. In Figure 5, the water drop images on the films, as well as the values of their contact angles, are shown for the electrospun PHBV films. In Figure 5A, one can observe that the neat PHBV film presented an angle of 103.61◦, which is characteristic of hydrophobic materials [60]. In all cases, the incorporation of the active substances resulted in a significant decrease in hydrophobicity (*p* < 0.05). Figure 5B shows that the OEO-containing PHBV film presented a water contact angle of 82.23◦, whilst these values were even lower for both the films containing RE (Figure 5C), that is, 73.86◦, and GTE (Figure 5D), that is, 71.26◦. The reduction achieved could be related to the presence of the oily molecules on the surfaces of the PHBV films, which decreased the surface tension. A similar decrease in the water contact angles was observed by Galus and Kadzi ´nska [61] in whey protein isolate (WPI) edible films when almond and walnut oils were added. In any case, following the terms "hydrophobic" and "hydrophilic", defined for θ > 65 ◦ and ≤65 ◦, respectively [62], the angles for each of the films studied were still within the hydrophobic range.

**Figure 5.** Water contact angle of the electrospun films 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.

## *3.6. Active Properties*
