*3.3. Mechanical Properties*

Since the resultant electrospun films may be subjected to various kinds of stress during use, the determination of the mechanical properties involves not only scientific but also technological and practical aspects. Table 4 displays the values of elastic modulus (E), tensile strength at yield (σy), elongation at break (εb), and toughness (T) of the electrospun films made of PHBV and PHBV/MCM-41 with eugenol calculated from their strain–stress curves. In general, all the electrospun films presented characteristics of a brittle material associated to the inherent low ductility of PHBV, showing εb and T values below 3% and 0.5 mJ/m3, respectively. The film specimens also presented a relative high mechanical strength. In particular, the mean values of E were comprised in the of 1250–2000 MPa range while <sup>σ</sup>y varied from approximately 18 to 30 MPa. The here-obtained mechanical properties of the PHBV films are similar to those recently reported in our group by Cherpinski et al. [54] for PHB films also prepared by electrospinning and thereafter thermally post-treated, having a E value of 1104 MPa and εb and T values of 2.9% and 0.3 mJ/m3, respectively.

**Figure 6.** Thermogravimetric analysis (TGA) curves for Mobil Composition of Matter (MCM)-41, eugenol, MCM-41 with eugenol, poly(3-hydroxybutyrate-*co*-3-hydroxyvalerate) (PHBV), and PHBV/Mobil Composition of Matter (MCM)-41 without and with eugenol.

It can be observed that the incorporation of MCM-41 with eugenol increased the mechanical strength of the PHBV films while the ductility was slightly reduced. This effect can be related to the reinforcing effect of MCM-41 as a filler in the PHBV matrix, while the smaller impact in ductility may be accounted for the plasticizing effect that the released eugenol may have in the polymer matrix. This mechanical enhancement of E and <sup>σ</sup>y indicates a good transfer of mechanical energy from the hard filler, that is, MCM-41, as well as the interaction between the biopolymer matrix and the silica nanoparticles. Considering both the low concentration of MCM-41 and the presence of eugenol, which acts as plasticizer, the mechanical reinforcement of the filler is thought to dominate the enhancement in E and <sup>σ</sup>y. However, a comparative reduction, change in trend, in mechanical strength was observed when the content of the antimicrobial filler exceeded 10 wt.-%. This effect may be ascribed to a balance between filler agglomeration and stronger plasticizing effect of the released eugenol. High tensile strengths are generally necessary for food packaging films in order to withstand the normal stress encountered during their application, subsequent shipping, and handling [66]. Similarly, Voon et al. [67] reported that the addition of 3 wt.-% of mesoporous silica nanoparticles to bovine gelatin films improved their mechanical resistant properties, that is, <sup>σ</sup>y, while it reduced εb. Others studies have also demonstrated that the incorporation of mesoporous silica nanoparticles can remarkably enhance the mechanical strength in PVOH-based materials due to the intermolecular interactions between the fillers and the polymer when prepared by in situ radical copolymerization [68,69]. As compared to commercial biopolymers for packaging applications, the here-developed electrospun films of PHBV/MCM-41 with eugenol are slightly less deformable but more elastic than thermo-compressed PHBV films, stiffer but less ductile than rigid polylactide

(PLA) films, and mechanically stronger but considerably more brittle than flexible poly(butylene adipate-*co*-terephthalate) (PBAT) [70].

**Table 4.** Mechanical properties in terms of elastic modulus (E), tensile strength at yield (σy), elongation at break (εb), and toughness (T) for the electrospun films of poly(3-hydroxybutyrate-*co*-3-hydroxyvalerate) (PHBV) and PHBV/Mobil Composition of Matter (MCM)-41 with eugenol.

