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Letter Cite This: ACS Macro Lett. 2018, 7, 840−845

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Setting the Stage for Fabrication of Self-Assembled Structures in Compact Geometries: Inside-Out Isoporous Hollow Fiber Membranes Kirti Sankhala,† Joachim Koll,† and Volker Abetz*,†,‡ †

Helmholtz-Zentrum Geesthacht, Institute of Polymer Research, Max-Planck-Strasse 1, 21502 Geesthacht, Germany University of Hamburg, Institute of Physical Chemistry, Martin-Luther-King-Platz 6, 20146 Hamburg, Germany

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S Supporting Information *

ABSTRACT: Fabrication of evaporation-induced self-assembled structures on easily accessible surfaces is an established strategy, while achieving such microphase-separated structures in compact geometries has been a longstanding goal. The requirement of comparatively less concentrated block copolymer (BCP) solution to pass through the compact geometries significantly reduces the stimulations required for self-assembly. The high polymer relaxation rates and decreased thermodynamic driving forces, as well as high capillary suction of dilute solutions in the porous substrates, complicates the BCP self-assembly and fabrication of the uniform coated layer, respectively. In this study, highly permeable robust poly(ether sulfone) hollow fiber membranes (PES HFM) with an inner diameter of approximately 1 mm are selected as compact geometries, and the isoporous structures are developed on top of ≤10 μm thin coated layer. This fabrication process introduces a technologically favored inside-out configuration for isoporous composite HFM with large bore diameters.

fficient separations offering high permeability along with sharp molecular weight cutoff is a demand of all the separations ranging from water purification to separation of expensive drugs.1−5 Moreover, integral asymmetric isoporous membranes providing high surface porosity along with a narrow pore size distribution on a thin selective top layer, which is supported by a more open porous asymmetric structure, promise to become the prime of ultrafiltration membranes.6−10 Hollow fiber membranes (HFM) hold a prospective to be applicable in a wide area of applications due to their self-supporting geometry and their ability to provide higher active surface area per unit volume. Furthermore, the inside-out HFM offers protection of the selective inner skin along with well-controlled flow hydrodynamics during crossflow in the lumen side, where the small sweeping friction of the inlet fluid flowing across the membrane surface limits the build-up of cake layers. Easy back-flushing and cleaning thereby minimize membrane fouling and maximizes process flux and product yield.11,12 However, fabrication of selfassembled structures on the lumen of HFM remains a challenge due to the requirement of additional structure controlling evaporation conditions. Our recent work on block copolymer (BCP) HFM has conveyed the notion for control over self-assembly by using nitrogen gas (N2) flow during a complex dynamic process of HF spinning,13 but the high costs and mechanical strength of BCP HFM is quite a challenge for

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large scale installations. Also, involving significant viscous stresses on the polymer chains during extrusion, the complex spinning mechanism restricts the use of low polymer concentrations and big variations of spinning parameters.14,15 Therefore, the preparation of composite membranes via coating of dilute BCP solutions onto a mechanically stable and inexpensive support HF is a method of choice. In the initial stage, a self-assembling polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymer was used to provide an integral asymmetric isoporous flat sheet membrane. It was prepared by a comparatively straightforward approach: the evaporation induced self-assembly on the top surface of a blade cast BCP solution that was then trapped by nonsolventinduced phase separation (SNIPS).16 BCP membranes offer attractive features like tailorable pore size,17,18 high surface porosity,19 sharp molecular weight cutoff,20 thermoresponsivity,21,22 pH-responsivity,21,23,24 catalytic activity,25,26 thermal stability,27 and so on. Moreover, it is hard to achieve nanometer-sized structures on a large scale using other materials or techniques. Therefore, in order to cover a broader range of applications, subsequently, the SNIPS process was extended to HFM with an isoporous outer15,25,28−32 and inner Received: May 23, 2018 Accepted: June 20, 2018

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Figure 1. (a−d) Schematic diagram demonstrating the mechanism of inside-out isoporous composite HFM fabrication. (a) Module holding H2O2treated PES support HF. (b) BCP solution is then pumped into the lumen of support fiber. (c) Afterward, N2 is conveyed in the lumen; the enlarged section shows the microphase separation on the lumen side. (d) Then, the coated fiber is exposed to water flow, and the enlarged section displays the pore opening on the inner surface of composite membrane. (e) Scheme of coated HFM.

mechanisms and adhesion of two chemically distinct polymer layers. The highly asymmetric membranes have a significant difference in pore sizes on inner and outer surfaces from about 25 nm to 2.5 μm, respectively, and show high water flux of 260 ± 50 L m−2 h−1 bar−1 and molecular weight cutoff (MWCO) of 300 kDa for poly(ethylene glycol) (PEG). A schematic depiction describing the composite isoporous HFM fabrication process is provided in Figure 1 (more details in the Supporting Information). Many factors such as substrate hydrophilicity/hydrophobicity, substrate roughness and porosity, composition of the coating solution, polymer solution flow rate Qp, time for polymer solution flow tP, N2 flow rate QN2, time for N2 flow tN2, and water flow rate Qw make this system, pumping coating in the lumen side, intricate and demanding. We discuss each factor in the following paragraphs. Influence of substrate HF: The key question in this study sought to get robust support membranes providing as high flux as possible along with a surface compatible with the coating solution. The inner surface, the porosity of the substructure near the inner surface, and the fiber diameter of support membranes are key parameters in fabrication of composite membranes. In contrast with the outside-in composite membranes, here, the capillary force acting on the polymer solution in the fiber-lumen makes the system different and more challenging. The capillary force in the lumen influences the thickness of the coated layer, which is controlled by gas flow. Additionally, the capillary behavior of pores affects the intrusion of polymer solution and changes the porosity of the support membrane. So, for easy flow of a polymer solution through the fiber lumen, it is preferred to have fibers of bore

Figure 2. SAXS patterns of polymer solutions having 4 and 8 wt % PS83-b-P4VP17168 BCP (one with 0.04 wt % MgAc2) in DOX and solvent mixture of DOX/DMF 95/5 (wt/wt). The curves are plotted in log/log scale and the y-offset is adjusted.

surface.13 Here, we highlight a successful pathway to develop inside-out isoporous composite HFM prepared by depositing a thin selective layer using dilute BCP solutions onto the lumen side of a mechanically robust poly(ether sulfone) (PES) support HFM, followed by the SNIPS method. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) helped us to elucidate the coating 841

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Figure 3. (a−g) PES2 HFM were coated with the polymer solution of 4 wt % PS83-b-P4VP17168 and 0.04 wt % MgAc2 in solvent mixture of DOX/ DMF 90/10 (wt/wt). Thickness of the coated layer is about 5 μm. The coating parameters were Qp, 1.0 mL min−1; tp, 10 s; QN2, 1.0 mL min−1; tN2, 60 s; and Qw, 3.0 mL min−1. (a, e) SEM micrographs show the surface of the cross-sectional morphology by using in-lens detector. (b) Topographical image using secondary electron detector (HE-SE2). (c, d) Compositional contrast is shown by energy specific separation of backscattered electrons using energy selective backscattered electrons (EsB) detector. Micrographs (a−c) have the same scale bar. (f, g) Bright-field TEM images of cross sections from I2-stained films showing the interface of PES support and coating layer. The selectively stained P4VP domains appear black and PS is dark gray. The yellow arrows and marked areas show intrusion of BCP into the PES matrix by forming connected pores.

diameter ≥1 mm. In this work, two different commercial PES HFM were chosen to validate our process: PES1 HFM (bore diameter: 1.0 mm; MWCO: 750 kDa) and PES2 HFM (bore diameter: 1.5 mm; pore size on inner surface ca. 0.2 μm; as mentioned by the suppliers); Figure S1. Prior to coating, these support HFM were rinsed with 30 wt % hydrogen peroxide (H2O2) solution in order to clean the inner surface and remove the influence of surface-treatments on commercial fibers, if there are any. Selection of polymer solution and the structure formation: The art of self-assembly and isoporous structure formation requires selecting a suitable block copolymer solution since this parameter governs the membrane morphology, solubility, and diffusivity in the support membrane, which in fact affect the complete membrane morphology and the performance. Moreover, uniform coating without dewetting or suction of the polymer solution remains the main challenge in coating of highly porous support HF. Here, we used PS83-b-P4VP17168 (subscripts denote the amount of respective block in wt % and superscript denotes the total molecular weight in kg mol−1). In the common isoporous membrane fabrication, 15−35 wt % BCP is dissolved in the solvent mixture of N,N-dimethylformamide (DMF)/tetrahydrofuran (THF) or DMF/THF/1,4dioxane (DOX).19,33,34 However, such viscous solutions cannot be easily pumped through a HFM. Since self-assembly is a result of segmental incompatibility between the repulsive blocks, therefore, by selecting a right solvent or solvent mixture, the structure formation in solution can be influenced.

The micellar structures can be achieved in 1−3 wt % of PS-bP4VP solution using DOX as a solvent offering rather rigid micellar cores for P4VP.35 As DOX is selective for PES substrate and thus penetrates into it, the BCP concentration in the coating solution increases which enhances the microphase separation. We could not obtain open pores when using DOX solutions with 1−8 wt % of BCP and evaporation times of 10− 60 s for both PES1 and PES2 HFM (Figure S2). These fibers were impermeable. A possible reason could be the slow evaporation rate in the lumen side that leads to almost closed domains of P4VP on the top surface, as compared to using DOX as a single solvent in flat sheet membrane fabrication by blade casting35 or by spray coating.36 Moreover, here, the absence of a commonly used more volatile cosolvent (THF) lacks a strong, highly directional field due to a fast increase in concentration gradient from the top surface, leading to a lesser segregated system, and overall, a comparatively relaxed system for self-assembly.6,37−39 Consequently, an increased swelling of the P4VP blocks was required. Therefore, by adding 5−10 wt % DMF and by this increasing the solubility of P4VP blocks, the micellization decreases, and the ordering transition in the solution shifts toward higher polymer concentration (Figure 2). By selecting the right BCP solution of 4 wt % PS83-bP4VP17168 in DOX/DMF 90/10 (wt/wt) and suitable coating parameters, isoporous structures could be fabricated on the inner surface (Figure S3). The addition of 5−10 wt % DMF in DOX solution leads to dissolution of the inner skin of the support fiber instead of just swelling it. This has an influence 842

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Figure 4. (a−f) H2O2-treated PES2 membranes were coated with the polymer solution of 4 wt % PS83-b-P4VP17168 and 0.04 wt % MgAc2 in solvent mixture of DOX/DMF 90/10 (wt/wt). (a, b) SEM micrographs show the morphology of inner surface of membranes. The coating parameters were Qp, 1.0 mL min−1; tp, 10 s; QN2 (0.7 and 3.0 mL min−1) and tN2 (20, 40, and 60 s); and Qw, 3.0 mL min−1; all micrographs have the same scale bar. (c) Inner view of coated HFM. (d−f) SEM micrographs show the cross-sectional morphology where thickness of the coated layer is around 10 μm. The coating parameters were Qp, 1.0 mL min−1; tp, 10 s; QN2, 0.7 mL min−1; tN2, 60 s; and Qw, 3.0 mL min−1.

the used BCP and PES2 HFM, the optimized polymer solution is 4 wt % PS83-b-P4VP17168 and 0.04 wt % MgAc2 in the solvent mixture of DOX/DMF 90/10 (wt/wt). However, the optimized solution is still comparatively dilute for selfassembly, which leads to high relaxation rates of the macromolecules and therefore requires a good control over kinetics and thermodynamics of microphase separation, as shown in Figure 4. Coating parameters influencing the fabrication of composite membranes: The homogeneous PS83-b-P4VP17168 BCP solution was incorporated into the lumen side of PES support HF (Figure 1b) using a high precision pump. Here, polymer solution flow rate Qp and time for flow tP influence the intrusion of BCP solution into the support membrane due to the capillary forces in the lumen side. Therefore, in order to reduce the time tP required to fill the fiber lumen and complete wetting of the inner skin with polymer solution a certain QP is required. Otherwise, either pinholes generate on the inner surface due to the capillary suction of polymer solution or

on the compaction of the top BCP layer after precipitation and resulted in a well-coated HFM and, also, assuring the strong adhesion of the two chemically distinct polymers by slight intrusion of BCP solution into the support (Figure 3). Further, depending on the nonuniform porosity on the inner surface of the support membrane and the coating solution, the coated layer provides spongy or macrovoid substructures (Figure 3a,e). The main work was focused on the more robust PES2 HFM because the highly porous and thinner PES1 HFM dissolves too fast in the DMF of the coating solutions. Further, the PS-b-P4VP supramolecular assemblies can be facilitated by addition of metal salts or carbohydrates.23,40−42 The SAXS results show that the addition of magnesium acetate (MgAc2; 1 wt % of BCP concentration) increases the structure ordering with a shift of the maxima in SAXS toward higher scattering vectors and thus decreases the domain spacing. The slightly more viscous BCP solution showed good spreading and controlled sinking of the solution into the HF support without dewetting and resulted in a uniform coated layer. For 843

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such as diffusion of different molecules in opposite directions, where the support fiber can trigger catalytic reactions as well.

more intrusion of solution into the substructure will result in a rather dense interface due to more dissolution of PES inner layer and will further deform the support. After some preliminary coating experiments with the selected PES2 support membranes, QP was fixed to 1 mL min−1 for tP 10 s. The controlled dissolution of the inner skin of PES HF and intrusion of BCP solution confirm the attachment of the two layers with interconnected pores (Figure 3f,g). Afterward, N2 flow is conveyed in the fiber lumen which is full of polymer solution (Figure 1c) to remove the surplus polymer solution while allowing a fine coating of BCP solution on the inner surface. The kinetics of the BCP chains and their self-assembly is influenced by the rate of evaporation, which is controlled by QN2, while the evaporation time is controlled by tN2. With increasing tN2, the increased dissolution of the inner skin of the PES HF deforms the inner surface morphology and changes membrane performance (Figure S4). The development of structure and increasing segregation of BCP with increasing QN2 and tN2 is shown in Figures 4 and S3. It was observed that QN2 higher than 1.0 mL min−1 tends to tear off the PES2 support HFM. However, the self-assembly and isoporous structure formation is facilitated. A support HFM that does not have a tightly packed substructure near the inner surface and a highly asymmetric open porous structure afterward is suitable for such coating experiments. Therefore, for the optimized polymer solution and the used PES2 support HFM, we optimized QN2 as 0.7−1.0 mL min−1, and the evaporation time around 60 s for the fabrication of a smooth, uniform, and well attached isoporous coating layer on the lumen side (Figures 3 and 4). After providing sufficient evaporation in a controlled manner for a predetermined period, water is introduced in the lumen in order to freeze the isoporous structure and complete exchange of solvents by nonsolvent water (Figure 1d). Here, the flow rate drives the exchange rate of solvents by nonsolvent, so, a higher flow rate is preferred, that is, Qw 3.0 mL min−1, to provide enough nonsolvent for replacing the solvents. Afterward, the module is being immersed into the coagulation bath of water for another 48 h. The method efficiency is shown by investigation of the morphology (Figures 3, 4, S3, and S5) of the modified membranes and investigating water flux (Figure S6). The composite membranes show a promising water flux of 260 ± 50 L m−2 h−1 bar−1 in wet conditions, higher than the previously reported outside-in isoporous composite HFM30 and comparable to single layer isoporous HFM.13,15 It must be noted that these coating parameters highly depend on the diameter of lumen, support membrane morphology and material, and the BCP solution used for coating. In summary, we demonstrated a method to develop selfassembled structures on the lumen side of compact geometries like the lumen of a HFM via a cost-effective and scalable coating method followed by SNIPS. We validated this strategy on the example of a 4 wt % PS-b-P4VP BCP solution to coat the inner surface of a microfiltration PES HFM of inner diameter 1.5 mm. Using N2 flow the self-assembled lateral order was developed on the lumen side and fixed by subsequent water flow. The membranes hold potential to be explored and applied in the fields of bioreactors, protein separation, pharmacy, food industry, and wastewater treatment, among others. Moreover, by coating on the outer surface, a bifunctional separation system can be fabricated which may offer a different venue toward biotechnological applications

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00402. Details on experimental methods, Figures S1−S6, and Table S1 (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Volker Abetz: 0000-0002-4840-6611 Funding

The work has been funded by Helmholtz-Zentrum Geesthacht (HZG) and all the authors are financed by this organization. Notes

The authors declare the following competing financial interest(s): An EU patent application has been submitted on behalf of this organization.

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ACKNOWLEDGMENTS The authors gratefully acknowledge Clarissa Abetz and AnkeLisa Metze for their kind support and microscopical characterization, Brigitte Lademann for synthesis of the BCP, Florian Wieland (DESY) for help with SAXS measurements and data analysis, Maryam Radjabian for helpful discussions, Silvio Neumann for NMR, and Barbara Bajer and Maren Brinkmann for GPC measurements. The authors would like to thank Cut Membrane Technology GmbH for kindly providing the PES2 support HFM.

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