Using the Green Solvent Dimethyl Sulfoxide (DMSO) to Partly Replace

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Materials and Interfaces

Using the Green Solvent Dimethyl Sulfoxide (DMSO) to Partly Replace Traditional Solvents and Fabricating PVC/PVC-g-PEGMA Blended Ultrafiltration Membranes with High Permeability and Rejection Wancen Xie, Tong Li, Chen Chen, Haibo Wu, Songmiao Liang, Haiqing Chang, Baicang Liu, Enrico Drioli, Qingyuan Wang, and John C. Crittenden Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 25 Mar 2019 Downloaded from http://pubs.acs.org on March 25, 2019

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Industrial & Engineering Chemistry Research

Using the Green Solvent Dimethyl Sulfoxide (DMSO) to Partly Replace Traditional Solvents and Fabricating

PVC/PVC-g-PEGMA

Blended

Ultrafiltration Membranes with High Permeability and Rejection Wancen Xie, † Tong Li, ‡ Chen Chen, § Haibo Wu, † Songmiao Liang, ∥ Haiqing Chang, † Baicang Liu, †,* Enrico Drioli, ⊥ Qingyuan Wang, † and John C. Crittenden # †

College of Architecture and Environment, State Key Laboratory of Hydraulics and Mountain

River Engineering, Institute of New Energy and Low-Carbon Technology, Institute for Disaster Management and Reconstruction, Sichuan University, Chengdu, Sichuan 610207, PR China

*

Corresponding author.

Tel.: +86-28-85995998; fax: +86-28-62138325; e-mail: [email protected]; [email protected] (B. Liu).

ACS Paragon Plus Environment

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Industrial & Engineering Chemistry Research 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

‡ Key

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Laboratory for Water Quality and Conservation of the Pearl River Delta, Ministry of

Education, Institute of Environmental Research at Greater Bay, Guangzhou University, Guangzhou 510006, PR China § Litree

Purifying Technology Co., Ltd., Haikou, Hainan 571126, PR China

∥

Vontron Technology Co., Ltd., Guiyang, Guizhou 550018, PR China

⊥

Institute on Membrane Technology ITM-CNR, Via P. Bucci 17/C, 1-87030 Rende, CS, Italy

#

Brook Byers Institute for Sustainable Systems, School of Civil and Environmental

Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA

KEYWORDS: Poly (vinyl chloride) PVC; Amphiphilic copolymer; Green solvents; Dimethyl sulfoxide DMSO; High-flux membrane; High rejection membrane

ABSTRACT: Traditional solvents are harmful to human health and the environment. Here, we use a green solvent, dimethyl sulfoxide (DMSO), to partly replace traditional solvents as well as improve membrane performance. The amphiphilic copolymer poly (vinyl chloride)-graft-poly (ethylene glycol) methyl ether methacrylate (PVC-g-PEGMA) is blended with PVC to improve the membrane performance. PVC can not dissolve in DMSO, so based on the Hansen solubility parameter calculation, we investigated the mixture solvents of traditional solvents and DMSO. We found that membranes fabricated by solvent 1-methyl-2-pyrrolidinone (NMP)/ N, Ndimethylacetamide (DMAc)/DMSO=4/3/3 had the highest pure water flux of 891.54 ± 64.41 L m-2 h-1 bar-1 and the highest sodium alginate (SA) rejection of 94.7 ± 1.3 %. Other studies


2

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have rarely reported modified PVC membranes with such good performance. This membrane was a successful attempt to use a green solvent in membrane fabrication, meeting the challenges of sustainability in chemical enterprises.

INTRODUCTION Poly (vinyl chloride) (PVC) is considered to be a good membrane material for its good physical, mechanical and chemical properties, such as its stiffness, low-cost, and good resistance to acids, alkalis and chlorine conditions.

1-3, 5

1-3

long lifetime

4

However, it is hydrophobic,

inducing severe membrane fouling mainly by colloids, microorganisms and natural organic matter (NOM). 4, 6 Therefore, proceeding hydrophilic modification is essential. 4 Among the various methods of hydrophilic modification (e.g., surface coating,

7, 8

surface

grafting, 8, 9 and blending 10-13 ), blending is regarded as the most suitable method for its various advantages, including being a single-step process, having long-term stability and being suitable for hollow fiber membranes.

3, 14, 15

The blending additives are usually derivative amphiphilic

copolymers of polymers due to their good compatibility and stability, 2 such as poly(vinylidene fluoride)-graft-poly(oxyethylene methacrylate) (PVDF-g-PEGMA),

16

polyacrylonitrile-graft-

poly(ethylene oxide) (PAN-g-PEO) 17 and polysulfone-graft-poly(ethylene glycol) (PSf-g-PEG). 18

However, PVC blended with its derivative amphiphilic copolymer has seldom been reported.

Ahn et al. 19-21 successfully synthesized PVC-g-PEGMA via atom transfer radical polymerization (ATRP) to fabricate graft copolymer membranes. Our group used PVC to blend with PVC-gPEGMA for the first time, fabricating membrane with high flux and good antifouling property. 22 This blending method is much cheaper than that of using copolymer only, because a small amount of copolymer is used for the blending method, and the copolymer is costly due to the complex synthesis process. Also, it effectively improved the membrane performance.


3


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When fabricating polymeric membranes via non-solvent-induced phase separation (NIPS), solvents play an important role in determining the membrane properties 15, 23 by influencing both polymer-solvent 15, 24, 25 and solvent-nonsolvent interactions. 25-27 Based on the Hansen solubility parameter (HSP), the affinity of the polymer-solvent can be investigated quantitatively. The better the compatibility, the stronger the solvent power, indicating that the polymer molecules can be easily dissolved in this solvent, thus a uniformly distributed membrane structure can be formed.

15

Li et al.

24

used mixed solvents to improve the solvent power to PVDF, making the

precipitation rate faster and the flux higher. If the solvent power is weak, polymer molecules will aggregate, resulting in a sponge-like structure, low porosity and low pure water flux

15, 24

Too

weak solvent poor will either fail to dissolve the polymer into solution or create a partially dissolved inhomogeneous mixture, neither of which would be fit for NIPS casting. Meanwhile, if the interaction between solvent and non-solvent is stronger, it will lead to an instantaneous liquid-liquid separation, forming finger-like macrovoids. 27 Solvents commit the vast majority of harm to the environment, health and safety (EHS) in membrane manufacturing.

28, 29

Common solvents for fabricating PVC membranes are DMAc,

tetrahydrofuran (THF) and NMP. However, all of these are toxic and hazardous,

30, 31

which is

contrary to the definition and principles of green solvents 29 and green chemistry. 32, 33 Therefore, it is essential to partly/overall substitute these solvents with green and sustainable ones and reduce the use of toxic solvents. As a paradigm of green solvents, DMSO is nontoxic and recyclable,

34

and there has been some researches on fabricating membrane by DMSO.

34-39

However, no PVC membranes are synthesized by DMSO because it cannot dissolve in DMSO directly. So trying to mix DMSO with traditional solvents to reduce harm to the environment and


4

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obtain membranes with good properties is an active research. The hazard statements of some solvents are summarized in Figure 1.

Figure 1. The hazard statements of common organic solvents. (Note: All information is according to Regulation (EC) NO. 1272/2008) Our previous study investigated the optimum addition of PVC-g-PEGMA blended with PVC using DMAc as a solvent to fabricate antifouling membranes,

22

but the effect of the solvent,

especially a green solvent, has never been investigated in this system. Therefore, in this study we added the green solvent DMSO to the membrane casting system for the first time and examined the effect of different solvents on the PVC blended PVC-g-PEGMA membranes. Four possible suitable solvents were used: DMAc, THF, NMP and the green solvent DMSO. Based on HSP, a certain proportion of DMSO was mixed with the traditional solvents. The properties and performance of the membranes were systematically investigated.


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MATERIALS AND METHODS Chemicals and Materials. Poly (vinyl chloride) (PVC, high molecular weight), poly (ethylene glycol) methyl ether methacrylate (PEGMA, Mn=500 g/mol), 1, 1, 4, 7, 10, 10 hexamethyltriethylenetetramine (HMTETA, 97 %), copper (I) chloride (CuCl, ≥ 99.995 %), 1methyl-2-pyrrolidinone (NMP, anhydrous, 99.5 %), N, N-dimethylacetamide (DMAc, 99 %), dimethyl sulfoxide (DMSO, anhydrous, ≥ 99.9 %), tetrahydrofuran (THF, anhydrous, 99.9 %), sodium alginate (SA) and sodium chloride (NaCl, reagent grade, 99 %) were obtained from MilliporeSigma (St. Louis, MO, USA). Methanol (99.9 %) was obtained from Kelong Chemical (Chengdu, China). Synthesis of the Graft Copolymer PVC-g-PGEMA. The steps for synthesizing the copolymer PVC-g-PEGMA were similar to Ahn et al. and our former work. 19-22 First, 6.75 g of PVC were dissolved in 50 mL of NMP in a conical flack at 60 ℃ for 24 h with a magnetic stirrer (Corning, USA), stirring at 500 rpm. After the solution was cooled to 25 ℃, 50 mL of PEGMA, 0.1 g of the catalyst CuCl, and 0.23 mL of the initiator HMTETA were added to the solution. Subsequently, nitrogen gas was bubbled through the reaction for 30 min with the magnetic stirrer stirring. Then, the flask was put into a 90 ℃ silicon oil bath. The reaction proceeded for 19 h with the magnetic stirrer stirring. Then, THF and methanol were used to dilute and precipitate the mixture solution, respectively. Finally, the PVC-g-PEGMA was obtained by drying in a vacuum oven (Memmert, Germany. The type of diaphragm pump: 412543, ILMVAC GmbH, Germany) for 12 h at 25 ℃. These steps did not take into account the removal of CuCl, because the amount of CuCl in the casting solution was relatively small and some was converted to copper (II) chloride (CuCl2), which was easily to be wash out. So it had negligible effect on the ultimate membranes. 40


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Membrane Casting. The steps of fabricating PVC ultrafiltration membranes were similar to the previous work of our group. 4, 41, 42 The components (see Table 1) were added to a 125 mL conical flask and heated at 60 ℃ with a magnetic stirrer stirring at 500 rpm for at least 24 h. After verifying the complete dissolution of all polymers, the solution was degassed for approximately 2 h to remove the gas bubbles. Subsequently, a doctor blade (purchased from Paul N. Gardner Company, Inc., Pompano Beach, Florida, USA) with a ~ 200 μm gate height was used to cast the solution on a first-grade surface optical mirror. Then, the mirror was immersed into a coagulation bath of deionized (DI) water for 24 h at 25 ℃. M5-NMP/THF/DMSO 4/3/3 had the volatile solvent THF, and 30 s of evaporation time before immersing was preceded due to it can improve the membrane performance. The details of the comparison are in the Supporting Information (SI). After that, the membrane was taken out of the coagulation bath carefully, and a washing bottle full of DI water was used to rinse the membrane surface. Finally, the membrane was stored in DI water at 4 ℃.

Table 1. Compositions of PVC membrane casting solutions.

Membrane

PVC (g)

PVC-gPEGMA (g)

NMP DMSO (g) (g)

DMAc (g)

THF (g)

Additive/ PVC (%, w/w)

M1-DMAc

12

1.2

-

-

86.8

-

10

M2-NMP

12

1.2

86.80

-

-

-

10


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M3-NMP/DMSO 7/31

12

1.2

60.76 26.04

-

-

10

M4-P2 NMP/DMSO 7/3

12

0

61.60 26.40

-

-

0

M5-NMP/THF/DMSO 4/3/33

12

1.2

34.72 26.04

26.04

10

M6-NMP/DMAc/DMSO 4/3/34

12

1.2

34.72 26.04

-

10

26.04

Note: NMP/DMSO=7/31 indicates that the weight ratio of NMP/DMSO is 7/3. P2 indicates pure PVC. NMP/THF/DMSO=4/3/33 indicates that the weight ratio of NMP/THF/DMSO is 4/3/3. NMP/DMAc/DMSO=4/3/34 indicates that the weight ratio of NMP/DMAc/DMSO is 4/3/3.

Model Foulant. Sodium alginate (SA), which was used as model for extracellular polymeric substances (EPS),

41

was added to DI water in a flask and mixed until being completely

dissolved, then the stock solution of 2 g/L was stored at 4 ℃. In the fouling experiments, the SA concentrations were determined using a UV-vis spectrometer (Thermo Orion Aquamate 8000, USA) at a constant wavelength of 210 nm. The wavelength was determined by full spectrum scanning of the SA solution. The rejection of SA (RSA) was calculated by equation (1).

 C  R SA  %  = 1- p  100  Cf  (1) where Cp and Cf (mg/L) refer to the concentrations of SA in the permeate and feed solutions. Membrane Characterization. The X-ray photoelectron spectroscopy (XPS) (Axis Ultra, Kratos Analytical Ltd., UK) was used to study the near-surface compositions of PVC membranes to a depth of less than 5 nm. Survey XPS spectra were obtained over 0-1100 eV with a resolution of 1 eV.


8

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The surface and cross-sectional morphologies of the membranes were obtained by fieldemission scanning electron microscopy (FESEM) (JSM-7500F, JEOL Ltd., Tokyo, Japan) with a 5 kV acceleration voltage. The samples for cross-sectional morphologies were pretreated by being frozen in liquid nitrogen for over 15 min. All samples were sputter coated with a ~ 2 nm gold layer. The overall membrane porosity was measured by its dry-wet weight. The 2 × 2 cm2 membrane sample was soaked in DI water for at least 24 h and then weighed after mopping. Next, the wet membrane was dried under a vacuum at 60 ℃ for 24 h, and then the dry weight was measured. Two repeat experiments for every membrane were taken. The porosity was calculated by equation (2). 5, 43, 44 ε % =

Ww -Wd 100 ρ w Aδ

(2)

where ε (%) is the membrane porosity, Ww (g) is the wet sample weight, Wd (g) is the dry sample weight, ρw (g/cm3) is the density of pure water (0.998 g/cm3), A (cm2) is the area of the wet membrane (4 cm2) and δ (cm) is the thickness of the wet membrane. A KRÜSS DSA25S instrument (KRÜSS GmbH, Germany) was used to measure the water contact angle of the membrane surface. In every experiment, 2 μL of DI water were dropped on the membrane surface, and the dynamic contact angle was measured every 10 seconds. The whole process lasted 170 s. For each membrane, at least 9 repeat measurements at different locations were taken. A Fourier transform infrared (FTIR) spectrometer with a germanium attenuated total reflection (ATR) (ALPHA, Bruker, Germany) was used to analyze whether PVC-g-PEGMA existed on the surface of the PVC membranes. The wavenumber range was 4000-650 cm-1 from 64 scans with a 2 cm-1 resolution.


9


Page 10 of 52

The surface roughness morphologies of the membranes were obtained by atomic force microscopy (AFM, Multimode 8, Bruker, Germany). The scan size was 5×5 μm2 with a 0.977 Hz scan rate. Six replicates were conducted for each sample at different locations. The filtration experiment was conducted using an Amicon 8200 stirred dead-end filtration cell (Millipore, USA) with a diameter of 63.5 mm, an effective area of 28.7 cm2 and a cell volume of 200 mL. It was attached to a 5.0 L dispensing vessel. A Pro Balance (AV8101, Ohaus Adventurer, USA) was used to weigh the permeate, and the permeate mass date was recorded every minute by Collect 6.1 software. In fouling experiment, a stirring plate (PC-410D, Corning, USA) was used to stir the liquid at 200 rpm to minimize the concentration polarization. In all tests, the transmembrane pressure (TMP) was 10 psi (0.7 bar) and the temperature was 25 ℃. The flux performance experiment procedures were similar to those of previous studies.

4, 41, 42

First, the membrane was compacted by DI water for 2 h and the flux (J1) was measured. Next, it was conditioned with 10 mM NaCl solution for 2 h. Then, the feed solution containing 20 mg/L of SA and 10 mM of NaCl was used in the fouling test for 7 h, with the flux defined as Jp. In the end, the testing membrane was physically cleaned by DI water for 3 min, and the flux of the cleaned membrane was J2. The flux performance experiment of every kind of membrane was repeated at least twice. The long-time fouling test before physically cleaning was adopted by many studies.

45, 46

This

method can reflect the antifouling properties of the membranes better. Some ratios were used to measure the antifouling properties: the flux recovery ratio (FRR), the total flux decline ratio (DRt), the reversible flux decline ratio (DRr) and the irreversible flux decline ratio (DRir). 47


10

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FRR=

J2 100% J1

 J DR t = 1- p  J1

DR r =

J 2 -J p J1

(3)

  100% 

(4)

100%

(5)

 J  DR ir = 1- 2  100%  J1 

(6)

Group Contribution Method to Calculate the Solubility Parameter of PEGMA. Based on the Hansen solubility parameters theory, the total solubility parameter δ t is divided into 3 parts:

δd , δ p and δ h , which quantitatively represent the dispersion parameter ( δd ), the polar parameter ( δ p ) and the hydrogen bonding parameter ( δ h ). 48

δ t 2 =δd 2 +δ p 2 +δ h 2

(7)

δd , δ p and δ h can be calculated by a group contribution method using the following equations: 49

δd = 

δp =

δh =

Fdi

(8)

V

F

2

pi

E

hi

V

(9)

V

(10)

The detailed calculating process is in the Supporting Information (SI), and the solubility parameter δ t of PEGMA is 18.47  MJ/m3  . 1/2


11


Page 12 of 52

Calculation of Polymer-solvent Affinity and Ternary Phase Diagram Determination. The polymer and solvent affinity can be described by the Ra value, which is calculated by the Hansen solubility parameters using the following equation: 48

R a = 4  δd1 -δd2  +  δ p1 -δ p2  +  δ h1 -δ h2  2

2

2

(11)

Specifically, the δ d , δ p and δ h values of mixed solvents can be calculated in the following way. 48 First, calculate the volume fraction of each solvent according to the mass fraction using equation (12).

 Wt .Fraction   Density   1  Vol.Fraction 1 =  Wt .Fraction   Wt .Fraction   Density  +  Density   1  2

(12)

Second, calculate the values of δ d , δ p and δ h of the mixed solvents.

δd =  Vol.Fraction 1  δd1 +  Vol.Fraction 2  δd2 δ p =  Vol.Fraction 1  δ p1 +  Vol.Fraction 2  δ p2 δ h =  Vol.Fraction 1  δ h1 +  Vol.Fraction 2  δ h2

(13)

Equations (12) and (13) use a binary mixture solvent as an example, but a ternary mixture solvent can be calculated in the same way. The detailed calculating process is in the Supporting Information (SI). The cloud points for the casting solutions of M1-M6 were measured by the titration method. After the complete dissolution of PVC and PVC-g-PEGMA in different solvents, the casting solutions were titrated using the DI water at 60 ℃ and 500 rpm. DI water was added to the


12

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solution slowly, until the solution can not become homogeneous within 24 h. The amounts of PVC, PVC-g-PEGMA, solvents and DI water were recorded. 50, 51

RESULTS AND DISCUSSION Polymer-solvent Affinity and Ternary Phase Diagram Determination. According to equations (11), (12) and (13), the results of the Ra values between the PVC and different solvents are listed in Table 2. Table 2. Hansen solubility parameter 48 and Ra values of PVC and different solvents.

δd

δp

δh

Ra

Polymer

PVC

18.7

10.0

3.1

-

solvents

DMAc

16.8

11.5

10.2

8.19

NMP

18.0

12.3

7.2

4.91

*DMSO

18.4

16.4

10.2

9.58

NMP/DMSO=7/3

18.11

13.48

8.06

6.17

NMP/THF/DMSO=4/3/3

17.70

11.20

8.29

5.69

NMP/DMAc/DMSO=4/3/3

17.72

13.18

9.01

6.99

Note: * indicates the PVC cannot dissolve in DMSO at 60 ℃.

A lower Ra value of the PVC-solvent exhibits a better compatibility of the system theoretically. 52, 53

The PVC-DMSO system had the highest Ra value, which is in good agreement with the

experiment, in which PVC cannot dissolve in DMSO at 60 ℃. The ternary phase diagram of PVC/solvent/DI water system with the additive PVC-g-PEGMA revealed the thermodynamic stability of the casting solutions. From Figure 2, It was obvious that M1-M6 all had low coagulation values, so instantaneous phase inversion will take place, leading


13


to porous membrane structure,

54

Page 14 of 52

which was consistent with the SEM surface images. M6, M5

and M4 were closer to the polymer–solvent axis than M1 and M2, indicating that less amount of water will needed to induce phase inversion.

55

So, M6 had the fastest precipitation rate, then

followed by M5, M3, M4, M2 and M1. This phenomenon indicated that the addition of DMSO can accelerate the phase inversion during NIPS, which can be explained by the lower solventnonsolvent diffusivity of DMSO making the phase separation occur at a lower PVC concentration.

27

Fast phase inversion will lead to high surface porosity, and M6, M5, M3 had

higher surface porosity than M1 and M2, which can be verified by Table 3. Compared M3 with M4, the additive can make the phase inversion rapidly, which was consistent with the work of Koros et al. 55, 56 Using ternary phase diagram for complex polymer dopes analysis was better than Ra value, because it contained the whole system of PVC/solvent/DI water with the additive PVC-gPEGMA. Ra value just reflected polymer and solvent affinity, so the effect of additive was neglected. However, Ra value was also very useful for solvent selection.

Figure 2. Ternary phase diagram of PVC/solvent/DI water system at 60 ℃ with the additive PVC-g-PEGMA.


14

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XPS Analysis. The XPS spectra of M1-M6 are shown in Figure 3. They indicate that Cl, C and O are the predominant elements of the membrane surface, and the relative proportion of them are different in membranes fabricated by different solvents. M3-NMP/DMSO 7/3 had the highest oxygen content of 15.73 %, followed by M2-NMP with 11.68 %, M1-DMAc with 11.39 %, M6-NMP/DMAc/DMSO 4/3/3 with 10.50 %, M5NMP/THF/DMSO 4/3/3 with 8.59 % and M4-P NMP/DMSO 7/3 with 1.75 %. The oxygen content reflects the number of hydrophilic PEGMA segments on the membrane surface, which are positively correlated. The PEGMA segments exist on the membrane surface because when a phase inversion occurs in DI water, the PEGMA segments of the PVC-g-PEGMA will migrate to the water and polymer interface,

57

so the results of the XPS also proved the successful grafting

of PVC-g-PEGMA. Different contents of O reflect different number of PEGMA segments under different solvent conditions. The results indicated that M3 had the most PEGMA segments on the membrane surface, followed by M2, M1, M6 and M5. The reason why M5 had a low content of O was that the 30 s evaporation time made the polymer concentration high near the surface, hindering the migration of PEGMA segments. M4 had no PEGMA on the membrane surface, but a small O 1s peak also existed, which was because of the oxidation by environmental oxygen and adsorption of H2O. 58 The migration of hydrophilic PEGMA improved the surface properties of the membranes. Theoretically, the more PEGMA segments, the better hydrophilicity and antifouling property of the membranes.

22

However, hydrophilicity and antifouling properties are also affected by

roughness, surface pore size and many other factors,

59, 60

making the order not accordant with

the contents of PEGMA segments.


15


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Figure 3. XPS spectra of PVC/PVC-g-PEGMA and pure PVC membranes with different solvents: (a) M1-DMAc, (b) M2-NMP, (c) M3-NMP/DMSO 7/3, (d) M4-P NMP/DMSO 7/3, (e) M5-NMP/THF/DMSO 4/3/3 and (f) M6-NMP/DMAc/DMSO 4/3/3.

Membrane Morphology. The SEM images obtained by field-emission scanning electron microscopy (FESEM) of all membranes are listed in Figure 4.


16

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Figure 4. SEM images of the surface and cross-section morphologies of PVC/PVC-g-PEGMA and pure PVC membranes with different solvents: (a and b) M1-DMAc, (c and d) M2-NMP, (e and f) M3-NMP/DMSO 7/3, (g and h) M4-P NMP/DMSO 7/3, (i and j) M5-NMP/THF/DMSO 4/3/3 and (k and l) M6-NMP/DMAc/DMSO 4/3/3. All surface images were under magnification of 20 K ×, and the cross-sectional images were under a magnification of 400 × except for (f), which was under a magnification of 270 ×.

Based on the surface SEM images, the statistics of the pore size properties were obtained via Image Pro Plus V.7.0 software (Media Cybernetics, USA). The results of the average pore size (Daverage), maximum pore size (Dmax), pore density and surface porosity are listed in Table 3. The overall porosity was calculated by equation (2).

Table 3. The statistics of the membrane pore size properties and pure water fluxes.


17


Membrane ID

Page 18 of 52

Daverage

Dmax

Pore

Surface

Overall

Pure water flux

(nm)

(nm)

density

porosity

porosity

(L m-2 h-1 bar-1)

(m-2)

(%)

(%)

M1-DMAc

22

28

1.1×1012

0.04

86.20±3.27

478.70±38.93

M2-NMP

27

33

1.6×1012

0.10

93.23±1.26

395.27±27.80

M3-NMP/DMSO 7/3

29

34

1.4×1012

0.11

88.65±2.55

333.66±50.90

M4-P NMP/DMSO 7/3

21

25

1.5×1013

0.48

82.63±1.35

261.07±27.53

M5-NMP/THF/DMSO 4/3/3

24

28

1.2×1013

0.84

89.26±0.51

667.74±56.43

M6-NMP/DMAc/DMSO 4/3/3

23

26

2.2×1013

0.93

88.20±0.72

891.54±64.41

As shown in Figure 4 and Table 3, M5 and M6 had large surface porosities of 0.84 % and 0.93 %, large pore densities of 1.2×1013 m-2 and 2.2×1013 m-2, and small average pore sizes of 24 nm and 23 nm, respectively, so the two kinds of ternary mixture solvents could increase the surface porosity and pore density, and decrease the pore size and pore size distribution, thus fabricating high flux PVC/PVC-g-PEGMA membranes. M2 possessed the second to last smallest surface porosity of 0.10 % but the largest overall porosity of 93.23 ± 1.26 %, indicating a dense surface but a large macrovoid structure of the sublayer. From Figure 4 d, the cross-section morphology of M2 verified the assumption: there were large macrovoids under the thin top layer. Comparing M3 with M4, the addition of PVC-g-PEGMA dramatically decreased the surface porosity and pore density but increased the average pore size and overall porosity. Because of the larger pore size and overall porosity, as well as better hydrophilicity, the flux of M3 was higher than M4, but the SA rejection was lower. The cross-section morphology images illustrated that M1, M3, M4 and M6 had interconnected finger-like voids across the membranes, while M2 had large macrovoids. M5 had figure-like


18

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voids too, but the voids were not across the membrane. There were large macrovoids under the figure-like voids. So M2 had the largest overall porosity of 93.23 ± 1.26 %, then followed by M5 of 89.26 ± 0.51 %. Wettability. For each membrane, the change in the water contact angle can be seen in Figure 5. The contact angles decreased linearly with time. M1-DMAc, M2-NMP, M3-NMP/DMSO 7/3, M5-NMP/THF/DMSO 4/3/3 and M6-NMP/DMAc/DMSO 4/3/3 had much smaller contact angles than M4-P NMP/DMSO 7/3, indicating the hydrophilic segments PEGMA of PVC-gPEGMA were on the surface of these membranes. M5 had the smallest contact angle and quickest decrease rate with time, showing that M5 was the most hydrophilic, followed by M2, M6, M1 and M3. The hydrophilicity result is the competition between the surface roughness and surface PEGMA segments amount.

46

A lower roughness and more surface PEGMA segments

will lead to a more hydrophilic surface. However, the order was consistent with the surface roughness order (see Figure 7), indicating that the roughness was the dominating factor influencing hydrophilicity.

Figure 5. The contact angles with the times of all PVC/PVC-g-PEGMA and pure PVC membranes: M1-DMAc, M2-NMP, M3-NMP/DMSO 7/3, M4-P NMP/DMSO 4/3/3, M5NMP/THF/DMSO 4/3/3 and M6-NMP/DMAc/DMSO 4/3/3.


19


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FTIR-ATR Analysis. The FTIR-ATR spectra of all PVC membranes are shown in Figure 6. This method was used to verify the presence of PVC-g-PEGMA on the surface. The bond at 1727 cm-1 exhibited the existence of the carbonyl (C=O) group of PEGMA.57, 61 Meanwhile, no bond at 1638 cm-1 exhibited the inexistence of C=C,

61

indicating that unreacted PEGMA

monomers were removed in the coagulation bath. These results proved that PVC-g-PEGMA was synthesized successfully and existed on the surface of all modified membranes. From Figure 6, the spectra of M1-DMAc, M2-NMP, M3-NMP/DMSO 7/3, M5NMP/THF/DMSO 4/3/3 and M6-NMP/DMAc/DMSO 4/3/3 all had a bond at 1727 cm-1, indirectly proving the successful grafting and blending of PVC-g-PEGMA. The direct proof of successful grafting was by nuclear magnetic resonance (1H NMR), which found the existence of chemical shifts from 3.2 to 4.3 ppm caused by O-CHx groups in PEGMA, which was conducted in our previous study. 22

Figure 6. FTIR-ATR spectra of PVC/PVC-g-PEGMA and pure PVC membranes: M1-DMAc, M2-NMP, M3-NMP/DMSO 7/3, M4-P NMP/DMSO 4/3/3, M5-NMP/THF/DMSO 4/3/3 and M6-NMP/DMAc/DMSO 4/3/3.


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Roughness. The root mean square (RMS) roughness values of blended membranes were higher than that of pure PVC membrane due to the PEGMA segments migrating to the membrane surface. More PEGMA segments led to an increase of the membrane surface roughness, except for M2-NMP, as shown in Figure 7. M2 had a lower roughness than M6NMP/DMAc/DMSO 4/3/3, although it had a higher oxygen content, which can be explained by its much lower surface density and surface porosity. The surface roughness will affect the hydrophilicity and antifouling properties of the membranes. A lower roughness tends to lead to a good antifouling property because of little foulants adsorbing on the surface, and can be cleaned easily. Bur antifouling property is also affected by many factors.

Figure 7. AFM images of PVC/PVC-g-PEGMA and pure PVC membranes: (a) M1-DMAc, (b) M2-NMP, (c) M3-NMP/DMSO 7/3, (d) M4-P NMP/DMSO 4/3/3, (e) M5-NMP/THF/DMSO 4/3/3 and (f) M6-NMP/DMAc/DMSO 4/3/3.


21


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Flux Performance. The effect of the solvents on the flux performance of membranes is presented in Figure 8 and the pure water fluxes for M1-M6 were listed in Table 3. Comparing M3 and M4, when the solvent was the same, modified membranes adding PVC-g-PEGMA had a higher flux than pure PVC membranes, which was due to the dramatic increase in the hydrophilicity and a larger pore size. The flux of M1, M2, M3, M5 and M6 reflected the significant effect of the solvents on the membrane flux performance. Solvents play an important role in determining the membrane’s morphology and hydrophilicity, which were discussed above, thus determining the flux performance. With different solvents, the flux was from 333.56 ± 50.90 L m-2 h-1 bar-1 (M3) to 891.54 ± 64.41 L m-2 h-1 bar-1 (M6), increasing by nearly 170 %. M5 and M6 had much higher fluxes than the other modified membranes because of their relatively good hydrophilicity and large surface porosities. Although M6 was more hydrophobic than M5, the surface porosity was much larger, thus the flux of M6 was higher than M5. The competition between the hydrophilicity and surface porosity determined the flux that also existed in M1, M2 and M3. The order of hydrophilicity was: M2 > M1 > M3, whereas the surface porosity order was: M3 > M2 > M1, and the flux order of them was: M1 > M2 >M3.


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Figure 8. Flux performance of PVC/PVC-g-PEGMA and pure PVC membranes: M1-DMAc, M2-NMP, M3-NMP/DMSO 7/3, M4-P NMP/DMSO 4/3/3, M5-NMP/THF/DMSO 4/3/3 and M6-NMP/DMAc/DMSO 4/3/3.

The SA rejection is depicted in Figure 9. All membranes had good rejection of SA particles higher than 92 %, which was because of the small average pore size. M6 had the highest rejection of 94.7 % due to the narrow pore size distribution and small average pore size (see Table 3), as well as fewer defects than other membranes, then followed by M4, which had the smallest average pore size of 21 nm. M3 had a relatively lower rejection rate of 92.8 % due to its larger average pore size of 29 nm.

Figure 9. Sodium alginate rejection of PVC/PVC-g-PEGMA and pure PVC membranes: M1DMAc, M2-NMP, M3-NMP/DMSO 7/3, M4-P NMP/DMSO 4/3/3, M5-NMP/THF/DMSO 4/3/3 and M6-NMP/DMAc/DMSO 4/3/3.

From Figure 10, it is obvious that when the SA solution was used to replace pure water, the fluxes of all membranes decreased rapidly and dramatically, causing membrane fouling. The


23


membrane fouling was affected by many factors, such as the additives,

Page 24 of 52

62

hydrophilicity,

63, 64

surface pore size, pore density 65 and roughness. 66 Figure 10 reflected the antifouling properties of M1-M6, covering four antifouling performance indexes: FRR (flux recovery ratio), DRr (reversible flux decline ratio), DRir (irreversible flux decline ratio) and DRt (total flux decline ratio). After 7 h of the SA solution filtration test, the fouled membrane was physically cleaned using DI water for 3 min. The FRR values of M1-M6 were 85.68 ± 2.09 %, 100 ± 7.18 %, 92.19 ± 3.77 %, 76.49 ± 9.63 %, 67.14 ± 3.84 % and 81.88 ± 0.39 %. DRr / DRt means relative reversible fouling. The higher values of FRR and DRr/DRt represent better antifouling properties,

60

so the

order of the antifouling properties was: M2 > M3 > M1 > M6 > M4 > M5. M2-NMP had an FRR value of 100 ± 7.18 %, a low DRt value of 70.03 ± 3.81 % and a DRir value of 0 ± 7.18 %, corresponding 100 ± 3.37 % of the DRr value, exhibiting the best antifouling property. This phenomenon could be explained by its relatively high oxygen content, good hydrophilicity and smooth surface. Additionally, there were large macrovoids under the thin top layer. Large macrovoids absorbed fewer SA particles to the wall and were easier to wash away with physical cleaning. 57 Comparing M1-M6, the addition of PVC-g-PEGMA indeed enhanced the antifouling properties of the PVC membranes, which was consistent with our previous study.

22

Because

hydrophilic PEGMA segments on the surface will promote the formation of the hydration layer near the surface via hydrogen bond between water molecules and the hydrophilic groups. 64 The hydration layer acts as a barrier to prevent foulant adsorption on the surface, so the membrane with more hydrophilic surface will have better antifouling property. This also explained why M4 had worse antifouling property than M2 even though it had smoother surface. But the DRt value of M4 was lowest due to its lowest pure water flux. M5 showed the worst antifouling properties.


24

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Although it was the most hydrophilic and smoothest, it could not compare with a pure PVC membrane. This can be explained by the oxygen content being lowest among all modified membranes. Another important reason was the top layer was thicker than the other membranes (see Figure 4), which would absorb more SA particles and be more difficult to remove.

Figure 10. FRR (flux recovery ratio), DRr (reversible flux decline ratio), DRir (irreversible flux decline ratio) and DRt (total flux decline ratio) for PVC/PVC-g-PEGMA and pure PVC membranes: M1-DMAc, M2-NMP, M3-NMP/DMSO 7/3, M4-P NMP/DMSO 4/3/3, M5NMP/THF/DMSO 4/3/3 and M6-NMP/DMAc/DMSO 4/3/3.

Comparison on the membrane performance and production costs. Compared with other modified PVC membranes which have been reported in literatures, this membrane, fabricated by PVC, PVC-g-PEGMA and solvent NMP, DMAc, DMSO, had the second highest pure water flux except the membrane fabricated by Zhang et al 62. They used Pluronic F127 as the additive, NMP as the solvent and the salt coagulation bath as the non-solvent, endowing the membrane with a remarkable flux (1405 L m-2 h-1 bar-1). However, the SA rejection of that membrane was lower


25


Page 26 of 52

than M6-NMP/DMAc/DMSO 4/3/3 in this study. More importantly, the salt coagulation bath will put up the production cost in industry and the solvent NMP is not as green as DMSO. The detailed comparison is in the Supporting Information (SI). Meanwhile, it also presented literatures reporting poly(vinylidene fluoride) (PVDF), polyacrylonitrile (PAN) and poly(ether sulfone) (PES) membranes fabricated by using the solvent DMSO or other green solvents, such as triethyl phosphate (TEP), acetyl tributyl citrate (ATBC), triethylene glycol diacetate (TEGDA) and methyl-5-(dimethylamino)−2methyl-5-oxopentanoate (PolarClean). The approximate prices of these green solvents are: 10.9 CNY/kg of DMSO, 17.5 CNY/kg of TEP, 12.5 CNY/kg of ATBC, 130 CNY/kg of TEGDA and 52.4 CNY/kg of PolarClean. DMSO has lowest price among these green solvents, and PVC is much cheaper than PVDF, PAN and PES polymers, too. So, the production cost of M6NMP/DMAc/DMSO was cheap. These better properties, lower price and greener fabrication method make it possible for M6 to applicate in water treatment industrial.

CONCLUSION In this paper, based on polymer-solvent affinity calculated using Hansen solubility parameters, we used the green solvent DMSO to replace a certain proportion of traditional toxic solvents for PVC blended PVC-g-PEGMA membrane fabrication. The solvents we investigated were binary mixture solvent NMP/DMSO=7/3 and ternary mixture solvents NMP/THF/DMSO=4/3/3, NMP/DMAc/DMSO=4/3/3, which was the first attempt to add the green solvent DMSO to the solvent system for PVC membrane fabrication. By a systematic investigation of all prepared membranes, we found that PVC/PVC-g-PEGMA membranes fabricated by ternary mixture


26

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solvent NMP/DMAc/DMSO=4/3/3 had the highest pure water flux of 891.54 ± 64.41 L m-2 h-1 bar-1 and highest SA rejection of 94.7 ± 1.3 %. The good performance of the NMP/DMAc/DMSO=4/3/3 membrane was attributed to the relatively good hydrophilicity and smooth surface, high surface porosity, low average pore size and pore size distribution. The effect of the solvent on membrane performance is very significant. Because of the poor solvent power, it is impossible to obtain a PVC membrane merely using the green solvent DMSO. But using DMSO to mix with a reasonable portion of traditional solvents, which needs to be guided by theoretical calculations, can achieve the dual purpose of environmental protection and improving membrane performance. This is also simultaneously in accordance with the requirements of sustainability. This study is a successful exploration on green chemistry in membrane fabrication process.

ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge. Figure S1. The morphologies and performances of the two membranes: (a) the surface morphology of membrane fabricated by evaporation for 30 s (20 K ×), (b) the surface morphology of membrane fabricated without evaporation (20 K ×), (c) the water contact angles of the two membranes, (d) the fluxes performance of the two membranes and (e) the antifouling properties and SA rejection rates of the two membranes. Table S1. The statistics of the pore size properties and thicknesses of the two membranes. Table S2.


27


Page 28 of 52

Solubility parameter component group contributions method (Hoftyzer – Van Krevelen). Table S3. Addition of the group contributions of PEGMA. Table S4. The 𝛿𝑑, 𝛿𝑝 and 𝛿ℎ values and densities of some solvents. Table S5. Comparison on the pure water flux, SA rejection and FRR ratio of the membranes in this study and from literatures.

AUTHOR INFORMATION Corresponding Author Tel.:

+86-28-85995998;

Fax:

+86-28-62138325;

E-mail:

[email protected];

[email protected] (B. Liu). ORCID Baicang Liu: 0000-0003-3219-1924 Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51678377,

51708371),

the

Sichuan

University

Outstanding

Youth

Foundation

(2015SCU04A35), the Applied Basic Research of Sichuan Province (2017JY0238), the Key Projects in the Science & Technology Program of Hainan Province (zdkj2016022), and


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China Postdoctoral Science Foundation (2018T110973, 2017M612965). This research was also supported by the Brook Byers Institute for Sustainable Systems, the Hightower Chair, and the Georgia Research Alliance at the Georgia Institute of Technology. The views and ideas expressed herein are solely those of the authors and do not represent the ideas of the funding agencies in any form.

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(46) Wang, S.; Li, T.; Chen, C.; Chen, S.; Liu, B.; Crittenden, J., Non-woven PET fabric reinforced and enhanced the performance of ultrafiltration membranes composed of PVDF blended with PVDF-g-PEGMA for industrial applications. Appl. Surf. Sci. 2018, 435, 1072-1079. (47) Zhao, X.; Su, Y.; Chen, W.; Peng, J.; Jiang, Z., Grafting perfluoroalkyl groups onto polyacrylonitrile membrane surface for improved fouling release property. J. Membr. Sci. 2012, 415, 824-834. (48) Hansen, C. M., Hansen Solubility Parameters: a user's handbook. CRC press, 2007. (49) Van Krevelen, D. W.; Te Nijenhuis, K., Chapter 7 - Cohesive Properties and Solubility in Properties of Polymers (Fourth Edition), Elsevier: Amsterdam, 2009, 189-227. (50) Tiraferri, A.; Yip, N. Y.; Phillip, W. A.; Schiffman, J. D.; Elimelech, M., Relating performance of thin-film composite forward osmosis membranes to support layer formation and structure. J. Membr. Sci. 2011, 367, (1), 340-352. (51) Shi, L.; Wang, R.; Cao, Y.; Liang, D. T.; Tay, J. H., Effect of additives on the fabrication of poly(vinylidene fluoride-co-hexafluropropylene) (PVDF-HFP) asymmetric microporous hollow fiber membranes. J. Membr. Sci. 2008, 315, (1), 195-204. (52) Jung, J. T.; Kim, J. F.; Wang, H. H.; di Nicolo, E.; Drioli, E.; Lee, Y. M., Understanding the non-solvent induced phase separation (NIPS) effect during the fabrication of


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Graphic Abstract

Synopsis: The green and sustainable solvent DMSO was used to partly replace traditional toxic solvents, fabricating membranes with improved performance.


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Graphic Abstract 84x47mm (300 x 300 DPI)


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Figure 1. The hazard statements of common organic solvents. (Note: All information is according to Regulation (EC) NO. 1272/2008) 119x79mm (300 x 300 DPI)



Figure 2. Ternary phase diagram of PVC/solvent/DI water system at 60 ℃ with the additive PVC-g-PEGMA. 79x57mm (300 x 300 DPI)


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Figure 3. XPS spectra of PVC/PVC-g-PEGMA and pure PVC membranes with different solvents: (a) M1-DMAc, (b) M2-NMP, (c) M3-NMP/DMSO 7/3, (d) M4-P NMP/DMSO 7/3, (e) M5-NMP/THF/DMSO 4/3/3 and (f) M6NMP/DMAc/DMSO 4/3/3. 160x78mm (300 x 300 DPI)



Figure 4. SEM images of the surface and cross-section morphologies of PVC/PVC-g-PEGMA and pure PVC membranes with different solvents: (a and b) M1-DMAc, (c and d) M2-NMP, (e and f) M3-NMP/DMSO 7/3, (g and h) M4-P NMP/DMSO 7/3, (i and j) M5-NMP/THF/DMSO 4/3/3 and (k and l) M6-NMP/DMAc/DMSO 4/3/3. All surface images were under magnification of 20 K ×, and the cross-sectional images were under a magnification of 400 × except for (f), which was under a magnification of 270 ×.


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Figure 5. The contact angles with the times of all PVC/PVC-g-PEGMA and pure PVC membranes: M1-DMAc, M2-NMP, M3-NMP/DMSO 7/3, M4-P NMP/DMSO 4/3/3, M5-NMP/THF/DMSO 4/3/3 and M6-NMP/DMAc/DMSO 4/3/3. 229x176mm (300 x 300 DPI)



Figure 6. FTIR-ATR spectra of PVC/PVC-g-PEGMA and pure PVC membranes: M1-DMAc, M2-NMP, M3NMP/DMSO 7/3, M4-P NMP/DMSO 4/3/3, M5-NMP/THF/DMSO 4/3/3 and M6-NMP/DMAc/DMSO 4/3/3. 203x162mm (300 x 300 DPI)


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Figure 7. AFM images of PVC/PVC-g-PEGMA and pure PVC membranes: (a) M1-DMAc, (b) M2-NMP, (c) M3NMP/DMSO 7/3, (d) M4-P NMP/DMSO 4/3/3, (e) M5-NMP/THF/DMSO 4/3/3 and (f) M6-NMP/DMAc/DMSO 4/3/3.



Figure 8. Flux performance of PVC/PVC-g-PEGMA and pure PVC membranes: M1-DMAc, M2-NMP, M3NMP/DMSO 7/3, M4-P NMP/DMSO 4/3/3, M5-NMP/THF/DMSO 4/3/3 and M6-NMP/DMAc/DMSO 4/3/3. 203x155mm (300 x 300 DPI)


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Figure 9. Sodium alginate rejection of PVC/PVC-g-PEGMA and pure PVC membranes: M1-DMAc, M2-NMP, M3-NMP/DMSO 7/3, M4-P NMP/DMSO 4/3/3, M5-NMP/THF/DMSO 4/3/3 and M6-NMP/DMAc/DMSO 4/3/3. 1213x954mm (96 x 96 DPI)



Figure 10. FRR (flux recovery ratio), DRr (reversible flux decline ratio), DRir (irreversible flux decline ratio) and DRt (total flux decline ratio) for PVC/PVC-g-PEGMA and pure PVC membranes: M1-DMAc, M2-NMP, M3NMP/DMSO 7/3, M4-P NMP/DMSO 4/3/3, M5-NMP/THF/DMSO 4/3/3 and M6-NMP/DMAc/DMSO 4/3/3. 272x208mm (300 x 300 DPI)


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Using the Green Solvent Dimethyl Sulfoxide (DMSO) to Partly Replace

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