Synthesis of PES membranes modified with polyurethane–paraffin wax nanocapsules and performance of bovine serum albumin and humic acid rejection

Water scarcity and pollution have become a worldwide concern as a result of growing urbanization, industry, and technological advancements, posing a threat to the entire ecosystem and harming human health (Arenas-Sánchez et al. 2016). Membrane technology is still the most effective treatment and water recovery system, with the best prospects in wastewater applications, as compared to other physical, chemical, physicochemical, or biological treatment systems (Nasir et al. 2022). Sadly, the most significant disadvantage of membrane-based separation is membrane fouling, which decreases the permeation flux and hence raises operational and maintenance costs (Barambu et al. 2021; Li et al. 2021; Qiu et al. 2022). Therefore, the membrane surface needs to be modified in various ways to prevent the fouling mechanism.

Phase change materials (PCMs) with a crystalline structure are organic or inorganic compounds that can absorb and store energy as latent heat and release energy depending on temperature differences during the phase change process, melting and freezing, evaporation and condensation, or changes in the crystal structure (Jin et al. 2008; Atinafu et al. 2020; Yang et al. 2021). In addition to the chemical properties of latent heat storage PCMs such as non-toxic, non-corrosive, non-flammable, non-explosive, and chemically stable for a long time, physical (high density, low vapor pressure, and small volume changes at phase transitions), thermal (high thermal conductivity, high latent heat, and high specific heat), and kinetic properties (absence of subcooling and having a suitable crystallization rate) also draw attention (Shchukina et al. 2018; Vadhera et al. 2018; Rodríguez-Cumplido et al. 2019). Moreover, the low cost and abundance of PCMs are important factors in their applicability in a variety of fields (Vadhera et al. 2018). PCMs have the potential to be used in application areas like energy (Gupta et al. 2020; Faraj et al. 2021; Zhi et al. 2022), environment (Lamnatou et al. 2018; Dibene et al. 2020; Roy et al. 2021), textiles (Aitali et al. 2016; Lu et al. 2019; Saraç et al. 2019; AitAli et al. 2023), and medicine (Li et al. 2015, 2019) due to their phase transition temperatures and characteristics. PCMs are classified as organic, inorganic, or eutectic. Among them, paraffin, which is one of the organic PCMs, stands out with its important properties such as non-corrosive, chemically inert, low vapor pressure during the melting phase, and non-toxicity (Magendran et al. 2019; Ghasemi et al. 2022). However, paraffin's limited heat conductivity, absence of a stable melting point, and flammability limit their application possibilities. As a result, various solutions have been created, especially to solve the problem of low thermal conductivity, and the most common of these is the method of keeping paraffin in polymeric mesh such as polyurethane and polyethylene (Vasu et al. 2019; Nikpourian et al. 2020). Microencapsulation and nanoencapsulation methods applied for this purpose are used to eliminate the disadvantages of paraffin. PCMs coated by microencapsulation or nanoencapsulation methods are small core-shell particles coated with inorganic or polymer materials. The nanoencapsulation approach improves the microencapsulation method by reducing the surface-to-volume ratio by shrinking the capsule to nanoscale size. Thus, the heat transfer rate increases with the nano-encapsulated PCMs obtained from Zhang & Yuan (2020), Peng et al. (2021), and Cao et al. (2022). Abdeali et al. employed a polymerization procedure to generate nano-encapsulated paraffin wax core/polyurethane shell (PW/PU) and determined that the sonication time and reaction temperature had a good effect on thermal performance by creating narrow particle size distribution, according to the findings (Abdeali et al. 2021). The incorporation of PCMs into the UF membrane matrix can increase its thermal stability and improve its resistance to temperature variations. This can be particularly useful in applications where the temperature of the feed solution needs to be maintained within a narrow range, as the PCM can help to absorb or release thermal energy and prevent fluctuations in the feed solution temperature (Wang et al. 2014). In addition, the use of PCM-modified UF membranes can enhance the separation performance of the membrane. PCMs can reduce fouling and enhance the permeate flux by reducing the adhesion of foulants to the membrane surface, resulting in better membrane cleaning and longer membrane lifespan (Fakhry et al. 2022).

In this study, nano-encapsulated paraffin wax core/thermoplastic elastomer polyurethane shell (PW/PU) was synthesized by the mini-emulsion polymerization method. The synthesized PW/PU nanocapsules were characterized and validated by scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and Fourier-transform infrared spectroscopy (FTIR) analyses. Confirmed by characterization methods, polyurethane–paraffin wax (PU/PW) nanocapsules were integrated into the polyethersulfone (PES) membrane to form a composite membrane with higher antifouling and permeability performance against humic acid (HA) and bovine serum albumin (BSA) foulants. To the best of our knowledge, no studies on PES membranes modified with PU/PW nanocapsules have been reported in the literature.

Isophorone diisocyanate, polyethylene glycol (PEG6000), cyclohexanone, paraffin wax, 1-dodecanol, glycerol, sodium dodecyl sulfate (≥99.0%), polycaprolactone (MW: 10,000), and HA were purchased from Sigma Aldrich and used without additional purification. BASF Company (Germany) provided polyethersulfone (PES, Ultrason E6020P, MW: 58,000 g/mol). Dimethyl sulfoxide (DMSO) and bovine serum albumin (BSA, MW: 66,000 g/mol) were supplied from Merck Company (Germany). All chemicals were analytical grade, and distilled water was obtained using a Millipore Direct-Q3UV purification system with two stages.

PU/PW nanocapsules were synthesized in accordance with the literature (Abdeali et al. 2021). In a typical synthesis of the PU/PW nanocapsules, 2 g of sodium dodecyl sulfate (0.006 mol) is first dissolved in 10 mL of deionized water under inert gas, and the organic phase containing 0.6 g of isophorone diisocyanate (IPDI), 0.1 g of paraffin wax, and 0.14 g of cyclohexanone (0.0014 mol) are mixed thoroughly in a separate beaker. Subsequently, both phases are brought together in an inert atmosphere and allowed to mix at 80 °C. The mixture was ultrasonicated for 15 min and 0.12 g of PEG6000 was added to the medium and mixed for 1 h. About 0.16 g of polycaprolactone and 0.16 g of glycerol were added to the medium, and the mixing was continued for another 3 h. After adding 0.3 g of 1-dodecanol (0.0016 mol) to the medium, PU/PW dispersion was filtered and washed through centrifugation (10,000 rpm, 15 min, several times) using hot deionized water and cyclohexanone. Finally, obtained PU/PW nanocapsules were freeze-dried in a vacuum. The zeta potential of PU/PW nanocapsules was determined using a zeta sizer (Malvern Zetasizer equipped with MPT-2 Titrator, Nano ZS). The samples were prepared using deionized water as a 0.1% solution, and the pH of the solution was adjusted to 7.0.

The phase inversion method was used to manufacture the neat and PU/PW nanocapsules blended membranes. Before using the PES beads, they were dried at 80 °C in an oven for 2 h. To manufacture functionalized membranes, first, the different ratios of PU/PW nanocapsules (0.5, 1.0, and 2.0 wt%) were added to DMSO solvent and dispersed for 15 min by using an ultrasonication bath, and PES beads were then added to this solution. The prepared solution was rapidly shaken at 60 °C for 6 h and after that at room temperature overnight then ultrasonicated for 20 min again to obtain a clear homogeneous solution and remove air bubbles from the casting solutions. When all bubbles were removed, casting solutions of PU/PW nanocapsules were spread on a glass plate with a casting knife gap setting of 200 μm at a speed of 100 mm/s. The glass plates were subsequently immersed in a deionized water bath to achieve polymer sedimentation. The membranes were withdrawn from the water bath and kept in fresh deionized water for at least 1 day after manufacture to ensure complete phase inversion. The casting solution's composition is shown in Table 1. Before the experiments, all membranes were operated at 5 bar with deionized water.

The surface morphology of the manufactured pristine PES membrane and PU/PW nanocapsules blended PES membranes were characterized by scanning electron microscopy (SEM, Gemini Zeiss Supra 55). The membranes were coated with a gold layer after drying at room temperature. The SEM images were taken using a 20.0 kV acceleration voltage. The contact angle of all the manufactured membranes is measured to determine their surface hydrophilicity. A dead-end flat sheet membrane module with a filtration area of 14.6 cm² was used to test the filtration performance of the manufactured membranes at a temperature of 25 ± 1 °C and an operating pressure of 1 bar.

The dead-end filtration system was filled separately with 100 mg/L of BSA solution and 50 mg/L of HA solution to test the efficiency of the manufactured membranes. To prevent the denaturation of protein, BSA solution was prepared in phosphate buffer solution (PBS, 50 mM, pH 7.4 ± 0.1). Both solutions were filtrated at 1 bar for 120 min. The Lowry method (Lowry et al. 1951) was used to determine the protein concentration of the feed (C_f) and permeate (C_p). The HA concentration in the solution is represented by total organic carbon (TOC) content, which was measured by using a TOC analyzer (Elementar, Vario TOC Cube). The desorption experiments for BSA and HA, which were adsorbed onto the membrane surface, were conducted by separately filtering them for 150 min at prepared concentrations. Afterward, the membranes were placed in pure water and agitated using an orbital shaker to measure the time-dependent amounts of BSA and HA pollutants on their surfaces that passed into the pure water.

According to the results of the TGA analysis, approximately 92% mass loss occurred, and from the DTG curve (red curve) given in the TGA analysis, it is seen that the maximum temperature value at which degradation occurs is 367.8 °C. This temperature value obtained from the DTG curve shows the highest material loss at 367.8 °C. DSC analysis of the synthesized material is shown in Figure 3(b). The endothermic peak at 49.9 °C is seen in the DSC analysis graph, and this peak belongs to the PU/PW nanocapsules. The use of PU/PW nanocapsules provided delayed temperature increases.

The energy dispersive spectrometry (EDS) mapping of pristine and PU/PW nanocapsules blended PES membrane depicted that the elementary components were composed of C, O, and S (Table 2). It can be seen the increase in the C ratio and the decrease in the O and S ratio indicate that the PU/PW is successfully involved in the membrane structure.

The porosity and mean pore size of the membranes are presented in Table 3. The results showed that the porosity of the membranes decreased from 62.5 ± 1.1 to 55.5 ± 1.2% after blended of PU/PW nanocapsules. Moreover, the mean pore radius decreased from 24.8 ± 0.35 to 17.7 ± 0.19 nm.

In this study, nano-encapsulated paraffin wax core/thermoplastic elastomer polyurethane shell (PW/PU) was synthesized by the mini-emulsion polymerization method and then it is used for manufacturing PU/PW nanocapsules blended PES membranes by the phase inversion method. PU/PW nanocapsules were characterized by SEM, TGA, DSC, and FTIR analyses. The use of PU/PW nanocapsules provided delayed temperature increases, which are confirmed by DSC. All membranes were characterized by SEM, SEM-EDS, and contact angle. After the PU/PW nanocapsules were dispersed into the PES membrane, the composite membranes' hydrophilicity increased. BSA and HA are used to test the antifouling and permeability performance of manufactured membranes. The pure water, BSA, and HA fluxes of the composite membranes increased compared to the pristine PES membrane. PU/PW nanocapsules blended PES membranes can be used to decrease membrane fouling.

B.S., G.K., Y.O. and A.A. performed methodology and data curation. K.O. and N.D. conceptualized, investigated, and wrote the original draft and did formal analysis and reviewed and edited the manuscript.

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.