3.1 Morphological and Porosity Properties
The morphological properties of PVC nanofibers have low diameter and high porosity. These properties depend on the type of solvent used for the polymer and solution parameters such as concentration, viscosity, surface tension, and electrical conductivity [20]. While the PS morphological properties, represented by the diameter of the fiber, are much larger than the diameter of the fibers for PVC and depend on the type of solvent used, the DMF solvent is considered the best type of solvent used to dissolve PS because it has a high electrical conductivity that reduces the diameter of the fiber and reduces the beads formed with the fibers [21]. In this research, blending two polymers improves the morphological properties of PS nanofibers by blending with PVC. Table 2 shows the average nanofiber diameters and pore size, which were determined using ImageJ software. Decreasing the average and standard deviation of the diameters of PS nanofibers from 1712 ± 472 nm to 543.33 ± 109.89 nm after blending with PVC, accompanied by a decrease in the average pore size with a standard deviation from 0.961 ± 0.316µm to 0.38 ± 0.196 µm with an increase in porosity by 29.105%. On the other hand, Fig. 1 shows the smoothness and uniformity of nanofibers after adding zirconia to PVC: PS, where the average diameter and standard deviation decrease from 543.33 ± 109.89 to 412 ± 107 after adding 5.5 wt.% of ZrO2 to PVC: PS accompanied by a decrease in the size and number of beads. This result is consistent with the EDX analysis of the nanofibers before and after the addition of zirconia; the atomic weight and atomic area percent of Zr element increased from 4.34 to 9.73 wt.% and 1.56 to 4.5 A % in the composition of the nanofibers after adding 5.5 wt.% of ZrO2. Zirconia also works to reduce the pore size and increase the porosity, as the average pore size decreased with the standard deviation from 0.38 ± 0.196 µm to 0.229 ± 0.147 µm and the porosity increased from 68.71–91.45% after adding 5.5 wt.% ZrO2. Increasing the concentration of nanoparticles of metal oxides such as ZrO2 works to increase the charge density and thus increase the electrical conductivity of the solutions used in electrospinning and produce nanofibers that have a higher elongation that helps in pulling the fibers and reducing the diameter in addition to the orientation of nanofibers [22, 23]. In addition, the ZrO2 caused a highly porous surface [24]. Despite the 5.5 wt.% PVC:PS: ZrO2 has a higher overall porosity (%) and low nanofiber diameter; it has high aggregations or beads on the surface of the nanofibers Fig. 1E.
Table 2
shows the results of average nanofiber diameters, average pore size, and porosity of pure, blend, and composite nanofibers.
Samples | Average Nanofiber Diameters (nm) | Average Pore Size (µm) | Overall Porosity (%) |
PVC | 390 ± 144 | 0.153 ± 0.044 | 86.64 |
PS | 1712 ± 472 | 0.961 ± 0.316 | 53.22 |
PVC: PS | 543.33 ± 109.89 | 0.38 ± 0.196 | 68.71 |
2.5 wt.% PVC:PS: ZrO2 | 533.467 ± 126 | 0.033 ± 0.008 | 77.33 |
3.5 wt.% PVC:PS: ZrO2 | 484.2 ± 137.25 | 0.29 ± 0.180 | 84.66 |
4.5 wt.% PVC:PS: ZrO2 | 446.87 ± 97.64 | 0.283 ± 0.163 | 87.86 |
5.5 wt.% PVC:PS: ZrO2 | 412 ± 107 | 0.229 ± 0.147 | 91.45 |
3.2 Surface wettability
To produce nanofibers that can absorb oils and treat water from oils contaminating the water surface, the surfaces of these fibers must be hydrophobic for water and hydrophilic for oils.
These materials can be classified as super-hydrophobic/super-oleophilic with water contact angle (WCA) > 150◦ and oil contact angle (OCA) ~ 0◦ [25]. Figure 2 shows the change in the degrees of contact with water and paraffinic oil with increasing concentration of zirconia, where the surface of the nanofibers becomes wetter and turns from a hydrophobic surface to a hydrophilic surface with increasing concentration of zirconia, where the average contact angle with water (WCA) decreases from 121.16 ± 2.65° to 77.1 ± 3.76° at concentration. 5.5 wt. %. On the other hand, we notice a high speed of absorption of paraffin oil by the surface of the fibers, as the average contact angle with the oil (OCA) decreases from 38.64 ± 25.83° to 4.91 ± 24.70° at a concentration of 5.5 wt.%. Zirconia nanoparticles improve the wettability of surfaces of polymeric materials using water and enhance their wettability at a high rate using oils [26–28].
3.3. XRD Analysis
Figure 3 shows the XRD patterns of pure PVC/PS mixture, as these patterns show the amorphous nature of the mixture, which is represented by the peaks 19.24°, 24.16°, and 40.72° at plans 101, 111, 221 respectively [59,60]. While the XRD patterns of PVC/PS show peaks at 21.12°,24.4°,28.24°,31.64°,34.36°,38.88°,45.44°,50.52°,61.36°,65.24°,73.32°, and 75.48° well indexed to the 101,111,111,200,210,211,221,311,222,321,322,400, and 332 plans respectively [28–30]. The sharp diffraction peaks indicate the effect of zirconia on the crystalline properties of PVC: PS nanofibers, transforming them from amorphous to highly crystalline fibers. On the other hand, the crystallite size of PVC: PS and PVC: PS: ZrO2 nanofibers depends on the half-width of the peaks (β) and is calculated using the Debye-Scherrer equation [31]:
$${L_{hlk}}=\frac{{K*\lambda }}{{\beta *\cos {\theta _{\hbox{max} }}}}$$
2
Where L (hlk) is the average size of crystallites, K represents the Scherrer constant (0.98), λ denotes the wavelength (Å), β denotes the full width at half maximum (FWHM), and is the angle for the maximum peak (rad).
The crystallinity (%) of nanofibers is calculated by calculating the area of the crystalline region, which includes all the crystallization peaks, as well as calculating the total area, which includes the crystallization area and the amorphous area, using the Origen software, according to the following relationship [32]:
$$C(\% )=\frac{{{I_c}}}{{{I_{total}}}}*100$$
3
C is the crystallinity (%), \({I_c}\) is the total crystalline area, and \({I_{total}}\) is the total area (crystalline and amorphous). Table 3 shows the increment in the crystalline size and crystallinity of PVC: PS when reinforced by zirconia nanoparticles were 81.06% and 12.646%, respectively.
Table 3
crystalline size and crystallinity of PVC: PS and PVC: PS: ZrO2 nanofibers
samples | Crystalline Size (nm) | Crystallinity (%) |
PVC: PS | 54.830 | 21.675 |
PVC:PS: ZrO2 | 99.273 | 24.416 |
3.4. Thermal Analysis
Figure 4 shows the thermal behavior of the nanofibers of the PVC: PS blend and the nanofibers of the mixture reinforced with zirconia PVC:PS: ZrO2, with a heating range from 25°C to 900°C and a heating rate of 10°C / min in atmospheric pressure. The glass transition point (Tg) is often used as a criterion in determining miscibility between polymers, as it is considered (Tg) for PS (89.33°C) and PVC (84.61°C), as the results of the thermal analysis showed that (Tg) for PVC: PS was 91.89°C, where the results indicate good miscibility between the polymers [33]. At the same time, zirconia contributed to increasing (Tg) to 105°C.On the other hand, the thermal degradation of blend with two stages and composite nanofibers included three stages. The first degradation temperature (Td1) of PVC: PS nanofibers was 300°C, and the second degradation temperature was 500°C; these results agree with a previous study [34]. In contrast, the first degradation of composite nanofibers at 308°C and the third degradation at 750°C. Incorporating the zirconia into polymeric nanofibers enhances the thermal properties of blend nanofibers with lower than 7 wt. % of ZrO2 [35].
3.5 Oils Sorption
3.5.1. Oil Sorption from Pure Oil Medium.
To verify the maximum absorption capacity of oils such as paraffin, motors, and transmission oils by absorbent materials such as PVC: PS and PVC: PS: ZrO2 nanofibers. Tests were conducted for the absorption of oils without water to compare the absorption efficiency in the oil medium and the oil medium mixed with water. According to the physical specifications of the oils used in Table 1, it appears that Paraffin oil has the lowest density and Transmission oil has the highest density. Therefore, the buoyancy rate on the surface of transmission oil is high, and its absorption capacity by the nanofibers is large compared to other types of oils. Therefore, the nanofibers' high porosity and low diameter, in addition to the crystalline and thermal properties, contribute to increasing the absorption capacity due to the addition of zirconia nanoparticles. Figure 5 shows the maximum absorption capacity of oils by PVC: PS: ZrO2 nanofibers was 529.364, 148.1, and 119.438 g/g, respectively, for transmission, motors, and paraffin oils. In contrast, the absorption capacity of PVC: PS nanofibers was 249.083, 125.396, and 119.433 g/g, respectively, for transmission, engine and paraffin oils. High viscosity increases absorption due to the increased adhesion of oil to the nanofiber surface and preventing oil penetration into the interior of the absorbent material [36, 37]. Nanofiber membranes with high porosity low density, and hydrophilic surface can absorb oils [38].
3.5.2. Sorption for Oil –Water
The selectivity in the ability of absorbent materials to absorb oils for a medium containing both oil and water, which is summarized by pouring oil on the surface of the water with thicknesses (3, 9, 15, and 21 mm) of paraffin, motors, and transmission oils, is shown in Fig. 6. The absorption results proved that the absorption capacity of PVC: PS and PVC: PS: ZrO2 nanofibers increases with the increase in the thickness of the oil layer floating above the surface of the water. The absorption capacity of oil is much greater than the absorption capacity of water, and the absorption capacity of nanofibers reinforced with zirconia is higher than that of nanofibers of PVC: PS.
The nanofibers of PVC: PS and PVC: PS: ZrO2 have an almost constant selectivity for absorbing oil/water ratio and for all oils, as the selectivity reaches 24,15.67,11.5, and 9 times at the thickness of oil layer 21, 15.9, and 3 mm for transmission, engine and paraffin oils as shown in Fig. 7. According to the above results, the maximum absorption capacity of oil and minimum absorption capacity of water by nanofibers for transmission at all layer thicknesses indicated that the transmission oil has a high viscosity [39]. Many main parameters increase the absorption capacity of oils, including high porosity, void volume, nanofiber diameter, viscosity and specific gravity of the oil, and surface roughness [11]. The increase in the thickness of the oil layer floating above the water provides an important driving force that overcomes all the resistance that occurs between the water molecules and the surface of the nanofibers, which enhances the increase in the oil absorption capacity compared to the water absorption capacity by the nanofibers [40].