Synthesis of well-defined PVDF-based amphiphilic block copolymer

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Materials and Interfaces

Synthesis of well-defined PVDF-based amphiphilic block copolymer via ITP for antifouling membrane application Luqing Zhang, Zhongkai Zhu, Umair Azhar, Jiachen Ma, Yabin Zhang, Chuanyong Zong, and Shuxiang Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00533 • Publication Date (Web): 09 May 2018 Downloaded from http://pubs.acs.org on May 9, 2018

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Industrial & Engineering Chemistry Research

Synthesis of well-defined PVDF-based amphiphilic block copolymer via ITP for antifouling membrane application Luqing Zhang,† Zhongkai Zhu,† Umair Azhar, Jiachen Ma, Yabin Zhang, Chuanyong Zong,* Shuxiang Zhang*

Shandong Provincial Key Laboratory of Fluorine Chemistry and Chemical Materials/Shangdong Engineering Research Center for Fluorinated Material, School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China

Abstract: An innovative strategy is devised, to obtain poly(vinylidene fluoride) membranes with splendid fouling resistance used amphiphilic block copolymer containing poly(vinylidene fluoride) (PVDF) segments as modification agent. The PVDF-based block copolymer with poly[2-(N,N-dimethylamino) ethyl methacrylate] as the hydrophilic segment (PVDF-b-PDMAEMA) was successfully fabricated by iodine transfer polymerization. This strategy not only improved the PVDF membrane structure and hydrophilicity but also assisted in fouling resistance properties as an ultrafiltration membrane. The obtained PVDF/PVDF-b-PDMAEMA blend membrane displayed excellent water flux, which was enhanced by 3 times compared with the original membrane. And the bovine serum albumin (BSA) rejection ratio (≥90%) maintained at a reasonable level. Meanwhile, by incorporation of the amphiphilic block copolymer, the PVDF based modified membrane possessed excellent protein

ACS Paragon Plus Environment

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

resistance and high flux recovery ability. The blend membranes with high BSA separation efficiency and excellent antifouling properties will find promising applications in the field of separation science specifically in sewage purification.

Keywords: ITP polymerization; PVDF membrane; Amphiphilic copolymer; Hydrophilic modification; Anti-fouling

1. Introduction PVDF is an excellent material for water treatment and separation processes, owing to its satisfactory mechanical properties, excellent thermal stabilities, good chemical resistances and easy processing.1-4 However, PVDF membrane encounters limitations due to its easy membrane fouling and strong hydrophobicity when treating aqueous solutions containing natural pollutants. To minimize the adverse effects of fouling, the contaminants need to be frequently removed from membrane through cleaning operation, which minifies the lifetime of the membrane and eventually increases the operational costs.5,6 Hydrophilic modification as an improvement strategy endows the PVDF membranes with better anti-fouling ability, and this method has been received extensive attention and research focus in recent years.7-9 Normally, the blending of hydrophilic modifier is regarded as a preferable and facile strategy to enhance the hydrophilic ability of original PVDF membrane, which acts simultaneously as a modifier for both membrane internal pores and membrane external surface during the


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preparation process.10,11 Generally, fouling resistance properties of PVDF membranes can be ameliorated by mixing with hydrophilic addition, for example, TiO2,12 Al2O3,13 graphene oxide,14 and polyethylene glycol (PEG) derivatives15 etc. However, the poor compatibility of these modifiers with the PVDF membrane matrix can cause membrane performance instability and further limits the actual applications of the blending modification membranes. To solve this problem, using amphiphilic copolymers consist of PVDF backbones and hydrophilic segments as the alternative blending modifiers is an optimal strategy.16,17 These amphiphilic copolymers exhibit excellent antifouling property, resulting from the distribution of hydrophilic segments onto the membrane and internal pore wall surface. Meanwhile, the hydrophobic PVDF segments guarantee the compatibility of copolymers with PVDF matrix, which anchors with the PVDF matrix to prevent the elution of the amphiphilic segment. Amphiphilic graft copolymers with PVDF segments as hydrophobic segments have been synthesized and discussed in many reports.18-20 Among these methods, amphiphilic

graft

copolymers

are

achieved

through

the

appropriate

chemical modification of the secondary fluorinated site of PVDF.21, 22 However, these methods cannot control the number of living initiated sites on each PVDF chain, which may result in an inconsistent number of hydrophilic segments grafted onto each PVDF chain. With the significant advancement in controlled radical polymerization technique, well-defined PVDF-based block copolymer obtained by ATRP, RAFT, and ITP is now possible.23,24 Among these developed processes, it is worth noting that ITP as a technique enables polymerization to proceed under air condition without the



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catalyst, and it also has been accepted as a universal industrial technique of copolymers synthesis.25,26 Valade et al. synthesized PVDF-based block copolymer containing polystyrene segments via ITP. It was found that these copolymers with different molecular weights could be synthesized via adding different concentrations of monomer as well as chain transfer agent, which demonstrated the good controlled character of styrene in ITP.27 A. Meskini et al. prepared various PVDF-based block copolymers by ITP, using cyan monomer of vinylidene cyanide, methacrylonitrile, and acrylonitrile. The results showed that as compared to the cyano-containing homopolymers, the incorporation of VDF units could improve the dielectric permittivity of the block copolymers.28 Boyer et al. had researched the reaction mechanism of methyl methacrylate via the reverse ITP and demonstrated that it was applicable to methacrylates.29 The above-mentioned studies provided some clues for a feasible method of preparing amphiphilic block copolymers consist of PVDF backbones to obtain hydrophilic modified PVDF membranes. However, it is to be noted that there is relatively less research attention being paid on the preparation of PVDF based amphiphilic copolymers for membrane modification by ITP.

In this study, synthesis of PVDF-b-PDMAEMA block copolymer, based on the ITP

of

VDF

and

DMAEMA

was

achieved.

Subsequently,

PVDF/PVDF-b-PDMAEMA blend modified membrane was cast via immersion precipitation phase inversion technique. PVDF segments of PVDF-b-PDMAEMA showed excellent compatibility with PVDF membrane matrix, and the PDMAEMA chains were migrated spontaneously towards membrane surface and pore wall,


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leading to enhance the wetting ability of PVDF membranes. The PVDF-based block copolymer modified membranes exhibited excellent permeability, selectivity and anti-fouling properties, which may have potential use in applications related to separation science.

2. Materials and methods

2.1 Materials Vinylidene fluoride and commercial PVDF (1010, Mn = 1.53 × 105 g·mol-1) were bought from Dongyue Group Co., Ltd, (China) and from Solvey Co., Ltd. (Belgium). DMAEMA (99%) was supplied by Aladdin Chemical Co. (Shanghai, China). tert-Butylperoxypivalate (TBPPI) and trichlorotrifluoroethane (F113) were bought from Huafu Chemical Co., Ltd. (Shandong, China). Perfluorohexyl iodide (CTA) was purchased from Fuxin Co., Ltd. (Liaoning, China). Phosphate buffered saline buffer solution (PBS) was provided from Sino pharm Chemical Reagent Co., Ltd. (Beijing, China). BSA was supplied by Yuanye Bio-Technology Co., Ltd. (Shanghai, China). All other reagents used in this paper were commercial analytical products unless otherwise specified.

2.2 Synthesis of block copolymer First, a certain amount of F113 (115 g), CTA (0.669 g, 1.50 × 10-3 mol), and TBPPI (0.0718 g, 4.12 × 10-4 mol) were added into the autoclave. After complete dissolution of the components, VDF (14.4 g, 2.25 × 10-1 mol) was put into the reaction system. The reaction process lasted 22 h at temperature 85 °C before unreacted VDF was



released and white polymer powder was obtained by drying the solvent. An iodine-end-capped polyvinylidene fluoride (PVDF-I) was obtained, which was used for the subsequent polymerization reaction.

In a typical synthesis procedure, a reactor with a magnetic stirrer was charged with DMAEMA, PVDF-I, TBPPI (PVDF-I: TBPPI = 3 mol: 1 mol) and DMF. The sealed reaction vessel was purged with nitrogen and performed at 85 ºC for 22 h, and then naturally cooled. Unreacted monomer and solvent were removed by dialysis method, and then dried at 50 °C for 48 h. Three different block copolymers (C1, C2, and C3) with content variation of DMAEMA ([PVDF-I]: [DMAEMA] (mol: mol) = 1:50, 1:100 and 1:150) were synthesized in this section (shown in Table S1).

2.3 Membranes preparation Membranes of PVDF and blend block copolymer were fabricated by the classical non-solvent induced phase separation process according to the previous reports.30,31 Commercial PVDF was mixed with PVDF-b-PDMAEMA block copolymers and PEG10000 as a pore-forming agent with different composition ratios in DMF casting solutions. The composition of polymers solution used for membrane casting was shown in Table 1. After stirring at 60 ºC for about 8 h, the casting solution was degassed for 1 h under vacuum. Blend membranes were prepared with different compositions, via scraping the solution onto glass substrate followed by immersion in deionized aqueous coagulation bath. In this paper, the block copolymer PVDF-b-PDMAEMA used to fabricate blend membranes was denoted as C3 unless


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otherwise stated.

Table 1. Compositions of the casting solutions for preparation of PVDF/PVDF-b-PDMAEMA blend membranes

Membrane type

Bulk composition

Casting solution composition (g)

PVDF

PVDF-b-PDMAEMA

PEG

DMF

M0

Pure PVDF

18

0

9

85.5

M5

5 wt% PVDF-b-PDMAEMA

18

0.95

9

85.5

M10


18

2

9

85.5

M20


18

4.5

9

85.5

M30


18

7.71

9

85.5

* The block copolymer PVDF-b-PDMAEMA used here is C3.

2.4 Polymer characterization The FT-IR spectra of PVDF-I and block copolymer were obtained from a Nicolet Is10 FT-IR spectrophotometer (Thermo Fisher, USA). The 1H and 19F NMR of block copolymers were analyzed using an ADVANCE III 400 MHz spectrometer with dimethyl sulfoxide-d6 (DMSO-d6) as solvent and tetramethylsilane as the internal standard. The molecular weight and dispersity (Ð) of block copolymer were determined by gel permeation chromatography (GPC) with a column of DMF solvent,



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Waters Model 1515 HPLC pump, and Waters Model 2414 refractive index (RI) detector.

2.5 Membrane characterization The chemical elementary composition of the membranes was analyzed by X-ray photoelectron spectroscopy (XPS, Geneis 60S, EDAX Inc, USA) and fourier transform infrared spectrometer (FT-IR, Nicolet iS1, Thermo Fisher Scientific, USA). Surface wettability of blend membrane was obtained via water contact angle (WCA) measurement (OCA40, Dataphysics, Germany) at room temperature. Dropped the water (3µL) on the membrane surface, and observed the change of WCA as drop age. The morphology of membranes surface and cross-section were characterized via scanning electron microscopy (SEM, S-2500, Hitachi, Japan).

2.6 Filtration Experiments Filtration experiments were carried on our homemade permeation test rig equipment referring to the presentation of procedures by Liu et al.19 The tested membrane was fixed in the filtration instrument and pre-compacted under 0.2 MPa (30 min) before measurement. The stabilized water permeation flux was determined under 0.10 MPa for 30 min. And the steady pure water flux (Jw, L m-2 h-1) was calculated by the following Eq (1):16,32

Jw =

V A∆t

(1)

where J is the volume of permeation flux (L m-2 h-1), V is the volume of filtered


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water (L), ∆t is the testing time (h), and A is the effective filter area (m2). Here, the effective membrane area of the home-made equipment was 2.25 × 10-4 m2. Rejection ratio (R) of the membrane was researched, that solution of BSA (1g·L-1) was permeated through the membrane under 0.1 MPa. The R was calculated based on the Eq (2):16,32

R=(1-

Cpermeate Cfeed

)×100%

(2)

where feed solution (Cfeed) and protein amount (Cpermeate) in permeate solution were measured by a UV-vis spectrometer (Persee TU-1910, China) at 280 nm wavelength.16 To test the antifouling properties of blend membranes, multi-cycle operations were performed by using BSA (1 g·L-1) solution as a model pollutant and followed by pure water washing. Each cycle filtration process consisted of three steps, pure water flux (Jw1) of the first step was obtained, second, BSA solution flux (JP) of simulation fouling process was measured, then the permeation flux (Jw2) after pure water cleaning was recorded. The flux recovery ratio (FRR), the reversible fouling value (Rr), the irreversible fouling value (Rir), and the total fouling value (Rt) were introduced to study the antifouling performance of membrane in water treatment process. All the FRR, Rr, Rir, and Rt values were calculated by the eqs 3, 4, 5, and 6, respectively.32

FRR= Rr = (

Jw2 Jw1

×100%

Jw2 JP Jw1

) × 100%


(3)

(4)


Rir = ( Rt = (

Jw1 Jw2 Jw1

Jw1 JP Jw1

) × 100%

) × 100%

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(5)

(6)

3. Results and discussion

3.1 Characterization of polymer

Figure 1. (A) 19F NMR spectroscopy of PVDF-I; (B) 1H NMR analysis of PVDF-I and block copolymer; (C) FT-IR analysis of PVDF-I and block copolymer; and (D) GPC analysis of PVDF-I and block copolymers.

The representative 19F NMR spectroscopy of PVDF-I was depicted in Figure 1 A. The spectrum of PVDF-I exhibited a significant signal peak at -92.5 ppm (c) was attributed to vinylidene fluoride groups (-CF2CH2-). The feature peaks of CTA group


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at -82.0 (a) and -122.0 to -128.0 (b) ppm were corresponded to CF3- and -CF2- groups, respectively. There was a significant peak at the chemical shifts of about -40 ppm (d) due to end groups (-CH2CF2dI) of PVDF-I, which indicated that the structure of PVDF-I still contained living initiated site of ITP.33 Figure 1 B show the 1H NMR analysis for PVDF-I and synthesized block copolymer. The characteristic signals of PVDF-I at 2.90 and 2.25 ppm were attributed to head-to-head (HH) and head-to-tail (HT) polymer structure, respectively. Compared with PVDF-I spectrum, the chemical shifts of methyl protons appeared at 0.93 ppm (signal b), and the peaks at chemical shift of 2.19 and 3.99 ppm were corresponded to the protons of -CH2-CH2-N(CH3)2 in PDMAEMA chains.34 These results

also

confirmed

successful

polymerization

of

PVDF-b-PDMAEMA

amphiphilic copolymer. The mole fraction of DMAEMA in block copolymer, defined as M(DMAEMA), was calculated by Eq (7):35

M(DMAEMA) = 1

1 A 6

(A +A ) 6 HT HH

×100%

(7)

where A, AHT and AHH represent feature-peak areas of -N(CH3)2, PVDF (HT) and PVDF (HH), respectively. The M(DMAEMA) of block copolymer was calculated to be 15.06%.

The FT-IR analysis of PVDF-I and copolymer were shown in Figure 1 C. The FT-IR spectrum of PVDF-I showed characteristic stretching vibration peaks at 1168 and 1411 cm-1 corresponded to the -CF2 and -CH2 group, respectively,36 revealed the



successful polymerization of VDF. The additional characteristic absorption band of amphiphilic PVDF-b-PDMAEMA at 1730 cm-1 compared to PVDF-I,37 resulted from the stretching vibration of carbonyl in the PDMAEMA chains. These results suggested successful synthesis of the PVDF-b-PDMAEMA block copolymer.

The GPC traces of the PVDF-I and block copolymers were exhibited in Figure 1 D. A negative trace of PVDF-I was observed, which was due to the low-refractive indices of fluoropolymers. However, with the increase in monomer concentration , the GPC traces of block copolymers were strongly overturned, indicated that the polymerization of DMAEMA occurred well.38 Nevertheless, the broad dispersity of the copolymers with high Mw was found, due to the existence of a small number of dormant PVDF chains (-CF2CH2I) in the macromolecular chain transfer agents, which were residues in the copolymers. Based on the GPC results, we have successfully synthesized the copolymer PVDF-b-PDMAEMA via ITP.

3.2 Characterization of membranes

3.2.1 Compositions of membranes The FT-IR spectra of pure PVDF membrane and PVDF/PVDF-b-PDMAEMA blend membranes were displayed in Figure 2. The FT-IR spectra of all blend membranes (M5, M10, M20, M30) exhibited characteristic absorption band at 1728 cm-1 compared to raw PVDF membrane (M0) due to the stretching vibration of the carbonyl group in PDMAEMA.36 Therefore, during the preparation process of the blend membranes, the PDMAEMA segments were migrated to the membrane surface,


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based on the FT-IR results.

(E) (D) (C) (B) (A) -1

1728cm

800

1200

1600

2000

2400

2800

Wavenumber(cm)-1

Figure 2. FT-IR spectra of the PVDF (M0) (A) and PVDF/PVDF-b-PDMAEMA (M5) (B), (M10) (C), (M20) (D), (M30) (E) blend membranes.

XPS

analysis

was

also

performed

to

further

ascertain

the

surface

element compositions of membranes (Figure 3). The pure membrane (Figure 3 A) showed major emission peaks correspond to C1s (285.8 eV) and F1s (689.4 eV) of PVDF.39 The emission peaks of N1s (397.9 eV) and O1s (532.1 eV) had been unexpectedly appeared, which were due to the residues of solvents and porogen PEG10000 in the preparation process.7,17 The C1s core-level spectrum of M0 (Figure 3 C) was resolved into four peaks, and the peaks at 286.6, 284.8, and 291.1 eV corresponded to the -CH2, -CH, and -CF2 groups, respectively.40 In addition, the -C-O- group species had a binding energy at about 287.0 eV from residual porogen (PEG10000). The blend membranes (M30) also exhibited the same major emission peaks (Figure 3 B). However, six C1s peaks of PVDF/PVDF-b-PDMAEMA blend



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membranes were observed, besides four components corresponded to pure PVDF membrane, where two peaks for C-N and -O=C-O- species from PDMAEMA segments, at the binding energies of about 286.7 and 288.9 eV (Figure 3 D), respectively.41,42 In addition, from the data of Table 2, it is clear that the content of nitrogen (N) detected in M30 increased significantly compared to the original film, which proved that the PDMAEMA segments have migrated onto the blend membrane surface. But it is worth noting, the O content of blend membrane (M30) was less than that in raw PVDF membrane as shown in Table 2, which was resulted from the higher residual quantity of PEG in pure PVDF membrane. The surface DMAEMA mole fraction (Ms) of PVDF-b-PDMAEMA membrane was calculated by Eq (8): 34

Ms =

[N]- 0.89 [N]- 0.89+ 0.5×[F]

×100%

(8)

Where [N] and [F] are the percentage coefficients of nitrogen and fluorine elements of the membrane surface, respectively (data shown in Table 2). The coefficient “0.5” means that every PVDF structural unit possesses two F atoms. Meanwhile, we assume that all samples are affected by the same amount of N (0.89%) (Table 2). The calculated result by Eq (8) indicated that the surface DMAEMA mole fraction (Ms) of M30 was 10.54%. As described above, the content of PVDF-b-PDMAEMA in the PVDF/PVDF-b-PDMAEMA blend membrane M30 was 30 wt%. Based on the molecular weight of the commercial PVDF (Mn = 153,000 g·mol-1) and PVDF-b-PDMAEMA (C3, Mn = 28775 g·mol-1) (as shown in Figure 1 D), the mole fraction of PVDF-b-PDMAEMA in the PVDF/PVDF-b-PDMAEMA blend membrane M30 was calculated to be 70%. Combined with the DMAEMA mole fraction in the ACS Paragon Plus Environment

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PVDF-b-PDMAEMA copolymer calculated by Eq. (7) (characterized by 1H NMR analysis, M(DMAEMA) = 15.06%), the theoretical surface DMAEMA mole fraction was about 10.47%, which was consistent with the data determined by XPS (Ms = 10.54%). This result indicated that the copolymers PVDF-b-PDMAEMA had superior compatibility with the commercial PVDF and maintained excellent dispersion stability during the film formation process in an aqueous medium.

Figure 3. XPS analysis (A, B), and their corresponding C1s core-level spectra (C, D) of membranes: M0 (A, C) and M30 (B, D).



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Table 2. Element mole percentages for M0 and M30 membranes analyzed via XPS.

C (%)

F (%)

O (%)

N (%)

M0

57.88

33.43

7.80

0.89

M30

60.27

29.87

7.21

2.65

3.2.2 Morphology of membranes The construction morphology of block copolymer blend membranes with different PDMAEMA content was obtained from SEM (Figure 4). It has been shown that the pristine PVDF membrane possessed a dense surface structure, while a growing number of pores were formed on the top surfaces of the membrane as the PVDF-b-PDMAEMA content in blend membranes increased (Figure 4 A-E). These results indicated that the block copolymers possessed excellent pore-forming capability.16,37 As shown in Figure 4 (a-e), the membrane blending with PVDF-b-PDMAEMA copolymers exhibited typical asymmetric membrane structure in cross-section, including a thin functional layer, long finger-like macroporous layer and spongy pores of the support layer, which was similar to the PVDF pure membrane. However, it is to be noted that the blend membranes exhibited larger and wider finger-like structures. That’s because fast phase inversion has been occurred in casting solution due to the hydrophilic segments in the amphiphilic copolymer,1,43 which conspicuously improved water flux of the blend membrane.5,8


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Figure 4. SEM images of the top surface (A-E) and cross-section (a-e) of prepared membranes: M0 (A, a); M5 (B, b); M10 (C, c); M20 (D, d); M30 (E, e).



3.2.3 Membrane wettability measurement According to previous literature, anti-fouling of PVDF membranes can be improved with increasing membrane hydrophilicity.7-9 Figure 5 illustrates the curves of WCA decay with drop age, as expected the hydrophilicity of blend membranes was significantly enhanced. The initial WCA of M0 was 84.5°, pointed out the inferior surface hydrophobicity of the PVDF membrane. Compared with M0, different WCAs of blend membranes were obtained, decreasing from 84.5° to about 75.0°. In addition, the blend membranes were wetted faster with increasing PVDF-b-PDMAEMA block copolymer blend ratio. For example, the WCA of M30 was dramatically decreased from 72.7° to 20.6° within 300 s. However, a small decrease in water contact angle was found in PVDF membrane (only about 11°). This revealed the block copolymer successfully compound to the membrane and subsequently reduced the interface energy,17,44 which improved the hydrophilicity of blend membranes. Another important observation was that, increasing the amphiphilic block copolymer amount to more than 10 wt%, led to only a slight further enhancement of surface hydrophilicity. These results can be explained by the distribution of the PDMAEMA segments onto the membranes surface and pore wall, as proved in the previous report.45


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90 75

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


60 45 M0 M5 M10 M20 M30

30 15 0

50

100

150

200

250

300

Drop Age (sec)

Figure 5. Curves of water contact angle decaying with drop age for M0, M5, M10, M20 and M30.

3.2.4 Filtration and antifouling performance of membrane The pure water flux and rejection ratio of block copolymer blend membranes were observed through filtration experiments and the result have been demonstrated in Figure 6 A. The water flux (JW) of M0 was only about 143.2 L m-2 h-1, while the JW of the modified membranes was obviously increased from 335.3 to 426.1 L m-2 h-1 with heightening amphiphilic block copolymer content. This improvement in JW can be attributable to the dual effect of the formation of hydrophilic membrane surface and modification of pores structure in the membranes.18,19 But it is noteworthy that increasing the amphiphilic block copolymer amount to 10 wt% led to relatively saturated JW., which was due to the similar membrane surface wettability (Figure 5) and internal pores structure (Figure 4) of the blend membranes. In conclusion, the PDMAEMA segments of the block copolymers were migrated onto surface and pore



wall of the membrane, and the hydrophobic PVDF chains of block copolymers were existed inside of the membrane matrix. This structure made the hydrophilic chains permanently distributed in the pore walls and membrane surfaces, which could guarantee the excellent performance of the blend membranes. Figure 6 A illustrated the BSA rejection rate of the membranes. The pristine membrane (M0) exhibited a satisfactory rejection rate (96%) resulted from the less quantity of pores in membrane surface.46 For the modified membranes, the formation of porous and low obstacle membrane structure resulted in decrease in BSA rejection rate theoretically. However, it still maintained at a high level (90%) due to the water molecules adsorbed to the hydrophilic membrane surface formed a water boundary layer, which acted as a barrier to hinder the protein molecules to penetrate.1,32 At the same time, a detailed study of the anti-fouling properties of modified membranes was carried out through three-cyclic filtration operations, to analyze the protein pollution parameters. Here, three types of block copolymers were used as modifiers with constant content at 30 wt% (labeled as M30, C1; M30, C2; M30, C3) in the preparation process of the membrane. As shown in Figure 6 B, all membranes fluxes decreased during the protein filtration. That’s because the protein was adsorbed and deposited on membrane surface and pores, heading to form filtration cake layer. Flux recovery and BSA solution filtration of all blend membranes maintained at higher level as compared to the original membrane, which was due to the hydrophilic segment could combine with water molecules to form anti-pollution hydration layer.19 At the same time, by comparing flux recovery of C1, C2, and C3, it was demonstrated that the length of the hydrophilic portion determined the ability of the blend membrane to resist contamination.4,32 As can be seen from the Figure 6 C, the reversible Rr and FRR values of pure PVDF membrane have gradually declined along with the increase of the filtration times, and on the other hand, Rir tend to increase obviously. In contrast, with


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increasing the cycle times, the FRR and Rr values of the modified membrane increased evidently, while the Rt and Rir values were constantly decreased as presented in Figure 6 D. Hydration layer on blend membrane surface through hydrophilic PDMAEMA segments in combination with water was formed, which hindered the deposition of protein molecules. Moreover, the protein molecules could be discharged simply by flushing.4,32 This shows the antifouling ability of pure PVDF membrane was effectively enhanced through composite with the PVDF-b-PDMAEMA block copolymer.

Figure 6. (A) Pure water flux and BSA rejection ratios of the membranes (M0, M5, M10, M20, and M30); (B) Time-dependent fluxes of pure PVDF (M0) and three types of blend membranes M30 (C1, C2, C3); Rt, Rr, Rir, and FRR of pure PVDF membrane (M0) (C) and PVDF/PVDF-b-PDMAEMA blend membranes (M30, C3) (D) during multicycle filtration.



4. Conclusion In summary, a block copolymer PVDF-b-PDMAEMA with amphiphilicity was successfully prepared via iodide transfer agent polymerization and used for the development of high-performance membranes. The PVDF/PVDF-b-PDMAEMA blend modified membranes have preponderances in simple preparation by immersion phase inversion method. PDMAEMA segments were uniformly enriched on the blend modified membrane surface and porous surface, which endowed the hydrophilicity of the membrane. Based on the introduction of block copolymers, it was advantageous to obtain a low resistance structure in membranes, which enhanced the filtration efficiency. Compared with pure PVDF membrane, PVDF/PVDF-b-PDMAEMA blend membrane exhibited excellent properties of flux and antifouling in the cyclic filtration process, which will find brilliant applications in sewage purification field.

ASSOCIATED CONTENT Supporting Information

Summary of copolymers with various concentrations of DMAEMA.

Author Information

Corresponding Author S. X. Zhang (E-mail: [email protected]), C. Y. Zong (E-mail: [email protected])


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Author Contributions †

L.Q.Zhang, Z.K.Zhu contributed equally to this work.

The manuscript was written through contributions of all authors. All authors read and approved the final version of the manuscript.

Acknowledgments S. X. Zhang, C. Y. Zong acknowledge financial supports from the Natural Science Foundation of China (21704033, 21304037), Natural Science Foundation of Shandong Province (ZR2017ZC0529), and Key Research Program of Shandong Province (2015GGX102011, 2018GGX102002).

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Table of Contents (TOC)


Synthesis of well-defined PVDF-based amphiphilic block copolymer

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