3.2.2. Thermal Stability

Figure 4 shows the evolution of the mass loss as a function of temperature for the PHB samples, including the curves of the first derivative analysis (blue lines). As shown in Table 2, it can be seen that the neat PHB initiated degradation at 207 ◦C while the biopolymer fully decomposed in two steps, seen at 262 ◦C and 348 ◦C, providing a residual mass of approximately 3%. Thermal degradation of the PHB films containing the PdNPs and the surfactants also occurred in two stages with a slight increase in thermal stability. In particular, the onset degradation took place in the 220–230 ◦C range that continued to the first degradation stage at 270–275 ◦C and, at a slower rate, to over 360–380 ◦C, remaining a residue of 3–4% of the initial mass of the sample. In this sense, Díez-Pascual et al. [29] demonstrated that the incorporation of zinc oxide (ZnO) improved the heat resistance of PHB, which was ascribed to the barrier effect of the nanoparticles that effectively hindered the diffusion of decomposition products during thermal degradation. Therefore, the PdNPs probably also functioned as a thermal barrier, absorbing heat, thus also resulting in an enhanced thermal stability.

It is also worthy to note the slight increase observed in the thermal stability of the PHB/PdNP films with the incorporation of both surfactants, especially in the case of CTAB. This suggests that the surfactant increased the matrix–nanoparticle interaction, inducing a positive delay in thermal degradation. Taking into account that the ammonium salts degrade at the temperature range of 150–460 ◦C, the surfactants could successfully provide a bonding effect between both components of the nanocomposite and, eventually, enhance the whole thermal stability of PHAs.

**Figure 4.** Thermogravimetric analysis (TGA) curves of the electrospun poly(3-hydroxybutyrate) (PHB) and palladium nanoparticles (PdNPs) films with and without hexadecyltrimethylammonium bromide (CTAB) and tetraethyl orthosilicate (TEOS) surfactants.

**Table 2.** Mean values of thermal stability obtained from the thermogravimetric analysis (TGA) curves of the electrospun poly(3-hydroxybutyrate) (PHB) and palladium nanoparticles (PdNPs) films with and without hexadecyltrimethylammonium bromide (CTAB) and tetraethyl orthosilicate (TEOS) in terms of degradation temperature at 5% of mass loss (T5%), degradation temperature (Tdeg), and residual mass at 500 ◦C (R500).

