3.1. Membrane Surface Modification
Microscopic investigations play an important role in determining the morphological properties of nanofiber surfaces [23]. The surface morphology and wettability of membranes such as PVDF: PAN, PVDF: PAN: Tipp, and PVDF: AOPAN: Tipp, as shown in Figure 2 and Table 2. The thickness of membranes was measured by coating thickness gauge, type (TT260, TIME GROUP Inc) with testing a many points on the nanofibers web. The membrane thickness increased from 7.26±1.121µm to120.45±5.643 µm and nanofiber diameters increased from 0.877±0.331 µm to 1.97±0.89 µm after modification of PVDF: PAN to PVDF: AOPAN: Tipp. On the other hand, using Equations 1 and 2 to calculate the porosity and pore size of membranes, the results showed an increment in porosity of about 30.25% accompanied by a decrease in pore size of about 33.28 % and converting the wettability of surface membrane from hydrophobic to hydrophilic by decreased the contact angle from 117.22±3.45° to 58.29±4.57°, this result is due to the presence of a carboxyl group [24]. The cross-section of FE-SEM images shows a decrease in the sizes of pores or cavities between the nanofibers with increased membrane thickness after modifying the PVDF: PAN to PVDF: AOPAN: Tipp, which indicated increased membrane porosity despite the increase in the diameters of nanofibers. According to the results above, it produces a highly porous membrane and increases the surface area [25]. In addition, increasing the nanofiber's web surface hydrophilicity enhanced the water flux [26, 27]; in contrast, hydrophobic surfaces of fibers reduce membrane performance [23]. Also, when converting PVDF: PAN to PVDF: AOPAN: Tipp, it was found that the increased nanofiber diameters as a result of improving the electrical conductivity of the solution [18, 20] and pore sizes are reduced to small parts and the porous structure is replaced by a homogeneous nanostructure [28].
Table 2 Results of surface membrane morphology such as nanofiber diameters, pore size, and porosity
Membrane Code
|
Membrane Thickness (µm)
|
Nanofiber Diameter (µm)
|
Pore Size
(µm)
|
Porosity
(%)
|
M1
|
7.26±1.121
|
0.877±0.331
|
18.263
|
70.4
|
M2
|
93.37±3.225
|
1.43±0.44
|
15.671
|
80.0
|
M3
|
120.45±5.643
|
1.97±0.89
|
13.809
|
91.7
|
3.2 FTIR Analysis
Figure 3 displays FTIR spectra ranging from 500 to 4000 cm-1 for structural characterization of samples such as PVDF: PAN blend, PVDF:PAN: Tipp, PVDF:AOPAN: Tipp. The results of spectroscopic analysis of polyacrylonitrile nanofibers showed peaks at 2925 cm-1, 2244 cm-1, 1735 cm-1, and 1666 cm-1, representing the vibration of CH2, -C≡N, -C=O, and -C=N groups, related to PAN, respectively. While the peaks at 1460 and 1270 cm-1 have multiple vibrations of CH and CO, and the peak at about 3618 cm-1 represents the extension band of CH [7, 29, 30]. On the other hand, FTIR spectroscopy showed that the peaks at 487 and 531 cm-1 indicate the α phase, and the peaks at 838, 1184 and 1274 cm−1 are consistent with the β phase, where the α and β phases indicate that the PVDF nanofibers are semi-crystalline [10]. The characteristic peak at 1,173 cm-1 represents the symmetrical stretching of the -CF2 group, and the peak at 876 cm-1 represents the C-F vibrational stretching of the amorphous phase [21]. FTIR spectroscopy at the 840 and 880 cm-1 peaks indicates that C-C -C and C-F represent the stretching vibrations of PVDF in the mixture. The results showed that the nitrile group moved from 2244 to 2240 cm-1 when the atomic percentage of fluorine in the PVDF: PAN mixture increased, which was previously indicated when analyzing the elements.
New peaks also appeared at 1182 and 1665 cm-1, representing the starch vibrations of the CN and C=N groups [31, 32]. FT-IR spectroscopic analysis of the developed membrane after the reaction of PAN with hydroxylamine hydrochloride to produce AOPAN in the PVDF: AOPAN: Tipp , which indicates the emergence of new peaks at 1043 and 1665 cm-1, and stretching vibrations of N – O and C = N. The result of the reduction of the absorption peaks in the spectra of AOPAN, such as reducing peak of -C=N groups at 2242 cm-1, and C-H stretching peak in CH and CH2 at 2936 and 2871 cm-1.
The peak at 3359 cm-1indicates the hydroxyl groups corresponding to Ti-O-Ti from 454 cm-1for PVDF: PAN: Tipp. While the absorption peaks such as the C=O stretches at 1735 cm-1 and the C=O groups in primary amines (NH2) 1665 cm-1, contributes to the shift of the symmetric stretching of the –CF2 group of PVDF from 876 cm-1to 879 cm-1 [20, 33,34]. On the other hand, the range of peaks 3150-3650 cm-1 indicates to the N-H and OH groups [35].
3.3 XRD Analysis
Figure 4 and Table 3 show the X-ray diffraction and crystalline properties results of the PVDF: PAN, PVDF: PAN: Tipp, PVDF: AOPAN: Tipp respectively.
The analysis shows the board peak with PVDF: PAN blend nanofibers at 2θ≈22.502° according to the plane (110) and d-spacing 3.950 Å. On the contrary, we notice an enhancement of the crystalline properties and the emergence of a sharper peak at 2θ≈16.725°, 20.275°, and 50.275° according to the plane (100),(110), (221) and d-spacing 5.299Å, 4.379Å, 1.814Å of the PVDF: PAN: Tipp after strengthening the blend nanofiber with Tipp. This result was agreed with the previous study [19]. We notice high crystallinity and a sharp peak due to the conversion of PAN into AOPAN with its strengthening with Tipp. A sharp peak appears at 2θ≈16.975 based on the plane (110) and d-spacing 5.299Å related to PVDF molecular chains [36,37]. Others peak as 2θ≈ 25.475°and 51.275° based on the planes (200) and (322) with accompaniment d-spacing as 3.495Å and 1.781Å related to PAN fibers [38].In other words, the average crystalline and crystallinity increased by 320.682 % and 46.93% after modifying the blended nanofibers.
Table 3 The crystalline properties of the membrane samples.
3.4 Thermal Analysis
Figure 5 shows the differential scanning calorimetry (DSC) thermographs and Thermogravimetric analysis (TGA) of PVDF: PAN (M1), PVDF: PAN: Tipp (M2), and PVDF: AOPAN: Tipp (M3) respectively. The TGA analysis results show three-step changes in mass loss (%), which occurred at 85 °C representing the stage of moisture loss, 375°C representing the original polymer chains of PAN and PVDF, and 475°C with a maximum mass loss of 90.69 % for M1, while decreased the mass loss at M2 and M3 to 87.06 % and 82.17 % respectively. Therefore, PVDF: AOPAN: Tipp has high chemical resistance, thermal stability, and excellent wettability due to the decomposition of amidoxime groups [18,20]. On the other hand, DSc thermograph results show the increased peak of melting temperature and enthalpy change (∆H) from 332.56 °C to 353 °C and 164.6 J/g to 376.4 J/ g after modifying the PVDF: PAN to PVDF: AOPAN: Tipp. The AOPAN membrane nanofibers exhibit excellent heat resistance due to the conversion of the molecular chain of PAN into a ladder-type structure [39].
3.5. Measurements of Parameters Filtration
3.5.1. Effect of Protein Milk on Membrane Surface Morphology.
Figure 6 shows the FE-SEM images with 500 nm and 10µm scale bares of membranes before and after filtration. The FE-SEM images show an increased accumulation of protein milk particles on the surface of membranes at PVDF: PAN: Tipp (M2) and PVDF: AOPAN: Tipp (M3), compared with an accumulation of protein milk particles on the surface of PVDF: PAN (M1). These results indicated an increased rejected percentage of protein after modifying and compositing the PVDF: PAN to PVDF: AOPAN: Tipp, and the fine fractions on the surface of the nanofibers are covered with protein particles [40]. In addition, the reduction in pore size and the presence of polar and ionized groups attract the protein and enhance the filtration process [41].
On the other hand, Figure 7 shows the Energy dispersive X-rays (EDX) elements on surfaces of membranes as M3 and M4 before and after filtration. EDX are used to identify elements in powders, including whole milk powders or powdery materials that do not contain free fats on the surface. Processed milk generally contains C, N, O, P, S, K and Ca, while carbohydrates contain C and O [42] and mineral elements [43]. The EDX analysis of PVDF: PAN: Tipp (M2) showed the elements as C, N, O, F, and Si before and after filtration. In addition, increased levels of O and Si elements after filtration indicated an accumulation of protein milk on the surface of the membrane [44]. Decreasing the N content led to PVDF: PAN nanofibers being successfully modified by titanium [45]. As a result of the emergence of new peaks at 1182 and 1665 cm−1 are assigned to the starching vibrations of C-N and C=N groups. On the other hand, the O and Si levels increased from 6.3 wt. % to 16.59 wt. % and 1.02 wt.% to 1.75 wt.% after filtration of PVDF: AOPAN: Tipp (M3) and PVDF: PAN: Tipp (M2) respectively. In addition, increased the mineral elements as Fe from 0.76 wt. % to 4.93wt. %. In contrast, the N and C levels decreased from 27.27 wt. % to 21.26 wt. % and 57.45 wt.% to 55.37 wt.% after filtration of M3, which indicated to high rejection rate for protein milk after modifying the PAN to AOPAN.
3.5.2. Effect of Protein Milk on Filtration Parameters.
With the increase in pollution, especially in water, ultrafiltration has become necessary for water treatment and use in the food industry. Therefore, the two most important factors in determining the efficiency of membrane filtration must be eliminated: the relatively low porosity, which means limited permeability and low water flux rate, and the high pollution rate due to the increased pore size [46]. Table 4 shows the flux recovery ratio, solute rejection, total fouling ratio, and water flux before and after using dry milk protein infiltration.
The results show an increase in the water flux rate before and after using the dry milk protein after modifying the PVDF: PAN to PVDF: AOPAN: Tipp nanofibers membrane from 737Lm-2.h-1) to 804 (Lm-2.h-1) and 442 (Lm-2.h-1) to 738 (Lm-2.h-1) respectively. The milk protein concentration before and after filtration is calculated using a spectrophotometer at 280 nm. The reason for the increase in water flux rates before and after using milk protein in the filtration process is due to the increase in membrane thickness, as shown in Table 2, in addition to the increase in the wettability of the membranes by changing them from hydrophobic to hydrophilic. Therefore, the high ability to absorb water is related to the thickness and improvement of the porosity of the membrane. Thin membranes have lower flux rates than thicker membranes [47, 48].On the other hand, the concentration of penetrated protein milk was reduced after modifying the membrane by 433.07 % compared with the protein milk concentration of the unmodified membrane. This result indicated that the protein milk flux was low than the water flux before and after using the protein milk in the filtration process and decreased with increased membrane thickness and porosity (%). In addition, the flux recovery ratio, solute rejection, and rejection rate were increased by 53.06 %, 100.8 %, and 59.07 % after modifying the membrane. Figure 8 shows the permeability of membranes at many filtration times and 500 ppm of protein milk concentration with 30 Psi operational pressure. The results show decreased permeability for M1 and M2 membranes with increased filtration times while slightly decreased at the modified membrane (M3). The permeability at 60 min and 30 Psi was 3500 L.m-2.h-1.atm-1 for the modified membrane while 1650 L.m-2.h-1.atm-1 for the unmodified membrane. These results indicated that the pore size of composite membranes was smaller than the molecule size of protein, which enhanced the high rejection rate and high flux recovery ratio when the thickness of membranes was increased [49]. Introducing Ti-O-Ti into PVDF: PAN and converting PAN into AOPAN contributes to increasing the wettability of the nanofiber surface and helps to increase the water flux into the membrane [19].
Table 4 The flux recovery ratio, solute rejection, rejection rate, and water flux before and after using dry milk protein.
JW2
(Lm-2.h-1))
|
JW1
(Lm-2.h-1)
|
Jp
(Lm-2.h-1)
|
Cp
(ppm)
|
Rt
(%)
|
Sr
(%)
|
FRR
(%)
|
Membrane
Code
|
442
|
737
|
357
|
276.88
|
51.53
|
44.62
|
59.97
|
M1
|
640
|
760
|
210
|
60.810
|
72.37
|
87.84
|
84.21
|
M2
|
738
|
804
|
145
|
51.940
|
81.97
|
89.61
|
91.79
|
M3
|
FRR: Flux Recovery Ratio; Sr: Solute Rejection; Rt: Rejection rate ; Cp: penetrate protein milk concentration; Jp: protein flux