Polyphenylsulfone Membrane | Encyclopedia MDPI

Polyphenylsulfone Membrane: Comparison

Please note this is a comparison between Version 1 by Javed Alam and Version 2 by Rita Xu.

Polyphenylsulfone (PPSU) membranes are of fundamental importance for many applications such as water treatment, gas separation, energy, electronics, and biomedicine, due to their low cost, controlled crystallinity, chemical, thermal, and mechanical stability. Numerous research studies have shown that modifying surface properties of PPSU membranes influences their stability and functionality. Therefore, the modification of the PPSU membrane surface is a pressing issue for both research and industrial communities.

membranes
modification methods
polyphenylsulfone

1. Introduction

Polyphenylsulfone (PPSU) belongs to sulfone-family polymers that have been thoroughly studied for their potential applications in membrane science and technology. The state-of-the-art PPSU-based membranes show superior properties, including excellent thermal and mechanical stability, high chemical resistance, impact resistance, and hydrolytic stability ^{[1][2][3][4][5][6][7][8]}[1,2,3,4,5,6,7,8]. This stability can be attributed to the difference in their backbone structure compared to other polymeric materials. However, the membrane morphological structure and properties are elaborated by the composition and operating conditions of a PPSU solution, including concentration, additives, solvent type, temperature, kinetic factors, the coagulation bath (phase inversion process), etc. ^{[9][10][11][12][13][14][15][16]}[9,10,11,12,13,14,15,16]. Thermodynamics and kinetics play significant roles during membrane development ^[17][18][19][17,18,19]. Thermodynamics determines whether a PPSU polymer solution is stable or not. Kinetics plays a key role in the phase separation speed. Despite the aforementioned important properties of PPSU polymers, there has been a limited number of studies concerning the preparation of PPSU membranes ^{[20][21][22][23]}[20,21,22,23]. However, these studies showed promising results in polymer applications. In particular, the membranes can be used for ultrafiltration, nanofiltration, and reverse and forward osmosis. At the same time, other polymer materials often prove more susceptible in terms of stability (chemical, thermal, and mechanical) and are often expensive ^{[24][25][26][27][28][29][30]}[24,25,26,27,28,29,30].

One disadvantage of PPSU-based membranes is their hydrophobic nature, which leads to reduced surface energy. The latter causes poor antifouling ability by foulant pollutants in water. Two more disadvantages of the PPSU membrane are its low water permeability and high fouling ability. These two have limited its application in aqueous phase separation. A number of studies have concluded that membrane fouling is directly related to hydrophobicity and surface charge, as reviewed by several researchers, while the opposite has also been reported ^{[31][32][33][34]}[31,32,33,34]. Membrane fouling is generally classified as organic fouling, inorganic fouling, or biofouling (nonpolar solutes, hydrophobic particles, microorganism, mineral scale). It can easily adhere to or accumulate on the membrane surface or plug membrane pores by hydrophobic interactions, hydrogen bonding, van der Waals attractions, and electrostatic interactions ^{[14][35][36][37][38][39]}[14,35,36,37,38,39]. As a result, the membrane separation process becomes more complex, and its permeability and selectivity are reduced. The latter leads to an increase in operating costs, energy demand, and shorter membrane lifetimes.

Thus, the current trend is to improve PPSU membrane materials and structures and to get membranes with both good separation and antifouling performance. Controlling the membrane surface properties and structure has been a common goal for improving membrane separation performance (Figure 1). Achieving this goal is not an easy task. However, various types of inorganic and organic materials have been used to improve the characteristics of the PPSU membranes ^{[28][40][41][42][43]}[28,40,41,42,43]. Several features, including the PPSU and additive concentrations, molecular weights, the miscibility characteristics, the compatibility with organic and inorganic materials properties, and the solvent type, can impact the performance of these additives.

Figure 1. PPSU membranes significant environmental applications: water purification, gas separation, decontamination of water, solvent separation, fuel cell, and biomedical.

Multiple studies have reported the fabrication of PPSU-based membranes in different configurations, including flat sheet and hollow fibers ^{[44][45][46][47][48]}[44,45,46,47,48]. However, to the best of theiour knowledge, there is no state-of-the-art report on PPSU-based membranes that summarizes their surface modifications and associated changes in performance. The current review serves to fill this research gap. More specifically, researcherswe bring together recent advancements in polymer and membrane development for the benefit of both academic and industrial researchers. ResearchersWe focus on various modification methods as well as performance evaluation. There are three main approaches for the modification of PPSU polymer or membranes with improved surface properties: (1) bulk modifications via sulfonation, amination, and chloromethylation; (2) blending with a synthetic polymer, inorganic nanomaterials, and biopolymer; and (3) surface modification via physical and chemical approaches (Figure 2).

Figure 2. Modification techniques for PPSU utilizing various modifiers.

2. Polyphenylsulfone Characteristics

PPSU is the abbreviation for Polyphenylsulfone. Also known as PPSF, PPSU is a new member of the sulfone polymers family that has multiple attractive properties such as high-temperature performance, good chemical resistance (maintaining its original properties after being exposed to a harsh chemical environment pH at 1–13), outstanding toughness, corrosion resistance, chlorine tolerance, excellent colorability, and very good dimensional stability [49]. The polymer can be distributed in two different families depending on the level of the molecular organization of the constitutive chains at the microscopic level. Compared to other sulfone polymers, PPSU is an amorphous polymer. Therefore, it features very good creep resistance, isotropic thermal and mechanical properties, and transparency. PPSU consists of an aromatic unit (phenylene) chain with a sulfone group and a benzene ring, connected by an oxygen atom. Due to this conjugated structure, the rigidity of the material can be maintained, and it gives good liquidity ^[28][50][28,50]. Figure 3 shows the molecular structures of PPSU.

Figure 3. General structure of Polyphenylsulfone polymer.

The presence of the electronegative sulfone group results in sulfur being in its highest oxidation state. The latter brings excellent thermo-oxidative stability and easy functionalization. The surface of PPSU resin has a negative charge over a pH of 3 [30]. The existence of a biphenylene unit in PPSU resin significantly elevates the impact strength and reduces the notch sensitivity of the material. The latter results in strength at break (tensile) values greater than 75 MPa, a glass transition temperature of 288 °C, and a heat deflection temperature of 274 °C. Therefore, PPSU is expected to become the next widely used polymer in various applications, including membrane filtration, plumbing, food services, medical, aerospace, wire and cable, etc. On the other hand, the PPSU-based membranes that have been widely used in water applications have several drawbacks. The main drawback is related to its relatively hydrophobic nature. It reduces membranes’ permeability and makes them more susceptible to fouling during water treatment. Chemical cleaning is an essential step to sustain the membrane life. At the same time, high standards of water quality should also be met. Table 1 summarizes different methods that were used in the modification of PPSU membrane, modifier agents, and performance of modified membranes.

Table 1. Summary of frequently used methods for modification of PPSU membrane and performance.

Description	Methods of Modification	Modifier Agents	Process of Membrane	Application	Performance	Ref.
Proton-conductive sPPSU membranes	Sulfonation	SO₃ and (CH₃)₃ SiSO₃Cl	Solvent evaporation	Electrochemical	(CH3)₃SiClSO₃ gave a homogeneous sPPSU with better control of the DS values as high as 1.0; asymmetric structure; high mechanical stability; proton conductivity about 55 mS/cm at 80 °C	[51]
Proton-conducting fuel cell sphPPSU membranes	Sulfophenylation	BuLi (metalating agent) and 2-sulfobenzoic acid cyclic anhydride	Vacuum dry	Fuel cells	sphPPSU showed DS values as 0.9; membranes have high thermal stability (300 and 350 °C); the proton conductivity about 60 mS/cm at 70 °C	[52]
PEI/PPSU sheet	Blending	PEI	Direct injection molding	Plasticization	PEI/PPSU blends are miscible; elasticity and yield stress changed linearly with PEI-rich blends composition	[53]
Proton exchange SPEEK/SiSPPSU membranes	Silylation and sulfonation; and blending	PhSiCl₃ and H₂SO₄; SPEEK	Solvent evaporation	Fuel cells	SiSPPSU showed DS values as 2.0; exhibited high and stable conductivity values at 120 °C when dry (6.1 × 10⁻³ S/cm) and wet conditions (6.4 × 10⁻² S/cm)	[54]
sPPSU-proton conducting membrane	Sulfonation	H₂SO₄ and ClSO₃Si (CH₃)₃	Sol-gel processes	Fuel cells	sPPSU reached the conductivity values as high as 1.1 × 10⁻² S cm⁻¹ at 130 °C	[55]
PPSU/PBNPI membrane	Blending	PBNPI	Solvent evaporation	Hydrogen separation	The gases H₂, CO₂ and CH₄ permeability increased up to 50%	[56]
PPSU/PBNPI membrane	Blending; immersion method	PBNPI; p-xylylenediamine (crosslinking reagent)	Solvent evaporation	Gas permeation	O₂ and N₂ permeation rates of 23.2 and 22.42	[57]
sPOSS/sPPSU composite proton exchange membranes	Blending	sPOSS	Dry	Fuel cells	sPOSS/sPPSU composites multilayered structure and reduce brittleness; conductivity 1 × 10⁻² S cm⁻¹ at 90 °C	[1]
Ionic exchange sPPSU/sPES membrane	Sulfonation; Blending	H₂SO₄; sPES	Solvent evaporation and dry	Fuel cells	The membrane surfaces show the smoother about 2 nm; stress–strain values 80 MPa and 7%	[5]
SPEEK/SiPPSU composite membranes	Silylation; Blending	SPEEK	Dry	Fuel cells	The presence of silicon enhances the temperature of loss of sulfonic acid groups; composites show superior behavior in terms of mechanical properties (higher elastic modulus and tensile strength)	[50]
PPSU/PEI membranes	Blending	PEI; PEG 200	Wet phase inversion	Ultrafiltration	Asymmetric and spongelike structure; water contact angle decreases significantly upto 64° and EWC 59.37%; IEP shifted pH 8 and shown positive charge; flux 545.54 kg m⁻² h⁻¹; rejection 56%	[20]
sPPSU positively charged membrane	UV grafting	[2-(methacryloyloxy)ethyl]trimethyl ammonium chloride; diallyldimethylammonium chloride		Nanofiltration; textile dyes	Spongelike morphology; MWCO 1627–1674 Da; PWP of 9–14 LMH bar⁻¹; rejection of MgCl₂ (95%) and Safranin O dye (99.9%)	[58]
PPSU thin-film composite membrane	Oxygen plasma (pretreatment); surface modification	2,5-bis(4-amino- 2-trifluoromethyl-phenoxy)benzenesulfonic acid; 4,4-bis(4-amino-2-trifluoromethyl-phenoxy)biphenyl-4,4-disulfonic acid	interfacial polymerization	Nanofiltration; dye removal	Water flux 63.9 and 71.3 L/m² h; dye rejection 48–80%	[59]
sPPSU/sPES membranes	Sulfonation; Blending	H₂SO₄; sPES	Crosslinking; heat and dry	Fuel cells	Maximum conductivity of 0.12 S/cm	[60]
sPPSU TFC membranes	Surface modification	MPD;TMC	Interfacial polymerization	Forward osmosis	Water flux up to 54 LMH with 8.8 gMH salt reverse flux under PRO mode	[61]
PPSU/PI solvent resistant membrane	Blending	PI	Phase inversion; solvent evaporation	Nanofiltration	Asymmetric structure with a dense skin layer; highest flux for alcohol and alkanes was achieved for a 50/50 wt.% blend;	[62]
PPSU/TiO₂ nanocomposites membrane	Blending	TiO₂	Solvent evaporation	Biomedical	Nanocomposites shown active inhibition against E. coli and S. aureus bacteria with and without UV irradiation; the stiffness, strength, toughness, hardness and heat distortion temperature increases	[63]
Anion exchange PyPPSU membrane	Blending	1-methyl-2-pyrrolidone	Solvent evaporation	Vanadium redox flow battery	Vanadium ions permeability (0.07 × 10⁻⁷–0.15 × 10⁻⁷ cm² min⁻¹); coulombic efficiency of 97.8% and energy efficiency of 80.2%	[64]
PPSU solvent resistant membrane	Blending	Cu-BTC	Phase inversion	Nanofiltration; methanol–dye separation	Improve tensile strength 29%; methanol flux 135 L m⁻² h⁻¹	[65]
PPSU nanofibrous membrane	Blending	PEG 400	Electrospinning	Wastewater treatments	Water contact angle 8.9°; porosity 72.4%; water flux 7920 L/m²h	[66]
PPSU membranes	Blending	sPPSU	Phase inversion	Ultrafiltration	Porosity 48%; MWCO 70 kDa; pure water flux 218 L m⁻² h⁻¹; FRR 79%; BSA rejection 85%	[49]
sPPSU/PIM-1 membrane	Blending	sDCDPS; PIM-1	Slower solvent evaporation	Gas Separation	The tensile strength up to 72 MPa and extension at break 3.5%; the gas separation performance above the Robeson upper bounds for O₂/N₂, CO₂/N₂, CO₂/CH₄	[67]
PPSU/FAC composite membrane	Blending	FAC	Phase inversion	Phenol filtration	Fragmented surface and spongy porous linkages; contact angle 43.8°; porosity 30%; pure water flux 26 Lm⁻² h⁻¹, phenol rejection 96.4%	[68]
MgO/sPPSU/PPSU membranes	Blending	MgO; sPPSU	Phase inversion	Ultrafiltration; Oil separation	Porosity 65% and MWCO 70 kDa; contact angle 48°; FRR 85% and HA rejection 63% and castor oil rejection 99%	[69]
PPSU/Cu-BTC solvent resistant nanofiltration	Blending	Cu-BTC	Phase inversion	Nanofiltration; dye and methanol separation	Contact angle 61°, and porosity 62%; Flux 19 L/m² h and rejection of methanol 93%	[70]
sPPSU proton exchange membrane	Sulfonation; Blending	H₂SO₄	Solvent evaporation	Fuel cells	Conductivity of 0.1 S/cm and power density of 471 mW/cm² at 80 °C	[71]
PPSU membrane	Blending	PVP; PEG; Tween 80	Phase inversion	Ultrafiltration	Water flux 148 L/m² h; BSA rejection increased from 53.2% to 81.5%	[30]
sPPSU asymmetric membranes	Sulfonation; Blending	TMSClS	Phase inversion	Ultrafiltration	Decomposition temperature at 510 °C; contact angle 33°, and porosity 51%; FRR 70%	[72]
sPPSU/f-SWCNTs mixed-matrix membranes	Sulfonation; Blending	3,3′-disulfonated 4,4′-dichlorodiphenyl sulfone; f-SWCNTs	Phase inversion	Gas separation	Enhanced the permeability for N₂, O₂, He, and CO₂ and the selectivity for O₂/N₂ and O₂/CO₂	[73]
Porous PPSU membrane	Blending	Carrageenan	Phase inversion	Ultrafiltration	Contact angle 43° and porosity 78%; zeta potential −24 mV at pH 7; permeability increased up to 29 Lm⁻² h⁻¹ bar⁻¹	[74]
PPSU/GO mixed matrix membrane	Blending	GO; PEG1000	Phase inversion	Ultrafiltration	Hydrophilicity and the thermal stability improved; pure water flux 132 L·m⁻²·h⁻¹ and the rejection 96.8%	[28]
PPSU/Zeolite mixed matrix membrane	Blending	Fe-ZSM-5; Cu-ZSM-5	Phase inversion	Organic compounds removal	Surface roughness increased (Ra- 18.52 nm); zeta potential about −57.2 mV at pH 7; water flux of 62 L·m⁻²·h⁻¹, lignin rejection up to 88.5%	[31]
PPSU/BiOCl-AC membrane	Blending	BiOCl-AC; PVP	Phase inversion	Ultrafiltration; oil separation	Asymmetric structures with thick top layer; contact angle 67°; pure water flux 465 L·m⁻²·h⁻¹; rejection diesel fuel 80% and 90% of crude oil	[42]
Alkali resisting PPSU membrane	Blending	PVP- 10, 55, 360, and 1300 kDa	Phase inversion	Ultrafiltration	Asymmetric and fingerlike structure; Tensile strength upto 2.53 MPa for 10 kDa; MWCO ranged from 2 kDa to 175 kDa; pure water flux 69 L·m⁻²·h⁻¹; better anti-alkali property in NaOH solution (pH = 13)	[13]
HBE–MMT/PPSU nanocomposite membrane	Blending	Functionalized montmorillonite	Phase inversion	Water treatment	Contact angle 53.6°; pure water flux about 380 L·m⁻²·h⁻¹ at 5 bar; rejection of salt 40–50%	[75]
Polyamide TFN PPSU membrane	Blending; Surface modification	GO (support layer); PIP and TMC	Interfacial polymerization	Nanofiltration; l kinetic hydrate inhibitor (KHI) removal	KHI rejection of 99% and permeation flux of 32.7 L/m² h (at 9 bar and feed concentration of 0.5 wt.% KHI)	[76]
sPPSU/TiO₂ mixed matrix hollow fiber membranes	Blending	TiO₂	Phase inversion	Ultrafiltration	Pure water flux 60 L·m⁻²·h⁻¹; contact angle 67°; rejection of BSA 91%	[77]
PPSU membrane	Blending	PEG 400; PEG 20000	Phase inversion	Filtration of aqueous media	Porosity 72%; tensile Strength at Break 7.75 MPa and elongation at Break 50.14%; Pure water flux 19 L·m⁻²·h⁻¹ (PEG400) and 183 L·m⁻²·h⁻¹ (PEG20000); 100% turbidity rejection	[10]
PPSU membrane	Blending	PEG 400; PEG 2000; PEG 6000; PEG 20000; PEG 35000; PEG 40000	Phase inversion	Ultrafiltration	Contact angle 50° to 90°; pure water flux of 486 Lm⁻² h⁻¹; human serum albumin rejection 90%	[78]
Ionic crosslinked sPPSU membrane	Surface modification	HPEI	Coating	Nanofiltration; organic solvent filtration	Ethanol permeability 1.47 L m⁻² h⁻¹ bar⁻¹; rejection of 99.9% to Rose Bengal dye	[79]
High-Flux PPSU membranes	Blending	PEG 6000–40000	Phase inversion	Ultrafiltration	Pure water flux 500–1000 L m^–2 h^–1 at 0.1 MPa; 90% rejection of human serum albumin (PEG20000)	[80]
PA-MOF/PPSU-GO TFN membrane	Blending; Surface modification	GO (support layer); MOF; PIP and TMC	Interfacial polymerization	Nanofiltration	Permeate flux 59.9 L/m²·h; KHI rejection 96%; FRR 97.8% and an excellent long-term stability	[81]
sPPSU/PBI membrane	Blending; crosslinking	PBI; DBX (crosslinker)	Heat and solvent evaporation	Nanofiltration; organic solvent removal	Permeability 11.8 Lm⁻² h⁻¹ bar⁻¹; rejection of tetracycline 97%.	[82]
Double crosslinked sPPSU/PBI membrane	Blending; crosslinking	PBI; DBX (crosslinker)	Heat and solvent evaporation	Nanofiltration; hydrogen purification	H₂ permeability of 46.2 Barrer and a high H₂/CO₂ selectivity of 9.9 at 150 °C	[83]
Amine functionalized PPSU membrane	Amination; Blending	SnCl₂; HNO₃	Phase inversion	Nanofiltration; dye removal	Pore size of 0.72 nm; positively charged active layers; contact angles 31°; pure water flux ∼54 Lm⁻² h⁻¹; CaCl₂ and AlCl₃ multivalent salts rejection 89% and 93.5%; crystal violet dye rejection > 99%	[84]
High-performance PPSU/sPANI membrane	Blending	sPANI	Nonsolvent induced phase separation	Ultrafiltration	Contact angle was 57°; porosity 81%; BSA adsorption value of 3.6 μg/cm²; water flux of 260 L/m² h; BSA rejection 95%	[40]
PPSU/carboxylated GO nanocomposite membrane	Blending	Carboxylated GO	Phase inversion	Nanofiltration; heavy metal removal	Surface charge of −70 mV; flux of 27 L m⁻² h⁻¹; rejection of As(V) 96%, Cr(VI) 93%, Zn²⁺(81%), Cd²⁺ (74%), Pb²⁺ (73%)	[85]
sPPSU membrane	Sulfonation	H₂SO₄	Phase inversion	Ultrafiltration; heavy metal and protein separation	Water flux of 190.33 Lm⁻² h⁻¹ and FRR of 86.56%; protein rejection of 66.3%, 74.0% and 91.2% for trypsin, pepsin, and BSA; Cd²⁺ and Pb²⁺ ions rejection of 75.2% and 87.6%;	[86]
PPSU/carboxylated GO nanocomposite membrane	Blending	Carboxylated GO	Phase inversion	Ultrafiltration; Antimicrobial and antifouling	Bacteriostasis rates of 74.2%,81.1% and 41.9% against E. coli, P. aeruginosa and S. aureus; FRR 95.3%	[87]
Porous PPSU/sPEEK membrane	Blending	sPEEK	Solvent evaporation	Vanadium flow batteries	Contact angle 47°; tensile strength 2.78 MPa; proton conductivity of 14.3 mS cm⁻¹ at 15 °C	[88]
PPSU/SnO₂ mixed matrix hollow fiber membrane	Blending	SnO₂	Vacuum evaporation	Ultrafiltration; dyes removal	Contact angle 63°; porosity 84%; pure water flux 362.9 L/m² h; dyes rejection about >94% for RB-5, and >73% for RO-16	[89]
PPSU/CuO/g-C₃N₄ membrane	Blending	CuO/g-C₃N₄	Nonsolvent induced phase inversion	Ultrafiltration; antifouling and protein separation	Smooth surfaces Ra-9.8 nm; increase pores on the top layer as well as in the sublayer; contact angle 48°; water flux 202 L/m²h; BSA protein rejection 96%; FRR 79%	[90]
Super-hydrophilic PPSU TFC membrane	Surface modification	MPD and TMC	Electrospun; plasma treatments; interfacial polymerization	Forward osmosis	Contact angle 0°; Osmotic water flux 14 L/m²h	[91]
PPSU hollow fiber membranes	Blending	CA; CAP	Dry-wet spinning	Ultrafiltration; arsenic removal	Contact angle 60° and 43°; arsenic removal 34% and 41%; pure water permeability 61.47 L/m²h bar and 69.60 L/m² h bar; FRR 88.67%	[92]
PPSU/silver-hydroxyapatite nanocomposite membrane	Blending	silver-hydroxyapatite	Phase inversion	Ultrafiltration; organic matter removal	Porous and honeycomblike structure; contact angle 60°; rejection 89%	[93]
Proton exchange sulfonated PPSU/PSU membrane	Sulfonation	Trimethylsilyl chlorosulfonate;	Vacuum dry	Fuel cells	Proton conductivity 34.1 mS cm⁻¹ at 70 °C; power density of 400 mW cm⁻²; current density of 1100 mA cm⁻²	[35]
PPSU/Ag-MWCNTs nanocomposite membrane	Blending	Ag-MWCNTs	Phase inversion	Nanofiltration; ion removal and antibacterial activity	Zeta potential −78 mV; contact angle 49°; porosity 73%; rejection of Na₂HAsO₄ 99.5% and Na₂Cr₂O₇ 100%	[87]
PPSU/MWCNTs membrane	Blending	MWCNTs	Phase inversion	Ultrafiltration; heavy metals removal	Dense skin layer on top and a porous supportive sub-layer; surface roughness Ra 21 nm; contact angle 61°; porosity 50%; flux 186 L/m² h rejection of Pb²⁺ (>98%), Hg²⁺ (>76%) and Cd²⁺ (>72%)	[94]
PPSU/ZnO nanocomposite membrane	Blending	ZnO	Phase inversion	Nanostructured- hybrid membranes; anionic dye; antimicrobial; wastewater treatment	Pore size 0.75 nm; zeta potential –65.7 mV at pH 7; methyl orange dye rejection 98% with a water flux 19 L/m²h; antibacterial activity of E. coli (6.2) and S. aureus (6.8)	[95]
Hydrophilic PPSU membranes	Blending	1,2-propandiol; PVP	Nonsolvent induced phase separation	Ultrafiltration	Contact angles of 46.4°;Water flux 674 kg m⁻² bar⁻¹h⁻¹	[96]
PPSU/PES/SiO₂ nanocomposite membrane	Blending	PES; SiO₂	Vapor induced phase separation; nonsolvent induced phase separation	Ultrafiltration	Water flux 76.65 L/m²·h; BSA retention of 82.01%;	[97]
Silica filled PPSU/PDMS Composite Membranes	Surface modification	PDMS; Silica	Coating	Biobutanol Separation	Weight loss starts from 400 °C; contact angle ∼130°; flux 536 g. m⁻² h⁻¹; butanol separation factor 30.6	[36]
PPSU/PANI hollow fiber membrane	Blending	PANI	Dry-jet wet spinning	Humic acid removal	Zeta potential −16 mV at pH 9; Water flux 127 L/m²h; Humic acid rejection 98%;	[98]
Proton exchange sPPSU membrane	Sulfonation	H₂SO₄; CNDs (crosslinker)	Vacuum dry	Fuel cells	Proton conductivity 10⁻² S/cm at 120 °C.	[99]
PPSU/Al-MOF mixed matrix membrane	Blending	Al-MOF	Phase inversion	Ultrafiltration,; dye separation; antifouling	Contact angle 63°; surface roughness Ra 21.9 nm; pure water flux 47 L·m⁻²·h⁻¹; FRR 93%; rejection of organic dye methyl violet 93.8%	[100]
PPSU/CA/ZrO₂ hollow fiber membranes	Blending	CA; ZrO₂	Dry-wet spinning	Arsenic Removal	Surface roughness Ra 43 nm; contact angle 48°; permeability of 89.94 L/m²h bar; removal of arsenic 87%	[45]
PPSU/CA hollow fiber membrane	Blending	CA	Dry–wet spinning	Removal of dyes	Permeability 64.47 L/m² h bar; removal of Reactive black 5 dye 95%	[101]
PPSU/Zn-MOF composite membrane	Blending	Zn-MOF	Phase inversion	Ultrafiltration; antifouling	Asymmetric structure and dense microporous active skin layer; surface roughness Ra 13.88 nm; porosity 72%; tensile strength 7.9 MPa; permeability 33 L m⁻² h⁻¹ bar⁻¹; FRR 98%	[102]
PPSU/CA/ZnO-MgO hollow fiber membrane	Blending	CA; ZnO-MgO	Dry–wet phase inversion	Arsenic removal	contact angle 60°; permeability 69.58 L/m²h bar; arsenic rejection 81.31%; FRR 91%	[103]
PANI coated PPSU Membranes	Surface modification	PANI	Coating	Dye separation; antibacterial activities	Surface roughness Ra-3.15 nm; contact angle 55°; zeta potential −1.7 mV at pH 6; permeability 53 L·m⁻²·h⁻¹·bar⁻¹; rejection of methylene blue dye 96%; bacteriostasis of E. coli 95% and S. aureus 88%	[104]

3. Bulk Modification

3.1. Polyphenylsulfone Sulfonation

Sulfonation is defined as an aromatic electrophilic substitution reaction used to attach the sulfonic acid group to the molecule of an organic compound via a chemical bond, wherein an ortho positions the aromatic ring in place of the hydrogen atom. It is attributed to the fact that this electron-donating oxygen atom activates the ortho position ^{[35][40][44][105][106][107][108]}[35,40,44,105,106,107,108]. Sulfonation is the accumulation of sulfonic groups at the aromatic backbone (including a phenyl and a sulfone unit as part of the backbone) of PPSU due to electron-donating substituents enhancing sulfonation. However, electron repulsing substituents have the opposite effect. PPSU is difficult to sulfonate because of the electron-withdrawing effect of the sulfone linkages that deactivate the adjacent aromatic rings for electrophilic substitution ^{[9][51][72][109][110][111]}[9,51,72,109,110,111]. Moreover, its sulfonation requires stronger reagents and/or longer times. However, sulfonated PPSU features increased hydrophilicity and proton conductivity in the presence of sulfonate groups in the polymer chain. The latter, however, can introduce negative charges.

The sulfonation of the aromatic backbone of the polymer is carried out before membrane fabrication using sulfonating agents. There are three groups of sulfonating agents that do not cause polymer chain degradation. Sulfuric acid (H₂SO₄), sulfur trioxide (SO₃), chlorosulfonic acid (ClSO₃H), fluorosulfonic acid (FHO₃S), amidosulfonic acid (H₃NO₃S) and its complexes, halogen derivatives of sulfuric acid, etc., form the first group and are derived from sulfur trioxide. They are referred to as electrophilic reacting agents and are most frequently used to sulfonate aromatic compounds. The second group includes nucleophilic agents such as sulfites (SO₃²⁻), hydrogen sulfites (HO₃S⁻), and sulfur dioxide (SO₂), which react with halogen derivatives and unsaturated compounds containing multiple bonds. The third group contains radically reacting agents. In particular, it includes sulfuryl chloride (SO₂Cl₂) and blends of gases (sulfur dioxide and chlorine (SO₂ + Cl₂), sulfur dioxide, and oxygen (SO₂ + O₂)). Sulfonation of polymer can be completed via either a heterogeneous reaction or a homogeneous reaction in hydrocarbons or chlorinated solvents.

The polymer sulfonation method works for most reagents. In the sulfonation protocol, the dried polymer is dissolved in a sulfonating agent and stirred at approximately 50 °C to produce a homogeneous solution in a nitrogen atmosphere. After the reaction, the solution is poured into a large volume of ice-cold deionized water under continuous stirring. As a result, a white precipitate is obtained. After standing overnight, the white precipitate is filtered and washed several times with cold deionized water to attain neutral pH level. The sulfonated polymer is then dried in a vacuum at room temperature ^{[5][55][86][107]}[5,55,86,107].

A typical procedure is described by Hartmann–Thompson et al. [112] PPSU was added to dichloromethane (CH₂Cl₂), mixed, and placed in an ice bath on a stirring plate. Next, the solution was cooled to 10 °C under agitation. ClSO₃H was added by drops over a one-hour period while stirring continuously. In the next step, acetic anhydride was added to the mixture by drops. The reaction was then allowed to continue for a period of time while stirring and maintaining the temperature. The reaction was stopped by gradually pouring the reacted solution into an ice-deionized water mixture. The resultant precipitate was recovered by pouring and washed repeatedly with deionized water until the wash water had a neutral pH. The PPSU was subsequently dried in an oven. Minor variations have been reported by other researchers, and a generalized overview of the sulfonation reaction scheme is shown in Figure 4a ^[107][113][107,113].

Figure 4. Synthesis of polyphenylsulfone. Reaction pathways: (a) sulfonation; (b) amination; (c) chloromethylation.

Liu et al. [9] demonstrated the sulfonation of PPSU random copolymers with various disulfonation levels. These were prepared via the direct polymerization method by aromatic nucleophilic substitution copolymerization. The reaction mechanism for this direct sulfonation has also been outlined. Karlsson et al. [52] prepared SPPSU by an anionic modification using n-butyllithium (BuLi) as a metalating agent. The preparation was performed via a one-pot synthesis in a reactor equipped with a gas inlet/outlet. In this method, the PPSU was dissolved in anhydrous tetrahydrofuran (THF) and cooled. The polymer solution was carefully titrated with a solution of BuLi until a faint reddish color was achieved. Subsequently, an amount of 2-sulfobenzoic acid cyclic anhydride corresponding to a twofold excess in relation to the lithiated sites of the polymer was quickly added in the form of a fine degassed powder and immediately dissolved and quenched the lithiated sites. Next, SPPSU was precipitated to remove the reactant residues via isopropanol. The precipitate was then filtered and dried in a vacuum and characterized by combining FTIR, ¹H NMR, and ¹³C NMR spectroscopy. Licoccia and coworkers [55] followed the same methodology for sulfonated PPSU with H₂SO₄ and ClSO₃Si(CH₃)₃.

H₂SO₄ is a low-cost sulfonating agent. However, it causes degradation of the main polymer chain when the reaction temperature is too high, or the reaction time is too long. This degradation may change the mechanical resistance of the membrane, therefore compromising its use in industrial applications. Among other reagents, the sulfonated PPSU with ClSO₃Si(CH₃)₃ showed better sulfonation control when compared to the one sulfonated with SO₃ ^[51][72][51,72]. However, SO₃ has a drawback in that side reactions may occur. Moreover, the reaction is heterogeneous because when a part of the polymer reacts with SO_3, it becomes insoluble in an apolar solvent. The rest of the reaction must be carried out in a dispersed system and not in a homogeneous solution. To solve the heterogeneity problem, ClSO₃Si(CH₃)₃ can be used.

3.2. Polyphenylsulfone Amination

In the core of sulfonic groups, the existence of an amine group can enhance the physicochemical properties of the membrane compared to the others formed by segments. This process involves a similar substitution reaction but with amine groups as substituents. PPSU can be simply nitrated to almost one nitrogen atom per reproductive unit via reactions with strong bases. The latter is due to the sulfonic group having a strong activation effect on the nitration process. In this nitration reaction, the acidic ortho-to-sulfone hydrogens are replaced with nitrogen atoms that result in a positive charge of the carbon atom in the phenylene unit. Arumugham et al. [84] used a two-step method for the amination of PPSU. The reaction scheme for amine-functionalized PPSU is shown in Figure 4b. In the first step, nitration of PPSU was performed with nitric and sulfuric acid. It resulted in nitrogen atoms being placed in the ortho position. In the second step, the intermediate was aminated using tin chloride with hydrochloric acid. A similar polymer amination procedure has also been applied by other researchers ^[114][115][114,115].

Aminated polymer synthesis was followed by the reaction in a nitrogen atmosphere. PPSU (10 g) was added to a mixture of sulfuric acid and nitric acid (1/4 mixing ratio, 200 mL) and then stirred for 2 h at 25 °C to produce nitrated PPSU (PPSU-NO₂). The resulting product was washed with five times the deionized water (300 mL) and dried in a vacuum oven at 30 °C for 24 h. To synthesize aminated PPSU, the tin chloride (20 g, 0.105 mol) in hydrochloric acid solution (20 g, 37%) was added in 60 mL of ethanol in a circular bottom flask. The flask was kept at 70 °C and then allowed to stir for 10 min. Subsequently, the synthesized PPSU-NO₂ (5 g) powder was added slowly to the flask. The color of the solution transformed from yellow to dark brown, indicating the progress of the reaction. The reaction was further carried out for 4 h with stirring at 70 °C. Afterward, the reaction mixture was poured into 400 mL of deionized water for precipitation. Finally, PPSU-NH2 was separated, washed with deionized water, and dried in a vacuum oven at 80 °C for 12 h [84]. Considered amino groups have a significant effect on the surface charge and hydrophilicity of the PPSU polymer, as was shown in ion exchange capacity and water absorption measurements.

3.3. Polyphenylsulfone Chloromethylation

PPSU is a polymer that has no functional groups for further chemical modifications. However, the chloromethylation reaction of aromatic polymers is of particular interest to researchers and includes attaching functional groups onto aromatic ring-like chloromethyl. Currently, chloromethylation is actively investigated, both theoretically and experimentally, in the context of the procurement of precursors for functional membranes. Chloromethyl generally provides higher flexibility with time since it can easily interact with any kind of amines. Once a functional group is attached to the aromatic ring, further reactions can occur, including immobilization of compounds for enhanced hemocompatibility resulting in antifouling capability [116]. Zhang et al. [64] carried out the chloromethylation of PPSU and cast anion exchange membranes from the resultant chloromethylated PPSU. Chloromethylation of PPSU was performed following the one-step procedure. In a typical reaction shown in Figure 4c, the PPSU was dissolved in tetrachloroethane, and then tin tetrachloride and chloromethyl ethyl ether were added to the solution. The reaction mixture was heated, and the temperature was maintained.

After the desired reaction time elapsed, the reaction mixture was precipitated in excess ethanol, and the chloromethylated PPSU polymers were isolated by filtration. The polymer was purified by dissolution in chloroform and precipitation with ethanol and then dried in a vacuum oven. Chloromethylation of the polymer was not easy to control, and the number of chloromethyl groups attached to the polymer could be very small. The latter affected the properties of the membrane. The reaction mixture often produced a gel if it was not properly controlled by adjusting the temperature and the reaction time. Most of the studies considered chloromethylation of the polymers with the system made from trioxane and chlorotrimethylsilane as an agent of chloromethylation in the presence of tin tetrachloride ^[117][118][117,118]. The chloromethylation of polymers was dependent primarily on the kind of chloromethylating agent, the polymer structure, the type and amount of solvent, the catalyst, and other parameters of the reaction.