*3.2. Thermal Properties*

Table 2 displays the main thermal transitions, obtained by DSC during the heating and cooling steps, of the annealed electrospun neat PHBV film and the films containing MCM-41 without and with eugenol. It can be observed that the neat PHBV film presented a glass transition temperature (Tg) of 2.6 ± 0.4, while the addition of MCM-41 without eugenol had a negligible effect on Tg. Interestingly, after the incorporation of MCM-41 with eugenol, the Tg values were reduced to 1.8–0.6 ◦C in the PHBV film samples. Reductions of Tg are habitually associated to a plasticization process by low-molecular weight (M W) molecules with high chemical affinity to the polymer matrix by which the free volume of the polymer is enlarged since they increase the distance between the polymer chains and then favor segmental motion [57]. Some previous studies have already reported the plasticizing effect of eugenol on different polymer matrices. For instance, Fernandes Nassar et al. [58] reported a reduction in Tg when eugenol was incorporated into soy protein isolate (SPI) films, ascribing this effect to the plasticizing role that the aroma compound played in the protein matrix. Also, Narayanan et al. [59] observed a reduction in Tg from 4 ◦C, for the neat PHB film, to −14 ◦C, for PHB films containing up to 200 μg/g of eugenol.

Whereas cold crystallization phenomenon was not observed in any of the PHBV films during heating, all the samples crystallized from the melt during cooling. In particular, the neat PHBV film showed a crystallization temperature (Tc) of 116.8 ± 0.5 ◦C. The presence of MCM-41 without eugenol increased the crystallization temperature of PHBV, up to reaching a maximum value of 120.5 ◦C for the film filled at 15 wt.-%. This result suggests that the nanoparticles provided a nucleating effect on the PHBV molecules, except for the film filled with 20 wt.-% MCM-41, possibly due to nanoparticles agglomeration as previously described during the morphological analysis. On the contrary, one can observe that the Tc values of the film samples containing MCM-41 with eugenol decreased as the filler content increased. Then, the Tc value was reduced up to a value of 113.8 ± 0.6 ◦C for the film filled with 20 wt.-% MCM-41 with eugenol. This restrained crystallization of PHBV can be ascribed to the above-described plasticizing effect of eugenol, which impair the packing of the polymer chains to form crystals.

During heating, the neat PHBV film melted in a single peak at 170.4 ± 0.2 ◦C while all the PHBV films containing MCM-41 without eugenol presented similar T m values in the 169–171 ◦C range. However, the T m values progressively reduced in the PHBV films containing MCM-41 with eugenol was as the filler content increased. Up to contents of 15 wt.-% MCM-41 with eugenol, the PHBV films presented a single melting peak in the 163–171 ◦C range, whereas the film filled with 20 wt.-% MCM-41 with eugenol showed two endothermic peaks, starting melting at 160.5 ± 1.5 ◦C. Therefore, the MCM-41 particles when loaded with eugenol were able to impair and induce some defects in the PHBV crystals, particularly at the highest tested contents. It is also worthy to note that the presence of MCM-41 without eugenol, up to fillings of 15 wt.-%, increased the values of enthalpy of melting ( Δ H m), confirming the formation of more perfect PHBV crystals with thicker lamellae by a

nucleation phenomenon. As opposite, all the PHBV films with MCM-41 with eugenol presented lower values of ΔHm, being this reduction significantly noticeable for the films filled with contents above 15 wt.-%. Therefore, the presence of MCM-41 with eugenol impaired the crystallization of PHBV due to the above-described reduction of the biopolymer segments packing. It has been similarly reported that the addition of mesoporous silica nanoparticles has a slight influence on Tg or Tm in polymer nanocomposites [60,61], therefore supporting that the here-observed suppressed effect on the melt behavior is ascribed to eugenol. In this sense, Garrido-Miranda et al. [62] showed that the Tm value of PHB/thermoplastic starch (TPS)/organically modified montmorillonite (OMMT) nanocomposites was reduced by approximately 4 ◦C when 3 wt.-% eugenol was incorporated, concluding that eugenol induces the formation of less perfect crystals. Woranuch et al. [63] also observed a ΔHm reduction when eugenol-loaded chitosan nanoparticles were incorporated into thermoplastic flour (TPF) made of cassava, rice, and waxy rice through an extrusion process. The reduction observed was related to a plasticization by eugenol.

**Table 2.** Thermal properties in terms of glass transition temperature (Tg), crystallization temperature (Tc), melting temperature (Tm), and normalized enthalpy of melting (ΔHm) for the electrospun films of poly(3-hydroxybutyrate-*co*-3-hydroxyvalerate) (PHBV) and PHBV/Mobil Composition of Matter (MCM)-41 without and with eugenol.


Figure 6 depicts the TGA curves of MCM-41 and MCM-41 with eugenol powders, the eugenol-free oil, and the electrospun films made of neat PHBV and PHBV/MCM-41 without and with eugenol. Table 3 gathers the main relevant thermal parameters obtained from the TGA curves. As one can observe in the graph, the neat MCM-41 particles presented a mass loss of ~5% at a temperature close to 100 ◦C, which can be ascribed to residual humidity on the surface and/or in the pores of the nanoparticles. In addition, the neat MCM-41 particles provided a residual mass of 95.0 ± 2.3% measured at 800 ◦C. On the contrary, the eugenol free oil had a relatively low thermal stability, showing full decomposition at approximately 200 ◦C. Moreover, comparison of the TGA curves of the MCM-41 nanoparticles with and without eugenol corroborated that the eugenol loading was 49.5 ± 1.2%. This loading capacity of MCM-41 was higher than other encapsulation techniques reported for polyphenols [64].

In relation to the neat PHBV film, a low-intense first weight loss process (<1%) was observed at 100 ◦C due to absorbed moisture and/or volatiles leaving the samples. Trapped solvent losses were discarded by Fourier transform infrared (FTIR) spectroscopy and TGA of the neat PHBV fibers (results not shown). One can also observe that the biopolymer presented the onset of degradation, measured at the temperature at which the mass loss was 5% (T5%), at 259.9 ± 1.2 ◦C. The degradation temperature (Tdeg) occurred at 277.3 ± 0.6 ◦C, degrading in a single step and producing a residual mass of 2.0 ± 0.2% at 800 ◦C. In addition, the weight loss process corresponding to thermal decomposition reaction of the biopolymer chain occurred sharply, approximately from 225 ◦C to 275 ◦C. The thermal degradation onset was shifted to lower temperatures when both the MCM-41 without and with eugenol, in all the

composition range, was incorporated. This result suggests that the nanoparticles catalyzed thermal degradation. Interestingly, the T5% and Tdeg values were slightly improved at the lowest content of MCM-41 with eugenol, which can be related to the above-described nucleating effect and restricted mobility of the biopolymer chains by the presence of MCM-41 and eugenol. However, the thermal stability was reduced at the higher filler contents, that is, 15 wt.-% and 20 wt.-% MCM-41 with eugenol, due to the high content of both MCM-41 and eugenol. Furthermore, the residual weight at 800 ◦C of the PHBV/MCM-41 with eugenol films increased due to the presence of the mesoporous silica nanoparticles. In any case, the incorporation of up to 10 wt.-% of MCM-41 with eugenol had a relatively low influence on the thermal stability of the PHBV films, which can be considered a positive result since they encapsulate an active component with low thermal stability. In this sense, Requena et al. [65] reported that the incorporation of carvacrol and eugenol enhanced the thermal sensitivity of PHBV, decreasing the onset temperature, whereas the incorporation of whole essential oils (oregano and clove) slightly promoted its thermal stability. The latter effect was suggested to occur due to a strong bonding of the eugenol with the polymer network.

**Table 3.** Thermal properties in terms of mass loss was 5% (T5%), degradation temperature (Tdeg), mass loss at Tdeg, and a residual mass at 800 ◦C for Mobil Composition of Matter (MCM)-41, MCM-41 with eugenol, eugenol free oil, and electrospun films of poly(3-hydroxybutyrate-*co*-3-hydroxyvalerate) (PHBV) and PHBV/MCM-41 without and with eugenol.

