Postmodification of PS-b-P4VP Diblock Copolymer Membranes by

Jun 19, 2014 - The surfaces of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymer membranes were modified in order to obtain polyme...
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Postmodification of PS‑b‑P4VP Diblock Copolymer Membranes by ARGET ATRP Damla Keskin,† Juliana I. Clodt,† Janina Hahn,† Volker Abetz,†,‡ and Volkan Filiz*,† †

Institute of Polymer Research, Helmholtz-Zentrum Geesthacht, Max-Planck-Str. 1, 21502 Geesthacht, Germany Institute of Physical Chemistry, University of Hamburg, Grindelallee 117, 20146 Hamburg, Germany

‡

ABSTRACT: The surfaces of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymer membranes were modified in order to obtain polymer brushes by using surface-initiated activators regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP). Isoporous membranes were prepared by the combination of self-assembly of PS-b-P4VP diblock copolymers and the nonsolvent induced phase separation process, also known as “phase inversion”. In order to allow further functionalization, the membranes were modified with an ATRP initiator, 2-bromoisobutyryl bromide (BIBB). Therefore, the mussel-inspired poly(dopamine) coating was used to attach BIBB on the membranes surface. In the next step the coated membranes were postmodified by using surface-initiated ARGET ATRP with the hydrophilic monomer 2-hydroxyethyl methacrylate (HEMA). HEMA as a hydrophilic methacrylate was chosen for the modification in order to enhance the membrane characteristics and to obtain a surface with antifouling properties. The surface-initiated ARGET ATRP reaction was carried out using different reaction times and environments. PHEMA could successfully incorporate on the membrane surface as confirmed by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), 1H nuclear magnetic resonance spectroscopy (1H NMR), scanning electron microscopy (SEM), and contact angle measurements. Furthermore, stability tests against heat and solvents were performed, and water flux was measured for the raw and modified membranes. Stability against heat and hydrophilicity could be increased with this type of modification for diblock copolymer membranes.

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INTRODUCTION In recent years, isoporous membranes have gained growing importance due to their versatile structures with many promising applications including micro- and nanofiltration, cell separation, controlled drug delivery, optics, and gas separation.1 The formation of isoporous membrane structures from polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymers is feasible by a targeted process. Herein, the combination of self-assembly of amphiphilic block copolymers and the nonsolvent induced phase-inversion process (SNIPS) provides an interesting way to prepare well-defined membranes with highly ordered surface structures, hexagonally arranged pores, high porosity, and adjustable, narrow disperse pore sizes.2−6 The SNIPS process could be adjusted to several diand triblock copolymer systems in the past couple of years.7−11 Since PS-b-P4VP diblock copolymer membranes are not stable in many solvents, mild conditions are required for postmodification. A versatile method includes a coating of another thin polymer layer in aqueous media followed by a © 2014 American Chemical Society

reaction with functional molecules. Previously, PS-b-P4VP diblock copolymer membranes were functionalized with a mild mussel-inspired poly(dopamine) coating and then reacted with an amino-terminated PNIPAM to prepare double stimuliresponsive membranes.12 With respect to surface modification bio-inspired poly(dopamine) coatings became fundamentally important in the past couple of years.13−20 Poly(dopamine) coating is based on the oxidative polymerization of dopamine hydrochloride and carried out under slightly basic conditions.21,22 Although poly(dopamine) coating has been used on various substrates, the structure of poly(dopamine) itself is still not completely understood.23,24 Poly(dopamine) deposition has been made on porous membranes to progress hydrophilicity,25 to control pervaporation,18 or as an adhesion layer in multilayer membranes.26 Moreover, the poly(dopamine) coatReceived: April 16, 2014 Revised: May 27, 2014 Published: June 19, 2014 8907

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ing offers an interlayer for further modification to improve fouling resistance and separation performances of the membranes.27 The broad applicability of poly(dopamine) coating can lead to a platform for producing initiator for SIP (surface-initiated polymerization) that may be applied to the membrane surfaces. In general, SIP is the growth of a polymer from initiators immobilized on a surface, creating layers of densely end-grafted chains which are identified as polymer brushes.28−30 Using SIP to grow these thin polymer films has turned into a very famous way to tailor interfacial properties for a variety of applications such as responsive surfaces,31,32 antibacterial coatings,33 and antifouling surfaces.34 Besides, surface-initiated atom transfer radical polymerization (SI-ATRP) is performed in the same way as solution ATRP except the initiating functional groups for the ATRP are immobilized on surfaces. The initiatinggroup-functionalized surface is dipped in a solution of monomer, catalyst, and ligand to initiate polymer growth from the initiating sites on the surface. Polymerization on the surface is stopped by removing the surface from the solution once the growth time is complete.35 Initiator immobilization is a significant stage in surface-initiated polymerization, as the vast majority of materials surfaces do not have readily reactive initiating groups. Several common methods are used for immobilization of initiators on surfaces for SI-ATRP systems; for instance, BIBB is a commonly used ATRP initiating group.30 More recently, several studies have stated the use of ARGET ATRP for SIP, an alternative type of conventional ATRP.36,37 In ARGET ATRP systems, excess reducing agent added to the polymerization solution generates the Cu(I) activator species in order to reduce the sensitivity of the system to oxygen and to allow reduced catalyst quantities to be employed.30,38,39 In order to compensate for the competitive complexation of the low amounts of copper species with monomer/solvent/ reducing agent, which are present in large molar excess compared to the copper concentration, strong and excess ligands may be required in an ARGET system. Thus, a common ligand used is tris[2-(dimethylamino)ethyl]amine (Me6TREN) in ARGET ATRP.38−40 Depending on the range of targeted degrees of polymerization, different ligands, catalyst, and ligand concentration can be used in ARGET ATRP systems including inexpensive ligands weaker than Me6TREN.41 Edmondson et al. have investigated a novel approach for surface modification on silicon wafers by using bromoester initiating groups for ATRP systems which can be incorporated into poly(dopamine) coatings by a reaction of the dopamine monomer with 2-bromoisobutyryl bromide (BIBB). They confirmed a successful modification with the initiator groups by the growth of PHEMA and PMMA polymer brushes by ATRP and studied the growth time of the polymer.30 ATRP of acrylic acid was carried out on anodic aluminum oxide membranes whereas BIBB was immobilized on a poly(dopamine) coated membrane and used as initiator.42 Similar catechol containing macroinitiators were used to modify substrates with functional polymer brushes by SI-ARGET ATRP.43 Immobilization of the ATRP initiator group BIBB and further ATRP reaction was carried out after oxidative hydrolysis of polyester track etched membranes44 by a multistep premodification of microfiltration membranes45 or by acylation reaction.46

To the best of our knowledge, an isoporous diblock copolymer membrane modified by an initiator group and further successfully used for ATRP has not been reported yet. In this work the Edmondson method30 for the modification of silicon wafers by SI-ARGET ATRP was adjusted to isoporous diblock copolymer membranes using mild reaction conditions that are essential for this type of membrane. The dopamine monomer and BIBB could be attached to each other by esterification or by amide formation, which was discussed in Edmondson’s paper,30 and immediately coated on the membranes by oxidative polymerization to obtain poly(dopamine)BIBB modified membranes (compare Scheme 1; Scheme 1. Reaction Pathway for the Modification of PS-bP4VP Membranes with Poly(dopamine)BIBB and Further ATRP with HEMA

here only esterification shown). In the next step the coated membranes were postmodified by using surface-initiated ARGET ATRP with the hydrophilic monomer HEMA. The SI-ARGET ATRP reaction was carried out under different reaction times and environments. The modified membranes were characterized by NMR, IR, and contact angle measurements. Clean water fluxes were carried out and compared with the raw precursor membranes. Furthermore, thermal and chemical stability was tested.

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RESULTS AND DISCUSSION Poly(dopamine)BIBB Coated Membrane after Reaction with HEMA for Different Reaction Times. SI-ARGET ATRP was successfully carried out on poly(dopamine)BIBB modified membranes under argon for different reaction times, namely 18 h, 3 h, 30 min, 10 min, 5 min, and 1 min. In order to locate the best reaction conditions for our purpose, the membranes were characterized by different techniques. Surface Structure of the Modified Membranes Using SEM. Figure 1 shows the SEM of the surface of a raw PS-bP4VP membrane (a), a poly(dopamine)BIBB modified membrane (b), and a poly(dopamine)BIBB coated membrane modified with HEMA for different reaction times (c, d, e, f, g, h). As can be seen from Figures 1c and 1d, rather dense surface structures were observed after 18 and 3 h of modification. In this case the pores are quite closed. Therefore, the duration of the modification was decreased to 30, 10, 5, and 1 min. When we decreased the modification time, SEM results illustrate that the pores on the surface are still open and retain their hexagonally packed structure. The pore sizes calculated from SEM pictures show that the average diameter of the pores of the poly(dopamine)BIBB modified membrane (∼57 nm) and poly(dopamine)BIBB membrane after reaction with HEMA for 30 min (∼46 nm) are smaller than the average pore diameter of the unmodified 8908

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Figure 1. SEM images, surfaces of (a) PS-b-P4VP membrane, (b) membrane after of poly(dopamine)BIBB coating, (c) poly(dopamine)BIBB coated membrane after reaction with HEMA for 18 h, (d) for 3 h, (e) for 30 min, (f) for 10 min, (g) for 5 min, and (h) for 1 min.

membranes (∼60 nm), as expected. When the reaction was carried out for 10 min, the average pore diameter is ∼54 nm, bigger than after 30 min of polymerization reaction and only slightly smaller than the precursor membrane (∼57 nm). Since the pore diameter of the membrane should decrease as less as possible after modification, we assume from the surface structures that a 1 to 10 min reaction is preferred. ATR-FTIR of the Modified Membranes. The unmodified membrane, the membrane coated with poly(dopamine)BIBB, and the membrane after further reactions with HEMA for different reaction times were analyzed by ATR-FTIR. The results are presented in Scheme 2. In the ATR-FTIR spectra of the poly(dopamine)BIBB coated membrane after modification with HEMA for 3 h and for 10 min, we observed major characteristic peaks for the carbonyl group (CO) stretching vibration at 1718 and 1260 cm−1 and peaks for the ester group (C−O) stretching vibration at 1155 cm−1. The result confirms

the presence of PHEMA on the membrane. A broad signal from 3500−3200 cm−1 was detected due to the hydroxyl (O− H) stretching vibration. After a reaction time of 1 min the characteristic peaks become very small. It is possible that the PHEMA chains are short because of a low polymerization rate after 1 min. A homogeneous distribution of PHEMA on the surface of the membrane is absolutely necessary with respect to membrane characterization. Therefore, we concluded a 10 min reaction time to be preferred. 1 H NMR of the Modified Membranes. The membranes were characterized by 1H NMR spectroscopy. In order to ensure that the HEMA is polymerized on the surface of the membrane and not just attached via hydrogen bonding, the membranes were rinsed with minimum 500 mL of deionized water in a water flux measurement system. Scheme 3 shows the 1 H NMR in pyridine-d5 of the poly(dopamine)BIBB coated

Scheme 2. ATR-FTIR Spectra of (a) PS-b-P4VP Membrane, (b) Membrane after Poly(dopamine)BIBB Coating, and (c) Poly(dopamine)BIBB Coated Membrane after Reaction with HEMA for 3 h, (d) for 10 min, and (e) for 1 min

Scheme 3. 1H NMR Spectra of (a) Membrane after of Poly(dopamine)BIBB Coating and (b) Poly(dopamine)BIBB Coated Membrane after Reaction with HEMA for 1 min and (c) for 10 min (Spectra Measured in Pyridine-d5)

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pores are more or less closed (Figure 3, a2, a3, b2, and b3). However, the membrane coated with poly(dopamine)BIBB is more stable than the precursor membrane. Poly(dopamine)BIBB coated membrane after reaction with HEMA (HEMA modifications for 10 and 5 min) provides a better thermal stability than the unmodified membrane and exhibits ordered hexagonally packed cylindrical structure on the surface (c2, c3, d2, and d3). Thermal stability of ultrafiltration membranes is very important for application. In order to sterilize the membranes, they are heated above 100 °C in general. The modified membranes can resist such conditions. All membranes show no changes of their pore structure, and the structures are similar to the precursor membranes without treatment, after keeping them in isopropanol for 16 h in order to prove the chemical stability (a4, b4, c4, and d4). In comparison with previous stability tests,9 these results are quite interesting since it was expected that the raw membrane is unstable in isopropanol. The stability test using isopropanol as a solvent could depend on the molecular weight and the P4VPcontent of the PS-b-P4VP block copolymer used for the membrane preparation. As compared to previous investigations the length of the P4VP block of the block copolymers used differs by approximately 20 kg/mol (ca. 58 kg/mol in this work compared to ca. 79 kg/mol before). Isopropanol can dissolve P4VP; therefore, a high P4VP content might decrease the stability of the membrane. In addition, the PDI of the block copolymer used in this study is considerably lower (PDI: 1.10) compared to former studies (PDI: 1.30), which could probably increase stability due to best arrangement of block copolymer chains. ATRP of Poly(dopamine)BIBB Coated Membrane with HEMA under Air. In order to analyze the membrane characteristics like water fluxes, retention, and fouling behavior, the modification of bigger membrane sheets is necessary. An upscaling process would be easier to carry out under air. Since the modification of PS-b-P4VP membranes with PHEMA by ATRP is already finished after 10 min under argon, we also tried to modify them under air. In this first attempt the substrates were reacted in an open glass jar for 30 min with unlimited air exposure. All other conditions were kept similar to the reaction under argon. Scheme 4 shows the ATR-FTIR spectra of the membrane after poly(dopamine)BIBB coating and further modification with HEMA under an argon atmosphere (c) and the same reaction under air (d). A very small characteristic peak for the carbonyl group indicates the presence of PHEMA even when the reaction was carried out under air. A further evidence for a successful reaction under air can be seen in Scheme 5 displaying the NMR spectra of the membrane (a) after poly(dopamine)BIBB coating, (b) after further reaction with HEMA under argon atmosphere, and (c) after further reaction with HEMA under air. Again chemical shifts at 4.4 and 4.2 ppm for the methylene protons of PHEMA were observed for both reaction conditions. The results of the 1H NMR and ATR-FTIR measurements demonstrate a successful reaction under air. It has to be mentioned that the reaction conditions used for this purpose cannot be controlled. The Cu(I) species made from Cu(II) reduced by ascorbic acid is exposed to unlimited air and will be oxidized. It is not possible to calculate a sufficient amount of reducing agent for unlimited air. Further investigation including a reaction under air in a closed jar in order to calculate the amount of oxygen and

membrane (a) after further modification with HEMA for 1 min (b) and for 10 min (c). The chemical shift at 4.4 ppm, a, is attributable to the methylene protons close to the ester group and the other peak at 4.1 ppm, b, is due to the methylene protons close to the hydroxyl group of PHEMA which can be observed for all reaction times. It cannot be guaranteed that the membrane was dissolved completely in pyridine-d5 after modification, so that this data is only qualitative and the degree of polymerization cannot be estimated quantitatively. The results are in agreement with the data of the ATR-FTIR leading to a favorite reaction time of 10 min. During the preparation of the solutions for the 1H NMR investigation, we found out that the membrane has a lack of solubility in CDCl3 after modification with PHEMA. It seems that the modified membranes are somehow more stable against solvents. However, for a better understanding, stability tests against heat and solvents were carried out. Dynamic Contact Angle Measurements. Figure 2 shows snapshots of the dynamic contact angle measurements of 5 μL

Figure 2. Snapshots of dynamic contact angle measurements of water droplets onto different membrane surfaces: (a) PS-b-P4VP membrane, (b) PS-b-P4VP membrane after poly(dopamine)BIBB coating, and (c) PS-b-P4VP membrane after poly(dopamine)BIBB coating and reaction with HEMA for 10 min at 25 °C (CA = contact angle).

water droplets onto the surface of the PS-b-P4VP membrane (a), poly(dopamine)BIBB coated PS-b-P4VP membrane (b), and the poly(dopamine)BIBB coated PS-b-P4VP membrane after further reaction with HEMA for 10 min (c). The water contact angle of the raw membrane becomes zero after 10 s. In case of the poly(dopamine)BIBB coated membrane the droplet needs more than 10 s to sink into the membrane. In the case of the more hydrophilic PHEMA modified membrane the droplet is totally sunk into the membrane already after 6 s even the pore size of this membrane is a bit smaller compared to the raw membrane. The contact angle measurements indicate clearly that the surfaces of the membranes are more hydrophilic after modification with PHEMA. Since a more hydrophilic membrane should increase water flux and could decrease fouling, the results are promising concerning membrane performance and lifetime. Stability Tests. Thermal and chemical stability of unmodified and modified membranes was tested under different conditions. A detailed examination of the temperature effect on the stability of a pure membrane and a poly(dopamine)BIBB coated membrane in dry state showed that these membranes are not thermally stable up to 125 °C because the surface structure of the pores became indistinct and the 8910

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Figure 3. SEM results of the stability test of (a1−a4) PS-b-P4VP membrane, (b1−b4) membrane after poly(dopamine)BIBB coating, (c1−c4) poly(dopamine)BIBB coated membrane after reaction with HEMA for 10 min, and (d1−d4) for 5 min. Stability was tested for thermal (T = 110 and 125 °C for 2 h) and chemical stresses (isopropanol for 16 h).

Scheme 5. 1H NMR Spectra of (a) Membrane after Poly(dopamine)BIBB Coating and (b) Poly(dopamine)BIBB Coated Membrane after Reaction with HEMA for 30 min under Argon Atmosphere and (c) Reaction for 30 min under Air (Spectra Measured in Pyridine-d5)

Scheme 4. ATR-FTIR Spectra of (a) PS-b-P4VP Membrane, (b) Membrane after Poly(dopamine)BIBB Coating, and (c) Poly(dopamine)BIBB Coated Membrane after Reaction with HEMA under Argon Atmosphere for 30 min and (d) Reaction under Air for 30 min

reducing agent are required. In this case an excess of ascorbic acid is not recommended since uncontrolled ATRP can occur again.36 In addition, the reaction time could be increased.

Clean Water Flux Measurements. Clean water fluxes were measured over 5 h for modified and unmodified 8911

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membranes in order to obtain first results concerning the membrane characteristics. For this purpose the modification with PHEMA was carried out under argon atmosphere for 10 min. Scheme 6 shows the results of the water flux measure-

Table 1. Clean Water Fluxes of Modified and Unmodified PS-b-P4VP Membranes

precursor (raw) membrane poly(dopamine)BIBB coated membrane poly(dopamine)BIBB coated membrane after reaction with HEMA for 10 min

flux after 5 h [L m−2 h−1 bar−1]

flux decline [%]

2107

1535

27

2055

1430

30

1606

1452

10

EXPERIMENTAL SECTION

Synthesis of Block Copolymers. Tetrahydrofuran (THF) was ordered from Th. Geyer. Styrene and 4-vinylpyridine (4VP) were ordered from Sigma-Aldrich and purified as per description. THF was purified by successive distillation from potassium under purified argon atmosphere. Styrene was stirred with dibutylmagnesium (MgBu2). 4VP was once distilled under reduced pressure from calcium hydride (CaH2) and twice from ethylaluminum dichloride. PS-b-P4VP was synthesized via sequential anionic polymerization at −67 °C in THF. The polymerization of styrene was initiated with sec-butyllithium (secBuLi). After 4 h 4VP was added, and the solution was stirred for another 16 h. The polymerization was quenched with degassed methanol and a mixture of methanol and hydrogen chloride. After partial removal of THF under reduced pressure, the polymer was precipitated in water. The composition of the block copolymer was determined by 1 H NMR spectroscopy and gel permeation chromatography (GPC). Preparation of PS-b-P4VP Diblock Copolymer Membranes. Membranes used in this work were prepared by a combination of the self-assembly of amphiphilic block copolymers and the phase inversion process. Therefore, 18.5 wt % of a PS-b-P4VP diblock copolymer (Mw = 264 kg/mol, 22.0 wt % P4VP content) was dissolved in a solvent mixture of THF and DMF according to Rangou et al.3 The solutions were cast on a polyester nonwoven support using a homemade casting machine. The membrane casting machine allows a continuously casting of the polymer solution onto a substrate, e.g. nonwoven, on rolls up to 30 cm width. The films were left for 10 s on air before immersing in water. The membranes were dried at 60 °C under reduced pressure before using for further modification. Modification of Membranes by Poly(dopamine)BIBB. PS-bP4VP diblock copolymer membranes were modified with poly(dopamine)BIBB in accordance to Edmondson’s method.30 In this method, the dopamine monomer is reacted with BIBB under base catalysis and immediately polymerized on the membrane surface. Dopamine hydrochloride (380 mg, 2 mmol) was placed in a flask which was degassed by purging with argon for 20 min. An excess of BIBB was used in our case in order to ensure an sufficient amount of initiator group on the membrane surface after coating. Degassed N,N′dimethylformamide (DMF, 20 mL) and triethylamine (Et3N, 0.55 mL, 4 mmol) were added under argon to the flask. 2-Bromoisobutyryl bromide (BIBB, 0.49 mL, 4 mmol) was added to the mixture and stirred at room temperature for 3 h, leading to precipitation of Et3NHBr and Et3NHCl. 10 mL of the remaining solution was decanted to a glass dish to which tris(hydroxymethyl)aminomethane (TRIS) (2.42 g, 20 mmol) and ultrapure water (50 mL) were added. A PS-b-P4VP diblock copolymer membrane (4 cm × 4 cm) was then immersed in this mixture which was continuously shaked at a speed of 90 rpm for 90 min under air. The reaction can be followed by a color change from colorless to brown of the membrane. The poly(dopamine)BIBB modified membrane was rinsed three times with 50 mL of ultrapure water for 30 min and dried at 60 °C under reduced pressure before using for further reactions. Surface-Initiated ARGET ATRP of Poly(dopamine)BIBB Modified Membranes. The poly(dopamine)BIBB modified membranes were used as a substrate for surface-initiated ARGET ATRP with 2-hydroxyethyl methacrylate (HEMA). HEMA (0.26 g, 2 mmol), CuBr2 (13.4 mg, 0.06 mmol), and Me6TREN (27.6 mg, 0.12 mmol) were placed in a glass tube with 15 mL of ultrapure water. The mixture was deoxygenated by purging with Ar for 30 min at room temperature. An Ar-purged aqueous solution of sodium L-ascorbate (23.8 mg, 0.12 mmol, 5 mL of ultrapure water) was added to the mixture. To dissolve all solids, the mixture was magnetically stirred for 5 min under an argon atmosphere. A poly(dopamine)BIBB coated membrane (2 cm × 4 cm) was placed in a glass reactor, and the tube was evacuated and degassed with argon; the reaction mixture was then syringed over the membrane. After the polymerization was allowed to proceed at room temperature for various times, the membrane was removed and washed three times with 50 mL of ultrapure water for 30 min and dried at 60 °C under reduced pressure.

Scheme 6. Water Flux Measurements for (a) the Pure Membrane, (b) Membrane after Poly(dopamine)BIBB Coating, and (c) Poly(dopamine)BIBB Coated Membrane after Reaction with HEMA for 10 min under Argon Atmosphere

initial flux [L m−2 h−1 bar−1]

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ments over 5 h. As listed in Table 1, the initial flux decreases after modification with PHEMA, which is in agreement with the decreasing pore diameters as calculated from SEM pictures. The pore diameters of the membranes modified with PHEMA are approximately 6 nm smaller than the precursor membranes. Interestingly, the clean water fluxes after 5 h are only slightly smaller for the modified membranes meaning a flux decline of 10% compared to for the raw membrane with a flux decline of 27%. Flux decline can be mainly caused by swelling of the P4VP blocks of the membrane, by compression of the membrane substructure due to transmembrane pressure, or by fouling. A low amount of fouling may occur using demineralized water as we discussed before since there are still small amounts of bacteria in the system.6 In the case of the PHEMA modified membranes the flux decline is less pronounced. Therefore, the swelling of the P4VP blocks seems to be suppressed in this case. In this context PHEMA grafted on the poly(dopamine) interlayer could insulate the P4VP blocks from contact with water. Furthermore, an increased stability of the membranes substructure associated with decreased compaction could be possible through modification. However, flux stability of the PHEMA modified membrane seems to be superior to the raw membrane which is essential for further studies in this direction. 8912

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Notes

Morphological characterization of all membranes was performed by scanning electron microscopy (SEM) using a LEO Gemini 1550 VP at a voltage of 3 or 5 kV. The samples were coated with 2.0 nm platinum. Average pore sizes values were determined using the software analysis (Olympus) on the basis of the SEM results. ATR-FTIR Measurements. Membranes were characterized and identified by using ATR-FTIR spectroscopy. Fourier transform infrared spectroscopy (FTIR) was conducted using a Bruker alpha, platinum ATR equipped with diamond ATR (Bruker Corporation, Alexandria, New South Wales, Australia). 1 H NMR Measurements. The compounds that were modified onto the membranes were analyzed by nuclear magnetic resonance spectroscopy. All 1H NMR measurements were performed on a Bruker Advance 300 NMR spectrometer at 300 MHz using pyridine-d5 as a solvent. Dynamic Contact Angle Measurements. The wettability properties of the membranes were determined by measuring dynamic contact angles on a KRUESS Drop Shape Analysis System DSA 100. Water-Flux Measurements. Water flux measurements were performed in dead-end mode using a homemade automatic testing device at pressures of 2.0 bar at room temperature. The volume was measured gravimetrically every 5 min for 5 h. The effective membrane area was 1.77 cm2. These studies were conducted employing demineralized water with an electrical conductivity of 0.055 μS cm−1.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Brigitte Lademann for the synthesis of the polymers, Kristian Buhr for the preparation of the membranes, Sofia Rangou for scientific discussion, Maren Brinkmann for the GPC measurements, Silvio Neumann for the NMR measurements, Anne Schroeder and Sofia Dami for the SEMs, and Jan Pohlmann and Carsten Scholles for the automatic water flux device.

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CONCLUSION We demonstrated that the ATRP initiator poly(dopamine)BIBB could be successfully fixed on isoporous PS-b-P4VP diblock copolymer membranes. Herein, a BIBB modified dopamine was oxidatively polymerized on the membrane surface in order to use it as interlayer for further functionalization. For a further reaction HEMA could be polymerized on poly(dopamine)BIBB membrane surfaces via surface-initiated ARGET ATRP. The modification was proven by ATR-FTIR, 1H NMR, and contact angle measurements. Furthermore, different reaction times and conditions were carried out, and the resulting pore surface structures of the membranes were studied by SEM. First, water flux measurements were carried out and indicate that the modified membrane is superior concerning flux stability. In a primary attempt it was shown that the reaction can also be carried out under air. In order to improve the reaction conditions for an upscaling process, other ligands than Me6TREN should be tested in the future to prevent disproportionation of Cu(I) in water.47 In this context, tris(2-pyridylmethyl)amine seems to be superior for aqueous ATRP.48 Since ATRP reactions can be carried out in aqueous media, this method broadens the tool to introduce functional groups on the surfaces of PS-b-P4VP membranes. The increased hydrophilicity of the obtained membranes could enhance the membrane characteristics. Additional studies in progress include the development of this methodology using other functional monomers and characterizations of the membranes by different methods such as adsorption, fouling tests, and retention measurements. The functional groups obtained by this method on the membrane surfaces offer possibilities for further reactions, like binding of bioactive molecules in aqueous solution or the polymerization of a second block.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail volkan.fi[email protected]; Tel +49 4152 872425; Fax +49 4152 872499 (V.F.). Author Contributions

D.K. and J.I.C. contributed equally to this work. 8913

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