Preparation of Membranes Based on Polysulfone (PSU) and Graphene Oxide (GrO) by Electrospinning

Nowadays, more than 700 million people lack access to drinking water sources according to reports of the World Health Organization (WHO) in 2014 [ 1 ]. The human water supplies are only 2.5 % of the total of water in the world, with the majority of it located in polar ice caps and glaciers, and just a small portion is surface water [ 2 ]. There are several conventional technologies for water purifi cation , such as disinfection, granular fi ltration, and distillation, among others. In addition, advanced technologies such as membrane fi ltration, multiple effect distillation, and advanced oxidation process have been recently used for water and wastewater treatment [ 3 ]. Desalination technology is based mainly on membranes for the production of drinking water [ 4 , 5 ] and industrial use. Thus, membrane technology has an important role in the advance of desalination processes and has garnered signifi cant attention in recent years for the development of new materials and production techniques. Nanofi bers have been recently used in order to obtain membranes due to their excellent properties such as high surface area to volume ratio, high porosity, and fl exibility [ 6 ]. The nanofi bers can be applied in microfi ltration, ultrafi ltration, nanofi ltration, membrane distillation, and reverse osmosis. This study focuses on evaluating the porosity and diameter of fi bers obtained by electrospinning to produce membranes with potential application in fi ltration process.


Introduction
Nowadays, more than 700 million people lack access to drinking water sources according to reports of the World Health Organization (WHO) in 2014 [ 1 ]. The human water supplies are only 2.5 % of the total of water in the world, with the majority of it located in polar ice caps and glaciers, and just a small portion is surface water [ 2 ]. There are several conventional technologies for water purifi cation , such as disinfection, granular fi ltration, and distillation, among others. In addition, advanced technologies such as membrane fi ltration, multiple effect distillation, and advanced oxidation process have been recently used for water and wastewater treatment [ 3 ]. Desalination technology is based mainly on membranes for the production of drinking water [ 4 , 5 ] and industrial use. Thus, membrane technology has an important role in the advance of desalination processes and has garnered signifi cant attention in recent years for the development of new materials and production techniques. Nanofi bers have been recently used in order to obtain membranes due to their excellent properties such as high surface area to volume ratio, high porosity, and fl exibility [ 6 ]. The nanofi bers can be applied in microfi ltration, ultrafi ltration, nanofi ltration, membrane distillation, and reverse osmosis. This study focuses on evaluating the porosity and diameter of fi bers obtained by electrospinning to produce membranes with potential application in fi ltration process.

Materials
Polysulfone (PSU, UDEL-P3500 ® ), Mn = 3.500 g/moL, was provided by Solvay Specialty Polymers; N -methyl-pyrrolidone (NMP) by Fisher Scientifi c (ACS grade); N , N -dimethyl formamide (DMF) by Fermont (ACS grade); and commercial graphene oxide solution (GrO) by SUPERMARKET (90 nm diameter, 1 nm thickness). All the materials were used as received without any further modifi cation or purifi cation. Only the graphene oxide solution was fi ltrated and then dried by vacuum desiccator.

Electrospinning
To obtain the membrane, the fi bers were processed by electrospinning. First, a polymer solution was prepared by dissolving PSU (20 % w/w) in a solvent mixture of DMF/NMP (30/70 w/w). Then, the PSU was added to the solvent mixture and kept under constant agitation at 60 °C for 24 h, until its dissolution. Finally, 0.15 % wt of dried GrO was added during stirring that continued for additional 15 min.
Electrospinning allowed the formation of fi bers through an electric potential difference that was applied to the PSU solution. The high voltage is applied to the PSU solution in order to induce polarization. As a result of such polarization, a Taylor cone is created. When the charge exceeds the surface tension of the solution, a droplet on the tip of the needle (jet) travels towards the region of low potential, in this case the metallic rotary collector. Figure 3.1 shows the schematic diagram of the electrospinning equipment. It consists of an adjustable DC power supply, which is able to generate a voltage in the range of 0-50 kV. This power supply is connected to a tip of the metallic needle (0.21 mm inner diameter) by two electrodes. The syringe, which contains the polymer solution, is connected to a syringe pump. A positive voltage of 20 or 25 kV was applied across a distance of 10 or 15 cm between the tip of the needle and the metallic collector (17 × 21 cm). The rotary collector was covered with aluminum foil on which the fi bers were deposited. The collector velocity was maintained constant at 36 or 65 rpm. Once the membranes were obtained, they were dried in a fume hood for 24 h at room temperature to remove residual solvent. Table 3.1 shows the conditions of the samples obtained.

Membrane Characterization
The morphology of the membranes was studied by scanning electron microscopy (SEM, QUANTA 200, ESEM FEI Co.). Samples of the membranes were gold coated for 25 s, at 40 mA under vacuum conditions, and placed on a pin support. SEM images were analyzed by the ImageJ software in order to determine the diameters and porosity of the obtained fi bers. To evaluate the diameter of each sample, three images at the same magnifi cation were considered. Thus, a total number of 200 fi bers were measured. In the case of porosity, for each sample the empty area was measured using the contrast difference in the image.
The contact angle of the membrane surface was measured at room temperature by a goniometer (Biolin Scientifi c, Theta Lite) equipped with the image analysis software One Attention. For this, deionized water drops of about 2.5 μL were placed on the membrane surface using a thin syringe. The contact angle was automatically calculated by the Young-Laplace equation fi tting the captured drop shape at both left and right sides of each drop.
The system to carry out the fi ltration experiments (Amicon 8400, Millipore) has an active membrane area of 0.0045 m 2 . It is composed of one cylindrical plastic chamber with the membrane located at the bottom of it; a pure air fl ow was applied on the top of the chamber. Different operating pressures were considered: 10, 20, 30, and 40 psi. The permeate fl ux was calculated by measuring the collected volume every 5 min. Finally, the hydraulic resistance was calculated using the Darcy Equation ( 3.1 ). The said equation relates the permeate fl ux J (m 3 /m 2 /s) as a ratio of the difference in pressures ∆ P (Pa) (applied pressure and osmotic pressure ∆∏) and the viscosity μ (kg/m s) and the hydraulic resistance R H (m −1 ). The viscosity of water was 0.001002 kg/m 2 s at room temperature and the osmotic pressure was negligible. Cellulose membranes with molecular weight cutoff of 1 and 100 kDa were used as support for every obtained membrane. Figure 3.2 shows the SEM images of the obtained membranes at different voltage, distance, fl ow, and collector velocity. As it can be seen, the surface exhibits a random arrangement of fi bers indicating its chaotic nature (nonwoven). The variation in the operating conditions affects the fi ber diameters. Some studies have reported that fi ber diameter depends on polymer concentration; in general, high polymer concentration increases viscosity, and therefore, fi bers of larger diameter [ 7 , 8 ] are obtained. Nevertheless, other groups [ 9 , 10 ] have reported that voltage plays a more important role in determining the fi ber diameters, e.g., when increasing the applied voltage, the electrostatic repulse force would increase the charge on the jet and thus the fi ber diameter decreases. The diameter for sample A is 592 nm and sample B is 261 nm; in this case, the applied voltage was higher for sample B, which is according to that reported by Mo [ 9 ] and Demir [ 10 ]. In addition, the low fl ow and the collector velocity give a stretched jet polymer and produce fi bers of lesser diameters. When the fl ow was increased, along with the collector velocity, there was not enough time to stretch the polymer jet, and thus the diameter of the fi bers increased, as in sample A. On the other hand, distance plays an important role; sample B (Fig. B2) has a more regular and opened nonwoven membrane unlike sample A (Fig. A2), which has fi bers overlapping between them with higher diameters.

Morphology
The GrO particles were poorly dispersed in the polymer solution; therefore, they are not well dispersed into the fi bers. It seems that the presence and distribution of GrO into the fi bers have no effect on the morphology of samples A and B.

Hydrophobicity
The hydrophobic nature of the membranes is a function of their chemical composition and rugosity. Polysulfone has a hydrophobic character due to the aromatic ring and the methyl groups, which have no interaction with the water molecules by hydrogen bridges (Fig. 3.3 ). The method for obtaining the membrane may change the contact angle. For example, the PSU fi lm obtained by phase inversion has a contact angle of 75°, while the fi lm of nonwoven membrane obtained by electrospinning has a contact angle of 135° [ 9 ]. This is because the surface area is greater, and there is a higher rugosity in the fi lms formed by the nonwoven fi bers. Figure 3.4 shows the sphericity of the water drops on the surface of samples A and B. As it can be seen, the sphericity is different in both samples. Contact angle values are shown in Table 3.2 . The difference in contact angles is due to the fact that membrane A has a greater contact area due to the overlaps originated in the electrospinning process. Larger diameters of fi bers mean a greater contact area and therefore a greater contact angle. Membrane B presents a more randomized distribution of fi bers of smaller diameters but with larger interfi ber spaces.

Test Filtration
Membranes were evaluated using commercial cellulose membranes (Millipore) as support. Such supporting membranes have a molecular weight cutoff of 100 and 1 kDa, and they were used in order to avoid ruptures of the obtained PSU membranes with low mechanical strength. The tests were carried out at pressures of 10, 20, 30, and 40 psi at room temperature. Figure 3.5 shows the obtained results.
From Fig. 3.5 , it is observable that both membranes, A and B, show a similar behavior with no apparent signifi cant difference between them regarding the permeated fl ow. Therefore, the hydraulic resistance between the two membranes is very similar. However, in Fig. 3.6 , it can be seen that membrane B shows a  hydraulic resistance of 16.7e5m −1 , which is 43 % greater than the value for the membrane A (10.2e5m −1 ). Thus, membrane B would have a better performance in water treatment system. The difference between them could be attributed to the differences in porosity.
The average size and porosity (% area) are reported in Table 3.3 below. Table 3.3 shows a slight difference in porosity; however, in terms of average size, sample B has a value of half of that for sample A. Thus, the hydraulic resistance is higher in sample B than in sample A.

Conclusions
It is possible to obtain hydrophobic membranes by electrospinning with different hydraulic resistances . The synthesis conditions infl uence the characteristics of the obtained membranes. As the fl ow and the distance are decreased and voltage is higher, the fi ber diameters are more homogeneous and have a better distribution in the membrane. The membranes exhibited a hydrophobic character due to polysulfone, and the graphene oxide did not have a noticeable effect on the obtained membranes since it  was not properly dispersed into them. The membrane with lower porosity showed a high hydraulic resistance. It seems that the obtained membranes could be used in the distillation by membrane process due to its hydrophobic nature, high fl ow, hydraulic resistance, and low membrane thickness.