Xinzhen Zhao†
,
Huixia Xuan†,
Aiwen Qin,
Dapeng Liu and
Chunju He*
State Key Lab for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China. E-mail: chunjuhe@dhu.edu.cn
First published on 24th July 2015
In this work, a novel modification method was introduced to improve the antifouling property of polyvinylidene fluoride (PVDF) ultrafiltration membranes, which were prepared with a one-pot process using plasma pretreatment and graft polymerization of PVDF powder. The amphiphilic copolymer of polyvinylidene fluoride-g-polyacrylic acid (PVDF-g-PAA) present in the modified PVDF membrane was demonstrated by infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS). The improved wetting ability was conducive to improve the water flux, bovine serum albumin rejection and antifouling ability of modified PVDF membranes. A totally novel pH quantitative method was employed to investigate the antifouling property based on the acid–base reaction of the amino acid residues of BSA (pollutant), according to the detected change of BSA adsorption amount on membrane surface and pore surface before and after modification, less irreversible fouling was detected in the modified PVDF membrane due to the existence of PVDF-g-PAA, and it was more convenient to remove the pollutants in the pores of modified PVDF membranes by water flushing.
Numerous studies have demonstrated that the hydrophilicity of the membrane surface plays an important role for improving the antifouling properties of hydrophobic separation membranes,8–10 it is generally accepted that the more hydrophilic the surface, the better the antifouling ability. As we all know, the hydrophobic membranes surface grafted with polyethylene glycol (PEG) based or zwitterionic based polymers11–13 is able to exhibit excellent antifouling property due to the formed hydration layer, which can restrain hydrophobic effect between pollutants and hydrophobic membrane materials to decrease the irreversible fouling. However, the fouling problem of ultrafiltration (UF) membranes cannot be completely eliminated merely via surface hydrophilic modification.14–16 Membrane fouling includes reversible and irreversible fouling and the fouled location includes membrane surface and membrane pores, the pollutants pushed into the pores and adhered to the pores surface will block the flow path of feed, leading to the great decline of flux,17 and these pollutants can't be washed away without chemical cleaning agent.8 Thus, the wetting ability of surface and pore surface of hydrophobic UF membranes are both ought to be improved though hydrophilic modification. The reported studies have shown that blending amphiphilic polymer is the most used method to improve the wetting properties of hydrophobic membrane materials,18–20 hydrophilic interface of membrane surface and pore surface can be fabricated though blending amphiphilic polymer based on the assembly of hydrophilic and hydrophobic polymer chain segments. In order to improve the compatibility of amphiphilic polymers and PVDF material, attention have been paid to prepare the PVDF based amphiphilic polymer as additive to modified PVDF membrane, which is beneficial to exhibit better compatibility with PVDF matrix material. Kang21 obtains PVDF-g-PAA copolymer by ozone pretreatment PVDF powder and prepared a hydrophilic PVDF membrane. Li22 uses amphiphilic copolymer of PVDF-g-PEGMA to modify hydrophilic PVDF membrane, which exhibits perfect fouling recover ratio by water flushing. There is no doubt that the prepared PVDF membrane though blending PVDF based amphiphilic polymer will obtain excellent antifouling effect.
The objective of this study is to introduce a novel PVDF powder modification method to prepare PVDF based amphiphilic polymer via one-spot process for improving the antifouling ability of PVDF membranes. As a valid activation method, plasma is employed to pretreat PVDF powder to generate uniformly active site23,24 and amphiphilic copolymer of PVDF-g-PAA are fabricated via followed polymerization, the modified PVDF membranes are prepared by one-pot process though non-solvent induced phase separation (NIPS) method. In order to investigate the antifouling performance of modified membrane, a totally new mean named pH quantitative method based on the acid–base reaction of the amino acid residues is proposed in this work for evaluating the improvement of antifouling property, cooperating flux recover ratio (FRR) to discuss the antifouling performance of the modified PVDF membrane.
Sample | Pretreatment PVDF powder (g) | PVDF powder (g) | AA (g) | DMAC (g) | PVDF powder of the second addition (g) | Coagulating bath (°C) |
---|---|---|---|---|---|---|
M1 | 0 | 1 | 4 | 30 | 5 | 20 |
M2 | 1 | 0 | 4 | 30 | 5 | |
M3 | 2 | 0 | 4 | 30 | 4 |
PVDF powder was added into the above mixture solution to obtain homogeneous casting solution, the casting solution was cast on a glass sheet with the casting knife of 200 μm and immediately immersed in water coagulation bath (20 °C). All the prepared membranes were stored in pure water for 24 h to remove the monomer and homopolymer of AA, the prepared membranes were stored in pure water for subsequent separation and antifouling test without drying process. All the casting solution compositions and casting conditions of PVDF membranes were showed in Table 1. In addition, different polymerization time of 1 h, 2 h and 4 h and coagulation bath temperature at 0 °C, 20 °C and 50 °C were applied to study the effect of prepared condition for the microstructures of modified membranes.
![]() | (1) |
![]() | (2) |
![]() | (3) |
![]() | (4) |
Checked the methods listed in the literatures, the antifouling properties of membranes were mainly evaluated with surface adsorption mass and the flux recovery rate (FRR) of dynamic fouling process. However, unacceptable errors were obtained regularly because of protein denaturation and impact of different preloading time for membranes flux. Therefore, a more accurate method named pH quantitative method for the evaluation of antifouling properties was proposed in this paper, which was based on the amino acid residues of BSA having the consumption capacity of H+ and OH− to effect the pH value of the solution contained BSA. Tuning the pH of BSA from 7.4 to 4.7 (isoelectric point of BSA), the consumed amount of acid was proportional to the molar amount of BSA. So the consumed acid amount of fouled membrane by BSA could exhibit the fouling situation. The specific details of pH quantitative method were described below, the tested membrane was placed in the dead-end filtration device (Fig. 1), and the BSA (1 g L−1, pH 7.4) solution as pollutant feed was filtered for 2 hours. Then the membrane was taken out and placed in 100 mL ultrapure water to adjust the pH to 4.7 using HCl (0.01 mol L−1), the consumed volume of HCl subtracting the consumed volume of membrane itself and water was assigned to the volume consumed by BSA. If the lower surface of the membrane was sealed with an impermeable support layer, the liquid was impermeable, so the membrane surface was the only fouled location, and the consumed volume by the adsorbed BSA on the membrane surface was labeled V1, and the volume was labeled V1W after flushing. When the feed solution could pass through the whole membrane with the applied pressure, BSA was able to contact the membrane surface and membrane pores, the consumed volume of HCl was labeled V, the consumed volume after cleaning was labeled VW, all the details were shown in Table 2 and μL was the volume units.
Whole membrane | Membrane surface | Membrane pores | |
---|---|---|---|
Before flushing | V | V1 | V − V1 |
After flushing | VW | V1W | VW − V1W |
Flushing coefficient | VW/V | V1W/V1 | VW − V1W/V − V1 |
Because the consumed amount of HCl was proportional to the molar amount of BSA, BSA adsorption amount could be calculated according to the eqn (5), which was determined in advance and the R2 is 0.95. Where V (μL) was the consumed HCl volume, S (cm2) is the effective area of fouling. Y (μg cm−2) refers to the adsorption amount of BSA.
![]() | (5) |
Fig. 3a shows the ATR-FTIR spectra of the pure and modified PVDF membranes, the strong peak at 1715 cm−1 appear in the spectra of M2 and M3 membranes, which indicate the existence of COOH, belonging to the PAA segment of amphiphilic PVDF-g-PAA. As a contrast, the characteristic peak of COOH cannot be viewed obviously in the spectrum of M1. To obtain further information about the composition of the modified membrane, the surface elemental composition percentages of pure and modified PVDF membranes are determined by XPS and the results are shown in Fig. 3b and c, the characteristic signal of oxygen in the XPS spectra are clearly observed, the surface the atomic percentages of oxygen increase from 1.05% to 16.49%, and the atomic percentages of fluorine element are significantly reduced, which means the oxygen-containing functional groups increase on the M2 and M3 membranes surface due to the added treated PVDF powder. In addition, the resolving results of M3 are shown in Fig. 3d and e, the C 1s core level spectra of M3 membrane consists of four peak components with binding energy at 285.7 eV, 287.0 eV, 288.5 eV and 291.3 eV, which are attributed to the CH2, C–O, O–CO and CF2 species, the C–O and O–C
O are all assigned to the COOH group. As for the resolving results of O 1s core level spectra, a lot of oxygen element is detected on the M3 surface, and the peaks at 532.5 eV and 533.5 eV are agreement with the O–C
O and C–O binding energy, which are also attribute to the COOH group. It can be inferred that the PAA segments enriches on the modified PVDF membrane surface, fluorinated segments with low surface energy move toward the inside of the modified membrane based on the difference in free energy, and the oxygen-containing segment chains with high surface energy move to the membrane surface. The calculated molecular weight of PVDF-g-PAA in M3 by HNMR is about 450
000, the hydrophobic PVDF segments with high Mw are anchored in the membrane matrix and the hydrophilic PAA segments migrate to the membrane surface. These results are consistent with those reported in the literature,21 indicating the grafted copolymer PVDF-g-PAA is successfully synthesized and exists in the modified membranes. PAA homopolymer is easily washed away as porogen using pure water according to the ATR-FTIR and XPS spectra of M1.
Hydrophilic property is one of the most important factors that affect the antifouling property. The wetting ability of membranes is usually determined by the static water contact angle on the membrane surface. As shown in Fig. 4, the M1 membrane shows the highest contact angle of 91° and the modified M3 membrane exhibits the lowest water contact angle of 52°, it is clear that the increased PVDF powder in the membrane solution can decrease the water contact angle of membranes surface. It is commonly accepted that hydrophilic polymer chains in hydrophobic material are driven by interface free energy and enriched on the membranes surface, which are already demonstrated by ATR and XPS test. So the increased hydrophilicity of PVDF membrane is due to the enriched PAA chains on the membrane surface, and the amount of PVDF-g-PAA is related with the added PVDF powder.
In order to investigate the effect of grafted copolymer PVDF-g-PAA on PVDF membrane surface morphology, the surface SEM images of pure and modified PVDF membranes are shown in Fig. 5. Obviously, more pores appear on the surface of M2 and M3 compared to smooth M1, indicating amphiphilic copolymer is conducive to form the flow channel on the surface skin layer. As a general rule, surface morphology of PVDF membrane are affected by the hydrophilic polymer porogen or amphiphilic polymer additive, the interaction between hydrophilic PAA segments and water molecules increase the probability of forming membrane pores in double diffusion process of membrane formation, the phase aggregation process of amphiphilic polymer can effectively change the microstructure of membranes surface.22,25 We also investigate the effect of formation condition for the whole structure of M3 membranes, viewed from the images of (a), (b) and (c) in Fig. 6, the cross section structure varies regularly depending on the coagulation bath temperature change from 0 °C to 50 °C, fewer finger like pores and thicker skin layer of the modified M3 membrane resulting from the high coagulation bath temperature are observed. In addition, Fig. 6b, d and e show a great number of spherulites formed in the matrix of the PVDF membrane with the increasing of the graft polymerization time, the generation of PVDF spherulites demonstrate that a large degree of micro-phase separation occurs due to the increased homopolymer of PAA in the graft polymerization process. Based on the different interfacial free energy of PAA and PVDF, PAA homopolymer with high molecular weight moving to the coagulation bath at low speed is conducive the to generate PVDF spherulites to change the bulk structure during NIPS process.
The permeation properties of all the membranes are tested by the self-made dead end filtration system, the results of pure water flux and the rejection of BSA solution are shown in Fig. 7, all the values are recorded when the flux of water or BSA solution are stable. Obviously, both the pure water flux and the BSA rejections of modified membrane (M2 and M3) are improved with the addition of treated PVDF powder, the pure water flux of M3 increases to 145 L m−2 h−1, and the BSA rejection increases by 18%. It may be attributed to the increased content of PVDF-g-PAA, the enriched hydrophilic PAA chains on the membrane surface are conducive to improve the wetting ability, so the hydrophilic surface can preferentially adsorb water molecules via hydrogen to enlarge the permeation flux and repel the hydrophobic BSA molecules to improve the rejection ratio,26 which breaks the traditional trade-off phenomenon in membrane separation process.
The reversible fouling can be cleaned by water or buffer solution flushing, but the irreversible fouling as the main reason inducing membrane fouling cannot be washed away without chemical cleaning agent, which will lead to the degradation or oxidization of polymer membrane materials.8 Therefore, reducing the irreversible fouling is the most important factor in the preparation of antifouling separation membranes. To investigate the antifouling property under dynamic fouling process, the FRR values are employed. As shown in Fig. 8a, the water flux of all the membranes decrease dramatically at the initial fouling filtration, after the cleaning process, the secondary water flux are significantly different, high water flux of the M2 and M3 membranes are obtained compared to M1 membrane. All the FRR values of M1, M2 and M3 are listed in the Fig. 8b, the FRR of M3 is as high as 85% (the IFR is 15%), but the FRR of reference M1 is only 46%. The enhanced FRR values prove the antifouling property of modified PVDF membrane are improved though one-spot modification of pretreated PVDF powder, and the surface hydrophilization effect resulting from the contribution of hydrophilic PAA chains are beneficial to change the original hydrophobic interface of PVDF membrane surface and pore surface. PAA segments can reduce the adsorption probability of BSA on the membrane surface and pore surface though diminishing the hydrophobic interaction between hydrophobic membrane interfaces and hydrophobic pollutant, and the weakened anchoring effect is contribute to clean the BSA pollutants easily by water flushing, water molecules are selectively adsorbed on the hydrophilic segments to form a protective hydration layer to exclude BSA molecules for decreasing the irreversible fouling on membrane surface and pores.
![]() | ||
Fig. 8 Time dependent flux variation (left) and FRR values (right) of prepared membranes at 0.1 MPa (0–60 minute: pure water, 60–120 minute: BSA solution, 160–200 minute, pure water). |
In order to further verify the antifouling effect of the modified membrane, the pH quantitative method, a totally novel evaluation method based on the acid–base reaction of amino acid residues is proposed. As the instruction in experimental detail, the adsorption mount of BSA in different locations of membrane are calculated according to the consumed volumes of HCl solution, and the smaller the flushing coefficient, the better the antifouling ability.
As can be seen from Table 3, the adsorbed BSA amount of modified PVDF membrane after fouling process is reduced, the adsorption amount of BSA are 103 μg cm−2, 68 μg cm−2 and 57 μg cm−2 for M1, M2 and M3 membranes, respectively. In addition, the flushing coefficient is also reduced, indicating the amount of irreversible pollutant in the modified membrane decrease effectively. According to the comparison of BSA adsorption amounts on membrane surface and membrane pores, it is found that the BSA amount in membrane pores are more than that on the membrane surface. According to the decreased flushing coefficient of modified membrane surface and pores, the pollutants in modified membrane pores and membrane surface are easily removed, indicating the adsorption capacity of BSA on the modified membrane surface and the membrane pores are weakened and the antifouling ability of the modified membrane was enhanced. All these results indicated that the introduction of hydrophilic PAA segments are beneficial to reduce the irreversible adsorption and accumulation of hydrophobic pollutants, PAA segment could be enriched on the membrane surface based on the anchoring effect of the hydrophobic PVDF segment to form the hydration layer structure near the surface of PVDF material to reduce the hydrophobic effect. Compared to the membrane surface, the pollutant in the membrane pores are more difficult to remove due to the physical barriers of structure and chemical effects of membrane pores, which is the important aggregation location of BSA pollutants. However, PAA segments can be employed to improve the hydrophilicity of membrane matrix including pore surface, so that the probability of pollutants being washed away from membrane pores is increased, and the irreversible fouling ratio is effectively reduced to enhance the overall antifouling ability of the PVDF membrane.
M1 | M2 | M3 | |||||||
---|---|---|---|---|---|---|---|---|---|
Whole membrane | Membrane surface | Membrane pores | Whole membrane | Membrane surface | Membrane pores | Whole membrane | Membrane surface | Membrane pores | |
Before flushing | 103 | 45.3 | 57.8 | 68 | 25.3 | 42.8 | 57 | 20.3 | 36.8 |
After flushing | 71 | 28 | 43 | 38 | 15.3 | 22.8 | 30 | 10.5 | 19.5 |
Flushing coefficient | 69% | 62% | 74% | 56% | 60% | 53% | 53% | 52% | 53% |
Footnote |
† These authors contribute equally. |
This journal is © The Royal Society of Chemistry 2015 |