Structural parameters reduction in polyamide forward osmosis membranes via click modification of the polysulfone support

Journal Pre-proof Structural parameters reduction in polyamide forward osmosis membranes via click modification of the polysulfone support Jin Zhou, Heng-Li He, Fei Sun, Yu Su, Hai-Yin Yu, Jia-Shan Gu

PII:

S0927-7757(19)31072-6

DOI:

https://doi.org/10.1016/j.colsurfa.2019.124082

Reference:

COLSUA 124082

To appear in:

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Received Date:

23 August 2019

Revised Date:

6 October 2019

Accepted Date:

6 October 2019

Please cite this article as: Zhou J, He H-Li, Sun F, Su Y, Yu H-Yin, Gu J-Shan, Structural parameters reduction in polyamide forward osmosis membranes via click modification of the polysulfone support, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2019), doi: https://doi.org/10.1016/j.colsurfa.2019.124082

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Structural parameters reduction in polyamide forward osmosis membranes via click modification of the polysulfone support Jin Zhoua,b, Heng-Li Hea, Fei Suna, Yu Sua, Hai-Yin Yua,*, Jia-Shan Gua

a. College of Chemistry and Materials Science, Anhui Normal University, 189 Jiuhua

ro of

Nanlu, Wuhu, Anhui 241002, China b. Key Laboratory of Micro-Nano Powder and Advanced Energy Materials of Anhui Higher Education Institutes, Chizhou University, Anhui 247000, China

-p

Revised version for Colloids and Surfaces A: Physicochemical and Engineering Aspects

re

October 6, 2019

Jin Zhou, [email protected]; Heng-Li He, [email protected]; Fei Sun, [email protected].

lP

[email protected]; Yu Su, [email protected]; Jia-Shan Gu,

*. Corresponding authour: Hai-Yin Yu, [email protected]; fax:+86 553 3869303;

na

Tel.:+86 553 3869303.

Jo

ur

Graphic abstract

1

Abstract To reduce the structural parameter of composite polyamide (PA) forward osmosis membrane, the support polysulfone (PSf) was prepared by blending PSf/PSf-g-methoxypolyethylene glycol (PSf/PSf-g-mPEG) with the mPEG weight molecular weights of 200, 500, 1000 and 1900 g/mol at the PSf/PSf-g-mPEG weight ratios of 4:1 and 9:1, respectively. The composite membranes were prepared as the following steps: (1) propargyl mPEGs preparation; (2) chloromethylation and azidation of PSf; (3) PSf-g-mPEG preparation via click chemistry; (4)

ro of

PSf/PSf-g-mPEG blended support membrane preparation by phase separation; (5) polyamide active layer formation through interfacial polymerization between

phenylenediamine and 1,3,5-benzenetricarbonyl trichloride on the support membrane.

-p

The successful synthesis of the polymers and preparation of the composite membranes were certified with Fourier transform infrared spectroscopy, nuclear magnetic

re

resonance, X-ray photoelectron spectroscopy analysis, field emission scanning electron microscope and energy dispersive spectrometer.

lP

The pure water fluxes of the blended support membranes were sharply increased; the maximum pure water flux was gained for the PSf/PSf-g-mPEG500 blended

na

membrane. For the composite polyamide (PA) membrane, the water flux was improved drastically, while the rejection to the salt remained unchanged in the reverse osmosis system; the pure water flux rose dramatically and the structure parameter (S)

ur

dropped significantly in the forward osmosis system. Keywords: forward osmosis; thin-film composite polyamide membrane;

Jo

methoxypolyethylene glycol; click chemistry; internal concentration polarization; structural parameter.

Abbreviations and nomenclatures: phenylenediamine (MPD), 1,3,5-benzenetricarbonyl trichloride (TMC), poly(ethylene glycol) (PEG), methoxypolyethylene glycol (mPEG), propargyl mPEG (mPEG-CCH), polyamide (PA), thin film composite (TFC) membrane, polysulfone (PSf), chloromethylated PSf 2

(PSf-CH2Cl), azido-polysulfone (PSf-CH2N3), X-ray photoelectron spectroscopy analysis (XPS), field emission scanning electron microscope (FE-SEM), energy dispersive spectrometer (EDS), Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (1H NMR), forward osmosis process (FO), reverse osmosis process (RO), liter per square meter per hour (L/m2h, LMH), salt retention rate (Rs), water permeability coefficient (A), salt permeability coefficient (B), internal concentration polarization (ICP), structural parameter (S), active layer facing the draw solution (AL-DS), active layer facing the feed solution (AL-FS), solute diffusion coefficient D, tortuosity (), porosity (ε).

ro of

1. Introduction

Forward osmosis (FO) technology is widely used in seawater desalination and

treatment, energy production, food processing and so on [1, 2]. Thin-film composite

-p

(TFC) aromatic polyamide (PA) membrane, the core component of FO, is a kind of

re

widely used polymer separation membrane [3-9]. However, the internal concentration polarization (ICP) tends to occur, reducing the membrane permeability and service

lP

life. In order to reduce ICP or the structural parameters of the FO membranes, many researchers have proposed various hydrophilic modifications of aromatic TFC PA

na

membranes physically and chemically. Nevertheless, these methods are complex and of high cost; the effect is not able to last long [10-12].

ur

ICP will appear within the support of the traditional TFC membrane, which cannot be easily alleviated by changing the flow conditions like the external concentration

Jo

polarization. There are two kinds of ICP: concentrative ICP (the active layer toward DS, the solute being accumulated within the PSf support) and dilutive ICP (the active layer toward FS, the draw solution being diluted inside the porous PSf support). The osmotic driving force will significantly be reduced, resulting in a significant reduction in water flux in the cases concentrative ICP or dilutive ICP takes place [13]. Overall the properties of the FO membranes are significantly affected by its support [14-16]. 3

Hence reducing tortuosity, increasing porosity and enhancing hydrophilicity of the support membrane are effective way to reduce ICP or the structural parameters. Polyamide-graphene oxide membrane was synthesized on a polyethersulfone support by intra-crosslinking graphene oxide via m-xylylenediamine and inter-crosslinking graphene oxide via trimethyle chloride. The hydrophilic and porous support rendered the PA FO membranes with low ICP [17]. A stable and hydrophilic polyethersulfone/poly(acrylic acid) composite support membrane was synthesized

ro of

through cross-linked polymerization in-situ of acrylic acid and the cross-linker

N,N-methylene-bis(acrylamide); then the polyamide active layer was prepared by interfacial polymerization of m-phenylenediamine (MPD) and trimesoylchloride

-p

(TMC); the as-prepared PA FO membrane possessed decreased structural parameter and increased water flux due to the more hydrophilic substrate [18]. Effect of the

re

structure (including pore diameter, surface roughness and thickness) of the support

lP

polyketone membrane on the performance of FO membrane was investigated, and the results showed that the decrease of surface pore size and the supporting membrane

na

thickness improve the permeability of FO and the anti-fouling performance of active layer; the superior antifouling property is mostly resulted from the more hydrophilic nature of the support polyketone membrane [19]. A highly permeable TFC FO

ur

membrane was successfully prepared by using PE membrane as the support. The PE

Jo

membrane was pretreated with O2 plasma; the aqueous solution of amine was impregnated, and the active layer of PA was formed. The high opening of PE supporting membrane and the combination of the connected pore structure and its thinness with the hydrophilicity due to O2 plasma treatment can reduce the ICP and improve the permeability of FO membrane [20].

4

So far, research on the use of poly(ethylene glycol) (PEG) to modify the commonly-used PSf support membranes for reducing ICP particularly is rather limited. Polyethylene glycol has high retention volume, flexibility and hydrophilicity. It is an effective material to improve the hydrophilicity of membrane. However, it is difficult to introduce PEG onto the PSf membrane due to the lack of functionalities. The main objective of the present work is to reducing ICP via the hydrophilization of the polysulfone (PSf) support membrane through the click grafting of

ro of

methoxypolyethylene glycol (mPEG) onto PSf. First the propargyl poly(ethylene glycol)s (mPEG-CCH) was prepared; then, the chloromethylation of PSf

(PSf-CH2Cl) and azidation of PSf-CH2Cl were performed to obtain azido PSf

-p

(PSf-CH2N3); after that the mPEG grafted PSf (PSf-g-mPEG) was obtained via click reaction between mPEG-CCH and PSf-CH2N3; the support layer of PSf was

re

prepared by non-solvent induced phase separation and blended with PSf-g-mPEG;

lP

finally, the polyamide active layer was prepared through the interfacial polymerization of phenylenediamine (MPD) with 1,3,5-benzenetricarbonyl trichloride

na

(TMC) on the porous PSf/PSf-g-mPEG blended support. Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (1H NMR), X-ray photoelectron

ur

spectroscopy analysis (XPS), field emission scanning electron microscopy (FESEM) and electron spectroscopy (EDS) confirmed their preparation and modification.

Jo

2. Materials and methods 2.1. Materials Sodium azide (NaN3), chloromethyl ether (a toxic carcinogen that must be carefully avoided from exposure or inhalation), propargyl bromide and copper bromide (CuBr) were bought (Sinopharm Chemical Reagent Co. Ltd). Methoxypolyethylene glycol 5

(mPEG, nominal Mw of 200, 500, 1000 and 1900), phenylenediamine (MPD), 1,3,5-benzenetricarbonyl trichloride (TMC) and N,N,N,N’,N’’-pentamethyldiethylenetriamine (PMDETA) were purchased from Aladdin Reagent Co. Ltd (Shanghai, China). 2.2 Preparation of functional polymers and TFC PA membranes 2.2.1 Synthesis of propargyl terminated mPEG

ro of

Alkynyl-mPEG (Mw = 500 g/mol) was synthesized as an example according to [21].

Briefly, NaOH (0.4 g, 10 mmol) and propyl bromide (1.2 g, 10 mmol) were added to

-p

methylbenzene solution (100 mL) of mPEG (5.0 g, 1 mmol) for 24 h at 50 oC. The

resulting solution was concentrated in vacuo. Then the mixture was washed several

re

times with water. The organic phase was dried over MgSO4, concentrated in vacuo,

lP

and purified by column chromatography. 2.2.2 Chloromethylation of PSf

na

The chloromethylation of PSf was performed according to [22]. Briefly, anhydrous zinc chloride (5.0 g) was dissolved in 100 mL chloromethylether, then dropped into

ur

the flask of 50.0 g PSf in 250 mL 1,2-dichloroethane and kept for 3-5 h at 40 oC. After

Jo

that, the polymer was precipitated by pouring into methanol, further dissolved in dimethyl acetamide, precipitated in water and further purified. The chloromethyl content is 2.4 meq/g measured by spectrophotometric determination [22]. 2.2.3 Azidation of PSf

6

The above obtained PSf-CH2Cl (3.85 g, 0.01 mmol) was dissolved in 30 mL of DMSO in a flask. Sodium azide (1.95 g, 0.03 mol) was added to the solution. After stirred for 24 h at 70 oC, the mixture was concentrated and precipitated into methanol/water mixture (4/1 v/v). The product (azido-PSf or PSf-CH2N3) was dried in a vacuum oven at 30 oC for 24 h, yielding a white amorphous solid [21, 23]. 2.2.4 Preparation of PSf-g-mPEG (1000 g/mol)

ro of

Coupling alkynyl-mPEG1000 (Mw 1000 g/mol) onto azido-PSf was catalyzed by

CuBr/PMDETA [24]. Alkynyl-mPEG1000 (319.0 mg, 0.28 mmol) and PSf-CH2N3

-p

(1.0 g, 0.28 mmol) was dissolved in 20 mL dimethyl formamide (DMF) in a bottle, three freeze-thaw cycles were carried out to remove oxygen and backfilled with N2

re

[25]. A solution of PMDETA (58 µL, 0.28 mmol) and CuBr (40 mg, 0.28 mmol) in 2

lP

mL DMF was injected through the stopcock to the mixture. The reaction was performed at 40 oC for 36 h, and then the reaction mixture was exposed to the air,

na

concentrated and then precipitated into methanol for three times. The product (PSf-g-mPEG) was dried in a vacuum oven at 25 oC for 24 h.

ur

2.2.5 Preparation of PSf/PSf-g-mPEG blended support membrane

Jo

PSf/PSf-g-mPEG blended support membrane was prepared by non-solvent induced phase inversion method. Total weight percentage 12 wt.% of PSf plus PSf-g-mPEG was dissolved in a premixed solvent (DMF:NMP, 60:20) at 70 oC and magnetically agitated for at least 12 h to ensure that the polymer was completely dissolved, and it was then stored at room temperature for at least 24 h without stirring to remove air

7

bubbles. The viscous solution was then poured onto a clean glass plate to cast a 152 µm film and was quickly dipped into a precipitation water bath (3.0 wt.% NMP) at room temperature. The membrane was put into a precipitation bath for 10 min, and then soaked in fresh distilled water for 24 h to remove excess solvents [26]. The casting compositions are tabulated in Table 1. The PSf part from mPEG-b-PSf acts as an anchor, it could not be leached out through the strong hydrophobic interaction with

ro of

the PSf substrate. The weight remained nearly unchanged after several times washing, and the results were not shown in the manuscript.

-p

Table 1. Casting solution composition (solvent, NMP:DMF = 60:20, w/w; total

Sample

Casting solution composition

No.

PSf (g)

re

weight percentage of PSf plus PSf-g-mPEG for all the samples is 12 wt.%; T=30 oC).

PSf-g-mPEG

Weight ratio of

Casting

mPEG Mw

mPEG *,

(g)

PSf to

solution

(g/mol)

(g)

null

transparent

null

0

9:1

transparent

1900

0

9:1

transparent

1000

0

10.91

0

2

9.81

1.1

3

9.81

1.1

4

9.81

5

9.81

6

8.72

7

8.72

na

1

lP

PSf-g-mPEG

9:1

transparent

500

0

1.1

9:1

transparent

200

0

2.2

4:1

transparent

1900

0

2.2

4:1

transparent

1000

0

8.72

2.2

4:1

transparent

500

0

8.72

2.2

4:1

transparent

200

0

5.45

5.45

1:1

translucent

1900

0

11

5.45

5.45

1:1

translucent

1000

0

12

5.45

5.45

1:1

translucent

500

0

13

5.45

5.45

1:1

translucent

200

0

14

10.91

0

null

transparent

1900

0.54

15

10.91

0

null

transparent

1000

0.29

16

10.91

0

null

transparent

500

0.15

17

10.91

0

null

transparent

200

0.07

8 9

Jo

10

ur

1.1

*. Pure mPEG as the additive. 8

2.2.6 Preparation of polyamide active membrane on the support PSf The polyamide active layer was formed on the PSf support by interfacial polymerization. The scaffolds were first immersed in MPD solution (3.4 wt.% in DI water) for 2 min. After the excess MPD was removed by air knife, the PSf support was immersed in the TMC solution (0.15 wt.% in Isopar-G) for 1 min, then the polyamide selective layer was formed on the PSf support, the excess TMC solution is

ro of

vertically drained for 2 min. After that it was cured in DI bath at 95 oC for 2 min,

soaked in 0.2 g/L NaClO solution for 2 min, and then immersed in NaHSO3 solution

-p

(1 g/L, 30 s), solidified again at 95 oC for 2 min. Finally, the TFC PA membranes was washed thoroughly and stored in deioned water at 4 oC [26].

re

2.3 Characterization of the prepared polymers and evaluation of the TFC PA

lP

membrane performances

2.3.1 Characterizaiton of the prepared polymers

na

The chemical structures of alkynyl mPEG and azide PSf were determined by Fourier transform infrared spectroscopy (FTIR-8400 S, Hitachi, KBr particle method) and

ur

X-ray photoelectron spectroscopy (XPS). Water contact angle was measured with

Jo

automatic double titrated contact angle analyzer (Theta, Biolin Scientific Inc.). The surface morphology of the membrane was photographed by field emission scanning electron microscope (FESEM, Hitachi S4800, Japan) at 5 keV; energy dispersive spectroscopy (EDS) was applied to determine the elements of PSf, PSf-CH2Cl and PSf-CH2N3. 1H NMR spectra of mPEG-CCH, PSf, PSf-CH2Cl and PSf-CH2N3 were

9

recorded by BrukerAV-500 NMR spectrometer (500 MHz, CDCl3, δ, in ppm). Evaluations of the TFC PA membrane performances, including membrane porosity, water flux of the polysulfone support membrane, transport parameters of the TFC PA membranes, the intrinsic property of a membrane to retain salt, were presented in Supplementary Material. 3. Results and discussion

ro of

3.1. Structural characterization of alkynyl-mPEG, chlorinated PSf, azido-PSf and PSf-g-mPEG

-p

The general procedure of preparation of alkynyl-mPEG, chlorinated PSf, azido-PSf

Jo

ur

na

lP

are presented schematically in Figure 1.

re

and the click reaction between alkynyl-mPEG and azido-PSf to obtain PSf-g-mPEG

Figure 1. Schematic representation of the preparation of alkynyl-mPEG, chlorinated PSf, azido-PSf and PSf-g-mPEG. The 1H NMR spectrum of alkynyl-mPEG has been collected by using the Bruker Avance 500MHz NMR spectrometer, as shown in Figure S1. 10

Alkynyl-mPEG (mPEG-CC): 1H-NMR (CDCl3, 500 MHz) δ: 4.17 (s, 2H, -CH2CC), 3.35 (s, 3H, CH3O), 3.53 (m, O-CH2-CH2-O), 2.42 (s, 1H, -CCH). 1

H NMR spectra of PSf, PSf-CH2Cl and PSf-CH2N3 (PSf-g-mPEG cannot be

dissolved in deuterated solvent) are shown in Figure S2. PSf: 1H-NMR (CDCl3, 500 MHz) δ: 7.00~8.06 (m, Ar-H), 1.87 (s, 6H, CH3-H). PSf-CH2Cl: 1H-NMR (CDCl3, 500 MHz) δ: 7.00~8.06 (m, Ar-H), 1.87 (s, 6H,

ro of

CH3-H), 4.72 (s, 2H, CH2Cl-H).

PSf-CH2N3: 1H-NMR (CDCl3, 500 MHz) δ: 7.00~8.06 (m, Ar-H), 1.87 (s, 6H,

-p

CH3-H), 4.25 (s, 2H, CH2N3-H).

FT-IR was used to characterize the chemical composition of alkynyl-mPEG, PSf,

re

PSf-CH2Cl, PSf-CH2N3 and PSf-g-mPEG (Figure S3). The characteristic band of

lP

chloromethylation group is located at 720-770 cm-1. The peak at 1100 cm-1 is ascribed to the telescopic vibration absorption of C-O, 2108 cm-1 is the telescopic vibration of

na

terminal alkynyl group, the 2100 cm-1 peak in the spectrum of azido-PSf is corresponding to azide groups [23], and this peak is obviously attenuated after the

ur

click reaction.

Jo

Analyses of the elemental contents of PSf, PSf-CH2Cl and PSf-CH2N3 were also performed using the FESEM-EDS technique (Figure S4 and Table S1). C, O, S elements could be observed in the blank PSf (Figure S4a). For the chlorinated PSf (PSf-CH2Cl, Figure S4b), a new element of Cl could be found, indicating the successful chloromethylation. After the nucleophilic replace of Cl with azido group

11

(Figure S4c), Cl disappears and new element of N appears for the azido-PSf (PSf-CH2N3), which indicates that Cl is completely replace with azido group. These results agreed with that of FTIR spectroscopy (Figure S4). From Table S1 it could also be observed that O/C mole ratios of the blended supports are higher than that of the pure PSf, indicating that more mPEG has been tethered into the blended membrane. The presence of N in PSf-g-mPEG1900 and the PSf/PSf-g-mPEGx

ro of

suggests that mPEG was successful linked with PSf via click reaction through the formation of aromatic 1,2,3-triazoles.

-p

The wide scan XPS spectra of PSf, chlorinated PSf (PSf-CH2-Cl), azido PSf

(PSf-CH2-N3) and PSf-g-mPEG1900 are shown in Figure 2. The peaks are at 284.7

re

eV, 533.0 eV, 401.0 eV, 170.0 and 203.0 eV, corresponding to the binding energies of

lP

C1s, O1s, N1s, S2p3 and Cl2p3, respectively. After chloromethylation, a new peak at 203.0 eV appears, which is corresponding to C12p3; for the azido PSf, the N1s

na

appears at the binding energy of 401.0 eV, and at the same time the peak of Cl2p3 disappears completely, which clearly indicates that the chlorine atoms have been

ur

substituted with azide groups; the N1s peak strength weakens after the click coupling

Jo

of mPEG1900 onto PSf, suggesting that the mPEG1900 chains have been tethered onto PSf, and thus shielding the N1s peak. O1s

C1s PSf PSf-CH2-Cl PSf-CH2-N3 PSf-CH2-mPEG1900 N1s

S2p3

Cl2p3

12

Figure 2. XPS spectra of PSf, chlorinated PSf (PSf-CH2-Cl), azided PSf (PSf-CH2-N3) and PSf-g-mPEG1900.

ro of

The C1s core level spectra of PSf, chlorinated PSf (PSf-CH2-Cl), azido PSf (PSf-CH2-N3) and PSf-g-mPEG1900 can be curve-fitted with three components at

about 284.7 eV, 285.6 eV and 286.4 eV, which are attributed to C-H/C-C, C-O/C-S,

-p

and C-Cl/C-N, respectively [27]. As shown in Figure 3, the C-Cl adsorption peak

re

appears for chlorinated PSf (PSf-CH2-Cl) compared with pure PSf, which is due to

lP

chloromethylation. After the azidation of the chlorinated PSf, this peak is completely replaced with C-N peak. These results confirm that the chlorination and azidation of

Jo 280

282

C1s for PSf-CH2-Cl

C-C/C-H

284

C-O/C-S

C-O/C-S

ur

C1s for pure PSf

na

PSf were successfully achieved.

C-C/C-H C-Cl

286

288

290

280

282

284

286

288

290

Binding energy, eV

Binding energy, eV

C1s for PSf/PSf-g-mPEG1900

C1s for PSf-CH2-N3

C-C/C-H

C-O/C-S

C-O/C-S

C-N

C-N

C-C/C-H

280

282

284

286

Binding energy, eV

288

290 280

282

284

286

Binding energy, eV

288

290

13

Figure 3. C1s core level spectra of PSf, chlorinated PSf (PSf-CH2-Cl), azido PSf (PSf-CH2-N3) and PSf-g-mPEG1900. 3.2 Morphologies of the blended PSf supports and the TFC PA membranes

PSf

-p

PSf

ro of

FESEM images of the surface and cross-section of membranes are shown in Figure 4.

re

10 m (A1)

100 m

(A2)

PSf-g-MPEG500

na

lP

PSf-g-MPEG500

100 m

10 m

ur

(B1)

(B2)

PSf-g-MPEG1900

PSf-g-MPEG1900

Jo

100 m

10 m (C1)

Active layer on PSf

(C2)

Active layer on PSf/PSf-g-MPEG1900 14

5 m

5 m

Figure 4. FESEM images of the surface (1) and cross-section (2) of membranes. A, the pure PSf membrane; B-C, the blended PSf/PSf-g-mPEG (with Mw weights of 500

ro of

and 1900 g/mol) support membranes at the weight ratio of 9:1; D and E, the active PA membrane on the pure PSf and PSf/PSf-g-mPEG1900 blended support membrane.

-p

It could be found that a finger-shaped pore forms on the non-solvent side owing to the instantaneous de-mixing and the porosity increases gradually as the membrane is

re

closer to the glass plate; a dense layer facing the glass plate is formed due to the retard

lP

phase separation; after blended with PSf-g-mPEG, a thick layer of spongy membrane formed on the sublayer of the membrane, and the spongy layer becomes thicker with

na

the molecular weight increasing. The finger-like macrovoids are largely suppressed when the molecular weight is increased to 1900 g/mol. After blended with

ur

PSf-g-mPEG, the water fluxes of the support membrane increase, this would be

Jo

discussed later. From Figure 4D and 4E it could be observed that the ridge-and-valley structure on the membrane surface are formed [26], which is the unique structure of the aromatic polyamide surface, form the active layer of aromatic PA on the surface of PSf support membrane. 3.3. Water contact angle measurements

15

The water contact angle is shown in Table S2, the water contact angle decreases for the blended PSf/PSf-g-mPEG membranes, which may contribute to the water flux increase of the blended membranes. The slight decrease of water contact angle may be due to the aggregation of the mPEG segments in PSf-g-mPEG block copolymer enriched onto the membrane surface during the membrane preparation process. 3.4. Permeation properties of the blended support PSf membrane

ro of

The changes of water flux through the pure PSf, the PSf/mPEG and the

PSf/PSf-g-mPEG blended support membranes with mPEG molecular weight changing

-p

are shown in Figure 5. It can be seen from Figure 5 that when mPEG is not added into the cast solution (the pure PSf), the membrane flux is very small; after blended with

re

pure mPEG, the membrane flux jumps quickly, which proves that mPEG can improve

lP

water flux of the PSf support membrane [28]. Interestingly, when the casting solution was prepared with the same proportion of click-chemically prepared PSf-g-mPEG, the

na

water flux is further improved. The water fluxes are remained higher for the membranes PSf/PSf-g-mPEG200 and PSf/PSf-g-mPEG500, and then it drops for the

ur

membranes PSf/PSf-g-mPEG1000 and PSf/PSf-g-mPEG1900. The higher water flux

Jo

may be due to the following two reasons: (1) it is easier for PSf-g-mPEG to concentrate on the membrane skin and inner pore wall surfaces, and more difficult for PSf-g-mPEG to leach out from the PSf/PSf-g-mPEG blended membrane, the hydrophilicity would be remained unchanged during the filtration process; (2) mPEG with lower molecular weights are relatively more hydrophilic than those with higher

16

molecular weights, which results in the less water permeation resistance, and thus the higher water fluxes. On the other hand, for the PSf/mPGE membranes, mPEG without hydrophobic interaction with PSf substrate is easier to leach out, resulting in the surface hydrophilicity decreased during the filtration process. The PSf/PSf-g-mPEG membranes at the ratio of 9:1 have high water flux and better membrane forming abilities, as a result, these membranes were chosen for further investigation.

ro of

1800

PSf/PSf-g-MPEG=9:1 PSf/PSf-g-MPEG=4:1 PSf/MPEG=9:1

1600

1200 1000 800

-p

Water flux, LMH

1400

600 400

0

500

re

200 1000

1500

2000

Molecular weight of MPEG, g/mol

lP

Figure 5. Changes of water flux for the pure PSf support membrane and PSf/PSf-g-mPEG blended membrane. 3.5. Permeation properties of the polyamide membrane

na

Under different transmembrane pressures, reverse osmosis was applied to determine the permeability and salt retention rate of the membrane [29]. As is shown in Figure

ur

S5, pure water flux increases as pressure increases; the PA membrane on the

Jo

PSf/PSf-g-mPEG blended support has higher pure water flux, moreover the salt retention rate was remained nearly unchanged. The increase of pure water flux may be due to the decrease of structural parameters caused by the hydrophilically blending with PSf-g-mPEG, and thus the reduction of water permeation resistance; on the other hand, the dense PA active layer is not altered, resulting in the nearly unchanged salt retention rate. The membrane permeability coefficient A was thus determined by 17

linearly fitting flux vs. pressure. The calculated results are shown in Table 2, which shows that the PSf/PSf-g-mPEG blended membranes have higher A values. The structure parameter S, one of the key properties of FO membrane, is defined as the product of the thickness of supporting layer (l) and the tortuosity () to the porosity (ε) [30] (equatin S10). According to Loeb et al, the larger S value is bound to result in serious ICP. Based on the classical ICP model [31], the FO water flux can be

ro of

predicted with equation S8 and S9. The experimental results of A, B and S were

accordingly obtained and tabulated in Table 2. It could be found in this work that S

-p

(the membrane structural parameters) is reduced and A (water permeability

coefficient) improved by blending PSf with PSf-g-mPEG, which indicates that the

re

internal concentration polarization (ICP) have been successfully alleviated. However,

lP

these values are higher than those reported in the literatures, it may be due to that the original composite PA membranes possess higher S values because of the

na

manufacturing methods. According to equation 10, the thickness of the membrane (l) plays an important role in the structural parameters (S), its relatively high thickness

ur

leads to relatively large S values in the present work.

Jo

Table 2. Porosity (), water permeability (A), salt permeability (B) and structure parameters (S) of TFC polyamide composite membrane with the PSf/PSf-g-mPEG blended membranes as the supports. Membrane (ratio, mPEG

Ref.

No. a

Mw)

 (%)

A (L/m2∙h∙bar)

B (L/m2∙h)

S (m)

1 (1)

PSf

28.63

7.67

4.35

1.90E-03

Present work

2 (2)

9:1, 1900

60.22

89.00

44.63

1.00E-03

Present work

3 (3)

9:1, 1000

70.96

66.83

35.66

1.30E-03

Present work

4 (4)

9:1, 500

77.05

30.28

19.67

9.00E-04

Present work

5 (5)

9:1, 200

72.01

29.87

15.31

1.10E-03

Present work

18

6 (14)

PSf/mPEG1900

50.87

9.84

7.65

1.60E-03

Present work

7 (15)

PSf/mPEG1000

65.83

8.00

4.46

1.80E-03

Present work

8 (16)

PSf/mPEG500

41.72

15.76

8.43

1.50E-03

Present work

9 (17)

PSf/mPEG200

75.27

7.76

5.39

1.50E-03

Present work

10

TFC polyamide membrane

-

1.61

0.24

4.00E-04

[32]

0.16

5.2010-4

[33]

0.12

1.6710-6

[34]

2.62

6.8910-4

[35]

0.10

3.0010-4

[36]

0.88

3.7010-4

[37]

0.32

5.7010-4

[38]

11 12 13 14 15 16

TFC polyamide membrane PSfco-TFC

-

4.70

-

1.65

TFC polyamide membrane TFC polyamide membrane TFC polyamide membrane TFC polyamide membrane

1.23 -

1.22

-

5.81

-

0.84

ro of

a: No.1-9, the experimental results of this work; No. in the bracket, the experiment No. in Table 1; No. 10-16, from the literatures. “-”: Not reported in the literatures. The water flux and salt flux through the thin-film composite (TFC) aromatic

-p

polyamide (PA) membrane with mPEG molecular weight changing are shown in

re

Figure 6. It could be seen that both the water flux and salt flux increase up to the

lP

molecular weight of mPEG500, then they decrease with further increase of the molecular weight, which has a similar trend to the water flux for the PSf/PSf-g-mPEG

na

blended support membrane. These results imply that the support membrane plays an important role on the composite PA FO membranes, hydrophilization of the support

ur

membrane would improve the performances of the PA FO membrane [39]. 3.0

100

80 2

Salt flux, mmol/m h

Water flux, LMH

Jo

2.5 2.0

60 1.5 40 1.0 20

0.5 0.0 0

500

1000

1500

0 2000

Molecular weight of mPEG, Da

19

Figure 6. Water flux and salt flux through the thin-film composite (TFC) aromatic polyamide (PA) membrane with mPEG molecular weight changing. 4. Conclusions To reduce the internal concentration polarization of thin-film composite (TFC) aromatic polyamide (PA) membrane in the forward osmosis process, the support

ro of

membrane was prepared by blending PSf and PSf-g-mPEG with the mPEG molecular weight of 200, 500, 1000 and 1900 g/mol at the PSf/PSf-g-mPEG ratios of 4:1 and 9:1, respectively. Fourier transform infrared spectroscopy, X-ray photoelectron

-p

spectroscopy analysis (XPS) and 1H nuclear magnetic resonance characterizations

re

confirmed the successful synthesis of alkynyl-mPEG, chlorinated PSf, azido-PSf and

lP

PSf-g-mPEG; energy dispersive spectrometer characterization showed that Cl element appeared after chlorination of PSf and then it disappeared completely after the

na

nucleophilic replace of Cl with azido group.

The pure water flux of the PSf/PSf-g-mPEG blended support membrane was sharply

ur

increased; the maximum value of the pure water flux was gained for the

Jo

PSf/PSf-g-mPEG500 blended support membrane. In the reverse osmosis (RO) system, the water flux for the TFC PA membrane increased significantly, while the salt retention rate was remained unchanged. In the forward osmosis (FO) system, the pure water flux increased significantly and the structure parameter (S) decreased sharply.

20

These experimental results demonstrate that by using the blended PSf/PSf-g-mPEG as the support membrane, the internal concentration polarization or the structural parameters could be substantially alleviated. The approach provides with a plausible way to the fabrication of TFC PA forward osmosis membrane with small structural parameters. This research opens a new avenue to reduce the internal concentration polarization of the TFC PA forward osmosis membrane by hydrophilization of its

ro of

support. Thus, new relative hydrophilic materials or modification methods could be applied to this end.

-p

Declarations of interest: none.

re

Acknowledgements

This work was supported by the National Natural Science Foundation of China [grant

lP

number 21371008] and key research project of Anhui provincial education department (KJ2019A0867). The authors also thank Mrs. Zhong-Xiang Ma for her

Jo

ur

na

English polishing.

21

References

Jo

ur

na

lP

re

-p

ro of

[1] P. Xiao, J. Li, Y. Ren, X. Wang, A comprehensive study of factors affecting fouling behavior in forward osmosis, Colloids Surf. A 499 (2016) 163-172. [2] X. Wang, X. Wang, P. Xiao, J. Li, E. Tian, Y. Zhao, Y. Ren, High water permeable free-standing cellulose triacetate/graphene oxide membrane with enhanced antibiofouling and mechanical properties for forward osmosis, Colloids Surf. A 508 (2016) 327-335. [3] Z. Tan, S. Chen, X. Peng, L. Zhang, C. Gao, Polyamide membranes with nanoscale Turing structures for water purification, Science 360(6388) (2018) 518-521. [4] Z. Yao, Z. Yang, H. Guo, X. Ma, Y. Dong, C.Y. Tang, Highly permeable and highly selective ultrathin film composite polyamide membranes reinforced by reactable polymer chains, J. Colloid Interf. Sci. 552 (2019) 418-425. [5] T. Zhang, Z. Li, W. Wang, Y. Wang, B. Gao, Z. Wang, Enhanced antifouling and antimicrobial thin film nanocomposite membranes with incorporation of Palygorskite/titanium dioxide hybrid material, J. Colloid Interf. Sci. 537 (2019) 1-10. [6] X. Ma, Z. Yang, Z. Yao, H. Guo, Z. Xu, C.Y. Tang, Tuning roughness features of thin film composite polyamide membranes for simultaneously enhanced permeability, selectivity and anti-fouling performance, J. Colloid Interf. Sci. 540 (2019) 382-388. [7] R. Pang, K. Zhang, Fabrication of hydrophobic fluorinated silica-polyamide thin film nanocomposite reverse osmosis membranes with dramatically improved salt rejection, J. Colloid Interf. Sci. 510 (2018) 127-132. [8] L. Gutierrez, C. Aubry, R.V. Linares, J.-P. Croue, Natural organic matter interactions with polyamide and polysulfone membranes: Formation of conditioning film, Colloids Surf. A 477 (2015) 1-8. [9] D. Saeki, T. Tanimoto, H. Matsuyama, Prevention of bacterial adhesion on polyamide reverse osmosis membranes via electrostatic interactions using a cationic phosphorylcholine polymer coating, Colloids Surf. A 443 (2014) 171-176. [10] X.L. Lu, S.R.V. Castrillon, D.L. Shaffer, J. Ma, M. Elimelech, In Situ Surface Chemical Modification of Thin-Film Composite Forward Osmosis Membranes for Enhanced Organic Fouling Resistance, Environ. Sci. Technol. 47(21) (2013) 12219-12228. [11] L. Setiawan, R. Wang, L. Shi, K. Li, A.G. Fane, Novel dual-layer hollow fiber membranes applied for forward osmosis process, J. Membrane Sci. 421 (2012) 238-246. [12] H. Isawi, M.H. El-Sayed, X. Feng, H. Shawky, M.S.A. Mottaleb, Surface nanostructuring of thin film composite membranes via grafting polymerization and incorporation of ZnO nanoparticles, Appl. Surf. Sci. 385 (2016) 268-281.

22

Jo

ur

na

lP

re

-p

ro of

[13] M. Shan, H. Kang, Z. Xu, N. Li, M. Jing, Y. Hu, K. Teng, X. Qian, J. Shi, L. Liu, Decreased cross-linking in interfacial polymerization and heteromorphic support between nanoparticles: Towards high-water and low-solute flux of hybrid forward osmosis membrane, J. Colloid Interf. Sci. 548 (2019) 170-183. [14] W.A. Suwaileh, D.J. Johnson, S. Sarp, N. Hilal, Advances in forward osmosis membranes: Altering the sub-layer structure via recent fabrication and chemical modification approaches, Desalination 436 (2018) 176-201. [15] M.J. Park, R.R. Gonzales, A. Abdel-Wahab, S. Phuntsho, H.K. Shon, Hydrophilic polyvinyl alcohol coating on hydrophobic electrospun nanofiber membrane for high performance thin film composite forward osmosis membrane, Desalination 426 (2018) 50-59. [16] H. Salehi, M. Rastgar, A. Shakeri, Anti-fouling and high water permeable forward osmosis membrane fabricated via layer by layer assembly of chitosan/graphene oxide, Appl. Surf. Sci. 413 (2017) 99-108. [17] L. Jin, Z. Wang, S. Zheng, B. Mi, Polyamide-crosslinked graphene oxide membrane for forward osmosis, J. Membrane Sci. 545 (2018) 11-18. [18] M. Qiu, J. Wang, C. He, A stable and hydrophilic substrate for thin-film composite forward osmosis membrane revealed by in-situ cross-linked polymerization, Desalination 433 (2018) 1-9. [19] L.-F. Fang, L. Cheng, S. Jeon, S.-Y. Wang, T. Takahashi, H. Matsuyama, Effect of the supporting layer structures on antifouling properties of forward osmosis membranes in AL-DS mode, J. Membrane Sci. 552 (2018) 265-273. [20] S.J. Kwon, S.-H. Park, M.S. Park, J.S. Lee, J.-H. Lee, Highly permeable and mechanically durable forward osmosis membranes prepared using polyethylene lithium ion battery separators, J. Membrane Sci. 544 (2017) 213-220. [21] J.-Q. Meng, T. Yuan, C.J. Kurth, Q. Shi, Y.-F. Zhang, Synthesis of antifouling nanoporous membranes having tunable nanopores via click chemistry, J. Membrane Sci. 401–402(0) (2012) 109-117. [22] J.H. Hao, C.X. Chen, L. Lin, L.X. Yu, W.J. Jiang, Preparation of solvent-resistant anion-exchange membranes, Desalination 129(1) (2000) 15-22. [23] H. Toiserkani, G. Yilmaz, Y. Yagci, L. Torun, Functionalization of Polysulfones by Click Chemistry, Macromol. Chem. Phys. 211(22) (2010) 2389-2395. [24] P. Zhao, Y. Yan, X. Feng, L. Liu, C. Wang, Y. Chen, Highly efficient synthesis of polymer brushes with PEO and PCL as side chains via click chemistry, Polymer 53(10) (2012) 1992-2000. [25] X.-M. Wu, L.-L. Wang, Y. Wang, J.-S. Gu, H.-Y. Yu, Surface modification of polypropylene macroporous membrane by marrying RAFT polymerization with click chemistry, J. Membrane Sci. 421–422(0) (2012) 60-68. [26] X. Lu, L.H. Arias Chavez, S. Romero-Vargas Castrillon, J. Ma, M. Elimelech, Influence of active layer and support layer surface structures on organic fouling propensity of thin-film composite forward osmosis membranes, Environ. Sci. Technol. 49(3) (2015) 1436-44.

23

Jo

ur

na

lP

re

-p

ro of

[27] X. Du, J. Meng, R. Xu, Q. Shi, Y. Zhang, Polyol-grafted polysulfone membranes for boron removal: Effects of the ligand structure, J. Membrane Sci. 476 (2015) 205-215. [28] Y. Chen, M. Wei, Y. Wang, Upgrading polysulfone ultrafiltration membranes by blending with amphiphilic block copolymers: Beyond surface segregation, J. Membrane Sci. 505 (2016) 53-60. [29] Z. Yang, D. Saeki, H. Matsuyama, Zwitterionic polymer modification of polyamide reverse-osmosis membranes via surface amination and atom transfer radical polymerization for anti-biofouling, J. Membrane Sci. 550 (2018) 332-339. [30] C.Y. Tang, Q. She, W.C.L. Lay, R. Wang, A.G. Fane, Coupled effects of internal concentration polarization and fouling on flux behavior of forward osmosis membranes during humic acid filtration, J. Membrane Sci. 354(1-2) (2010) 123-133. [31] S. Loeb, L. Titelman, E. Korngold, J. Freiman, Effect of porous support fabric on osmosis through a Loeb-Sourirajan type asymmetric membrane, J. Membrane Sci. 129(2) (1997) 243-249. [32] A. Tiraferri, N.Y. Yip, A.P. Straub, S.R.-V. Castrillon, M. Elimelech, A method for the simultaneous determination of transport and structural parameters of forward osmosis membranes, J. Membrane Sci. 444 (2013) 523-538. [33] M. Xie, L.D. Nghiem, W.E. Price, M. Elimelech, Relating rejection of trace organic contaminants to membrane properties in forward osmosis: Measurements, modelling and implications, Water Res. 49 (2014) 265-274. [34] P. Xiao, L.D. Nghiem, Y. Yin, X.-M. Li, M. Zhang, G. Chen, J. Song, T. He, A sacrificial-layer approach to fabricate polysulfone support for forward osmosis thin-film composite membranes with reduced internal concentration polarisation, J. Membrane Sci. 481 (2015) 106-114. [35] Y.C. Kim, M. Elimelech, Adverse Impact of Feed Channel Spacers on the Performance of Pressure Retarded Osmosis, Environ. Sci. Technol. 46(8) (2012) 4673-4681. [36] N.Y. Yip, M. Elimelech, Performance Limiting Effects in Power Generation from Salinity Gradients by Pressure Retarded Osmosis, Environ. Sci. Technol. 45(23) (2011) 10273-10282. [37] N.Y. Yip, A. Tiraferri, W.A. Phillip, J.D. Schiffman, L.A. Hoover, Y.C. Kim, M. Elimelech, Thin-Film Composite Pressure Retarded Osmosis Membranes for Sustainable Power Generation from Salinity Gradients, Environ. Sci. Technol. 45(10) (2011) 4360-4369. [38] W. Luo, M. Xie, F.I. Hai, W.E. Price, L.D. Nghiem, Biodegradation of cellulose triacetate and polyamide forward osmosis membranes in an activated sludge bioreactor: Observations and implications, J. Membrane Sci. 510 (2016) 284-292. [39] D.L. Shaffer, J.R. Werber, H. Jaramillo, S. Lin, M. Elimelech, Forward osmosis: Where are we now?, Desalination 356 (2015) 271-284.

24

25

ro of

-p

re

lP

na

ur

Jo

Figure captions Figure 1. Schematic representation of the preparation of alkynyl-mPEG, chlorinated PSf, azido-PSf and the click reaction between alkynyl-mPEG and azido-PSf to obtain PSf-g-mPEG. Figure 2. XPS spectra of PSf, chlorinated PSf (PSf-CH2-Cl), azided PSf

ro of

(PSf-CH2-N3) and PSf-g-mPEG1900. Figure 3. C1s core level spectra of PSf, chlorinated PSf (PSf-CH2-Cl), azided PSf (PSf-CH2-N3) and PSf-g-mPEG1900.

-p

Figure 4. FESEM images of the surface (1) and cross-section (2) of membranes. A,

the pure PSf membrane; B-C, the blended PSf/PSf-g-mPEG (with Mw weights of 500

re

and 1900 g/mol) support membranes at the weight ratio of 9:1; D and E, the active PA

lP

membrane on the pure PSf and PSf/PSf-g-mPEG1900 blended support membrane. Figure 5. Changes of water flux for the pure PSf support membrane and

na

PSf/PSf-g-mPEG blended membrane.

Figure 6. Water flux and salt flux through the thin-film composite (TFC) aromatic

Jo

ur

polyamide (PA) membrane with mPEG molecular weight changing.

26