Simultaneous zwitterionization and selective swelling-induced pore generation of block copolymers for antifouling ultrafiltration membranes

Journal Pre-proof Simultaneous zwitterionization and selective swelling-induced pore generation of block copolymers for antifouling ultrafiltration membranes Chenxu Zhang, Congcong Yin, Yanjie Wang, Jiemei Zhou, Yong Wang PII:

S0376-7388(19)32848-0

DOI:

https://doi.org/10.1016/j.memsci.2020.117833

Reference:

MEMSCI 117833

To appear in:

Journal of Membrane Science

Received Date: 11 September 2019 Revised Date:

23 November 2019

Accepted Date: 9 January 2020

Please cite this article as: C. Zhang, C. Yin, Y. Wang, J. Zhou, Y. Wang, Simultaneous zwitterionization and selective swelling-induced pore generation of block copolymers for antifouling ultrafiltration membranes, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/j.memsci.2020.117833. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Table of Content

1

Simultaneous zwitterionization and selective swelling-induced pore generation of block copolymers for antifouling ultrafiltration membranes

Chenxu Zhang, Congcong Yin, Yanjie Wang, Jiemei Zhou* and Yong Wang*

State Key Laboratory of Materials-Oriented Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, College of Chemical Engineering, Nanjing Tech University, Nanjing 211816, Jiangsu (P. R. China) *Corresponding author Fax: 0086-25-8317 2292 Email: [email protected] (J. M. Zhou), [email protected] (Y. W.)

1

Abstract Membranes with zwitterionic surfaces are highly desired in water-based separations for their distinguished fouling resistance. Herein, a facile strategy for the preparation of zwitterion-functionalized membranes is developed by coupling the pore-making and zwitterion functionalization in one step. We soaked

thin

films

of

poly(2-dimethylaminoethyl

methacrylate)-block-polystyrene (PDMAEMA-b-PS) in the mixture of ethanol and

1,

3-propanesultone

(1,

3-PPST).

In

this

mixture,

PDMAEMA

microdomains were selective swollen leading to nanoporous structures with PDMAEMA chains enriched on the surface. Simultaneously, the quaternization reaction between PDMAEMA and 1, 3-PPST took place. Unexpectedly, in addition to function as the zwitterionic agent, 1, 3-PPST also contributed to the selective swelling induced pore generation. It was found that the presence of 1, 3-PPST in ethanol slowed down the swelling process at the early stage, but evidently enhanced the swelling of PDMAEMA-b-PS films in longer duration. Thus-prepared zwitterion-functionalized membranes exhibited several times higher water permeance than non-zwitterionized counterparts, which should be attributed to increased pore size and strongly improved hydrophilicity as a result of zwitterion functionalization. Moreover, the membranes showed exceptional fouling resistance because of the presence of zwitterions. Keywords: zwitterion, membrane, selective swelling, block copolymers, fouling resistance.

2

1. Introduction Membrane-based processes have been extensively employed to address the water scarcity with high separation efficiency, low energy consumption, environmental friendliness and reliability [1, 2]. However, during separation processes, membranes usually suffer from problems of organic and biological fouling caused by various foulants, which are particularly serious for those polymeric membranes with intrinsically hydrophobic nature [3-5]. Unexpected adsorption and aggregation of foulants on membrane surfaces result in significant degradation of separation performance, increased costs for cleaning and shortened membrane lifetimes. To address this issue, surface modification to membranes with hydrophilic materials has been demonstrated to be an efficient way to reduce the interactions between the membrane and foulants so as to render fouling resistance to membranes [6, 7]. Commonly used hydrophilic materials for membrane surface modification include poly(ethylene glycol) (PEG), polyacrylic acid (PAA), zwitterionic polymers (polyzwitterions), etc [8-11]. A zwitterionic polymer contains both a positively charged group and a negatively charged group within one structural unit, and the overall charge of the polymer is neutral [12]. Polyzwitterions exhibit much stronger affinity toward water molecules via electrostatic interaction and hydrogen bonding than conventional hydrophilic polymers like PEGs [13]. Hydration layers thus-formed on membranes surface could greatly resist foulants absorption [14, 15]. Furthermore, owing to the charge neutrality, it is 3

difficult for charged foulants (for example, proteins) to adhere on the surface [16, 17]. Therefore, polyzwitterions are considered as highly promising polymers for the preparation of hydrophilic membranes with strong fouling resistance. A large number of works have been reported for surface zwitterionization to membranes, where “grafting to” and “grafting from” approaches are usually employed to anchor polyzwitterions onto existing membrane surfaces [18, 19]. However, grafting techniques often require pretreatments like ozone pre-activation [20], plasma [21, 22] or UV exposure [23, 24], which are disadvantageous in terms of practical applications on account of high costs and complicated manufacturing steps at large scale. To develop more convenient processes towards surface zwitterionization, Zhou and co-workers co-deposited poly(sulfobetaine methacrylate) (PSBMA) and polydopamine to improve the fouling resistance of polypropylene (PP) membranes [25]. This work presents a facile and efficient approach for the preparation of zwitterionic membranes.

Alternatively,

Chang

et

al.

introduced

zwitterions

on

poly(vinylidene fluoride) (PVDF) membranes by surface adsorption of random copolymers of styrene and sulfobetaine [26]. Thus-modified membranes possess very hydrophilic surface and can be effectively separate oil-in-water emulsions. However, these methods are essentially based on the physical adsorption via hydrophobic interactions, and consequently the adsorbed polyzwitterions are prone to leach out from the membrane surfaces during

4

long-term use. That is, current methods either require tedious multi-step syntheses, as in the case of chemical bonds via grafting techniques, or lead to unstable modification, as in the case of simple coating process via deposition or adsorption. Therefore, it remains highly demanded for direct and convenient strategies to prepare zwitterionic membranes embodied with advanced antifouling performance. Recently, we reported a selective swelling strategy to prepare fouling-resistant membranes. In this strategy, block copolymers (BCPs) composed of a hydrophilic block (e. g., PEG) and a hydrophobic block are employed as membrane-forming materials, and simply swelling the BCPs in selective solvents to the hydrophilic leads to nanoporous membranes with PEG blocks simultaneously enriched on the membrane surface including pore walls [27, 28]. Inspired by the unique features of this selective swelling method including extreme ease and simultaneous surface enrichment of hydrophilic blocks, we attempt to apply this strategy to the preparation of zwitterionic membranes as the hydrophilic blocks may be in situ zwitterionized during the process of selective swelling-induced pore generation. However, directly introducing polyzwitterions into BCP precursors for membrane fabrication is nearly impossible because polyzwitterions cannot be dissolved in common organic solvents [29, 30]. Fortunately, it is found that poly(sulfobetaine methacrylates), which are the most commonly used zwitterionic polymers, can be synthesized from the reaction between poly(2-dimethylaminoethyl

5

methacrylate) (PDMAEMA) and 1, 3-propanesultone (1, 3-PPST) [31]. By coupling this reaction to the selective swelling process, a new pathway to fabricate zwitterion-functionalized membranes in one step is realized in this work. We soaked dense films of PDMAEMA-b-PS in ethanol mixed with 1, 3-PPST. Selective swelling-induced pore formation took place, and the enrichment of PDMAEMA chains on the membrane surface and pore walls would occur in parallel with the reaction between PDMAEMA and 1, 3-PPST, thus in situ producing nanoporous membranes with zwitterionic surfaces (Figure 1). As expected, thus-produced membranes exhibited superior fouling resistance,

and

they

presented

unexpectedly

enhanced

ultrafiltration

performances.

Figure

1.

(a)

Schematic

illustration

of

the

preparation

of

zwitterion-functionalized PDMAEMA-b-PS membranes by a simultaneous selective swelling-induced pore generation and zwitterionization strategy; (b) quaternization between 1, 3-PPST and the PDMAEMA blocks.

6

2. Experimental section 2.1 Materials PDMAEMA-b-PS (MnPDMAEMA = 20.8 KDa, MnPS = 60 KDa, PDI = 1.26) was synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization according to our previous work [32]. 1, 3-PPST was purchased from Macklin and purified by ethanol washing followed by vacuum drying overnight at room temperature. Silicon wafers were cleaned twice in fresh ethanol under ultrasonication and dried prior to use. Polyvinylidene fluoride (PVDF) membranes (diameter of 25 mm and nominal pore size of 0.22 µm) serving as the macroporous substrates were purchased from Millipore. Bovine serum albumin (MnBSA = 67 KDa) was purchased from Aladdin Inc. Deionized water (conductivity: 8-20 µs cm-1) was used in all tests. All other chemicals and reagents

including

tetrahydrofuran

(≥

99%),

ethanol

(≥

99.7%),

dichloromethane (≥ 99.5%), chloroform (≥ 99.5%), petroleum ether (60-90°C) were obtained from local suppliers and used as received.

2.2 Membrane preparation PDMAEMA-b-PS was dissolved in chloroform to form a 2 wt% solution. Subsequently,

the

as-obtained

solution

was

filtered

through

polytetrafluoroethylene (PTFE) filters (nominal pore size of 0.22 µm) three times in order to remove big aggregates or impurities which may exist. For the investigation of swelling behaviors and surface properties, the BCP solution

7

was spin-coated on the silicon substrates at 2000 rpm for 30 s to produce dense films. The spin-coated films were then soaked in ethanol mixed with different percentages of 1, 3-PPST at 60°C to generate pores and also to initiate the reaction between PDMAEMA and 1, 3-PPST. The films were taken out from ethanol after pre-set durations, washed with pure ethanol for three times to remove 1, 3-PPST remaining in the films and subsequently dried at room temperature. As a comparison, selective swelling in the absence of 1, 3-PPST was also conducted in pure ethanol while all other conditions maintained unchanged. Composite membranes were prepared following our previous fabrication procedures [33]. Briefly, PVDF substrate membranes were first immersed in deionized water for at least 30 min to ensure the complete filling of the macropores in the substrates with water. Then BCP solutions were spin-coated onto the water-filled PVDF substrates. The BCP-coated PVDF membranes which were subsequently soaked in ethanol with or without 1, 3-PPST at set temperatures for different durations, followed by ethanol washing and drying as described above.

2.3 Characterization Fourier transform infrared spectroscopy (Nicolet 8700 FT-IR spectrometer, Thermo Scientific) was used to characterize the chemical groups in the BCP films. A field-emission scanning electron microscope (FESEM, Hitachi S4800)

8

was performed to observe the surface and cross-sectional morphologies of the samples at the accelerating voltage of 5 kV. Prior to SEM characterizations, the samples were sputtering coated with a thin layer of gold to enhance conductivity. At least 50 pores on the surface SEM image of each sample were measured

to

estimate

average

pore

sizes

by

using

the

software

Nanomeasurer. Static water contact angles (WCAs) were measured by a contact angle goniometer (Dropmeter A100, Maist). At least 5 regions on each film were probed and the average value was reported. Films on silicon wafer before and after swelling were examined using a spectroscopic ellipsometer (Complete EASE M-2000U, J. A. Woollam) with an incident angle of 75°. Cauchy model was adopted to determine film thickness and refractive index. The films with various swelling duration were characterized by X-ray photo-electron spectrometer (K-alpha X-ray photoelectron spectroscopy, Thermo Fisher) to examine the surface chemical compositions.

2.4 Filtration tests Water permeabilities and BSA rejections of different membranes were performed on a dead-end filtration module (Amicon8003, Millipore). Before permeance measurement, each membrane was pressurized in deionized water for at least 10 min at 1 bar to guarantee the permeance stability. Permeance (Perm) can be calculated by Eq. (1). =

× × 9

(1)

where V (L) is the volume of water permeated through the membranes, A (m2) is the effective membrane area, t (h) is the filtering time, p (bar) is the operation pressure. BSA was dissolved in phosphate buffer saline (PBS) aqueous solutions at the concentration of 1 g/L to test the separation performance. Stirring at 700 rpm was carried out during the rejection test to reduce the effect of concentration polarization. BSA concentrations of the permeate and feed solutions were measured by a UV–vis absorption spectrometer (NanoDrop 2000C) at the wavelength of 280 nm, and the BSA rejections were calculated according to Eq. (2): =

−

× 100%

(2)

where Cp and Cf is the BSA concentration (g/L) of the permeate and feed solution.

2.5 Anti-fouling tests The dynamic BSA filtration tests were performed to evaluate the anti-fouling performance of the prepared BCP membranes. Deionized water and the BSA solution were filtrated alternately through the membrane for three cycles in the dead-end filtration module. The membranes were cleaned by thoroughly washing the membrane surface with the PBS buffer solution for several times at the end of each BSA filtration. Marking the initial steady-state water permeance of membrane with J0, the recovery water permeance after 10

each cycle was record as JR. The BSA solution permeance of membrane at each cycle was recorded as Jp, and the water flux recovery ratio (FRR), the reversible fouling ratio (Rr), the irreversible fouling ratio (Rir) and the total fouling ratio (Rt) were calculated by Eqs. (3)-(6), respectively: = = =

× 100%

−

× 100%

(4)

× 100%

(5)

− =

(3)

+

(6)

The antifouling properties of corresponding BCP membranes without zwitterion modification were identically tested for comparison.

3. Results and Discussion 3.1 Swelling behaviors Our previous study has demonstrated that nanoporous membranes with PDMAEMA chains covered on the surface and pore walls can be formed by immersing PDMAEMA-b-PS dense films into hot ethanol based on the mechanism of selective swelling-induced pore generation [32]. In this work, 1, 3-PPST was introduced into ethanol for the purpose of zwitterion modification during the swelling process. Considering that the presence of 1, 3-PPST may have effect on the selective swelling process, we first examined the swelling behaviors of BCP films under different concentrations of 1, 3-PPST mixed with ethanol. The 2 wt% chloroform solution of PDMAEMA-b-PS was spin-coated

11

onto silicon wafers to produce thin films. The as-coated films exhibited a dense and nonporous morphology (Figure S1). For subsequent swelling treatment, the swelling agents were prepared by dissolving 1, 3-PPST in ethanol with the concentration varying from 0 wt% (pure ethanol), 0.5 wt%, 1 wt%, 2 wt%, 5 wt% to 10 wt%. After immersion in swelling agents at 60°C for 6 h, the films all showed porous structures, but different morphologies were observed for samples prepared with different concentrations of 1, 3-PPST (Figure 2 and Table S1). In the case of pure ethanol, isolated pores with an average diameter of ~33 nm sparsely appeared on the film surface (Figure 2a), revealing a low swelling degree of the films. With 0.5 wt% and 1 wt% 1, 3-PPST, more pores formed on the films despite almost no change in the pore diameter (Figure 2b and 2c). When the concentration of 1, 3-PPST was increased to 2 wt% and 5 wt%, the films experienced stronger swelling. The pores were enlarged to ~39 nm and ~45 nm, respectively, and the pores densities were also increased (Figure 2d and 2e). However, further extending the concentration of 1, 3-PPST to 10 wt% led to slight micellization of the BCP (Figure 2f), and thus the pore diameter was decreased to 38 nm. These results suggest that the presence of 1, 3-PPST accelerates the swelling process of PDMAEMA-b-PS films to some extent, and higher concentrations of 1, 3-PPST result in larger swelling degrees.

12

Figure 2. SEM images of PDMAEMA-b-PS films soaked in swelling agents with different 1, 3-PPST concentrations at 60 °C for 6 h: (a) 0 wt%, (b) 0.5 wt%, (c) 1 wt%, (d) 2 wt%, (e) 5 wt%, and (f) 10 wt%. All images have the same magnification and the scale bar corresponding to 500 nm is given in (f).

To further investigate the kinetics of selective swelling process in the presence of 1, 3-PPST, BCP films were subjected to swelling in the 5 wt% 1, 3-PPST/ethanol solution for different durations, and then their surface and cross-sectional morphologies were examined. Due to numerous pores formation, the films always experience significant volume expansion during the course of swelling, therefore the change in film thickness with swelling durations was also monitored by ellipsometry to indicate the swelling degree of the films. Furthermore, the selective swelling of the film in pure ethanol without 1, 3-PPST was conducted under identical conditions as a comparison to give an insight of the role of 1, 3-PPST in the swelling process. For the case of pure ethanol as the swelling agent, isolated pores with an average size of ~25 nm 13

were sparsely distributed on the film surface after a swelling duration for 0.5 h (Figure S2 and Table S2), and the cross-sectional image showed a slightly porous structure (Figure S3). Its thickness exhibited a sharp increase from initial ~220 nm to ~330 nm (Figure 3). However, the film thickness had no obvious increase thereafter, and surface pores only showed a slight growth in pore size with essentially unchanged pore morphology even if the swelling duration was extended up to 20 h, revealing that the pore-forming process subjected to ethanol swelling mainly occurred in the initial swelling period of 0.5 h. In contrast, there was a clear trend that the pores were increased both in sizes and densities with the prolonged durations in the case of swelling in the presence of 1, 3-PPST (Figure S2). With a short swelling for 0.5 h, irregular nanopores with an average pore size of 15 nm were generated on the films surface. Subsequently, the pore sizes were significantly increased to 27 nm and 38 nm after swelling for 2 h and 5 h, respectively, then slowly enlarged to 40 nm after swelling for 10 h, and finally remained almost unchanged upon further extending the swelling duration to 20 h. From the cross-sectional images (Figure S3a1-e1), it was observed that formed porous structures all spanned the entire thickness of the films. In terms of the thickness change, the increase in thickness of film treated by 5wt% 1, 3-PPST/ethanol was determined to be ~50 nm in the initial 0.5 h, which was less than the thickness increase in pure ethanol (Figure 3). This implied that the films swelling in the presence of 1, 3-PPST had a lower

14

swelling degree at this stage. However, the film thickness experienced a pronounced increase and exceeded the thickness of ethanol-swelling films after swelling for 2 h, then it was increased slowly until a little drop at a swelling duration of 20 h. The results indicate that 1, 3-PPST slows down the swelling process at the early stage, but induces higher swelling degrees of PDMAEMA-b-PS films after a long period of swelling. This interesting observation can be understood according to the mechanism of selective swelling-induced pore generation. Unlike conventional selective swelling systems where the component of swelling agent will not undergo any chemical reaction with BCP chains, 1, 3-PPST here not only constitutes the swelling agent together with ethanol but also works as modifying agent that can react with PDMAEMA chains to form zwitterions. In the swelling process, ethanol permeates into the films and is enriched in the PDMAEMA microdomains as ethanol is a good solvent to PDMAEMA but a nonsolvent to PS, leading to strong swelling of PDMAEMA microdomains. However, as one component of swelling agent, 1, 3-PPST is a nonsolvent to both PDMAEMA and PS blocks, consequently its presence would have a negative effect to the swelling process, resulting in the lower swelling degree of the BCP film compared to swelling in pure ethanol. Conversely, as the modifying agent, 1, 3-PPST is covalently bonded to PDMAEMA chains and will increase the volume fraction of the original PDMAEMA microdomains. It is known that higher volume ratios of hydrophilic blocks lead to stronger swelling degrees of BCP films based on

15

previous reports [34], therefore the presence of 1, 3-PPST as the role of modifying agent is able to facilitate the swelling process. The former negative effect plays the major role in the earlier stage of the swelling process because 1, 3-PPST has not diffused into the deep inside of the BCP film yet and its reaction with PDMAEMA chains primarily takes place on the film surface. With prolonged swelling durations, the latter positive effect of 1, 3-PPST dominates the swelling process, so higher swelling degree and larger pore sizes are achieved. This is consistent with the above discussion that the presence of 1, 3-PPST may facilitate the swelling process of PDMAEMA-b-PS film at a swelling duration of 6 h. The final decrease of thickness for a swelling duration of 20 h should be due to the micellization of BCP under strong swelling-induced by long swelling durations.

16

Figure 3. The change in thickness of PDMAEMA-b-PS films for various swelling durations soaked in ethanol with and without 1, 3-PPST (5wt% concentration), respectively.

3.2 Surface compositions and hydrophilicity The variation in chemical structure of PDMEMA-b-PS films during the course of swelling was investigated by ATR-FTIR characterizations (Figure 4). Compared with the pristine film, the prominent peaks located at 1037 cm-1 and 1160 cm-1 [35] after swelling in the presence of 1, 3-PPST can be attributed to the symmetric stretching vibration and asymmetric stretching vibration of the sulfonate anions. These characteristic peaks were intensified with increasing 1, 3-PPST concentrations. The FTIR results imply that the tertiary amines in the PDMAEMA chains are engaged in the nucleophile substitution reaction with 1, 3-PPST thus forming zwitterionic PSBMA. Also, it clearly demonstrates the feasibility of this in situ zwitterion functionalization during the selective swelling process. It should be noted that the weak peaks located at around 1037 cm-1 and 1160 cm-1 in the spectra of pristine membrane were resulted from C=S stretching vibration of RAFT agent group and C-O stretching vibration of DMAEMA units, respectively.

17

Figure 4. ATR-FTIR spectra of pristine PDMAEMA-b-PS films and the films soaked in ethanol with different concentration of 1, 3-PSST for 6 h.

To obtain the degree of quaternization reaction between 1, 3-PPST and PDMAEMA chains during the swelling process, surface chemical compositions of the BCP films were investigated by XPS wide-scan spectra (Figure S4). The S2p peak in the spectrum of the pristine film was originated from the RAFT agent terminating PDMAEMA-b-PS [36]. The N1s peak at 399.7 eV was referred to the N of the tertiary amine. After converting PDMAEMA chains into zwitterions by reacting with 1, 3-PPST, an additional peak at 402.5 eV appeared, which was attributed to the N+ in the quaternary amines [37]. Therefore, compared to the pristine film, the N1s peak in the spectrum of

18

functionalized films was curved-fitted into two peaks. The conversion of tertiary amines to quaternary amines can be calculated using Eq. 7 by based on the XPS-peak-differentation-imitating analysis. Conversion$%% =

&N +( 100% &N +( + &N(

(7)

where [N] and [N+] represent areas of the different peak components originated from N1s core-level spectrum of the films. The zwitterions conversion of PDMAEMA chains were determined to be 54.3%, 58.9%, 59.6%, 60.8%, 71.2%, respectively, as the swelling duration was extended to 0.5 h, 2 h, 5 h, 10 h, 20 h (Figure 5).

Figure 5. N1s spectra of PDMAEMA-b-PS films after swelling in 5wt% 1, 3-PPST/ethanol for various durations: (a) 0 h, (b) 0.5 h, (c) 2 h, (d) 5 h, (e) 10 h and (f) 20 h.

19

The change in surface hydrophilicity of the BCP films with swelling treatment was evaluated by testing their water contact angles (WCAs) (Figure 6). A clear tendency of enhanced hydrophilicity was observed for the films treated both in ethanol and in the 5 wt% 1, 3-PPST/ethanol solution. The WCA of the pristine film was ~75°. For the cases of swelling in ethanol, the WCA was decreased to ~51° after swelling for 0.5 h, which was attributed to the formed porous structure and enrichment of hydrophilic PDMAEMA chains on the surface [38]. However, the film soaked in the 5 wt% 1, 3-PPST/ethanol solution showed a sharp decrease of WCA from 75° to 35° under an identical swelling condition. In addition to the enrichment of PDMAEMA, this enhanced hydrophilicity also came from the zwitterionic groups which are capable of binding water molecules firmly through electrostatic interaction and hydrogen bonding [39]. As swelling duration was prolonged from 0.5 h to 20 h, WCA of BCP films was gradually reduced to 34°, implying more PDMAEMA chains were migrated to the surface. Meanwhile, WCA of the film subjected to swelling with 1, 3-PPST was further decreased down to 29°. The WCA values were always lower than that of films treated in pure ethanol, indicating that zwitterionic modification also contributes to enhance surface hydrophilicity. It is notable that the major variation of surface hydrophilicity took place in the initial 0.5 h swelling duration. This is in good consistence with the results from XPS characterization that high conversion of quaternization reaction over 50% can be achieved at 0.5 h. After 5 h swelling, the WCA values of film treated by 1,

20

3-PPST/ethanol were almost unchanged, while the zwitterions conversion still increased according to the XPS results. This is because the surface porosity and roughness also contribute to the change of WCAs except for zwitterionic groups on surface. With increasing swelling duration, pores were generally enlarged, leading to the increase of surface porosity and roughness. According to Cassie model [40], the increasing surface porosity results in increasing apparent WCA of a porous surface. The competing effect of surface porosity and zwitterionic groups eventually results in unchanged WCAs after swelling for 5 h.

Figure 6. Water contract angles of PDMAEMA-b-PS films after soaking in ethanol and 5 wt% 1, 3-PPST/ethanol for various durations.

21

3.3 Separation performance To reveal the implication of zwitterionization with the permselectivity of membranes, we prepared composite membranes by coating BCP solutions on macroporous PVDF substrates and investigated their separation performances after

swelling.

Swelling

in

5

wt%

1,

3-PPST/ethanol

resulted

in

zwitterion-functionalized BCP composite membranes (abbreviated as ZCP membranes thereafter), while ethanol swelling gave non-zwitterionic BCP composite membranes (abbreviated as NCP membranes thereafter) as a comparison. No water permeated through the composite membranes at a pressure of 0.5 bar prior to swelling treatment, confirming a dense and defect-free nature of as-coated membranes before swelling. As shown in Figure 7, the ZCP membrane exhibited a water permeance of ~89 L/(m2·h·bar) and a BSA rejection as high as 95.5% after swelling for 0.5 h, demonstrating the formation of interconnected nanopores throughout the entire BCP layer. With a swelling duration to 2 h, the water permeance was increased to ~220 L/(m2·h·bar), and BSA rejection was slightly decreased to 95%. As the duration was prolonged to 5 h and 10 h, the permeance was further increased to 472 and 775 L/(m2·h·bar) with BSA rejection correspondingly decreased to 80% and 60%, respectively. This change in permselectivity should be attributed to the enlarged membrane pores with extending swelling durations. Finally, the ZCP membrane reached a permeance up to ~1000 L/(m2·h·bar) and its BSA rejection was decreased to 55% after a long swelling duration of 20 h.

22

Compared to the NCP membranes, it can be observed that ZCP membranes displayed much higher permeance than NCP membranes under same swelling durations. For instance, the water permeance of NCP membranes prepared at a swelling duration of 2 h and 5 h is ~63 and ~89 L/(m2·h·bar), which are less than one third of the permeance of corresponding ZCP membranes. Moreover, with a BSA rejection of ~95%, the NCP membranes showed a permeance of ~167 L/(m2·h·bar) at a swelling duration of 10 h, which is also lower than the permeance of the ZCP membrane having the same BSA rejection. The significant increase in permeability of ZCP membranes should be attributed to the enlarged pores and enhanced hydrophilicity both of which were caused by the zwitterionation. Compared with commercial ultrafiltration membranes, the permeance of prepared ZCP membrane has ~2-4 times improvement with similar rejections [41, 42].

23

Figure 7. The water permeance and BSA rejection of zwitterionized and non-zwitterionized membranes prepared for different swelling durations.

3.4 Anti-fouling performance In view of the strong interaction with water molecules of polyzwitterions, the prepared ZCP membranes here are expected to exhibit excellent anti-fouling performance. Then ZCP and NCP membranes with close water permeance and rejection were selected to contrast their antifouling performance. The former was obtained by swelling in 5 wt% 1, 3-PPST/ethanol for 2 h, and the latter was prepared by ethanol swelling for 10 h. Herein, BSA was chosen as a model foulant because it is easy to form adhesion layer with PDMAEMA chains due to electrostatic interaction [43]. The membranes were

24

first subjected to deionized water filtration for 60 min until reaching a stable water permeance. Then 1 g/L BSA solution (pH 7.4) was used to foul the membranes for another 60 min. After fouling, the membranes were washed with PBS buffer thoroughly, subsequently tested for next cycle again. Figure 8 illustrates time-dependent membrane permeances by filtering deionized water and BSA solution alternately for three cycles. The ZCP membrane showed an initial water permeance of ~131 L/(m2·h·bar). After fouling by BSA, the permeance was reduced to 102 L/(m2·h·bar). PBS cleaning could recover the permeance of membrane to 112 L/(m2·h·bar), indicating a FRR of 86% as shown in Table S3. As a comparison, the permeance of NCP membrane was significantly declined from ~115 to ~25 L/(m2·h·bar) once fouled by BSA and the FRR after cleaning is only 41%. Notably, the FRR of ZCP membranes remained unchanged in the second and third test cycle, implying that protein adsorption approached saturation in the first cycle and the irreversible permeance recovery was determined to be ~14%. In contrast, the FRR of NCP membranes was declined to 29% and 27%, respectively after the second and third cycle, indicating more protein was accumulated on membrane surfaces with the proceed of fouling test. These results demonstrated the outstanding fouling resistance of ZCP membranes, which is due to the formation of hydration layer between betaine-based zwitterionic polymers and water molecules on one hand, and charge neutrality of zwitterion groups capable of preventing BSA nonspecific adhesions on the other. Considering the facts that

25

the zwitterionic polymers are covalently bonded to the membrane surface and would not leach our during separation operation, such excellent fouling resistance of the produced membranes is expected to be maintained during long-term use.

Figure 8. Time-dependent permeance of water and the BSA solution (1 g/L) of zwitterionized and non-zwitterionized membranes.

4. Conclusion In summary, we develop a novel one-step approach to prepare zwitterion-functionalized BCP membranes with excellent fouling resistance. Simply soaking PDMAEMA-b-PS films into the mixture of ethanol and 1, 3-PPST can produce nanoporous membranes with zwitterionic polymer chains

26

covered on the film surface and pore walls. Herein, 1, 3-PPST acts as both one component of swelling agent and the quaternization agent capable of transforming PDMAEMA chains into polyzwitterions. Within the range of certain concentrations (<10 wt%), higher concentrations of 1, 3-PPST in ethanol result in larger pores. The kinetic investigation of the swelling process indicates that 1, 3-PPST leads to a higher swelling degree of BCP films while it slows down the swelling process in the early stage. XPS and FTIR characterization evidence the successful zwitterion functionalization to the membrane

surface

through

such

a

simultaneous

pore-making

and

zwitterionization strategy. Owing to the strong hydrophilicity and enlarged pores, the zwitterion-functionalized membranes exhibit significantly improved water permeance. Furthermore, the outstanding anti-fouling performance enabled by zwitterionic groups has been confirmed via dynamic BSA fouling tests, and the water permeance recovery can reach as high as 86% after three cycles of BSA filtration.

Acknowledgments Financial support from the National Basic Research Program of China (2015CB655301) and Natural Science Foundation of China (21776126,

27

21825803), the Jiangsu Natural Science Foundation (BK20150063) are gratefully acknowledged. We also thank the support from the Program of Excellent Innovation Teams of Jiangsu Higher Education Institutions and the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Highlights Selective swelling and zwitterion functionalization are coupled into one step. 1, 3-PPST works as both one component of the swelling agent and modifying agent. Zwitterionic membranes exhibit excellent anti-fouling performance. Separation performance can be facilely regulated by swelling durations.

Author Statement

Chenxu Zhang: Investigation, Data curation, Writing-orginal draft preparation. Congcong Yin: Investigation Yanjie Wang: Validation. Jiemei Zhou: Methodology, Writing-review & editing. Yong Wang: Conceptualization, Supervision. All authors have approved to the final version of the manuscript.

The authors declare no conflict of interest.