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Hyperbranched Poly (ether amine)@Poly (vinylidene fluoride) Hybrid Membrane with Oriented Nanostructure for Fast Molecular Filtration Kejia Ji, Hongjie Xu, Xiaodong Ma, Jie Yin, and Xuesong Jiang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03676 • Publication Date (Web): 12 Mar 2018 Downloaded from http://pubs.acs.org on March 12, 2018
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Langmuir
Hyperbranched
Poly
(ether
amine)@Poly
(vinylidene fluoride) Hybrid Membrane with Oriented
Nanostructure
for
Fast
Molecular
Filtration Kejia Ji, Hongjie Xu, Xiaodong Ma, Jie Yin, 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. E-mail:
[email protected] Keywords: poly (vinylidenene fluoride) (PVDF); hyperbranched poly (ether amine) (hPEA); dye adsorption; CCD method
Abstract Porous membranes with the uniform nanostructure and selective adsorption can realize molecular filtration with high flux, and raise great concern for its wide application in water treatment and industrial separation. Herein, a novel hyperbranched poly (ether amine)@poly (vinylidene fluoride) (hPEA@PVDF) porous membrane with the oriented nanostructure and selective adsorption of guest molecules was fabricated by firstly applying combined crystallization and diffusion (CCD) method for the functionalization of PVDF membrane. The resulting hPEA@PVDF porous membranes were fully characterized by Scanning electron microscope (SEM) and X-ray photoelectron spectra (XPS) et al. The results indicated that the hPEA@PVDF membrane exhibited oriented open channels structure and high water flux up to 2116 L m-2 h-1, in which the PVDF skeleton were covered by the amphiphilic hPEA layer. The adsorption behavior of hPEA@PVDF porous membranes to twelve hydrophilic dyes including batch adsorption and molecular filtration were systematically investigated. The 1 / 31
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results revealed that hPEA@PVDF membrane possessed high adsorption capacity toward ETB (577 µmol g-1) and EB (511 µmol g-1), while low adsorption capacity toward Cal (76 µmol g-1) and MB (hardly adsorbed), indicating the selective adsorption behavior toward dyes in aqueous solution. Based on this selective property, the hPEA@PVDF could be used to separate the dye mixtures very efficiently through molecular filtration. And the separation efficiency maintained 100% after five adsorption-desorption cycles, indicating that it had great potential in practical.
Introduction The rapid growth of population and global economy 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 hydrogen-bonding,1 size-sieving,2-3 electrostatic interaction,4-5 ionic-bonding6-8 and host-guest interaction.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 virus in water-treatment through filtration with high flux. Generally, however, most pristine MF membranes such as PP,10-11 PES12-13 and PVDF14-15 do not exhibit the selective adsorption to guest molecules and can not separate molecules since molecules smaller than pore size can flow through the pore structure. Providing the selective adsorption to the porous MF membranes is of great significance, which involves in modification of MF membranes with the functional polymers in their preparation. For example, the MF membranes of PVDF are usually modified by blending functional polymers to get the selective adsorption to guest molecules16. Due to the 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 2 / 31
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porous structure of modified PVDF membranes is beneficial to large permeation flux,20-22and the selectivity is dependent on the chemical features of blending functional polymers. Usually, MF membranes of various polymers are prepared by non-solvent induced phase-separation (NIPS)17, 23-25 method, which is one of the most commonly used methods in 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, NIPS process involves the complex physical and chemical factors, leading to the difficulty in controlling the quality of the final membranes products. Recently, Wang and the coauthor28 proposed a novel method called combined crystallization and diffusion (CCD) method to prepare porous membranes of PVDF with oriented nanostructure, which exhibited the superior water permeation flux in comparison to the traditional NIPS membranes with similar pore sizes. Compared with NIPS approach, moreover, the CCD process was much less affected by the various factors in fabrication of PVDF membrane with the highly uniform-sized pores. Therefore, CCD method might be good alternative approach to prepare the hybrid PVDF membranes with the selective adsorption modified by the functional polymers. In this text, to combine 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 structure by firstly applying the CCD method, in modified membranes which can be used for separation of dyes in aqueous solution through molecular filtration. Multifunctional hyperbranched poly(ether amine) (hPEA) developed by our group exhibited a unique selective adsorption and separation of dye molecules with similar backbones and the same 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 3 / 31
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The layer of hPEA hydrogel covered on the PVDF skeleton can provide the selective adsorption of the resulting porous membranes to dyes and increase the water flux; while PVDF as the porous skeleton for hPEA hydrogel could enhance the adsorption capacity for hPEA due to its large surface area. As highly crosslinked polymers are usually synthesised by photoinitiated polymerisation 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 conventional NIPS approach, the obtained hPEA@PVDF membranes by CCD possessed more stable nanostructure with fully opened, very welloriented channels, whose structure was much less affected by addition of hPEA. Besides, the hPEA@PVDF membrane by CCD exhibited excellent permeation performance overwhelming hPEA@PVDF membranes by NIPS.35 Also the anti-fouling and regeneration tests proved that the membrane had good performance in anti-protein pollution and maintained the separation efficiency of 100% after regeneration for five times, thus was durability in practical. Resort to these superior performances, hPEA@PVDF membrane fabricated via CCD method has great potential in dye removal and wastewater treatment.
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 was illustrated in Figure 1 and Scheme S2. Due to 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 benzophenone (BP) moieties were grafted to the backbone of hPEA through nucleophilic substitution/ringopening reaction. The detailed synthesis and characterization of hPEA-BP-CF6 are shown in the Supporting Information (Supporting Information 1.2, 2.1, Scheme S1, Figure S1 and S2), 4 / 31
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and hPEA-CF6-BP is abbreviated to hPEA in the following experiments. The chemical structure of hPEA is shown in Figure 1(a). 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 hPEA layer by irradiation of UV-light, consequently enhancing the stability of hPEA@PVDF porous membranes.
Figure 1. (a) Chemical structure of amphiphlic hPEA; (b) Photograph of hPEA@PVDF (P20@hPEA7) membrane; (c) Proposed model of hPEA@PVDF membrane; SEM images of P20@hPEA7 membrane: (d) Cross-sectional overview, (e) Pore structure in the separation layer, (f) Pores on the surface, (g) Cross-sectional view of inter-connected micro-channels at the back side; (h) Opened micro-channels on the back surface. The scale bar in d is 100 µm; in e, f, h is 5µm; and in g is 20 µm. The red arrow in Figure 1(d) means the orientation of the membrane structure.
Typically, the mixture of hPEA and PVDF in DMSO was first cast on a 6 mm thick aluminium casting plate, and then the casting plate was contacted with a pre-cooled 5 / 31
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aluminium plate (-18℃) on a freezing board (Scheme S2). Due to the good thermal conductivity of the aluminium 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 remain 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 1(b) shows the image of the resultant membrane. Then the membrane was irradiated by 365nm 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 benzophenone moieties (Figure S3(b), Supporting Information). The amino groups in the backbone of hPEA could be used as the hydrogen donor. Upon irradiation of UV-light, one benzophenone moieties can abstract hydrogen from the hPEA backbone to produce a ketyl radical (from benzophenone) and a radical derived from hPEA backbone (Figure S3(a), 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, SEM images were applied to reveal the morphology of the membranes. As shown in Figure 1(d), P20@hPEA7 membrane prepared by the modified CCD method
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exhibited separation layers which were supported by numerous oriented micro-channels. The size of these micro-channels gradually increases from the separation layers. The separation layer of P20@hPEA7 membrane consisted of tortuous pores in the top layer and intensively scattered pores on membrane surface (Figure 1(e,f)), and the supporting layer was composed of fully opened, oriented and inter-connected micro-channels (Figure 1(g,h)). The element distribution on the surface of P20@hPEA7 membrane was investigated by XPS analysis. As shown in Figure 2, there were two new peaks at 400 eV 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 mircochannels and layers of the membrane, F/C ratio on the P20@hPEA7 surface would be less than the theoretical F/C ratio; while 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 theoretical ratio of F/C 0.96. Moreover, the content of N in 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 hPEA@PVDF membrane during CCD process when the DMSO was removed by water. The analysis above proved that hPEA wrapped on the surface of PVDF skeleton to form thin layer of hydrogel, whose model was proposed in Figure 1c.
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Figure 2. XPS spectra of pure PVDF and P20@hPEA7 membrane.
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 CCD method exhibited the similar oriented nanostructure. No matter 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 S4(a), 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 S4(b), Supporting Information). Besides, the NIPS membranes had largely closed pores at the backside, which might have negative effects for water permeation. Based on the comparison, the hPEA@PVDF porous membranes via CCD exhibited the same oriented morphology, which suggested that CCD method was a general and facile approach to fabricate modified 8 / 31
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membranes with 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 mainly ascribed to the enhancement of hydrophilicity, which could increase the opportunity for water molecule inflow. When the concentration of hPEA was over 5wt%, the higher polymer concentration became the leading factor for causing the denser structure and narrower pore size. Table 1 Composition of the casting solution for fabricating hPEA@PVDF membranes, pore size on the back surface, porosity, water flux and water contact angle results. Sample
PVDF
hPEA
DMSO
Pore size
Porosity
Water flux
Contact
[wt%]
[wt%]
[wt%]
[nm]
[%]
[L m-2 h-1]
angle [°]
20.0
0.00
80.0
375
63.3
422
87.3
P20@hPEA1
20.0
1.00
79.0
392
72.3
1227
83.7
P20@hPEA3
20.0
3.00
77.0
490
70.6
1596
70.1
P20@hPEA5
20.0
5.00
75.0
532
66.0
1847
22.6
P20@hPEA7
20.0
7.00
73.0
382
65.1
2116
