Postfunctionalization of Nanoporous Block Copolymer Membranes via

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Postfunctionalization of nanoporous block copolymer membranes via Click reaction on polydopamine for liquid phase separation Christian Höhme, Volkan Filiz, Clarissa Abetz, Prokopios Georgopanos, Nico Scharnagl, and Volker Abetz ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00289 • Publication Date (Web): 19 Jun 2018 Downloaded from http://pubs.acs.org on June 26, 2018

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ACS Applied Nano Materials

Postfunctionalization

of

nanoporous

block

copolymer membranes via Click reaction on polydopamine for liquid phase separation Christian Höhmea, Volkan Filiza *, Clarissa Abetza, Prokopios Georgopanosa, Nico Scharnaglb, and Volker Abetza,c * a. Helmholtz-Zentrum Geesthacht, Institute of Polymer Research, Max-Planck-Str.1, 21502 Geesthacht, Germany. b. Helmholtz-Zentrum Geesthacht, Institute of Materials Research, Max-Planck-Str.1, 21502 Geesthacht, Germany. c. University of Hamburg, Institute of Physical Chemistry, Martin-Luther-King-Platz 6, 20146 Hamburg, Germany. *Corresponding Authors: E-Mail: [email protected], [email protected], Tel: +49 4152 872461 KEYWORDS: block copolymer, self-assembly, isoporous membrane, postfunctionalization, polydopamine, “Click”-reaction, anti-fouling

ACS Paragon Plus Environment

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In this work, an azido-modified dopamine derivative was synthesized and subsequently used to postfunctionalize the surface of nanoporous poly(styrene)-block-poly(4-vinylpyridine) diblock copolymer membranes. Based on this layer a continuative modification was realized by performing a “Click” reaction, namely the Cu(I)-catalyzed 1,3-dipolar cycloaddition, with different alkynes. While the “Click” reaction was monitored by X-ray photoelectron spectroscopy, the morphology of the membranes in the different states of modification was examined with scanning electron microscopy and atomic force microscopy. The membrane properties were characterized by measurements of contact angle and clean water permeance, retention tests and protein adsorption. Independent from the alkyne applied during the “Click” reaction, the clean water permeance is approx. 1200 L·m-2·bar-1·h-1 and therefore slightly below the permeance of the pristine membrane. While the sharp molecular weight cut-off of the pristine membrane and all modified membranes is similar, anti-fouling properties as studied on the interaction of two model proteins (bovine serum albumin, hemoglobin) with the membranes turned out to be best for the membranes modified with 1-nonyne.

Introduction The ever-growing world population and the associated demand for clean water embody a key challenge for our society. Moreover, the upcoming environmental problems such as water and air pollution demand for more energy-efficient and sustainable technologies. Towards these problems, separation processes using ultrafiltration (UF) membranes have drawn immense attraction, as they are an excellent alternative to energy consuming techniques like distillation. The ideal ultrafiltration membrane combines a sharp molecular weight cut-off with a high flux. These properties can be achieved by a thin isoporous top layer and a mechanically stable, highly porous substructure. Here, amphiphilic block copolymers offer a tremendous potential, since


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integral asymmetric membranes with nanopores and a narrow pore size distribution can be prepared from these polymers in a fast one-step casting process. By combining the block copolymers’ ability to self-assemble with the non-solvent induced phase separation process (SNIPS), membranes with a high surface regularity and open pores can be obtained directly.1-3 Besides in flat sheet geometry nanoporous block copolymer membranes have also been prepared in hollow fiber geometry.4-5 An increasing number of block copolymers has been investigated on the ability to form nanoporous membranes via the SNIPS process since the initial development of poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP) membranes in 2007.6 In this, membranes were not only prepared from diblock7-9 but also from triblock copolymers.10-12 Furthermore, the effect of additives on the structure formation was investigated13-14, as well as the possibility to tune the pore size of the membrane by blending block copolymers of different molecular weights and compositions.15 In order to vary their properties, porous membranes were prepared from block copolymers with different chemical functionalities like hydroxyl- or aminogroups.16-19 Nevertheless, the introduction of an alternative functional group (e.g., zwitterionic group) also necessitates the time consuming redetermination of the ideal membrane casting conditions. Postfunctionalization of membranes is a powerful toolbox to overcome this problem and allows adding functional groups on the membrane’s surface, pore walls and substructure. Hence, surface properties of the membrane material can be adjusted to the application requirements and negative effects on the membrane performance (e.g., fouling) can be reduced.20 For the modification of membranes, strategies like blending of additives21-22, chemical modification of the membrane material23-25, plasma treatment26, grafting27-28, and coating2, 29-31 are well described.


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The bio-inspired and catecholic chemistry based self-polymerization of dopamine has attracted enormous scientific interest as a promising surface modification method in recent years.32-35 Coating with polydopamine occurs under slightly alkaline conditions and air atmosphere in an oxidative reaction. Although different polymerization mechanisms were proposed, the detailed structure of polydopamine is not fully understood.32 Nevertheless, coating with polydopamine became a versatile surface modification technique, as it is applied under mild conditions (i.e. in aqueous solution) and shows strong adhesion on a variety of materials.36 Related to polymeric membranes, Yu et al. reported that a layer of polydopamine increases the wettability effectively accompanied by an anti-fouling effect.37 Due to its modifiability and reactivity, polydopamine is also used as an intermediate layer for further modification reactions. Strategies including Michael addition reaction, e.g. with amines38, or grafting reactions39 on polydopamine layers were performed successfully. In relation to postfunctionalization reactions, the number of approaches using “Click” chemistry increased in the last decade. According to Sharpless et al., organic reactions are classified as “Click” reactions if they allow high yields under simple conditions, are stereospecific and generate only by-products that can be easily removed.40 For instance, the thiol-ene and Diels-Alder reactions as well as the Cu(I)-catalyzed variant of the 1,3-dipolar cycloaddition of azides and alkynes (CuAAC) meet these requirements. Here, the CuAAC tolerates a variety of functional groups and can be performed under mild conditions, e.g. in water/alcohol mixtures at room temperature.41 Due to its flexibility, this reaction type is used for a broad range of applications as for modification of peptides and pharmaceuticals, but also for synthesis and modification of polymer surfaces42-43, to name just a few.


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Since the deposition of polydopamine as well as the CuAAC (during the following “Click” will be used as synonym for CuAAC) can be performed in water-based solutions under mild conditions, both approaches are suitable for the postfunctionalization of block copolymer membranes.

Figure 1. Chemical structure of the azido-modified dopamine derivative (1) and dopamine hydrochloride (2) In this work, nanoporous, integral asymmetric PS-b-P4VP block copolymer membranes with a narrow pore size distribution were postfunctionalized by a combination of a polydopamine-based coating and a subsequent “Click” reaction. First, the membranes were coated with a mixture of a “Click”-reactive azido-modified dopamine derivative (1) and dopamine hydrochloride (2) (see Figure 1 & Figure 2). Based on this coating, “Click” reactions were carried out with two different alkyne components to form the corresponding 1,2,3-triazoles (see Figure 2). In one case, the poly(dop-N3/dopamine) layer was functionalized with the hydrophobic 1-nonyne (3), in another case

with

the

zwitterionic,

hydrophilic

compound

3-((2-propyne)dimethylammonio)propane-1-sulfonate (4) (PDMAPS).


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Figure 2. “Click” reaction on membranes coated with poly(dop-N3/dopamine) and the chemical structures of 1-nonyne (3) and 3 ((2-propyne)dimethylammonio)propane-1-sulfonate (4) (PDMAPS) The pristine and modified membranes were investigated thoroughly via a variety of characterization methods in order to reveal their properties, as it is described in the next parts.

Results and Discussion Deposition of polydopamine and its derivatives According to Khanal et al.44, an azido-modified dopamine derivative (1) (dop-N3) was synthesized (see experimental section). Subsequently, this dopamine derivative and dopamine hydrochloride (2) were used for coating the block copolymer membranes (see Figure 2). The membranes were prepared by the SNIPS process from a PS76-b-P4VP24197 diblock copolymer


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which contains 76 wt.% PS, 24 wt.% P4VP and has a total molecular weight of 197 kg·mol-1 (for details see experimental section). The copolymerization of dopamine and azido-modified dopamine was carried out in open vessels to supply sufficient oxygen. Due to the higher hydrophobicity of dop-N3 compared to pure dopamine, a mixture of ethanol and Tris-buffer (1:1, v/v) instead of pure Tris-buffer was used. Compared to pure Tris-buffer, the solvent mixture also provides higher oxygen solubility and increases the wetting of the membrane substructure during the coating procedure due to its lower surface tension. Moreover, as reported by Yue et al.45 the presence of ethanol reduces the polymerization rate of polydopamine and this leads to a better controllability of the surface modification. By applying the coating, a certain concentration of the azido group should be generated at the membrane’s surface to allow the “Click” reaction in the second modification step. At the same time, the pore diameter of the block copolymer membrane should decrease as little as possible to preserve the membrane properties (e.g., water permeance, retention behavior). In order to face this trade-off and find out the best coating conditions, the weight fraction of dop-N3 in the coating solution was varied from 50 wt.% to 90 wt.% and the influence of the coating time was also evaluated (1-3 h).

“Click” reaction on poly(dop-N3/dopamine) coated membranes In the second modification step the azido groups of the poly(dop-N3/dopamine) layer were functionalized with alkyne compounds. For the Cu(I) catalyzed cycloaddition reaction literature procedures were adapted.46-47 Here, the catalytically active Cu(I) species was produced in situ by the reaction of CuSO4 and sodium ascorbate in water-based solutions. Additionally, N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDTA) was added as a ligand to prevent


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nonspecific chelation and deactivation of the copper ions (e.g. by amino groups of polydopamine). The two alkyne compounds used for the “Click” reaction differ significantly in their polarity. Hence, the influence of the surface polarity on the membrane performance, especially on the protein adsorption behavior, can be determined. In a first series of experiments, the “Click” reaction was carried out with the hydrophobic 1-nonyne (3) in a mixture of ethanol and water (1:1,

v/v).

In

contrast,

the

zwitterionic

and

hydrophilic

compound

3-((2-propyne)dimethylammonio)propane-1-sulfonate (4) (PDMAPS, for synthesis details see experimental section) was used for the second modification step. Because the thickness of the applied layer is in the range of several nanometers, a gravimetric determination of the amount of applied poly(dop-N3/dopamine), 1-nonyne and PDMAPS was not possible.

Scanning electron microscopy (SEM) SEM images of the pristine PS-b-P4VP membrane show hexagonally ordered pores with a narrow pore size distribution on the surface (Figure 3a) and a spongy substructure underneath (Figure 3e). Image analysis reveals an average pore diameter of 50 ± 2 nm. Figure 3b depicts the surface structure and Figure 3f the cross-section of a membrane coated with a mixture of 90 wt.% dop-N3 and 10 wt.% dopamine hydrochloride for 2 h. This example represents a successfully coated membrane with a well-preserved structure, though the pore diameter decreases to 46 ± 5 nm. In additional experiments with different coating times (1-3 h), it was found that in order to ensure a certain layer thickness with “Click”-reactive azido groups and a well preserved membrane structure, a coating time of 2 h is most suitable (pore sizes of membranes coated for varied times are shown in Figure S1 in supporting information). The same


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behavior was observed for a mixture of 50 wt.% dop-N3 and 50 wt.% dopamine hydrochloride (see Figure S2). As can be seen in Figure 3c, after the “Click” reaction with 1-nonyne, the pore diameter is not noticeably smaller. Here, image analysis reveals a pore diameter of 45 ± 5 nm independent of the weight fraction of dop-N3 in the coating solution. Furthermore, no significant change in the cross-section was noticed (Figure 3g). In contrast, the “Click” reaction with PDMAPS leads to a significantly decreased pore diameter of 36 ± 10 nm (Figure 3d) and a denser cross-section (Figure 3h). Additionally, the pore size distribution becomes broader and the surface appears rougher. Membranes initially coated from a solution with 50 wt.% dop-N3 have a pore diameter of 43 ± 8 nm after the same treatment.

Figure 3. SEM images: surface and cross-sections of (a, e) pristine PS-b-P4VP membrane, (b, f) PS-b-P4VP membrane modified with 90 wt.% dop-N3 for 2 h, (c, g) PS-b-P4VP membrane + 90 wt.% dop-N3 + “Click” 1-nonyne, (d, h) PS-b-P4VP membrane + 90 wt.% dop-N3 + “Click” PDMAPS


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The reason for the morphological changes and the broader pore size distribution is most probably the fact that the codeposition of dop-N3 and dopamine which form the poly(dop-N3/dopamine) layer, is random in terms of the azide group distribution. Consequently, the coating contains domains with higher azide group concentration, whereas other areas exhibit a higher concentration of unfunctionalized dopamine. After the “Click” reaction with PDMAPS, attractive or repulsive forces can arise within this network depending on whether unlike or like charges are facing each other. The strong interactions of the poly(dop-N3/dopamine) layer (e.g., H-bonding, C-C bonds and π-π stacking) limit an energetically favorable rearrangement of the network and consequently arising electrostatic repulsive forces cannot be reduced effectively.32, 48

The poly(dop-N3/dopamine) network can expand due to repulsive forces leading to a change of

the membranes’ morphology.49 This effect is more pronounced in domains with higher azide group concentration and the pore diameter becomes noticeably smaller while in domains with a lower dop-N3 concentration the change in pore diameter is less significant. This assumption is supported further by the fact that, compared to membranes initially coated with 90 wt.% dop-N3, the described morphological changes are less distinct for membranes coated with 50 wt.% dopN3 because the concentration of the charges is lower in these coatings.

X-ray photoelectron spectroscopy (XPS) In order to investigate the progress of the “Click” reaction, the membranes were analyzed by XPS. To focus the signal detection to the very first nanometers from the surface of the material, angle

resolved

XPS

(ARXPS)

was

carried

out.

Because

the

thickness

of

the

poly(dop-N3/dopamine) layer is much less (only 1 to 2 nm) compared to the bulk layer, overlap with signals of the block copolymer support phase still takes place. For example, the large


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number of different carbon species leads to a complex C1s region. Therefore, a detailed and quantitative analysis of these signals is not possible. Thus, we focused the evaluation on the N1s signals, particularly on the signal of the azido group that gives a specific signal at ~405 eV binding energy, to obtain a qualitative analysis of whether the “Click” reaction is complete. An overview on the deconvoluted N1s signals of the pristine membrane and the modified membranes is given in Figure 4 (all membranes shown here are based on a modification with 90 wt.% dop-N3 as the membranes coated with 50 wt.% dop-N3 show a comparable tendency; results of membranes coated with 50 wt.% dop-N3 are shown in Figure S3 in supporting information).


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Figure 4. XPS spectra of the N1s region for (a) pristine PS-b-P4VP membrane (b) PS-b-P4VP membrane modified with 90 wt.% dop-N3 for 2 h, (c) PS-b-P4VP membrane + 90 wt.% dop-N3 + “Click” PDMAPS, (d) PS-b-P4VP membrane + 90 wt.% dop-N3 + “Click” 1-nonyne Figure 4a depicts the spectrum of the pristine PS-b-P4VP membrane with the signal of P4VP at 399.0 eV. In Figure 4b the spectrum of the membrane modified with 90 wt.% dop-N3 is


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shown. The spectrum clearly reveals the signal of the electron-poor nitrogen of the azido group at 404.7 eV.44 Furthermore, the signal at 399.1 eV is assigned to P4VP and to secondary (R2NH) and tertiary/aromatic (=NR) amine functionalities in polydopamine, whereas primary amino functionalities of polydopamine are visible at 403.0 eV.50 The N1s spectra of the membranes after the “Click” reaction with PDMAPS and 1-nonyne are shown in Figure 4c and 4d, respectively. Here, the disappearance of the signals at 404.7 eV suggests a conversion of the azido

group

during

the “Click” reaction.

Consequently,

the

modification

of the

poly(dop-N3/dopamine) layer with PDMAPS and 1-nonyne was successful.

Atomic force microscopy (AFM) The surface roughness of a membrane has significant impact on its wetting and fouling properties. Vrijenhoek et al. showed that higher surface roughnesses correlate with higher fouling rates.51 The surface roughness values of the membranes were determined by AFM measurements. To correlate the roughness measured by AFM with the fouling experiment results, the membranes were investigated not only in dry state but also in phosphate-buffered saline (PBS) (10 mM, pH=7.4). Since the nanoporous structure of the membranes influences the roughness values due to topography, a height threshold was set in order to exclude all pores from the calculation of the mean square roughness (Rq).


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Table 1. Mean square roughness values (Rq) of the pristine and the modified membranes PS-b-P4VP membrane pristine

90 wt.% dop-N3

90 wt.% dop-N3 + “Click” 1-nonyne

90 wt.% dop-N3 + “Click” PDMAPS

10.4 ± 2.1

9.3 ± 1.5

10.2 ± 1.8

11.1 ± 1.7

13.2 ± 2.4

10.2 ± 2.7

10.7 ± 2.5

16.8 ± 2.7

Roughness Rq dry state [nm] Roughness Rq PBS-buffer [nm]

In Table 1 only the roughness values of the pristine and the modified membranes coated with 90 wt.% dop-N3 are presented as the membranes coated with 50 wt.% dop-N3 show a comparable tendency (for details see Table S1 & Figure S6 in supporting information). The analysis of these values reveals that the differences of roughness in the dry state are not significant (for AFM height images measured in dry state see Figure S4 & S5). By coating the membrane with poly(dop-N3/dopamine), the roughness decreases from 10.4 nm to 9.3 nm. This observation can be associated with the uniform coating of the membrane with poly(dop-N3/dopamine).38


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Figure 5. AFM height images measured in PBS-buffer (a) pristine PS-b-P4VP membrane, (b) PS-b-P4VP membrane modified with 90 wt.% dop-N3 for 2 h, (c) PS-b-P4VP membrane + 90 wt.% dop-N3 + “Click” 1-nonyne, (d) PS-b-P4VP membrane + 90 wt.% dop-N3 + “Click” PDMAPS After the “Click” reaction with 1-nonyne, the roughness value is in the range of the pristine membrane (10.2 nm) whereas the membrane functionalized with PDMAPS shows a slightly increased roughness (11.1 nm). As mentioned, a possible explanation are repulsive forces inside the poly(dop-N3/dopamine) layer caused by interactions of the ionic groups of PDMAPS.


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In Figure 5, the AFM height images of the membranes measured in PBS-buffer after an equilibration time of at least 24 h are presented. Compared to the roughness values in dry state, the different modifications have a more substantial influence under liquid. The pristine membrane shows a higher roughness value of 13.2 nm (Figure 5a). This is attributed to the different swelling of the PS and the P4VP domains in the aqueous PBS-buffer since PS is hydrophobic while P4VP is rather hydrophilic. After coating with poly(dop-N3/dopamine) (see Figure 5b), the difference in roughness between dry state (9.3 nm) and under liquid (10.2 nm) is less significant. Assuming a uniform poly(dop-N3/dopamine) coating, the result suggests a uniform swelling of this additional layer in the buffer solution. After the “Click” reaction with 1-nonyne and PDMAPS the Rq values of the membranes in PBS-buffer are 10.7 nm and 16.8 nm, respectively. According to this, the surface is not noticeably rougher after the reaction with 1-nonyne (Figure 5c) and the Rq value is still lower compared to the pristine PS-b-P4VP membrane. Among the investigated systems, the membrane coated with poly(dop-N3/dopamine) and functionalized with PDMAPS (Figure 5d) shows the highest roughness value (16.8 nm). In this case it has to be mentioned, that the codeposition of dop-N3 and dopamine that form the poly(dop-N3/dopamine) layer, is random in terms of the azide group distribution. It is possible that there are domains with higher azide group concentration, whereas other areas exhibit a higher concentration of unfunctionalized dopamine. By functionalizing the poly(dop-N3/dopamine) layer with PDMAPS positive and negative charges were introduced and the behavior of these areas would be different compared to areas rich in unfunctionalized dopamine. Consequently, the higher roughness value might be caused by a combination of a different level of swelling connected to inhomogeneously distributed azido groups and interactions of the zwitterionic PDMAPS with ions of the PBS-buffer.52 The white


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areas in Figure 5d are poly(dop-N3/dopamine) particles which are also visible in the SEM image of the respective membrane (Figure 3d). These particles are already present on the membrane’s surface before the “Click” reaction with PDMAPS but become more pronounced during the second modification step. Although the particles contribute to the Rq value listed in Table 1, the roughness is not significantly lower when measuring in particle-free areas in Figure 5d.

Contact angle For determining the water wettability of the membranes, dynamic contact angle measurements were conducted using the sessile drop technique. Figure 6 shows the time dependent contact angle values of the membranes in the different states of modification.

Figure 6. Time dependence of the water contact angle on the pristine PS-b-P4VP membrane and the modified PS-b-P4VP membranes The effect of surface roughness on the contact angle is negligible since the roughness values of the different membranes in the dry state are comparable. As can be seen, the water contact angle of the pristine PS-b-P4VP membrane is 51 ° directly after the water droplet was applied and


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reduces to < 10 ° after 25 s. Compared to this, the poly(dop-N3/dopamine) coated membranes show a lower value in the beginning and the water droplet also penetrates faster. Most probably this effect is caused by the hydrophilicity of the poly(dop-N3/dopamine) coating. As expected, the modification with the hydrophobic 1-nonyne leads to an increased contact angle and a longer time for the water droplet to penetrate into the membrane. Thereby, the effect of hydrophobization is more pronounced on membranes coated with a higher content of dop-N3 and consequently on those with a higher concentration of “Click”-reactive azido groups. When poly(dop-N3/dopamine) coated membranes were functionalized with the hydrophilic PDMAPS, the contact angle decreases only slightly. Regarding this, it must be considered that the penetration speed of the water droplet is different on these membranes due to the smaller average pore diameter. The water droplet starts to soak the membrane structure once it is applied and the change of the contact angle over time depends on the penetration speed. Although the measurement was started directly after the droplet was applied, this effect influences the starting value and is most probably the reason for the comparably slight decrease of the contact angle after the modification with PDMAPS.

Clean water permeance The clean water permeance is an important characteristic of ultrafiltration membranes. Besides chemical properties of the membrane material (e.g., polarity), parameters like the pore size, surface porosity, and membrane thickness influence this value.53 The permeances of the pristine and the modified PS-b-P4VP membranes were measured in dead-end mode with 1 bar trans-membrane pressure (TMP).


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The initial permeance of the pristine PS-b-P4VP membrane is 1360±25 L·m-2·bar-1·h-1, as presented in Figure 7. The permeance drop of the PS-b-P4VP membrane over time is attributed to the swelling of the pore forming P4VP. The AFM images in Figure S7 support this assumption by indicating that the water induced swelling of P4VP in a PS-b-P4VP film is most significant within 1.5 h. Here, a spin-coated film was used as model system because the discriminability of the two blocks allows a detailed investigation of the swelling behavior. Membranes coated with 90 wt.% dop-N3 and 50 wt.% dop-N3 exhibit initial permeance values of 1412±51 L·m-2·bar-1·h-1 and 1409±44 L·m-2·bar-1·h-1, respectively. Compared to the pristine membrane, the coated membranes show comparable permeance values although smaller pores were observed in the SEM images. In this case, considering the results of the contact angle measurements, the more hydrophilic surface leads to an increased wetting of the porous structure and consequently to higher permeance values.54

Figure 7. Time dependence of water permeance of the pristine PS-b-P4VP membrane and the modified PS-b-P4VP membranes


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After the “Click” reaction with 1-nonyne, the membranes coated with 90 wt.% dop-N3 and 50 wt.% dop-N3 in the first step show an initial permeance of 1150±34 L·m-2·bar-1·h-1 and 1290±35 L·m-2·bar-1·h-1, respectively. Compared to the pristine membrane, the lower permeance values are attributed to the decrease of the pore diameter from 50 nm to 45 nm. However, this explanation is not applicable when comparing the permeance values before and after the “Click” reaction with 1-nonyne since the pore diameter does not change significantly. In contrast, the more hydrophobic surface presumably causes an incomplete wetting of the membrane structure leading to lower permeance values. This assumption is supported by the contact angle values. Membranes that were first coated with 90 wt.% dop-N3 and 50 wt.% dop-N3 and subsequently modified with PDMAPS show an initial permeance of 1208±37 L·m-2·bar-1·h-1 and 1321±40 L·m-2·bar-1·h-1, respectively. No significant change of surface polarity was observed for these membranes after the “Click” reaction with PDMAPS. Considering the observations made in the respective SEM images, the reason for the permeance drop is most probably the decrease of the pore diameter during the modification with PDMAPS. Furthermore, we investigated the dependence of the water permeance on the pH value. The results of the pristine membrane and membranes initially coated with 90 wt.% dop-N3 and further modified with 1-nonyne and PDMAPS are shown in Figure S8 (membranes initially coated with 50 wt.% dop-N3 were not investigated). The membranes showed a pH-responsive behavior which is reversible. The applied functionalization influences the permeance at larger pH values. Both the hydrophilicity and size of the added functional groups affect the water permance (large hydrophobic nonyne group leads to lowest permeance and the membrane with only dopamine coating shows the highest permeance).. Not only the investigated membranes show a pH-responsive behavior but also the properties of proteins depend on the pH value. To


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avoid an overlay of too many different effects, a fixed pH value close to physiological conditions (pH~7) was chosen for characterization methods related to proteins (e.g., AFM in liquid, protein adsorption and fouling experiments).

Retention measurements In order to further investigate the separation performance of the prepared membranes, retention experiments with aqueous solutions of poly(ethylene glycol)s with different molecular weights were carried out in dead-end mode to determine the molecular weight cut-off (MWCO, retention ˃ 90%). The results are shown in Figure S9. The MWCO of all membranes is between 280 kg·mol-1 and 290 kg·mol-1. For most of the investigated membranes, this value is in good agreement with the average pore diameter since the estimation of the hydrodynamic radius (Rh) of the poly(ethylene glycol)s gives similar values for all membranes of Rh= 23±1 nm.55 However, for the membrane coated with 90 wt.% dop-N3 and further modified with PDMAPS a lower MWCO could be expected considering the smaller average pore diameter (36±10 nm). Here it is important to note, that not only the average pore diameter is smaller but also the pore size distribution is broader. The SEM image of the mentioned membrane (Figure 3d) reveals that there are several pores with a diameter up to 46 nm. These pores are assumed to affect the retention behavior and are most probably the reason for the fact that this membrane exhibits a similar MWCO compared to the other investigated membranes.


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Static protein adsorption The static protein adsorption behavior of an ultrafiltration membrane can be a valuable indication of its fouling properties.56 Once adsorption occurs on the surface, the pore walls of the selective layer, or the substructure of the membrane, the effective pore diameter will be reduced down to a complete blocking of the pores. Consequently, the performance of the membrane is influenced negatively. The static adsorption experiments were carried out with hemoglobin and bovine serum albumin (BSA) at 25 °C in PBS-buffer (pH=7.4). At the given pH value, when taking into account the isoelectric points (IEP) of the proteins, hemoglobin is not significantly charged (IEP = 6.8), while BSA (IEP = 4.9) shows a negative overall charge.57 This is important because, apart from hydrophobic interactions and hydrogen bonding, electrostatic interactions of the proteins with the membrane material also have an influence on the adsorption behavior. The hydrodynamic diameters (Dh) are 6.4 nm for hemoglobin and 6.9 nm and for BSA as stated by supplier.


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Figure 8. Protein adsorption of hemoglobin (red, striped pattern) and BSA (blue) of the pristine PS-b-P4VP membrane and the modified PS-b-P4VP membranes at 25 °C, pH = 7.4 The results of the adsorption experiments are shown in Figure 8. On the pristine PS-b-P4VP membrane, hemoglobin and BSA show an adsorption of 171±9 µg·cm-2 and 148±6 µg·cm-2, respectively. Additionally to the above mentioned interaction types, an important factor for adsorption of hemoglobin is chelation of the hemoglobin’s iron ion by the free electron pair of the P4VP nitrogen atom.58-59 After applying the poly(dop-N3/dopamine) layer, the adsorption values for hemoglobin hardly change. Membranes coated with 90 wt. % dop-N3 and 50 wt.% dop-N3 show adsorption values of 190±10 µg·cm-2 and 177±10 µg·cm-2, respectively. One reason for this behavior might be the fact that by applying the poly(dop-N3/dopamine) layer, additional functional groups capable of metal ion chelation are introduced into the system.60 Compared to that, for BSA, lower adsorption values of 137±4 µg·cm-2 on membranes coated with 90 wt.% dop-N3 and 113±6 µg·cm-2 on membranes coated with 50 wt.% dop-N3, were measured. In this context,


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lower values could be expected due to the anti-fouling properties of polydopamine. Here, not only adsorption, but also the fact that the free amino and thiol groups of BSA can undergo a Michael reaction with the poly(dop-N3/dopamine) layer can lead to these values.61 The “Click” reaction with the hydrophobic 1-nonyne leads to a significant drop of the protein adsorption. After the reaction, the membranes coated with 90 wt.% dop-N3 and 50 wt.% dop-N3 in the first step show adsorption values for hemoglobin of 68±3 µg·cm-2 and 127±7 µg·cm-2, respectively. In contrast, the BSA adsorption values are lower for both types of membranes (

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