Highly permeable nanoporous block copolymer membranes by machine-casting on nonwoven supports: An upscalable route

Highly permeable nanoporous block copolymer membranes by machine-casting on nonwoven supports: An upscalable route

Journal of Membrane Science 533 (2017) 201–209 Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier...

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Journal of Membrane Science 533 (2017) 201–209

Contents lists available at ScienceDirect

Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

Highly permeable nanoporous block copolymer membranes by machinecasting on nonwoven supports: An upscalable route Xiansong Shi, Zhaogen Wang, Yong Wang

MARK



State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, and Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing 210009, Jiangsu, PR China

A R T I C L E I N F O

A B S T R A C T

Keywords: Block copolymer Machine casting Selective swelling Composite membrane Fractionation

Block copolymer (BCP) membranes are distinguished for their well-defined porosities, tunable pore geometries, and functionable pore walls. However, it remains challenging to produce robust BCP membranes by affordable, convenient methods. Herein, we demonstrate a facile and easily upscalable approach to produce highly permeable BCP membranes in large areas. The membranes possess a bi-layered composite structure with nanoporous polystyrene-block-poly(2-vinylpyrdine) BCP layers directly supported on macroporous nonwoven substrates. The BCP layers are machine-cast on the water-prefilled nonwoven, and interconnected nanoporosities are created in the BCP layers by ethanol swelling. The nanoporous BCP layers exhibit a thickness of ~10 µm and are tightly adhered to the nonwoven. Changes in the swelling temperatures and durations modulate both pore sizes and surface hydrophilicity of the BCP layers, and consequently the permselectivity of the membranes. By increasing swelling duration from 15 min to 12 h, the permeability of the membrane swollen at 65 °C can be increased from ~100 to ~850 L m−2 h−1 bar−1 with the retention to 15-nm gold nanoparticles reduced from ~93% to ~54%. Moreover, we demonstrate that the composite membrane can efficiently fractionate nanoparticles and narrow down their size distribution from ~3–20 nm to ~3–10 nm.

1. Introduction Membrane separation is playing an increasingly significant role in industry as well as daily life for their wide applications in dynamic fields such as water treatment [1], biotechnology processes [2], and health care [3]. To improve separation performances and also to expand the applications of membrane separation in new areas, it is particularly attractive to fabricate new membranes or modify existing membranes. New materials, for example, carbon nanotubes [4,5], graphene and its derivatives [6,7], metal-organic frameworks [8,9], and self-assembled polymers [10,11] have been used to prepare advanced membranes with fast permeability and/or sharp selectivity. Among them, block copolymers (BCPs) are of particular interest because they can readily microphase-separate into highly ordered nanoscopic structures leading to homogeneous membrane pores [12–14]. Moreover, BCPs-based membranes enjoy the merit of availability of membrane pores with different geometries and the flexibility in functionalization of membrane pores. Selective etching/extraction of the minority phases [15–17], nonsolvent-induced phase separation (NIPS) of concentrated BCP solutions [18–20], and selective swelling of amphiphilic BCPs [21–23] are the three main strategies for the



preparation of nanoporous membranes from BCP precursors. However, BCP membranes are currently suffering from several challenges including high cost of the BCP precursors, the weak mechanical stability, and complicated manufacturing process, which severely limit their upscalability and real-world applications. To tackle the issue of high cost and also mechanical strength, the bi-layered composited membrane structure consisting of a thin BCP selective layer on the top of a macroporous substrate is typically used [24]. Such an approach allows to design and prepare the two layers independently with the nanoporous thin skins ensuring high selectivity at small consumption of BCP raw materials and the underlying macroporous substrates promising strong mechanical robustness and low flow resistance. Recently, a number of methods have emerged for the preparation of composite membranes having a nanoporous BCP selective layers. Transferring spin-coated BCP thin films onto macroporous substrates is a versatile method to produce ultrathin layers of various BCPs [25–27]. However, it is very tedious and is difficult to produce large-area membranes. Alternatively, direct coating dilute BCP solutions onto the surface of liquid-filled porous substrates is a much convenient process capable of making BCP layers with the thickness down to a few micrometers directly on the substrate [16,28,29]. Hillmyer et al. prepared composite membranes by manu-

Corresponding author. E-mail address: [email protected] (Y. Wang).

http://dx.doi.org/10.1016/j.memsci.2017.03.046 Received 14 January 2017; Received in revised form 27 March 2017; Accepted 28 March 2017 Available online 31 March 2017 0376-7388/ © 2017 Elsevier B.V. All rights reserved.

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tration of 10 wt%. A clean and dust-free glass plate was then placed onto the horizontal surface of the casting machine (AFA-II, Shanghai Xiandai Environmental Engineering Technique Co., Ltd). A piece of nonwoven with the size of 15 cm×20 cm, used as the supporting layer, was immersed in deionized water for about 5 min, allowing water to percolate through the pores in it. The nonwoven was then withdrawn from the water bath, gently shaken to remove excessive water and placed on the top of the glass plate. Afterwards, the BCP solution was machine-cast onto the nonwoven by using a casting knife with ~200 µm gate height to evenly spread the solution over the substrate. The as-cast nonwoven was then kept in the fume hood at room temperature for about 6 h to remove both the residual organic solvent and the water filled in the pores, followed by heating in vacuum at 100 °C for another 1 h to dry the membrane. Evaporation of the solvent led to a thin and dense BCP layer on the top of the nonwoven, producing a bi-layered composite structure. To generate pores in the BCP layer, the selective swelling process was applied [10,25]. Briefly, the BCP-coated nonwoven was immersed in warm ethanol for desired periods of time, followed by air drying at room temperature.

ally coating the polystyrene-block-polylactide (PS-b-PLA) solution on polyethersulfone microfiltration (MF) membranes with pores previously filled with water, followed by selective etching the sacrificial PLA blocks [30]. Recently, we manually coated dilute polystyreneblock-poly (2-vinylpyrdine) (PS-b-P2VP) solutions on water-filled polyvinylidenefluoride (PVDF) MF membranes, and immersed the coated membranes into hot ethanol for hours to generate nanoporosities in the BCP layers by the selective swelling mechanism, thus producing BCP/ PVDF composite membranes delivering an ultrafiltration (UF) function [21,28]. However, the direct coating method is predominantly using UF or MF membranes as the macroporous supports which are much less permeable and more expensive than nonwoven extensively used as supports for UF and MF membranes. Moreover, the composite membranes are only available with small areas because coating of BCP solutions on porous substrates is typically manually performed with poor controllability. These issues significantly hamper the reproducible production of BCP membranes in large scale and their usages in realworld applications. Polyester nonwoven fabrics can be cheaply sourced and they are mechanical strong and ductile, and also highly porous and water permeable. Therefore, they might serve as a good macroporous substrate to support nanoporous BCP layer. However, their pore sizes are much larger than that of UF/MF membranes typically used as support for BCP membranes, and their surface structure and chemistry are also significantly different. We notice that polyester nonwoven has been directly used to support BCP membranes produced through the NIPS process in which highly concentrated BCP solutions are involved [31,32]. However, polyester nonwoven has not been used in other two methods (selective etching and selective swelling) because they start from much thinner BCP solutions which easily leak out of the nonwoven. Therefore, it is necessary to have a focused study to reveal the feasibility and possible merits of production of swelling-resultant BCP membranes using nonwoven as the support. To this end, we first machine-cast PS-b-P2VP solutions on the nonwoven fabrics followed by thermal treatment, and cavitate the BCP layers by selective swelling, thus producing composite membranes with nanoporous BCP as the selective layers and macroporous nonwoven as the supporting substrates. Thanks to the controllable machine-casting of BCP solutions on polyester nonwovens, thus-produced BCP composite membranes exhibit excellent permselectivity tunable by altering the swelling temperatures and/or durations. Moreover, we demonstrate that the membranes can be used to fractionate nanoparticles by reducing their size distributions.

2.3. Characterizations The morphological features of composite membranes were examined with a Hitachi S-4800 field emission scanning electron microscope (SEM) operated at 5 kV. In the preparation of the samples for crosssectional examination, the membranes were fractured in liquid nitrogen. Prior to SEM observations, the samples were sputter-coated with a thin layer of platinum to reinforce their conductivity. A contact angle goniometer (DropMeter A100, Maist) was used to measure the dynamic water contact angles (WCAs) of the BCP layers coated on the nonwoven before and after swelling, and the test time was fixed at 3 min. For each sample, at least five randomly chosen locations of the sample surface were measured and the mean value was reported. The size-distribution curves of gold nanoparticles in the feed and filtrate were measured by dynamic laser scattering method (Nano-ZS90, Malvern). 2.4. Filtration tests Membrane coupons with the diameter of 2.5 cm were cut from large membrane sheets and were used for the filtration tests. All the filtration tests were carried out on a stirred filtration cell (Amicon 8010, Millpore) with a 10 mL working volume and an effective membrane area of 4.1 cm2. For the tests of pure water permeability (PWP), each membrane was initially pressurized in deionized water for 10 min at 0.5 bar to ensure a stabilized permeability, and then the PWP was measured at 0.2 bar. The hydraulic permeability of the membrane was determined by the ratio of the volumetric filtrate flux (L m−2 h−1) to the trans-membrane pressure (bar). To measure separation properties, BSA was first dissolved in the phosphate buffered solution at a concentration of 0.5 g L−1, which was then used as the feed solution. Concentrations of the feed and filtrate solutions were measured with a UV–vis absorption spectrometer (NanoDrop 2000c, Thermo Scientific) at ~280 nm. Aqueous solutions of colloidal gold nanoparticles with monodispersed size of 15 nm were also used as the feed solutions to test the retention properties of membranes prepared under different swelling conditions. To eliminate any adsorption effects of gold nanoparticles on the membrane surface, we followed our previous work [33] and conditioned the composite membranes in anionic Acid Orange 7 for 30 min to form a thin adsorption layer on the membrane, making the membrane negatively charged. Therefore, the adsorption of the negatively charged gold nanoparticles on the membrane can be neglected. Gold nanoparticles with polydispersed particle sizes in the range of ~3–20 nm were used to test the size fractionation capability of the membrane prepared by swelling at 60 °C for 3 h. The gold concentrations in feed, filtrate and retentate were also determined with the UV–vis absorption spectrometer at ~520 nm.

2. Experimental section 2.1. Materials The block copolymer of PS-b-P2VP [Mn (PS)=53,000 g mol−1, Mn (P2VP)=21,000 g mol−1, polydispersity index (PDI)=1.17] was obtained from Tubang Polymer Materials Co., Ltd. The polyester nonwoven (E055094-74) was purchased from Suzhou Holykem Automatic Technology Co., Ltd and used as received. Organic solvents including chloroform and ethanol with analytical grade were sourced from local suppliers. The protein, bovine serum albumin (BSA) with the molecular weight of 67 kDa and a negatively charged dye, Acid Orange 7, were purchased from Aladdin Industrial Corporation. Monodispersed gold colloidal nanoparticles with the diameter of 15 nm were obtained from British Biocell International, and polydispersed gold nanoparticles with the size in the range of ~3–20 nm were purchased from Shanghai Huzheng Nanotechnology Co., Ltd. All the chemical reagents were used without further purification. 2.2. Membrane fabrication The PS-b-P2VP BCP was first dissolved in chloroform at a concen202

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Fig. 1. Schematic diagram for the fabrication process of the nanoporous BCP membranes machine-cast on the nonwoven support. The drying step contains solvent evaporation at room temperature and heating treatment at 100 °C.

3. Results and discussion

spaces initially occupied by the expanding P2VP chains are immobilized because of the PS matrix which is in the glassy state. Consequently, bicontinuous porosity is generated in the BCP layer. We note that the BCP-coated nonwoven maintained good structural integrity and no cracks or peeling-off can be observed after the swelling treatment. Meanwhile, the BCP-coated side of the nonwoven exhibited a uniform smooth and shining surface while the opposite side retained the initially dull appearance after the coating, heating, and swelling treatment.

3.1. Preparation of nanoporous BCP membranes on nonwoven supports Fig. 1 shows the schematic diagram for the fabrication process of the PS-b-P2VP composite membranes supported on the nonwoven. The water prefilled nonwoven was first obtained and placed onto the glass plate (Fig. 1a). The 10 wt% PS-b-P2VP solution was then machine-cast onto the nonwoven to obtain a liquid BCP layer (Fig. 1b). After solvent evaporation and air drying, a thin and dense BCP layer was formed on the top of the nonwoven with an area of ~150 cm2 (Fig. 1c), demonstrating the efficiency and scalability of this method. The SEM results present significant differences between the original nonwoven and the BCP-coated nonwoven (Fig. S1). The feature size of the voids in the nonwoven is ranging from 2 µm to 30 µm, and a dense BCP layer can be observed right on the support after drying. The BCP-coated nonwoven was then immersed in warm ethanol for various durations to cavitate the BCP layer by the selective swelling-induced pore generation process. However, early attempts to generate pores were plagued by macroporous cracks in the BCP layer during the swelling process because of the residual stress in the coated BCP layers [30,34]. To address this problem, pretreatment of heating in vacuum at 100 °C for 1 h followed by natural cooling to room temperature was introduced before swelling. During heating, the mobility of the polymer chains are significantly enhanced as the temperature is near the glass transition temperature (Tg) of both PS [35] and P2VP [36] blocks, so as to relax the residual stress [37]. The subsequent cooling minimizes the stress caused by the difference in the coefficients of thermal expansion between the BCP layer and the nonwoven support. Moreover, the heating treatment is also helpful to enhance the adhesion between the BCP layer and the nonwoven support. After such a heating treatment, bicontinuous nanoporosities can be successfully generated in the BCP layer without any cracking after ethanol swelling (Fig. 1d). The cavitation of the BCP selective layer follows the mechanism of selective swelling (magnified illustration in Fig. 1). Upon immersion of PS-bP2VP in ethanol, ethanol diffuses into the BCP layer and is preferentially enriched in the P2VP microdomains as ethanol is a good solvent to P2VP but a nonsolvent to PS. The P2VP microdomains are accordingly swollen and expanded to merge with their neighbors, resulting in a continuous phase distributed in the PS matrix. Upon drying, the swollen P2VP chains collapse with the evaporation of ethanol, while the

3.2. Morphology of nonwoven-supported nanoporous BCP membranes We investigated the morphologies of the membranes prepared under different swelling conditions. Fig. 2a–e show the surface morphologies of the membranes prepared by ethanol swelling at 60 °C for various durations from 15 min to 12 h. As can be seen from Fig. 2a, a swelling duration as short as 15 min is sufficient to introduce pores into the coated BCP layers. These pores are mainly present in two different geometries: circular pores and narrow channels. The coexistence of the two geometries of pores are reminisces of the perpendicularly and in-plane oriented P2VP cylinders embedded in the PS matrix before swelling, because the BCP coating layers were not subjected to any alignment and the P2VP microdomains were randomly distributed in the PS matrix [38]. These two types of pores are present in all membranes prepared under different swelling conditions. Although the pore size is difficult to be accurately determined, there is a clear evident trend that the pores are enlarged with prolonged swelling durations at the same swelling temperature. As can be seen from Table S1, the average pore size of the membranes and the standard deviation (σ) values were obtained by evaluation of minimum 100 nanopores randomly selected in SEM images using the software Nano Measurer (for channel-like pores, the pore width is considered to be the pore diameter) [24]. The average pore size in Fig. 2a is 13.9 nm (σ= ± 3.2 nm) whereas the average pore size of the membrane prepared by swelling at 60 °C for 12 h is 21.2 nm (σ= ± 4.3 nm) (Fig. 2e). In addition, the surface morphology of the BCP layer becomes increasingly rough after longer swelling durations due to a higher condition of swelling, which leads to the drastic volume expansion and overflow of the P2VP chains. A moderate increase of the swelling temperature to 65 °C noticeably enlarges the pore sizes, and at this swelling temperature the increase in pore diameters with swelling 203

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Fig. 2. Surface SEM images of nanoporous BCP layers machine-cast on nonwoven supports after ethanol swelling under different conditions. (a-e) Swelling at 60 °C for 15 min, 1 h, 3 h, 6 h, and 12 h, respectively. (f-j) Swelling at 65 °C for 15 min, 1 h, 3 h, 6 h, and 12 h, respectively.

Fig. 3. SEM images of the membrane prepared by ethanol swelling at 60 °C for 6 h. (a) Large-field view of the membrane surface, (b and c) Cross-sectional view of the membrane. (c) is the corresponding magnified images of the boxed areas in (b).

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change of microstructure and surface chemical constitution after swelling, the advantage of improved wettability came along. Hence, the WCAs of composite membranes before and after the treatment were measured. Fig. 4a presents the typical WCA curves of composite membranes with and without swelling. For the pristine sample (without swelling), an initial WCA of ~87° was measured. This value is close to that of PS homopolymers [39], implying that the surfaces of BCP layers are preferentially enriched with PS blocks after casting. Meanwhile, the WCA barely changes during a measurement period of 1 min (insets in Fig. 4a for the pristine membrane) implying a complete coating layer on the macroporous nonwoven support. However, as for the membranes subjected to swelling at 60 °C/65 °C for 3 h, the initial WCA decreases to ~60° and ~54°, respectively. The results indicate that the surface chemical composition changes due to the enrichment of the hydrophilic P2VP blocks on the surface after swelling, which was confirmed in our previous work by X-ray photoelectron spectroscopy [21]. Furthermore, the water droplet on membrane surface quickly penetrates into the membrane after 1 min time interval (insets in Fig. 4a for the swollen membranes), demonstrating the presence of accessible pores in the BCP layer after swelling. For further comparison, the WCAs for membranes prepared under various swelling conditions were tested. To have a direct comparison for the change of WCA of different membranes, the requisite time for the WCA decreased to 40° of each membrane was recorded. As can be seen from Fig. 4b, the requisite time decreases steadily with the swelling duration for membranes prepared at both 60 °C and 65 °C, as the BCP layers become more porous and more P2VP chains migrate to the surface as the swelling goes on. Meanwhile, the requisite time for the membranes prepared by ethanol swelling at 65 °C is always much shorter than that of the membranes prepared by swelling at 60 °C for the same duration. This can be ascribed to the pronounced pore generation and the drastic increase of surface roughness when the swelling temperature is increased to 65 °C. The hydrophilicity of the BCP layer is enhanced with higher surface roughness according to the Wenzel state [40]. As a shorter requisite time implies that the membrane possesses a better wettability, Fig. 4b also reveals that elevation in the swelling temperature is more effective to increase the wettability of the membrane than the extension of the swelling duration.

durations becomes more pronounced (Fig. 2f–j). For instance, the average pore size is enlarged from 20.4 nm (σ= ± 4.3 nm) to 33.4 nm (σ= ± 8.2 nm) with the swelling durations increased from 15 min to 12 h. By comparing the membranes prepared with the same swelling durations but at different temperatures, it is clear that swelling at 65 °C is always producing larger pores than swelling at 60 °C. This can be easily understood as higher temperatures enhance both the solvation of the P2VP chains and the mobility of the PS matrix in ethanol, leading to higher osmotic pressure accumulated in the P2VP microdomains confined in the PS matrix which has stronger deformability. Consequently, within an identical swelling duration larger pores are generated in the membrane prepared at higher swelling temperatures. Fig. 3a gives a large-field view of the surface of the membrane prepared by swelling at 60 °C for 6 h, and it is clear that the porous morphology with interconnected porosity exists homogeneously throughout the whole area of the BCP surface. Moreover, as can be seen from Fig. 3b, the membrane possesses a double-layered composite structure with a thin, nanoporous top layer supported on a thick, macroporous nonwoven bottom layer. Clearly, the top layer is a swelling-induced PS-b-P2VP layer with a thickness of ~10–15 µm uniformly adhered on the nonwoven. Magnified SEM examinations on the cross section of the membrane show interconnected porosity running from the top surface to the bottom surface, and we do not notice a gradient in porosity along the cross section.

3.3. Surface wettability of nonwoven-supported nanoporous BCP membranes We prepared porous membranes with the BCP selective layers subjected to ethanol swelling under various conditions. With the

3.4. Permselectivity of nonwoven-supported nanoporous BCP membranes As discussed in the last section, the hydrophilic surface, the porous structure, and the increasing roughness contribute to improve the surface hydrophilicity of the membranes after swelling, which is helpful to improve the water permeation. Herein, the permeability of the

Fig. 4. (a) Dynamic water contact angles of the BCP membranes prepared under different swelling conditions. (b) The requisite time for water contact angles of the BCP membranes dropped to 40°.

Fig. 5. The pure water permeability of membranes prepared by swelling in ethanol at 60 °C and 65 °C for various durations.

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membranes prepared under various swelling conditions is given in Fig. 5. Note that prior to swelling the BCP membrane was impermeable to water under 0.5 bar, indicating the nonporous and dense nature of the as-coated BCP layers. In contrast, the membrane shows a slight PWP of 33 L m−2 h−1 bar−1 after swelling at 60 °C for only 15 min, which implies formation of accessible pores in the BCP layer allowing the permeability of water throughout the membrane. After swelling for 1 h, the PWP is increased to 88 L m−2 h−1 bar−1. As expected, with extending the swelling duration from 3 through 6 to 12 h, the membrane exhibits enhanced permeability from 152 through 202 to 309 L m−2 h−1 bar−1, respectively. Impressively, remarkable increase in the PWP is observed when the swelling temperature is increased to 65 °C. For instance, The PWP is greatly increased to 106 L m−2 h−1 bar−1 after ethanol swelling at 65 °C for 15 min. The PWP is continuously increased to ~850 L m−2 h−1 bar−1 when the swelling at 65 °C was prolonged to 12 h. Clearly, the PWP curves display an increasing tendency with the extension of swelling duration, which coincides with WCA results. However, the increase in PWP is not linear with the swelling duration. As can be seen in Fig. 5, PWP is increased faster in the first 1 h and 3 h for the membrane prepared by swelling at 60 °C and 65 °C, respectively, and continues to increase but at a lower rate. The nonlinear increase in PWP with swelling durations is believed to be due to the opposite effect of water diffusibility and membrane thickness. In the initial stage of swelling, water diffusibility through the membrane is playing the dominated role in determining PWPs because the pore size and surface hydrophilicity are dramatically increased. With prolonged swelling, the surface hydrophilicity remains almost unchanged and the enlargement in pore size also turns to be less pronounced. Thus, a tradeoff effect would appear as the increased membrane thickness would take the leading role because of greater mass-transfer resistance, resulting in reduced increase in PWP. Furthermore, membranes prepared by swelling at 65 °C present larger pore sizes compared to membranes prepared by swelling at 60 °C and the PWP is proportional to the pore diameter. Therefore, the composite membranes subjected to swelling at a higher temperature require a longer swelling duration to achieve the shift point. We then investigated the separation performance of the machinecast BCP membranes fabricated under different swelling conditions. BSA and monodispersed gold nanoparticles with the size of 15 nm were chosen as two model materials to probe the size-sieving separation capability of the membranes. As can be seen from Fig. 6, all these membranes exhibit modest retention rates ranging from 50% to 10% to BSA, and the retention rate declines with the increase of both swelling durations and swelling temperatures as a result of enlarged pore sizes, which is opposite to the change of PWP as discussed above. In contrast, when 15-nm gold nanoparticles were used, the membranes prepared by swelling at 60 °C for various durations exhibit a similar retention of ~95%. Such a high retention to 15-nm nanoparticles reveals that the membranes are free of defects and the effective pore size of the membranes is lower than 15 nm. As shown in Fig. 2 the pore sizes characterized by SEM are typically larger than 15 nm, however, the effective pore sizes for the membranes used in aqueous systems are expected to be reduced because of the swelling of P2VP chains enriched on the pore wall in water. The BSA protein is reported to be in a ellipsoidal shape with the size of 14 nm×3.8 nm×3.8 nm [41], while the shape of the gold nanoparticles are in the spherical shape. During the rejection tests, the isotropic gold nanoparticles are more effective to be intercepted by the bicontinuous pores in the BCP layers, while the BSA molecules have more chance to percolate through the pores by changing their orientation toward the pores [42], leading to modest BSA retentions. For the membranes prepared by swelling at 65 °C the retention to 15-nm gold nanoparticles is decreased from ~93% to ~54% when the swelling duration is increased from 15 min to 12 h as a result of increased pore sizes. It is worth noting that the swelling duration increased from 15 min to 12 h leads to decreased retention to 15-nm gold nanoparticles (< 50%), however, this decrease in retention

Fig. 6. The retention rates to BSA and 15-nm gold nanoparticles of the membranes prepared under various swelling conditions.

Fig. 7. The permeability of pure water and the solution containing 15-nm gold nanoparticles across the membrane prepared by swelling at 60 °C for various durations.

is greatly paid back by remarkable increase in the PWP for nearly 8 times (Fig. 5). Moreover, Fig. 5 and 6 suggest that adjustable separating properties in wide ranges can be achieved simply by altering the swelling conditions including both swelling durations and temperatures. The permeabilities of the membranes prepared by swelling at 60 °C for different durations to solutions of 15-nm gold nanoparticles were also recorded and compared with their pure water permeabilities. As can be seen from Fig. 7, the permeability of gold solutions is approximately 12–40% lower than PWP for all the membranes. Considering that gold nanoparticles hamper the transport of water across the membrane pores such reduction in permeabilities is acceptable.

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Fig. 8. The pressure-dependent water flux of the membrane prepared by ethanol swelling at 60 °C for 6 h.

Fig. 10. Nanoparticles fractionation with the membrane prepared by swelling at 60 °C for 3 h. (a) The UV–vis spectra of the feed, filtrate, and retentate of polydispersed gold nanoparticles. Inset shows the picture of the three solutions displaying various color appearance. (b) The size distributions of the feed and filtrate solutions collected from the size-fractionation of ~3–20 nm gold nanoparticles. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

Fig. 9. The time-dependent water permeability of composite membranes prepared by ethanol swelling for 6 h at 60 °C and 65 °C, respectively. Insets are cross-sectional SEM image of the corresponding membranes.

pressure-dependent and time-dependent PWP results indicate that thus produced composite membranes possess a reasonably good mechanical stability and durability.

3.5. Mechanical stability of nonwoven-supported nanoporous BCP membranes

3.6. Fractionation of nanoparticles by nonwoven-supported nanoporous BCP membranes

As the nanoporous BCP layers are only about ten micrometers in thickness and the nonwoven supports possess pores with sizes up to tens of micrometers we should demonstrate that thus produced composite membranes are sufficiently strong for real applications. We first tested the pressure-dependent PWPs of the membrane prepared by ethanol swelling at 60 °C for 6 h. As shown in Fig. 8, the PWP is linearly increased with the trans-membrane pressure, and neither significant drop nor abrupt increase of the PWP is observed when the pressure rises up to 2.5 bar. It is obvious that the good resistance to pressures should be attributed to the robust nonwoven substrate as well as the interconnected porosities in the BCP layer which help to dissipate stresses. The influence of test duration on the PWP behavior was also investigated with results given in Fig. 9. As for the membrane prepared by swelling at 60 °C for 6 h, the PWP shows no noticeable drop within 1 h of filtration test. However, for the membrane prepared by swelling at 65 °C for 6 h the PWP is decreased by about 20% after filtration for the first 20 min. The permeability decline is mainly caused by the compaction of the membrane. The membrane prepared at higher swelling temperature possesses larger pore sizes and higher porosities (insets in Fig. 9), and therefore, tolerate pressures to a less degree. However, the PWP is soon stabilized with continuing filtration and maintains at high level of ~710 L m−2 h−1 bar−1. Therefore, the

Two general strategies have been employed to produce size-uniform nanoparticles during the practical synthesis process. One method is the direct particle size management during the synthesis by adjusting fabrication parameters [43], the other is the post-fractionation with the membrane from the polydispersed nanoparticle samples [44]. By taking advantage of the relatively narrow pore size distribution in the BCP layer, the composite membranes may be used for the size fractionation of nanoparticles. As demonstrated in the filtration of protein solutions and gold nanoparticles, the composite membranes showed a tunable size-sieving performance. Furthermore, we explored their applications in the fractionation of nanoparticles. A colloidal solution of polydispersed gold nanoparticles with sizes ranging from ~3 to 20 nm was used to challenge the membrane prepared by ethanol swelling at 60 °C for 3 h. Fig. 10a displays the UV–vis spectra as well as the color appearance (inset) of the feed, filtrate, and retentate involved in the fractionation test. A moderate absorption peak around 530 nm can be observed in the UV–vis spectrum of the feed solution and the peak is intensified in the spectrum of the retentate, while an ignorable peak exists in the spectrum of the filtrate. These results indicate that the membrane exhibits a certain retention to the particles. The difference in color appearance of the three solutions also directly reveals a noticeable 207

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retention of the polydispersed gold nanoparticles. Note that both the absence of the peak and colorless appearance in the filtrate do not necessarily imply that the retention solution is free of any gold particles, as the gold particles with sizes lower than 10 nm do not give a characteristic adsorption peaks because of their weak plasmonic effect. To further investigate the existence of the particles in the filtrate, the filtrate solution was tested by dynamic laser scattering. As shown in Fig. 10b, the feed solution contains nanoparticles with a wide size distribution in the range of ~3–20 nm, which is confirmed by the curve of the feed. A small amount of particles with the size of ~30 nm can also be observed due to the aggregation of some small gold nanoparticles. However, a much narrow size distribution ranging from ~3 to 10 nm was measured in the filtration, whereas the large particles are repelled. After filtration, the mean particle size is reduced from ~7 nm in feed to ~5 nm in the filtration. The results imply the membrane is able to narrow down the size distribution of nanoparticles, and a more precise fractionation is expected by using a series of membranes with tunable effective pore sizes. 4. Conclusions In summary, we explore the fabrication of nanoporous BCP composite membranes via mechanically casting amphiphilic BCP solutions over robust nonwoven supports. Solvent evaporation followed by swelling-induced pore generation results in nanoporous size-sieving layers with the thickness of about ten micrometers tightly adhered to nonwoven supports. Changes in temperature and duration during the selective swelling allow flexible regulation of porous structures as well as surface properties, leading to tunable separation performances. The composite membranes are also demonstrated to be mechanically stable and durable in pressurized filtrations. Moreover, the membranes can efficiently fractionate polydispersed nanoparticles to narrow down their size distribution. This convenient and controllable strategy offers the possibility for the production of composite nanoporous BCP membranes with good performance, low cost, and excellent upscalability. Acknowledgements Financial support from the National Basic Research Program of China (2015CB655301), the Jiangsu Natural Science Foundation (BK20150063), and the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) is gratefully acknowledged. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.memsci.2017.03.046. References [1] M. Elimelech, W.A. Phillip, The future of seawater desalination: energy, technology, and the environment, Science 333 (2011) 712–717. [2] C. Charcosset, Membrane processes in biotechnology: an overview, Biotechnol. Adv. 24 (2006) 482–492. [3] E. Gultepe, D. Nagesha, S. Sridhar, M. Amiji, Nanoporous inorganic membranes or coatings for sustained drug delivery in implantable devices, Adv. Drug Deliv. Rev. 62 (2010) 305–315. [4] H.J. Kim, K. Choi, Y. Baek, D.G. Kim, J. Shim, J. Yoon, J.C. Lee, High-performance reverse osmosis CNT/polyamide nanocomposite membrane by controlled interfacial interactions, ACS Appl. Mater. Interfaces 6 (2014) 2819–2829. [5] W.Z. Li, X. Wang, Z.W. Chen, M. Waje, Y.S. Yan, Carbon nanotube film by filtration as cathode catalyst support for proton-exchange membrane fuel cell, Langmuir 21 (2005) 9386–9389. [6] G.P. Liu, W.Q. Jin, N.P. Xu, Graphene-based membranes, Chem. Soc. Rev. 44 (2015) 5016–5030. [7] X. Zhao, Y. Su, Y. Liu, Y. Lip, Z. Jiang, Free-standing graphene oxide-palygorskite nanohybrid membrane for oil/water separation, ACS Appl. Mater. Interfaces 8 (2016) 8247–8256.

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