The synthesis and characterization of demulsifying poly vinylidene Fluoride (PVDF) Disc

Oil/water separation is still a challenging universal task. Here we fabricated an amphiphilic polymeric (APD) PVDF disc that can be used to separate free oil/water mixtures and recover the pure water from W/O emulsions. The modification of the disc includes deposition of copper oxide (CuO) was integrated on the surface via the impregnation method; the CTAB was used as a porogen to increase the roughness of the surface, Morphology of the disc was investigated by SEM and the result showed the nanoparticles were entrapped on the surface and inside the disc. FTIR results strongly predicted the correlation of CTAB, PVDF, and CuO nanoparticles of the modified disc. Separation and wettability test of the FRR% (Flux recovery ratio), permeate flux/oil rejection coefficient (R %) and the water-uptake test confirmed the antifouling properties, permeability of water, oil-rejection, and porosity of the disc. The purpose of impregnation CuO enhanced the hydrophilicity and anti-fouling properties of the PVDF disc.


Introduction
Oily wastewater is one of the constituents of industrial effluent, which is drained into the freshwater stream.The oily wastewater in the presence of different salts/surfactants turned into oil/water emulsions as produced in industries including petroleum, food, textile, and leather.The emulsified water ultimately finds its way into the ecosystem and harms the general public health.Many oil-spill accidents for example at Bohai Bay in (2011) once happened, affected enormous damages by way of the leaked oil diversified through seawater (containing a high fraction of different salts) producing emulsion, and severely affected the oceanic flora and fauna [1].On the analytical side, petroleum crude often presents complex composition, which is difficult to analyze with high reliability and precision and ultimately affects the valuation of crude in the market.One of the main problems in such type of scenario is the stability of the emulsion, which varies from well to well and ultimately affects the analytical protocol.Numerous approaches are in place to separate water from oil such as air flotation, adsorptive, electrochemical, and ultrasonic methods [2].Besides, all these methods suffer from one another drawback, as electrochemical and ultrasonic methods are difficult to materialize for handling material in bulk due to safety concerns.Therefore, an alternative method/material, which is economical, efficient method and material has desirable, and has a high recyclability ratable.Membrane technology offers the aforesaid benefits and has been widely used [3,4].Different polymers find application in this area and were successfully used with pores in the ultra, micro, and nano dimension including polyether-sulfone (PES) [5], poly (vinylidene fluoride) (PVDF), cellulose acetate (CA) and polyacrylonitrile (PAN) to break the emulsified matrix and offer effective demulsification [6].
Among these polymers the poly (vinylidene fluoride PVDF-based membranes secured a distinctive place in the separation area due to high toughness, inertness, relative transparency, and ease of processing [7].But with other man-made synthetic materials PVDF's membrane also suffers shortcomings of fouling [8][9], especially when employed in demulsification area [10].Being hydrophobic, organic or oily fraction stick to PVDF's membrane surface, this is ted as fouling.Fouling resultantly appears in flux reduction due to pores clogging the need arises to continuously regenerate the surface otherwise making the process tedious and time-consuming [11][12][13].However, in protocols where speed is a major concern the membranebased separation process suffers from mechanical stability.Due to fouling the clogged pores of extremely high back pressure deteriorated the membrane surface often leading to its rupturing.One method to counter this drawback is to make the surface of VDF membrane hydrophilic [14,15].Mohan et al successfully modified the PVDF nano-filtration membrane with carboxymethyl chitosan (CMC) and reported that the addition of CMC not only increase the hydrophilicity but also augmented the mechanical strength of the membrane [16].But it's still not enough mechanically strong to sustain the high flow rate, especially in automated method design.On the other hand, the disc or monolithic format of separation material minimized or even eliminated the cited problem, especially enabling the analyst to work with a high flow rate with maximum throughput [17].
In this work PVDF in Disc format was fabricated and modified its internal structure with hydrophilic copper nano-particles to counter the hydrophobicity and mechanical strength of the material [18].

Experimental
Material polyvinylidene fluoride (PVDF) blobs and CTAB (Cetrimonium bromide) were obtained from Sigma Aldrich, USA.Dimethylformamide (DMF) and ethanol (C2H5OH) were of analytical grade and achieved from Sigma Aldrich, USA.Chloroxylenol (C8H9ClO), Copper sulphate (CuSO4), and Sodium tetrahydridoborate (NaBH4) were purchased from Sigma Aldrich, Germany.Commercially available distilled water bought from Aqua Art institute of Pakistan.All chemicals used remained of analytical grade and were used as received.

Preparation and Fabrication of PVDF Disc
According to the method, the polyvinylidene fluoride solution was ready in dimethylformamide at a suitable temperature [18].The calculated quantity of Cetrimonium bromide as porogen has been added to the desired solution and extracted from the scaffold using distilled water as a leaching agent.Polyvinylidene fluoride disc was prepared in a disposable syringe used as rot.PVDF solution was packed into a specified syringe to control the sample thickness and width.The solution gelation has achieved in more than 2 Hrs.The required PVDF solution in the syringe was dipped into cold water for coagulation.After clotting, the disc was rained with refined water up to the whole elimination of CTAB.The obtained disc became dry for 40 minutes at 60 °C.

Synthesis of Nanoparticles
CuSO4 solution was synthesized in distilled water.A suitable amount of C8H9ClO as a structure-directing agent was added to the CuSO4 solution [19].The weighted amount of Sodium tetrahydridoborate solution was added to the solution at freezing temperature till the color of the mixture has been converted from blue to blackish dark, which demonstrates the formation of Copper nanoparticles (CNPs).The CNPs prepared were isolated by centrifugation at 6000 rpm, washed with distilled water followed by ethanol in triplicate, and dried in a vacuum oven at 60 °C for 3 hours.

Modification of Polymeric PVDF Disc
The pore modification of the PVDF disc was performed by the impregnation method.A nanostructure was prepared inside the pores of the disc.In a typical experiment, the disc was suspended in CuSO4 containing chloroxylenol in an ultrasonic bath, and then NaBH4 was added according to the method discussed earlier.
Finally, the discs were sprayed with de-ionized water and dried at 60 •C for 24 hrs.

Characterization
To investigate the chemical composition of a polymeric amphiphilic PVDF disc FTIR was employed.The morphology of the disc was determined by scanning electron microscope (JSM5910, JEOL Japan).The hydrophilicity of the resulting amphiphilic polymeric disc was measured by the different filtration experiments.The filtrate sample was characterized by UV.

Antifouling test
The disc was placed in the filtration cell.The flux of the disc Jw1 was gauged based on the absorbed water weight at 0.4 MPa pressure for 30 minutes.The process was carried out in the following steps: In step 1 the pure PVDF disc was tested through a water-oil mixture.Pure disc flux was measured to examine the fouling level.In step 2 the modified disc was tested with water-oil emulsion for 30 min for measuring their water flux, Jw2.To check out the antifouling property of the prepared disc, the flux recovery ratio (FRR) was employed, high FRR showed a greater fouling-resistant ability of the disc.The equation (1) used for FRR calculation was as FRR%= (Jw2/Jw1) ×100 ……….( 1) Where Jw1 and Jw2 are the fluxes of the original PVDF-Disc and the Polymeric Amphiphilic-disc, respectively

Rejection and Permeation Flux of the Amphiphilic Disc
A calculation was made for the polymeric disc's flow.The following equation was used to calculate the permeation's volume per unit of time (2).Flux=V/At ……………… (2) Where the volume of the permeate V, the valid area of the disc A (cm 2 ), and the retention time t, has been shown in eq (2).The filter was filled by definite volume of water-in-oil (15 mL) and every sample was measured to get the average result for each test.According to the given Equation (3) the separation efficiency was calculated and defined by oil rejection coefficient R (%).R (%) = (1-CP/C0) ×100% ……….(3) The concentration in the original oil/water mixture shown by C0 and Cp and the collected water similarly.

Extractive-gravimetric method
This method was used for the determination of water uptake by the disc, using a rounded disc sample and weighted it W0, the discs were soaked in a beaker containing 15 mL of swelling medium (water) and shaken at 22 °C for 8 hrs.The disc was then taken out of the beaker, additional water was strained away and the disc was weighed again to get its final weight, Wf (Water uptake) was then calculated by following Equation (4).W uptake= (Wf -W0) / ×100% ……….(4) Where Vsample is the volume of wastewater sample (dm 3 ) and ml is the mass of a round flask with oil (in grams), m 2 is the mass of a dry and empty round flask (in grams).As shown in Figure 10.

Fourier Transform Infrared (FTIR) Analysis
At 833 cm -1 confirmed the C-F starching vibration and PVDF material is characterized by an 872 cm -1 peak shown to the carbon C-C-C chain asymmetric vibration [24].Shown in Fig 1.The 1408 cm -1 absorption peak was attributed to the CH2 wagging vibration.The peak observed at 1230 cm -1 and 1074 cm -1 was ascribed to the γ and β phase of C-F out-of-plan deformation.The peak near 514 cm -1 is attributed to the bending vibration of CF2's phase [25].It can be inferred from the data presented in Figure 1 that not only does CTAB attenuate the crystal structure of PVDF but the DMF can also have profound effects on it as can be seen in Figure 1 (F0).
As reported earlier peaks arising at 1657, 1172, 1098, 880, 662, and 469 cm -1 different phases of PVDF were attributed.Still, the presence of 1393 cm -1 band has displayed the characteristic vibration of the (N-CH3) group, which is a principal component of CTAB [26] observed in Figure 1 (F1).The PVDF sample containing different loading of CTAB are designated from F1 to F6.It can be observed from the data presented in Figure 1 that more CTAB facilitates the formation of different phases of PVDF [27,28].Although, CTAB was leached out from the PVDF disc the band at 880 cm -1 band suggests the presence of α phase of PVDF, and the (N-CH3) stretching vibration of CTAB was also observed at 1389 cm -1 while the peaks at 1257, 1172 cm -1 indicates β phase of C-F2 out-of-plan deformation.It is suggestive of the fact that CTAB has strongly interacted with the polymeric chain and caused permanent deformation of the structure and tends to remain attached to a chain.It may be possible that the coil structure of the polymer inhibits its complete removal with the help of a leaching agent like H2O.This observation is confirmed with EDX as in the sub-set of Figure 2.3 (B), where the presence of Br confirms that some traces of CTAB are still present in the PVDF disc.The head-to-tail/head-to-head polymer chain flaw was apparent in the peak absorbed at 479 cm -1 [29].From the modified PVDF disc, FTIR data shows that there is a confirmation of CuO absorption at the peak 433 cm -1 from this peak it is predicted that the surface of the PVDF disc is altered by CuO nanoparticles [30,31].The stretching vibration of CuO is predicted by the band 472 cm -1 in the above-mentioned Fig 1 .The absorption bands at 880, 837, and 612 cm -1 , on the other hand, show that the alpha phase is still present.The (-C-C-) bond found in PVDF and CTAB has a similar symmetric attitude, as demonstrated by the peak absorbed at 1164 cm -1 once more.According to the FTIR data, which showed that the peaks assigned to the original PVDF had changed from absorption peaks at 608 and 476 cm -1 conforming to the CuO functional group, all of these samples were subjected to impregnation of nanoparticles inside the disc after leaching of the CTAB in good solvent (H2O).Hence, the presence of nano-sized Cu-O particles is confirmed by the highest absorbed peaks.

Morphological Analysis
To investigate the surface microstructure of the disc samples were identified under an SEM (scanning electron microscope).The morphological observation of the surface of the original PVDF sample is presented in Figure 2. The morphology in Figure 2 (A) is not smooth and the crystal is deformed.It means that earlier observation in the case of FTIR, that DMF being polar in nature, it can also have affected the crystalline structure of PVDF disc [32].It can also be inferred from the micrograph that there is competition in different forms of PVDF, which crystallized from DMF during the process of de-mixing in the non-solvent-induced phase separation process.This competition leads to the generation of a small porous structure of the original PVDF disc.It may be predicted from the surface morphology that the disc when dried and DMF was evaporating may have created these nanocavities.
In Figure 3 the morphology observed had shown that CTAB is acting not only porogen but also as a nucleating agent for PVDF chains.The chains tend to attain structure morphology and this is beneficial in terms of regular pores formation.
As the chain gets crystallized on a point-to-point basis, where CTAB is available nearby of the PVDF chain [21].The void volume is extended and the disc becomes more porous.When the amount of CTAB was increased, such behavior becomes more pronounced and now spherical morphology can be seen in Figure 4. Once these porous structures were developed the sample was subjected to in situ generation of copper (Cu) nanoparticles inside generated pores.The CTAB while self-assembled in the hydrophobic PVDF environment shows curvy nature and when it leaches out the pores remain in that shape.It was observed from the synthesis of nanoparticles in a separate experiment that shows random morphology with a curvy shape as reported earlier [33].
On the contrary, when nanoparticles are prepared inside confinement generated in PVDF with the removal of CTAB, attain the shape of the pores [26] and appear in that shape as shown in Figure 5. Scanning electronic microscopic images (Figure 5) shows that the pores of the disc were filled out by the inorganic nanoparticles (CuO).The amphiphilic properties of the PVDF were improved and the fouling properties were typically reduced by the nanoparticles [34].The average pore size has embedded by CuO particles.

Permeability and Separation of Amphiphilic Polymeric Disc
The inorganic nanoparticle was discovered to be an important factor in the modification of hydrophobic materials, according to several methods.The results of laboratory tests have verified that CuO nanoparticles have been deposited on the PVDF disc's surface.For the separation and permeability of the amphiphilic disc used different approaches, and many experiments were performed [19][20][21].Equation ( 1), examined the disc's antifouling characteristics and explained the flux recovery ratio (FRR %).Greater FRR% indicates high fouling resistance.The concentration of nanoparticles increases along with the flux recovery ratio.The outcomes from equation ( 1) are described in Figure 6.From the FRR% graphical results, it's confirmed that the disc shows antifouling properties.In the second experiment, the mass flux rate of water was calculated using equation ( 2), and the flow rate per unit area in relation to the time of the disc was examined.Figure 7 illustrates how the flow rate enhanced as CuO (nanoparticles) were gradually added to the disc.
As the concentration of nanoparticles is raised, the mass flux increases, and vice versa, the water retention time in the APD (amphiphilic polymeric disc) decreases.The experiment was conducted under standard pressure, and the quantity of nanoparticles had an impact on the APD's retention period.Hence, the finding indicated that the disc's water permeability increased continuously as inorganic particles were added (Cu-O).

Fig. 7. Flux of water through APD
The PVDF disc's Cu-O impregnation demonstrated hydrophilicity and improved separation performance tested by the method are as shown in eq (3).The separation efficiency increased throughout the duration of the 0.6 gm of nanoparticles adding, and over 95% of the oil was rejected, allowing water to flow onto the APD alternatively. Figure 10 depicts the rejection ratio R% grows as the concentration of CuO rises.The oil-water combination and Amphiphilic disc were shown in Figure 8 before separation.Purified water and oil were obtained after experimentation with the APD's separation efficiency, and oil were rejected by the disc from the emulsion as shown in Figure 9 (A) below.One of the major aspects of the water uptake experiment in equation ( 4) that defined the hydrated disc mass ratio to that of the dry disc was the porosity of the disc.The water intake for APD was depicted in Figure 10(B).As the number of nanoparticles added to the PVDF disc increased, so did the weight of the wetted disc.The average weight of the hydrated disc revealed that its surface has tiny pores filled with CuO nanoparticles that have the ability to absorb water, increasing the disc's weight also.

UV-Visible-analysis
The differential absorbance spectra of water-in-oil emulsions before and after separation were measured by using a UV-visible spectrophotometer [35].Several solutions with parts per million (ppm) concentrations in the range of 0.1, 0.2, 0.3, 0.4, and 0.5 were prepared, as well as standard solutions of crude oil in cyclohexane.A UV-Vis spectrophotometer was used to examine each solution in triplicate for the signal absorbance (A) versus wavelength (λ) using benzene as a reference, as illustrated in figure 10.

Fig. 10. Calibration curve of crude oil sample
The crude oil emulsion was passed to various disc samples and processed on a UV-visible spectrophotometer to evaluate how well the manufactured disc performed.In figure 11, the collected data were plotted.According to the data collected, it can be shown that as the amount of nanoparticles inside the PVDF disc grows, the absorbance of the permeate oil tends to decrease.This increases the disc's hydrophilic nature and significantly reduces the amount of oil that may pass through it.

CONCLUSION
In order to remove oil from water samples that are high in oil numerous pore structures were developed in the amphiphilic polymeric PVDF disc (APD).CTAB was applied to the APD as a porogen to create pores.With more CTAB amount, the function of (APD) surface is raised.To create the disc more hydrophilic, nanoparticles (CuO) were applied.Cu-O was added to the disc using the impregnation technique.Nanoparticles considerably improved the disc surface's antifouling characteristics.Experimental tests on the permeation of water and oil successfully separated the water from the water-oil emulsion.The graphic analysis of the impact of different nanoparticle concentrations revealed that the flow recovery ratio (FRR %) improved with increasing nanoparticle concentration; the higher FRR% demonstrated maximal fouling resistance.Also, the mass flux rate result demonstrated that dramatically nanoparticles enhanced the flow of water over time through the disc.To support the APD's rejection of oil, experiments on the oil-rejection coefficient and water uptake were also conducted, briefly addressed in the results and conclusions.Any polymeric hydrophobic surface can readily be converted into a hydrophilic surface using the approach that is employed in polymers modification.Also, the manufactured disc has excellent potential for treating emulsion/wastewater in practical applications in industry and daily life.

Fig. 2 .
Fig. 2. SEM images (Scanning electronic microscopic) of the sample F0 at a magnification of 10 and 100μm represented as A and B.

Fig. 8 .
Fig. 8. (a) Oil-water Mixture, (b) Disc Before Separation, (c) Photographs of water and (d) disk after separation of the oil-water emulsion.

Fig. 9 .
Fig. 9. (A) R % Oil rejection coefficient of the amphiphilic polymeric disc (B) Water-up taken by APD.One of the major aspects of the water uptake experiment in equation (4) that defined the hydrated disc mass ratio to that of the dry disc was the porosity of the disc.The water intake for APD was depicted in Figure10(B).As the number of nanoparticles added to the PVDF disc increased, so did the weight of the wetted disc.The average weight of the hydrated disc revealed that its surface has tiny pores filled with CuO nanoparticles that have the ability to absorb water, increasing the disc's weight also.

Table 1 .
Recipe for preparing the modified polymeric amphiphilic PVDF disc