To enhance the anti-fouling and separating properties of polyvinylidene fluoride (PVDF) membranes, an amphiphilic copolymer of methyl methacrylate and 2-acrylamido-2-methylpropane sulfonic acid, poly(MMA-co-AMPS), was designed and synthesized. Through a phase-inversion process, the poly(MMA-co-AMPS) were fully dispersed in the PVDF membrane. The properties of membrane including the surface and cross-section morphology, surface wettability and fouling resistance under different pH solutions were investigated. Compared to the unmodified PVDF membranes, the contact angles of modified PVDF membranes decreased from 80.6° to 71.6°, and the pure water flux increased from 54 to 71 L·m−2·h−1. In addition, the hybrid PVDF membrane containing 0.5 wt% copolymers demonstrated an larger permeability, better fouling resistance and higher recovery ratio via pure water backlashing, when it was compared with the other blend membranes, and the virgin one in the cyclic test of anti-fouling. The modified membranes with the copolymers possessed an outstanding performance and may be used for further water treatment applications.
Water pollution is a global problem. Access to clean water affects everything in our life, including health, transport, work opportunity and education. Membrane technology has been extensively used for the process of liquid filtration, such as drinking-water purification (Shannon et al. 2008), treatment of wastewater (Hidalgo et al. 2018), etc. The application of membranes can be a solution for the water pollution problems. Poly(vinylidene fluoride) (PVDF) membrane has taken the limelight due to its extraordinary membrane-forming properties, better thermal resistance, excellent chemical resistance and high mechanical performance (Ayyaru & Ahn 2017). Nevertheless, the fouling of PVDF membrane, which leads a reduction of liquid flow contrary to an increase of the energy cost in the water-filtration operation, has become one of the bottlenecks and suppression factors, and is hindering the development of membrane application (Wang et al. 2016). Hence, it is extraordinarily important to enhance the anti-fouling performance by adopting the numerous modifications for PVDF membrane (Kang & Cao 2014). Modification of PVDF membrane from the hydrophobic nature into the high hydrophilic performance to increase antifouling properties, based on the scientific of research works in membrane field, can be divided into three major classes: (I) surface modification by chemical methods (Xiao et al. 2015); (II) surface modification adopted with physical methods (Ma et al. 2015); (III) blending modification during the membrane preparation progress (Bera et al. 2015).
The blending modification changes both the surface structures and the more deep layers of the membrane. However, it is not so popular due to two main drawbacks. One is the different solubilities between the membrane casting solution and the modified molecule solution. The other is the poor stability of interactions between polymer and copolymer. To deal with the two disadvantages of in-situ modification, the copolymer in blending casting solution should satisfy two conditions. (I) Regulating a certain amount ratio of hydrophilic/hydrophobic moieties located in the amphiphilic copolymer. For example, the modification membrane with PPOm-b-PSBMA n zwitterionic copolymer (Hsiao et al. 2014) that had an increased number of a hydrophilic repeating unit could not only diminish pollution but also enhance the mixability of species. (II) The copolymer comprised hydrophilic moieties is water-insoluble, and it can capture water at a considerable scale (Liu et al. 2013). A right choice for copolymer in blending casting solution is a deciding factor in the matrix modification.
The choice of MMA in the copolymer excogitation is because of the compatibility and affinity between PVDF with MMA. (Minehara et al. 2014; Bera et al. 2015). Although the PMMA can improve compatibility of PVDF, and it is also a hydrophobic polymer. Another copolymer for modification is expected to have high hydrophilicity. The AMPS is considered as a proper candidate because of its hydrophilic nature (Sun et al. 2016), good solvent resistance (Natu & Van De Mark 2015) and easy polymerization (Rastogi et al. 2014). Therefore, both MMA and AMPS can be used as the comonomers of the amphiphilic and water-insoluble copolymer. With the above-mentioned advantages of MMA and AMPS, the poly(MMA-co-AMPS) is highly desirable as a molecule of surface modification to prepare the modified PVDF membranes by phase-inversion process. In this work, a hydrophilic copolymer of poly(MMA-co-AMPS) is synthesized and used as the modified material of PVDF membrane to increase hydrophilicity and anti-fouling property of membrane. The microstructure morphology of the membrane surface and cross-section, anti-fouling performances and separation capabilities of prepared blend PVDF membranes and the virgin one are studied. Moreover, the evaluation of the fouling resistance and flux recovery ratio are performed after running cyclic tests of protein filtration in different pH environments.
Synthesis of poly(MMA-co-AMPS)
Details of chemicals in this work are presented in the Supplementary Material (available with the online version of this paper). The copolymer of poly(MMA-co-AMPS) was synthesized by free radical polymerization (Rastogi et al. 2014). The organic synthesis process is shown in Figure S1 (Supplementary Material). A three-neck flask reactor was put into 19.02 g MMA monomer and 2.07 g AMPS monomer (molar ratio of 19:1). First, to the round bottom was entered N2 for 45 min to evacuate air. Secondly, the reactor was heated, and when the temperature of polymer solution came up to 60 °C, K2S2O8 initiator (0.1 g, dissolved in 5.0 mL deionized water) was gradually dropped into the reactor to make continuous copolymerization for 8 h at 60 °C. Finally, the copolymer of MMA and AMPS was purified by repeating precipitation of ethanol and repeating flushing of pure water to remove unreacted monomer and impurity. Then the product was dried under a vacuum at 90 °C and pulverized to reserve. The synthesis process was repeated at least three times and the experimental results were consistent.
Fabrication of blend PVDF membrane
Transparent casting solutions of poly(MMA-co-AMPS)/PVDF hybrid membranes were prepared from 20 wt% dried PVDF 2 wt% PVP and a certain content of poly(MMA-co-AMPS) dissolved in a homologous quantity of DMAc. The mass fraction of the copolymer was required to occupy for 0, 0.1 wt%, 0.3 wt%, 0.5 wt% and 1 wt%, then these above-mentioned integrative membranes were respectively flagged as M-0, M-0.1, M-0.3, M-0.5 and M-1. Briefly, a representative example of hybrid membrane preparation process, for instance, M1 was shown as below. DMAc (60 g) and dried PVDF (20 g) dissolved each other to form a unified solution continuously stirred for 6 h at 60 °C, then, in the same way, copolymer of 1 g was added to the DMAc of 10 g under unintermittent stirring for 6 h at 60 °C. After that, the polymer solution was completely poured into PVDF solution. The flask of containing polymer solution was repeatedly washed by 7 g of DMAc to ensure that copolymer was completely transferred into the PVDF solution. Stirring of the mixed solution resumed at the same temperature for 6 h; then it was placed in the darkness at room temperature for 12 h to remove the air bubbles. In the end, the casting solution was poured on a glass pane by a film knife to prepare a blend membrane of 100–110 μm thickness. The glass pane equipped with casting solution was exposed to the air for 30 min and immersed in a water tank of 25 °C. At last a thin film was gradually separated from the pane and persisted in pure water to standby application.
Characterization of membranes
The test methods of membranes included attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy measurements, X-ray photoelectron spectroscopy (XPS) analysis, scanning electron microscopy (SEM) observations, water contact angle (CA) measurements, membrane permeability experiments, molecular weight cut-off (MWCO) test and protein antifouling tests. The Supplementary Material lists the details of the test methods.
RESULTS AND DISCUSSION
Characterization of the copolymer and membranes
The poly(MMA-co-AMPS) copolymer was successfully synthesized. The chemical structural characterizations and group compositions of the copolymer were analyzed by FTIR and 1H NMR (Figures S2 and S3 in the Supplementary Material, available with the online version of this paper).
The virgin and hybrid membrane-microstructure morphology of the right-top surface and cross-section were observed by SEM. Figure 1 shows the right-top surface microstructures of a, b, c, d and e, and the cross-section microstructures of A, B, C, D and E. All the membrane structures were made up of two parts of a finger-like porous support layer and a dense skin layer that played a decisive role in the low flux and anti-fouling. First, the surface structure images of a, b, c, d and e were analyzed and compared to the unmodified PVDF membrane (a), and all the modified PVDF membranes (b, c, d and e) exposed a high porosity. A large number of noticeable pores could be observed, and, as well, as the increasing contents of poly(MMA-co-AMPS), the quantities of film pore increased rapidly. It should be expository that there were observable copolymers on the membrane surfaces and in the porous matrix of the modified PVDF membranes. The membrane with 0.5 wt% copolymers (d) had uniformly distributed and huge numbers of pores than the 1 wt% one (e). The phenomenon was owing to that polymer reunion causes blocking of membrane surfaces or channels (e.g. the image of E), which result in that the membrane (e) presented a phenomenon of extraordinary asymmetric distributed and a small number of pores. Then, from the cross-section images of A, B, C, D and E, compared to the unmodified PVDF membrane (A), the finger-like porous support layer of all the modified PVDF membranes (B, C, D and E) had a longer and wider microstructure. Because the MMA had good compatibility and the AMPS had good hydrophilicity, the dissolving capacity of PVDF in the casting membrane solution was increased by adding poly(MMA-co-AMPS) copolymer.
Figure 2 shows that unmodified PVDF membranes exhibited the largest contact angle. With the addition of copolymers filler, the water contact angle of the modified membranes showed a downward trend. The modified PVDF membrane possessed improved wettability and excellent hydrophilicity because of strong water uptaking capacity of P(MMA-co-AMPS) copolymer. The improved hydrophilicity of membrane was beneficial to promoting the permeability of the membrane and the antifouling performance.
Permeation of membranes
The pure-water flux (J0) of the virgin and modified PVDF membranes at 0.1 MPa are shown in Figure 3. The copolymers hybrid membranes possessed a higher water permeation flux than the virgin PVDF membrane. The water flux of virgin PVDF membrane was 54 L·m−2·−h−1, and with the addition of copolymers, the water permeation flux of the copolymers hybrid membranes (M-0.1, M-0.3, M-0.5) rose up and reached the maximum value (71 L·m−2·h−1). The rejection of modified membranes did not fluctuate dramatically, and its variation range was within 7%. The increase in the number of pores played an important role in improving water flux (J0) values. The amphiphilic copolymer of poly(MMA-co-AMPS) was embedded in the PVDF membrane surfaces and appeared in the membrane porous matrix (e.g. Figure 1), and the sulfonic groups segments on the membrane surfaces can form a thick water–hydrogen layer. So the copolymer hybrid PVDF membranes showed high water flux and excellent water permeability. However, the hybrid membrane of the highest copolymer content with 1 wt% showed a drastic decrease in the water flux, and even had a lower flux than the virgin PVDF membrane. At the highest concentration, the copolymer formed polymer clusters block the membrane pore channel, and the clusters are observed in Figure 1 with a decrease in membrane porosity and non-uniform pore density. The molecular weight cut-off curves of the membranes are shown in Figure S6 (Supplementary Material, available online). It indicated that the pore size distribution of PVDF membranes decreased with the addition of copolymers. The cut-offs of 0.5 wt% and 1 wt% membranes were over 0.9 with PEO100 K.
Anti-fouling property of hybrid membranes
Cyclic filtration experiments of the virgin and hybrid PVDF membranes were carried out to ascertain the fouling resistance effects of the poly(MMA-co-AMPS). The bovine serum albumin (BSA) buffer (0.4 g/L, pH = 5.5, 7.0 and 8.2) solutions were served as three kinds of model protein foulants. After passed through a constant BSA concentration, the permeation flux of all membranes began decreased rapidly owing to protein fouling. In 120 min and 240 min, the membranes were backflushed using pure water for 2 min at 0.1 Mpa. Figure 4 shows the results of BSA filtration operated in the phosphate buffer solution. Figure 4(a)–4(c) represent the testing environment of acidic solution, neutral solution and alkaline solution, respectively. The relative protein permeability (J/J0) of the PVDF membranes is displayed in Figure S7 (Supplementary Material, available online).
During the filtration process of acidic solution, as shown in Figure 4(a), the hybrid membranes had a very high anti-fouling property and separation performance compared to the virgin PVDF membrane. From Figure S7(a), the films greatly enhanced the acid resistance, the water permeability and the recovery rate of the back-flushing process. This reason should be attributed to the increased coverage of the –SO3H groups located on the membrane surfaces. The sulfonic group possessed a high ability of release and repulsion fouling and can assuage the nonreversible fouling problem brought about by the cyclic filtration tests of the acidic buffer.
Combining Figure 4(b) and Figure S7(b), the modified PVDF membranes possessed the larger water flux, better antifouling properties and water permeabilities than the virgin PVDF membrane in the filterable operation of neutral solution. The permeation flux of copolymers hybrid membranes rose up with the growth of the polymer concentrations. Although the 1 wt% hybrid membrane had the best protein permeability, its flux was the lowest and was even lower than the virgin one. The 0.5 wt% hybrid membrane had a maximum water flux and excellent protein permeability, and it was recognized as the best modified PVDF membrane.
Compared to the filtration process of acid buffer solution and neutral buffer solution, the modified PVDF membranes had not a particular increase value of the permeation flux and relative protein permeability in the alkaline buffer solution. However, compared to the virgin PVDF membrane, in Figure 4(c) and Figure S7(c), the hybrid membranes still had a preferable fouling resistance and higher recovery rate of water permeation in the alkaline solution.
The polluted flux recovery ratio (FRR) was used to appraise the anti-fouling performances of the virgin and hybrid PVDF membranes. Regarding the recovery rate of membrane fouling, as shown in Figure 5, the recovery percentages of protein filtration with the hybrid PVDF membranes increased first and then decreased with the concentration of the copolymers. The hybrid membrane of 0.5 wt% copolymers has the maximum recovery rates of the first and the second in the three different buffer solutions. In acidic conditions, the fouling recovery rates of the first and the second (FRR-1 and FRR-2) were 0.77 and 0.66. In the neutral and alkaline solutions, the values were 0.73 and 0.65, 0.69 and 0.58, respectively. The membranes possess the best recovery rate in the acidic buffer solution. In addition, the FRR-2 of hybrid PVDF membrane dropped more slowly than the bare membrane, and with the increase of copolymer concentration, the decline of FRR-2 tended to be slow. This means that the modified membrane had good anti-fouling ability. Together with Figures 4 and 5 and Figure S7, the 0.5 wt% hybrid PVDF membrane had the greatest water permeability and the best resilience of fouling in the filtration process. The sequence of anti-fouling ability in different buffer solutions were as follows: acidic solution > neutral solution > alkaline solution.
In this work, we developed a novel PVDF hybrid membrane with the amphiphilic copolymer of AMPS and MMA. The copolymers were confirmed to disperse in the PVDF membrane by the XPS analysis and ATR-FTIR spectroscopy. From the results of water contact-angle measurement, the hydrophilicity of the membrane surfaces was obviously enhanced. The modified hybrid membranes have high water-flux rate and good antifouling through the flux and pollutant filtration tests, and the 0.5 wt% hybrid PVDF membrane presents the greatest water-flux, best antifouling performance and highest recovery rate. The MMA-co-AMPS copolymer is expected to supply a new chance for improving the performance of modified PVDF membranes in pollutant water treatment.
This work is financially supported by the National Natural Science Foundation of China (NNSFC, No. 21476172), the Program for Innovative Research Team in University of Tianjin (No. TD13-5042) and Tianjin Science Technology Research Funds of China (16JCZDJC37500).