*3.3. FTIR Analysis*

FTIR is a powerful tool, which is very sensitive to the molecular environment, to investigate the structural changes that occur in the material during any chemical or physical process. In Figure 5, the FTIR spectra are presented for the above-mentioned PHB-based fibers and films. From the given spectra of the PHB materials, the band centered at 1722 cm<sup>−</sup><sup>1</sup> is an indicative of the C=O stretching vibration for the biopolyester molecule. The absorption bands in the 1200–1300 cm<sup>−</sup><sup>1</sup> range were previously related to the presence of the C–O–C stretching vibrations whereas the band centered at approximately 980 cm<sup>−</sup><sup>1</sup> has been ascribed to stretching bands of the carbon–carbon single bond (C–C) in PHAs [21,30]. In the spectra of the pure surfactants, it is interesting to be noted that the bands at 2873 cm<sup>−</sup><sup>1</sup> and 2977 cm<sup>−</sup><sup>1</sup> correspond to the anti-symmetric C–H stretching of CTAB [19] while the bands located at 1168 cm<sup>−</sup>1, 1103 cm<sup>−</sup>1, and 1080 cm<sup>−</sup><sup>1</sup> have been assigned to the CH3 rocking and C–O asymmetric stretching of TEOS, respectively [31]. For the surfactant-containing PHB films, the above surfactant bands or others could not be detected most likely due to the low concentration in which these additives are present in the composite.

**Figure 5.** Fourier transform infrared (FTIR) spectra of: (**a**) Hexadecyltrimethylammonium bromide (CTAB); (**b**) Tetraethyl orthosilicate (TEOS); (**c**) Poly(3-hydroxybutyrate) (PHB) fibers; (**d**) PHB film; (**e**) PHB/palladium nanoparticles (PdNP) fibers; (**f**) PHB/PdNP film; (**g**) PHB/PdNP/CTAB fibers; (**h**) PHB/PdNP/CTAB film; (**i**) PHB/PdNP/TEOS fibers; (**j**) PHB/PdNP/TEOS film. Arrows indicate the chemical bonds and/or groups discussed in the text.

Table 3 gathers the band area ratio A1230:A1453 and the 1722 cm<sup>−</sup><sup>1</sup> carbonyl band width at half maximum, which have been recently associated with crystallinity content in electrospun PHB materials [21]. Both a 1722 cm<sup>−</sup><sup>1</sup> band broadening and reduction in the A1230:A1453 band area ratio have been previously connected with a reduction in molecular order and, hence, in crystallinity. Figure 5 and also Table 3 indicate that the band at 1722 cm<sup>−</sup><sup>1</sup> tended to broaden somehow in the case of the film samples as compared to their respective fibers, which can be indicative of a phenomenon of molecular disorder due to thermal treatment in the material. The band ratio seems to be less sensitive, as previously discussed [21], than the carbonyl band. These results may be in close agreemen<sup>t</sup> with those previously reported by Pachekoski et al. [32], in which it was indicated that band widening with annealing correlates with the changes in molecular backbone stability and, hence, crystallinity in PHB. Similarly, Mottin et al. [33] also reported a change in crystallinity in PHB by annealing process.



## *3.4. Mechanical Properties*

Table 4 presents the mechanical properties, obtained from the tensile tests, of the electrospun PHB films. The incorporation of the PdNPs into PHB caused an increase in both the modulus of elasticity and tensile strength, therefore increasing the elastic deformation and stiffness of the PHB film. The enhancement in mechanical resistance attained in the nanocomposite films can be attributed to the combination of fairly good nanoparticle dispersion and strong interfacial adhesion between both phases through interactions via H-bonding of PHB [29]. In relation to elongation at break, all films presented values of around 3%, which confirms the intrinsic brittleness of PHB. In any case, the presence of the PdNPs did not alter the film ductility and toughness characteristics of PHB while the effect of the surfactants addition on their mechanical performance was also not statistically significant.

**Table 4.** Mechanical properties in terms of elastic modulus (E), tensile strength at break (σb), elongation at break (%εb), and toughness (T) of the electrospun poly(3-hydroxybutyrate) (PHB) and palladium nanoparticles (PdNPs) films with and without hexadecyltrimethylammonium bromide (CTAB) and tetraethyl orthosilicate (TEOS).


a–c: Different superscripts within the same column indicate significant differences among the samples (*p* < 0.05). \* Obtained in previous work [21].

## *3.5. Barrier Properties*

#### 3.5.1. Water Vapor Permeability

Measuring the loss or gain in water content is a common method to estimate the WVP of film samples. The WVP values of the electrospun PHB films are gathered in Figure 6. It can be observed that the neat PHB film showed a higher barrier performance to water vapor than their respective nanocomposites with the PdNPs. In particular, while the neat PHB film showed a WVP value of 5.2 × 10−<sup>15</sup> kg·m·m<sup>−</sup>2·Pa−1·s<sup>−</sup>1, this value was 1.2 × 10−<sup>14</sup> kg·m·m<sup>−</sup>2·Pa−1·s<sup>−</sup><sup>1</sup> for the PHB/PdNP film. The observed permeability increase in the nanocomposite films can be related to the existence of not bonded interfacial regions acting as preferential paths, especially in the vicinity of agglomerates. These preferential pathways could accelerate the diffusion of gas molecules, thus increasing the diffusion coefficient [34]. Interestingly, the WVP values of the PHB/PdNP films was lower in the case of the CTAB-containing film, i.e., 8.0 × 10−<sup>15</sup> kg·m·m<sup>−</sup>2·Pa−1·s<sup>−</sup>1, and considerably higher for the

film with TEOS, i.e., 6.6 × 10−<sup>14</sup> kg·m·m<sup>−</sup>2·Pa−1·s<sup>−</sup>1. This result suggests that the dispersion in the PHB/PdNP/CTAB film was higher and, then, the sizes of such unbonded interfacial regions were lower. In addition, the high WPV value observed for the PHB/PdNP/TEOS film suggests that this film sample could be also plasticized by the surfactant, increasing the free volume of the film and favoring the diffusion of water vapor molecules through the film sample [35,36].

**Figure 6.** Values of water vapor permeability (WVP) of the electrospun poly(3-hydroxybutyrate) (PHB) and palladium nanoparticles (PdNPs) films with and without hexadecyltrimethylammonium bromide (CTAB) and tetraethyl orthosilicate (TEOS) surfactants. Different letters indicate significant differences among the samples (*p* < 0.05).
