Amphiphilic Block Copolymer of Poly(dimethylsiloxane) and Methoxypolyethylene Glycols for High-Permeable Polysulfone Membrane Preparation.

Poly(dimethylsiloxane)-block-methoxypolyethylene glycols (PDMS-b-mPEG) were synthesized by Steglich esterification. The high-permeable membrane (PSf/PDMS-b-mPEG) was prepared by using PDMS-b-mPEG as additives. The successful synthesis of PDMS-b-mPEG was confirmed by nuclear magnetic resonance. Field emission scanning electron microscopy images show that the distribution of finger-like macroporous and sponge-like macroporous can be modulated by controlling the ratio of the hydrophilic/hydrophobic components of additives. The distribution of additives and membrane wettability are validated with X-ray photoelectron spectroscopy and water contact angle test. The permeability of the blended membrane, especially for the membrane PSf/PDMS-b-mPEG1900 (M3), was remarkably improved. The water permeability of M3 (239.4 L/m2·h·bar) was 6.6 times that of the unblended membrane M0 (42.5 L/m2·h·bar). The findings of protein BSA filtration show that the flux recovery ratio of M3 is 89.2% at a BSA retention rate of about 80%, which demonstrates that the polysulfone membranes blended with PDMS-b-mPEG have excellent antifouling performance and extraordinary permeability.

Introduction

The polysulfone (PSf) membrane is widely used for its excellent chemical inertia, thermal stability, mechanical properties, and membrane forming ability.[1] PSf membranes are frequently applied in gas separation,[2] protein purification and fractionation, osmotic evaporation, hemodialysis,[3] plasma separator, cell culture, and artificial liver. Nevertheless, the PSf membrane is vulnerable to fouling owing to its hydrophobic nature.[4] Fouling is caused by the deposition of retained inorganic, organic, and biological substances within the membrane pores or on the membrane surface. The fouling would reduce the membrane flux and the service life and increase maintenance costs. The fundamental solution to the fouling is to improve the surface hydrophilicity.[5−8] However, a large proportion of the water-soluble additives will seep out in the water environment.

Amphiphilic additives have hydrophobic anchoring and hydrophilic segments; the former part would anchor with the hydrophobic membrane matrix tightly, while the latter part can improve the wettability of the blended membrane.[9,10] The hydrophilic segments are apt to migrate to the nonsolvent phase, thus enrich on the membrane surface during the NIPS.[11] PSf-b-PEO (polysulfone-block-polyethylene oxide) was synthesized and blended it with PSf for ensuring the hydrophilicity.[12] PEGMEMA-PSf-PEGMEMA (poly(ethylene glycol) methyl ether methacrylate) triblocks were prepared and used as additives to blend with PSf. The mixed PSf membranes exhibited better hydrophilicity and protein resistance.[13] Gao and co-workers synthesized a comb-like PEG-r-PDMS (poly(dimethylsiloxane), PDMS) and then mixed with PES,[14] the foulants can be readily flushed off by water currents.[15,16] Xu et al. prepared the polyethersulfone-blended membrane (PES/TPQP-Cl) with more excellent antifouling properties due to the gradient concentration of the additive (tris(2,4,6-trimethoxyphenyl)polysulfone-methylene quaternary phosphonium chloride, TPQP-Cl), and the permeability was improved 34 times.[17]

In the present paper, we synthesized a series of hydrophobic/hydrophilic amphiphilic block copolymer PDMS-b-mPEG with well determined portion of hydrophilic mPEG and low surface energy PDMS by simple and efficient Steglich esterification, and then with the block copolymer PDMS-b-mPEG as the additives, we prepared PSf/PDMS-b-mPEG blended membranes by NIPS (nonsolvent induced phase separation).[18] PDMS exhibits low surface energy owing to the flexible siloxane bonds. On account of hydrogen bonding interaction, mPEG segments can tightly hold a layer of water to prevent foulant adhesion to the membrane surface and thus possess excellent antifouling ability.[19] Therefore, hydrophobic PDMS plays an anchoring role and avoids the leaching of PDMS-b-mPEG, and the mPEG segment spontaneously migrates toward the membrane surface by surface segregation to render the membrane surface hydrophilicity.[2,20]

Experimental Section

Materials

PSf (P3500, Solvay, 22,000 Da) was bought. mPEG (weight-average molecular weight: 500, 1000, 1900, and 3000 Da), DCC (N,N′-dicyclohexylcarbodiimide, >99%), DMAP (4-dimethylaminopyridine, >99.0%), succinic anhydride (>99%), and BSA (bovine serum albumin) were bought (Aladdin Reagent Co., Ltd., Shanghai, China). α-Hydroxyethoxypropyl-ω-butyl terminated poly(dimethylsiloxane) (Mw = 5 kDa; PDMS-mono-OH, 96%) was purchased from Gelest. Analytical reagent ethanol was employed as the precipitation agent and cyclopentanone as the solvent. Dichloromethane (DCM, AR) and cyclopentanone were purchased from Sinopharm Chemical Reagent; DCM was purified by distillation over calcium hydride.

PDMS-b-mPEG Synthesis

PDMS-b-mPEG was synthesized according to the reported method.[18] Typically, PDMS–OH (PDMS with one end hydroxyl group, 5.00 g, 1.0 mmol, Mn 5.0 kDa), succinic anhydride (0.15 g, 1.5 mmol), DMAP (0.01 g, 0.10 mmol), and pyridine (0.12 mL, 1.5 mmol) were dissolved in anhydrous DCM (5 mL) and refluxed for 16 h. Then, the mixture was cooled to room temperature and precipitated into hexane (50 mL). The unreacted succinic anhydride was removed by centrifuge. The hexane phase was washed with aqueous HCl (0.25 M, 3 × 50 mL) and H2O (3 × 50 mL), dried with NaSO4, filtered, and finally dried in vacuo (0.1 mbar) at 30 °C to afford PDMS–COOH as a colorless oil. As-synthesized carboxylic acid-terminated PDMS (PDMS–COOH, 0.20 g, 0.20 mmol), mPEG (0.18 mmol; 500, 1000, 1900, and 3000 Da), DCC (0.05 g, 0.24 mmol), and DMAP (2.2 mg, 0.02 mmol) were dissolved in anhydrous DCM (10 mL) and stirred at room temperature for 48 h. The mixture was precipitated into cold diethyl ether (50 mL), collected by centrifugation, and dried in vacuo (0.1 mbar) at 30 °C to yield PDMS-b-mPEG, a colorless solid. The specific recipes are shown in Table .

Table 1

Recipes for Amphiphilic Block Copolymersa

	molecule weight (Da)
expt.	PDMS	mPEG	content of PDMS (%)
PDMS-b-mPEG500	5000	500	90.9
PDMS-b-mPEG1000	5000	1000	83.3
PDMS-b-mPEG1900	5000	1900	72.5
PDMS-b-mPEG3000	5000	3000	62.5

PDMS-b-mPEG500 stands for the block copolymer with the mPEG molecular weight of 500; PDMS-b-mPEG1000 stands for the block copolymer with the mPEG molecular weight of 1000; PDMS-b-mPEG1900 stands for the block copolymer with the mPEG molecular weight of 1900; PDMS-b-mPEG3000 stands for the block copolymer with the mPEG molecular weight of 3000.

PSf/PDMS-b-mPEG and PSf/PDMS/mPEG Blended Membranes Preparation

All the membranes were prepared by the nonsolvent induced phase separation method (NIPS), using cyclopentanone as the solvent and ethanol as the coagulation bath. The casting solutions of 12 wt % polymers were prepared by dissolving PSf and PDMS-b-mPEG in cyclopentanone at desired proportions (PDMS-b-mPEG percentages ranging from none to 9 wt % in the total amount of polymers) under mechanical stirring at 70 °C for 24 h and were kept in a vacuum oven at least 48 h at 70 °C to remove air bubbles. Then, the liquid dope was cast onto a glass plate with a casting knife to obtain a wet film with a thickness of 200 μm, coagulated in absolute ethanol at room temperature. The obtained PSf/PDMS-b-mPEG binary blended membrane was thoroughly washed with ethanol to remove the residual cyclopentanone. For comparison, a set of ternary blended PSf membranes with mPEG and PDMS as the additives were prepared, the contents of mPEG and PDMS in the casting solutions were equal to the contents of mPEG and PDMS segments in the casting solutions (12 wt %) for the preparation of the binary blended membrane with PDMS-b-mPEG. The specific compositions of membrane casting solutions are shown in Table . The PSf/PDMS-b-mPEG blended membranes are designated as M0, M1, M2, M3, and M4 (as indicated in Table ), while the PSf/PDMS/mPEG blended membranes are designated as S1, S2, S3, and S4.

Table 2

Compositions of Membrane Casting Solutions

membrane ID	PSf/PDMS-b-mPEG	cyclopentanone (80 wt %)
M0 12 wt % PSf	2.4 g/0 g	16.42 mL
M1 12 wt % PSf/PDMS-b-mPEG500 94/6	2.256 g/0.144 g	16.42 mL
M2 12 wt % PSf/PDMS-b-mPEG1000 94/6	2.256 g/0.144 g	16.42 mL
M3 12 wt % PSf/PDMS-b-mPEG1900 94/6	2.256 g/0.144 g	16.42 mL
M4 12 wt % PSf/PDMS-b-mPEG3000 94/6	2.256 g/0.144 g	16.42 mL

Characterization

The chemical composition of the synthesized polymers and the prepared membranes were characterized with NMR (nuclear magnetic resonance), ATR-FTIR (attenuated total reflectance-Fourier transform infrared spectroscopy) and XPS (X-ray photoelectron spectroscopy). The membrane surface wettability was examined by the water contact angle with an automatic double titrated contact angle analyzer (Theta, Biolin Scientific Inc.). FESEM (field emission scanning electron microscopy) observations of the surface and cross section of the membrane were recorded. Thermal gravimetric analysis (TGA) and the mechanical strength of the polymers and membrane samples were measured. Please see the Supporting Information for the details of the characterizations.

Filtration Experiments

Membrane permeation and antifouling performance were then examined with ultrafiltration of pure water and BSA solution.[21] The details of the procedure are shown in the Supporting Information.

Results and Discussion

Characterization of PDMS-b-mPEG and Membranes

PDMS-b-mPEG containing hydrophilic mPEG and low surface energy PDMS blocks was synthesized by simple and efficient Steglich esterification. The hydrophilic and hydrophobic ratios of amphiphilic diblock copolymers were controlled by using mPEG with different chain lengths (average molecular weight: 500, 1000, 1900, and 3000 Da). PSf-b-mPEG (weight ratios of mPEG to PSf are 9.1, 16.7, 27.5, and 37.5%) was served as the additive. The chemical structures of the materials used in the esterification process and the resulted diblock copolymer are shown in Figure .

Figure 1

Schematic representation for PDMS-b-mPEG synthesis.

The 1H NMR spectra of these polymers (PDMS, PDMS–COOH, and PDMS-b-mPEG1900) are shown in Figure S1, ATR-FTIR (attenuated total reflectance-Fourier transform infrared spectra) for the samples are shown in Figure S2. These two spectra clearly confirm the successful synthesis of these polymers.

The broad scan XPS spectra of the membranes are shown in Figure . The peaks are at 285.0, 533.0, 103.8, and 171.2 eV corresponding to the binding energies of C 1s, O 1s, Si 2p, and S 2p, respectively. The membrane surface elemental mole percentages and O/S atomic ratio are presented in Table . The mole percentages of O and S of pure PSf are 20.25 and 1.84%, respectively. In Table , the increase of O and the decrease of S can be observed obviously for the PSf/PDMS-b-mPEG membranes compared with the pristine PSf membrane. This indicates that the mPEG segment exists in the membrane. The mPEG segments segregate onto the membrane surface preferentially leading to the increased surface O/S ratio.[2] The XPS peak of 103.8 eV (Si 2p) suggests the PDMS block on the surfaces be resulted from segregation.[18] In other words, it is confirmed that the additive PDMS-b-mPEG migrated toward the membrane surface in the membrane preparation process;[22] moreover, the additive content of the upper surface is higher than that of the lower surface. In order to further determine the chemical contents quantitatively, the C 1s core level spectrum is resolved into multipeaks and shown in Figure : the binding energy positions of C–C at 285.0 eV, C–Si at 284.2 eV, and C–O at 286.0 eV.[14,22]

Figure 2

XPS wide scans of the different membranes.

Table 3

Mole Percentage of Elements on the Surface of the Membrane Samples

	atomic percent (%)
membrane sample	C	O	S	Si	atomic ratio O/S
M0 top	73.70	20.25	1.84		11.01
M1 top	63.32	22.25	1.35	13.13	16.48
M1 bottom	64.95	21.46	1.24	12.35	17.31
M3 top	66.35	21.01	1.48	11.16	14.20
M3 bottom	68.32	20.25	1.68	9.76	12.05

Figure 3

C 1s core level spectra of the membrane samples on top and bottom surfaces.

The surface and the cross section morphologies of the membrane are displayed in Figure . It can be observed that the PSf/PDMS-b-mPEG membrane pore structure after adding additives shows a typical asymmetric structure. The asymmetric membrane appears a skin layer of finger-like pores and a bottom layer of spongy pores.

Figure 4

FESEM observations of the upper surface, cross section, and lower surface of the membranes PSf (M0), PSf/PDMS-b-mPEG (M1–M4) and PSf/PDMS/mPEG1900 (S3).

With the additive added, the pore size and porosity of the top surface increase but not as obvious as the bottom surface. Meanwhile, the proportion of finger-like pores increases, while the spongy pores decrease. The pristine membranes are almost spongy pores due to the strong chain activity and sufficient adjustment time during the phase inversion process. The addition of the additive PDMS-b-mPEG (equivalent to surfactant) impedes the interaction between solvents and nonsolvents, which means that the adjustment time of the chain, is shortened. Hence, the addition of the additive leads the finger-like pores and macrovoids to increase. With the increase of molecular weight of PDMS-b-mPEG in the casting solution, there is a trend from delayed demixing to instantaneous demixing.[23,24] From Figure C, it can be observed that this membrane is almost finger-like and the pore size is larger. This is mainly due to the mPEG chain migrating into the coagulation bath in the membrane-making process.

Figure 5

Cross-sectional FESEM image of the polysulfone membrane in different (a, b) solvent–nonsolvent systems and (c) blended membrane M3.

Solvent plays a critical role in the membrane morphology formation, by using different solvents, the finger-like and spongy membrane pores could be obtained. For comparison, NMP (a usually used solvent, which could only dissolve pure PSf but could not dissolve PDMS nor PDMS-b-mPEG) and cyclopentanone (could dissolve both PSf and PDMS) were applied in the work to dissolve PSf and the diblock PDMS-b-mPEG. It could be clearly found from Figure that the membranes with totally different morphologies have been obtained, which might pose an important impact on the membrane performances.

For further confirmation, the membrane pore size (average diameter, measured at intervals of 2° and through the center of mass of the object) and surface porosity (hole ratio, the ratio between the hole area and the total area of the object) are calculated by Image-Pro Plus (Figure S3, Image-Pro Plus automatically generate the red mark when calculating the pore size and surface porosity). Table shows that the average pore size of the membrane surface is smaller on the upper surface than that on the lower surface, and the mean membrane pore size becomes larger at both of the upper and lower surfaces with PDMS-b-mPEG addition. This result is very consistent with the observations of the FESEM image (Figure ).

Table 4

Membrane Pore Size (Mean Diameter: Average Length of Diameters Measured at 2° Intervals and Passing through Object’s Centroid) and Surface Porosity (Hole Ratio: Ratio of Hole Area Excluding Holes to Total Area of Object) Obtained by Image-pro Plus Calculating

surface	parameter	M0	M1	M2	M3	M4	S3
membrane top surface	mean pore size	0.1201 ± 0.6342	0.1294 ± 0.7696	0.1084 ± 0.2213	0.1339 ± 0.1698	0.1000 ± 0.1888	0.1078 ± 0.1699
	hole ratio	0.9680 ± 0.1407	0.9993 ± 0.0172	1.0000 ± 0.0011	0.9999 ± 0.0015	0.9967 ± 0.0439	0.9914 ± 0.0704
membrane bottom surface	mean pore size	2.5047 ± 3.2945	5.5999 ± 43.4916	3.1610 ± 4.8922	3.1629 ± 4.8054	1.9884 ± 3.5295	2.6922 ± 8.3799
	hole ratio	1.0000 ± 0.0003	1.0000 ± 0.0001	0.9999 ± 0.0020	0.9998 ± 0.0079	0.9994 ± 0.0174	0.9999 ± 0.0028

Figure shows that the water contact angle increases for the PSf/PDMS-b-mPEG membrane with low-molecular part of mPEG due to PDMS segments segregated on the surface.[25] With mPEG molecular weight increasing, that is, the proportion of PDMS component decreases or the mPEG component increases, and the static contact angle gradually decreases. The water contact angles of M1–M3 decrease gradually, while that of M4 increases. The significant hydrophilicity improvement is attributed to the migration of mPEG fragments to the membrane surface.[26] The decreased hydrophilic property of the membrane M4 is owing to the high molecular weight of mPEG in the additive PDMS-b-mPEG3000 with low water solubility or water affinity, which leads the migration ability of the additive to weaken during the phase inversion process.[27,28]

Figure 6

Water contact angle of the membranes.

The thermal degradation of pure PSf, PDMS-b-mPEG1900, PSf/PDMS-b-mPEG500, PSf/PDMS-b-mPEG1000, PSf/PDMS-b-mPEG1900, and PSf/PDMS-b-mPEG3000 was performed up to 800 °C at 10 °C/min under an argon flow rate as presented in Figure . It could be found that the pure PSf is the most stable, its degradation starts at about 500 °C. While the diblock copolymer of PDMS-b-mPEG1900 is the most unstable, the major weight loss of approximately 40% was observed around 400 °C, which may be attributed to the disintegration of the mPEG chains attached to PDMS.[29] For the blended membranes of PSf/PDMS-b-mPEG500, PSf/PDMS-b-mPEG1000, PSf/PDMS-b-mPEG1900, and PSf/PDMS-b-mPEG3000, the thermal stability is obviously improved. These results show that the prepared blended PSf membranes are thermally stable.

Figure 7

TGA analysis of pure PSf, PDMS-b-mPEG1900, PSf/PDMS-b-mPEG500, PSf/PDMS-b-mPEG1000, PSf/PDMS-b-mPEG1900, and PSf/PDMS-b-mPEG3000.

Mechanical properties of the PSf membrane and PSf/PDMS-b-mPEG1900 blended membrane are shown in Figure . It could be found that the tensile strength and the elongation rate at break of the PSf membrane are 17.16 MPa and 12.5%, respectively, while they are 23.3 MPa and 13.7% for the PSf/PDMS-b-mPEG1900 blended membrane, respectively. These obtained results reveal that the mechanical properties are improved with the blending of PDMS-b-mPEG1900 with PSf, indicating that the blended membranes are mechanically stable.

Figure 8

Strain–stress curve for the PSf membrane and PSf/PDMS-b-mPEG1900 blended membrane.

Membrane Filtration Performance

The BSA retention rate and the pure water flux for M0 and M1–M4 are exhibited in Figure , which shows that the flux dramatically rises with the molecular weight of mPEG component in the additive PDMS-b-mPEG up to 1900 Da and then it decreases with further raising the molecular weight to 3000 Da. The flux ascends from 42.5 LMH through the pure PSf membrane to roughly 239.4 LMH through the PSf/PDMS-b-mPEG1900 membrane and then falls to about 115.7 LMH through the PDMS-b-mPEG3000 membrane. The PSf/PDMS-b-mPEG1900 membrane shows the highest water flux. The retention rate to BSA of these membranes remains stable and high at 82% regardless of the molecular weight of mPEG in the additive PDMS-b-mPEG.

Figure 9

Pure water flux and BSA retention rate of the pure PSf membrane M0 and the blended membranes M1–M4.

For the sake of comparison, as shown in Figure , water permeability is determined (the slope of the linear fitting curve of each membrane flux with transmembrane pressure). The permeability of the 12 wt % PSf/PDMS-b-mPEG1900 94/6 membrane was 142.5 LMH/bar, almost three times than that of the 12 wt % PSf pure membrane (47.9 LMH/bar).

Figure 10

Pure water flux of M0 and M1–M4 vs transmembrane pressure.

The permeability and protein retention rate have a synergistic effect on the hydrophilicity and pore microstructure. The corresponding microstructure is shown in Figures and 5. Previous work has demonstrated that membrane fouling with larger pore sizes is more serious than the membrane with smaller pore sizes, and the fouling of the macroporous membrane is mainly caused by pore blockage.[30] The pore size of the PSf/PDMS-b-mPEG membrane is larger than that of the pure PSf membrane M0. Nevertheless, PSf/PDMS-b-mPEG membranes display an improved antifouling property. The results show that the PDMS-b-mPEG segregation on the surfaces of the membrane skin and the inner pore wall can effectively prevent fouling. It is reported that a variety of additives have been used to modify polysulfone membranes. The performances of our blended membrane PSf/PDMS-b-mPEG1900 are compared with those of published works (Table ).

Table 5

Comparison of the Performances of the PSf/PDMS-b-mPEG1900 Membrane with Those of the Previously Reported Additives

additives	pure water permeability (LMH/bar)	BSA rejection	FRR (%)	ref
PBIa	178	68.7	93.5	(31)
PANIa	115	98.0	78.5	(32)
SPSf/CNFa	137.6	95.8		(33)
PDMS-b-mPEG	239.4	83.5	89.2	this study

PBI, poly[2,2′-(m-phenylene)-5,5′-dibenzimidazole]; PANI, polyaniline nanoparticles; CNF, cellulose nanofibers.

The pure water flux (J0), BSA flux (Jp) at the steady state, and pure water flux after cleaning (J1) are recorded and shown in Figure .[34] After cleaning, the flux of the M0 membrane decreased seriously, and the flux recovery ratio (FRR) is low (30.2%, Figure ), with poor antifouling performance. For membranes M1–M4, FRR values are much higher than M0. The M3 FRR is up to 89.2%. The high FRR value means that the membrane has higher hydraulic cleaning efficiency and better antifouling performance. This indicates that M3 has the best antifouling performance.

Figure 11

Water fluxes for M0 and the blended membranes M1–M4.

Figure 12

FRR of the pure PSf membrane M0 and the blended membranes M1–M4.

To better understand the effect of PDMS-b-mPEG on microstructure and permeable property, PSf membranes with mPEG and PDMS homopolymers as blend components were prepared. The PSf/PDMS-b-mPEG binary blended membrane and the PSf/PDMS/mPEG ternary blended membrane, with the same content of mPEG chains, were prepared. The morphology and permeability of PSf/PDMS/mPEG membranes were studied, as shown in Figures and 13. It can be observed that this membrane is almost finger-like and the pore size is very large. It is mainly due to the mPEG homopolymer that easily leaching out toward the coagulation bath in the membrane-preparing process. As is found from Figure , the water flux of the membrane remains basically unchanged, indicating that mPEG homopolymers may leach out into the coagulation bath. On the other hand, PDMS-b-mPEG would be stagnated in the membrane matrix due to its insolubility in ethanol.[2]

Figure 13

Pure water flux through the PSf/PDMS/mPEG blended membrane with various molecular weights of mPEG (S1/mPEG500; S2/mPEG1000; S3/mPEG1900; S4/mPEG3000).

The top membrane surface is more hydrophilic (as shown in Figure ) with smaller pore size than that of the bottom membrane surface (as shown in Figure , Figure S3, and Table ). As a consequence, the membrane performances, including pure water flux and antifouling characterizations were tested with the top membrane surface toward the feed solution. On the other hand, the pore sizes of the top surface for the blended membranes are larger than those of the top surface for the pure PSf membranes, but it changes very little. As a result, the blended membranes display good antifouling property than that of the pure PSf membrane, though the blended membranes have larger top membrane pore sizes due to the above reasons.

Conclusions

The amphiphilic diblock copolymer PDMS-b-mPEG was prepared by the Steglich esterification method. The copolymers as additives were incorporated with polysulfone to fabricate blended membranes (PSf/PDMS-b-mPEG) through the nonsolvent induced phase separation technique. To better prepare the PSf/PDMS-b-mPEG blended membrane, cyclopentanone was used as the solvent and ethanol as the coagulant.

FTIR, XPS analysis, and water contact angle measurement showed that PDMS-b-mPEG was segregated to the membrane surface. By taking advantages of mPEG hydrophilicity and PDMS low surface energy, the blended membrane has excellent antifouling performance and extraordinary water permeability. The pore size and porosity on the upper membrane surface increased with the additive adding but not as obvious as the ones on the lower surface. Meanwhile, the proportion of finger-like pores becomes large, and that of spongy pores becomes small, which was characterized by FESEM observation. The PSf/PDMS-b-mPEG1900 blended membrane exhibits the highest permeability of 142.5 LMH/bar. The flux recovery ratio of M3 significantly enhanced to a high value of 89.2%, indicating that the blended membranes has a good long-term application prospect.