Impact of PCLNPG Nanopolymeric Additive on the Surface and Structural Properties of PPSU Ultrafiltration Membranes for Enhanced Protein Rejection

Abstract

This research explored the use of a partially cross-linked graft copolymer (PCLNPG) as an innovative nanopolymer pore-forming agent to enhance polyphenylsulfone (PPSU) membranes for protein separation applications. The study systematically examined the impact of incorporating PCLNPG at varying concentrations on the morphological and surface properties of PPSU membranes. A thorough characterization of the resulting PPSU-PCLNPG membranes was performed, focusing on changes in morphology, water affinity, porosity, pore size, and pore size distribution. The experimental findings demonstrated that the use of PCLNPG led to a significantly more porous structure, as confirmed by SEM analysis, with notable increases in porosity and pore size (nearly double). Additionally, the hydrophilicity of the PPSU membrane was remarkably enhanced. Performance evaluations revealed a substantial improvement in pure water flux, with the flux nearly tripling. The BSA retention was directly correlated with the concentration of the PCLNPG pore former for a loading range of 0.25–0.75 wt.%. The incorporation of PCLNPG also reduced the membrane fouling propensity by reducing both cake layer resistance (Rc) and pore plugging resistance (Rp). These results underscore the potential of PCLNPG-PPSU membranes for wastewater reclamation and nutrient recovery applications.

1. Introduction

In both industrial and environmental settings, wastewater containing bovine serum albumin (BSA) offers special potential and problems for resource recovery and treatment. BSA is a widely used protein in biotechnology and medicine [1]. Nevertheless, these applications’ effluent can allow this important protein to infiltrate wastewater systems, generating many environmental problems. The chemical and biological oxygen demands (COD and BOD) of wastewater may rise when BSA is present. Elevated levels of BOD and COD can cause oxygen depletion in aquatic environments, which can negatively impact aquatic organisms by decreasing the available dissolved oxygen. Furthermore, different contaminants can interact with BSA and other proteins, changing their toxicity and bioavailability. The efficiency and treatment procedures of traditional water purification systems may become more difficult as a result of this interaction. In order to preserve healthy aquatic ecosystems and guarantee the security and dependability of water sources, it is crucial to mitigate the effects of BSA on water quality. In the present scenario, BSA separation calls for effective treatment plans in order to retrieve the high-purity protein, preserve the environment from pollution, and adhere to regulatory standards. By permitting selective passage based on molecule size and charge, membrane-based separation processes are among the potential treatment methods for effective protein separation [2].

The practicality and sustainability of wastewater treatment systems aimed at BSA removal are being improved by breakthroughs in membrane technology and process optimization, underscoring the relationship between resource recovery and environmental management in contemporary wastewater treatment techniques. BSA can be safely disposed of or used again in secondary applications thanks to the effectiveness of membrane filtration techniques like microfiltration and ultrafiltration in recovering such a resource from wastewater. These techniques can handle complex wastewater matrices because of their great selectivity and efficiency. The separation of BSA from wastewater not only reduces its environmental impact, but also advances the circular economy [3]. However, membrane fouling is an intrinsic major challenge because of its complexity. Membrane fouling can occur at different locations within the membrane, such as the pore walls and the selective layer. Depending on the properties of the target organic molecules and the membrane, fouling mechanisms may manifest to varying degrees. Given the membrane’s large pore size range, it is possible that during the first filtration stage, BSA adsorption may occur both on the membrane’s surface and inside its pores. As the filtration continues, a larger protein layer builds up on the first monolayer due to BSA adsorption, which, in turn, diminishes the flux [4,5]. Modifying the membrane’s morphology is believed to help reduce the fouling issue.

In the past few decades, an immense amount of work has been put into developing synthetic polymeric membranes with the appropriate permeability, structure, physiochemical characteristics, and selectivity. Many synthetic materials and techniques have been used to create a wide range of synthetic membrane structures [6]. The membrane structure and physio-chemical properties of target components are the key factors that determine the separation efficiency [7]. The phase-inversion procedure serves as the foundation for most of the popular techniques for fine-tuning polymeric membranes with desired characteristics [8]. Both organic and inorganic compounds have been documented as adaptable additives in the literature that aim to improve the internal structure and membrane surface during this process [9,10,11,12,13,14,15,16,17]. In this context, the integration of various water-soluble additives into the membrane structure is a standard approach for polymeric membrane enhancement because of their low toxicity, solubility in conventional solvents and water, and pore-forming capabilities [18]. Common water-soluble polymers that have been widely used as pore-forming agents include polyethylene glycol (PEG), polyethyleneimine (PEI), and polyvinylpyrrolidone (PVP) [19]. Due to their restricted porosity, variable pore size distribution, and susceptibility to fouling, conventional porous polymeric membranes have poor selectivity, which reduces their usefulness in high-efficiency applications [20]. Incorporating pore formers is a proposed method to enhance membrane separation efficiency and selectivity by modifying the membrane surface and its internal structure.

This study utilized an innovative water-soluble co-polymer (PCLNPG) as a pore-forming agent to improve the structural morphologies and surface PPSU membranes. The PCLNPG content (0, 0.25, 0.5, 0.75, 1.0, and 1.25 weight percent) inside the polymeric matrix was optimized based on the resultant membrane features. The chemical structure and morphological changes of the prepared membranes were evaluated utilizing field emission scanning electron microscopy (FE-SEM), Fourier-transform infrared (FTIR) spectroscopy, and contact angle (CA) measurements. The porosity, average pore size, and pore size distribution were examined employing the dry/wet technique, the Guerout–Elford–Ferry equation, and Image J software, respectively. The effectiveness of the membranes was assessed using bovine serum albumin (BSA) as a protein model.

2. Experimental Work

2.1. Materials

Polyphenylsulfone (PPSU) (Radel R-5000) with a specific gravity of 1.28 and an average molecular weight of 50,000 g/mol was provided by BASF (Ludwigshafen, Germany). N-methyl-2-pyrrolidinone (NMP) (C5H9NO) with a 99.5% purity was obtained from Sigma-Aldrich (St. Louis, MO, USA). Glycerol, phthalic anhydride, and dimethyl sulfoxide were employed for PCLNPG synthesis (Sigma Aldrich, St. Louis, MO, USA). Bovine serum albumin (BSA, MW = 66.4 KDa, Alpha Chemika Co., Mumbai, India) was used as a model foulant; it was acquired from Alpha Chemika Co. (Mumbai, India). The chemicals utilized were of high grade and were used exactly as received.

2.2. PCLNPG Synthesis

The synthesis of PCLNPG was established following previous work disclosed elsewhere [20]. In brief, 70 mL of dimethyl sulfoxide (DMSO) was used to dissolve 740.5 g of phthalic anhydride at 110 °C until the solution turned clear. This process produced PCLNPG. Then, 184 g of glycerol was added, followed by the gradual addition of 15 mL of P-xylene to separate the water molecules formed during the reaction [20]. Finally, cold deionized water was added to the mixture. Filtration was utilized to extract the solid white precipitate, and the resultant suspension was dried for two hours at 80 °C to produce PCLNPG powder.

2.3. Membrane Preparation

Non-induced phase separation (NIPS) was employed in the membrane synthesis process. Flat sheet membranes are primarily composed of PCLNPG and polyphenylsulfone (PPSU). The PCLNPG percentage varied from 0 to 1.25 weight percent (wt.%). The membrane composition is given in

Table 1. Codes and constituents of prepared PPSU membranes.

Membrane Code	PPSU wt.%	NMP wt.%	PCLNPG wt.%
M1	16	84	0
M2	16	83.75	0.25
M3	16	83.5	0.5
M4	16	83.25	0.75
M5	16	83	1.0
M6	16	82.75	1.25

. First, the solvent was used to independently disperse the desired content of PCLNPG for an hour. The mixture was then gradually stirred at 50 °C with a magnetic stirrer while a predetermined quantity of 16% PPSU was added. The mixing continued until the casting solution had a consistent, transparent, and yellowish look. By applying a vacuum to the mixture, air bubbles were removed. The resultant polymeric solution was then cast onto a 200-micron dry and clean glass substrate with a clearance gap using a film applicator (AFA-IV, Changsha, China). The cast film was immediately submerged in a water bath at room temperature to complete the phase inversion. The prepared membrane was carefully removed from the bath, rinsed multiple times under a flowing stream of water, and then placed in distilled water for further characterization.

2.4. Membrane Characterization

A Field Emission Scanning Electron Microscope (FE-SEM) (Inspect F-50, FEI, Alkhora Company, Holland) was used to examine the surface and cross-section morphologies of the prepared membranes. The membrane samples were fractured in liquid nitrogen for taking the cross-sectional micrographs. All samples were sputtered with gold before SEM imaging to reduce the charge effects. Additionally, a SEM was used to measure the membrane’s thickness. The pore size distribution of virgin PPSU and PPSU/PCLNPG membranes was assessed using the Image J software (Image J version 1.54d).

Membrane wettability measures were assessed utilizing the sessile drop method using an optical contact angle (CAM110, Tainan, Taiwan). A 3 μL droplet of deionized water was applied to the flat membrane sample. Using the instrument software, the contact angle between the flat base and the drop was determined. Greater hydrophilicity values are generally indicated by smaller contact angles, and vice versa. For each membrane sample, three drops of water were inspected at different locations. The mean contact angles of these drops were measured, and the average results were reported.

Fourier-transform infrared (FTIR) spectroscopy was used to analyze the chemical structure of the prepared membranes. A Bruker (Germany) Tensor 27 FT-IR spectrometer was used for the measurements. The membrane sample spectra were obtained after recording a background spectrum to create a baseline. The spectral range covered by all measurements was 4000 cm⁻¹ to 400 cm⁻¹.

The dry/wet approach was utilized to evaluate the porosity of the prepared membranes. Membrane samples measuring 10 × 10 cm² were cut, and the dry weights of the samples were noted. After that, the samples were soaked in deionized water (DI) for a whole day. The excess water was removed using soft tissue, and the samples’ wet weights were measured. Equation (1) was utilized to calculate the produced membranes’ porosity (ε) [21].

$$\mathsf{\epsilon }={\displaystyle \frac{(\mathrm{W}\mathrm{w}-\mathrm{W}\mathrm{d})/{\mathsf{\rho }}_{{\mathrm{H}}_2\mathrm{O}}}{(\mathrm{W}\mathrm{w}-\mathrm{W}\mathrm{d})/{\mathsf{\rho }}_{{\mathrm{H}}_2\mathrm{O}}+\mathrm{W}\mathrm{d}/{\mathsf{\rho }}_{\mathrm{p}}}}\times 100\%$$

(1)

where W_w and W_d are the wet and dry weight of the membrane (g), respectively, and ρ_H2O = 1.0 g/cm³ and ρ_{p (PPSU)} = 1.29 g/cm³ are the water and polymer densities, respectively.

The Guerout–Elford–Ferry Equation (2) was utilized to determine the average pore size of the prepared membranes [22].

$${\mathrm{r}}_{\mathrm{m}}=\sqrt{{\displaystyle \frac{\left(2\cdot 9-1.75\mathsf{ϵ}\right)\ast \left(8\eta \mathrm{L}\mathrm{Q}\right)}{\mathsf{\epsilon }\ast \mathrm{A}\ast {\mathsf{\Delta }}_{\mathrm{P}}}}}$$

(2)

where r_m is mean pore size (nm), ε is the porosity of the membrane, $\eta$ is the water viscosity (8.9 × 10⁻⁴ Pa. s), L is the membrane thickness, Q is the permeate flow rate of water (m³/s), A is the membrane area (m²), and ∆p is the operating pressure (Pa).

2.5. Performance Tests

Membrane filtration studies were carried out using a specially designed cross-flow filtration system with an effective membrane area of 17.68 cm². Every experiment was carried out with the feed solution at 25 ± 2 °C, compacted at 2 bars, and then decreased to 1 bar of operating pressure. Every test was run at least three times for each sample, and the collected data were averaged. Equation (3) was employed to compute the permeate flux [23].

$$\mathrm{J}={\displaystyle \frac{\mathrm{V}}{\mathrm{A}\ast \mathrm{t}}}$$

(3)

where J is the permeate flux (L/m²·h), V is the collected permeate volume (L), A is the effective area of membrane (m²), and t is the filtration time (h).

Tests were carried out with 1000 parts per million (ppm) BSA solution to evaluate membrane separation performance. A UV spectrophotometer (UV-1900, Shimadzu Co., Kyoto, Japan) was used to measure the BSA concentrations of the feed and permeate samples. Equation (4) [24] was utilized to calculate the rejection (R%):

$$\mathrm{R}=\left(1-{\displaystyle \frac{\mathrm{C}\mathrm{p}}{{\mathrm{C}}_{\mathrm{F}}}}\right)\times 100\mathrm{\%}$$

(4)

where ${\mathrm{C}}_{\mathrm{F}}$ and $\mathrm{C}\mathrm{p}$ are BSA concentrations in the feed solution and the permeate, respectively (ppm).

The different resistances were computed using Equations (5)–(9) to assess the degree of membrane fouling during filtration [25,26].

$$J={\displaystyle \frac{∆P}{\mu \ast R_t}}$$

(5)

$$R_t=R_m+R_C+R_P$$

(6)

$$R_m={\displaystyle \frac{∆P}{\mu \ast J_{w_1}}}$$

(7)

where R_t is total membrane resistance (m⁻¹), R_m is the hydraulic/inherent resistance (m⁻¹), R_c is the cake layer resistance (m⁻¹), and R_p is the pore plugging resistance (m⁻¹). These resistances can be calculated using the following equations:

$$R_P+R_c={\displaystyle \frac{∆P}{\mu \ast J_P}}-R_m$$

(8)

$$R_P={\displaystyle \frac{∆P}{\mu \ast J_{w_2}}}-R_m$$

(9)

where Jw₁ and Jp are the initial pure water and BSA solution fluxes (L/m²·h), Jw₂ is the pure water flux after washing the membrane (L/m²·h), ∆P is the operating pressure, and µ is the feed water viscosity.

The flux recovery ratio (FRR) can be calculated using Equation (10), which establishes the amount of permeate flux that may be recovered following the BSA filtration process [23].

$$FRR={\displaystyle \frac{J_{w_2}}{J_{w_1}}}\times 100$$

(10)

where FRR is the flux recovery ratio, Jw₁ is the pure water flux (L/m²·h) of virgin membrane, and Jw₂ is the pure water flux (L/m²·h) of fouled membrane after backwashing.

3. Results and Discussion

3.1. Membrane Morphology

SEM images were used to examine the impact of PCLNPG pore former content on the membrane surface and cross-sectional morphologies. Figure 1A shows that the membrane made from pristine PPSU (0 wt.% PCLNPG) had a solid, flat layer with tiny pores that were hardly noticeable on the surface. However, Figure 1B–F demonstrate that the PCLNPG significantly altered the surface morphology of the modified membranes. As the PCLNPG was increased to 1 wt.%, the changed membranes showed signs of increased pore density and larger pores when compared to the top surface of virgin PPSU. A denser surface began to emerge once more beyond this percentage (Figure 1F). This is most likely to be brought on by the casting solution’s high viscosity, which was made with the highest possible PCLNPG concentration. Denser morphologies were the outcome of the increased viscosity, which caused a delayed mixing–demixing between the solvent (NMP) and nonsolvent (water) during phase separation [27].

The cross-section morphology of the modified membranes exhibited a distinct finger-like structure visible at the top layer, and a sponge-like structure with varying numbers and sizes of macrovoids at the bottom layer (Figure 2). The cross-section morphology of the control PPSU membrane was notably denser than PCLNPG-PPSU membranes (Figure 2A). Based on the additive content, morphological changes started to appear gradually when PCLNPG was added to the PPSU polymeric matrix. When 0.25 wt.% PCLNPG was added to the casting solution, macrovoids began to appear at the bottom layer beneath the finger-like structure, whereas the control PPSU membrane only showed a sponge-like structure. The size and quantity of macrovoids increased even more when the PCLNPG weight percentage increased to 0.75 and 1.00 wt.%. At 1.25 wt.% PCLNPG, the majority of the macrovoids were suppressed, revealing a denser structure once more beyond this additive level (Figure 2F). The findings of Simphiwe et al. [28] align well with these observations. The inclusion of PCLNPG leads to the enhanced thermodynamic instability of the dope solution, which logically explains the observed morphological alterations upon adding the pore former. The instantaneous demixing in the coagulation bath is caused by the increased thermodynamic instability of the casted polymeric film caused by the presence of hydrophilic polymer as pore-forming agent. This phenomenon makes it easier for the bottom layer’s broader finger-like structure to form. Moreover, this phenomenon causes the process of precipitation in the coagulation bath to reach completion faster. This, in turn, accelerates the solidification of the casted polymeric film and creates a membrane with greater porosity [29,30].

During the phase separation process, the formation of pores creates a more porous structure by facilitating better mixing–demixing between the solvent and nonsolvent [31]. The improved hydrophilicity, porosity, and pore size in the modified membranes are attributed to the alteration in the phase inversion upon the addition of PCLNPG to the casting solution. The PPSU precipitates during the interchange of water and DMSO in the coagulation process. For relatively fast interchange rates, pores with large diameters and finger-like void structure are formed. However, with the slow interchange rate, pores with small diameters and sponge-like structures are formed. The addition of PCLNPG to the PPSU solution modifies the membrane skin layer and sublayer due to the increase in the hydrophilic character of the surface caused by the additive. The enhanced hydrophilic character of the membrane reduces the thermodynamic stability of the PPSU solution, resulting in an instantaneous water/DMSO demixing. Consequently, pores with larger diameters are formed in the support layer with the increase in the membrane porosity.

3.2. FTIR Analysis

The potential of FTIR to distinguish between different functional groups on membrane surfaces and their possible molecular interactions makes it a highly valued and frequently used characterization tool. Figure 3 presents the FTIR spectra of the unmodified PPSU and modified membranes with varied PCLNPG additions. All of the membranes displayed the distinctive PPSU FTIR spectra, which were the same as those of the pristine membrane. In particular, the anti-symmetric stretching peak of the O=S=O group was found at 1234 cm⁻¹, whereas the symmetric stretching absorption peak of the same group in PPSU was observed at around 1148.69 cm⁻¹ [32,33]. Moreover, two distinct peaks at 1493 cm⁻¹ and 1590 cm⁻¹ were found to be caused by the stretching vibrations of the benzene ring (C=C). Similar characteristic peaks were also reported in [34,35,36]. The bands at 897 and 1669 cm⁻¹, which were only visible on the PCLNPG, completely disappeared from the membranes that were ready for the PPSU/PCLNPG, according to the PCLNPG spectra. The peak at 1669 cm⁻¹ displays the C=O stretch. This double bond permits interaction with the PPSU polymer, suggesting that pores were left behind when the PCLNPG was hydrolyzed. The PCLNPG structure’s high hydroxyl group (OH-) content also contributed to this kind of degradation by promoting hydrogen bonding with water during phase inversion. The observations made here agree with the conclusions made by Kadhum et al. [20]. When the pore former content was changed, the spectra of the membranes incorporating PCLNPG did not significantly change. This result was anticipated, since PCLNPG is expected to leach after film formation.

3.3. Hydrophilicity of Membranes

Determining a membrane surface’s wettability is essential for studying its fouling behavior. The membrane surface affinity with water is evaluated by the water contact angle [37]. The contact angle data for both virgin and modified PPSU membranes are shown in Figure 4. Increasing the PCLNPG increased the membrane hydrophilicity. When compared to membranes modified by PCLNPG, the pristine PPSU membrane’s contact angle (CA) measurement was determined to be the highest at 81.7°. A small decrease in the CA value was seen when PCLNPG was added to the polymeric matrix at a wt.% of 0.25, recording 80°, compared to 60.3° when the same content was doubled to 0.5 wt.%. Using 0.75 wt.% PCLNPG produced the lowest CA value of 52.2°. This decline could be caused by changes in the pore density and size. At 1.0 and 1.25 wt.%, the membrane contact angle rose to 61.0° and 63.1°, respectively. The increases in contact angles for 1.0 and 1.25 wt.% PCLNPG as opposed to 0.75 wt.% PCLNPG were caused by the reduction in both porosity and mean pore size.

3.4. Thickness, Porosity, and Pore Size of Membranes

The thickness of a membrane is a critical parameter that significantly influences its performance as it presents the main barrier for water transfer across the membrane. The membrane’s thickness determines the rate at which substances can permeate through it, thereby affecting its efficiency and suitability for various applications. As shown in Figure 5 and

Table 2. The pore size, porosity, and thickness of prepared membranes.

Membrane	Pore Size (nm)	Porosity (%)	Contact Angle (°)	Thickness (µm)
M1	25.22	60.16	81.7	64.33
M2	36.68	72.61	80.0	110.1
M3	40.92	76.02	61.8	114.8
M4	45.32	79.49	52.2	111.53
M5	42.22	74.81	61.0	110.2
M6	42.42	69.48	63.1	130.3

, the pristine PPSU membrane demonstrated the lowest thickness, measuring approximately 64.3 microns. This measurement serves as a baseline for comparison against the modified membranes. The introduction of a small amount (0.25 wt.%) of the nanopolymer PCLNPG into the casting solution led to a notable increase in membrane thickness, reaching around 110 microns. This significant increase underscores the high impact of even minor modifications of the casting solution composition on the resulting membrane’s structural characteristics. Further increments in the PCLNPG content did not induce similarly substantial increases in membrane thickness. Specifically, membranes modified with 0.5 wt.% PCLNPG exhibited a thickness of approximately 114 microns, those with 0.75 wt.% PCLNPG measured around 111 microns, and those with 1 wt.% PCLNPG showed a thickness of about 110 microns. These measurements suggest a plateau effect, where additional nanopolymer beyond a certain concentration does not significantly alter the membrane’s thickness. However, an exception was observed at a higher concentration of 1.5 wt.% PCLNPG, where the membrane thickness increased to approximately 130 microns. This further increase in thickness can be attributed to the rise in solution viscosity. The higher viscosity likely induces a delayed mixing–demixing of the solvent and nonsolvent during casting. This delay results in a thicker membrane structure and a more robust active layer, as the components have more time to interact and form a denser matrix [38].

Another crucial factor that affects membrane performance is membrane porosity. The incorporation of PCLNPG had a significant impact on the porosity of the membranes, as shown in Figure 5. The pristine PPSU membrane exhibited an initial porosity of 60.1%. With the addition of 0.25 wt.% and 0.5 wt.% PCLNPG, the porosity increased slightly to approximately 72.6% and 76%, respectively. However, a notable increase in porosity was observed when the PCLNPG content was raised to 0.75 wt.%, with the porosity rising significantly from 60.16% in the neat PPSU membrane to 79.49%. This increase in porosity can be attributed to the water solubility of the PCLNPG additive, which may leach out of the membrane matrix during the exchange between the nonsolvent and solvent in the coagulation bath. As the concentration of PCLNPG increased to 1.0 wt.% and 1.25 wt.%, the porosity of the PPSU/PCLNPG membranes decreased to 74.81% and 69.48%, respectively. This decrease is likely due to the increased casting solution viscosity, which restricted the movement of PCLNPG nanopolymer molecules, thereby impeding the creation and expansion of membrane pores. These findings are consistent with those reported by other researchers [31,39], highlighting the complex relationship between additive concentration, solution viscosity, and membrane porosity.

Alongside other surface characteristics, the pore size and its distribution are important in determining a membrane’s permeability and selectivity. Figure 6 illustrates the impact of varying PCLNPG percentages on PPSU membranes’ pore size. The pristine PPSU membrane demonstrated an average pore size of 25.2 nm, the smallest among all modified membranes. However, incorporating the PCLNPG nanopolymer significantly increased the pore size in the nanocomposite membranes, with the largest pore size of 45.3 nm observed in the 0.75 wt.% PCLNPG membrane. This increase is likely due to the enhanced thermodynamic stability of the casting solution induced by the PCLNPG addition, which accelerates the solvent and nonsolvent exchange rate, leading to the formation of larger pores and higher pore density. Conversely, increasing the PCLNPG content beyond this concentration led to a slight decrease in pore size, reducing it to around 42.2 nm and 42.4 nm for the 1 wt.% and 1.25 wt.% membranes, respectively. This reduction is attributed to the slight viscosity increase of the casting solution at higher PCLNPG concentrations, which curtails the solvent and nonsolvent exchange during the phase separation process. This results in reduced porosity and pore sizes, and decreased membrane thickness. The results observed in the membrane morphology as captured by SEM images support these findings. According to Hong and He [40], the solvent and nonsolvent (water) exchange velocity is a key determinant of pore size. Previous studies have demonstrated that rapid or immediate solvent–nonsolvent demixing processes tend to produce membranes with larger pore sizes [41,42,43]. These observations align with the results obtained in this study, highlighting the intricate balance between casting solution composition, phase separation dynamics, and the resulting membrane properties.

Figure 7 illustrates the impact of PCLNPG incorporation on the distribution of pore sizes within the membranes. As the PCLNPG content increased to a specific threshold, a corresponding pore size increase was observed. The pore size distribution for all PPSU-PCLNPG membranes shifted to the higher side of the distribution scale. Additionally, the incorporation of PCLNPG broadened the pore size distribution and increased pore density. These results validate the effectiveness of PCLNPG as a pore-forming agent. The underlying mechanism for this enhancement is tied to the solvent and nonsolvent exchange rate during casting. The inclusion of PCLNPG accelerated this exchange rate, leading to the formation of larger pores. This is due to the strong hydrophilic properties of the PCLNPG nano-additive, which facilitates the creation of a more porous membrane structure. However, when the PCLNPG content in the casting solution exceeded 0.75 wt.%, a decrease in pore size was observed. This phenomenon can be attributed to the viscosity increase of the PPSU-PCLNPG solution at higher PCLNPG concentrations, which delays the mixing–demixing process and consequently reduces pore size. These findings highlight the delicate balance between additive concentration and membrane morphology, demonstrating that while PCLNPG effectively increases pore size and density at lower concentrations, excessive amounts can hinder this effect by altering the solution’s viscosity and slowing down phase separation dynamics. This study confirms that the optimal PCLNPG concentration for enhancing membrane porosity is 0.75 wt.%.

3.5. Membrane Performance

The prepared membranes were tested at 1 bar and 25 ± 3 °C for 60 min using pure water and a synthetic BSA solution. Figure 8A illustrates the effect of varying PCLNPG concentrations in the PPSU casting solution on the permeate flux of PPSU membranes. A clear proportional relationship was observed between permeability and PCLNPG concentration up to 0.75 wt.%. The pristine PPSU membrane exhibited the lowest permeability for pure water, recording 40.83 L/m²·h. This low permeability can be attributed to the hydrophobic nature of pure PPSU, which results in the formation of a dense layer on the membrane’s surface, coupled with small pore size and porosity, thereby reducing permeability [25].

Incorporating 0.25 wt.% PCLNPG nanopolymer into the casting solution increased the pure water flux (PWF) of membranes to 68.9 L/m²·h. Further increasing the concentration to 0.5 wt.% resulted in a PWF value of 89.42 L/m²·h for the M3 membrane. The highest PWF of 115.5 L/m²·h was achieved with a 0.75 wt.% nanopolymer concentration. This was expected, as this membrane exhibited the highest hydrophilicity, porosity, and mean pore size. According to the existing literature [16,44], the primary factors enhancing membrane performance include improved membrane structure and increased hydrophilicity from the addition of hydrophilic additives. However, the pure water flux decreased to 96.27 and 72.21 L/m²·h for membranes with 1.0 and 1.25 wt.% PPSU/PCLNPG, respectively. Vatanpour et al. [45] explained that the higher nanopolymer concentrations in the casting solution, the lower are the porosity and mean pore size leading to decreased permeate flux.

The BSA solution flux increased from 15.3 to 65.41 L/m²·h as the PCLNPG load in the membrane composition increased from 0 wt.% (M1) to 0.75 wt.% (M4). However, membranes with 1.0 wt.% and 1.25 wt.% PCLNPG showed a sharp decline in flux to 49.73 L/m²·h and 36.8 L/m²·h, respectively. As shown in Figure 8A, the neat PPSU membrane had a lower BSA flux of 15.3 L/m²·h. This decline was attributed to the hydrophobicity of the PPSU membrane surface, which encourages BSA molecules to adsorb and accumulate on the membrane surface, blocking some pores [23].

The BSA rejection of the fabricated membranes was also investigated, and the results are presented in Figure 8B. The results revealed that all modified membranes showed an increasing trend in BSA retention with higher PCLNPG nanopolymer content. Incorporating 0.25 wt.% of PCLNPG into the PPSU polymeric matrix resulted in a significantly higher rejection rate of 91.5% for BSA, compared to approximately 88.4% for the pristine PPSU membrane. Herein, the retention potential of the neat PPSU membrane was the lowest among all fabricated membranes. When the PCLNPG content in the PPSU membrane increased to 0.75 wt.%, the rejection rate rose to 95.14% (M4). This improvement was due to the hydrophilicity imparted by PCLNPG, which facilitated water absorption by the membrane’s surface forming a water layer between the membrane and BSA (foulants). This not only increased the water flux, but also improved BSA rejection [22]. Meanwhile, additive content beyond 0.75 wt.% resulted in a gradual decline in the separation performance, but was still higher than neat PPSU membrane. This outcome corroborates well with the findings reported in [24], which indicate that protein adsorption is more favorable on hydrophobic surfaces than hydrophilic surfaces.

3.6. Antifouling Analysis

Membranes developed with different PCLNPG concentrations are employed in BSA solution filtering tests to evaluate the membranes’ resistance capabilities. The data for each membrane are shown in Figure 9 and correspond to the following resistance metrics: total resistance (Rt), hydraulic resistance (Rm), cake layer resistance (Rc), and pore plugging resistance (Rp). The Rt values of all of the modified membranes are clearly lower than those of the pristine membrane, which demonstrated an Rt value of about 45 m⁻¹. Diminished Rt values imply enhanced antifouling characteristics and increased water flux in the modified membranes. Moreover, the reduced Rm values suggest that the hydrophilic surface of the modified membranes facilitates easier passage of water molecules, thereby reducing resistance [46]. During filtration, the wet surface of these hydrophilic membranes tends to minimize the adhesion of BSA molecules. Consequently, the M4 membrane, compared to the unmodified PPSU membrane, exhibited significantly lower Rp and Rc values, recorded at 1.9 m⁻¹ and 6.1 m⁻¹, respectively. A lower Rc value indicates reduced BSA adhesion on the membrane surface, while the decreased Rp values highlight that the adsorbed BSA molecules on the modified membranes can be easily removed through simple cleaning with pure water.

The flow recovery ratio (FRR) was calculated to gauge the membranes’ antifouling potential in greater depth. Measurements of pure water flux were made following BSA filtration to evaluate the reusability of the membrane. All membranes were washed with deionized water for an hour under non-pressurized conditions prior to recording the PWF. Better antifouling properties are correlated with a larger FRR, which is suggestive of a reduced degree of BSA adherence to the membrane surface [47]. All composite membranes including PCLNPG nanopolymer disclosed greater FRR values than the pristine PPSU membrane (M1), which had the lowest FRR of 55.62%, as shown in Figure 10. The incorporation of 0.25 wt.% and 0.5 wt.% of PCLNPG in the polymeric matrix resulted in FRR values increasing to 63.2% and 66.1%, respectively. The M4 membrane, which incorporated 0.75 wt.% PCLNPG, exhibited the highest FRR value of 69.45%. However, a slight reduction in FRR values to 66.33% and 65.8% was observed when the PCLNPG concentration was further increased to 1.0 wt.% and 1.25 wt.%, respectively. Despite this decline, the FRR values of the modified membranes remained significantly higher than those of the unmodified PPSU membrane.

These results highlight that the addition of PCLNPG enhances the surface and morphological properties of the composite membranes, reducing the tendency of protein molecules to accumulate and adsorb on the membrane’s active layer. In addition to the improved antifouling properties, the PCLNPG additive improved the membrane’s surface hydrophilicity marked by a decrease of the contact angle by 36% with the increase of PCLNPG percentage from 0% to 0.75%. This increase in surface hydrophilicity of membranes was accompanied by an increase in the membrane porosity and pore diameter by 32% and 80%, respectively. All of these changes led to a higher water flux by 68%. Additionally, the modified membrane exhibited better protein rejection efficiency compared to pristine membranes, in the range of 91–95%, which is on par or better than the rejection levels reported for modified PPSU-UF membranes in other studies [25,48]. These enhancements make PPSU-PCLNPG a suitable candidate for water treatment applications that require high water flux with adequate rejection of large molecules such as wastewater reclamation for agricultural reuse [49] and nutrient recovery from wastewater [50].

4. Conclusions

In this study, a polyphenylsulfone ultrafiltration membrane was modified by incorporating a novel water-soluble polymeric nano-additive, PCLNPG, which was utilized as a pore-forming agent. The investigation focused on the effects of varying PCLNPG concentrations (ranging from 0 to 1.25 wt.%) on various membrane properties, including surface and cross-sectional morphology, porosity, hydrophilicity, and overall performance. The modification resulted in significant changes in the cross-sectional morphology compared to the pristine PPSU membrane. Additionally, the hydrophilicity of the membrane improved significantly, as evidenced by the reduction in contact angle from 81.7° to 52.2°, while the porosity increased from 60.16% to 79.49% when 0.75 wt.% PCLNPG was incorporated. Moreover, the integration of PCLNPG into the polymeric matrix led to a noticeable increase in both membrane thickness and average pore size, with these enhancements becoming particularly pronounced as the PCLNPG content was increased up to 0.75 wt.%. The performance of the modified membranes was also significantly enhanced by the presence of PCLNPG. Specifically, the PWF and BSA rejection rates for the membrane with 0.75 wt.% PCLNPG were markedly higher, recording 115.5 L/m²·h and 95.4%, respectively, compared to 40.83 L/m²·h and 88.4% for the unmodified PPSU membrane. The optimized PCLNPG content (0.75 wt.%) resulted in significantly lower filtration resistance and a higher flux recovery ratio (FRR) compared to the pristine membrane. These findings suggest that modifying PPSU membranes with PCLNPG enhances the membrane’s surface characteristics, which, in turn, improves its performance in protein separation applications and other water treatment applications that require high water flux.