Article Cite This: Langmuir 2018, 34, 3787−3796
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Hyperbranched Poly(ether amine)@Poly(vinylidene fluoride) Hybrid Membrane with Oriented Nanostructures for Fast Molecular Filtration Kejia Ji, Hongjie Xu, Xiaodong Ma, Jie Yin, and Xuesong Jiang* School of Chemistry & Chemical Engineering, State Key Laboratory for Metal Matrix Composite Materials, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China S Supporting Information *
ABSTRACT: Porous membranes with uniform nanostructures and selective adsorption can realize molecular filtration with high flux and have gained great attention because of their wide application in water treatment and industrial separation. Herein, a novel hyperbranched poly(ether amine)@poly(vinylidene fluoride) (hPEA@PVDF) porous membrane with oriented nanostructures and selective adsorption of guest molecules was fabricated by applying the combined crystallization and diffusion method for the functionalization of the PVDF membrane. The resulting hPEA@PVDF porous membranes were fully characterized by scanning electron microscopy and X-ray photoelectron spectra. The results indicated that the hPEA@PVDF membrane exhibited oriented open channel structure and high water flux up to 2116 L m−2 h−1, in which the PVDF skeleton was covered by the amphiphilic hPEA layer. The adsorption behavior of hPEA@PVDF porous membranes to 12 hydrophilic dyes including batch adsorption and molecular filtration was systematically investigated. The results revealed that the hPEA@PVDF membrane possessed high adsorption capacity toward erythrosin B (577 μmol g−1) and eosin B (511 μmol g−1), while low adsorption capacity toward calcein (76 μmol g−1) and methylene blue (hardly adsorbed), indicating the selective adsorption behavior toward dyes in aqueous solution. On the basis of this selective property, the hPEA@PVDF could be used to separate the dye mixtures very efficiently through molecular filtration. In addition, the separation efficiency remained 100% after five adsorption−desorption cycles, indicating that it had great potential in practical applications.
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INTRODUCTION The rapid growth of population and global economy has lead to the increase of wastewater, which contains a variety of contaminants with different sizes and chemical features. In that case, water purification membranes for molecular filtration are of great importance in water treatment. The key characteristic of membranes for molecular filtration is selectivity, whose mechanisms usually include hydrogenbonding,1 size-sieving,2,3 electrostatic,4,5 ionic-bonding,6−8 and host−guest interactions.9 Microfiltration (MF) membranes of polymers with pores on the order of 1 μm can realize separation essentially by size exclusion and are usually useful for removing large colloids, microbes, cells, and viruses in water treatment through filtration with high flux. Generally, however, most pristine MF membranes such as polypropene,10,11 polyethersulfone, 1 2 , 1 3 and poly(vinylidene fluoride) (PVDF)14,15 do not exhibit selective adsorption to guest molecules and cannot separate molecules because molecules smaller than the pore size can flow through the pore structure. Therefore, it is necessary to provide selective adsorption to the porous MF membranes, which involves the modification of MF membranes with the functional polymers. For example, the MF © 2018 American Chemical Society
membranes of PVDF are usually modified by blending functional polymers to provide selective adsorption to guest molecules.16 Because of its good mechanical properties, ease of processing, thermal stability, and chemical resistance,17,18 PVDF is one of the most commonly used membrane materials in water treatment.19 The porous structure of modified PVDF membranes is beneficial to large permeation flux,20−22 and the selectivity is dependent on the chemical features of blending functional polymers. Usually, MF membranes of various polymers are prepared by the nonsolvent-induced phase separation (NIPS)17,23−25 method, which is one of the most commonly used methods in the fabrication of MF membranes for separation. However, the MF membranes by NIPS usually possess tortuous porous structures and broad pore distributions, which might limit the enhancement of permeation and the improvement of selectivity.26,27 In addition, the NIPS process involves the complex physical and chemical factors, leading to the difficulty Received: October 23, 2017 Revised: December 21, 2017 Published: March 12, 2018 3787
DOI: 10.1021/acs.langmuir.7b03676 Langmuir 2018, 34, 3787−3796
Article
Langmuir
Figure 1. (a) Chemical structure of amphiphilic hPEA; (b) photograph of the hPEA@PVDF (P20@hPEA7) membrane; (c) proposed model of the hPEA@PVDF membrane; SEM images of the P20@hPEA7 membrane: (d) cross-sectional overview, (e) pore structure in the separation layer, (f) pores on the surface, (g) cross-sectional view of interconnected microchannels at the back side; (h) opened microchannels on the back surface. The scale bar in (d) is 100 μm; in (e,f,h) is 5 μm; and in (g) is 20 μm. The red arrow in (d) means the orientation of the membrane structure.
in controlling the quality of the final membrane products. Recently, Wang and the coauthor28 proposed a novel method called the combined crystallization and diffusion (CCD) method to prepare porous membranes of PVDF with oriented nanostructures, which exhibited superior water permeation flux in comparison to the traditional NIPS membranes with similar pore sizes. Compared with the NIPS approach, moreover, the CCD process was much less affected by the various factors in fabrication of the PVDF membrane with the highly uniformsized pores. Therefore, the CCD method might be a good alternative approach to prepare the hybrid PVDF membranes with the selective adsorption modified by the functional polymers. In this text, to combine the characteristics of the selective adsorption of amphiphilic hyperbranched poly(ether amine) (hPEA) and porous structure of PVDF, we prepared hPEA@ PVDF porous membranes with oriented structures by applying the CCD method, in modified membranes which can be used for separation of dyes in aqueous solution through molecular filtration. Multifunctional hPEA developed by our group exhibited a unique selective adsorption and separation of dye molecules with similar backbones and charge states.29,30 Dye molecules are common water contaminants in the industrial effluents from leather tanning, textile dying, paper, plastic, rubber, concrete, and medicine.31 The extensive use of dyes often causes serious pollution, which may potentially be toxic or mutagenic and carcinogenic for biological diversity.32,33 The layer of hPEA hydrogel covered on the PVDF skeleton can provide selective adsorption of the resulting porous membranes to dyes and increase the water flux, whereas PVDF as the porous skeleton for the hPEA hydrogel could enhance the adsorption capacity for hPEA because of its large surface area. As highly crosslinked polymers are usually synthesized by photoinitiated polymerization of multifunctional monomers and telechelic polymers to enhance the stability, benzophenone (BP) moieties34 were used as the photo cross-linker for the
membrane. Compared with the conventional NIPS approach, the obtained hPEA@PVDF membranes by CCD possessed more stable nanostructure with fully opened, very well-oriented channels, whose structure was much less affected by the addition of hPEA. Besides, the hPEA@PVDF membrane by CCD exhibited excellent permeation performance overwhelming hPEA@PVDF membranes by NIPS.35 Also, the antifouling and regeneration tests proved that the membrane had good performance in antiprotein pollution and maintained the separation efficiency of 100% after regeneration for five times, thus exhibiting durability for practical applications. Because of these superior performances, the hPEA@PVDF membrane fabricated via the CCD method has great potential in dye removal and wastewater treatment.
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RESULTS AND DISCUSSION Fabrication and Characterization of hPEA@PVDF Membranes. The whole strategy to prepare hybrid hPEA@ PVDF porous membranes through a modified CCD approach is illustrated in Figure 1 and Scheme S2. Because of its unique selective adsorption behaviors to the hydrophilic dyes in water, hPEA was chosen as the functional amphiphilic polymer. To enhance the compatibility to PVDF and durability of hPEA for the porous membrane, fluorocarbon chains (CF6) and photosensitive BP moieties were grafted to the backbone of hPEA through nucleophilic substitution/ring-opening reaction. The detailed synthesis and characterization of hPEA−BP−CF6 are shown in the Supporting Information (1.2, 2.1, Scheme S1, Figures S1 and S2), and hPEA−CF6−BP is abbreviated to hPEA in the following experiments. The chemical structure of hPEA is shown in Figure 1a. The incorporation of hydrophobic CF6 rich in fluorine atoms might enhance the compatibility between hPEA and PVDF, while BP could undergo well-known hydrogen abstraction to generate radicals to lead to cross-linked network of the hPEA layer by irradiation of UV light, 3788
DOI: 10.1021/acs.langmuir.7b03676 Langmuir 2018, 34, 3787−3796
Article
Langmuir consequently enhancing the stability of hPEA@PVDF porous membranes. Typically, the mixture of hPEA and PVDF in DMSO was first cast on a 6 mm thick aluminum casting plate, and then the casting plate was contacted with a precooled aluminum plate (−18 °C) on a freezing board (Scheme S2). Because of the good thermal conductivity of the aluminum plate, the casting solution along with the casting plate cooled at a fast rate. Thus, a temperature gradient was built in the polymer solution film when the polymer solution film was cooled from one side. Upon demixing/phase separation, the polymer started to precipitate, the hPEA/PVDF solute diffused to the cold end accompanying the nucleation/crystallization of the DMSO solvent and filled into the space between DMSO crystallites, thus sterically hindering the agglomeration of the crystallites to maintain their small size. In that case, polymer concentration in the remaining liquid phase was smaller than the adjacent polymer solution at higher temperatures, forming a denser layer than the warmer parts. After cooling, the frozen membrane was immersed into the iced water to remove DMSO for 3 days and dried at room temperature. Figure 1b shows the image of the resultant membrane. Then, the membrane was irradiated by 365 nm UV light and cross-linked, and this process was traced by UV−vis spectra (Figure S2, Supporting Information). Taking the sample P20@hPEA7 membrane (the blend ratio between PVDF and hPEA was 20/7) as an example, the peak ascribed to the adsorption of BP in UV−vis spectra decreased gradually with the increasing irradiation of 365 nm UV light, suggesting the photolysis of the BP moieties (Figure S3b, Supporting Information). The amino groups in the backbone of hPEA could be used as the hydrogen donor. Upon irradiation of UV light, one BP moieties can abstract hydrogen from the hPEA backbone to produce a ketyl radical (from BP) and a radical derived from hPEA backbone (Figure S3a, Supporting Information). These radicals could couple each other to make the hPEA layer to be cross-linked and enhanced the stability of the resulting porous membranes. To investigate whether the P20@hPEA7 membrane could possess the typical oriented nanostructure, scanning electron microscope (SEM) images were applied to reveal the morphology of the membranes. As shown in Figure 1d, the P20@hPEA7 membrane prepared by the modified CCD method exhibited separation layers which were supported by numerous oriented microchannels. The size of these microchannels gradually increases from the separation layers. The separation layer of the P20@hPEA7 membrane consisted of tortuous pores in the top layer and intensively scattered pores on the membrane surface (Figure 1e,f), and the supporting layer was composed of fully opened, oriented, and interconnected microchannels (Figure 1g,h). The element distribution on the surface of the P20@hPEA7 membrane was investigated by X-ray photoelectron spectra (XPS) analysis. As shown in Figure 2, there were two new peaks at 400 and 533 eV in the full-range XPS spectra of P20@hPEA7, which were assigned to signals of N and O, respectively. The appearance of N and O signals derived from hPEA confirmed the successful modification of PVDF by hPEA. It was easy to understand that if hPEA wrapped on the surface of microchannels and layers of the membrane, the F/C ratio on the P20@hPEA7 surface would be less than the theoretical F/C ratio, whereas the content of N atom would be higher than the theoretical value. As determined by XPS spectra, the experimental mass concentration ratio of F/C was 0.61, which was less than the
Figure 2. XPS spectra of pure PVDF and P20@hPEA7 membrane.
theoretical ratio of F/C 0.96. Moreover, the content of N in the P20@hPEA7 membrane was 0.83%, which was higher than the theoretical value based on the blend ratio. These results revealed that amphiphilic hPEA was segregated and enriched onto the surface of the hPEA@PVDF membrane during the CCD process when the DMSO was removed by water. The analysis above proved that hPEA wrapped on the surface of the PVDF skeleton to form a thin layer of the hydrogel, whose model was proposed in Figure 1c. To verify the feasibility of our strategy to fabricate the oriented nanostructure of hPEA@PVDF membranes, we prepared a series of hPEA@PVDF membranes by changing the feed ratio between hPEA and PVDF in casting solutions, whose formulation and parameters are summarized in Table 1. According to SEM analysis (Figure 3), it was clear that all membranes prepared by the CCD method exhibited the similar oriented nanostructure. No matter how the membranes were modified by hPEA or how much hPEA was added for modification, the structures were all similar. By comparison, the morphologies of PVDF membranes prepared by NIPS (Figure S4a, Supporting Information) exhibited typically asymmetric structure with skinned top layer supported by a region of finger-like voids and then a sponge-like layer. The morphology of hPEA@PVDF membranes through NIPS was affected significantly by the addition of amphiphilic hPEA, as it showed asymmetric structure constituted by a top layer, macrovoids, and sponge-like sublayer (Figure S4b, Supporting Information). Besides, the NIPS membranes had largely closed pores at the back side, which might have negative effects for water permeation. On the basis of the comparison, the hPEA@ PVDF porous membranes via CCD exhibited the same oriented morphology, which suggested that the CCD method was a general and facile approach to fabricate modified membranes with an oriented pore structure. The pore size on the back surface and porosity of the resulting hPEA@PVDF membranes were measured (Table 1). Compared with the pristine PVDF membrane, the porosity of hPEA@PVDF membranes with different blend ratios was larger, which might be ascribed to the addition of amphiphilic copolymers. The pore size increased with the increasing hPEA content until the hPEA content reached 5 wt %; then the pore sizes decrease to 382 nm for
[email protected]. Both polymer concentration in casting solution and hydrophilicity of system might have an effect on the porosity during the process of removing DMSO by ice water. With the addition of hPEA, the pore channels of the inner structure became wider, which is mainly ascribed to the enhancement of hydrophilicity, which could increase the opportunity for water molecule inflow. When the concentration of hPEA was over 5 wt %, the higher polymer concentration 3789
DOI: 10.1021/acs.langmuir.7b03676 Langmuir 2018, 34, 3787−3796
Article
Langmuir
Table 1. Composition of the Casting Solution for Fabricating hPEA@PVDF Membranes, Pore Size on the Back Surface, Porosity, Water Flux, and WCA Results sample
PVDF (wt %)
hPEA (wt %)
DMSO (wt %)
pore size (nm)
porosity (%)
water flux (L m−2 h−1)
contact angle (deg)
PVDF P20@hPEA1 P20@hPEA3 P20@hPEA5 P20@hPEA7
20.0 20.0 20.0 20.0 20.0
0.00 1.00 3.00 5.00 7.00
80.0 79.0 77.0 75.0 73.0
375 392 490 532 382
63.3 72.3 70.6 66.0 65.1
422 1227 1596 1847 2116
87.3 83.7 70.1 22.6
