Reactive Copolymer

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Multi-functionalization of Poly(vinylidene fluoride)/Reactive Copolymer Blend Membranes for Broad Spectrum Applications Dixit V. Bhalani, Anupam Bera, Arvind Kumar Singh Chandel, Sweta Binod Kumar, and Suresh Kumar Jewrajka ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13235 • Publication Date (Web): 23 Dec 2016 Downloaded from http://pubs.acs.org on December 25, 2016

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ACS Applied Materials & Interfaces

Multi-functionalization

of

Poly(vinylidene

fluoride)/Reactive

Copolymer

Blend

Membranes for Broad Spectrum Applications a

Dixit V. Bhalani,

a,b

a

a,b

Anupam Bera,

a,b

Arvind K. Singh Chandel,

b,c

Sweta B. Kumar and

Suresh K. Jewrajka*

Reverse Osmosis Membrane Division, CSIR-Central Salt and Marine Chemicals Research

Institute, G. B. Marg, Bhavnagar, Gujarat 364002, India. bAcSIR, CSIR-Central Salt and Marine Chemicals Research Institute, G. B. Marg, Bhavnagar, Gujarat 364002, India. c

Marine Biotechnology and Ecology Division, CSIR-Central Salt and Marine Chemicals

Research Institute, G. B. Marg, Bhavnagar, Gujarat 364002

*Corresponding author Suresh K. Jewrajka Fax: +912782566511; Tel.: +912782566511; Email: [email protected]

1 ACS Paragon Plus Environment


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ABSTRACT Simultaneous immobilization and crosslinking of antifouling/low toxic polymers, e.g. polyethyleneimine (PEI), dextran (Dex), agarose (Agr), polyethylene glycol (PEG), PEI-Dex, PEI-PEG conjugates, and stimuli responsive copolymers on porous membrane surface in mild reaction conditions is desirable for the enhancement of hydrophilicity, antifouling character, cytocompatibility, and inducing stimuli responsive behaviour. Grafting to technique is required, since; the precursors of the most of these macromolecules are not amenable to undergo surface initiated polymerization. In this work, we report a versatile process for the simultaneous immobilization and crosslinking of a library of macromolecules on and into the blend membrane (PVDF-blend) of poly(vinylidene fluoride) and poly(methyl methacrylate)co-poly(chloromethyl styrene). Sequential nucleophilic substitution reaction between activated halide moieties of the copolymer and amine groups of different macromolecules readily provided series of modified membranes. These membranes exhibited superior antifouling property than that of the unmodified membrane. The effectiveness of this technique has been demonstrated by the immobilization of pH or both pH and temperature responsive copolymer on PVDF-blend membrane for responsive separation of poly(ethylene oxide) and bovine serum albumin. Silver nanoparticles were also anchored on the select modified membranes surfaces for the enhancement of anti-biofouling property. Our approach is useful to obtain verities of functional membranes and selection of membrane for particularly application.

Keywords: Reactive copolymer blend membranes, sequential nucleophilic substitution reaction,

multi-functionalization,

antifouling

membranes,

applications 2 ACS Paragon Plus Environment

responsive

membranes,

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INTRODUCTION Long term performance, antifouling property, responsive behaviour and biocompatibility of a membrane largely depend on its surface feature. These properties can be manipulated through surface modification. Literature has witnessed significant progress on the surface modification of membranes by the covalent anchoring of macromolecules.1-5 Particularly, biocompatibility of a membrane surface can be improved by grafting of zwitterionic poly(sulfobetaine methacrylate),1 glycopolymer [poly(D-gluconamidoethyl methacrylate)]2 and betaines3 respectively. Covalent immobilization of enzymes for the enhancement of catalytic activity was also reported.4,5 Immobilization of macromolecules by the surface initiated graft polymerization significantly enhanced the antifouling property of the membranes [1, 2, 6-10]. Li et al. reported immobilization of Ag nano cluster on the aminated polyacrylonitrile nanofibers followed by surface modification with n-hexadecyl mercaptan for oil/water separation.11 Clickable membrane surface had been created by preparing membrane from graft copolymer of poly(vinylidene fluoride) (PVDF) and poly(propargyl methacrylate). The thiol-yne click reaction or alkyne-azide click reaction was then performed for the functionalization of the membrane.12 Temperature-13-15 and photo-responsive16 membranes were also reported. Coating of antifouling gels on the surface of PES and PVDF or coating of TiO2/PVA was reported for the oil-water separation and other applications.17-19 PVDF membrane coated with thin layer of copolymer was used for the oil-water separation.20. Recently, muscle inspired polydopamine coating on the membrane surface was introduced for the improvement of surface property.21-23 Surface tailored silica nanoparticles were grafted on PVDF membrane for the preparation of highly hydrophilic PVDF membrane via the post fabrication grafting process.24 Surface property of membranes can also be improved by blending with suitable copolymers or use of copolymers.15,25-34 Direct



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modification of the backbone of PVDF by poly(acrylic acid) followed by salt induced phase separation produced surface engineered membranes for the efficient water-oil separation.33 There are some problems with the blending and surface initiated grafting techniques. In case of blending, compatible polymer blend component is required for the preparation of casting solution. There is also a possibility of leaching out of the blend components, if there is a lack of interfacial adhesion between the membrane matrix and the blend components. The surface initiated polymerization of vinyl monomers on the surface of membrane is feasible either by photo-initiated or heat induced polymerization. Immobilization of polysaccharides, polyethyleneimine (PEI), and other amphiphilic copolymers are not feasible through surface initiated polymerization. The crosslinking is also difficult in both the surface initiated polymerization and the blending techniques. To address the problems of the limited variety of polymer attachment and difficulties for crosslinking in surface initiated polymerization, it is proposed to create reactive surface for simultaneous crosslinking and covalent immobilization of a library of macromolecules through grafting to technique. We propose blend membranes containing activated alkyl halide groups for the sequential nucleophile substitution reaction with amine containing macromolecules. Herein, we report a strategy which is based on the preparation of reactive blend membranes of PVDF and PMMA-co-poly(chloromethyl styrene) (PMMA-co-PCMSt) followed by sequential nucleophilic substitution reaction of CMSt moieties of the copolymer with amine moieties of PEI, PEI-dextran conjugate, PEI-PEG conjugate, amine functionalized agarose, amine terminated PEG and different stimuli responsive copolymers.35 We have demonstrated first time that the sequential nucleophilic substitution reaction between activated benzyl halide of the blend component with the above mentioned amine functionalized macromolecules leads to formation of partially cross-linked membranes with


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enhanced hydrophilic and low fouling characteristics. The versatility of this approach was demonstrated by the grafting and crosslinking of the pH- and temperature-responsive poly(N,N-dimethyl amino ethyl methacrylate)-b-poly(N-isoproyl acrylamide) copolymer for the preparation of antifouling and stimuli responsive membrane. As a broad application, silver nanoparticle was anchored on surfaces of the copolymer and the thiol treated membranes for simultaneous antimicrobial surface. The process is versatile due to easy access of amine (primary or secondary or tertiary) functional macromolecules for crosslinking and grafting.

EXPERIMENTAL SECTION Materials PVDF was obtained from Solvay Chemicals. MMA (98%) and CMSt (97%) were purchased from Aldrich. MAA was passed through basic alumina while CMSt was distilled under reduced pressure to remove the inhibitor prior to the polymerization. PEI (Mn=2000 g/mol), dextran (Dex, Leuconostoc spp., Mn=11000 g/mol), agarose (Agr, Type I-A, low EEO), (dimethylamino) propylamine (98%), 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU; 98%, absolute, over molecular sieve), carbonyl diimidazole (98%, CDI) were from Aldrich. N,N-dimethylformamide (DMF) was purchased from Fisher Scientific. The protein, bovine serum albumin (BSA, 98%), and polyvinyl pyrrolidone (PVP) were from SDFC, India. Phosphate buffer salts (SRL, India) and the polyester fabric (Nordlys, TS-100, France) were used as supplied. Poly(ethylene oxide) (PEO) of molecular weights 400000 g/mol, 300000 g/mol and 100000 g/mol

as well as poly(ethylene glycol) bis(3-aminopropyl)

terminated (H2NPEG-NH2 Mn=1500 g/mol) were from Aldrich and used as received. PMMA-co-PCMSt with MMA to CMSt ratio 68:32 (mol/mol) was synthesized by the free radical copolymerization of mixture of MMA and CMSt (supporting information). The



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composition of the copolymer was obtained by 1H NMR spectroscopy (Figure S1A, supporting information). The molecular weight (Mn) and PDI of the copolymer were 22300 g/mol and 1.9 respectively as obtained by the gel permeation chromatography (GPC). GPC was performed using DMF as eluent at flow rate 0.8 mL/min. Polystyrene samples were used for calibration (Figure S1B, supplementary information). Preparation of Amine Functionalized Macromolecules poly(dimethylamino ethyl) methacrylate (PDMA) as well as block copolymer of poly(Nisopropyl acrylamide) and PDMA i.e. PDMA-b-PNIPAM were synthesized by the reversible addition fragmentation chain transfer polymerization (RAFT) (supporting information).36 The PDMA and the copolymer were characterized by 1H NMR spectroscopy (Figure S2A, supporting information) and by GPC (Figure S2B, supporting information). The DMA to NIPAM ratio in the copolymer was 32:68 mol/mol. The GPC derived Mn and the PDI of the copolymer were 25000 g/mol and 1.4 respectively. Multifunctional Agr (Agr-NMe2) was synthesized by the reaction of hydroxyl groups of Agr with tertiaryamine-carbonylimidazole derivative.37 The degree of tertiary amine substitution in the Agr-NMe2 was calculated to be 0.41 (Figure S3, supporting information). Conjugates of PEI and dextran (PEI-Dex) as well as PEG (PEI-PEG) have been synthesized and confirmed by 1H NMR (Figure S3, supporting information). Preparation of blend membranes of PVDF and PMMA-co-PCMSt A typical example of preparation of blend membrane is as follows. Dried PVDF (9 g) and PVP (4 g) were dissolved in DMF (60 g) under continuous stirring for 7 h at 50 °C. PMMAco-PCMSt (4 g) was also added in DMF (15 g) and stirred at ca. 60 oC for 5 h. The solutions were then mixed. The flask containing the copolymer was thoroughly washed with DMF (8 g) and was added to the blend solution. The blend solution was allowed to settle for overnight


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at room temperature to remove the bubbles. The solution was then cast on a non-woven fabric (thickness ca. 100 µm) by semiautomatic casting machine at rate of 3 m/min. The gap between the blade and the surface was adjusted to obtain cast membranes of thickness in the range 50-60 µm. The casting was performed at humidity of ca. 30%. The temperature of the gelation bath was ca. 25 oC. The contact time between cast solution and the gelation bath was ca. 10 s. The membranes were then removed from the gelation bath and were washed with fresh water. The prepared membrane dimension was 30 cm in width and 10 m in length. This membrane is abbreviated as PVDF/blend-1. An ultrafiltration blend membrane (PVDF/blend2) without PVP was also prepared by dissolving PVDF (9 g) and PMMA-co-PCMSt (4 g) in total DMF (83 g) followed by phase inversion. This membrane is abbreviated as PVDF/blend-2. Functionalization of PVDF/blend-1 and PVDF/blend-2 with PEI, PEI-Dex, Agr-NMe2, H2N-PEG-NH2, PEI-PEG, PDMA, and PDMA-b-PNIPMA by Sequential Nucleophilic Substitution Reaction PVDF/blend-1 and PVDF/blend-2 membranes were treated with different concentrations of amine functional polymers and copolymer. The treatment time was varied for standardization experiment. A typical example of functionalization of blend membranes is as follows. First, PEI, PEI-Dex, PEG-PEI and H2N-PEG-NH2 were separately dissolved in distilled water (5%, w/v). The pH of the PEI, PEI-Dex, and PEG-PEI solutions was adjusted to ~7.8. The AgrNMe2 solution was prepared by first dissolving 6 g of Agr-NM2 in DMF (15 mL) at 70 0C and then addition of distilled water (285 mL) to a obtain 2% (w/v) solution. This was done due to low solubility of Agr-NMe2 in water and to lower the viscosity of the solution. The solutions were then taken in cylinders separately. The rolls (20 cm x 90 cm) of blend membranes were soaked with water for 48 h and then submerged in the different reactant solutions (250 mL) separately in a cylinder. The necks of the cylinders were then covered 7 ACS Paragon Plus Environment


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with alumina foil and sealed with adhesive tape. The cylinders containing membranes and reactant solutions were then placed in oven for 48 h at ca. 35 oC. After that, the membrane rolls were removed from the solutions and submerged in water for 24 h. The water was changed after 12 h to ensure the removal of unreacted polymer form the membranes. The modified membranes set obtained from sequential substitution reaction of PVDF/blend-1 with PEI, PEI-Dex, Agr-NMe2, H2N-PEG-NH2, and PEI-PEG, respectively were abbreviated as PVDF/PEI-1, PVDF/PEI-Dex-1, PVDF/Agr-NMe2-1, PVDF/PEG-1 and PVDF/PEI-PEG1. Similarly, the PVDF/blend-1 was treated with copolymer-1 or PDMA (3% w/v, ca. pH 7.8) in water+DMF (90/10 v/v) and the obtained modified membranes were abbreviated as PVDF/copolymer-1 or PVDF/PDMA-1. Similar procedure was used for the modification of PVDF/blend-2 and the obtained membranes were abbreviated as PVDF/PEI-2, PVDF/PEIDex-2, PVDF/Agr-NMe2-2, PVDF/PEG-2, PVDF/PEI-PEG-2, PVDF/copolymer-2 and PVDF/PDMA-2 respectively. Membranes were then stored in 5% (w/v) glycerol solution. PVDF/blend-2 also treated with cystamine hydrochloride in similar way except that the pH of the solution was kept to 8 by the addition of triethyl amine. This process gave membrane with thiol functional groups. The membrane was abbreviated as PVDF/cys-2. Quaternization and Zwitterionization of Membrane For quaternization reaction, PVDF/PDMA-1 was extracted with isopropanol for 24 h and then reacted with CH3I (10%, w/w) in isopropanol for 48 h at 45-50 oC. This membrane is abbreviated as PVDF/QPDMA-1. For zwiterionization reaction, the PVDF/PDMA-1 was extracted with methanol and then reacted with propane sultone (5%, w/v) in methanol at 60 o

C for 48 h. This membrane is abbreviated as PVDF/SPDMA-1.


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Extraction of Modified Membranes by DMF The modified membranes were detached from fabric and dried in a vacuum oven for 48 h at 60 0C. Membranes mass (2 g) was then dissolved in DMF (20 mL) under stirring for 12 h. The insoluble mass was filtered off. The insoluble mass was again stirred with DMF for 24 h. The insoluble mass was thoroughly washed with DMF. The DMF was removed from the collected mass by washing with methanol. The collected mass was subjected to drying in a vacuum oven. The DMF soluble mass was also recovered by removing the DMF with rotary evaporator. The collected DMF soluble and DMF insoluble fractions were weighed and were further characterized. Wetting behaviour, Surface morphology, Attenuated total Reflection Fourier Transform Infrared (ATR-IR) Spectroscopy, and Skin Layer Pore Radius (rp) Water θ, SEM, AFM and ATR-IR analyses, evaluation of pore radius and porosity were performed according to earlier report.34,37 Loading of Ag NPs on the Modified Membranes Surfaces, Leaching Experiment and Antimicrobial Activity A 20 cm x 10 cm of each PVDF/copolymer-1, PVDF/copolymer-2 and PVDF/cys-2 were extracted with distilled water for overnight. The membranes were then dipped in AgNO3 solution (0.1%, w/w in distilled water) for 15 min. The unbound Ag+ ions were removed by washing the membranes with water for several times. After thorough washing of the AgNO3, membranes were then contacted with NaBH4 aqueous solution (0.05%, w/w). The colour of the membrane surface was changed to yellow immediate after contact with NaBH4 solution due to formation of Ag NPs. The membranes were then washed with water. The loading of



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Ag NPs on membrane surface and leaching as well as the antimicrobial activity was determined by the standard procedure (supporting information).38, 39 UF Experiments and Evaluation of Antifouling and Responsive Properties UF

experiments

were

undertaken

using

dead-end

filtration

cell

as

described

elsewhere.7,10,22,34,40,41 The antifouling test, pH and temperature responsive behaviour was also evaluated.24,34,40 The details of experimental procedures are given in the supporting information. Notably, the membranes swatches were extracted with water for 48 h before filtration experiments.

RESULTS AND DISCUSSION Simultaneous Grafting and Crosslinking of Macromolecules on and into PVDF/blend-1 and PVDF/blend-2 Membranes Reactive blend membranes viz. PVDF/blend-1 and PVDF/blend-2 were obtained by the nonsolvent induced phase separation of PVDF+PVP+PMMA-co-PCMSt and PVDF+PMMA-coPCMSt blends respectively (Scheme 1A). The PMMA-co-PCMSt was selected as a blend component due to its miscibility with PVDF. The PMMA part also provides anchoring with the PVDF matrix. This is due to the miscibility between PMMA and PVDF.42,34 The PVDF to PMMA-co-PCMSt ratio was 70:30 (w/w) to ensure homogeneity of the casting solutions.


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(A )

(B)

Scheme 1. (A) Multi-functionalization of Blend Membrane, and (B) Simultaneous Immobilization and Crosslinking of Macromolecules by PMMA-co-PCMSt in the PVDF/blend-1 Through Nucleophilic Substitution Reaction The PVDF/blend-1 was then subjected to the sequential nucleophilic substitution reaction with a series of amine functional polymers e.g. PEI, PEI-Dex, PEI-PEG, Agr-NMe2, H2N-



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PEG-NH2, PDMA and PDMA-b-PNIPAM separately to obtain series of modified membranes (PVDF/PEI-1, PVDF/PEI-Dex-1, PVDF/PEI-PEG-1, PVDF/Agr-NMe2-1, PVDF/PEG-1, PVDF/PDMA-1, and PVDF/copolymer-1). Similarly, PVDF/blend-2 was also treated with above mentioned amine functional macromolecules to obtain PVDF/PEI-2, PVDF/PEI-Dex2,

PVDF/PEI-PEG-2,

PVDF/Agr-NMe2-2,

PVDF/PEG-2,

PVDF/PDMA-2

and

PVDF/copolymer-2. The activated halide (Cl−CH2−C6H4−) groups of the copolymer undergo nucleophilic substitution reaction with the amine groups of the macromolecules (Scheme 1 B) as confirmed by ATR-IR analysis, water θ measurement and by the solvent extraction process (vide infra). Standardization experiments revealed that 5 % (w/v) solution of amine was sufficient for grafting and cross-linking reaction. The initial water θ of the membranes prepared with 5% (w/v) PEI solution reached minimum value after 48 h of post-treatment at 35 oC (Figure S4, supporting information). Furthermore, the nucleophilic substitution reaction was performed for 48 h at 35 oC, since the initial water θ of the membrane attained minimum value after 48 h of treatment (Figure S5, supporting information). Higher the amount of reactive chloromethyl styrene group in the blend membrane matrix, higher will be the amount of macromolecule attachment. PVDF/PDMA-1 was also treated with methyl iodide and propane sultone separately to obtain modified membranes (PVDF/QPDMA-1 and PVDF/SPDMA-1) containing quaternized and zwiterionized moieties respectively (Scheme 1A). The above mentioned process is general since the reactive copolymer blend membrane can be grafted by the varieties of amine functionalized polymers and copolymers. Further modification and substitution of unreacted amine groups present on the membrane surface was also possible. Simultaneous grafting and crosslinking provided good stability of the tethered macromolecules as confirmed by the ATR-IR and solvent extraction experiments (vide infra). 12 ACS Paragon Plus Environment

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Cross-linking, Distribution, and Stability of the Macromolecule The blend and the modified membranes showed characteristic carbonyl (-C=O) stretching vibration at 1730 cm-1 due to presence of ester groups of PMMA (Figure 1A). The PVDF/blend-1 and the modified membranes showed IR band at 1650 cm-1 owing to C=O stretching vibration of PVP. The intensity ratio of 1650 cm-1 to 880 cm-1 (from PVDF backbone) bands lowered by about ca. 23% after extraction with water for 15 days at 30 oC (Figure S6). This indicated that the PVP remained in the membrane matrix. This is due to entanglements between PVDF and PVP chains in the membrane matrices.43,44 The N-H bending vibration cannot be distinguished due to appearance of band ca. 1660 cm-1 for PVP. This was confirmed by the appearance of N-H bending vibration at 1650 cm-1 in the spectrum of PVDF/PEI-2 (no PVP) (Figure S7, supporting information). The PVDF/PEG-1 and PVDF/PEI-PEG-1 showed intensity enhanced bands at 943 cm-1 and 1075 cm-1 due to the CO and C-C stretching vibration of PEG chain. The PVDF/Q-PEI-1 showed additional band at 1010 cm-1 due to presence of quaternized nitrogen (Figure 1A). An additional band appears at 1031 cm-1 for PVDF/SPDMA-1 due to S=O stretching vibrations. On the other hand, PVDF/copolymer-1 (Figure 1A) showed an additional band at 1550 cm-1 which is ascribed to the N-H stretching (amide II band) vibration of the O=C–N–H group in the PNIPAM chains. The values of I1730 to I880 ratio for the PVDF/copolymer-1 and PVDF/copolymer-2 were higher than that of other membranes (Table S1). This is simply due to the higher amounts of ester -C=O groups in these membranes. The expanded ATR-IR spectra of PVDF/PEI-1, PVDF/QPDMA-1,

PVDF/PEI-Dex-1 and PVDF/SPDMA-1 showed intensity enhanced

band ca. 2850 cm-1 due to C(sp3)-H stretching vibration of methyl groups (Fig. 1B).45 The PVDF/PEG-1 also showed broad band (2850-2860 cm-1) due to -CH2 symmetric stretch of PEG.



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The values of I1730 to I880 ratio for the top and bottom surfaces of the membranes were similar (Figure S8A, supporting information). The FT-IR spectrum (Figure S8B, supporting information) of PVDF/blend-1 shows that the I1730 to I880 ratio (~1) was much higher than that obtained (~0.35) from ATR-IR spectra. This indicated relatively higher concentration of PMMA-co-PCMSt in the bulk than that of membrane surface. Membranes were subjected to water exposure for 30 days at temperature ca. 30 oC. The water exposed membranes also showed similar (I1730/I880) ATR-IR spectra to that of Figure 1A (Table S1, supporting information). The attached macromolecules were stable due to their crosslinking with PMMA-co-PCMSt through the sequential nucleophilic substitution

Figure 1. ATR-IR spectra of different membranes in scale 1800 cm-1 to 600 cm-1 (A), and in an extended scale (B). Conditions: Phase inversion for 30 min, and then washing with distilled water for 15 min, followed by drying under vacuum for 24 h at 50 0 C for ATR-IR analysis. reaction. For example, extraction of PVDF/PEI-1 with DMF showed exclusive crosslinking of PMMA-co-PCMSt. Most of the PVDF component remained free, since the ratio of soluble mass to insoluble mass was close to that of PVDF to PMMA-co-PCMSt ratio (w/w) used for


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the preparation of the casting solution. The I1730 to I880 ratio was almost lowered by 90% in the soluble mass. On the other hand, the insoluble mass mostly contained cross-linked PMMA-co-PCMSt (Figure S9, supporting information). The insoluble mass also showed prominent IR band at 1650 cm-1 due to N-H bending vibration (Figure S9). The exclusive crosslinking of PMMA-co-PCMSt is attributed to the diffusion of functional macromolecules into the bulk of the membranes. The molecular weight cut-off of PVDF/blend-1, PVDF/blend-2 and the corresponding modified membranes were about 100000-400000 g/mol while the molecular weight of the macromolecules is in the range 2000-45000 g/mol. A controlled experiment was conducted by treating neat PVDF (14%, w/w in DMF) membrane with PEI under similar experimental conditions. The treated membrane mass was majorly soluble in DMF. Only 1-2% (w/w) insoluble mass was obtained. This indicated negligible reaction between PVDF backbone and PEI under the set reaction condition. Effect of Modification on the Wetting Property, Porosity/Pore Size and Morphology of the Membranes The values of water Ө decreased with time on the post-treated membranes surfaces. The complete wetting time varied depending on the type of modifications (Figure 2). The lowest wetting time of PVDF/QPDMA-1 and PVDF/SPDMA-1 is attributed to the development of charges through qaternization and zwitterionization which enhanced the hydration (hydrophilic character) of the membranes (Figure 2 and Table S2, supporting information). The wetting time>150 sec was not included in the Table S2 due to chance of evaporation of water from the membrane surface. Similar, trend of water Ө values were observed for PVDF/blend-2 and representative modified membranes (Figure S10, supporting information). The digital images (right side, Figure 2) of the water droplets on the membranes surfaces after 20 sec of residual time indicate effect of hydration on wetting behaviour of the membranes.



Contact angle (O)

80

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PVDF/blend-1 PVDF/PEI-1 PVDF/PEI-Dex-1 PVDF/PEG-1 PVDF/QPDMA-1 PVDF/SPDMA-1

60

40

PVDF/Agr-NMe2-1

20

PVDF/copolymer-1

0 0

50 100 150 200 250 300 350 400

Time (sec)

Figure 2. Variation of water Ө with time for PVDF/blend-1 and corresponding modified membranes. Right: images of water droplets on the membranes surfaces recorded after ca. 20 sec of residual time. The pore radius (rp) values and the porosity of the PVDF/blend-1 (rp=26.1 nm, porosity=76%) and the corresponding modified membranes (rp=22.7-25.5 nm, porosity=64%71%) were much higher than that of PVDF/blend-2 (rp=12 nm, porosity=39%) and the corresponding modified membranes (rp=10.5-10.7 nm, porosity=22%-25%) (Table S2, supporting information). The PVP additive caused such differences. The role of PVP for the enhancement of both porosity and rp was reported in the literature.44,46 Furthermore, bulk porosity of the modified membranes was lower than that of corresponding PVDF/blend-1 and PVDF/blend-2 (Table S2). Presumably, crosslinking (bulk and surface) of reactive PMMAco-PCMSt by the hydrophilic amine-functional macromolecules as well as swelling of the cross-linked copolymer reduced the effective free volume in the membrane matrix. The additional crosslinking and pore-filling by the macromolecules also lowered the rp values of the modified membranes as confirmed by the PEO (Mn=400000 g/mol and 100000 g/mol) permeation experiments (Table S2 and Figure S11, supporting information). Enhanced surface and bulk grafting also lowered the porosity and rp of the electrospun polypropylene fibrous membranes.40 Notably, rp values of our PVDF/Agr-NMe2-1, PVDF/copolymer-1 and 16 ACS Paragon Plus Environment

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PVDF/copolymer-2 decreased to a greater extent than that of other modified membranes (Table S2). This indicates that the skin layer pores are comparatively more affected by the high molecular weights Agr (Mn>100000 g/mol) and PDMA-b-PNIPAM (ca. 38000 g/mol) compared to that of low to moderate molecular weights PEI (2000 g/mol), PEI-Dex (Mn= ca. 21000 g/mol) and H2N-PEG-NH2 (ca. 800 g/mol) (co)polymers. Presumably, covalently attached high molecular weight Agr and PDMA-co-PNIPAM induced steric congestion on the skin layer of the membranes. The AFM height images of PVDF/blend-1 and the corresponding representative modified membranes showed the lowering of surface roughness after modification (Figure S12, supporting information). The average surface roughness values were ca. 45 nm for the PVDF/blend-1 and 15-25 nm for the modified membranes. SEM images show (Figure S13, supporting information) somewhat bigger surface pore of PVDF/blend-1 than that of PVDF/PEI-1,

PVDF/copolymer-1,

and

PVDF/Agr-NMe2.

The

surface

pores

of

PVDF/copolymer-1 and PVDF/Agr-NMe2 were still smaller than that of other two membranes as described above. Applications Tuneable Permeation Property and Protein Resistance Behaviour The membranes obtained by post treatment showed lower permeate flux than that of the untreated membranes (Figures 3 A and B). The pure water flux of the modified membranes varied more or less depending on the type of macromolecules used for the functionalization. For example, PVDF/Agr-NMe2-1 and PVDF/copolymer-1 showed lower permeate flux to that of other membranes due to the lowering of the skin layer rp. As discussed earlier, rp and porosity of the modified membranes depend on composition of casting solution (Table S2). For example, the flux of the PVDF/blend-2 and its corresponding modified membranes 17 ACS Paragon Plus Environment


PV D F/ bl en d2

PV D F/ PE I-D PV ex D -2 F/ co po ly m er -2

60

PV D F/ PE I-2

-2 -1

Water flux (LM h

100

0

B

)

PVDF/SPEI-1

PVDF/QPEI-1

PVDF/copolymer-1

PVDF/PEI-PEG-1

PVDF/Agr-NMe2-1

-2

200

80

PVDF/PEI-Dex-1

-1

)

300

A

PVDF/PEI-1

400

PV D F/ bl

(Figure 3A).

en d1

(Figure 3B) was lower than that of PVDF/blend-1 and its corresponding modified membranes

Water flux (LM h

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

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40

20

0

Membrane

Membrane

Figure 3. Permeate pure water flux through (A) PVDF/blend-1 and its corresponding modified membranes and (B) PVDF/blend-2 and its corresponding modified membranes. The permeate flux was measured at applied pressure 15 psi and at temperature 26±2 oC. The pH of the feed water was ca. 7. The membranes were pre-pressurized with pure water at applied pressure 25 psi for 1 h to obtain steady flux. The modified membranes exhibited much better antifouling property than that of pristine membranes during filtration of BSA solution (Figure 4A). The post-treated membranes showed flux recovery ratio (FRR) in range ~80%-93% after 4 h of filtration operation. This value was as low as ca. 30% for PVDF/blend-1. The PVDF/SPDMA-1 and PVDF/QPDMA-1 showed FRR as high as ca. 94-95% (Figure S14, supporting information). The average zeta potential of PVDF/blend-1 was -9.5 mV whereas representative PVDF/PEI1 and PVDF/copolymer-1 showed zeta potential of +4.5 mV and +6.7 mV respectively at pH 7 (supporting information). The presence of quaternized nitrogen moieties makes the surface of PVDF/copolymer-1 more positive. Protonation of amines of the modified membranes also


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makes the zeta potential values of the two membranes positive. The BSA is negatively charged at pH 7. Good fouling resistance property of the modified membranes may be due to the hydrogen bond formation between water and different functional groups of the macromolecules as well as enhancement of surface wetting property. The membrane resistance increased in a short filtration time and then remained almost unaltered during antifouling experiments with BSA solution. This indicates predominant external fouling .34

80

B

PVDF/copolymer-1

2

PVDF/PEG-PEI-1

1

PVDF/Agr-NMe2-1

20

PVDF/Dex-PEI-1

40

PVDF/PEI-1

60

PVDF/blend-1

BSA rejection (%)

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


3

4

5

6

0

Membrane

Figure 4. Permeate flux of the membranes during filtration of BSA solution and subsequent flux recovery at applied pressure 15 psi and at temperature 26±2 oC (A), and BSA rejection by the membranes (B). The pH of the feed solution was 7 (phosphate buffer solution). BSA rejection was measured by collecting the permeates after 15 min of filtration for 10 min time periods. Before experiment at 15 psi, the membranes were equilibrated with pure water at 25 psi for 1h period. BSA rejection (after 15 minutes of permeation test) by the modified membranes (62%-71%) was higher than that of PVDF/blend-1 (54%) (Figure 4B). The PVDF/AgrNMe2-1 and PVDF/copolymer-1 gave higher BSA rejection for the same reason as discussed 19 ACS Paragon Plus Environment


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earlier. Furthermore, BSA rejection at early stage (1-5 minutes of permeation) of filtration was about ca. 41% for PVDF/blend-1 and 50%-60% for the modified membranes. This indicates skin layer pore blocking at the initial stage of filtration in a dead-end filtration system. We have also determined the pure water permeate flux and fouling resistance property of a representative PVDF/PEI-1 in a cross-flow filtration system (Fig. S15, supporting information). The results indicates higher permeate flux and somewhat better FRR in a cross-flow set up than that of dead-end system. The modified membranes obtained with PVDF/blend-2 also gave superior antifouling property. For example, PVDF/copolymer-2 showed good antifouling property during UF of BSA (vide infra). Temperature

and

pH

Responsive

Behaviour

of

PVDF/Copolymer-1

and

PVDF/Copolymer-2 During UF of Water Besides exhibiting good antifouling property, PVDF/copolymer-1 and PVDF/copolymer-2 membranes exhibited pH- and temperature-responsive permeation behaviour. Figures 5 A and 5B shows the combined effect of both pH and temperature on the flux of these membranes. The flux of the membranes increases with either increasing pH or temperature or at fixed temperature by increasing pH. The permeate fluxes show hysteresis in heating and cooling cycles with good reproducibility. This indicates on-off switching of pore size with variation of temperature and pH. Such an effect during filtration operation is due to the pH and temperature responsive behaviour of PDMA and PNIPAM respectively. The DMA moieties in the PDMA-b-PNIPAM copolymer undergo swelling and de-swelling with decrease and increase of pH. This is due to the protonation and de-protonation of the tertiary nitrogen atoms. As a result, lowering of rp occurred either by pore swelling (partially blocking) with the copolymers at relatively low feed pH whereas de-blocking of pore occurs at relatively high feed pH. Similarly, the PNIPAM part of the copolymer causes swelling and


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de-swelling of the copolymer with lowering and increasing of the feed temperature. Below lower critical solution temperature (LCST), swelling of the PNIPAM segments causes

Figure. 5. Cyclic variation of pure water flux of (A) PVDF/copolymer-1 and (B) PVDF/copolymer-2 with temperature at pH 5, 7 and 9 respectively. The membranes were equilibrated at each temperature for 15 min by permeating the feed water before data collection. The permeation in each temperature was collected for 10 min. GPC traces (C) of feed and permeates showing effect of pH during permeation of PEO of molecular weight 300000 g/mol through PVDF/copolymer-1. The temperature of feed PEO solution was 30 oC. The membranes swatches were equilibrated with water at pressure 25 psi for 1 h to attain constant flux. The filtration experiments were then performed at applied pressure 15 psi Averages of three membrane swatches were taken.



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lowering of rp of the membrane. The thermo-responsive property of PNIPAM has been well described in the literature.13-15,34 The ATR-IR spectra as well as reversible permeate flux indicate good stability of the anchored copolymer. The PVDF/copolymer-1 showed 68% and 98% rejections of PEO (300000 g/mol) at feed pH ca. 7 and 5 respectively and at temperature 25 oC (Figure 5 C). The membrane also showed pH responsive permeates flux (42 Lm-2h-1 at pH 5 and 55 Lm-2h-1 at pH 7) during PEO filtration. Hence, this type of membrane can be used for tuneable filtration of macromolecules. Temperature- and pH-Responsiveness of PVDF/copolymer-1 During Filtration of BSA solution Akin to PVDF/copolymer-1 (Figure 4A), PVDF/copolymer-2 also exhibited good antifouling property during UF of BSA with FRR values in the range 92-94% (Figure 6A). The BSA rejection by the PVDF/copolymer-2 (ca. 83%) was higher than that of PVDF/copolymer-1 (71%) at neutral feed pH and at ca. 28 oC. Hence the former membrane was selected to evaluate the pH and temperature responses during UF of BSA. The permeate water flux through the PVDF/copolymer-2 decreased with decreasing temperature and pH of the BSA feed solution (Figure 6A) while BSA rejection (R) increased (Figure 6B). After washing the membrane with water, permeate pure water flux also varied with pH and temperature of the feed water. High BSA rejection ~100% with FRR value ca. 93% can be achieved at pH 5 and at temperatures 20 oC. On the other hand, enhancement of BSA rejection from 83% to 94% with FRR value ca. 94% were achieved by lowering the temperature of the BSA feed from 28 oC to 20 oC at feed pH 7 (Figure 6B). Hence, good rejection and antifouling property can be achieved at low feed temperature.


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Our result on the low protein fouling property during ultrafiltration of BSA was compared with some of the reported membranes. The FRR values of our membranes are compaable with the reported PVDF/PEGMA-b-PS-b-PEGMA blend membrane after first pH=7, T=28 oC pH=7, T=20 oC

A 100

Flux (Lm-2h-1)

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


pH=7, T=42 oC pH=5, T=28 oC pH=5, T=20 oC pH=5, T=42 oC

80

60

40 Water

0

50

BSA

100

Water

150

200

250

Time (min)

Figure 6. Temperature and pH responsive permeate flux (A), and BSA rejection (B) by the PVDF/copolymer-2 during UF of BSA. The BSA solutions were prepared in PB (0.1 M). The membranes were pre-pressurized with pure water at applied pressure 40 psi and then the experiments were conducted at applied pressure 25 psi. The rejection of BSA was measured after permeation for 10 minutes. cycle of operation as reported by Carretier et al.28 Our post-treated membranes exhibited superior fouling resistance property than that of the hydrophilic PVDF membrane (commercial) and hydrophobic PVDF membranes.28,34 The membranes obtained with present process also exhibited better fouling resistant property than that of hydrophilic PVDF membrane obtained by simple blending approach. The antifouling behaviour of our membranes was also similar to that of reported polysulfone/sulfonated polyaniline blend membrane.47 23 ACS Paragon Plus Environment


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Immobilization of Ag NPs for Inducing Anti-biofouling Property on the Membrane Surface As a broad application, Ag NPs were immobilized on PVDF/copolymer-2. Since, PDMA48 has the ability to stabilized Ag NPs, we used representative PVDF/copolymer-2 membrane for this purpose. Furthermore, PVDF/blend-2 was also treated with cystamine to obtain modified membrane (PVDF/cys-2) for the immobilization of Ag NPs. The PVDF/copolymer2 and PVDF/cys-2 were treated with AgNO3 solution followed by removal of unbound Ag+ ions and subsequent reduction of bound Ag+ with NaBH4 produced embedded Ag NPs on these membranes surfaces. The PVDF/copolymer-2 and PVDF/cys-2 were whitish (Figure 7, photographs A and B). On the other hand, formation of yellowish color (Figure 7, photographs C and D) confirmed the presence of Ag NPs on the surfaces of PVDF/copolymer-2-Ag and PVDF/cys-2-Ag. ICP measurements showed that about 3-3.3

Figure 7. Digital photographs of (A) PVDF/copolymer-2, (B) PVDF/cys-2, (C) PVDF/copolymer-2-Ag, and (D) PVDF/cys-2-Ag membranes. Photographs E and F are for PVDF/copolymer-2-Ag and PVDF/cys-2-Ag NCs membranes recorded after subjected these membranes to leaching in water for 30 days at 30 oC at pH 7. Effect of Ag NP on diffusion inhibition zone test against Gram-positive B. subtilis (G-J) and Gram-negative E. coli (K-N) after 24 h exposure of PVDF/copolymer-2-Ag (G and I), PVDF/copolymer-2 (H and J), PVDF/cys-2-Ag (K and L), and PVDF/cys-2 (M and N). 24 ACS Paragon Plus Environment

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µg/cm2 silver loading occurred under the set conditions (supporting information). We examined the dissolution of the Ag NPs in water from the membrane surface. Our results indicated that the amount of silver ions that were leached out in the media was about 6-7% relative to the Ag loading. The persistent yellow color (Figure 7, photographs E and F) even after leaching experiment also indicated stability of the Ag NPs. The leaching amount was increased to about 9-11% in the acidic water (pH 5) after 30 days of exposure. The good stability of the Ag NPs is due to their stabilization by the DMA or thiol moieties present on the membrane surface. It is interesting to note that the whitish color of the unmodified PVDF/blend-2 remained almost unchanged after treatment with AgNO3 and NaBH4. Clearly, the role of PDMA or Cys moieties towards the stabilization of Ag NPs has been established. Hence, proper post-treatment of unmodified membranes is necessary for the immobilization and stabilization of Ag NPs. The diffusion disk experiment confirmed the formation of inhibition zone around the PVDF/copolymer-2-Ag and PVDF/cys-2-Ag after culturing B-subtilis and E. coli in presence of these membrane swatches. The Ag NC membranes (swatches G, I and K and L) showed inhibition zones of area ca. 14 cm2 for B. subtilis (photographs G and H) and ca. 30 cm2 for E. coli (photographs K and L). On the other hand, PVDF/copolymer-2 (H and J) and PVDF/cys-2 (M and N) membranes swatches were heavily fouled by the bacteria. The formation of bacterial colony around and on the surfaces of PVDF/copolymer-2 and PVDF/cys-2 was clearly seen by the disk experiment. PVDF/copolymer-1-Ag and PVDF/cys-1-Ag showed similar inhibition zone to that of PVDF/copolymer-2-Ag and PVDF/cys-2-Ag. The formation of inhibition zone may be explained by the leaching of even very low concentration of Ag ions as well as by contact killing mechanism.49,50 Figure 8 (bar diagrams A and B) shows the quantitative of the bacteria attachment on the surface of Ag NC membranes and membranes without Ag NP. The Ag NCs membranes 25 ACS Paragon Plus Environment


showed much lower propensity towards attachment of bacteria. It was noted that CFU/mL also lowered significantly in bacterial solution after exposure of Ag NCs membranes for 7 days (Table S3, supporting information). Since, the leaching of Ag in form of ions is negligible, most probably, contact killing mechanism is majorly responsible for the

1x103

E. coli B. subtilis

PVDF/cys-2-Ag

1x104

PVDF/cys-2

PVDF/copolymer-2

B

CFU/mL

PVDF/cys-1

1x105

PVDF/copolymer-2-Ag

1x103

E. coli B. subtilis

PVDF/cys-1-Ag

1x104

A

PVDF/copolymer-1-Ag

1x105

PVDF/copolymer-1

antimicrobial effect of these NC membranes.49,50

CFU/mL

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

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1x102

1x102

Membrane

Membrane

Figure 8. Quantitative of bacterial attachment on the surfaces of the membranes. Bar diagrams (A) for PVDF/blend-1 and the corresponding modified membranes, and (B) for PVDF/blend-2 and the corresponding modified membranes.

CONCLUSION Covalent functionalization of PVDF membranes was successfully accomplished through post-nucleophilic substitution reaction between activated benzyl chloride groups of PVDF blend membrane and amine groups of macromolecules. Good stability of the grafted macromolecules was realized by this method owing to covalent crosslinking. The pore size


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and porosity decreased in the modified membranes. The resulted modified membranes exhibited enhanced hydrophilicity and low protein fouling characteristics. Simultaneous pH and temperature responsive membranes were obtained by treating the PVDF blend membranes with poly(N,N-dimethyl amino ethyl methacrylate-b-(N-isoproyl acrylamide). Protein rejection as high as ca. 100% and flux recovery ratio ca. 94% have been achieved during bovine serum albumin filtration at pH 5 and at temperatures 25 oC and 28 oC. The quaternized and zwiterionic membranes were also obtained by treatment of poly(N,Ndimethyl amino ethyl methacrylate) modified membranes with quaternizing and zwiterionizing agents. Suitable choice of macromolecules containing di- or multi-functional amine provided membrane for desirable application. Our further work also showed that immobilization of silver nanoparticle is possible for obtaining responsive membranes with good anti-biofouling and anti-organic fouling properties. Hence, library of modified membranes have been obtained by this technique. This work thus highlights a convenient approach for the immobilization varieties of macromolecules on and into reactive blend membranes. Based on the approach, our future goal is the preparation of charged membranes for electrodialysis applications. ACKNOWLEDGMENTS CSIR-CSMCRI Registration No. 200. We thank Department of Science and Technology (project grant number: EMR/2015/000843), Government of India for financial support. This work is also supported by CSIR (Project No. ESC-0306), Government of India. We also thank Centralized Analytical Facility for all round analytical support. AB and AK thanks CSIR, India for providing research fellowship. ASSOCIATED CONTENT Supporting Information 27 ACS Paragon Plus Environment


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The Supporting Information is available free of charge on the ACS Publications website. Experimental details Synthesis and characterization of PMMA-co-PCMSt and PDMA-bPNIPAM copolymers, preparation and characterization of PEI-Dex and PEI-PEG conjugates, characterizations of membranes (SEM, AFM, ATR-IR and FT-IR), antifouling property of PVDF/SPDMA-1 and PVDF/QPDMA-1, loading of Ag NPs, stability of Ag NPs, and evaluation of antimicrobial activity. AUTHOR INFORMATION Corresponding Author Suresh K. Jewrajka Fax: +912782566511; Tel.: +912782566511; Email: [email protected] Notes The authors declare no competing financial interest.

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Table of Content


Reactive Copolymer

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