Effect of amine spacer of PEG on the properties, performance and antifouling behavior of poly(piperazineamide) thin film composite nanofiltration membranes prepared by in situ PEGylation approach

Journal of Membrane Science 472 (2014) 154–166

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Effect of amine spacer of PEG on the properties, performance and antifouling behavior of poly(piperazineamide) thin film composite nanofiltration membranes prepared by in situ PEGylation approach Ravindra M. Gol a, Anupam Bera a, Semire Banjo b,1, Bishwajit Ganguly b,n, Suresh K. Jewrajka a,nn a

Reverse Osmosis Discipline, CSIR-Central Salt and Marine Chemicals Research Institute, G. B. Marg, Bhavnagar 364002, Gujarat, India Computational and Simulation unit, Analytical Discipline and Centralized Instrument Facility, CSIR-Central Salt and Marine Chemicals Research Institute, G. B. Marg, Bhavnagar 364002, Gujarat, India

b

art ic l e i nf o

a b s t r a c t

Article history: Received 20 May 2014 Received in revised form 15 July 2014 Accepted 6 August 2014 Available online 27 August 2014

In continuation of work (Gol et al., J. Membr. Sci., 2014, 455, 271–282) for in situ PEGylation of thin film composite (TFC) membrane, herein, effect of polyethylene glycol (PEG) end-group on the properties and performance of PEGylated TFC nanofiltration (NF) membranes is reported. In situ PEGylation of conventional poly(piperazineamide) TFC NF membranes was performed by interfacial polymerization between TMC and PIPþ PIP-terminated polyethylene glycol (PIP–PEG–PIP), PIPþm-phenylenediamineterminated PEG (MPD–PEG–MPD) and PIPþ alkyl amine terminated-PEG (H2N–PEG–NH2) mixtures respectively. Among these three processes, PEGylated membranes prepared with TMC and PIPþ PIP– PEG–PIP mixture exhibited excellent antifouling property, similar performance, close pore radius and pore structure factor compared to conventional poly(piperazineamide) TFC NF membrane. This is attributed to the closer reactivity of PIP and PIP–PEG–PIP towards TMC. Membranes prepared with TMC and PIPþMPD–PEG–MPD mixtures exhibited superior antifouling property compared to conventional TFC NF membrane, nevertheless, rejection and permeate flux were decreased. TFC membranes prepared with PIPþ H2N–PEG–NH2 mixtures/TMC exhibited similar performance compared to conventional membrane with low degree of PEGylation and hence showed only marginal improvement of antifouling property. Conceptual Density Functional Theory (DFT) and average local ionization potential energy calculations performed on the lowest energy conformations of PIP and different amines (PIP, MPD and alkyl amine) terminated PEG type of molecules indicated closer reactivity of PIP and PIP-terminated PEG type of molecule compared to PIP and other amines terminated PEG type of molecules towards TMC. & 2014 Elsevier B.V. All rights reserved.

Keywords: Low fouling TFC NF membranes In situ PEGyltaion PEG-end group structure and membrane performance Reactivity Conceptual DFT calculation and local ionization potential

1. Introduction Module system comprising polyamide thin film composite (TFC) membrane has already been emerged as a reliable system for water purification. The efficiency of a TFC membrane to operate either in the reverse osmosis (RO) or in the naofiltration (NF) range depends on types of monomer employed for the preparation of its active layer on top of ultrafiltration membrane support by interfacial polymerization (IFP) [1–9]. The active layer of TFC RO membrane is usually dense with pore size smaller than hydrated

n

Corresponding author. Tel.: þ 91 2782567760; fax: þ 91 2782566511. Corresponding author. Tel./fax: þ 91 2782566511. E-mail addresses: [email protected] (B. Ganguly), [email protected] (S.K. Jewrajka). 1 Permanent address: Department of Pure and Applied Chemistry, Ladoke Akintola University of Technology, Ogbomoso, Oyo State, Nigeria. nn

http://dx.doi.org/10.1016/j.memsci.2014.08.015 0376-7388/& 2014 Elsevier B.V. All rights reserved.

Na þ and Cl  ions and larger than water [10,11]. Thus the rejection efficiency of TFC RO membrane depends predominantly on steric factor, whereas, rejection efficiency of NF membrane depends both on steric factor and charge–charge repulsion between membrane surface and ions (Donnan exclusion principle) [5–9]. Characteristic features such as pore radius, effective charge and selective layer thickness to porosity ratio (pore structure factor) of NF membranes can be obtained from steric exclusion and transport models [8,9]. Polyamide active layer formed by IFP between multifunctional acid chlorides, more preferably; TMC and diamines more preferably; MPD produces TFC RO membrane with high NaCl rejection efficiency [1–3]. On the other hand, IFP between TMC and a diamine, e.g. PIP produces TFC NF membrane with high multivalent salt rejection efficiency and usually much lower NaCl rejection efficiency [5,6]. After successful demonstration of these polyamide RO and NF membranes for water purification, efforts were also made to improve their performance [4,5,10,11].

R.M. Gol et al. / Journal of Membrane Science 472 (2014) 154–166

One serious drawback of TFC membrane is the fouling due to deposition of proteins, colloidal particles, formation of biofilms and concentration polarization [12,13]. As regards to fouling, hydrophilic TFC membranes have attracted wide attention due to many advantages: (i) enhances the membranes life by lowering the organic fouling which is a commonly occurring phenomenon, (ii) lowering of biofouling by reducing the adhesion of organics followed by adhesion of bacteria and (iii) lowers the operation cost by reducing the extra cleaning step and maintaining steady permeate flux. Surface modification is a convenient approach for hydrophilization of TFC membrane which can be categorized into two major classes: (i) physical adsorption or coating over the surfaces [14–19] and (ii) covalent attachment of antifouling polymer chains on premade TFC membrane surfaces [20–28]. The first approach includes, surface modification of polyamide TFC membranes (NF or RO) by self-deposition of polycations [14], coating by PEG [15], coatings by solution of thermo-responsive copolymers such as poly(N-isopropylacrylamide-co-acrylamide), poly (N-isopropylacrylamide-co-acrylic acid) [16,17] and coating of silverPEGylated dendrimer nanocomposite [18]. Such modifications enhanced the antifouling property; nevertheless, permeate flux declines more or less compared to corresponding pristine membranes. On the other hand, two main approaches have been put forward for covalent grafting of hydrophilic polymers, e.g. “grafting from” route, where polymer chains grow from surface by the surface initiated polymerization of monomers [20–25] and “grafting to” approach [26–28], where polymer chains are directly

HN

N

CH2 CH2 C O

O

O

PIP-PEG-PIP (A)

C

CH2 CH2 N

the immobilized PEG chains depends on concentration of unreacted –COCl or –NH2 groups present on the polyamide film surface. To cope with the lower concentration and lower reactivity of free aromatic amine available on the premade TFC, Castrillón et al. proposed a rout to enrich TFC membrane surface by primary aliphatic amine to facilitate reaction with poly(ethylene glycol) di-epoxides [28]. PEG was also deposited on the RO membrane surface by plasma polymerization of triethylene glycol dimethyl ether for enhancement of antifouling property of the membrane [29]. Such modified membranes showed 10–15% of reduction in permeability. Besides coating and covalent attachment, hydrophilic surface modifying macromolecules such as combination of reactive diisocyanate and PEG were reported to enhance the antifouling property of the membranes when added into the TMC solution before IFP [30]. Addition of polyvinyl alcohol into the PIP solution prior to IFP produced NF membranes with enhanced antifouling properties and performance [31]. Zhang et al. reported the preparation of NF membrane on the surface of modified pluronic/polyethersulfone ultrafiltration membranes by the IFP between PIP and TMC. This approach resulted TFC membrane with antifouling property due to exposure of PEO segments [32]. An et al. reported a novel way of preparation of NF membranes by the IFP between TMC and mixture of PIPþ N-aminoethyl piperazine propane sulfonate with enhanced flux and antifouling character [33]. Hence, in order to mitigate membrane fouling, design of novel low fouling TFC membranes without much compromising the performance is highly desirable.

NH

CH2 CH2 C HN

O

O

NH2

H2N

CH2 CH2

O

155

O

CH2 CH2

O

O

MPD-PEG-MPD (B)

C O

CH2 CH2 NH

NH2

NH2

H2N-PEG-NH2 (C) attached to the surface. The “grafting from” approach e.g. surface initiated (Red-Ox) polymerization was effective for surface modification of TFC membranes; however, high concentration of monomers and longer reaction time are required for maintaining proper grafting density [20,21]. Such process was further modified by performing polymerization under applied pressure to induce concentration-polarization-enhanced graft polymerization on the membrane surface for enhancement of antifouling property [22]. Himstedt et al. reported surface modification of commercially available NF membrane by UV initiated polymerization to grow pH-responsive poly(acrylic acid) chains on membrane surface without much affecting the performance of the membrane [23]. Surface initiated Atom Transfer Radical Polymerization (ATRP) of N-isopropylacrylamide/ethylene glycol methacrylate and sulfobetaine methacrylate on TFC NF and TFC RO membranes (carrying surface bound initiator) resulted grafting of membranes by fouling resistant Poly(N-isopropylacrylamide)/poly(N-isopropylacrylamide-block-ethylene glycol methacrylate) and zwitterionic poly (sulfobetaine methacrylate) respectively [24,25]. PEG is hydrophilic polymer and widely used to improve the antifouling property of a membrane. This is owing to its ability to form hydrogen bond with water molecules there by rendering the surface hydrophilicity and lowering the interaction with incoming fouling molecules. Surface, immobilization of PEG (“grafting to” approach) on TFC polyamide membrane was achieved by treating premade polyamide surface with amine-terminated PEG and glycidyl ether-terminated PEG [26,27]. The grafting density of

Our earlier publication reported a convenient approach for in situ PEGylation of TFC RO membranes for improvement of antifouling property [34]. Herein, we report a protocol for selecting end-functional PEG for in situ PEGylation of poly(piperazineamide) TFC NF membranes without much affecting the performance of PEGylated membranes compared to corresponding non-PEGylated membranes. The effect of end-group structure (amine types) of PEG (structures A–C) on membrane physical properties, performance and antifouling behavior was evaluated by employing PIPþA, PIPþB and PIPþC mixtures respectively as amine sources for IFP with TMC. Based on performance, pore radius, pore structure factor of the membranes, together with conceptual DFT and local ionization potential energy calculations on functional PEG type of molecule, herein, it is proposed that amine-terminated PEG, the end-group of which resembles the structure of employed excess conventional monomer (PIP in this case) is well suited for preparation of PEGylated TFC-NF membranes having similar performance and much better antifouling property compared to membrane prepared with PIP and TMC under similar experimental conditions.

2. Experimental 2.1. Materials Polysulfone (PSf, Udel P-3500, Solvay Polymers), dimethylformamide (DMF, Spectro Chem, India) and anhydrous PIP (98%, SRL,

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R.M. Gol et al. / Journal of Membrane Science 472 (2014) 154–166

India), MPD (granules 98%, Across), were used as received. TMC (98%), poly(ethylene glycol)diacrylate (AA–PEG–AA, Mn ¼700 g/ mol) and poly(ethylene glycol)methyl ether acrylate (MeO–PEG– AA, Mn ¼480 g/mol) are from Aldrich used as received. H2N–PEG– NH2 was prepared by the reported procedure [26]. Dichloromethane (DCM), hexane and bovine serum albumin (BSA) from Spectrochem, India, were of analytical grade and used without further purification. Non-woven polyester fabric (Nordlys, TS100) was used as received. 2.2. Preparation and in situ PEGylation of poly(piperazineamide)based TFC NF membranes by different approaches PSf support membrane was prepared on non-woven fabric by a phase inversion method using semi-automated casting machine as reported earlier [34,35]. All the TFC membranes were prepared on PSf ultrafiltration support membranes. First, separate (i) PIPþPIP– PEG–PIP or PIPþMeO–PEG–PIP, (ii) PIPþMPD–PEG–MPD and (iii) PIPþ H2N–PEG–NH2 solutions were prepared for in situ PEGylation. A representative example of preparation of PIP solution (3% w/v, 0.35 mol/L) containing PIP–PEG–PIP (0.5% w/v, 0.006 mol/L) or MeO–PEG–PIP (0.5% w/v, 0.0085 mol/L) is as follows. PIP (15 g) was dissolved in water (400 mL) and then either AA–PEG–AA or MeO–PEG–AA (2.5 g) (in water 100 mL) was added drop wise into PIP solution under stirring for 5 min at temperature 25 1C and was additionally kept for 5 min to obtain PIPþPIP–PEG–PIP or PIPþ MeO–PEG–PIP mixture separately. The reaction between PIP and AA–PEG–AA or MeO–PEG–AA was very fast which led to complete conversion of acrylate functionality within 5 min. A standardization procedure was performed to evaluate the time period up to which PIP–PEG–PIP remained stable, without side reaction such as amination type of reaction between excess PIP and β-amino ester bond of the formed adduct. For this purpose, the reaction mixture was additionally kept for 1 h, 2 h, and 3 h. It was found that the PIP–PEG–PIP remained stable up to  1 h with minimum side reaction. Hence, IFP reaction may be extended up to ca. 1 h after preparation of PIPþPIP–PEG–PIP or PIPþ MeO– PEG–PIP mixture (vide infra). Thus the PIPþ PIP–PEG–PIP or PIPþ MeO–PEG–PIP mixture (containing 0.25–0.75% w/v i.e. 0.003–0.009 mol/L of PIP–PEG–PIP or 0.25–0.75% w/v i.e. 0.004– 0.012 mol/L of MeO–PEG–PIP) obtained after  5 min reaction time was directly used for IFP with TMC. The pHs of the solution were 10.470.2 to 10.570.1. On the other hand, MPD–PEG–MPD was prepared by Michael addition between AA–PEG–AA and excess MPD at 70 1C for 4 h. The adduct MPD–PEG–MPD was extracted from reaction mixture. Detailed of obtaining purified MPD–PEG–MPD adduct was described in our earlier publication [34]. The purified adduct was then co-solubilized with PIP to obtain PIP þMPD–PEG–MPD solutions (containing 0.25–0.75% w/v i.e. 0.003–0.009 mol/L of MPD– PEG–MPD) for the IFP with TMC. The pHs of the solutions were 10.5 70.1 to 10.7 7 0.1. The H2N–PEG–NH2 was prepared by multistep reactions as reported in the literature [18] and solutions of PIP þH2N–PEG–NH2 (containing 0.25–0.75% w/v i.e. 0.003–0.009 mol/L of H2N–PEG– NH2) was prepared for IFP with TMC. The pHs of the solutions were 10.4 70.1 to 10.6 70.1. The pH of solution containing only PIP (3% w/v) was 10.6 70.1. Polyamide TFC NF membranes on top of PSf support were prepared via IFP between TMC taken in hexane at temperature 30 1C and (i) in situ generated PIP þPIP–PEG–PIP or PIP þMeO– PEG–PIP, (ii) PIP þMPD–PEG–MPD (premade) and (iii) PIP þH2N– PEG–NH2 (premade) taken in water at 20–25 1C. A typical example of preparation of TFC membrane with PIPþPIP– PEG–PIP or PIPþMeO–PEG–PIP mixture is as follows. TMC (1.25 g) was dissolved in hexane (1000 mL) at room temperature (30 1C).

Next, the water wet PSf support membrane was carefully attached on a glass slide, and was placed into the previously prepared aqueous solution of PIPþPIP–PEG–PIP or PIPþ MeO–PEG–PIP mixture (prepared just before the IFP). The membrane was allowed to soak for at least 20 s. The attached PIPþPIP–PEG–PIP or PIPþ MeO–PEG–PIP soaked membrane was then removed from the solution. The surface of the membrane was then gently rolled with a rubber roller to eliminate small bubbles which may have formed during the process of soaking. Afterwards, the PIPþPIP–PEG–PIP or PIPþ MeO–PEG–PIP soaked membrane was gently placed into the solution of TMC and kept for 60 s which resulted in the formation of a thin film over the PSf support. The membrane was heat cured at 60 1C for 2 minutes. The temperature and humidity during a coating process were 3073 1C and 6277% respectively. The prepared membrane was then washed with water and stored in water containing glycerol (10% w/v). All the TFC membranes were prepared on PSf support using exactly the same procedure as mentioned above. The average TFC membrane preparation time starting from PIPþAA–PEG–AA solution preparation was 12–15 min for all cases. Table 1 summarizes the concentration of constituents used for the preparation of different TFC NF membranes and their abbreviations. 2.3. Characterization 2.3.1. Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) Thoroughly washed membranes were used for ATR-IR analysis. The area 3  6 cm2 of each TFC membrane was taken for analysis. The membranes were dried in vacuum at 40 1C for 24 h. ATR-IR of the polymer membranes were recorded on an Agilent instrument (Agilent Cary 600 series FTIR) at room temperature. A Germanium crystal was used for recording ATR-IR spectra. ATR-IR spectra on 5 different positions were recorded. The average degree of PEGylation (DGPEGylation) was determined by ATR-IR as follows: Average DGPEGylation ¼ ðI1080 =I1585 ÞPEGylated  ðI1080 =I1585 ÞnonPEGylated ð1Þ where I1080 is the intensity of C–O and C–C stretching vibrations of PEG chains and also C–C aromatic stretching. The I1585 is the intensity of aromatic CQC stretching of PSf support membrane. Table 1 Concentration of constituents used for the preparation of select TFC membranes and their abbreviations. [TMC]¼ 0.125% (w/v) in hexane; curing temperature and curing time for all the cases were 60 1C and 2 min respectively. Entry

Abbreviation

PIP (%, w/v)

X–PEG–X Or MeO–PEG–X (%, w/v)

Y–PEG–Y (%, w/v)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

NF-0 NF-1 NFAA–PEG–AA(0.25%) NFAA–PEG–AA(0.5%) NFAA–PEG–AA(0.75%) NF0 AA–PEG–AA(0.25%) NF0 AA–PEG–AA(0.5%) NFMeO–PEG–AA(0.25%) NFMeO–PEG–AA(0.5%) NFMeO–PEG–AA(0.75%) NFMPD-PEG–MPD(0.25%) NFMPD–PEG–MPD(0.5%) NFMPD–PEG–MPD(0.75%) NF0 MPD–PEG–MPD(0.5%) NFH2N–PEG–NH2(0.25%) NFH2N–PEG–NH2(0.5%) NFH2N–PEG–NH2(0.75%) NF0 H2N–PEG–NH2(0.5%)

3 2 3 3 3 2 2 3 3 3 3 3 3 2 3 3 3 2

0 0 0.25 0.5 0.75 0.25 0.5 0.25 0.5 0.75 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0.25 0.5 0.75 0.5 0.25 0.5 0.75 0.5

X¼–AA; Y¼–MPD or –NH2. pH of all aqueous solutions were in the range 10.4-10.7.

R.M. Gol et al. / Journal of Membrane Science 472 (2014) 154–166

Usually, the intensity of 1585 cm  1 band changes insignificantly upon modifications, unless the thickness of the top layer exceeds the penetration depth of evanescent IR wave (  1 μm) [36]. Hence the DGPEGylation is obtained by subtracting the calculated I1080 to I1585 ratio of non-PEGylated TFC membrane from (I1080 to I1585 ratio of PEGylated TFC membrane. The reported DGs are the average of at least five spots on each sample. 2.3.2. Water contact angle (θ), zeta potential, scanning electron microscope (SEM) and atomic force microscope (AFM) All these surface characterizations were carried out as described in our earlier publications (Supplementary material) [34,35]. 2.4. Membrane performance The performances of the membranes were evaluated in a reverse osmosis test kit for water desalination using 1500 mg/L Na2SO4, NaCl, and MgCl2 as feed solutions. The test kit consisted of four SS361 cells connected in series. The cell could accommodate circular membrane coupons of 3.5 cm dia. The kit was additionally equipped with a high pressure pump, pressure-gauge, pressurecontrol valve and conductivity meter. The feed flow occurred in the cross-flow mode in the test kit. After fixing the membrane circles in the test cells, pure water was permeated initially at 1.03 MPa for 1 h, to obtain steady flux. Then, the permeation of the each salt solution was undertaken at 0.51 MPa, 0.69 MPa, 1.03 MPa and 1.37 MPa. The flux and rejection were measured at each pressure. The permeate flux, J (Lm  2h  1), was calculated using J¼

V At

ð2Þ

where V is the volume of water permeated (L), A is the membrane area (m2) and t is the permeate time (h). The salt concentrations in the feed and permeate were determined by measuring the electrical conductivity of the solutions using digital conductivity meter. The salt rejection (%SR) was determined using   Cp SR% ¼ 1   100 ð3Þ Cf where Cf and Cp are the salt concentrations in the feed and permeate, respectively. Average value of 4–6 coupons was taken. 2.5. Estimation of permeability coefficient (Lp), effective pore radius (rp) and effective thickness to porosity ratio (lp/εp) Separation performance of TFC membranes was evaluated using dead end filtration system with a feed volume of 500 mL and an effective membrane area of 13.8 cm2. The solute rejection (SR) experiments were performed using 500 mg/L solutions of glucose and sucrose. The pore size was estimated from SR under limiting conditions where the SR reaches a limiting value as described earlier [8,9] (Supplementary material). The pore structure factor (lp/εp) (lp ¼thickness and εp ¼porosity) was calculated from the Hagen–Poiseuille pore flow model [8,9]. 2.6. Evaluation of antifouling property and stability of PEGylated TFC NF membranes Water extracted TFC membrane coupons (4 coupons of each type of membranes) were first pressurized at 1.03 MPa pressure for 1 h with Na2SO4 solution (1500 mg/L) and then initial flux and rejection were measured at pressure 0.69 Mpa. Coupons of each type of membranes were then tested by permeating Na2SO4 (1500 mg/L) solution spiked with BSA protein (250 mg/L). The pH of the feed solution was 7.1. The filtration experiment was then carried out for 20 h. The temperature during testing was  24 1C.

157

The rejection efficiency and flux during filtration experiments were evaluated. Antifouling property was determined in terms of flux reduction ratio (%FR) by the following equation: %FR ¼

J0  Jt  100 J0

ð4Þ

where J0 is the initial flux during water desalination (containing 1500 mg/L Na2SO4) (after 1 h of pressurization) and Jt is the flux at a given time of desalination of water (1500 mg/L Na2SO4) contaminated by BSA (250 mg/L). After 20 h of filtration, membranes were washed with deionized water for 10 min and the flux (Jc) and rejection efficiency of the cleaned membranes were measured again by permeating water (containing 1500 mg/L Na2SO4). In order to evaluate the fouling property of the membrane, the flux recovery ratio (FRR %) was also calculated by the following equation: FRR% ¼

Jc  100 J0

ð5Þ

A similar experiment was repeated with feed water containing Na2SO4 and BSA at feed pH 4.7 7 1.

2.7. A computational method Conformational search was performed on all the molecules employing a semi-empirical AM1 method with the Monte Carlo search algorithm. For each conformational search, 1000 conformers were examined, only conformers within 710 kJ/mol of energy window were considered. The lowest-energy conformer of this conformational search was taken for further DFT calculations [37]. This involves a restricted systematic search that walks through all possible conformers within the specification in order to enforce a fixed number of conformers (50 conformers). The lowest equilibrium conformations were used for DFT calculations. All calculations were performed on the molecules with DFT of Becke's three parameter hybrid functional with correlation of Lee et al. [38]. Optimization of molecules was performed at B3LYP/ 6-31þ Gn level of theory in both gas and aqueous phases (coordinates of the calculated compounds, Supplementary material). Frequency calculation at the same of theory was used to confirm that the optimized molecules were minima, as characterized by positive harmonic frequencies [38,39]. The solvent phase optimization of the molecules was performed with polarized continuum salvation model (PCM) using the integral equation formation variant (IEF-PCM) [40] as implemented in Gaussian 09 [41]. The conceptual DFT method provides definitions of important universal concepts of molecular structure stability and reactivity. Popular qualitative chemical concepts such as hardness (η), chemical potential (μ) and global nucleophilic index (ω) within the DFT concept are use as described as follows: 1 2

μ ¼ ðEHOMO þ ELUMO Þ

ð6Þ

η ¼ EHOMO  ELUMO

ð7Þ

ω¼

μ2

2η

ð8Þ

where EHOMO and ELUMO are the energies of the highest occupied and lowest unoccupied molecular orbitals respectively. The nucleophilic and electrophilic indices of molecules correspond to opposite extremes of the scale of global reactivity indices. A more reactive nucleophile is characterized by a lower value of both μ and ω.

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R.M. Gol et al. / Journal of Membrane Science 472 (2014) 154–166

Route A

Route B

Route C

PEGylation of polyamide TFC NF membranes Route D

Route E

Amination reaction

Scheme 1. PEGylation of TFC NF membrane via IFP between TMC and (A) PIP þ PIP–PEG–PIP (in situ generated), (B) PIP þ MPD–PEG–MPD and (C) PIPþ H2N–PEG–NH2 mixtures. Routes D and E: possibility of amination reaction.

3. Results and discussion 3.1. PEGylation of TFC-NF membrane by different end-functional PEG In situ PEGylation of TFC-NF membrane was conveniently achieved by the addition of AA–PEG–AA or MeO–PEG–AA (0.003– 0.012 mol/L) into aqueous PIP solution (0.25–0.35 mol/L) followed by stirring for 5 min for further IFP reaction with TMC. AA–PEG–AA or MeO–PEG–AA rapidly reacts with excess PIP via the Michael addition reaction at  25 1C which produced an adduct called β-amino ester (PIP–PEG–PIP) and excess PIP remained in solution (Scheme 1, route A). Formation of adduct was evident from 1H NMR spectra of reaction mixture as well as purified product (Fig. S1, Supplementary material). It is seen that acrylate signals of AA–PEG– AA or MeO–PEG–AA at δ ¼5.8–6.4 were completely disappeared by the Michael addition reaction. The intensity ratio of signal at δ ¼3.6–3.9 for –O–CH2– of PEG chain to signal at δ ¼4.4 due to –OCOCH2– remained almost unaltered before and after reaction, indicates no detectable side reaction. Formation of β-amino ester was also evident from appearance of new signals at δ ¼2.2–2.6 for the protons of the adduct (Fig. S1, Supplementary material). In addition, Michael addition reactions for 10 min, 1 h, 2 h and 3 h (times when spectra recorded) were conducted to observe the stability of the adduct by 1H NMR (Fig. S2, Supplementary material). After 1 h,  12% of amination took place (Scheme 1, route D) whereas  30% amination reaction took place after 3 h as calculated from 1H NMR (from intensity ratio of signals δ ¼4.4 to δ ¼3.6–3.9). Thus, PIPþPIP–PEG–PIP or PIPþMeO–PEG–PIP mixture should completely be used within 1 h. This is one problem in the current process for large scale preparation of TFC which takes several hours for continuous preparation. However, such problem in large scale

preparation may be overcome by (i) use of freshly prepared PIPþAA–PEG–AA solution after 1 h of continuous production of TFC NF membrane or by (ii) continuous addition of suitable amount of AA–PEG–AA into the continuously used PIPþAA–PEG–AA solution for large scale purpose. Presently, all the membranes were prepared with freshly prepared solution of PIPþPIP–PEG–PIP mixture with average membrane preparation time (12–15 min). Such amination reaction under this experimental conditions was not observed for MPD–PEG–MPD based system due to lower reactivity of MPD towards ester bond (Scheme 1, route E) owing to lower nucleophilicity of amine attached to aromatic ring compared to aliphatic amine (PIP). The PIP þPIP–PEG–PIP or PIP þMeO–PEG–PIP mixture undergoes poly(co-condensation) with TMC via IFP reaction (Scheme 1, route A) to generate PEGylated TFC NF membranes (Table 1, entries 3–10). Total 2–3% (w/v) PIP and 0.125% (w/v) TMC concentrations were selected to achieve reasonable rejection and flux of the membrane during salts removal. PEGylation of TFC NF membranes was also performed by extraneous addition of premade MPD–PEG–MPD (Table 1, entries 11–14) and H2N–PEG–NH2 (Table 1, entries 15–18) respectively into the PIP solutions to understand the influence of type of end-group on characteristic features and performance of the formed membranes (Scheme 1, routes B and C respectively). 3.2. Characteristic features of TFC-NF membranes prepared by different approaches 3.2.1. ATR-IR analysis Fig. 1 shows ATR-IR spectra of select membranes prepared by three different approaches. All the NF membranes show characteristic

R.M. Gol et al. / Journal of Membrane Science 472 (2014) 154–166

1585 cm1 1625 cm-

1

1080 cm-

159

1

2872 cm-1

1800

1600

1400

1200

1000

800

3200

3100

0.30

2900

2800

2700

MeO-PEG-AA AA-PEG-AA MPD-PEG-MPD H2N-PEGNH2

0.25

DGPEGylation

3000

Wavenumber (cm-1)

Wavenumber (cm-1)

0.20 0.15 0.10 0.05 0.00 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Functional PEG (% w/v) Fig. 1. (A and B) ATR-IR spectra (bottom to top) of NF-0, NFMeO–PEG–AA(0.25%), NFMeO–PEG–AA(0.5%), NFAA–PEG–AA(0.25%), NFAA–PEG–AA(0.5%), NFMPD–PEG–MPD(0.5%) and NFNH2–PEG–NH2(0.5%). (C) Variation of DGPEGylation with the increased amount of different functional PEG added into the PIP solution prior to IFP with TMC.

bands at 1620 cm  1 and 1585 cm  1 (Fig. 1A) for amide and aromatic –CQC– stretching vibrations respectively. PEGylated NF membranes also show intensity enhanced peaks at 1080 cm  1, 943 cm  1, and 2875 cm  1 which are ascribed to the C–O and C–C stretch, CH2 rock and C–C stretch and the CH2 symmetric stretch of PEG respectively [26]. Enhancement of intensity of band at 1080 cm  1 (Fig. S3, Supplementary material) and 2872 cm  1 can be seen from expanded ATR-IR spectra (Fig. 1B). The PEGylation of TFC membranes can be visualized by the enhancement of calculated DGPEGylation values with increasing amount of added functional PEG into the PIP solution (Fig. 1C). The NFAA–PEG–AA membranes show highest values of DGPEGylation and NFH2N–PEG–NH2 membranes show lowest values. Relatively lower values of DGPEGylation of NFH2N–PEG–NH2 membranes were probably due to the relatively low degree of diffusion of –NH2 moiety attached to PEG towards the interfacial zone compared to –PIP and – MPD moieties attached to PEG.

3.2.2. Effect of PEG end-functionality on morphology of NF membranes Surface SEM images of NF-0, NFAA–PEG–AA(0.25%), NFAA–PEG–AA(0.5%) and NFMPD–PEG–MPD(0.5%) show “globular type” of morphology (Fig. 2). The size, shape and penetration (height) of these globules are of course different in these membranes. The size of globules seems to be higher in NF-0. Both NF-0 and NFMPD–PEG–MPD(0.5%) (Fig. 2 images A and D) show less surface penetrated globules than that of NFAA–PEG–AA(0.25%) and NFAA–PEG–AA(0.5%) (Fig. 2 images B and C). The AFM images of Fig. 3 show typical ridge-valley morphology for all the membranes (images A–E). It is seen that the surface roughness of all PEGylated membranes were less than that of NF-0 (Table 2). The lower surface roughness of the PEGylated membranes may be due to the collapsed PEG chain present in the polyamide network and due to smaller sizes of the globules in the

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Fig. 2. SEM images showing surface morphology. Images: (A) NF-0, (B) NFAA–PEG–AA(0.25%), (C) NFAA–PEG–AA(0.5%) and (D) NFMPD–PEG–MPD(0.5%).

PEGylated membranes than that of NF-0 as seen from SEM images. Surface roughness of NFH2N–PEG–NH2(0.5%) was less but closer to that of NF-0 which is most likely due to lower amount of incorporation of PEG in NFH2N–PEG–NH2(0.5%) as evident from low DGPEGylation (Fig. 1C) and close θ value between NFH2N–PEG–NH2(0.5%) and NF-0 (vide infra).

3.2.3. Effect of PEG-end functionality on water θ, zeta potential, rp and lp/εp Fig. 4 shows the variation of water θ with increasing amount of added different functionalized PEG into the PIP solution. The average θ values of the membranes prepared by MeO–PEG–AA (plot 1), AA–PEG–AA (plot 2) and MPD–PEG–MPD (plot 3) respectively shows decreasing trend up to addition of 0.5% (w/v) of respective functionalized PEG and then the effect level off. In contrast, θ values of the membranes prepared with H2N–PEG–NH2 do not change after the addition of 0.25% (w/v) of H2N–PEG–NH2 and were higher than other PEGylated membranes (plot 4). The θ value follows the order for the membrane as NFAA–PEG–AA o NFMeO–PEG–AA  NFMPD–PEG–MPD oNFH2N–PEG–NH2. This order in θ values is well supported from calculated DGPEGylation data which are highest for NFAA–PEG–AA and lowest for NFH2N–PEG–NH2. Along with surface roughness, Table 2 also summarizes the zeta potential, rp and lp/εp values of the membranes. Zeta potential values of different membranes were close (ranging from  29 to 32 mv at pH 7) which indicates negatively charged surface. Both rp and lp/εp values of NFAA  PEG  AA0.5 and NFMeO  PEG  AA0.5 were closer to NF-0 whereas these values for NFMPD  PEG  MPD(0.5) were higher. Interestingly, like NFAA  PEG  AA(0.5%), the rp and lp/εp values of NFNH2  PEG  NH2(0.5%) were also closer to NF-0, although the reactivity and diffusibility of amine end-group of H2N  PEG  NH2

are different than that of PIP (vide infra). The characteristics of a film formed by IFP depends both on rate of diffusion and reactivity difference of monomers (mixture of amine monomers in this case) towards another monomer (TMC in this case). The reactivity difference of PIP and MPD  PEG  MPD towards TMC may cause substantial formation of discrete network which generated larger pore (aggregate pore) among the networks but may simultaneously lower the number of network pore as proposed in Scheme 2. Closer rp and lp/εp values of membranes prepared with PIP-terminated PEG is mainly ascribed to the formation of relatively lesser number of discrete network and the soft hydrophilic PEG chains remained in collapsed form in water wet state in the resulted PEGylated network. This may be ascribed to the closer reactivity of PIP  PEG  PIP and PIP towards TMC (DFT calculations). On the other hand, the diffusibility of end group of H2N  PEG  NH2 towards the reaction zone may probably be limited. 3.3. Effect of PEG-end functionality on performance of TFC NF membranes TFC-NF membranes were prepared by varying the amount of PIP, keeping the TMC concentration fixed (0.125%, w/v). The Na2SO4 rejection efficiency of membrane (NF-0) was highest (94%) when 3% PIP was used. This membrane gave 30 Lm  2h  1 permeate flux at 0.69 MPa during NF of water (containing 1500 mg/L Na2SO4). The rejection efficiency of the membranes decreased and flux increased with decreasing concentration of PIP. For example, membrane prepared with 2% (w/v) PIP exhibited 42 Lm  2h  1flux and 90% rejection of Na2SO4 (Fig. S4, Supplementary material). Hence, all PEGylated membranes were prepared using 2–3% PIP. Fig. 6A D compare the rejection of NaCl, MgCl2, Na2SO4 and corresponding

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Fig. 3. AFM height (left) and topological (right) images of (A) NF-0, (B) NFAA–PEG–AA(0.25%), (C) NFAA–PEG–AA(0.5%), (D) NFMPD–PEG–MPD(0.5%) and (E) NFH2N–PEG–NH2(0.5%).

Table 2 Characteristic parameters of different NF membranes prepared with different PEGylation routes. Membrane

Roughness rms/average (nm)

Pure water Flux (Lm  2h  1)

Zeta potential (mV)

Lp (μm/MPa s)

rp (nm)

lp/εp (μm)

NF-0 NFAA–PEG–AA0.5 NFMeO–PEG–AA0.5 NFMPD–PEG–MPD0.5 NFH2N–PEG–NH20.5

170/143 143/119 130/103 117/98 160/134

30 37 41 26 28

 30  31  30  35  29

11.9 14.68 16.27 10.32 11.11

0.53 0.56 0.61 0.77 0.60

2.95 2.3 2.86 7.18 4.05

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permeate flux for membranes prepared with PIP (3%, w/v) containing different amounts of in situ generated MeO PEG PIP (Fig. 5A), PIP PEG PIP (Fig. 5B), extraneously added MPD PEG MPD (Fig. 5C) and NH2  PEG NH2 (Fig. 5D) respectively. The relative salts rejection followed the trend NaCloMgCl2 oNa2SO4 for all the membranes. This is consistent with the results reported in the literature for PIP-based NF membranes [5,42]. Such rejection order is explained by the greater electrostatic repulsion between bivalent SO4 2 and membrane surface. The higher rejection of MgCl2 salt compared to NaCl is due to the higher hydrated radius of Mg þ 2 than Na þ . It can be seen from Figures that Na2SO4 and NaCl rejection efficiencies of NFMeO PEG  AA (Fig. 5A) and NFAA PEG  AA (Fig. 5B) are marginally lower compared to NF-0 membrane. For example, a reduction of Na2SO4 rejection from 94% to 93% and 91% was observed for NFAA PEG  AA(0.25%) and NFAA PEG  AA(0.75%) respectively. This is due to the somewhat higher rp values of NFAA PEG  AA and NFMeO  PEG  AA compared to NF-0. Permeate flux of these membranes remained almost unaffected due to close lp/εp value among these membranes. In contrast, rejection efficiency and permeate flux of NFMPD PEG  MPD membranes were significantly lowered compared to NF-0 (Fig. 5 C) which is ascribed to the higher rp and lp/εp values of NFMPD PEG  MPD compared to NF-0. As regards to NF-0, salt rejection efficiency of NFH2N  PEG  NH2 membranes was less affected than that of NFMPD  PEG  MPD which is consistent with the DGPEGylation, θ and rp values of NFH2N  PEG  NH2 (Fig. 5D). Membranes prepared with mixture of PIP (2% w/v) and PIP  PEG  PIP or MPD  PEG  MPD also

48 46 44 42 40

4

38 36 34 32

1 3

30 28 26

MeO-PEG-AA AA-PEG-AA MPD-PEG-MPD H2N-PEGNH2

24 22 20

2

18 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Functional PEG (% w/v) Fig. 4. Change of water θ of membranes prepared with addition of varying amount of different end-functionalized PEG into PIP solution prior to IFP with TMC.

Collapsed PEG chain

Poly(PIP-TMC)

exhibited similar trend in their salt rejection efficiency and permeate flux (Fig. S5, Supplementary material). Variation of permeate flux and rejection of salts were tested at different applied pressures for NF-0, NFAA  PEG  AA, NFMeO  PEG  AA, and NFMPD  PEG  MPD (Figs. S6–7, Supplementary material). The flux of the membranes increases with the applied pressure whereas marginal increase of rejection efficiency occurred for all membranes. The fluxes were almost similar with marginal lowering of salt rejection for NFAA  PEG  AA and NFMeO  PEG  AA compared to that of NF-0 in all tested pressure was observed. NFMPD  PEG  MPD showed much lower rejection of Na2SO4 and permeates flux among the membranes at wide range of applied pressure (Fig. S6, Supplementary material).

3.4. Effect of PEG-end functionality on antifouling property of NF membranes Fig. 6 shows the change of FR of NF-0, NFAA PEG AA(0.25%), NFAA PEG AA(0.5%), NFOMe PEG AA(0.5%), NFMPD PEG MPD(0.5%) and NFH2N PEG NH2(0.5%) during NF of water (containing 1500 mg/L Na2SO4) and additionally spiked with BSA (250 mg/L) at pH 7.171. NFAA PEG AA(0.25%), NFAA PEG AA(0.5%) and NFMeO PEG AA(0.5%) showed only 8–11% FR after 20 h of permeation whereas NF-0 showed  43% FR. Although, NFMPD PEG MPD(0.5%) and NFH2N PEG NH2(0.5%) showed superior antifouling property compared to NF-0, nevertheless, their FR values (21% and 34% respectively) were higher compared to NFAA  PEG  AA and NFOMe  PEG  AA. Table 3 summarizes the characteristic parameters which describe the fouling behavior of the membranes. The FR values follows the order for membranes NFAA PEG AA  NFOMe PEG AA oNFMPD PEG MPD(0.5%) o NFH2N PEG NH2(0.5%) oNF-0 whereas the FRR follows opposite trend. Thus NFAA PEG AA and NFOMe PEG AA exhibited best antifouling property which indicates that addition of only 0.25–0.5% of AAPEG AA or MeO PEG AA is sufficient to improve the antifouling property of the membranes. The anti-adhesion property of a membrane depends on electrostatic forces between membrane surface and foulent, surface roughness, surface hydrophilicity and steric hindrance between surface and incoming fouling agent. In present case, membrane surfaces and the BSA are negatively charge at operating pH (PH 7.1 71). Since, all the membranes are negatively charged and NF-0 suffered from significant fouling (FR¼  43% and FRR¼  70%), hydrophilicity and surface roughness of PEGylated membranes play significant role on antifouling behavior at this pH. Interestingly, NFH2N–PEG–NH2(0.5%) showed poorest antifouling property among the PEGylated membranes. This is once again attributed to the higher θ value (Fig. 4, plot 4) as a consequence of low DGPEGylation (Fig. 1C) in NFH2N–PEG–NH2(0.5%) compared to other

Network pore

Aggregate pore

Scheme 2. Proposed structures of polyamide network formed via IFP between TMC and (A) PIP þ PIP–PEG–PIP mixture and (B) PIP þ MPD–PEG–MPD mixture.

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100

100

90

90

90

80

80

70

70

Na2SO4 MgCl2 NaCl

Flux (Lm-2h-1)

80 70 60

60

50

50

40

40

30

Flux (Lm-2h-1)

100

100 90

Na2SO4 MgCl2 NaCl

80 70

60

60

50

50

40

40

30

30

30

20

20

20

20

10

10

10

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

10 0.0

0.1

0.2

MeO-PEG-AA (% w/v)

Na2SO4 MgCl2 NaCl

70

100

100

90

90

80

80

70

70

60

60

50

50

40

40

30

30

20 10 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Flux (Lm-2h-1)

90 80

0.3

0.4

0.5

0.6

0.7

0.8

AA-PEG-AA (% w/v)

100

Flux (Lm-2h-1)

163

100 90

Na2SO4 MgCl2 NaCl

80 70

60

60

50

50

40

40

30

30

20

20

20

10

10

10 0.0

0.8

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

H2N-PEG-NH2(% w/v)

MPD-PEG-MPD (% w/v)

Fig. 5. Rejection efficiency (filled symbols) and permeate water flux (open symbols) of TFC membranes prepared by addition of different functional PEG into the PIP solution (3% w/v) prior to IFP with TMC (0.125% w/v).

by lower standard deviation in θ value and lower rp of the former membrane than that of later membrane. It was evident that protein aggregates and adsorbs severely on a membrane surface at its isoelectric point [43]. Hence, the pH of feed solution was further adjusted to 4.7, which is the isoelectric point of BSA. It can be seen that NFAA–PEG–AA(0.5) shows almost comparable antifouling property even at pH 4.7 (Fig. 6, red open symbol, Table 3, entry 4) as was exhibited by it at pH 7.1 (Fig. 6, red filed symbol). Hence, it may be concluded that the PEGylated membrane (NFAA–PEG–AA) even under favorable fouling condition also exhibits excellent antifouling property due to enhanced hydrophilicity, lowering of surface roughness and probably due to effect of steric hindrance caused by covalently attached PEG chains.

60 50

FR (%)

40 30 20 10 0 0

2

4

6

8

10

12

14

16

18

20

22

Time (h) Fig. 6. FR(%) values of various membranes with time (filled symbols) during NF of water (containing 1500 mg/L Na2SO4 and spiked with 250 mg/L BSA at pH 7.1 71 and at operating pressure 0.69 MPa). FR(%) vs. time plot for a representative NFAA–PEG–AA (open symbol) at pH 4.7 7 1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

PEGylated membranes. Relatively better antifouling property of NFMeO–PEG–AA(0.5%) compared to NFMPD–PEG–MPD(0.5%) may be ascribed to the greater uniformity of PEG distribution as judged

3.5. Reactivity of PIP and three different amine-terminated PEGtype of molecules towards TMC by conceptual DFT calculations and local ionization potential energy Global nucleophic index, a conceptual DFT method was used to probe into the reactivity of PIP (M1), MPD-terminated (M2), PIPterminated (M3) and alkyl amine-terminated (M4) PEG type of molecules towards TMC (Fig. 7). The optimized conformations of M1–M4 were used for the calculation. Table 4 summarizes the global nucleophicity calculated in gas and aqueous phases. The reactivity of the molecules towards TMC followed the order for the molecules M14M3 4M44M2. Thus the reactivity of M1 and M3 is closer than the reactivity of either M1 and M4 or M1 and M2.

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Table 3 Summary of antifouling properties of different TFC NF membranes after filtration of water containing Na2SO4 (1500 mg/L) and BSA (250 mg/L) for 20 h. Membrane

a

NFPEG-0 NFAA–PEG–AA(0.25%) NFAA–PE–AA(0.5%) b NFAA–PE–AA(0.5%) a NF Meo–PEG–AA(0.5%) a NFMPD–PEG–MPD(0.5%) a NFNH2–PEG–NH2 (0.5%) a a

a b

FR (%)

43.5 71 10.5 70.6 7.5 70.3 11 70.5 7.7 70.5 21 71 34 72

FRR (%)

707 3 967 1 987 1 967 1 977 1 867 1 787 2

Jc (Lm  2h  1)

Flux (Lm  2h  1)

22.4 7 1 28.8 7 0.5 31.3 7 1 327 0.5 297 1 237 1 257 1

Na2SO4 rejection (%)

Before

After (20 h)

Before

After (20 h)

327 2 307 1 327 1 337 0.5 307 0.5 277 1 327 1

18 71 27 71 29.6 70.5 29.5 71 27.7 71 20.5 70.5 21 70.7

947 1 937 1 937 1 937 1 927 1 817 1 917 1

957 0.5 937 0.5 937 0.5 947 1 92.5 7 0.5 837 1 937 0.5

Feed pH¼ 7.2 7 1. Feed pH 4.7 7 1.

Fig. 7. Optimized structure of the molecules at B3LYP/6-31þ Gn level of theory in aqueous phase. Table 4 Electronic properties of the four nucleophilic amines calculated at B3LYP/6-31 þGn level of theory in gas (aqueous). Parameters

M1

Rel E (kcal/mol) Homo (au) Lumo (au) η μ ω (Global)

0.00  0.2239 0.0026 0.2265  0.1107 0.740

M2 (0.00) (  0.2336) (  0.0042) (0.2294) (  0.1147) (0.776)

M3

0.00  0.1954  0.0174 0.1781  0.1064 0.865

(0.00) (  0.1973) (  0.0141) (0.1832) (  0.1057) (0.829)

This result correlates well with Domingo theorem [44], the lower global nucleophilic index value corresponds to higher reactivity towards electrophile. Therefore, much enhanced antifouling property, closer values of rp, lp/εp and similar performance of NFAA–PE–AA or NFMeo–PEG–AA membrane compared to NF-0 can be explained by closer nucleophilicity of PIP and PIP-terminated PEG. Furthermore, we use the average local ionization energy I(r) introduced in 1990 by Sjoberg et al. [45] as a means of identifying and ranking the sites of the more reactive electrons in a molecular system by the following equation: ∑ ρ ðrÞjεi j I ðr Þ ¼ i i ρðrÞ

ð9Þ

where ρi(r) is the electronic density of orbital i, having energy εi, at the point r of interest; and ρ(r) is the total electronic density of the system. The summation is over all occupied orbitals. The I(r) an overlaying of energy of electron removal onto the electron density is another indicator of nucleophilic attraction has provided by the local ionization potential energy surface [46]. The regions with red color represent regions in the molecular surface where electron removal goes (with minimal energy) most easily. Therefore, the difference in reactivity of the nucleophiles could be judged from the values of I(r) presented in Fig. 8 which show that a molecule with lower I(r) on nitrogen atom is more reactive towards TMC. The I(r) values correlate well with the global

0.00  0.2034  0.0071 0.1963  0.10525 0.768

M4 (0.00) (  0.1966) (  0.0092) (0.1873) (  0.1029) (0.769)

0.00  0.23693  0.00034 0.2366  0.1186 0. 805

(0.00) (  0.2402) (0.040) (0.2434) (  0.1265) (0.810)

nucleophilicity in relation to their reactivity towards TMC as shown in Fig. 8. Hence, reactivity of PIP and PIP-terminated PEG is expected to be much closer as evident from DFT calculation and computed I(r) values.

4. Conclusion The structure of reactive end-group (amine-spacer) of an antifouling hydrophilic polymer chain e.g. PEG is an important factor when used as comonomer with the commonly used diamine such as PIP for IFP with commonly used acid chloride, TMC for the preparation of PEGylated TFC membrane. It turned out that rp, lp/εp, DGPEGylation and surface hydrophilicity were influenced by this factor and consequently, performance and antifouling property of the membrane were also affected. PIPþPIP-terminated PEG mixtures gave PEGylated TFC membranes (TFCAA–PEG–AA or TFCMeO–PEG–AA) with almost similar performance and much enhanced antifouling property compared to membrane prepared with PIP as sole amine monomer. The similar performance of PEGylated NF membranes prepared in this way is attributed to the close values of rp and lp/εp to that of NF-0 owing to relatively closer reactivity of amine groups in PIP and PIP-terminated PEG as confirmed by I(r) value and global nucleophilicity calculated using

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Fig. 8. The local I(r) calculated at B3LYP/6-31 þGn level of theory in aqueous phase for M1–M4; red color represents region in the molecular surface with minimal energy. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

the conceptual DFT method. Excellent antifouling property and good performance of TFCAA–PEG–AA or TFCMeO–PEG–AA clearly indicate beneficial effect of added AA–PEG–AA or MeO–PEG–AA into the PIP solution. On the other hand, membrane prepared with extraneous addition of MPD-terminated PEG into the PIP solution exhibited superior antifouling property compared to NF-0 but still the antifouling character was inferior to that of NFAA–PEG–AA and NFMeO–PEG–AA. Moreover, performance in terms of salt rejection efficiency and permeate flux of NFMPD–PEG–MPD were also lower compared to NF-0, NFAA–PEG–AA and NFMeO–PEG–AA due to relatively greater difference of reactivity between PIP and MPD–PEG–MPD which probably caused formation of discrete network as evident from higher rp values of these membranes. The performance and antifouling property were less influenced in the membranes prepared with PIP þH2N–PEG–NH2 mixtures which was evident from low DGPEGylation in the resultant membranes presumably due to lower rate of diffusion of alkyl amine towards the interfacial zone. Hence, it may be concluded that proper end-functional reactive PEG is desirable for in situ PEGylation of TFC membranes for enhancement of antifouling property without much affecting the performance of the membranes compared to conventionally prepared membrane. This finding thus highlights an approach for selecting reactive end-functional antifouling macromolecules for effective in situ modification of TFC membrane. Work is continued for the preparation of stimuli responsive low fouling ultrafiltration and NF membranes.

Acknowledgment CSIR-CSMCRI communication No. 079/2014. We thank the Centralized Analytical Facility for all round analytical support and, particularly, Babulal Rebary, Jayesh Chaudhury, and Batuk Bhil for carrying out AFM, SEM, and NMR respectively. This work was carried out under the projects MLP-0013/OLP-0032 CSIR, India. Semire Banjo thanks JNCASR-CICS for three months fellowship awarded to him.

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