3.2.1. Melting Profile

The thermal properties of the electrospun PHB fibers and films containing the PdNPs nanoparticles were firstly investigated by DSC analysis. Table 1 gathers the melting temperatures (Tm1 and Tm2) and enthalpies (ΔHm) obtained from the first heating run. Likewise, the crystallization temperature (Tc) was also obtained from the cooling run. One can observe that when thermal annealing was applied to the fibers, the melting profile of the PHB materials, both temperature and enthalpy, slightly decreased. In particular, the Tm values of the neat PHB-based fibers varied in the 168–170 ◦C range, melting in a single peak, whereas these values were around 3 ◦C lower in their counterpart film samples. In addition, the incorporation of the PdNPs into PHB induced a slight decrease in both the melting temperature and enthalpy as well as resulted in the formation of multiple melting peaks. This observation may sugges<sup>t</sup> that the nanoparticles interfered with the crystallization process of the homopolyester, producing more imperfect crystals composed of thinner lamellae that melted over a wider range at lower temperatures and with lower enthalpies [28]. This effect was more intense in the case of the surfactant-containing film samples, which suggests that the PdNPs were better dispersed then highly influencing the packing process of the PHB chains during cooling. In relation to the crystallization from the melt, the PHB film samples incorporating the PdNPs also showed slightly lower values of Tc than the fibers, which confirms that the nanoparticles provided an anti-nucleating effect on PHB during the film formation. In any case, given that PHB is known to exhibit cold crystallization during the dynamic DSC runs, it becomes difficult to discuss with certainty potential changes in crystallinity and crystalline morphology [21,26].

**Table 1.** Thermal properties obtained from the differential scanning calorimetry (DSC) curves in terms of melting temperature (Tm), normalized melting enthalpy (ΔHm), crystallization temperature (Tc), and normalized crystallization enthalpy (ΔHc) for the various poly(3-hydroxybutyrate) (PHB) and palladium nanoparticles (PdNPs) fibers and films with and without hexadecyltrimethylammonium bromide (CTAB) and tetraethyl orthosilicate (TEOS).


a–e: Different superscripts within the same column indicate significant differences among the samples (*p* < 0.05).
