Tailoring Membrane Surface Properties and Ultrafiltration

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Article

Tailoring Membrane Surface Properties and Ultrafiltration Performances via Self-assembly of Polyethylene glycol-block-Polysulfone-blockPolyethylene glycol Block Copolymer upon Thermal and Solvent Annealing Ning Wang, Tao Wang, and Yunxia Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06997 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 14, 2017

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ACS Applied Materials & Interfaces

Tailoring Membrane Surface Properties and Ultrafiltration Performances via Self-assembly of Polyethylene glycol-block-Polysulfone-block-Polyethylene glycol Block Copolymer upon Thermal and Solvent Annealing

Ning Wang,1# Tao Wang,1# and Yunxia Hu1, 2*

1. CAS Key Laboratory of Coastal Environmental Processes and Ecological Remediation; Research Center

for Coastal Environmental Engineering and Technology of Shandong Province; Yantai Institute of Coastal

Zone Research, Chinese Academy of Sciences, Yantai, Shandong Province 264003, P. R. China

2. State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin Polytechnic

University, Tianjin 300387, P. R. China

# Ning Wang and Tao Wang equally contributed to this work

*Corresponding author, Tel: +86-22-83955129; E-mail: [email protected]

KEYWORDS:

Ultrafiltration,

Membrane,

Amphiphilic

block

Hydrophilicity, Antifouling.

ACS Paragon Plus Environment

copolymers,

Self-assembly,


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ABSTRACT

Recently, ultrafiltration (UF) membranes faced great challenges including the fine control of the

membrane surfaces for high filtration performances and antifouling properties in treating complex

solution systems. Here, a particular type of amphiphilic block copolymers polyethylene

glycol-block-polysulfone-block-polyethylene glycol (PEG-b-PSf-b-PEG) was synthesized through

one-pot step-growth polymerization with mPEG as two ends to achieve the mobility of hydrophilic

polymer chains. Without any other polymers or additives involved, the PEG-b-PSf-b-PEG triblock

copolymer UF membrane was fabricated through the non-solvent induced phase separation (NIPS)

method. The surface properties and filtration performances of UF membranes were tailored through

the self-assembly of PEG-b-PSf-b-PEG triblock copolymers combining the thermal and solvent

annealing treatments in water at 90 oC for 16 h. The annealed PEG-b-PSf-b-PEG triblock copolymer

membrane significantly enhanced its water flux resulting from the increased mean pore size with the

improved porosity, as well as the decreased skin layer thickness, upon annealing. More importantly, the

PEG-b-PSf-b-PEG triblock copolymer membrane surface turned from hydrophobic to hydrophilic upon

annealing with the PEG enrichment on the surface, and exhibited the improved protein antifouling


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performances. Our research opens a new avenue to tailor the membrane structure and surface properties

by self-assembly of amphiphilic block copolymers upon the thermal and solvent annealing treatments.



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1

INTRODUCTION

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Membrane separation processes have been extensively used as an industrially practicable and

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sustainable clean technology to selectively separate and recover valuable products in various fields.1-4

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The major industrial-scale membrane separation processes include microfiltration (MF), ultrafiltration

5

(UF), nanofiltration (NF), and reverse osmosis (RO), which are distinguished by their increasing fine

6

separation with decreasing pore sizes in the membrane.5-8 UF membrane is a semipermeable barrier

7

with pore sizes in the range of 0.1 to 0.001 micron to remove large molecules, bacteria, viruses,

8

pathogens, pyrogens, fine particles, and colloidal materials.9,

9

separations at low or ambient temperatures with no phase change and work as a cost-effective solution

10

to have many important medical, biological, chemical, and environmental applications including

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hemodialysis, food and beverage processing, chemical and pharmaceutical manufacturing, water

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purification and wastewater reuse.11-14

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UF can produce very selective

13

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Now, the UF membranes are facing great challenges including the limited control of the membrane

15

structure and surface properties for antifouling property and high filtration performance in treating


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complex solution systems.15,

17

materials such as polysulfone (PSf), polyvinylidene difluoride (PVDF) and polyvinyl chloride (PVC)

18

requires hydrophilic surface modification through blending or grafting hydrophilic materials to

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improve the filtration performances and anti-fouling properties. However, the grafting strategy suffers

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from complicate surface reaction procedures and compromises filtration performances of the

21

membrane.17 The blending strategy is very simple but may deteriorate the membrane hydrophilicity

22

over time due to the leaking of hydrophilic additives.18 The in situ surface segregation approach was

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reported to provide a robust and simple fouling resistant surface tailoring strategy to fabricate UF

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membranes.19-22

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polyacrylonitrile-graft-poly(ethylene oxide) (PAN-g-PEO),23, 24 polysulfone-block-poly(ethylene oxide)

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copolymers (PSf-b-PEO),25-27 Poly(vinylidene fluoride-poly(ethylene oxide)) (PVDF)-PEO) comb

27

copolymers,28 and Poly(vinylidene fluoride-Polyethylene glycol dimethacrylate (PVDF-PEGMA),29

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were synthesized and blended with their hydrophobic membrane materials to cast the UF membranes

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through the non-solvent induced phase-inversion process (NIPS). The hydrophilic blocks such as PEG

30

and PEGMA could segregate to polymer/water interface and thus endowed UF membranes with the

A

16

The intrinsic hydrophobicity of widely-used polymeric membrane

particular

type

of

amphiphilic

block


copolymers,

such

as


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enhanced surface hydrophilicity for the improved fouling-resistance, and the hydrophobic block such as

32

PAN, PSf, and PVDF could provide strong anchoring with the membrane skeleton to maintain

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mechanical strength.23-29 However, due to the very limited content of hydrophilic blocks in the overall

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membrane materials, the enhancement of surface hydrophilicity and filtration performances of UF

35

membranes was not yet optimized through blending amphiphilic copolymer with popular membrane

36

materials.25, 26

37

38

Recently, to increase the hydrophilic block content in the membrane materials and surfaces, the

39

amphiphilic copolymer of PSf-b-PEG was also used as the only membrane material to cast the UF

40

membrane through NIPS.30 It was very interesting to find that the microphase separation of PSf-b-PEG

41

block copolymers could promote the formation of the interconnected PEG microdomains and

42

dramatically improve the permeability of the fabricated UF membranes.30 Although the PEG surface

43

aggregation was observed on the PSf-b-PEG membrane surface, the water contact angle of the

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membrane surface was not increased much compared with the PSf membrane, which may because of

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no enough PEG enrichment on the membrane surfaces. The thermal and solvent annealing methods


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have been well-established to further promote the microphase separation of block copolymers, and

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thus to strongly drive the surface migration of one block on the thin film surfaces,31-34 which provides

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a simple and convenient approach to tailor the membrane surfaces through the directed self-assembly

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of block copolymers. To date, no work has been done to tailor the membrane structure and surface

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properties of the phase inversion polymeric membranes through the thermal or solvent annealing

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approach.

52

53

In this work, we attempt to enhance the surface enrichment of hydrophilic polymers through the

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annealing induced self-assembly of amphiphilic block copolymers for the improved filtration and

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antifouling performances of the phase-inversion polymeric membranes. Thus, a well-defined

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amphiphilic triblock copolymer of polyethylene glycol-block-polysulfone-block-polyethylene glycol

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(PEG-b-PSf-b-PEG) was synthesized through one-pot step-growth polymerization and used as a model

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of the amphiphilic block copolymer membrane material. Without any other polymer involved, the

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non-solvent induced phase separation (NIPS) method was used to fabricate the UF membranes of

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PEG-b-PSf-b-PEG triblock copolymer. The surface properties and filtration performances of UF



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membranes were tailored through the self-assembly of PEG-b-PSf-b-PEG triblock copolymers

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combining the thermal and solvent annealing treatments in water at high temperature. X-ray

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photoelectron spectroscopy (XPS), confocal microscopy, and scanning electronic microscopy (SEM)

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were used to monitor the surface segregation process of PEG during the annealing. Membrane

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properties and performances including hydrophilicity, pure water flux, and fouling resistance to Bovine

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Serum Albumin (BSA) protein were also investigated. Our research provides a simple and effective

67

approach to tailor the membrane structure and surface properties via the self-assembly of amphiphilic

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block copolymers through the thermal and solvent annealing treatments.

69

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2. MATERIALS AND METHODS

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2.1 Materials and Chemicals

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Bisphenol A (99 %), 4, 4’-Chlorophenyl sulfone (98 %) and N-methyl-2-pyrrolidone (NMP, 99 %)

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were purchased from Acros (New Jersey, USA). Polysulfone Udel P-3500 (PSf, Mn: 21 kg mol-1,

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measured by GPC in THF solvent) was supplied by Solvay Advanced Polymers (Alpharetta, GA,

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USA). Poly(ethylene glycol) (PEG, Mn: 150 kg mol-1, PDI 1.20 and 275 kg mol-1, PDI 1.17) was


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purchased from Polymer Source Inc. (Dorval, Quebec, Canada). Monomethylpoly(ethylene glycol)

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(mPEG, Mn: 5 kg mol-1, PDI 1.05) and bovine serum albumin (BSA, Mn : 67 kg mol-1) were

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purchased from Sigma-Aldrich (St. Louis, MO, USA). Dimethylacetamide (DMAC), toluene and

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potassium carbonate (K2CO3) were purchased from Sinopharm Chemical Reagent Co., Ltd.

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(Shanghai, China). All the materials and reagents were used as received.

81

82

2.2 Synthesis of PEG-b-PSf-b-PEG Triblock Copolymers

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One-pot approach was adopted with a slight change from the reported method to synthesize triblock

84

copolymers of PEG-b-PSf-b-PEG.35 The detailed procedure is described as follows: Bisphenol A

85

(45.01 g, 0.197 mol), 4, 4’-Chlorophenyl sulfone (57.82 g, 0.201 mol), and potassium carbonate

86

(82.60 g, 0.598 mol) were charged to a 1 L three-neck round-bottom flask fitted with a mechanical

87

stirrer under a nitrogen atmosphere, followed by the addition of a mixture solvent of DMAC and

88

toluene (500 mL DMAC, 100 mL toluene). The above reagent mixture was heated to 155 °C until

89

toluene refluxed to remove a total of 10.5 mL water through the Dean-Stark trap over 3 h. The

90

reaction temperature was further raised to 190 °C to promote the polymerization for 6 h, followed by



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the addition of mPEG (50.05 g, 0.01 mol, Mn: 5 kg mol-1). 2 h later, the polymerization was stopped

92

upon cooling the reaction mixture to room temperature. The viscous polymer solution was precipitated

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in 2 L 0.6 M HCl aqueous solution to neutralize K2CO3. The precipitated polymer was washed

94

thoroughly using water till its pH reached to 7.0. Upon filtration, the polymer was collected and dried

95

in a vacuum oven at 60 °C to a constant weight.

96

97

2.3 Characterization of PEG-b-PSf-b-PEG Triblock Copolymers

98

1

99

and Si(CH3)4 as an internal standard. Triblock copolymer composition was calculated from the area

100

ratio of the methylene protons (4H, δ 4.16 ppm) from the mPEG blocks and the aryl protons (4H, δ

101

7.80 ppm) from the PSf blocks. Gel permeation chromatography (GPC) was employed to determine

102

Mn and Mw using Waters 410 GPC system equipped with two Varian PL gel 5 µm Mixed-C columns

103

and RI detector, THF was used as eluent at a flow rate of 1.0 mL min-1 at 35 oC. The GPC system was

104

calibrated using 12 narrow polystyrene standards from Polymer Source, Inc. (Montreal, Canada) with

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Mn ranging from 200 to 3 × 106 g mol-1. Differential scanning calorimetry (DSC) was performed on a

H NMR spectra were recorded on a Bruker 500 MHz spectrometer using Chloroform-d as the solvent


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Perkin-Elmer 7 Series Thermal Analysis System with a scan rate of 10 °C min-1 from - 50 °C to +

107

250 °C. Thermal gravimetric analysis (TGA) was performed on a Mettler 5MP/PF7548/MET/400W

108

instrument (Mettler Toledo Co., Switzerland) under nitrogen flow with an increased temperature rate

109

of 10 °C min-1 from 200 °C to 800 °C with a purge rate of 60 mL min-1. Approximately 3 mg

110

specimens were heated first from room temperature to 200 °C with 10 minutes incubation to remove

111

any moisture.

112

113

2.4 Preparation of PEG-b-PSf-b-PEG Triblock Copolymer UF Membrane

114

The non-solvent induced phase separation (NIPS) method was applied to cast the UF membrane of

115

PEG-b-PSf-b-PEG triblock copolymers. In a typical procedure, 12 g of PEG-b-PSf-b-PEG triblock

116

copolymers was dissolved in N-methyl-2-pyrrolidone (NMP, 88 g) by stirring at 40 °C for 12 h, and

117

then stored in a desiccator for 12 h to degas at room temperature. Then the solution was cast on a

118

clean glass plate using a casting knife with a 250 µm gate height. The glass plate was immediately

119

immersed into the precipitation water bath at 40 ± 2 oC. After 10 min in the precipitation bath, the

120

casted membrane was transferred to a fresh deionized water bath for 2 h to remove the residual solvent.



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Subsequently, the fabricated membranes were annealed in deionized water at 90 oC for a period of

122

time from 8 h, 12 h, 16 h, to 24 h, respectively. PSf (Udel P-3500, Mn: 21 kg mol-1, Solvay) was also

123

used to prepare the membrane as controls under the same conditions as above. In addition, the dilute

124

PEG-b-PSf-b-PEG triblock copolymer solution and PSf solution in NMP and in THF solvent were

125

measured separately using a Malvern zetasizer (Nano ZS90, Malvern Instruments Ltd., UK).

126

127

2.5 PEG Segregation Observation on Membrane Surface

128

Confocal scanning fluorescence microscopy (CSFM, FluoView FV1000, Olympus, Japan) was used to

129

investigate the surface segregation of mPEG fragment from triblock copolymers on the membrane

130

surface. The mPEG could be dyed selectively with Dragendorff reagent (see supporting information),

131

thus visualized at 461 nm under the excitation of 405 nm.36 To dye membrane samples, the membrane

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coupon (1 × 1 cm2) was incubated with 10 mL Dragendorff reagent (BiI3·KI) at 0.2 M HAc-NaAc

133

buffer solution (pH 4.8) for 12 h, followed by a brief rinse with water. The dyed membrane coupon

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was transferred to a glass slide with a covering slide on top, and then was observed by CSFM

135

(excitation: 405 nm, emission: 461 nm). All images were taken with a fixed exposure time of 1.0 s.


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2.6 Characterization of Membrane Morphology and Property

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Field emitting scanning electronic microscopy (SEM, Hitachi S-4800, Japan) was used to observe

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membrane morphologies. Cross-sectional samples were prepared by snapping the wet membranes in

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liquid nitrogen. Prior to observations, all of the samples were dried in a vacuum oven overnight and

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sputtered with 10 nm thick platinum (EMITECH SC7620 sputter coater) at the argon pressure of 0.1

142

Torr. The mean pore diameter on the membrane surface and the thickness of skin layer were measured

143

using the software Nano Measure based on the scale bar of SEM images. For each data, more than 50

144

pores were randomly selected from the SEM images of three individual parallel specimens, and errors

145

were given as the standard deviation. X-ray photoelectron spectroscopy (XPS, Thermo Escalab 250Xi,

146

Thermo Fisher Scientific, USA) was employed to detect membrane surface chemistry with

147

monochromatic Al Kα excitation (hν =1486.6 eV) on a spot size of 500 µm. Spectra were collected at

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60 ° takeoff angle relative to sample surface for 4 scans. The whole spectra were collected by survey

149

scan from 0 ~ 1000 eV (step size: 1). High resolution scans were performed by narrow scan (step size:

150

0.1) for chemical analysis of oxygen element (O, 526.0 ~ 544.0 eV). Water contact angle



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151

measurements were performed using the sessile drop method to study the surface hydrophilicity on an

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optical instrument (ADS300, Data physics, Germany). The dried membrane coupon (1 × 5 cm2) was

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mounted on a clean glass slide and then put on the sample holder. A deionized water droplet (2.0 µL)

154

was dropped from a micro-syringe to the membrane surface, and its dynamic droplet shape was

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recorded immediately through video using the automated Drop Shape Analysis software (SCA20

156

Version 2) for at least 20 minutes with 5 frames per minute. Each data point was reported as the

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average value and the standard deviation from more than 15 measurements tested on at least three

158

individual parallel specimens. AFM imaging was performed using a Multimode 8 (Veeco/Digital

159

Instruments, Santa Barbara, USA). Image collection and data analysis were carried out with the

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NanoScope software version 8.10, and NanoScope Analysis software version 1.10. For the roughness

161

of membrane surfaces, three individual parallel specimens were measured and errors were given as the

162

standard deviation.

163

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2.7 Membrane Porosity Measurement

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The fully-wetted membranes coupons were weighed using the electronic balance (Mettler-Toledo


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ME104E, Switzerland) after mopping superficial water with air-laid paper. The wetted membrane was

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dried in a vacuum oven at 60 °C for 24 h to achieve a constant weight before measuring the dry

168

weight. Membrane porosity was determined following the reported equation (1):37

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ε = (Ww- Wd)/(ρw*V)

(1)

170

Where Ww is the weight of wet membrane coupons (g), Wd is the weight of dry membrane coupons

171

(g), ρ w is the density of pure water at 25 oC (g cm-3) and V is the volume of the wet membrane coupon

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(cm3). For each data point, three individual specimens were measured for parallel experiments, and

173

errors were given as the standard deviation.

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2.8 Membrane Filtration and Antifouling Performances

176

The membrane filtration properties were tested using a stirred filtration cell (Amicon 8010, Millipore,

177

USA) following the reported protocol.38 Prior to tests, each membrane sample was pre-compacted at

178

1.5 bar with DI water for 15 min, and the water flux was subsequently tested at 1.0 bar for 30 min.

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The membrane selectivity was tested using 0.1 g L-1 of PEG (Mn: 275 kg mol-1, PDI: 1.17) aqueous

180

solution as the feed at 1.0 bar, and the PEG rejection was determined as (R = 1-CPermeate/CFeed). The



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PEG concentration in the permeate solution was measured using a UV-VIS spectrophotometer

182

(PERSEE, TU-1810, China) as PEG was dyed with Dragendorff reagent (BiI3·KI, see supporting

183

information) to have a strong absorption at 510 nm.39 The linear calibration curve was obtained

184

through measuring the absorbance of the PEG and Dragendorff reagent solution at a series of PEG

185

concentrations from 0, 5, 10, 15, 20, 25, 30, and 50 mg L-1, with the fixed ratio of PEG and

186

Dragendorff reagent (Figure S1). All tests were taken in triplicate with separately prepared samples.

187

The membrane flux (J) was calculated according to the following equation:

J=

188

m ρ×S×t

(2)

189

Where J (LMH bar-1, L m-2 h-1 bar-1) is the water flux, the permeate water volume over a period of

190

time across the certain membrane area under the fixed pressure, m is the weight of the permeate water

191

(g), ߩ is the density of water (g cm-3), S is the membrane sample area (cm-2), and t is the filtration

192

time (hour), respectively. The average value of membrane flux was calculated from three individual

193

specimens.

194

The antifouling properties of membranes were investigated using BSA as a well-used model protein

195

foulant based on the reported protocol.40 Three sequential cycles of the fouling-cleaning process were


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performed on each membrane sample simulating practical membrane filtration operations. The extents

197

of water flux decline during fouling and water flux recovery after cleaning are two important

198

parameters of the membrane antifouling performances. In a typical run, the membrane with the skin

199

layer at the top was pre-compacted at 1.5 bar with DI water for 30 min until the flux stabilized as a

200

constant. In order to keep all membranes suffer from the same amount of BSA contact with membrane

201

surfaces, the initial flux of all filtration runs was set to 200 ± 10 LMH bar-1 by adjusting the applied

202

pressure, and the feed solution was switched from pure water to foulant solution (15 mg L-1 BSA

203

solution with 10 mM ionic strength including 9.9 mM NaCl and 0.1 mM NaHCO3), after 5 min of

204

stabilization under the fixed pressure. The fouled water flux was continuously monitored for 60 min.

205

Subsequently, the membrane was reversed with the skin layer facing to the bottom in the cell and

206

backwashed for 30 min with 500 mL DI water filtered through under 500 rpm stirring. Then, the water

207

flux of the cleaned membrane was measured with DI water at the end of one filtration cycle. All

208

filtration experiments were conducted at room temperature (25 oC) under 300 rpm stirring. The

209

samples were PSf UF membranes, triblock copolymer UF membranes and annealed triblock

210

copolymer UF membranes in duplicate.



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211

2.9 BSA Absorption and Desorption on Membrane Surfaces

212

An Anton Paar SurPASS electrokinetic analyzer (Anton Paar, Austria) was used to monitor the zeta

213

potential change of membrane surfaces during the absorption and desorption process of BSA protein.

214

All the samples were wetted with DI water, and then were assembled on the adjustable gap cell

215

apparatus. The equipped Ag/AgCl electrodes were used to measure their streaming potential and the

216

zeta potential data was determined using the associated software from the streaming potential slope

217

versus pressure plots based on the Helmholtz-Smoluchowski approach.41 The experimental

218

temperature was maintained at 25 °C. For each run, one experiment cycle included three sequential

219

steps: baseline, BSA adsorption, and BSA desorption. During each step, the zeta potential of each

220

membrane sample was measured and recorded with running time using different background

221

electrolyte solutions, till reached to a plateau after 600 s. For both baseline and BSA desorption

222

measurement, the background electrolyte was 1 µM phosphate buffered saline (PBS, pH 7.2 ~ 7.4).

223

For BSA adsorption step, the background electrolyte was 0.5 g L-1 BSA in 1 µM PBS solution. All the

224

measurements were at 300 mbar.

225


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3. RESULTS AND DISCUSSION

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3.1 Synthesis and Characterization of PEG-b-PSf-b-PEG Triblock Copolymers

228

The PEG-b-PSf-b-PEG triblock copolymers have been demonstrated to be synthesized through the

229

nucleophilic substitution polymerization at a weak base when monomethyl poly(ethylene glycol)

230

(mPEG) was used as an end-capping reagent to react with chloro-terminated polysulfone.42

231

Polymerization stoichiometry was manipulated to ensure PSf with chlorine as end groups using a little

232

excessive 4, 4’-Chlorophenyl sulfone monomer during the condensation polymerization of 4,

233

4’-Chlorophenyl sulfone and Bisphenol A.26 As shown in Figure 1A, the one-pot approach was

234

applied to synthesize the chloro-terminated polysulfone first, followed by the in situ addition of mPEG

235

to obtain the PEG-b-PSf-b-PEG triblock copolymers. GPC was employed to measure the

236

time-dependent molecular weight change of the synthesized polysulfone. Figure 1B shows that the

237

number-average molecular weight of the synthesized polysulfone increased significantly from 2.5 ×

238

103 g mol-1 to 1.9 × 104 g mol-1 with the prolonged polymerization time from 1 h to 6 h. Further

239

prolonging reaction time to 12 h, the molecular weight of polysulfone did not change much compared

240

with the obtained polysulfone at 6 h, suggesting the polymerization of Bisphenol A and 4,



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

241

Page 20 of 55

4’-chlorophenyl sulfone completed in 6 h.35

A

O

CH3 C CH3

HO

OH

O Cl

O

S O

+

Cl

Cl

S O DMAc/Toluene K2CO3 155-190 oC O

CH3 C CH3

O

S O

n

Cl

H3C OCH2CH2 OH m 190 oC

O CH3O CH2CH2O

m

242 243

O

CH3 C CH3

O

S O

B

O n

S O

O CH2CH2 OCH2CH2 OCH3 m

C 1h 3h 6h 12 h

15

244 245

16

17

18

19

20

21

Retention time (min)

246

Figure 1. Synthesis and characterization of PEG-b-PSf-b-PEG triblock copolymer: (A) Synthesis scheme of

247

PEG-b-PSf-b-PEG triblock copolymer; (B) GPC traces of chloro-terminated polysulfone (Cl-PSf-Cl) products upon 1 h,

248

3 h, 6 h and 12 h polymerizations, respectively; (C) 1H NMR spectrum of PEG-b-PSf-b-PEG triblock copolymer. The

249

insets display the triplet peak at 4.16 ppm from the methylene protons of mPEG and the peak at 6.76 ppm from the aryl

250

protons at terminal 4, 4’-Chlorophenyl sulfone unit of the achieved product after the certain time of nucleophilic

251

substitution polymerization between Cl-PSf-Cl and mPEG varying from 0.5 h (①), 1 h (②), 1.5 h (③), to 2 h (④).

252

(Solvent: Chloroform-d, Temperature: 25 oC).


Page 21 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


253

Upon the complete polycondensation of Bisphenol A and 4, 4’-Chlorophenyl sulfone at 190 oC for 6 h,

254

mPEG was added as the end-capping reagent. 1H NMR was employed to monitor the nucleophilic

255

substitution of Cl-PSf-Cl with the hydroxyl groups of mPEG. The triplet peaks at 4.16 ppm from the

256

methylene protons of mPEG (4H, denoted as h in the polymer backbone shown in Figure 1C)

257

increasingly grew big with the prolonged reaction time from 0.5 h, 1.0 h, 1.5 h, to 2 h, indicating the

258

increasing mPEG was linked with PSf backbone (Figure 1C). Meanwhile, the peak at 6.76 ppm (4H,

259

denoted as g in Figure 1C) from the aryl protons at terminal 4, 4’-Chlorophenyl sulfone unit kept

260

constant as a marker even with the increased reaction time since the number of PSf chains did not

261

change any more with time once synthesis.26 The integrated area ratio of aryl protons at 6.76 ppm

262

from the PSf and methylene protons at 4.16 ppm from PEG was used as an indicator to determine the

263

incorporation of PEG into the PSf backbone, which should be 1.0 as the PEG worked as an

264

end-capping reagent to terminate both ends of the PSf backbone.43 With the prolonged reaction time

265

from 0.5 h to 2 h, the integrated area ratio of aryl protons from the PSf and methylene protons from

266

PEG remarkably decreased from 11.8 to 1.0, suggesting that the reaction of mPEG and Cl-PSf-Cl

267

completed within 2 h (Figure 1C inset). In addition, from 1H NMR analysis, the weight percentage of



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

268

mPEG in the PEG-b-PSf-b-PEG triblock copolymer was 28.5 wt. % based on the molecular weight of

269

PEG and PSf in the copolymer.44

270

271

To characterize the final product of PEG-b-PSf-b-PEG triblock copolymer, GPC, DSC, and TGA

272

were used to analyze its molecular weight, glass transition temperature, and the thermal

273

decomposition behavior, respectively. GPC results show that apparent molecular weight of the

274

synthesized PEG-b-PSf-b-PEG triblock copolymer was 1.6 × 104 g mol-1 with a relatively narrow

275

polydispersity index (PDI) 2.05 (Figure 2A). However, compared with the Mn of the intermediate

276

product Cl-PSf-Cl as 1.9 × 104 g mol-1, the apparent molecular weight of the final product

277

PEG-b-PSf-b-PEG triblock copolymer did not increase, which may due to the PEG incorporation in

278

the triblock copolymer increasing its solubility in THF solvent based on the like-dissolves-like

279

principle (Hansen solubility parameters of PSf, PEG, and THF are 22.9 MPa1/2, 21.6 MPa1/2, and

280

19.4 MPa1/2, respectively).43, 45 In addition, the size of PEG-b-PSf-b-PEG triblock copolymer was 5

281

nm in diameter in THF, measured through the scattering method using the Malvern zetasizer (Nano

282

ZS90), which was much smaller than the size of Cl-PSf-Cl as 10 nm in diameter in THF (Figure S3).


Page 22 of 55

Page 23 of 55

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283

Furthermore, different from the blending of PEG and PSf having their individual Tg peaks, the

284

PEG-b-PSf-b-PEG triblock copolymer has only one Tg peak, which further confirms that the PEG

285

block and PSf block are covalently bonded together, and the synthesized PEG-b-PSf-b-PEG triblock

286

copolymer has high purity without free PEG in the product. In addition, the PEG incorporation in the

287

PEG-b-PSf-b-PEG triblock copolymers significantly decreased its glass transition temperature (Tg) to

288

133.8 oC, compared with the Tg of Cl-PSf-Cl as 192.4 oC and the melt point of PEG as 66.6 oC

289

(Figure 2B), which confirms that the successful PEG incorporation in the triblock copolymer lowered

290

the temperature where polymer chain can move.43 Moreover, the PEG-b-PSf-b-PEG triblock

291

copolymer has great thermal stability to be used as the UF membrane material, since its Tg is still far

292

higher than the working temperature of UF membranes (usually below 60 oC).

293

294

TGA results present that the degradation of PEG-b-PSf-b-PEG triblock copolymers passed through

295

two stages including the thermal decomposition of PEG blocks at a low temperature between 390 oC

296

and 485 oC, and the thermal decomposition of PSf blocks at a high temperature between 510 oC and

297

685 oC (Figure S2). The weight percentage of mPEG blocks in triblock copolymer could be



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

298

calculated as 35.5 wt. % from the weight loss of PEG blocks from 390 oC to 500 oC (Table 1), which

299

was more reliable than 28.5 wt. % from the above 1H NMR analysis. Considering that the proton

300

signals in 1H NMR from PSf and PEG depend on their microenvironment in polymer chain and

301

solvent,46 the weight percentage of mPEG blocks in triblock copolymer could be underestimated. In

302

addition, the molecular weight of PEG-b-PSf-b-PEG triblock copolymer could be calculated when the

303

weight percentage of PEG was determined in the copolymer and the molecular weight of PEG was

304

measured accurately as 0.5 × 104 g mol-1 (Mn) in GPC with narrow PDI. As shown in Table 1, the Mn

305

of PEG-b-PSf-b-PEG triblock copolymer was calculated to be 3.5 × 104 g mol-1 when the weight

306

percentage of PEG was determined to be 28.5 wt. % from the 1H NMR analysis, which may

307

over-estimate the Mn of PEG-b-PSf-b-PEG triblock copolymer since the weight percentage of PEG

308

could be underestimated in 1H NMR. Moreover, from the TGA results, the weight percentage of PEG

309

was 35.5 wt. %, and the Mn of PEG-b-PSf-b-PEG triblock copolymer was calculated to be 2.8 × 104 g

310

mol-1, agreeing well with the molecular weight sum of Cl-PSf-Cl (1.9 × 104 g mol-1) and two chain

311

ends of mPEG (0.5 × 104 g mol-1) as 2.9 × 104 g mol-1 (Table 1). Above all, the PEG-b-PSf-b-PEG

312

triblock copolymer was successfully synthesized with 2.8 × 104 g mol-1 molecular weight containing


Page 24 of 55

Page 25 of 55

313

35.5 wt. % of PEG.

314

315

More important, based on the synthesis scheme, a series of PEG-b-PSf-b-PEG triblock copolymers

316

with different PEG contents could be prepared through tailoring the molecular weight of

317

chloro-terminated polysulfone backbone and PEG by manipulating the monomer ratios. The

318

PEG-b-PSf-b-PEG triblock copolymer with 35.5 wt. % of PEG was selected for the membrane

319

fabrication because high PEG contents in the copolymers give us a large window to investigate how

320

the PEG-b-PSf-b-PEG triblock copolymer behaves differently from PSf during the fabrication

321

process.

322 A

C

B )

100% o

Tg=192.4 C

80% Weight (%)

Heat flow (Exo

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


o

Tg=133.8 C o

Tm=66.6 C

60% 40% 20% 0%

14

16 18 20 Retention time (min)

0

50

100 150o 200 Temperature ( C)

250

400

600 o Temperature ( C)

800

323 324

Figure 2. Characterizations of various polymers including GPC traces (A), DSC curves (B), and TGA plots (C) of the

325

original homopolymer PEG (blue line), the intermediate product Cl-PSf-Cl (black line), and the final product

326

PEG-b-PSf-b-PEG triblock copolymer (red line), respectively.

327 328



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

329

Table 1. Characterizations of PSf, mPEG, Cl-PSf-Cl, and PEG-b-PSf-b-PEG triblock copolymer. Mn

Mn

Mn

PDI

mPEG

mPEG

Tg (oC)

(g mol-1)

(g mol-1)

(g mol-1)

(GPC)

(wt. %)

(wt. %)

(DSC)

(GPC)

(1H NMR)

(TGA)

(1H NMR)

(TGA)

PSfa

2.1×104

---

---

2.17

---

---

---

mPEG

0.5×104

---

---

1.05

---

---

66.6 (Tm)

Cl-PSf-Cl

1.9×104

---

---

2.05

---

---

192.4

PEG-b-PSf-b-PEG

1.6×104

3.5×104

2.8×104

2.05

28.5

35.5

133.8

Samples

330

Page 26 of 55

a. Solvay Udel P-3500

331

332

3.2 PEG Segregation on the UF Membrane Surfaces through the Self-assembly of

333

PEG-b-PSf-b-PEG Triblock Copolymer upon the Thermal and Solvent Annealing

334

Upon the synthesis of the well-defined PEG-b-PSf-b-PEG triblock copolymers, the non-solvent

335

induced phase separation (NIPS) method was applied to fabricate the UF membrane of

336

PEG-b-PSf-b-PEG triblock copolymer, without any other polymers or additives involved. It has been

337

well documented that the selectivity and water permeability of the phase-inversion polymeric UF

338

membranes could be tailored through varying the concentration of polymer casting solution, and the

339

membrane pore size was generally smaller and the water permeability was lower with the higher

340

concentration of polymer casting solution.47, 48 In order to make such a UF membrane with high


Page 27 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


341

bacteria rejection and high water permeability, the optimized block copolymer concentration was 12

342

wt.% in NMP. The casting solution was clear pale yellow homogeneous solution in NMP, similar as

343

the PSf casting solution. But, the viscosity of PEG-b-PSf-b-PEG in NMP was 43 cP, much less

344

viscous than the same concentration of PSf casting solution as 121 cP. The scattering results from

345

Malvern zetasizer found the dilute PEG-b-PSf-b-PEG triblock copolymer formed micelles of 46 nm in

346

diameter in NMP solvent (Figure S4), which were much larger than 5 nm in diameter of the polymer

347

size in THF solvent. According to the Hansen solubility parameters of PSf, PEG, and NMP as 22.9

348

MPa1/2, 21.6 MPa1/2, and 22.9 MPa1/2, respectively, the micelles of PEG-b-PSf-b-PEG triblock

349

copolymer might have PSf as the shell and PEG as the core in NMP solvent.43, 45, 49

350

351

After the precipitation of the PEG-b-PSf-b-PEG triblock copolymer casting solution in coagulation

352

water bath at 40 oC, the white membrane was obtained. SEM was used to study the surface and

353

cross-section morphologies of the membranes. Figure 3 presents that the PEG-b-PSf-b-PEG triblock

354

copolymer membrane had a highly porous surface with 10 ~ 20 nm pore size in diameter and an

355

asymmetric membrane structure with a thin spongy-like skin layer and a finger-like matrix, similar as



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

356

the PSf control membrane. It can be concluded that both of the PSf and PEG-b-PSf-b-PEG triblock

357

copolymers could form porous membranes without any pore-making reagent involved, at a low

358

concentration of 12 wt. % polymer casting solution. Moreover, the PSf membrane had a smooth

359

surface, but the PEG-b-PSf-b-PEG triblock copolymer membrane exhibited a clear micellar

360

morphology, which was due to the micelle formation of the PEG-b-PSf-b-PEG triblock copolymer in

361

the casting solution. The surface roughness measurement by AFM also shows that the Ra of the

362

PEG-b-PSf-b-PEG membrane is 21.5 nm, much higher than 13.4 nm of the PSf membrane, as shown in

363

Figure S6.

364

365

366

367

368

369

370


Page 28 of 55

Page 29 of 55

371 PSf

PEG-b-PSf-b-PEG Mean pore diameter 12 ± 3 nm

Mean pore diameter 16 ± 4 nm

Thickness 474 ± 108 nm

Thickness 363 ± 85 nm

Surface

Mean pore diameter 11 ± 3 nm

Annealed PEG-b-PSf-b-PEG

372 Thickness 466 ± 120

nm

Cross-section

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


373 374

Figure 3. SEM micrographs of the surface and cross-section morphologies of the PSf and PEG-b-PSf-b-PEG triblock

375

copolymer membranes as prepared and annealed at 90 oC in water for 16 h. The scale bars are 400 nm (top images) and

376

2 µm (bottom images) respectively.

377

378

To investigate the surface hydrophilicity of PEG-b-PSf-b-PEG triblock copolymer membranes, the

379

water contact angle of the membrane surfaces was measured to be 83.8 o against water, similar as 83.5

380

o

381

PEG-b-PSf-b-PEG triblock copolymer membrane surface had 15.6 % oxygen content, very close to

382

16.2 % oxygen content of the PSf membrane surface (Table 2). These results illustrate that the PEG

383

surface segregation was not strong enough to significantly improve the membrane surface

384

hydrophilicity during the NIPS, which agrees with the reported phenomena as not obviously

of the PSf membrane surface. The membrane surface elemental analysis by XPS found that the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 55

385

decreasing its water contact angle of the phase-inversion PSf-b-PEG membrane.30 Different from the

386

in situ surface segregation of hydrophilic polymers from the amphiphilic copolymers during the

387

phase-inversion membrane formation process,10,

388

PEG-b-PSf-b-PEG triblock copolymer in NMP made PSf open on the shell and made PEG squeeze

389

inside the core to minimize the entropy, and thus prevented the surface migration of PEG on the

390

membrane surfaces during the NIPS, since water could not easily and quickly pass through the PSf

391

shell to meet PEG. Thus, it needs extra energy of thermal annealing for water to meet and dissolve the

392

PEG blocks and to bring them out to the membrane surface.

26,

50,

51

the micelle formation of the

393 394

Table 2. XPS surface elemental composition analysis of the PEG-b-PSf-b-PEG triblock copolymer membranes upon

395

the annealing in water at 90 oC for a period of time. Surface elemental Oxygen area value (%)

Annealing

C-O-C/Ph-O-Ph

composition (Atomic %)

Membrane time (h)

C

O

S

Ph-O-Ph

C-O-C

S=O

PSf

0

81.0

16.2

2.9

41.4

0.0

58.6

0.0

PEG-b-PSf-b-PEG

0

82.0

15.6

2.8

30.5

28.0

41.5

0.9

8

80.3

17.2

2.5

29.6

30.4

40.0

1.0

12

79.0

18.9

2.1

29.4

30.7

39.9

1.0

16

78.2

19.8

2.0

26.0

37.9

36.1

1.5

24

81.2

16.8

2.1

27.7

33.7

38.6

1.2

396 397

The self-assembly of the PEG-b-PSf-b-PEG amphiphilic triblock copolymer was taken as a great


Page 31 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


398

advantage to tailor its membrane surface hydrophilicity and microstructure. The hydrophobic rigid

399

blocks of PSf and the hydrophilic flexible blocks of PEG do not like each other and phase separate on

400

a mesoscopic length scale allowing for the enrichment of PEG in its favorite thermodynamic

401

conditions.34 Combining the solvent and thermal annealing treatments of immersing the

402

PEG-b-PSf-b-PEG triblock copolymer membrane in water at 90 oC for a period of time, the membrane

403

surface hydrophilicity and chemical compositions were monitored to characterize the surface

404

segregation of PEG.

405

406

Figure 4A shows that the water contact angle of the PEG-b-PSf-b-PEG triblock copolymer membrane

407

surface dropped down significantly from 83.8 o to 30.0 o with the prolonged annealing time from 0 h to

408

16 h, illustrating that the PEG-b-PSf-b-PEG triblock copolymer membrane surface achieved the

409

increasing hydrophilicity upon the annealing treatments. The membrane surface elemental

410

composition shown in Table 2 presents that the element oxygen content increased from 15.6 % to

411

19.8 %, and the element sulfur content decreased from 2.8 % to 2.0 %, correspondingly, suggesting

412

that the increasing PEG amount was observed on membrane surface. To fit the O1s peak into Ph-O-Ph



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

413

at 533.1 eV, C-O-C at 532.3 eV, and S=O at 531.6 eV,44 the peak of C-O-C from PEG segments on the

414

membrane surface grew bigger, and its area percentage increased from 28.0 %, 30.4 %, 30.7 % to

415

37.9 % with the prolonged annealing time from 0 h, 8 h, 12 h, to 16 h (Figure 5). Meanwhile, the ratio

416

of C-O-C/Ph-O-Ph also increased from 0.9, 1.0, 1.0 to 1.5, correspondingly, on the PEG-b-PSf-b-PEG

417

triblock copolymer membrane surface upon the annealing treatments, further confirming the

418

increasing PEG enrichment upon the increasing annealing time. Interestingly, after correlating the

419

water contact angles and the surface elements on the PEG-b-PSf-b-PEG triblock copolymer

420

membrane surface (shown in Figure 4B), it was found that the water contact angles of the membrane

421

surface decreased with the increase of PEG enrichment on the membrane surface, further proving that

422

the improved hydrophilicity was attributed to the increased PEG on the membrane surface. All these

423

results conclude that the surface segregation and enrichment of PEG occurred and the membrane

424

surface hydrophilicity could be tailored upon the annealing treatments. However, further prolonging

425

the annealing time to 24 h, the area percentage of C-O-C decreased down to 33.7 %, the ratio of

426

C-O-C/Ph-O-Ph also dropped to 1.2, and the water contact angle of the membrane surface increased

427

up to 50.0 o, suggesting less PEG and more PSf was present on the membrane surface with the


Page 32 of 55

Page 33 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

428


improved hydrophobicity.

429

430

It is very interesting to find that the PEG surface segregation and membrane surface hydrophilicity

431

was greatly dependent on the annealing time. The annealing condition of cooking membrane at 90 °C

432

water is favor for PEG to activate and to migrate since water is a good solvent of PEG and 90 °C is

433

high above the Tg of PEG, but not for PSf. For the pristine PEG-b-PSf-b-PEG triblock copolymer

434

membrane, PSf dominated on the membrane surface as micelles and PEG was squeezed inside the

435

micelles to minimize the entropy. It was difficult for water to diffuse through PSf and to dissolve PEG

436

in the micelles. Thus, it took quite long time and high temperature for water to penetrate the PSf shell

437

to meet PEG, and the initial stage was generally long for most of the block copolymer films under the

438

thermal annealing.52 Once PEG solubilized in water and gained extra energy to move, it could reverse

439

the micelles with PEG on the shell and PSf on the core to significantly improve the membrane surface

440

hydrophilicity. Two chain ends of PEG in the PEG-b-PSf-b-PEG triblock copolymer give PEG

441

freedom to migrate on the membrane surface, while, they could drag PSf to move passively due to the

442

chemical bonds of PEG and PSf.34



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

443

Thus, under the equilibrium state, all PEG blocks microphase separated from PSf blocks and stayed in

444

a favorable membrane surface to squeeze PSf underneath the surface in a lamellar structure and to

445

minimize the entropy. While, excess energy was given to break the thermodynamic equilibrium of the

446

PEG and PSf, all PEG loved to move on the membrane surface with dragging PSf to move on the

447

surface and breaking their ideal lamellar structure. Generally, the equilibrium state is quite fragile and

448

easy to break. The membrane after 16 h annealing treatment might be close to the equilibrium state of

449

the microphase separation of PEG-b-PSf-b-PEG triblock copolymer. For 12 h annealing treatment, the

450

membrane surface has limited PEG enrichment to improve its hydrophilicity. But for 24 h annealing

451

treatment, PSf block would be dragged to move towards the membrane surface passively when two

452

ends of PEG had excess energy and driving force to enrich on the membrane surfaces under the

453

overcooked excess annealing conditions. Thus, the surface movement of PSf was observed on the

454

membrane surface when the annealing time was 24 h even at 90 oC lower than its Tg.

455


Page 34 of 55

Page 35 of 55

A

B 1.6

C-O-C to Ph-O-Ph Ratio

100

o

Water contact angle ( )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 60 40 20 0 0h

8h

12h

16h

24h

1.4 1.2 1.0 0.8 0.6

Annealing time (h)

100

80

60

40

20

0

o

Water contact angle ( )

456 457

Figure 4. (A) Water contact angles of the PEG-b-PSf-b-PEG triblock copolymer membranes upon the annealing in

458

water at 90 oC for 0 h, 8 h, 12 h, 16 h and 24 h, respectively. The water droplets on the membrane surface were imaged

459

upon touching the membrane surfaces for 40 s. (B) The C-O-C to Ph-O-Ph ratio versus water contact angles of the

460

annealed PEG-b-PSf-b-PEG triblock copolymer membrane surfaces.

461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 55

479 PSf

Ph-O-Ph, 533.1 eV

S=O, 531.6 eV Area (%) 58.6

Area (%) 41.4

480

540

538

536

534

532

PEG-b-PSf-b-PEG, Annealed 0 h

530

528

526

C-O-C, 532.3 eV

Area (%) 30.5

Area (%) 28.0

Area (%) 29.6

Area (%) 30.4

Area (%) 29.4

Area (%) 30.7

Area (%) 26.0

Area (%) 37.9

Area (%) 27.7

Area (%) 33.7

481 Annealed 8 h

482 Annealed 12 h

483 Annealed 16 h

484 Annealed 24 h

485 486 487

540

538

536

534

532

530

528

526

488

Figure 5. The O1s XPS spectra of PEG-b-PSf-b-PEG triblock copolymer membrane surface upon annealing in water at

489

90 oC for 0 h, 8 h, 12 h, 16 h, and 24 h, respectively.

Binding Energy (eV)


Page 37 of 55

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


490

To directly visualize the PEG surface enrichment on the membrane surface, the Dragendorff reagent

491

was used to selectively stain the PEG blocks on the membrane surface and was visualized at 461 nm

492

under the excitation of 405 nm using CSFM.36 Confocal images shown in Figure 6 found that no

493

fluorescence was observed on the PSf membrane, but a well-distributed green fluorescence was

494

observed all over on the PEG-b-PSf-b-PEG triblock copolymer membrane surface, and the green

495

fluorescence from the stained PEG blocks was much stronger for the annealed membrane surface

496

upon 16 h annealing in water at 90 °C than the pristine triblock copolymer membrane due to the

497

thermal and solvent induced self-assembly of block copolymers.52 These confocal results clearly

498

demonstrate the surface segregation and the enrichment of PEG blocks upon annealing.

499 PSf

PEG-b-PSf-b-PEG


500 501 502 503 504 505 506

Figure 6. Confocal scanning fluorescence images of the PSf and PEG-b-PSf-b-PEG triblock copolymer membrane

507

before and after 16 h annealing in water at 90 °C. The samples were stained with the Dragendorff reagent and then

508

visualized under the excitation of 405 nm and the emission of 461 nm. The scale bars are 100 µm.

509



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

510

3.3 Filtration Properties of PEG-b-PSf-b-PEG Triblock Copolymer Membrane upon Annealing

511

To investigate the influence of annealing on the membrane properties and filtration performances, the

512

water flux and rejection of the PSf and PEG-b-PSf-b-PEG triblock copolymer membrane were

513

measured before and after the same annealing conditions as cooking membranes in water at 90 °C for

514

16 h. For the PSf membrane, the annealing treatment did not change its water flux much, but

515

decreased its PEG rejection from 87.6 ± 1.0 % to 83.1 ± 2.0 % as shown in Table 3. Interestingly, the

516

annealing treatment dramatically changed the filtration performances of the PEG-b-PSf-b-PEG

517

triblock copolymer membrane with improving its water flux by 194.8 % from 397 ± 5 LMH bar to

518

1231 ± 115 LMH bar, and decreasing its PEG275kDa rejection by 5.6 % from 83.3 ± 1.0 % to 77.7 ±

519

3.4 %, compared with the pristine triblock copolymer membrane before annealing. The annealing

520

process slightly increased its surface pore size from 12 ± 3 nm to 16 ± 4 nm in diameter, improved its

521

porosity from 85.2 ± 1.3 % to 87.1 ± 0.5 %, and relatively decreased its top skin layer thickness from

522

474 ± 108 nm to 363 ± 85 nm for the PEG-b-PSf-b-PEG triblock copolymer membrane. Therefore, it

523

was a good advantage for the thermal and solvent annealing treatment to improve the membrane

524

surface hydrophilicity and filtration performances simultaneously. The tailored UF membrane with


Page 38 of 55

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


525

high flux and super hydrophilicity can be used for the concentration of the fermentation broth, which

526

is shown in Figure S5. In addition, the annealed PEG-b-PSf-b-PEG triblock copolymer membrane

527

demonstrated to present good mechanical strength withstanding the pressure of 0.3 MPa under the

528

dead-end filtration operation and the shear stress under the cross-flow of the UF operation.

529

530

It has been reported that the self-assembly of amphiphilic block copolymers could be induced through

531

solvent or thermal annealing where one of the blocks improved its chain mobility and phase separated

532

from the other blocks.53, 54 Under the annealing condition, water is a good solvent for PEG blocks but

533

a non-solvent for PSf blocks, and 90 °C is far higher than the Tm of PEG blocks (66.6 °C) but lower

534

than the Tg of PSf blocks (192.4 °C, Table 1). Thus, the PEG chains locally solubilized in water and

535

achieved their mobility to assemble at the interface of water/membrane pore, leading to the PEG

536

enriched on the membrane surface and the pore surface. Meanwhile, the chemical bonding with PEG

537

chains and the conformational entropy loss of the unlike blocks drove the PSf chains squeeze

538

underneath the PEG layer, like lamellar structure with PEG on top (shown in TOC). Furthermore,

539

upon the evaporation of water in the following air dry process at room temperature, the PEG chains



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

540

dehydrated and shrank, leading to the increase of pore size, and the thinning of the skin layer, but PSf

541

chains lost their mobility and squeezed in the membrane skeleton. Both the surface elemental analysis

542

by XPS and surface hydrophilicity characterization by contact angle measurement had confirmed that

543

PEG was assembled and enriched on the membrane surface upon the annealing, suggesting the surface

544

rearrangement of PEG and PSf. Moreover, PEG is also known as a pore-making agent, and its

545

mobility could cause the rearrangement of PEG and PSf chains to release the free volume and thus

546

produce pores, effectively increasing the porosity of the PEG-b-PSf-b-PEG triblock copolymer

547

membrane.25, 34 Above all, the self-assembly of PEG-b-PSf-b-PEG triblock copolymers provided a

548

simple and unique way to tailor the membrane surface structure and properties upon the annealing

549

process in water at high temperature. Compared with other popular surface modification strategies, the

550

PEG surface enrichment was expected to occur homogeneously at both external and internal pore

551

surfaces (three-dimensional surfaces) through this unique self-assembly process. The covalent linkage

552

between the hydrophilic blocks PEG and the hydrophobic membrane forming blocks PSf gives

553

another key advantage of preserving the membrane surface integrity during use and avoiding the

554

leaking of additives to the permeate and thus to induce the associated toxicity issues.


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555

Table 3. The annealing influences on pure water flux (PWF), porosity, mean pore size, skin layer thickness, and PEG

556

rejection of the PSf and PEG-b-PSf-b-PEG triblock copolymer membranes. Annealing condition: in water at 90 °C for

557

16 h. PWF

Rejection

Average Pore

Skin layer

Porosity

(LMH bar)

(%)a

Diameter

Thickness

(%)

(nm)

(nm)

Membrane

PSf

344 ± 23

87.6 ± 1.0

11 ± 3

466 ± 120

83.0 ± 1.2

Annealed PSf

328 ± 12

83.1 ± 2.0

11 ± 3

472 ± 113

82.9 ± 0.5

PEG-b-PSf-b-PEG

397 ± 5

83.3 ± 1.0

12 ± 3

474 ± 108

85.2 ± 1.3

Annealed

1231 ± 115

77.7 ± 3.4

16 ± 4

363 ± 85

87.1 ± 0.5

PEG-b-PSf-b-PEG

558

a.

559

3.4 Antifouling Performances of the PEG-b-PSf-b-PEG Triblock Copolymer Membrane

560

PEG segregation on the membrane surface could endow the PEG-b-PSf-b-PEG triblock copolymer

561

membrane with excellent hydrophilicity and provide a hydration layer acting as a repulsive barrier to

562

foulant with great potential to achieve the antifouling performance.16 In the current study, the fouling

563

behavior of the annealed PEG-b-PSf-b-PEG triblock copolymer membrane was systematically

564

investigated through the surface zeta potential monitoring upon the absorption and desorption of

565

protein and also through the flux monitoring during filtration runs using a model protein foulant BSA.

566

Dynamic adsorption and desorption process of model protein BSA on the membrane surface was

567

investigated to determine whether the PEG enriched membrane surface could have antifouling

275 kDa PEG Rejection.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

568

properties towards the protein. The zeta potential of the membrane surface was monitored upon the

569

adsorption and desorption of BSA. The results shown in Figure 7 found that all three different

570

membranes shared the similar trends of their zeta potential increasing upon the absorption of BSA,

571

and decreasing after the desorption of BSA. It is easy to understand that the membrane surfaces were

572

covered with BSA upon its absorption and their surface zeta potential increase was attributed to the

573

accumulated BSA since BSA had the zeta potential of -10.6 mV in PBS solution, higher than all of the

574

three membrane surfaces. Furthermore, the membrane surface with the excellent protein antifouling

575

performance could recover to its pristine state and recover its original zeta potential if the absorbed

576

BSA could be washed out and removed completely during the desorption process.41 Figure 7 presents

577

that the annealed PEG-b-PSf-b-PEG triblock copolymer membrane surface could recover 90.3 % of

578

zeta potential from -19.5 mV to its original value of -20.0 mV after the BSA absorption and

579

desorption process, exhibiting much better zeta potential recovery than 46.0 % of the pristine

580

PEG-b-PSf-b-PEG triblock copolymer membrane without PEG enrichment on surface from -19.3 mV

581

to its original value of -24.9 mV. Thus, it can be concluded that the annealed PEG-b-PSf-b-PEG

582

triblock copolymer membrane surface greatly improved the protein antifouling performance once PEG


Page 42 of 55

Page 43 of 55

583

was enriched on the membrane surface. However, the pristine PEG-b-PSf-b-PEG triblock copolymer

584

membrane showed a worse zeta potential recovery than 65.3 % of the PSf membrane, which may be

585

due to the observed rough micellar surface, shown in Figure 3 and Figure S6.

586 PSf

PEG-b-PSf-b-PEG

Adsorption

Baseline

Desorption

Adsorption

Baseline

-10


Desorption

-30

0

600

587

1200

-15 -20 5.6 mV -25 -30

-20 0.5 mV

-30

-35

1800

0

Time (s)

Sample

Desorption

-10

Zeta Potential (mV)

1.7 mV

-20

Adsorption

Baseline

-10

Zeta Potential (mV)

Zeta Potential (mV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


600

1200

1800

0

Time (s)

600

1200

1800

Time (s)

Baseline

Adsorption

Desorption

Recovery

(mV)

(mV)

(mV)

percentage (%)

PSf

-19.2

-14.2

-17.5

66.0

PEG-b-PSf-b-PEG

-24.9

-14.6

-19.3

45.6

Annealed

-20.0

-14.8

-19.5

90.4

PEG-b-PSf-b-PEG

588

Figure 7. The zeta potential change of the PSf membranes, pristine PEG-b-PSf-b-PEG triblock copolymer membranes,

589

and annealed PEG-b-PSf-b-PEG triblock copolymer membranes upon the absorption and desorption of BSA on the

590

membrane surfaces. For both of the baseline and BSA desorption measurements, the background electrolyte was 1 mM

591

PBS (pH 7.2 ~ 7.4). For the BSA adsorption step, the background electrolyte was 1 g L-1 BSA in 1 mM PBS solution.

592

The annealed condition: in water at 90 °C for 16 h.

593 594

To study the protein fouling effect on the membrane filtration performance, the water flux was

595

monitored in the dead-end filtration process with 15 mg L-1 BSA solution in 10 mM ionic strength



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

596

(9.9 mM NaCl and 0.1 mM NaHCO3) as the feed. For all filtration runs, an initial flux of 200 LMH

597

bar-1 was maintained through the adjusted operating pressure to ensure an equitable comparison of

598

antifouling property for all different membranes,55 and the rigorous stirring (500 rpm) near the

599

membrane surface was carried out to minimize the concentration polarization of BSA.56 Therefore, the

600

flux decline was mainly caused by protein fouling. Physical cleaning was performed to remove the

601

protein foulants through backwashing the membrane with skin layer facing to the bottom in the cell,

602

and the recovered flux was attributed to the reversible fouling (Rr), and the non-recovered flux was

603

recognized as irreversible fouling (Rir). Figure 8 presents that all three different membranes suffered

604

their flux decline upon the deposition of BSA, and recovered their flux partially upon the physical

605

cleaning of removing BSA in the tested three cycles. For example, the pristine PEG-b-PSf-b-PEG

606

membrane and PSf membrane suffered an instant rapid flux decline in 20 minutes to 58.7 % and 58.9 %

607

of their initial flux due to instant fouling, and then linearly decreased water flux to 51.3 % and 39.7 %

608

of their initial flux at the end of the fouling cycle, respectively, where the flux still exhibited a

609

decreasing tendency. After the backwashing operation, the pristine PEG-b-PSf-b-PEG triblock

610

copolymer membrane and PSf membrane achieved only a slight increase of Rr (72.9 % and 74.3 %)


Page 44 of 55

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611

and the Rir (27.1 % and 25.7 %), indicating their poor antifouling property. In addition, the pristine

612

PEG-b-PSf-b-PEG triblock copolymer membrane did exhibit weaker fouling resistance than the PSf

613

homopolymer membranes due to its rough micellar surface morphology. The AFM images shown in

614

Figure S6 illustrate that the surface roughness of PEG-b-PSf-b-PEG triblock copolymer membrane is

615

21.5 nm (Ra), much higher than 13.4 nm of PSf membrane. In a sharp contrast, the annealed

616

PEG-b-PSf-b-PEG triblock copolymer membrane maintained 95.0 % of the initial flux after the instant

617

flux decline, then soon attained a plateau at 61.7 % as BSA foulant accumulated on the membrane

618

surface, and recovered more than 86.4 % of the initial water flux with only 13.6 % of Rir upon the one

619

cycle backwashing operation, demonstrating the enhanced antifouling property of the annealed

620

PEG-b-PSf-b-PEG triblock copolymer membrane. Moreover, this high water flux recovery (71.1 %

621

and 70.8 %, Figure 8) was consistently obtained in the further two filtration cycles, suggesting the

622

antifouling characteristics of the annealed PEG-b-PSf-b-PEG triblock copolymer membrane were

623

greatly improved once PEG was enriched on the three-dimensional membrane surfaces and pore

624

walls.



PSf

PEG-b-PSf-b-PEG

100

Cycle-3

Cycle-2

Cycle-1

Normalized Flux (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 55


Rinse Rinse

Rinse

80

60

40 0

50

100

150

200

250

Time (min)

625 Cycle

Reversible fouling (%) PSf

PEG-b-PSf-b-PEG

Irreversible fouling (%) Annealed

PSf

PEG-b-PSf-b-PEG

PEG-b-PSf-b-PEG


I

74.3

72.9

86.4

25.7

27.1

13.6

II

64.8

57.9

71.1

35.2

42.1

28.9

III

62.4

55.3

70.8

37.6

44.3

29.2

626

Figure 8. The flux decline of the PSf membranes, pristine PEG-b-PSf-b-PEG triblock copolymer membranes, and

627

annealed PEG-b-PSf-b-PEG triblock copolymer membranes with time upon BSA fouling in the three sequential

628

dead-end filtration cycles. The filtration operation included three steps and three cycles: pure water permeation, BSA

629

solution filtration (15 mg L-1 BSA solution with 10 mM ionic strength including 9.9 mM NaCl and 0.1 mM NaHCO3),

630

and backwashing of the membranes with 500 mL DI water filtered through under 500 rpm stirring for 30 min. The

631

recovery water flux (Jr) was measured to determine the reversible fouling (Rr) using the following equation: Rr=(Jr/Jo)

632

×100 %, where J0 is the initial pure water flux. The initial flux of all the membranes is 200 LMH bar, and the

633

normalized flux was defined as the relative percentage of real-time flux to the initial pure water flux (J/J0).

634

Experimental conditions for all filtration cycles were 500 rpm stirring at room temperature (25 ± 1 °C). The annealing

635

condition: in water at 90 °C for 16 h.

636


Page 47 of 55

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637

4. CONCLUSIONS

638

In conclusion, a particular type of PEG-b-PSf-b-PEG amphiphilic block copolymers was synthesized

639

with mPEG as two ends to achieve the mobility of hydrophilic polymer chains. Without any other

640

polymer or additive involved, the UF membranes of PEG-b-PSf-b-PEG triblock copolymer was

641

fabricated through NIPS. The thermal and solvent annealing treatments in water at 90 °C for 16 h was

642

performed to induce the microphase separation of PEG-b-PSf-b-PEG amphiphilic triblock copolymers

643

and to direct the PEG surface segregation on the membrane surfaces. The annealed PEG-b-PSf-b-PEG

644

triblock copolymer membrane significantly enhanced its water flux resulting from the increased mean

645

pore size with the improved porosity, as well as the decreased skin layer thickness, upon annealing.

646

More importantly, the PEG-b-PSf-b-PEG triblock copolymer membrane surface turned from

647

hydrophobic to hydrophilic upon annealing with PEG enrichment on the surface, and exhibited the

648

improved protein antifouling performances. Our research opens a new avenue to fabricate high

649

performance UF membranes using amphiphilic block copolymers and to tailor their pore structure and

650

surface properties through a simple and effective annealing treatment.

651

652



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

653

ACKNOWLEDGMENTS

654

The authors gratefully acknowledge the funding support from National Natural Science Foundation of

655

China (No. 21476249), Key Science and Technology Program of Shandong Province (No.

656

2014GHY115021), and Key Science and Technology Program of Yantai City (No. 2015ZH063).

657

ASSOCIATED CONTENT

658

Supporting Information

659

Preparation of the Dragendorff reagent, derivative thermogravimetry curves of Cl-PSf-Cl, mPEG and

660

PEG-b-PSf-b-PEG triblock copolymers, the size of Cl-PSf-Cl and PEG-b-PSf-b-PEG triblock

661

copolymers in THF and NMP solvent. This material is available free of charge via the Internet at

662

http://pubs.acs.org.

663

AUTHOR INFORMATION

664

Corresponding Authors

665

*Yunxia Hu, E-mail: [email protected].

666

Author Contributions

667

All authors have given approval to the final version of the manuscript. Ning Wang and


Page 48 of 55

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668

Tao Wang contributed equally to this work.

669

Notes

670

The authors declare no competing financial interest.

671 672

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673

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Surface Segregation of PEG Blocks 279x236mm (72 x 72 DPI)


Tailoring Membrane Surface Properties and Ultrafiltration

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