Contact Angle

The wettability and water absorption of material were determined by measuring the contact angle of a 4 μL water drop using a Fibrodat 1100 (Fibro System AB, Molenbaan, The Netherlands) dynamic contact angle tester. The change in the contact angle in the first 30 s was measured and evaluated.

#### Surface Characterization with a Scanning Electron Microscope (SEM)

The surfaces/structures of electrospun materials were examined with electron scanning microscopy to acquire information about fiber arrangemen<sup>t</sup> and the material network in general. The micrographs were taken with a scanning electron microscope JSM-6060 LV (Jeol Ltd., Akishima, Japan). The instrument was operated at 10 kV at 1500× magnification.

#### **3. Results and Discussion**

In the first part of this study, WTPS obtained from a local wastewater treatment plant was chemically characterized then purified and prepared for the electrospinning process. This process produced a new thin material that could be used for packaging applications. In the final part of the study, the newly obtained material was characterized in terms of tensile strength and wettability.

#### *3.1. FTIR and 1H NMR Analysis of Wastewater Treatment Plant Sludge*

In order to determine the presence of sodium polyacrylate in purified WTPS, FTIR spectra of purified WTPS were analyzed and compared to the sodium polyacrylate spectra (Figure 1).

**Figure 1.** Fourier-transform infrared (FTIR) transmission spectra (%T) at the wavelength (cm−1) of purified WTPS and sodium polyacrylate.

As shown, three peaks were observed in the pure sodium polyacrylate at 1464, 1571 and 1625 cm<sup>−</sup>1. The peaks at 1464 and 1625 cm<sup>−</sup><sup>1</sup> are related to the bending of –CH2– and the C=O stretching mode in the carboxylic acid group of sodium polyacrylate [30,31]. From the spectra, it is evident that purified WTPS and sodium polyacrylate share common peaks at 1464, 1571, and 1625 cm<sup>−</sup><sup>1</sup> corresponding to the bands of sodium polyacrylate. As previously reported in the literature, the strong peak at 1571 cm<sup>−</sup><sup>1</sup> remains in the FITR spectra of WTPS, which is also masked by the carboxylate ions of WTPS and sodium polyacrylate [31]. At the same time, in the FTIR spectra of sodium polyacrylate and purified WTPS, the peaks in the range of 1625 and 1800 cm<sup>−</sup><sup>1</sup> are assigned to the carbonyl groups [30–32]. In the purified WTPS sample, the addition of sodium polyacrylate was detected as expected.

To further analyze the unpurified and purified WTPS, a comparison of the spectra was performed (Figure 2). As can be seen from the spectra, in unpurified WTPS, the peak at 1554 cm<sup>−</sup><sup>1</sup> is shifted to 1571 in purified WTPS. These two peaks correspond to the carboxylate ions [31]. In purified WTPS, more distinct peaks were observed at 2911, 1362, 1181, and 915 cm<sup>−</sup><sup>1</sup> wavelengths.

**Figure 2.** Fourier-transform infrared (FTIR) transmission spectra (%T) at the wavelength (cm−1) of unpurified and purified WTPS.

Figure 3 illustrates the 1H-NMR of purified WTPS and sodium polyacrylate. Sodium polyacrylate was used for the purposes of WTPS analysis. Therefore, the presence of this component was also detected in the purified WTPS sample. The characteristic solid peaks on the purified WTPS appeared between 1.6 and 2.6 ppm, which are also typical for sodium polyacrylate. A signal at 1.6 ppm belongs to the hydrogen of methylene. The spectra also show the resonance signal CH2O–COOH bond at 2.580 ppm. As described in the literature on wastewater analysis, peaks between 3.3 and 4.6 ppm could correspond to glycine (3.5 ppm), glycerol (3.6 ppm), serine (3.7 ppm), 2-aminopropanol (3.9 ppm) and 3-hidroxibutyric acid (4.2 ppm) [33]. For the purified WTPS, the strong peaks were detected at 3.7 ppm corresponding to serine and 4.2 ppm corresponding to 3-hidroxibutyric acid.

Figure 4 presents 1H-NMR spectra of unpurified WTPS and purified WTPS. The strong peak at 1.53 ppm corresponds to water in chloroform. The peaks of unpurified WTPS are stronger, indicating the addition of carboxylic acid in the sample compared to purified WTPS. At the same time, the minor addition of PHA was observed in both samples, as shown in Figure 4. PHA polymers contain hydrogen and carbon; therefore, typical peaks such as CH2 at 1.35 ppm and CH3 at 0.85 ppm were detected [34]. Typical peaks for PHA also include CH at 5.2 ppm and CH2 at 2.55 ppm, which were not detected

in our samples [35]. The peak CH2 at 1.6 ppm was present but was also masked by the water and chloroform.

**Figure 3.** 1H-NMR spectra of purified WTPS and sodium polyacrylate.

**Figure 4.** 1H-NMR spectra of unpurified and purified WTPS.

In general, the quantitative estimate of PHA could be determined by the intensity ratio of the signals, and as before, the unpurified sample had higher peaks compared to the unpurified WTPS, especially in the detection of PHA groups. This could be due to the purification process and the reduction of the amount of PHA in the purified sample.

#### *3.2. Determination of Optimal Viscosity and Electrospinning Parameters*

The optimal viscosity for the WTPS suspension was experimentally determined to be 20,000 mPas ( ±500 mPas). This viscosity provided satisfactory nanofiber morphology without visible defects, beads or droplets within the fiber structure (Figure 5 with marked arrows). Purified WTPS with lower viscosities did not lead to uniform structure of the electrospun material. During the electrospinning process of such suspensions, undesirable electrospraying effects occurred in the form of tiny droplets. Higher viscosities (above 20,000 mPas), on the other hand, clogged the electrospinning device system and thus disrupted the production process.

**Figure 5.** SEM micrographs of final material (**a**) optimal-electrospun from suspension with a viscosity of 20,000 mPas; (**b**) electrospun from suspension with a viscosity of 5000 mPas; (**c**) electrospun from suspension with a viscosity of 2000 mPas at 1500× magnification.

> As described previously, the optimal parameters of the electrospinning process were a voltage of 19 kV, a flow rate of 200 μL/h and a distance of 15 cm between needle and collector. Only the sample prepared from a suspension with optimal viscosity was further analyzed. Based on the results of the preliminary tests, it shows better performance compared to the other two (sample b and c).

#### *3.3. Differential Scanning Calorimetry Characterization of the Final Material*

The DSC heating curve is shown in Figure 6. The analysis was done in one heating cycle only, since the nanofiber morphology of the material is disrupted by the heating and further cooling, reheating serves no purpose. A small peak on the derivative curve at 62.60 ◦C indicates the glass transition temperature of the new material. PHAs usually have a bit lower glass transition temperature, but since this material contains a considerable number of impurities, the increase might be a consequence. Low-intensity peaks around 130 and 137 ◦C might also indicate the presence of impurities in the final material. The melting point of the material was detected at around 150 ◦C, which is in accordance with the research published by Lorini et al. [36].

**Figure 6.** DSC of purified WTPS.

#### *3.4. Tensile Properties and Contact Angle Analysis of Final Material*

The results of the tensile analysis and contact angle analysis of the final material are summarized in Table 1 and presented in Figure 7. As can be seen from the results, the sample exhibited lower stress and strain than the results obtained in the literature [24–26]. The results confirmed a brittle sample, as only 0.422 N/mm<sup>2</sup> was applied to break it. The same trend was observed for strain, which was only 2.07%. This was more than 50% lower compared to the literature results. The tensile tests confirmed that purified WTPS was suitable for performing electrospun material, but it was very brittle. Therefore, one of the solutions could be a combination with other waste biopolymers or recycled polymers to obtain flexible material in further research.

**Table 1.** Stress–strain with standard deviation and contact angle measurements for final material.


As shown in Table 1, the newly produced material reached a meagre value of the contact angle determined with water at a time of 1 s (0◦). According to the solubility of the base material (WTPS) in the presence of EDTA in water, such a material property was expected. The presence of many accessible OH groups on the surface, in combination with other properties (size and interweaving of electrospun fibers), resulted in a material with high wettability. This property could prevent it from being used for packaging applications. However, with an appropriate blending method and/or surface treatment or coating, this could be drastically improved or adjusted. Since we wanted to present the properties of material derived only from WTPS in this study, these options were not used. Other chemicals that could improve this and possibly other properties of the material were also not used due to the preference for developing the new material in the most sustainable and environmentally friendly way possible. In the further development of the material, methods such as plasma and UV treatment and the possible use of other substances will also be used in accordance with these principles.

**Figure 7.** Stress–strain curve of the final material.
