Construction of Hierarchical Fouling Resistance Surfaces onto Poly

Publication Date (Web): April 28, 2017 ... Owing to the highly hydrophobic nature, fluoropolymer membranes usually suffer from serious fouling problem...
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Construction of Hierarchical Fouling Resistance Surfaces onto Poly(vinylidene fluoride) Membranes for Combating Membrane Biofouling Xue Li,† Xuefeng Hu,‡ and Tao Cai*,† †

Key Laboratory of Biomedical Polymers of Ministry of Education, College of Chemistry and Molecular Science, Wuhan University, Wuhan, Hubei 430072, P. R. China ‡ National Engineering Research Center for Biomaterials, Sichuan University, Chengdu, Sichuan 610065, P. R. China S Supporting Information *

ABSTRACT: Owing to the highly hydrophobic nature, fluoropolymer membranes usually suffer from serious fouling problem, and therefore largely limited their practical applications. Also, the development of environmentally benign and nonreleasing antifouling coatings onto the inert fluoropolymer membranes remains a great challenge and is of prime importance for various scientific interests and industrial applications. In the present work, a facile and effective approach for the construction of hierarchical fouling resistance surfaces onto the poly(vinylidene fluoride) (PVDF) membranes was developed. Graft copolymers of PVDF with poly(hyperbranched polyglycerol methacrylamide) side chains (PVDF-g-PHPGMA copolymers) were synthesized via reversible addition−fragmentation chain transfer (RAFT) graft copolymerization of pentafluorophenyl methacrylate (PFMA) with the ozone-preactivated PVDF, followed by activated esteramine reaction of PPFMA chains with amino-terminated hyperbranched polyglycerol (HPG-NH2). The copolymers could be simply processed into microfiltration (MF) membranes with surface-tethered PHPGMA side chains on the membrane and pore surfaces by nonsolvent induced phase inversion. Furthermore, the PVDF-g-PHPGMA-g-PSBMA membrane was prepared via surface-initiated atom transfer radical polymerization (SI-ATRP) of zwitterionic monomer, N-(3-sulfopropyl)-N-(methacryloxyethyl)-N,N-dimethylammonium betaine (SBMA) from the PVDF-g-PHPGMA membrane and pore surfaces. Arise from a synergistic effect of the dendritic architecture of PHPGMA branches and “superhydrophilic” nature of PSBMA brushes, the PVDF-g-PHPGMA-g-PSBMA membranes exhibit superior resistance to protein and bacteria adhesion with insignificant cytotoxicity effects, making the membranes potentially useful for water treatment and biomedical applications. One may find the present study a general and effective method for the fabrication of antifouling fluoropolymer membranes in a controllable and green manner.

1. INTRODUCTION Fluoropolymer membranes have been widely applied in microfiltration (MF) and ultrafiltration (UF) processes because of their outstanding mechanical properties, thermal stability and chemical resistance.1−4 Owing to their hydrophobic nature, fluoropolymer membranes tend to be contaminated by the unintended accumulation of foulants and consequently gives rise to the decreased performance and efficiency of the membrane separation system, among which microbes are one of the most difficult to be cleaned up because of their selfduplication nature.5−8 Membrane fouling would become much more complicated once the porous structures are involved, as in that case, membrane fouling not only takes place on the exterior surface but also entraps into the membrane matrix. Foulants over the molecular weight cutoff (MWCO) are brought inside of a membrane by permeation, leading to the internal membrane fouling. Therefore, in case of MF and UF membranes, it is of crucial importance to modify both the © XXXX American Chemical Society

membrane surface and the inner pore surfaces in the membrane matrix, other than simple surface modification. To solve the membrane biofouling problems, three main strategies are commonly considered. The first strategy is fouling release coatings, where the surfaces actively release entrapped antibiotics to kill biofoulants. The second strategy is foulingdegrading coatings, where the surfaces with antimicrobial polymer brushes are capable of degrading biofoulants through direct contact. The third strategy is fouling resistance coatings, where the surfaces with hydrophilic polymer brushes could prevent biofoulants from attaching to surfaces.9−11 Despite the high efficacy in fouling mitigation, fouling release coatings that release and bring increasing resistance to toxic antibiotics while fouling-degrading coatings inevitably accompany with cumuReceived: January 19, 2017 Revised: April 3, 2017

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Scheme 1. Schematic Illustration of the Processes of Ozone Pretreatment and RAFT Graft Copolymerization of PVDF with PFMA, Activated Ester-Amine Reaction of PVDF-g-PPFMA Copolymer with HPG-NH2, Preparation of PVDF-g-PHPGMA Membrane by Nonsolvent Induced Phase Inversion (NIP), Preparation of PVDF-g-PHPGMA-g-PSBMA Membrane via SIATRP of SBMA from the PVDF-g-PHPGMA Membrane

philic polymer brushes have been developed as a supplementary approach to the ablative/erodible biocide coatings. Unlike dendrimers, which usually require tedious multistep syntheses, hyperbranched polyglycerol (HPG) offer a facile one-step synthetic protocol from anionic ring-opening multibranching polymerization, while still maintaining many outstanding features of dendrimers including tunable molecular

lative cytotoxicity, can bring about detrimental effects to the surrounding environment and human beings. These limitations promote the development of environmentally benign and nondepleting means for surface functionalization of membrane materials in biomedical devices, water treatment systems, and chemical industries. Fouling resistance coatings with hydroB

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America Inc. Triethylamine (NEt 3 , ≥99%), 4-cyano-4(phenylcarbonothioylthio)pentanoic acid (CTP, > 97%), N-(3sulfopropyl)-N-(methacryloxyethyl)-N,N-dimethylammonium betaine (SBMA, 97%), 2-bromo-2-methylpropionyl bromide (BIBB, 98%), 2,2′-bipyridine (Bpy, ≥99%), 1-methyl-2-pyrrolidone (NMP, 99.5%), γ-globulin (≥97%), and thiazolyl blue tetrazolium bromide (MTT, 98%) were obtained from Sigma-Aldrich Chemical Co. and were used as received, unless otherwise stated. Pentafluorophenyl methacrylate (PFMA, 95%) was purified by passing through an inhibitor-removal column and stored at −10 °C. Cuprous bromide (CuBr, 99%) was purified by washing alternatively with acetic acid, ethanol and acetone prior to use. A commercially available hydrophilic PVDF microfiltration (MF) membrane with a comparable pore size of 0.22 μm was purchased from Millipore Co. Hyperbranched polyglycerol with amino terminal (HPG-NH2, Mn,NMR = 1200 g/mol) was synthesized following a procedure described elsewhere.30,31 The synthetic methods and characterization of HPG-NH2 are described in detail in the Supporting Information. Two bacterial species, Staphylococcus epidermidis (ATCC 35984) and Escherichia coli (ATCC DH5α) were purchased from American Type Culture Collection, Manassas, VA. 2.2. Synthesis of PVDF-g-PPFMA Copolymers via RAFT Graft Copolymerization. The PVDF-g-PPFMA copolymers were prepared by the reversible addition−fragmentation chain transfer (RAFT) graft copolymerization of vinyl monomers from the ozone-preactivated PVDF as described in the literature.32,33 A transparent homogeneous solution was formed by the dissolution of PVDF powder (2.0 g) into 30 mL of NMP. Mixed gas of O3/O2 gaseous with an optimal ozone concentration of approximately 0.027 g/L from an Azcozon RMU1604EM ozone generator (the inlet oxygen flow rate of 5 L/min) was continuously supplied to activate the PVDF NMP solution at 25 °C for 15 min. Afterward, argon was purged for 30 min to release the bound oxidized gases from the reaction mixture. The PFMA monomer (1.7 mL, 9.4 mmol), CTP (13.1 mg, 0.047 mmol) and NMP (10 mL) were then quickly injected to the reaction mixture and the reaction was allowed to proceed at 70 °C for a specific period. At the end of the reaction, the reaction mixture was quenched in an icy water bath, and the PVDF-g-PPFMA copolymer was precipitated in excess ethanol. The raw product was purified by three cycles of redissolution in NMP, washing alternatively with THF and ethanol, and reprecipitation in ethanol. The copolymer was dried in a vacuum oven overnight before use. Scheme 1 describes the synthetic routes of ozone pretreatment of PVDF and RAFT polymerization of PFMA. 1H NMR (DMSO-d6, δ, ppm, TMS): 7.4−8.0 (5H, aromatic ring), 2.05−3.2 (2H × (x + y + z), −CH2CF2−, −CH2C(CH3)C(O)O−), 1.3−1.65 (3H × z, −CH3). 2.3. Synthesis of PVDF-g-PHPGMA Copolymers via Activated Ester-Amine Reaction. The selective and quantitative chemical transformation of pentafluorophenyl esters with primary amines in the presence of diols leads to the inhibition of extensive gelation reaction.14,30,31 The use of a molar feed ratio [HPG-NH2]/ [-PFMA-]/[NEt3] of 20:1:1 with excess HPG-NH2 resulted in the complete transformation of pentafluorophenyl esters into HPG-based amides or PVDF-g-PHPGMA copolymers. PVDF-g-PPFMA copolymer (1.0 g), HPG-NH2 and DMF (40 mL) were introduced into the reaction flask. NEt3 was quickly injected to the solution and the mixture was allowed to proceed at 50 °C for 24 h. Afterward, the reaction flask was placed in cold water bath, followed by pouring into a copious amount of ethanol. The dissolution and precipitation process was performed on the raw product thrice. The copolymer was dried in a vacuum oven overnight before use. 1H NMR (DMSO-d6, δ, ppm, TMS): 7.4−8.0 (5H, aromatic ring), 4.35−4.85 (H × (m + 3) × z, −OH), 3.25−3.85 (H × (5m + 6) × z, −OCH−, −OCH2−), 2.7−3.2/ 2.2−2.4 (2H × (x + y), −CH 2CF2 −), 1.7−2.0 (2H × z, −CH2C(CH3)C(O)N−), 0.75−1.2 (3H × z, −CH3). 2.4. Fabrication of MF Membranes. Microfiltration (MF) membranes were fabricated by the nonsolvent induced phase inversion (NIP) from a 15 wt % NMP solution of the PVDF or PVDF-gPHPGMA polymers. The casting solution was prepared and stirred at 60 °C for complete dissolution. Then the solution was cooled down to room temperature and left for 24 h of degassing. The polymer solution was spread onto a glass plate with a blade, and the plate was

weight, narrow molecular weight distribution, abundant endgroup functionality, high solubility, thermal stability, chemical resistance and biocompatibility.12−20 Theoretically, a branched polymer provides more shielding area than its linear counterpart at an equivalent grafting density. As the chemically analogues to linear poly(ethylene glycol) (PEG), HPG with hyperbranched globular form holds the capability to take place of PEG across a wide range of practical applications once appropriate grafting techniques have been developed. To the best of our knowledge, HPG have yet to be fully explored for the modification of membrane materials.12−14 As expected, the incorporation of HPG branches to fluoropolymers paves the way for simply adjusting the surface compositions and wettability, reactivity, morphology, and porosity of the fluoropolymer membranes fabricated by nonsolvent-induced phase inversion (NIP). On the other hand, comprising equivalent amounts of cations and anions within the same monomers, zwitterionic polymers have also been extensively considered as unique ultralow fouling coatings, owing to their capability to form a hydration layer via electrostatic interaction between zwitterions and water molecules.21−25 Various attempts to construct ultralow fouling surfaces were preformed ever since Zwaal et al. reported the nonthrombogenic nature of phosphorylcholine moiety.26 Later, various zwitterionic monomers including carboxybetaines, sulfobetaines, phosphobetaines, were grafted on material surfaces including fluoropolymer membranes, and their antiprotein behaviors were assessed.4,27−29 Arise from a synergistic effect of the outstanding properties from HPG and zwitterionic polymers, the construction of hierarchical fouling resistance surfaces onto the fluoropolymer membranes will fully shield the underlying membranes, preventing direct contamination of membrane matrix from foreign biofoulants and maximally mitigating protein adsorption and bacterial fouling. In this work, poly(vinylidene fluoride) (PVDF) was selected as the model fluoropolymer and the synthesis of PVDF with grafted poly(hyperbranched polyglycerol methacrylamide) (PHPGMA) side chains was carried out (Scheme 1). The PVDF-g-PHPGMA copolymers are prepared from reversible addition−fragmentation chain transfer (RAFT) graft copolymerization of pentafluorophenyl methacrylate (PFMA) with the ozone-pretreated PVDF, followed by activated ester-amine reaction of pentafluorophenyl esters in the PPFMA side chains of PVDF-g-PPFMA with amino-terminated hyperbranched polyglycerol (HPG-NH2). The resultant copolymers can be readily processed into MF membranes with surface-enriched PHPGMA graft chains by NIP in an aqueous coagulant. The PVDF-g-PHPGMA membranes were further functionalized via surface-initiated atom transfer radical polymerization (SIATRP) with N-(3-sulfopropyl)-N-(methacryloxyethyl)-N,Ndimethylammonium betaine (SBMA) to obtain the PVDF-gPHPGMA-g-PSBMA membranes. The present work has demonstrated that covalently attached PHPGMA branches is an outstanding alternative to their linear analogues in improving the fouling mitigation behaviors of membrane surfaces with additional sites for further surface functionalization. Performance of the modified PVDF membranes in protein adsorption and filtration, bacterial fouling, and cytotoxicity tests were investigated.

2. EXPERIMENTAL SECTION 2.1. Materials. Poly(vinylidene fluoride) (PVDF, Kynar K-761 powders, Mw = 441 kDa) was purchased from Elf Atochem of North C

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Langmuir immediately placed into a coagulation bath at room temperature. Solvent exchange between the polymer solution and the coagulation bath finally renders the formation of porous membranes. The as-cast MF membranes were soaked and thoroughly rinsed with deionized water to complete remove the residual solvent. The purified membranes with thickness of about 120 ± 10 μm were obtained by lyophilization. 2.5. Preparation of PVDF-g-PHPGMA-g-PSBMA Membranes via SI-ATRP. The ATRP initiation sites were introduced to the PVDFg-PHPGMA membrane and pore surfaces via postmodification of the multiple diols of PHPGMA with BIBB. The PVDF-g-PHPGMA membranes were cut into small pieces of size 2 × 2 cm2. Ten pieces of the PVDF-g-PHPGMA membranes, NEt3 (138 μL, 1 mmol), and anhydrous CH2Cl2 (20 mL) were introduced into a 50 mL flask equipped with a constant-pressure dropping funnel containing BIBB (136 μL, 1.1 mmol) in anhydrous CH2Cl2 (5 mL). The reaction flask was placed in an icy water bath. After slow dosing of the solution over 30 min, the reaction mixture was allowed to stir at room temperature for an additional 24 h. Afterward, the membranes were collected and then alternatively extracted with acetone and deionized water, followed by freeze-drying overnight. In a typical reaction, SBMA (0.56 g, 2 mmol), deionized water (15 mL), and five pieces of the above as-functionalized PVDF-g-PHPGMA membranes of size 2 × 2 cm2 were added to a 50 mL round-bottom flask. Catalyst CuBr (14.4 mg, 0.1 mmol) and Bpy (31.2 mg, 0.2 mmol) were quickly dosed to the reaction mixture after the reaction mixture had been purified by argon for 30 min. The flask was then sealed with a rubber stopper and the polymerization reaction was performed at 40 °C for 4 h. Afterward, the resulting membranes were collected and then alternatively extracted with methanol and deionized water to remove any copper catalyst residues and bound homopolymer, followed by lyophilization overnight. 2.6. Polymer Characterization. Fourier transform infrared (FTIR) spectroscopy analysis of the polymers was detected on a Bio-Rad FTS 135 FTIR spectrophotometer. Each spectrum was collected by cumulating 64 scans. 1H NMR spectra of the polymers was measured by accumulation of 1000 scans at a relaxation time of 2 s on a Bruker ARX 300 MHz spectrometer at room temperature. The chemical shifts were referenced to the DMSO-d6 deuterated solvent peak at δ = 2.50 ppm. The thermal stability of the polymers was investigated by thermogravimetric analysis (TGA). The samples were heated from 25 °C to around 700 °C with a heating rate of 10 °C/min under a dry nitrogen atmosphere in a Shimadzu DTG-60AH TGADTA Analyzer. Gel permeation chromatography (GPC) analysis of the polymers was performed on a Waters GPC system, coupled with a Waters 1515 isocratic HPLC pump, a Waters 717 plus autosampler injector, and a Waters 2414 differential refractometer detector, and an Agilent PLgel 5 μm MIXED-D column (P/N: 79911GP-MXD). N,NDimethylformamide (DMF) was used as the mobile phase at a flow rate of 1.0 mL/min. Number-average molecular weights (Mn,GPC) and polydispersity indices (PDI) were calculated from a calibration curve based on a series of linear polystyrene standards. 2.7. Membrane Characterization. Surface compositions of the membranes were determined by X-ray photoelectron spectroscopy (XPS) measurements on a Kratos AXIS Ultra DLD spectrometer with a monochomatized Al Kα X-ray source (1486.71 eV photons). All binding energies (BEs) were referenced to that of the neutral C 1s hydrocarbon peak at 284.6 eV. Surface morphology of the membranes was observed by scanning electron microscopy (SEM, JEOL JSM5600LV). Prior to the SEM measurements, the membranes were freeze-dried and sputtered with platinum. The measurements were performed at an accelerating voltage of 15 kV. Static water contact angles of the pristine and modified membranes were measured at 25 °C by a contact angle geniometer (Rame Hart). To minimize the experimental error, each number reported was the mean value from three samples, with the value of each sample obtained by averaging the contact angles from at least three surface locations. To evaluate the antifouling efficacy of the modified membranes, static protein adsorption assays were carried out using γ-globulin as a model protein pollutant. The membranes were immersed and wetted

in methanol for 30 min, followed by washing thrice with the phosphate buffer saline (PBS, 0.01 mol/L, pH 7.4). The membranes were subsequently incubated in PBS containing 2 mg/mL of γ-globulin for 24 h. After that, the postincubated membranes were gently cleaned three times with ultrapure water. The surface coverage of γ-globulin was quantified by XPS, using the nitrogen signal associated with γglobulin as a marker. The relative [N]/[C] ratios before and after protein attachment were compared. The average data from measurements on at least four similar membranes were calculated. Permeation experiments were performed using a microfiltration cell (Tokyo Roshi UHP-25, Tokyo, Japan). Prior to tests, stabilization of either the commercial or modified PVDF membranes in deionized water was conducted at 5.9 kPa for 20 min. To initiate the fouling test, 500 mg/L of γ-globulin was dosed into the feed solution. After fouling tests, back washing was performed by feeding deionized water into the lumen side of the membranes. The effective membrane area was 3.14 cm2. The average data from measurements on at least four similar membranes were calculated. Two bacterial species, Gram-positive S. epidermidis (ATCC 35984) and Gram-negative E. coli (ATCC DH5α), are commonly used model microorganisms in literature and were employed in this study. The concentrations of viable bacteria were calculated by colony-forming units (CFU) counting. Log phase growth bacteria were adjusted to about 1 × 108 CFU/mL with sterilized PBS. All lab wares were autoclaved at 120 °C for 20 min, and the samples were irradiated with UV for 1 h to thoroughly remove microbial contamination from the surfaces. The dynamic bacterial adhesion, contamination and proliferation were conducted in a circulative flowing cell.18,19 The pristine and modified PVDF membranes of size 2 × 2 cm2 were first exposed to a bacterial suspension in PBS at a concentration of 1 × 108 CFU/mL for 4 h at 37 °C to simulate microbe-infected points. The inoculated membranes were then mounted on a glass slide and put inside a circulating cell. Sterilized culture medium was smoothly pumped through the cell for 18 h, followed by PBS for 0.5 h at room temperature, at a constant flow rate of 0.5 mL/min. After the incubation period, the bacterial layer was fixed by immersing each membrane in 3 vol % aqueous glutaraldehyde at 4 °C overnight, followed by serial dehydration in 20, 40, 60, 80, and 100% ethanol for 10 min each. SEM was used to exam the membrane after being vacuum-dried and subsequently sputtered with platinum. Bacterial adhesion and viability on both pristine and functionalized PVDF membranes was quantified by the spread plate method. The bacterialadhered membranes were washed in 2 mL of sterilized PBS under mild ultrasonication for 7 min, followed by a rapid vortex mixing for 20 s, to release the attached bacteria from the membrane surfaces. After 10fold serial dilution of the bacterial solution, a 100 μL aliquot was spread onto the plate containing the culture medium and the bacteria were incubated at 37 °C overnight. Finally, the amount of grown cell colonies on the spread plate were calculated and expressed as the relative viability of bacteria on the membranes. MTT assays were used to evaluate the cytotoxicity of the membranes in 3T3 fibroblasts and raw macrophages cell lines using the procedures similar to those described earlier.19 The membranes were first sterilized with 75% ethanol and dried under a reduced pressure before use. The cells were seeded in a 24-well culture plate and incubated at 37 °C for 24 h in the medium. Then, the medium was replaced with a fresh medium containing different samples of size 1 × 1 cm2. Control experiments were carried out using the complete growth culture medium without exposure to the MF membranes. The cells were incubated for another 24 h in the medium. After that, the culture medium in each well was removed and 90 μL of the medium and 10 μL of the MTT solution (5 mg/mL in PBS) were then added to each well. After 4 h of incubation at 37 °C, the medium was removed and the formazan crystals (a purple color dye from reduction of MTT in living cells) were solubilized with 100 μL of dimethyl sulfoxide (DMSO) for 15 min. The optical absorbance was measured at 560 nm on a microplate reader (Tecan GENios). The cell viability (%), relative to that of the control cells cultured in a medium without the membranes, was calculated from [As]/[A0] × 100%, where [As] and [A0] are the absorbance values of the wells (with the membranes) D

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Langmuir and control wells (without the membranes), respectively. For each sample, the final absorbance was the average of those measured from four wells in parallel. Before bacterial and cytotoxicity tests, the modified MF membranes were aged by immersion in deionized water at 37 °C for 30 days to assess the stability of surface modification.

copolymers via the activated ester-amine reaction was accompanied by the emergence of absorption band of amide groups at around 1650 cm−1 and hydroxyl groups in the range of 3100−3600 cm−1, with no remaining signals of the activated ester group discernible, indicating the complete conversion of pentafluorophenyl esters into HPG-based amides. On the other hand, the strong adsorption band in the range of 1120−1320 cm−1, representative of the −CF2− functional groups of PVDF, is clearly visible in all the copolymers.18,36−38 Thus, the FTIR spectroscopic results confirms the successful grafting of PHPGMA chains to the PVDF skeleton. The representative structures of PVDF, PVDF-g-PPFMA4, and PVDF-g-PHPGMA4 were characterized by 1H NMR spectra as demonstrated in Figure 2a-c, respectively. The wide and strong chemical shifts within the range from 2.7 to 3.2 ppm (a1 in Figure 2a) are consistent to the head-to-tail (ht) structure of the PVDF skeletons, whereas the narrow and weak chemical shifts within the region from 2.2 to 2.4 ppm (a2 in Figure 2a) correspond to the head-to-head (hh) or tail-to-tail (tt) stereoregularities.38−40 After RAFT-mediated graft polymerization of PFMA from the PVDF skeletons, the signals in the range of δ = 1.3−1.5 ppm (c in Figure 2b) correspond to the methyl protons adjacent to the carbonyl moieties of the ester linkage in the PPFMA chains. The other typical chemical shifts associated with the methylene protons (b in Figure 2b) in the PPFMA chains overlapped completely with those of the PVDF backbones. The selective and quantitative chemical transformation of pentafluorophenyl esters with HPG-NH2 at the focal amino functionality leads to the inhibition of extensive gelation reaction.14,30,31 The use of a [HPG-NH2]/[-PFMA-] feed molar ratio of 20:1 resulted in the complete conversion of pentafluorophenyl esters into HPG-based amides, as the methylene and methyl protons adjacent to the carbonyl moieties of the ester linkage (b and c in Figure 2b) in the PPFMA chains shifted sharply toward lower field (b and c in Figure 2c). The chemical shifts in the region of δ = 3.25−3.85 ppm (d and e in Figure 2c) are assigned to the methine protons and methylene protons adjacent to the diols or ether groups, whereas the chemical shifts in the region of δ = 4.35−4.85 ppm ( f in Figure 2c) are attributable to the hydroxyl groups of the PHPGMA graft chains.18,19,40 The integral area of peak from the methylene protons adjacent to the carbonyl moieties of the amide linkage (2H, b in Figure 2c) to the peaks from the methylene protons (2H, a1 and a2 in Figure 2c) in the PVDF backbone was determined to be about 1:40, manifesting that the bulk molar ratio of PHPGMA units per repeat PVDF unit in the copolymer is about 0.025. The molecular structures for the PVDF-g-PHPGMA copolymers have been determined and the characteristic results are summarized in Table 1. The outstanding thermal stability of HPG provides an added benefit for membrane materials used in water treatment and biomedical applications.41 Figure 3 depicts the respective thermogravimetric analysis (TGA) curves of the PVDF homopolymer (curve a), three PVDF-g-PHPGMA copolymers from different RAFT polymerization times (curves b−d), and PHPGMA copolymer (curve e). A small amount of weight loss was observed below 120 °C, which was attributable to the discharge of moisture from the samples. Two obvious weight loss steps under elevated temperature are observed for the PVDF-g-PHPGMA copolymer samples with reference to the PVDF and PHPGMA polymers. The first gross weight loss taking place at around 275 °C is assigned to the thermal degradation of PHPGMA side chains, whereas the second

3. RESULTS AND DISCUSSION 3.1. Preparation of the PVDF-g-PHPGMA Copolymers via RAFT Graft Polymerization and Activated EsterAmine Reaction. The absence of initiation sites and incompatibility with strong alkaline catalysts make it inaccessible to perform anionic ring-opening multibranching polymerization directly from pure PVDF polymers and PVDF membranes.2−4,14 This issue can be overcome by RAFT graft polymerization of pentafluorophenyl ester precursor with the ozone-preactivated PVDF, followed by activated ester-amine reaction of HPG-NH2 with pentafluorophenyl ester moieties. The postpolymerization modification technique allows the controlled radical polymerization of activated ester monomers but can be selectively and quantitatively converted to other functional groups in subsequent steps.34−36 Ozone pretreatment of partially fluorinated polymers usually led to the formation of certain polar groups, such as carbonyl or peroxide groups, which has been regard as one of the most straightforward and effective methods to activate partially fluorinated polymers for subsequent graft copolymerization.4 Usually, the amount of peroxides employed by ozone pretreatment depends on the treatment temperature, treatment time and ozone concentration. In the present work, the ozone treatment was maintained at 25 °C for 15 min and the inlet ozone concentration was set at 0.027 g/L of the O2/O3 mixture. Based on the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay, the content of peroxides in the ozone-pretreated PVDF is about 1.1 × 10−4 mol/g, which is comparable with the result reported in the literature.4 Figure 1 displays the representative FTIR spectra of PVDF, PVDF-g-PPFMA4, PVDF-g-PHPGMA4, and HPG-NH2 polymers. Differing from that of the pure PVDF polymer, the spectrum of PVDF-g-PPFMA4 copolymer is composed of distinctive absorption bands of the activated ester group at about 1784 cm−1 and the perfluorinated aromatic at about 1515 cm−1.36−38 As expected, the formation of PVDF-g-PHPGMA4

Figure 1. FTIR spectra of the (a) PVDF homopolymer, (b) PVDF-gPPFMA4 copolymer, (c) PVDF-g-PHPGMA4 copolymer, and (d) HPG-NH2 homopolymer. E

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Figure 2. 1H NMR spectra of the (a) PVDF homopolymer, (b) PVDF-g-PPFMA4, and (c) PVDF-g-PHPGMA4 copolymers in DMSO-d6.

major weight loss commencing at around 475 °C is assigned to the thermal degradation of PVDF backbones.19,38 The bulk graft concentrations of the PHPGMA side chains in the PVDFg-PHPGMA copolymers can be calculated on the basis of the weight remaining at 700 °C of all the polymer samples (Table 1), which are consistent with those derived from 1H NMR results. With the increase in RAFT polymerization time from 6 to 24 h, the number-average molecular weight (Mn,GPC) of the PVDF-g-PHPGMA graft copolymers, as determined from gel permeation chromatography (GPC) analysis, increased from 3.9 × 105 to 5.1 × 105 g/mol (Table 1). Besides, the polydispersity indices (PDI) of all the copolymer samples approximates to that of the initial PVDF, which is in good

agreement with the fact that RAFT copolymerization of PFMA from ozone-pretreated PVDF performed under a controlled manner and activated ester-amine reaction of HPG-NH2 from PVDF-g-PPFMA graft copolymers is highly quantitative. The observed Mn,GPC of branched polymers are usually slightly underestimated in GPC with respect to linear polystyrene standards. This discrepancy probably results from the differences of hydrodynamic volume of branched polymer with the densely packed structure.14,19,30,31 The 1H NMR results are more reliable and were therefore used for the subsequent characterization of the membrane samples. 3.2. Preparation of PVDF-g-PHPGMA Copolymer Membranes by Nonsolvent Induced Phase Inversion. F

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Table 1. Characterization of the PVDF Homopolymer and PVDF-g-PHPGMA Copolymers and the Resulting Microfiltration Membranes .polymer reaction time (h) PVDF PVDF-g-PHPGMA1a PVDF-g-PHPGMA2 PVDF-g-PHPGMA3 PVDF-g-PHPGMA4

6 12 18 24

membrane

Mn,GPC (g/mol)b

PDIb

([-HPGMA-]/ [−CH2CF2-])bulk

([−C-O]/ [−CH2CF2-])bulk

([−C-O]/ [−CH2CF2−])surfacee

Da (μm)f

SD (μm)f

× × × × ×

1.26 1.29 1.38 1.34 1.28

0.005c (0.006)d 0.012 (0.014) 0.018 (0.019) 0.025 (0.025)

0.23c (0.26)d 0.58 (0.65) 0.88 (0.91) 1.23 (1.22)

0.41 0.84 1.29 1.72

0.22 0.49 0.95 1.29

0.03 0.07 0.12 0.21

3.5 3.9 4.3 4.6 5.1

105 105 105 105 105

CA (deg)g 108 67 51 39 26

± ± ± ± ±

3 2 4 3 3

Reaction condition: 30:100:0.15 molar ratio of [PFMA]:[−CH2CF2-]:[CTP] in NMP at 70 °C, 20:1:1 molar ratio of [HPG-NH2]:[−PFMA−]: [NEt3] in DMF at 50 °C for 24 h, PFMA = pentafluorophenyl methacrylate, CTP = 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid, HPG-NH2 = amino-terminated hyperbranched polyglycerol, NEt3 = triethylamine, HPGMA = hyperbranched polyglycerol methacrylamide. bDetermined from GPC results (calibration using linear polystyrene standards). Polydispersity index (PDI) = Mw/Mn. cDetermined from 1H NMR spectroscopy results. d Determined from TGA results. Molecular weights of PFMA = 252 g/mol, −CH2CF2− = 64 g/mol, HPG-NH2 = 1200 g/mol, and HPGMA = 1268 g/mol. eDerived from the curve-fitted C 1s peak component area ratio of [−C−O]/[−CF2−] of the respective sample in Figure 5. fFrom SEM a

images, using the relationship average pore diameter Da = (Σin= 1Di)/n and absolute standard deviation of pore diameter SD = based on 100 pores (n = 100) of diameter Di. gStatic water contact angle (CA).

n

∑i = 1 (Di − Da)2 /n ,

correspondingly from 0.03 to 0.21 μm. The increase in Da and SD with the increase in PHPGMA content verifies that the membrane morphology relies on the PHPGMA graft density, and is therefore tunable. Further increase in the content of hydrophilic PHPGMA component will result in the formation of unsteady reverse micelles. Figure 5 demonstrates the respective XPS wide scan and C 1s core-level spectra of (a,b) PVDF, (c,f) PVDF-g-PHPGMA4, (d) PVDF-g-PHPGMA2, (e) PVDF-g-PHPGMA3 membranes. With regard to the pristine PVDF membrane, only C 1s and F 1s signals are detected in the wide-scan spectrum (Figure 5a). The curve-fitted XPS C 1s core-level spectrum comprises of three peak components with binding energies (BEs) at 284.6 eV for the normal −CH−/−C−C species, 285.8 eV for the −CH2− species (adjacent to the −CF2− species in PVDF) and 290.5 eV for the −CF2− species (Figure 5b).18,38,44 The [−CH2−]/[−CF2−] peak component area ratio approximates 1:1, which is in accordance with the molecular structure of PVDF. In the case of the PVDF-g-PHPGMA membrane, the emergence of characteristic O 1s and N 1s signals in the widescan spectrum of Figure 5c illustrates that PFMA have grown from the PVDF backbones via RAFT-mediated copolymerization and the pentafluorophenyl ester groups in the PPFMA chains have been transformed into amide groups via activated ester-amine reaction. Concomitantly, two new C 1s peak components with BEs at 286.2 eV for the -C-O species and at 287.4 eV for the NH−CO species in Figure 5d-f can also attributable to the PHPGMA segments. In comparison to the −CH2− and −CF2− reference peaks, the −C−O peak area of the PVDF-g-PHPGMA membrane surfaces exhibits a progressive increase with the increase in grafting density of PHPGMA branches.19,44 On the other hand, the graft content on the membrane surface, or the ([−C−O]/ [−CH2CF2−])surface ratio, as derived from the ([−C−O]/ [−CF2−]) peak component area ratio in the C 1s core-level spectrum of the copolymer membranes, is greater than the corresponding bulk ratio determined from 1H NMR results (Table 1). Owing to the affinity with the aqueous coagulant, the more hydrophilic PHPGMA components have been brought and accumulated on the membrane and pore surfaces during the construction of PVDF-g-PHPGMA membrane matrix in the NIP process.

Figure 3. TGA curves of the (a) PVDF homopolymer, (b) PVDF-gPHPGMA2, (c) PVDF-g-PHPGMA3, (d) PVDF-g-PHPGMA4, and (e) PHPGMA copolymers.

Direct formation from the integration of hydrophilic components into membrane polymers simplifies the formation process of membranes with antifouling properties and makes it more feasible, efficient and promising for industrial manufacture of antifouling polymeric membranes with currently existing equipment.42,43 The PVDF-g-PHPGMA copolymers were processed into microfiltration (MF) membranes by nonsolvent induced phase inversion (NIP) in aqueous coagulant at room temperature from 15 wt % NMP solutions. Figure 4a−d illustrates the SEM images of the MF membranes fabricated from the PVDF and three PVDF-g-PHPGMA polymers. After lyophilization, the pristine PVDF membrane exhibits flat, smooth surface structure with very few pores while the ascast PVDF-g-PHPGMA membranes possess well-structured pores with increased porosity, as induced by the hydrophilic PHPGMA side chains during NIP process, which is consistent with our previous work.18,32 The PVDF-g-PHPGMA4 membranes have asymmetric structures with finger-like macrovoids, as shown in Figure 4d′. The average pore diameters (Da) and the corresponding standard deviation (SD) of four PVDF-gPHPGMA membranes, as derived from statistical analysis of the SEM images, are present in Table 1. With the increase in HPGMA content of the PVDF-g-PHPGMA copolymers from 0.5 to 2.5 mol %, Da of the PVDF-g-PHPGMA membranes increases from 0.22 to 1.29 μm while the SD increases G

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Figure 4. SEM images of the microfiltration (MF) membranes cast from a 15 wt % NMP solution of the corresponding polymers by NIP in deionized water at room temperature: (a) PVDF, (b) PVDF-g-PHPGMA2, (c) PVDF-g-PHPGMA3, and (d,d′,e) PVDF-g-PHPGMA4 membranes; (f,f′) PVDF-g-PHPGMA4-g-PSBMA membrane prepared from the PVDF-g-PHPGMA4 membrane via surface-initiated ATRP of SBMA. Images (a)−(d) and (f) are the respective surfaces in contact with the glass substrate during membrane casting by NIP process. Image (e) is the surface in contact with coagulation bath. Inset images (d′) and (f′) are the respective cross sections of corresponding membranes.

3.3. Functionalization of the PVDF-g-PHPGMA Membranes via SI-ATRP of SBMA: the PVDF-g-PHPGMA-gPSBMA Membranes. One key benefit in using PHPGMA grafting over linear PEG grafting is the high level of diol endfunctionalities, which have been brought to the PVDF-gPHPGMA membrane and pore surfaces during the NIP process, afforded straightforward chemical handles for facile immobilization of ATRP initiation sites to the inert PVDF membrane surfaces. The emergence of a representative Br 3d signal at the BE of around 70 eV in the wide-scan and Br 3d core-level spectra in Figure 6a,b suggests that bromoisobutyryl groups has been successfully coupled onto the PVDF membranes. By taking into account the molar ratio of element bromide and nitrogen, the density of the bromoisobutyryl groups tethered to the PVDF-g-PHPGMA4-Br membrane

surface is about 2.3. The subsequent SI-ATRP of zwitterionic monomers (SBMA) on the PVDF-g-PHPGMA membrane renders the covalent attachment of PSBMA brushes to the membrane and pore surfaces. Figure 6c−f demonstrates the XPS wide-scan, C 1s, N 1s and S 2p core-level spectra of the PVDF-g-PHPGMA4-g-PSBMA membrane. The dense and thick coverage of PSBMA graft layer on the PVDF-gPHPGMA4 membrane surfaces led to signal shielding of the underlying elements (F 1s signal) in the XPS wide-scan spectrum (Figure 6a).25,44 The same result can also be reflected by the signal elimination of two carbon species, viz., (−CH2−)PVDF and (−CF2−)PVDF, in the C 1s core-level line shape of the PVDF-g-PHPGMA4-g-PSBMA membrane. The C 1s core-level spectrum (Figure 6d) can be curve-fitted into three peak components having BEs at 284.6, 286.5, and 288.5 H

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Figure 6. XPS wide-scan, C 1s, N 1s, S 2p, and Br 3d core-level spectra of the (a,b) PVDF-g-PHPGMA4-Br and (c−f) PVDF-g-PHPGMA4-gPSBMA membranes.

Figure 5. XPS wide-scan and C 1s core-level spectra of the (a,b) pristine PVDF, (c,f) PVDF-g-PHPGMA4, (d) PVDF-g-PHPGMA2, and (e) PVDF-g-PHPGMA3 membranes.

of biofouling due to nonspecific protein/bacteria attachment on the membrane surfaces. Generally, the steric hindrance owing to high conformational entropy of the electroneutral hydrophilic polymer brushes controls protein adsorption/repulsion.5,6 The relative content of surface-adsorbed protein, after exposure to a 5 mg/mL γ-globulin solution for 24 h, can be readily calculated as the net increase in surface composition of [N]/[C] ratio over that springing from the original activated ester-amine reaction, as displayed in Figure 7A. As expected, the hydrophobic PVDF membrane surface was bristled with a large quantity of γ-globulin, suggested by the high level of surface [N]/[C] ratio of 0.11. The proportion of static adsorption of γ-globulin on the PVDF-g-PHPGMA4-gPSBMA membrane is less than 50% of that of the corresponding PVDF-g-PHPGMA4 membrane and less than 5% of that of the pristine PVDF membrane. For membranes from 12 h or more of RAFT polymerization time, the amount of protein adsorption is further reduced. Dead-end membrane filtrations were performed to evaluate fouling resistance and flux-recovery ability of the functionalized membranes. The effect of γ-globulin fouling on the permeability of commercial (Millipore) hydrophilic PVDF membrane, PVDF-g-PHPGMA4 and PVDF-g-PHPGMA4-gPSBMA membranes were investigated as a function of time under a fixed filtration pressure of 5.9 kPa (Figure 7B). Similar trends can be observed for all the membranes. Since the molecular weight of γ-globulin is smaller than the molecular weight cutoff (MWCO) of current membranes, protein fouling not only takes place on the exterior surface but also entraps into the porous membrane matrix. With reference to the commercial Millipore hydrophilic PVDF membrane of

eV and with an area ratio of 4.2:5.9:1, attributable to the −CH−/−C−C, −C−O/−C−SO3−/−C−N+ and O−CO, respectively, which is in fairly good agreement with the theoretical ratio for the chemical structure of the PSBMA polymers.25,44 The corresponding S 2p (with BE of 167 eV) and N 1s (with BE of 402 eV) core-level spectra of PVDF-gPHPGMA4-g-PSBMA membranes are shown in Figure 6e and f, respectively. The XPS results have thus confirmed the successful surface grafting of the PSBMA brushes onto the PVDF-g-PHPGMA membranes. PSBMA was graft copolymerized from the PVDF-g-PHPGMA4 membrane to endow the surface with superhydrophilic property, leading to a marked decrease of water contact angle from 26 ± 3° to 12 ± 2°. As derived from statistical analysis of the SEM images (Figure 4f and f′), the D a of the PVDF-g-PHPGMA4-g-PSBMA membranes is 1.25 μm while the corresponding SD is 0.18 μm, which are comparable to those of starting PVDF-gPHPGMA4 membranes, indicating the introduction of ATRP initiation sites and subsequent surface grafting of PSBMA brushes have negligible effects on membrane structure. The copper signals in the BE scope of 910−940 eV arising from ATRP catalyst residues are barely detective in the XPS wide scan spectrum of PVDF-g-PHPGMA4-g-PSBMA membrane surfaces (Figure 6c). Thus, complete removal of copper residues from the membrane and pore surfaces ensure the reliability of subsequent antifouling, antibacterial and cytotoxicity assays. 3.4. Characteristics of the PVDF-g-PHPGMA and PVDF-g-PHPGMA-g-PSBMA MF Membranes. A challenging problem in membrane filtration processes is the prevention I

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PSBMA membrane surfaces (Table 1). After protein permeation arrays, back washing by deionized water was carried out from the permeation side to the previous feed side of the membrane for 20 min. The fouling tests were repeated twice on the cleaned membranes. In the case of PVDF-gPHPGMA4-g-PSBMA membrane, the flux recovery is around 93%, which is greater than that of 86% for the PVDF-gPHPGMA4 membrane and that of 74% for the commercial hydrophilic PVDF membrane after three runs of protein permeability. Obviously, the construction of hierarchical fouling resistance surfaces onto the PVDF membranes achieves superior antifouling performance. Biofouling arising from undesirable bacterial adhesion and biofilm formation on the membrane surface via deposition, growth and metabolism of microorganisms, is an intractable problem for polymeric membranes used in the long term separation process.5,6 In water treatment, convective mass transport, other than sedimentation or diffusion of suspended microbes, dominates the rate of microbial adhesion. A circulative flowing cell under continuous-flow conditions is employed to mimic dynamic microbial adhesion, growth and proliferation on surfaces.18,19 A stream of sterilized culture medium smoothly flows through the membrane surface bearing bacteria-infected points over an extended incubation period to allow bacterial growth, proliferation and biofilm formation. The pristine PVDF membrane surface is highly vulnerable to contamination and colonization for both bacteria species. A massive amount of bacterial cells attached and occupied almost the entire area of the pristine PVDF membrane surfaces, either individually or in small clusters (Figure 8a,d). S. epidermidis (ATCC 35984) exhibits a strong proliferation and biofilm formation capability on the pristine PVDF membrane surfaces, which are coincident with our previous study.25 In contrast, only a spot of bacterial cells were found to distributed sparsely on the PVDF-g-PHPGMA4 (Figure 8b,e) and PVDF-gPHPGMA4-g-PSBMA membranes (Figure 8c,f), with the latter membrane displaying a extraordinarily higher antimicrobial adhesion ability due to much denser and thicker hierarchical fouling resistance surfaces generated by the dendritic PHPGMA branches and “superhydrophilic” PSBMA chains. The hierarchical hydration layers on the membrane and pore surfaces effectively maintain a continuous dilution state in aqueous

Figure 7. (A) Dependence of the extent of γ-globulin adsorption (expressed as the increase in [N]/[C] ratio after γ-globulin exposure) on the PVDF-g-PHPGMA membrane (open circle, ○) and PVDF-gPHPGMA-g-PSBMA membrane (solid square, ■) at different RAFT polymerization time, after exposure to 2 mg/mL of γ-globulin solution for 24 h. Zero h RAFT polymerization time corresponds to the pristine PVDF membrane cast by NIP. (B) Normalized flux through the Milipore hydrophilic PVDF membrane with a standard pore size of d = 0.22 μm (open triangle, △, average pore size of d = 0.56 μm, as determined from mercury porosimetry), PVDF-g-PHPGMA4 membrane (solid circle, ●) and PVDF-g-PHPGMA4-g-PSBMA membrane (solid square, ■) as a function of time of alternatively exposure to the flux of deionized water and a 500 mg/L γ-globulin solution under an imposed pressure of 5.9 kPa.

comparable average pore size, the PVDF-g-PHPGMA4 and PVDF-g-PHPGMA4-g-PSBMA membranes exhibited slower decreases in permeation rate under prolonged flux (Figure 7B), which is attributed to the repulsive hydrodynamic forces arising from strong solvation of the more hydrophilic surface grafted

Figure 8. SEM images of (a, d) pristine PVDF, (b, e) PVDF-g-PHPGMA4, and (c, f) PVDF-g-PHPGMA4-g-PSBMA membranes after exposure to PBS suspensions of S. epidermidis (top) and E. coli (below) at an initial cell concentration of 1 × 108 CFU/mL for 4 h at 37 °C, followed by incubation in the culture medium in a circulative flowing cell at a constant flow rate of 0.5 mL/min for 18 h at room temperature. J

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Langmuir media and thereby assume a self-cleaning effect against microfouling. The inhibition of bacteria adhesion endured the extended incubation period and eventually prevented the formation of biofilm on the membrane surfaces. To quantitatively investigate bacteria fouling on the pristine and modified PVDF membranes, the survival of S. epidermidis and E. coli bacteria colonies attached on the membrane surface after rinsing was calculated using the spread plate method (Figure 9). The amount of E. coli adhesion decreases sharply

Figure 10. Cell viability assays of (A) pristine PVDF, (B) PVDF-gPHPGMA4, and (C) PVDF-g-PHPGMA4-g-PSBMA membranes in 3T3 fibroblasts and raw macrophages culture medium after 24 h of incubation. Cell viability was determined by the MTT assay and expressed as a percentage of the control cell culture. Error bars represent the standard deviation of four measurements.

by Food and Drug Administration (FDA) for biomedical applications.14,45−47 Prior to the long-term antifouling applications, the stability of the polymer coatings has to be verified for the safety consideration. To evaluate the stability of polymer coating layers, the PVDF-g-PHPGMA and PVDF-gPHPGMA-g-PSBMA membranes were immersed in PBS at 37 °C for 30 days. As can be seen from Figures 9 and 10, there was no significant difference in the antibacterial adhesion and cytotoxicity assays on the aged membrane samples, indicating the grafting of biocompatible PHPGMA and PSBMA brushes onto the PVDF membranes are relatively stable and durable in an aqueous environment.

Figure 9. Viable adherent fractions of (A) pristine PVDF, (B) PVDFg-PHPGMA4, and (C) PVDF-g-PHPGMA4-g-PSBMA membranes after exposure to PBS suspensions of S. epidermidis and E. coli at an initial cell concentration of 1 × 108 CFU/mL for 4 h at 37 °C, followed by incubation in the culture medium in a circulative flowing cell at a constant flow rate of 0.5 mL/min for 18 h at room temperature. The cell number was determined by the spread plate method. Error bars represent the standard deviation of four measurements.

4. CONCLUSIONS A facile and effective method for the construction of hierarchical fouling resistance surfaces onto the inert and hydrophobic PVDF membranes has been successfully developed. PVDF copolymers with grafted PHPGMA side chains (PVDF-g-PHPGMA copolymers) were synthesized through RAFT graft copolymerization of PFMA from the ozonepreactivated PVDF skeleton, followed by activated ester-amine reaction of the pentafluorophenyl ester groups in the PPFMA side chain with HPG-NH2. MF membranes were fabricated from the NMP solution of the PVDF-g-PHPGMA copolymers with different graft concentrations by NIP. The incorporation of bromoisobutyryl groups to the hydrophilic PHPGMA branches, which have been brought and accumulated on the PVDF-g-PHPGMA membrane and pore surfaces during membrane casting by NIP, afforded functionalities for the SIATRP of zwitterionic SBMA to generate the PVDF-gPHPGMA-g-PSBMA membranes. The present work has demonstrated that covalently attached PHPGMA branches is an outstanding alternative to their linear analogues in improving the fouling mitigation behaviors of membrane surfaces with additional sites for further surface functionalization. The construction of hierarchical fouling resistance surfaces from PHPGMA branches and PSBMA brushes to the PVDF membranes thus imparted comprehensive antifouling and antibacterial adhesion properties to the PVDF-g-PHPGMA-gPSBMA membranes without introducing any significant

after surface functionalization, with a viable bacterial proportion of 7.9% for the PVDF-g-PHPGMA4 membranes and 4.3% for the PVDF-g-PHPGMA4-g-PSBMA membranes, compared to that of the pristine PVDF membranes. The same results can also be deduced from the quantitative adhesion on the membrane surfaces after S. epidermidis exposure, the PVDF-gPHPGMA4 and PVDF-g-PHPGMA4-g-PSBMA membranes reduce the bacterial adhesion to 9.8% and 5.1%, respectively. The above results indicate that the PVDF-g-PHPGMA-gPSBMA membrane surfaces are highly effective in preventing bacterial adhesion, growth and proliferation, and thus biofilm formation. Cytotoxicity is one of the most essential parameters that should be given prior consideration in the design of antifouling polymeric membranes for water treatment and biomedical applications.5,6,45 The in vitro MTT assay results of the nonaged and aged PVDF-g-PHPGMA4 and PVDF-gPHPGMA4-g-PSBMA membranes in 3T3 fibroflasts and raw macrophages cell lines are shown in Figure 10. After being contaminated by specific MF membranes at 4 °C for 24 h, Dulbecco’s modified Eagle’s medium (DMEM) was then utilized for culturing the 3T3 fibroflasts and raw macrophages for another 24 h. Cell viabilities exceeded 95% in all cases and control experiment, indicating that the PVDF-g-PHPGMA and PVDF-g-PHPGMA-g-PSBMA membranes have negligible cytotoxicity effects toward 3T3 fibroflasts and raw macrophages. Nevertheless, HPG and PSBMA have been approved K

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Properties Based on a Dynamic Disulfide Exchange Reaction. Polym. Chem. 2015, 6, 7027−7035. (12) Calderon, M.; Quadir, M. A.; Sharma, S. K.; Haag, R. Dendritic Polyglycerols for Biomedical Applications. Adv. Mater. 2010, 22, 190− 218. (13) Khandare, J.; Calderon, M.; Dagia, N. M.; Haag, R. Multifunctional Dendritic Polymers in Nanomedicine: Opportunities and Challenges. Chem. Soc. Rev. 2012, 41, 2824−2848. (14) Herzberger, J.; Niederer, K.; Pohlit, H.; Seiwert, J.; Worm, M.; Wurm, F. R.; Frey, H. Polymerization of Ethylene Oxide, Propylene Oxide, and Other Alkylene Oxide: Synthesis, Novel Polymer Architectures, and Bioconjugation. Chem. Rev. 2016, 116, 2170−2243. (15) Zhou, L.; He, B.; Huang, J.; Cheng, Z.; Xu, X.; Wei, C. Multihydroxy Dendritic Upconversion Nanoparticles with Enhanced Water Dispersibility and Surface Functionality for Bioimaging. ACS Appl. Mater. Interfaces 2014, 6, 7719−7727. (16) Li, X.; Cai, T.; Chung, T. S. Anti-Fouling Behavior of Hyperbranched Polyglycerol-Grafted Poly(ether sulfone) Hollow Fiber Membranes for Osmotic Power Generation. Environ. Sci. Technol. 2014, 48, 9898−9907. (17) Li, X.; Cai, T.; Chen, C. Y.; Chung, T. S. Negatively Charged Hyperbranched Polyglycerol Grafted Membranes for Osmotic Power Generation from Municipal Wastewater. Water Res. 2016, 89, 50−58. (18) Cai, T.; Yang, W. J.; Neoh, K. G.; Kang, E. T. Poly(vinylidene fluoride) Membranes with Hyperbranched Antifouling and Antibacterial Polymer Brushes. Ind. Eng. Chem. Res. 2012, 51, 15962− 15973. (19) Cai, T.; Li, M.; Neoh, K. G.; Kang, E. T. SurfaceFunctionalizable Membranes of Polycaprolactone-click-Hyperbranched Polyglycerol Copolymers from Combined Atom Transfer Radical Polymerization, Ring-Opening Polymerization and Click Chemistry. J. Mater. Chem. B 2013, 1, 1304−1315. (20) Li, X.; Cai, T.; Amy, G. L.; Chung, T. S. Cleaning Strategies and Membrane Flux Recovery on Anti-Fouling Membranes for Pressure Retarded Osmosis. J. Membr. Sci. 2017, 522, 116−123. (21) Xiang, T.; Wang, R.; Zhao, W.-F.; Sun, S.-D.; Zhao, C.-S. Covalent Deposition of Zwitterionic Polymer and Citric Acid by Click Chemistry-Enabled Layer-by-Layer Assembly for Improving the Blood Compatibility of Polysulfone Membrane. Langmuir 2014, 30, 5115− 5125. (22) Chang, C.-C.; Letteri, R.; Hayward, R. C.; Emrick, T. Functional Sulfobetaine Polymers: Synthesis and Salt-Responsive Stabilization of Oil-in-Water Droplets. Macromolecules 2015, 48, 7843−7850. (23) Chang, Y.; Chang, W. J.; Shih, Y. J.; Wei, T. C.; Hsiue, G. H. Zwitterionic Sulfobetaine-Grafted Poly(vinylidene fluoride) Membrane with Highly Effective Blood Compatibility via Atmospheric Plasma-Induced Surface Copolymerization. ACS Appl. Mater. Interfaces 2011, 3, 1228−1237. (24) Chou, Y.-N.; Chang, Y.; Wen, T.-C. Applying Thermosettable Zwitterionic Copolymers as General Fouling-Resistant and ThermalTolerant Biomaterial Interfaces. ACS Appl. Mater. Interfaces 2015, 7, 10096−10107. (25) Cai, T.; Li, X.; Wan, C.; Chung, T. S. Zwitterionic Polymers Grafted Poly(ether sulfone) Hollow Fiber Membranes and Their Antifouling Behaviors for Osmotic Power Generation. J. Membr. Sci. 2016, 497, 142−152. (26) Zwaal, R. F. A.; Comfurius, P.; van Deenen, L. L. M. Membrane Asymmetry and Blood Coagulation. Nature 1977, 268, 358−360. (27) Leng, C.; Hung, H.-C.; Sun, S.; Wang, D.; Li, Y.; Jiang, S.; Chen, Z. Probing the Surface Hydration of Nonfouilng Zwitterionic and PEG Materials in Contact with Proteins. ACS Appl. Mater. Interfaces 2015, 7, 16881−16888. (28) Sundaram, H. S.; Han, X.; Nowinski, A. K.; Ella-Menye, J.-R.; Wimbish, C.; Marek, P.; Senecal, K.; Jiang, S. One-Step Dip Coating of Zwitterionic Sulfobetaine Polymers on Hydrophobic and Hydrophilic Surfaces. ACS Appl. Mater. Interfaces 2014, 6, 6664−6671. (29) Zhu, Y.; Sundaram, H. S.; Liu, S.; Zhang, L.; Xu, X.; Yu, Q.; Xu, J.; Jiang, S. A Robust Graft-to Strategy to Form Multifunctional and

cytotoxicity effects. The present approach provides a versatile means for tailoring the membrane structures, morphology, and surface functionality of MF membranes for water treatment and biomedical applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b00191. Experimental details on the synthesis and characterization of HPG-NH2 from anionic ring-opening multibranching polymerization (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tao Cai: 0000-0001-7200-5343 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the National Natural Science Foundation of China (51503155), the Natural Science Foundation of Jiangsu Province (BK20160384), the Natural Science Foundation of Hubei Province (2016CFB381), and Opening Project of Key Laboratory of Biomedical Polymers of Ministry of Education at Wuhan University (20160402).

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REFERENCES

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DOI: 10.1021/acs.langmuir.7b00191 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.7b00191 Langmuir XXXX, XXX, XXX−XXX