Symmetrical Permeable Membranes Consisting of Overlapped Block

Apr 22, 2016 - Meimei Zhou , Yi-nan Wu , Pingping Luo , Jiqiang Lyu , Dengrui Mu , Aowen Li , Fengting Li , Guangtao Li. RSC Advances 2017 7 (78), 495...
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Article pubs.acs.org/Macromolecules

Symmetrical Permeable Membranes Consisting of Overlapped Block Copolymer Cylindrical Micelles for Nanoparticle Size Fractionation Zhuan Yi,†,‡ Pei-Bin Zhang,† Cui-Jing Liu,† and Li-Ping Zhu*,† †

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China ‡ Department of Ocean, Zhejiang University of Technology, Hangzhou310014, China S Supporting Information *

ABSTRACT: Free-standing symmetrical porous membranes consisting of uniform cylindrical micelles were prepared from a polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymer following a process combining self-assembly and nonsolvent-induced phase separation (SNIPS). The fabricated membranes displayed an impressive ultrastructure by the overlapping of the cylindrical micelles throughout the whole membrane profile (∼65 μm). The effects of copolymer molecular weight and membrane-formation condition on membrane morphology were investigated and discussed in detail. It was concluded that the cylindrical micellar membranes could be obtained in a wide window of fabrication conditions as low molecular weight of PS-b-P4VP was used as the raw material. The permeation tests indicated that the water permeability of the symmetrical membranes reached 400 L/(h m2 bar) in neutral condition, which was comparable to that of the block copolymer (BCP) isoporous membrane reported in the literature. Significantly, the permeation and separation properties of the symmetrical membranes exhibiting a strongly pH-dependent character as the pH of feed solution varied between 1 and 6. This phenomenon was attributed to the pH-responsive conformational change of P4VP corona in the micellar aggregates of BCP. The potential application of the developed membranes in the size fractionation of nanoparticles with a wide size distribution was demonstrated.

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INTRODUCTION In the 1960s, Loeb and Sourirajan developed a method to prepare artificial permeable membranes that show high efficiency in seawater demineralization, and the process by immersing the cast films of polymer solution into nonsolvent is latterly known as the L−S phase inversion or the nonsolventinduced phase inversion (NIPS).1−4 In the following decades the NIPS has been one of the most important membranemanufacturing techniques that contributes greatly to the development of membrane technology. Actually, many permeable materials used in pervaporation, gas separation, reverse osmosis, and ultrafiltration (UF) are prepared by the NIPS process.5−10 In membrane separation, high permeability is desirable under an adequate selectivity to achieve high separation efficiency. However, the NIPS process with a typical polymer (i.e., commercially available poly(vinylidene fluoride) (PVDF), polysulfone (PSF), etc.) often leads to a wide pore size distribution and poor control on membrane structure. As a result, it is difficult to reach the optimization between the selectivity and permeability.11,12 In 2007, Peinemann and Abetz reported the preparation of block copolymer (BCP) membranes having long-range ordering pore arrays through the combination of the self-assembly and nonsolvent-induced phase inversion (so-called SNIPS process).13−17 The ordered isoporous morphology in these membrane is advantageous to realize high permeability and high separation accuracy simultaneously due to its monodisperse pores. In a SNIPS © XXXX American Chemical Society

process, the ordered structure formed in self-assembly of block copolymer was trapped by the subsequent NIPS. Because of the high efficiency and the facility, recently the SNIPS has attracted much attention in the preparation of isoporous membranes. In membrane fabrication by SNIPS, the organization of spherical micelles into cylinders during solvent evaporation is a key step for the formation of isoporous membrane. The isopores are perpendicular to membrane surface due to the evaporation induced assembly and the solvent concentration gradient.13,17−19 Various factors including the chemical composition, the molecular weight, and the aggregate strength of BCP as well as the preparation conditions (e.g., the BCP concentration, the evaporation time, and the additives) have impacts on the formation of isopores. In general, isoporous membrane can be successfully obtained only in a very narrow window involving in a good match of above-mentioned factors.20−22 To our knowledge, it remains an empirical experience to prepare isoporous membranes from new BCPs that are different in molecular structure and physiochemical characters from that of well-studied copolymers. However, Phillip and co-workers predicted recently that the growth of isopores during the membrane formation was a competitive result of the kinetics and thermodynamics.23 In their work, the Received: January 24, 2016 Revised: April 5, 2016

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Figure 1. SEM morphologies, XPS N1s spectra, and water contact angles of the membranes prepared from pure DMF, pure THF, and their mixed solvent (50:50 wt %). Polymer concentration and solvent evaporation time were 24 wt % and 60 s, respectively.

Theoretically, the growth of cylinders in SNIPS process can be facilitated by various methods including (1) prolong the solvent evaporation time, (2) decrease the viscosity of polymer solutions, (3) reduce the molecular weights of BCP, and (4) combine the methods showed above. In comparison with other methods, reducing the molecular weights of BCP is a three-inone design including decrease of the aggregate strength of BCP, enhancement of the chain relaxes, and improvement of molecular mobility. Especially, longer solvent evaporation time contributes positively to the block copolymer selfassembly and the full growth of cylindrical micelles. By the design of BCP molecular characteristics and the optimization of fabrication parameters, here we demonstrate the fabrication of symmetrical PS-b-P4VP micellar membranes with interconnected pores. Because of the facility of SNIPS, the manufacturing technique of symmetrical membrane developed in this work is convenient to be scaled up. Compared to electrospun fiberous membrane, the cylindrical micellar membrane is easier to be fabricated with high efficiency and in large scale. This symmetrical membrane shows the strong potential to be used as a substitute of electrospun porous membranes.29−31 The objective of this work is to fabricate a highly permeable and wholly symmetrical PS-b-P4VP cylindrical micellar membrane with interconnected pores by SNIPS process. The effects of BCP molecular weight and formation conditions on membrane morphology were investigated in detail, and the preparation window of symmetrical micellar membrane was briefly discussed. The pH-dependent water permeability and the applications in nanoparticle size fractionation of the developed membranes were finally demonstrated.

perpendicular orientation (i.e., isoporous) morphology of BCPs was observed as the solvent was evaporated fast, while parallel orientation was obtained in a slow solvent evaporation. Although the mechanism suggested by Philip et al. was concluded from the isopore formation for polystyrene-bpoly(lactide) (PS-b-PLA), it is believed that the same mechanism is also responsible for the isopore formation of other BCP systems such as polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP), which displays a stronger aggregation strength than that of the PS-b-PLA. Micellar porous membranes have attracted much attention because of their strong hydrophilicity, tunable porosity, stimuliresponsiveness, and controllable swelling/deswelling behaviors, and the membranes with these properties hold the great promise to be applied in precise separation, controlled drug delivery, etc.24−27 In diverse micellar membranes, fibrous membranes consisting of cylindrical micelles overlapped each other are rather favorable since they can be used as ideal templates for nanofabrication, the substrates for cells growth, and the permeable membranes. As far as we know, in comparison with the extensive attentions on preparation of isoporous membrane, rare work has been done on the fabrication of fiberous micellar membranes by SNIPS route. Recently, Nunes and Schacher have observed that well-defined cylinders were generated in membrane fabrication via SNIPS process.16,28 However, in their work, the cylinders can be formed only on either the upper or the bottom surface of the membranes. The deepness of cylindrical assemblies was less than 200 nm. The bulk structure of their membranes was created by the typical NIPS process and different from the assembled surface skin layer. A wholly symmetrical structure with interconnected pores consisting of cylindrical micelles overlapped each other has not been achieved by the welldefined SNIPS process as far as we know.

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RESULTS AND DISCUSSION Formation of Cylindrical Micelles in Mixed Solvents. BCPs are hybrid macromolecules consisting of chemically B

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Figure 2. Upper (u) and bottom (b) surface SEM images of PS-b-P4VP membranes fabricated with various solvent evaporation time. Mixed solvent: DMF/THF = 50/50 wt %; polymer concentration: 24.0 wt %.

displays the determining impact on BCP self-assembly and membrane formation. Although cylindrical and spherical aggregates appeared in membrane showing different geometry, they displayed a similar diameter of 55−60 nm. The surface chemistry of the membrane prepared from different solvents was analyzed by XPS, and the self-assembly of BCP was further investigated (Figure 1). We can see that the nitrogen concentrations on cylinders and nanospheres are 4.2 and 4.7 atom %, respectively. This results show that the cylindrical and spherical aggregates have not only an approaching diameter but also a similar surface composition. The nitrogen concentration on the membrane prepared from THF is only 2.7 atom %, which is much lower than that of cylinders and nanospheres. The value is close to that of the theoretical composition (∼2.5 atom %) of PS-b-P4VP-1 (the details of PS-b-P4VP-1 synthesis and characterization are shown in the Experimental Section and the Supporting Information), suggesting that the self-assembly is kinetically trapped in cast films. As we know, the P4VP chains are the sole source of nitrogen, the high nitrogen concentration detected on the membranes prepared from DMF and DMF/THF mixed solvent indicates that the polar P4VP block has segregated and organized onto the outer surface of micelles during micellation. This explanation is further supported by the contact angle (CA) measurements. The membranes prepared from DMF and mixed solvent show the CAs lower than 60°, but the membrane prepared from THF has a CA of 86 ± 1.7°, which is close to the intrinsic CA of polystyrene homopolymer.36 The good solubility of P4VP blocks in DMF and the affinity to precipitate (H2O) is considered to be the reason responsible for the organization of P4VP blocks onto the corona of BCP micelles during membrane formation. Solvent Evaporation Induced Cylinder Formation. The exposure in air to allow solvent evaporation prior to precipitation has been developed as an effective tool to tune the self-assembly of BCP in cast film. The isopores generated from SNIPS has been ascribed to the reason of solvent

different homopolymers which were connected by covalent or pseudocovalent linkages.32 Generally, they can be processed into well-defined nanoarrays of microdomains including spheres, cylinders, or lamellae by self-assembly, depending on the composition and segregation strength (incompatibility) of different blocks. The self-assembly of BCPs makes them as attractive candidates for nanolithography, heterogeneous catalysis, templates, high performance separation membranes, and many other critical applications.33,34 Amphiphilic di-BCP PS-b-P4VP have been developed as an ideal material that can be fabricated into isoporous membrane through different ways. In these methods, the pore generation by swelling and the SNIPS have attracted the most attentions since of their direct pore generation and nondestructive nature.17,25 In the SNIPS process of the PS-b-P4VP as membrane material, the mixed solvents of DMF and THF (or dioxane) are a sophisticated designing that makes the self-assembly of BCP controllable due to controlled evaporation of THF from cast films.21,22,35 In this work, the effect of solvent properties on membranes formation was first investigated. The cast films were exposed in air to allow solvent evaporation for 60 s before immersion in coagulation bath. During the solvent evaporation, the selfassembly of BCP has been activated. As shown in Figure 1, when pure DMF was used as the solvent, the as-prepared membranes showed typical spherical morphologies, and uniform nanospheres were clearly observed. However, only a dense and flat surface was obtained when the membranes were prepared using pure THF as solvent. It is easy to learn from these observations that the solvent property has played a key role in the assembly of BCP. Actually, chain relax and organization of BCP in films cast from THF were kinetically trapped due to the rapid evaporation of solvent, and the film became solidified before it was immersed into precipitate.22 However, when membranes are cast from the mixed solvent of DMF and THF, the morphologies become entirely different, and rod-like aggregates with length of 200−1200 nm are clearly observed. This is another evidence that the solvent property C

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Figure 3. Cross-sectional morphologies and XPS N1s spectrum of the assembled membranes (mixed solvent: DMF/THF = 50/50 wt %, solvent evaporation time of 60 s): (a-1) the local cross section close to the upper surface; (a-2) the local cross section close to the bottom surface; (a-3) the local cross section in the membrane bulk; (b) the whole cross section, (c) TEM image of the local cross section. (d) XPSN1s spectrum of the bottom surface.

Figure 4. Surface SEM images of the cylindrical micellar membranes fabricated from various DMF/THF mixed solvent (DMF/THF = 60/40 or 40/ 60 w/w). The BCP concentration in the casting solutions is 24 wt % unless otherwise specified. Characters of U and B denote the upper and bottom surface, respectively.

evaporation induced self-assembly.23 Figure 2 presents the upper and bottom surface morphologies of the membranes prepared with different evaporation times. We can see that with the prolonging of the solvent evaporation time the morphology of upper surface changed from typical NIPS pores to uniform cylindrical aggregates. The minimum evaporation time leading to cylinders is in range of 45−60 s. This phenomenon indicates that the solvent evaporation induced assembly is also responsible for the generation of cylinders. In this work, we have found that a fully symmetrical crosssectional morphology can be obtained by the tuning of solvent nature and evaporation time. As show in Figure 3, besides having the typical cylindrical micellar structure, the membranes prepared from DMF/THF mixed solvent exhibit a completely network ultrastructure by the overlapping of the cylindrical micelles throughout the whole membrane (thickness of about 65 μm). To the best of our knowledge, this membrane structure has never been reported for the membranes prepared from SNIPS so far. This characteristic is substantially different from that of the isoporous membrane, implying that the SNIPS can be designed as a powerful method to fabricate assembled

membranes with different structures. In published literatures, the isoporous membrane morphology is readily generated on membrane surface layer in a short solvent evaporating time less than 20 s, and the finger-like macrovoids are often observed in the sublayer of isoporous membranes prepared by SNIPS.13,20,21 The lower molecular weight of BCP and longer evaporation time contribute to the formation of cylindrical micelles and symmetrical structure. It is worth noting that the micellar structure in this work as shown in Figure 2 is rather stable, and the micellar morphology will remain interact when evaporation time is prolonged to 120 s. This indicates that the symmetrical cylindrical membrane morphology can be created in a broader time window than that of the isoporous membranes. Compared to the morphology of membrane bulk, that of the upper and bottom surfaces was more affected by the interface effect. Thus, from Figure 3, we can observe that the orientation of the micelles in the near surface (100−150 nm of deepness) is parallel to the surfaces (a-1 and a-2), while cylinders in the bulk of membrane are isotropy (a-3). Figure 3c,d shows the TEM image of the local cross section and the XPS N1s spectrum of D

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finding is rather appealing. Generally, it is believed that the cylindrical micelles grow through a nucleation/growth mechanism. The nucleation begins at the free surface first and then propagates into membrane bulk.19,23 Once the free BCPs on solution film surface was exhausted, it became impossible for the further growth of cylindrical micelles into larger diameters. As a result, the prolonging of the solvent evaporation time contributed slightly to the diameters of cylinders, but it would contribute to the formation of symmetric membranes. Compared to the solvent evaporation time, the mixed solvent composition had a quite obvious impact on the cylinder diameter. The diameter increased with the THF fraction in the mixed solvent. It is believed that the stretching of PS blocks in THF (good solvent for PS) during micellation process is responsible for the larger diameters of micelles. This result also indicates that the self-assembly of BCP and the formation of cylinders were accomplished during the solvent evaporation period, and the subsequent NIPS process in nonsolvent only played a role of freezing the assembled cylinders. This phenomenon also allows us to conclude that the formation of cylindrical micelles in membrane is a result of solvent evaporation induced self-assembly. Discussions on Cylinder Formation and Growth of Symmetric Structure. It has been theoretically predicted that cylinders will be generated from the prolonging of solvent evaporation time in the preparation of isoporous structure via SNIPS.16,19 To verify this prediction, a high molecular weight of PS-b-P4VP-2 was used to fabricate similar micellar membranes following the same SNIPS procedure. The surface SEM images of the obtained membranes are shown in Figure 5. As shown in Figure 5a, a typical isoporous membrane was prepared under optimized conditions. This result indicates this PS-b-P4VP-2 is potential to be processed into the cylindrical micelles by the evaporation induced self-assembly. As the solvent evaporation time increased to 60 s, the cylinders were observed in spite of their poor quality. The result was also found from an enhanced chain relax in a lower BCP concentration which led to a higher quality of cylinders, as shown in Figure 5b-1. In combination of Figures 5b and 5b-1, we can see that the formation of isoporous morphology is a dispensable process for the growth of cylinders. Moreover, in the present work, PS-b-P4VP-1 cannot be processed into an isoporous structure under all the investigated conditions, implying that the formation of symmetric membranes consisted of cylinderical micelles does not require the copolymer to be assembled into isoporous membranes. From Figure 5, we find that compared to PS-b-P4VP-1, it is difficult to obtain well-grown cylinders for the PS-b-P4VP-2. This can be attributed to the higher molecular weight and thus the lower chain mobility of PS-b-P4VP-2. The lamellar structure observed on the membranes cast from the 30 wt % of solution demonstrates the effect of concentration and solution viscosity on polymer assembly. In the films cast from solutions with higher viscosity, the solvent evaporation was slower and the concentration gradient was not fully induced. As a result, the membranes prepared by immersion precipitation displayed a typical asymmetrical structure similar to that prepared by the NIPS process. In contrast, the BCP (PS-bP4VP-1) with smaller molecular weight and lower viscosity was readily fabricated into symmetrical micellar morphology by SNIPS. Compared to the PS-b-P4VP copolymers used for the preparation of isoporous membrane reported in the literature,14−18,21,34,37 the molecular weight of the BCP used in this

the bottom surface. The TEM image displays an obvious phase contrast between I2 stained P4VP blocks (cylinder corona) and PS blocks (cylinder core) that tend to aggregate into the core of micelles. The morphological contrast consists well with the structure observed by SEM. The nitrogen concentration on the bottom surface is only 3.4 atom %, less than that on the upper surface (4.3 atom %), indicating that less P4VP blocks are segregated onto the bottom than onto the upper surface. This phenomenon can be attributed to the solvent concentration gradient which propagates from the upper to bottom surface during self-assembly. Although the nitrogen concentration on the bottom surface is lower than that on the upper surface, it is higher than that in PS-b-P4VP. This result demonstrates again the self-assembly of P4VP on both the upper and the bottom surfaces. Effects of BCP Concentration and Mixed Solvent Composition on Cylinder Formation. From the results showed above, a macrovoid-free symmetrical membrane of PSb-P4VP cylindrical micelles can be fabricated by a typical SNIPS process. It will become rather useful if this membrane can be fabricated from a wide preparation window. The solvent property and the polymer concentration generally show great effects on the self-assembly and membrane formation. As shown in Figure 4, similar cylindrical micellar membranes are obtained as the mixed solvent DMF/THF ranged from 40/60 to 60/40 (w/w) (cross-sectional SEM images are shown in the Supporting Information). However, as the DMF fraction increased to 70 wt %, the membrane formation became difficult and BCP dispersed into precipitate. So the DMF fraction in the mixed solvent should be not higher than 70 wt % to obtain cylindrical micellar membrane for our designed copolymer. The polymer concentration has also displayed a great influence on the growth of cylindrical micelles. The membranes can be conveniently prepared from a a higher concentration of 27 wt %. However, as the polymer concentration was raised to 30 wt %, cylinders growth was constrained and some defects were found on membranes (the SEM images of the membranes prepared from higher polymer concentrations are shown in the Supporting Information). So the polymer concentration in casting solution should be lower than 30 wt % for the preparation of defect-free micellar membrane. Poor formation of cylinders in membranes cast from polymer solutions with higher concentration can be attributed to the hindrance of BCP self-assembly in concentrated solutions. The diameters of cylinders prepared at different conditions were compared, and the results are shown in Table 1. It was found that the evaporation time had shown little influence on diameters of cylinders once assembly was completed. This Table 1. Effects of Mixed Solvent Compositions and Solvent Evaporation Time on Diameters of Assembled Cylinders diameters of cylindrical micelles (nm) mixed solvent DMF/THF (w/w) a

60/40 50/50a 40/60a 50/50b a b

60 s 44.8 52.7 60.2 50.7

± ± ± ±

100 s 5.2 4.4 5.2 3.4

46.4 52.3 62 51.2

± ± ± ±

4.5 4.6 7.2 1.9

120 s 47.2 50.9 63.8 49.8

± ± ± ±

5.1 5.1 7.2 2.3

Membranes prepared from BCP concentration of 24 wt %. Membranes prepared from BCP concentration of 27 wt %. E

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Figure 5. Surface SEM images of the PS-b-P4VP-2 membranes prepared in various conditions.

Figure 6. (A) Surface SEM image of the cylindrical micellar membrane, (B) the size distributions of Au NPs in the feed and the filtrate at pH 6, (C) pictures of the feed (1), the filtrate (2, at pH 6), and the filtrate (3) at pH 2, (D) water fluxes at various pH, (E) the size distributions of Au NPs in the feed and the filtrate at pH 2, and (F) UV spectra of the feed and the filtrate.

considering the >60 μm of thickness of the free-standing membranes. The theoretical water permeability would get to 4000 L/(h m2 bar) if the thickness of symmetric membrane decreases to 6.0 μm. Actually, the thickness of most fibrous membranes reported in the literature are only several micrometers, even less than 1 μm, and the high permeability of this membrane are obtained with an expense of the mechanical integrity.40 Obviously, the high water permeability of the cylindrical micellar membranes with free-standing property are attributed to their high porosity, well wettability, and interconnected pore structure. The effective pore size of the membranes was determined by the filtration experiments of polydisperse gold nanoparticles (Au NPs). The results indicate that the membrane reject the Au NPs with averaged diameters larger than 25.4 nm at neutral condition, indicating the effective pore size of the micellar membrane is about 25.4 nm (Figure 6B). The effects of the pH of feed solution on the permeation and separation properties of the assembled membrane were further investigated. The results show that as the pH of the feed changed from 6 to 1, the water permeability decreased drastically to about 10 L/(h m2 bar), as shown in Figure 6D.

work is the lowest. This allows us to conclude that the BCP with lower molecular weights and stronger mobility is beneficial for the preparation of symmetrical membrane with interconnected porous structure consisted of cylinderical micelles. Water Permeability and pH-Mediated Size Separation. In this work, the interconnected porous structure by random overlapping of cylinders endows the symmetrical membranes with a great potential to be used as separation material. The membrane prepared from 24 wt % of PS-b-P4VP1 solution (DMF/THF (w/w) = 50/50, solvent evaporation time of 60 s) was used as a typical sample to evaluate the permeation and separation properties of the assembled micellar membranes. The results are shown in Figure 6. The porosity was evaluated by the water adsorption and the density analysis methods, respectively, and they both showed the porosity ranging from 52 to 55%, demonstrating that the pores in the membrane are well accessible for water permeation. The permeation tests showed that the pure water permeability of the membranes reached 400 L/(h m2 bar) due to the porous and strongly hydrophilic characteristics. This permeability is comparable to that of isoporous films prepared by the solvent or thermal annealing,38,39 and this value is rather impressive F

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It is worth noting that the responsiveness is reversible. The effective pore size of the membrane decreased sharply to about 2.3 nm as the pH of the feed decreased to 2 (Figure 6E). This result shows that the separation ability of the membranes prepared in this work can transform from ultrafiltration to nanofiltration by the adjustment of the feed pH. The pHsensitive permeation and separation characters can be attributed to the swelling and stretching of P4VP blocks in acidic conditions.41 This result demonstrates that the developed cylindrical micellar membranes with strong pH-responsiveness can find their applications in the controlled and continuous fractionation of nanoparticles by the online regulation the pH of feed solution. As far as we know, similar applications has not been reported for the fibrous membranes prepared by deposition of nanostrands or nanofibers.40,42 In summary, the unique ultrastructured membranes consisting of the BCP cylindrical micelles throughout the whole profile reported in this work is really different from isoporous membranes prepared by the well-defined SNIPS process, demonstrating that novel assembled membranes different from isoporous structure can be created by the SNIPS. Compared with the perpendicular orientation of cylinders to membrane surface in an isoporous membrane, the orientation of the cylindrical micelles in the symmetrical BCP membranes here is isotropy in bulk. However, our membrane is the first example that the symmetric assembled membrane has been prepared by the SNIPS, from which the integral asymmetric membranes were frequently generated before. The homogeneous interconnected pore structure brings the symmetrical cylindrical micellar membranes high water permeability. The use of relatively low molecular weight of PS-b-P4VP as the raw material and longer evaporation time are critical in membrane structure control, and the cylindrical micellar membranes can be made in a wide fabrication window.

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EXPERIMENTAL SECTION

Materials and Reagents. Tetrahydrofuran (THF) and dimethylformamide (DMF) were purchased from Sinopharm Chemical Reagent Co., Ltd. The trace water in THF and DMF was removed by vacuum distillation. The gold nanoparticles dispersed in aqueous solution was purchased from Shanghai Huzheng Nano Technology Co., Ltd and diluted before application. PS-b-P4VP was synthesized through a well-defined reversible addition−fragmentation chain transfer polymerization, the process of which was showed previously in the literature.43 Details of polymer characterization and synthesis are shown in the Supporting Information. Molecular weights and compositions of PS-b-P4VP applied are summarized in Table 2. PS-b-

Table 2. Molecular Characteristics of PS-b-P4VP Used in This Work di-BCPs

Mna (kg/mol)

Mnb (kg/mol)

PDI

Weight fraction of P4VP (%)

PS-b-P4VP-1 PS-b-P4VP-2c

40.2 −

62.6 100

1.20 1.10

20.6 25

a

Molecular weight was determined by size-exclusion chromatography with the DMF as the eluant and monodisperse PMMA as the calibration standard. bMolecular weights calculated from nuclear magnetic resonance spectra. cPurchased from Polymer Source Corp.

P4VP-1 was used to fabricate the cylindrical micellar membranes normally, and PS-b-P4VP-2 was used as a control polymer to study the effect of molecular weight on membrane formation. Membrane Fabrication. PS-b-P4VP assembled membranes were fabricated following a typical SNIPS procedure. Typically, casting solutions were prepared by dissolving PS-b-P4VP-1 into mixed solvents consisted of THF and DMF, and the polymer concentration in solution ranged from 24 to 30 wt %. Viewable contaminates in the casting solution were removed by centrifugation, and films were then cast on a clean glass plate using a doctor blade with a gap height of 150 μm. The cast films were remained in air for a predesigned period to evaporate the THF from the cast films. The cast films were subsequently precipitated in a water bath to obtain the BCP porous membranes. The time exposed in air prior to precipitation was set as 10, 30, 45, 60, 100 and 120 s. Temperature (24.5 °C) and relative humidity (45%) remained constant during the fabrication. General Characterizations. The surface and bulk morphologies of fabricated membranes were visualized by a field emission scanning electron microscope (FESEM, Hitachi S4800) that was operated at a voltage of 3 kV. The membrane samples were sputtered with a thin Pt layer (∼2 nm) before observation. The SEM images were analyzed with the aid of Image Pro. Plus software, and the diameters as well as the aspect ratio of cylindrical micelles were calculated. Transmission electron microscopy (TEM) observation was conducted on the FEI Tecnai G2 F20 in bright-field mode. The sample was first embedded in epoxy resin, and then thin sections (thickness 50−100 nm) were cut using a Reichert-Jung Ultracut E microtome. Selective staining of the P4VP domains was accomplished by exposure of the thin sections to I2 vapor for 2 h. Surface chemistry of the membranes was determined by X-ray photoelectron spectroscopy (XPS) which was conducted with a takeoff angle of 60°, and the chemical composition within 5 nm in outer surface was obtained. Permeability and separation characteristics of the membranes were determined under the pressure of 1 bar; the procedure can be found in our previous report.44

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CONCLUSIONS A symmetrical membrane consisting of cylindrical micelles overlapping each other was successfully fabricated via a SNIPS process. The solvent property and molecular weights had played important roles on BCP self-assembly and membrane formation. When the mixtures of DMF and THF were used as the solvent for the PS-b-P4VP, the cylindrical micellar membranes could be obtained in a wide window of fabrication conditions. With the prolonging of the solvent evaporation time, the upper surface morphology of the membranes transformed gradually from typical NIPS pores to uniform cylindrical aggregates. Compared to the solvent evaporation time, the mixed solvent composition had a quite obvious impact on the cylinder diameter. The diameter increased with the THF fraction in the mixed solvent. The lower molecular weight of PS-b-P4VP was a key factor in the preparation of cylindrical micellar membranes with an interconnected symmetrical structure. The water permeability of the assembled micellar membranes reached 400 L/(h m2 bar) with a pore size of about 25.4 nm in neutral condition. Significantly, the permeation and separation properties of our symmetrical membranes exhibited a reversible pH-responsive character as the pH of feed solution varied between 1 and 6. The developed cylindrical micellar membranes can find their great potentials to be applied in fractionation of nanoparticles.

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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/acs.macromol.6b00166. G

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Details of PS-b-P4VP-1 characterization and the crosssectional SEM images of the membranes prepared with longer evaporation time (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (L.-P.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from the National Natural Science Foundation of China (51573159, 51273176) and the China Postdoctoral Science Foundation (2014M551722).

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DOI: 10.1021/acs.macromol.6b00166 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.6b00166 Macromolecules XXXX, XXX, XXX−XXX