Conferring pH-sensitivity on poly (vinylidene fluoride) membrane by poly (acrylic acid-co-butyl acrylate) microgels

Reactive & Functional Polymers 74 (2014) 58–66

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Conferring pH-sensitivity on poly (vinylidene fluoride) membrane by poly (acrylic acid-co-butyl acrylate) microgels Yang He, Xi Chen ⇑, Shiyin Bi, Weigui Fu, Congcong Shi, Li Chen School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, PR China State Key Laboratory of Hollow Fiber Membrane Materials and Processes, Tianjin Polytechnic University, Tianjin 300387, PR China

a r t i c l e

i n f o

Article history: Received 27 July 2013 Received in revised form 23 October 2013 Accepted 31 October 2013 Available online 9 November 2013 Keywords: Poly (vinylidene fluoride) Microgel pH-sensitivity Hydrophilicity Antifouling performance

a b s t r a c t In this paper, cross-linked poly (acrylic acid-co-butyl acrylate) microgels were utilized to impart pH-sensitivity to poly (vinylidene fluoride) membranes by phase separation of a casting solution of poly (vinylidene fluoride)/poly (acrylic acid-co-butyl acrylate)/DMF in aqueous solution. The effect of microgels content on morphologies, surface composition, and chemistry of the as-prepared membranes was studied by varieties of spectroscopic and microscopic characterization techniques. By using the filtration of water and protein aqueous solution, the performance of the membrane was evaluated. Results indicated that the as-prepared membrane was pH-sensitive to water flux, bovine serum albumin rejection and antifouling property. Besides, the as-prepared membrane showed an obvious improvement of water flux and proper bovine serum albumin rejection ratio, compared to the pristine PVDF membrane. Meanwhile, dynamic bovine serum albumin fouling resistance and flux recovery property were also greatly enhanced due to the improvement of surface hydrophilicity. Hopefully, the hydrophilic microgels additive would be favorable to fabricate other polymer membranes for water treatment. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Stimuli-responsive membranes, also called ‘‘intelligent membranes’’, have continuously studied over several years [1,2]. These membranes can reversibly change their pore dimensions/size, permeability, and surface wettability on receiving an external signal such as pH value, temperature, ionic strength, chemical cues, light or magnetic fields in environmental conditions [3–5]. Thus, the mass transfer and separation properties of the membranes can be easily regulated by adjusting external stimuli. Of all stimuliresponsive ones, pH-sensitive membranes are one of the most popular classes and their unique feature has been studied [6]. The most popular component for fabricating pH-sensitive membranes is the poly (acrylic acid) (PAA) which has an acid dissociation constant (pKa). At pH below the pKa, the carboxyl groups of PAA become protonated, leading to volume shrinkage. Whereas at pH above the pKa, carboxyl groups dissociate into carboxylate ions, resulting in a volume swelling [7]. Such a fascinating feature makes PAAbased membrane promising for wastewater treatment, bioseparation, drug delivery devices, tissue engineering [8–10]. To prepare a pH-sensitive membrane, one method is introducing pH-sensitive components into membrane materials before or after membrane formation [11–14]. Belfer [11] prepared a pH⇑ Corresponding author. Address: School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, PR China. E-mail addresses: [email protected], [email protected] (X. Chen). 1381-5148/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.reactfunctpolym.2013.10.012

sensitive polyacrylonitrile (PAN) membrane by surface grafting of various hydrophilic monomers. Ying et al. [12] prepared a pHsensitive membrane from PAA-g-PVDF copolymer. Ulbricht and co-workers [13] prepared a pH-sensitive membrane by photoinitiated copolymerization of PAA onto polypropylene (PP) microfiltration membranes. Ferro et al. [14] prepared a pH-sensitive membrane by radical grafting of PAA on PVDF porous membranes using supercritical carbon dioxide (scCO2) as a solvent. The above mentioned method often requires a complicated condition, has time-consuming and high cost [15]. In addition, a functional layer is generally achieved on the top and/or bottom surface of the membrane, not inside the membrane pores, leading to an imperfect modification of membranes. Preparing pH-sensitive membranes can be also achieved by using pH-sensitive polymers (linear or cross-linked polymers) as constituents of blends or as additives during the membrane formation process [16,17]. Since this method integrates both a phase inversion preparation and a subsequent modification procedure together into a single-step process, it is especially attractive for fabricating functional membranes [18,19]. By this method, Mbareck et al. [20] prepared a polysulfone (PSf)/PAA ultrafiltration membrane with a pH-sensitive rejection for lead, cadmium and chromium. P(AA-co-BA) microgels are widely studied and applied in many fields as functional network polymers [21,22]. They can reversibly change their structure at a fast rate according to external pH values. If P(AA-co-BA) microgels are included in a PVDF membrane,

Y. He et al. / Reactive & Functional Polymers 74 (2014) 58–66

it is possible to fabricate a new membrane with adjustable filtration endowed by microgels. Based on this consideration, we hope to prepare pH-sensitive membranes from blends of PVDF and P(AA-co-BA) microgels by phase inversion. If this is possible, it will provide a simple but reliable method for fabricating pH-sensitive PVDF membranes. However, to our knowledge, no similar attempt has been reported up to now. In the present paper, P(AA-co-BA) microgels were blend with PVDF to prepare pH-sensitive PVDF/P(AA-co-BA) membranes by phase inversion. Chemical composition and morphology of the membrane were then studied by using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), elemental analysis and field-emission scanning electron microscope (FESEM). The porosity, zeta potential, water contact angles, pore sizes as well as the pH-sensitivity and protein antifouling performance of the membranes were also investigated in detail. 2. Experimental 2.1. Materials Acrylic acid (AA), butyl acrylate (BA), potassium peroxodisulfate (KPS), N,N-methylene-bis-acrylamide (MBAA) and N,N-dimethylformamide (DMF) were purchased from Guangfu Fine Chemical Research Institute (Tianjin, China). Prior to use, AA and BA were purified by vacuum distillation, and MBAA was re-crystallized from ethanol. PVDF powders (Solef 1010, Mw = 3.52  102 kg/mol, Mw/Mn = 2.3) were obtained from Solvey Company of Belgium. Bovine serum albumin (BSA) having a molecular weight of 68,000 was purchased from Solarbio Science & Technology Co. Ltd., Beijing, China. Other reagents were all analytical grade and used without further purification. 2.2. Synthesis of P(AA-co-BA) microgels P(AA-co-BA) microgels were synthesized by a free radical polymerization method. A mixture of AA (4.00 g), BA (4.00 g), MBAA (0.10 g), and KPS (0.20 g) in 350 mL deionized water was first formed in a three-neck flask. After the solution was purged with ultra-high-purity nitrogen gas for 30 min to remove any dissolved oxygen, the reactor was then maintained at 70 °C for 8 h at an agitation speed of 600 rpm. After the reaction, the obtained suspension was centrifuged. The precipitate was then re-suspended in ethanol with ultrasonic-bathing and centrifuged again. This procedure was repeated for three times to remove the residual monomers and homopolymers. At last, the microgels were washed by deionized water and then dried at 60 °C in a vacuum oven for 48 h.

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2.4. Membrane characterization Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) spectra of the pristine PVDF membrane M00 and the blend membranes were recorded using a Bruker TENSOR37 instrument. X-ray photoelectron spectroscopy (XPS) measurements were employed to study the surface composition. XPS was conducted on K-alpha spectrometer (Thermo Fisher, UK) using a monochromatic Al Ka X-ray source (1486.6 eV photons). All binding energies were referenced to the C 1s hydrocarbon peak at 284.6 eV. The bulk C, F, O and N contents of the samples were determined by elemental analysis (Vario EL/micro cube elemental analyzer, Elementar Co., Ltd., Germany). The microscopic morphology of various membrane samples was observed by a Hitachi S-4800 field-emission scanning electron microscope (FESEM, Japan) operating at 10 kV. For cross-sectional observation of membranes, the dried samples were freeze-fractured using liquid nitrogen, and coated in gold by chemical vapor deposition. Zeta potentials of membranes were measured at 20 °C and difference pH values based on the streaming potential method. The measurement device was described in a literature [23]. A membrane sample was first fixed on a filtration membrane cell, which links two Ag/AgCl electrodes at the entrance and exit: one near the feed side and another at the permeate side. As an electrolyte solution flowed through the membrane sample, a voltage difference between the electrodes was measured with a digital multimeter (VC97, Shenzhen Victor Hi-tech Co., Ltd., China). The zeta potential (f) was then calculated using the Helmholtz-Smoluchowski equation as given below [24]:

f¼

U g jB DP ee0

ð1Þ

where U was the steam potential, DP was the operation pressure, g was the viscosity of the electrolyte solution, e was the dielectric constant of electrolyte, e0 was the vacuum permittivity and KB was the electrolyte conductivity. Before measurement, the membrane samples were immerged into a 0.01 M KCl solution for at least 24 h. The pH values were adjusted by 0.1 M HCl and 0.1 M NaOH solution. The obtained Zeta potential value was an average of three measurements.

2.5. Porosity and water contact angle Porosity (e) of a membrane was determined by wet-dry weighting method. The porosity is calculated as the volume of the pore divided by the total volume of the membrane [25]:

2.3. Membrane preparation PVDF powders and the synthesized microgels were dispersed in 48 mL DMF by vigorous stirring until clear homogeneous solution was obtained at 60 °C. After vacuum degassed, the solution was cast on a glass plate by a steel knife. Then, the glass plate was immediately immersed in a coagulation bath of deionized water at room temperature. When the nascent membrane was peeled off from the substrate, it was rinsed with deionized water thoroughly to remove the residual solvent and then stored in deionized water until use. In the experiments, the concentration of solid content (PVDF and P(AA-co-BA) microgels) was 16 wt%. The membranes prepared from the blends of PVDF/P(AA-co-BA) containing the microgels content of 0 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt% and 25 wt% were labeled as M00, M05, M10, M15, M20 and M25 respectively.

e¼

ðw1  w2 Þ=qw ðw1  w2 Þ=qw þ w2 =qp

ð2Þ

where w1 is the weight of the wet membrane, w2 is the weight of the dry membrane, qw is water density and qp is the polymer density. Water contact angle of a membrane surface was performed on the basis of contact angle measurements by using Jinshenxin JYSP-180 contact angle instrument (Beijing, China). The membrane was fixed on a glass slide using double-sided tape and dried for 2 h in a vacuum oven before measurement. At room temperature, a water droplet (about 4.0 lL) was dropped on the top surface, and the contact angle was recorded after 20 s. The measured values were the average of five measurements.

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2.6. pH-sensitive permeation 2.6.1. Water flux through PVDF/P(AA-co-BA) membrane Water flux through a membrane was investigated in an apparatus as illustrated in our previous study [26]. Water flux as a function of pH was performed using a cross-flow pattern with an effective membrane area of 17.9 cm2. To eliminate the influence from the membrane itself and external, pH-sensitive water flux was measured by the same piece of membrane. The pH values of the feed solution were adjusted using 0.1 M NaOH and 0.1 M HCl. Prior to measurement, a membrane was pre-pressurized for 1 h under 0.2 MPa to maintain the membrane in the steady state. Then, water was driven to permeate through the membrane at an operating pressure of 0.1 MPa at 20 °C. The flux was obtained in the average of three measurements by weighing permeate solution according to Eq. (3):

J¼

Q At

ð3Þ

where Q was the quantity of water permeated (L), A was the effective of the membrane area (m2), and t was the sampling time (h). Based on the water filtration velocity method, average pore size d (nm) of membrane was approximately calculated by the Guerout–Elford–Ferry equation [27]:

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð2:9  1:75eÞ  8glQ D¼ e  A  DP

ð4Þ

where g was the water viscosity (1.0087  103 Pa s) at 20 °C, l was the membrane thickness (m), DP was the operation pressure (0.1 MPa). 2.6.2. BSA rejection In the BSA rejection measurement, the permeation experiment was carried out at room temperature using the identical apparatus and operating conditions as described in experiment Section 2.6.1. A BSA aqueous of 1 g/L was permeated through a membrane for 0.5 h, and the BSA rejection (R) was calculated by Eq. (5):

  Cp  100% R¼ 1 Cf

ð5Þ

where Cp and Cf are the concentration of permeate and feed solutions, respectively. The concentration of solutions was determined by a UV–vis spectrophotometer (TU-1810PC, Beijing Purkinje General Instrument Co., Ltd., China) based on the absorbance wavelength of 280 nm for BSA. The BSA rejection is the average of three measurements. Based on the rejections (R), the average pore size of the membrane could be calculated by Eq. (6) [28]:

 4  2 dBSA dBSA R¼ 1 2 1 þ1 d d

Antifouling performance of membrane can be primarily evaluated by JBSA and FR. The antifouling performance of the membrane can be further quantified with the reversible fouling ratio (Rr), the irreversible fouling ratio (Rir) and the total fouling ratio (Rt), which are calculated by the following equations [30]:

Rr ¼

J w2  J BSA  100% J w1

ð8Þ

Rir ¼

J w1  J w2  100% J w1

ð9Þ

Rt ¼ Rr þ Rir ¼

J w1  J BSA  100% J w1

Here, Rr describes as recoverable flux decline; Rir describes the non-recoverable flux decline caused by pore plugging and adsorption or deposition of foulant on the membrane surface or wall pore. 3. Results and discussion 3.1. Chemical composition of the blend membrane Existence of P(AA-co-BA) microgels on the membrane surface could be confirmed by the ATR-FTIR analysis. ATR-FTIR spectra for the pristine PVDF and PVDF/P(AA-co-BA) blend membranes are depicted in Fig. 1. All the spectra have peaks at 1396 and 1180 cm1, which corresponded to CH2 and CF2 of PVDF, respectively. In addition, an intense absorption at 1738 cm1 attributed to the stretching vibration of C@O [31], which is the major characteristic of P(AA-co-BA) microgels, is found for the blend membranes. Furthermore, the intensities associated with C@O are gradually enhanced with the increase of P(AA-co-BA) microgels in casting solution, indicating more microgels on membrane surface. To further characterizing the surface composition of the blend membrane, XPS was carried out. Fig. 2 shows the XPS results of pristine PVDF membrane and blend membranes. The C 1s corelevel spectra of M00 (Fig. 2A) can be well fitted with three components at 284.8, 286.2 and 290.6 eV which are assigned to the neutral CH species, the CH2 species and CF2 species, respectively [32]. The ratio for the CH2 species and CF2 species corresponds to about 1.05, in good agreement with the stoichiometry of PVDF. A representative C 1s core-level spectra of blend membranes in Fig. 2B shows a significantly enhanced peak at 284.8 eV, arising

ð6Þ

where dBSA is the diameter of solute molecules, for BSA, it was 6.96 nm [29]. 2.7. Antifouling performance assessment In the antifouling performance tests, the initial water flux (Jw1) was obtained with deionized water (pH 7.4) at 0.1 MPa. Then, the flux (JBSA) was measured after 1 g/L BSA solution (pH 7.4) was being permeated for 0.5 h. After the membranes were washed thoroughly for 1 h, the flux Jw2 of the cleaned membrane was reevaluated again. The relative flux recovery ratio (FR) was obtained from Eq. (7):

FR ¼

J w2  100% J w1

ð10Þ

ð7Þ Fig. 1. ATR-FTIR spectra of M00, M05, M10, M15, M20 and M25.

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Fig. 2. C 1s core-level of membrane M00 (A) and M25 (B), and XPS survey spectrum (C).

both from the neutral CH species of PVDF, and from the hydrocarbon backbone of P(AA-co-BA) microgels. Compared with the M00, it can be clearly seen that two new component peaks, namely the peak at 288.6 eV corresponding to OAC@O species and the peak at 287.0 eV corresponding to CO species appear in the C 1s core-level spectra of M25 [12]. The OAC@O and CO species can be designated to the blending of P(AA-co-BA) microgels on the membrane surface. In addition, an obvious O 1s signal appears in the survey scan spectrum of the blend membranes but disappears in that of pristine PVDF membrane (Fig. 2C), further indicating existence of P(AA-co-BA) microgels on the membrane surface. Since there are two oxygen atoms per monomer unit of AA or BA and two fluorine atoms per repeat unit of PVDF, the content of P(AA-co-BA) microgels on membrane surface can be evaluated from the XPS-derived oxygen to fluorine ratio according to the following relationship:

½PðAA  co  BAÞ ½O / ½PVDF ½F

ð11Þ

Meanwhile, bulk composition was also evaluated based on the determination of O and F by elemental analysis. As shown in Table 1, the bulk [P(AA-co-BA)]/[PVDF] is higher than the corresponding surface [P(AA-co-BA)]/[PVDF] for the blend membranes. However, a totally different result was observed by Ying et al. [12], they found that the hydrophilic PAA in PVDF-g-PAA membranes had a trend of surface enrichment, causing a significantly higher surface [PAA)]/[PVDF] molar ratio than the corresponding bulk. The low surface [P(AA-co-BA)]/[PVDF] is possible that the

Table 1 Surface and bulk [O]/[F] molar ratios of blend membranes.

a b

[O]/[F] molar ratio

M05

M10

M15

M20

M25

On the surfacea In the bulkb

0.0112 0.0113

0.0205 0.0287

0.0329 0.0543

0.0473 0.0764

0.0592 0.0817

Determined from the XPS-derived oxygen to fluorine ratio. Determined from the elemental analysis.

three-dimensional structure of microgels suppresses their migrating from membrane bulk into membrane surface, leading to more microgels being immobilized inside the membrane. 3.2. Morphology analysis of the blend membrane The surface and cross-section morphologies of the membranes are observed by FESEM. Fig. 3A–F shows the surface morphology changes with the membrane composition. It can be seen that the blend membrane has a porous top surface compared to M00 membrane with a nonporous top surface. Furthermore, with increasing P(AA-co-BA) microgels content in the blend membranes, the pore number and size increases, while the pore distribution becomes broader. According to previous studies, the pools of solvent in polymer-poor phase are the sources of surface pores during the liquid– liquid phase separation process [33]. The exchange of solvent and nonsolvent makes the pools of solvent form the surface pores [34]. The evolutions found in surfaces of membranes confirmed that hydrophilic microgels additive was capable of increasing the

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surface segregation of segments changes the morphology (Fig. 3A–F) and composition of blend membrane surface (specifically described in Section 3.1). Fig. 3a–f shows the cross-section morphology of the membrane affected by P(AA-co-BA) microgels. As shown, all the membranes exhibit a typical structure during phase separation process: a skin layer on the top surface, a finger-like middle layer and a spongelike bottom layer. From the magnification of the finger-like structures, it can be clearly seen that the P(AA-co-BA) microgels with diameters of 150–450 nm are immobilized inside the blend membranes, increase with increasing microgels content in the casting solution, good agreement with the elemental analysis results. Additionally, it can be also noted that adding P(AA-co-BA) microgels results in an increase of macrovoid structure in blend membranes. This structure alteration can be interpreted by the mechanism of phase inversion, which depicts a porous top layer and finger-like structure induced by an instantaneous onset of demixing [31]. When casting solution is immersed into water, liquid–liquid demixing of a poor polymer concentration at top side causes the formation of pores in skin layer. This porous structure leads to a high ratio of water inflow versus DMF outflow, which further results in the formation of finger-like pores in the support layer [37]. At the same time, an indiffusion of PVDF and water, which can be regarded as a flow of a polymer lean phase relative to a polymer rich phase, rapidly appears [38]. This indiffusion leads to the microvoid formation. Since displacement of DMF and water is improved by increasing hydrophilic P(AA-co-BA) microgels in the casting solution, more macrovoid structures can be observed in the cross-section of the blend membranes. The membrane porosity is further characterized and the results are shown in Table 2. It can be seen that the porosity increases with increasing microgels in blend membranes, in good agreement with the FESEM observation. 3.3. Zeta potential and water contact angle measurements

Fig. 3. FESEM images of membranes: surfaces of M00 (A), M05 (B), M10 (C), M15 (D), M20 (E) and M25 (F); cross-sections of M00 (a), M05 (b), M10 (c), M15 (d), M20 (e) and M25 (f). Top right corner is a partial enlarged view of cross-section.

exchange of DMF and water and then favored pore formation, as a novel pore-forming agent. During the exchange of DMF and water in phase inversion process, the hydrophilic segments of microgels tend to segregate toward the top surface of just-formed membrane and simultaneously the hydrophobic segments of microgels tend to hinder this segregation from the as-prepared membrane, which was similar to the behavior of amphiphilic materials [35,36]. The

Fig. 4 shows the zeta potentials of the pristine membrane M00 and blend membranes with respect to pH. For the pristine membrane M00, an initial positive zeta potential passes through an isoelectric point (pI) at approximately pH 4 and then changes negative above pH 4. In contrast, the zeta potentials of blend membranes approached zero at lower pH values (the M05 and M10 have a lower isoelectric point at approximately pH 3.2 and 2.5 respectively). Furthermore, the zeta potentials show a decrease tendency with increasing microgels content and pH value in the whole measurement region. This decrease of zeta potential is associated to the charges from the dissociation of carboxyl acid groups of P(AA-co-BA) microgels on the membrane surface. Contact angle measurements are widely used for assessing the hydrophilicity of membrane. In general, if a water droplet has a lower contact angle on a polymer membrane surface, the membrane has a better hydrophilic surface [18]. Table 2 shows that the contact angles of the blend membranes. From M00 to M25, the static contact angle decreases from 74.8° ± 1.4 to 63.4° ± 3.8, indicating the contact angles decrease with increasing P(AA-coBA) microgels content in membranes. According to the contact angle results, the blend membranes show better hydrophilic surface possibly due to the existence of hydrophilic P(AA-co-BA) microgels in the membrane skin layer. 3.4. Water flux as a function of pH value Fig. 5 presents the effect of pH value changing from 2 to 13 on water permeating through the membranes. As shown in the figure, the flux through the M00 is 23.5 L/(m2 h) without variation under different pH conditions. By comparison, flux through the blend

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Y. He et al. / Reactive & Functional Polymers 74 (2014) 58–66 Table 2 Porosities and water contact angles of membranes. Sample

M00

M05

M10

M15

M20

M25

Porosity Water contact angle (°)

0.613 74.8 ± 1.4

0.653 73.6 ± 2.7

0.684 68.3 ± 1.9

0.705 65.4 ± 3.3

0.795 63.9 ± 2.1

0.886 63.4 ± 3.8

Fig. 4. Zeta potentials of membranes at various pH values: M00 (h), M05 (s), M10 (4), M15 (5), M20 (/), M25 (.).

membranes shows a decrease with the increase of pH value. For instance, when pH values increase from 2 to 9, the fluxes through the M05 and M25 decrease from 80.5 to 42.3 L/(m2 h) and from 268.9 to 148.8 L/(m2 h) respectively. The fluxes through blend membranes exhibit pH-sensitivity between pH 2 and 9, and hardly changes at pH higher than 9. The pH-sensitive flux is related to the effective pore channels which are resulted from the conformation change of the P(AA-co-BA) microgels in blend membranes. It is well known that the pKa of AA, which is defined as the pH at which 50% of the carboxyl acid groups in the polymer chains are protonated, is approximately 4.5–4.9 dependent on different measurement methods [39]. When pH is lower than pKa, the carboxyl acid group of P(AA-co-BA) microgels is protonated, causing the microgels collapse. The collapsed microgels then lead to an increase of the effective pore size, which favors a higher water flux. When pH is higher than pKa, the carboxyl acid group of P(AA-co-BA) microgels is deprotonated. The increased electrostatic interaction resulted from a higher degree of ionization segments brings the swelling of microgels. As a result, the membrane pore void fraction is diminished and the water flux thus decreases. The water flux of the membrane is further evaluated at various pH values and pressures. As shown in Fig. 6, the water flux exhibits decreased water permeability from pH 2–11 under different pressures, which shows that the membrane pore sizes are highly tunable by the microgels in blend membranes. In addition, the linear relationship between water flux and operation pressure is always kept at a same pH, showing that the physical structure of the blend membranes is stable. Similar results are observed for other blend membranes (data not shown). The effect of microgels content on water flux can be also clearly observed from Fig. 5. The water flux through the membrane shows an obvious increase with increasing P(AA-co-BA) microgels, possibly stemming from the increased pore size, porosity and the hydrophilicity of the membrane. Assuming the membrane thickness is not changed and no pores are completely blocked in the

Fig. 5. Water flux as a function of pH values for membranes: M00 (h), M05 (s), M10 (4), M15 (5), M20 (/), M25 (.).

Fig. 6. pH-reversibility and pressure stability for the M10.

experiment, the pathway through the membrane can be supposed as finger-like pores, and can be approximately calculated according to Eq. (4) based on the water filtration velocity method [25]. As shown in Table 3, the pore size calculated by Eq. (4) is smaller compared to the FESEM observation. This is possibly because the former is obtained from the wetting state of the membrane pore where polymer chains swell while the latter is obtained from the dried state of the membrane pore where polymer chains shrink. In addition, some smaller pores exist on the skin layer of the membrane but they are not examined by the FESEM, which is also the possible reason. The pore size shows no difference at pH 2 and 9 for M00. However, the pore size at pH 2 is about 1.4 times that at pH 9 for all the blend membranes, indicating that the ‘‘open’’ (pH 2) and ‘‘closed’’ (pH 9) pore can be tuned by changing the conformation of P(AA-co-BA) microgels in blend membranes.

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Table 3 Average pore sizes of membranes. Sample

Average pore size (nm) dw,

M00 M05 M10 M15 M20 M25 a b

a pH = 2

15.3 27.0 29.8 33.9 35.1 37.1

dw,

pH = 9

a

15.3 19.6 21.0 23.6 25.4 27.6

dB,

pH = 2

b

8.9 9.5 10.9 13.6 18.2 23.8

dB,

b pH = 9

8.9 9.0 9.3 9.6 9.9 10.1

Calculated from the Eq. (4) based on water flux experiments. Calculated from the Eq. (6) based on BSA rejection experiments.

Fig. 7. BSA rejection for the prisitine membrane and the blend membranes at pH 2 (4), 7.4 (s) and 9 (h).

3.5. Transport property of BSA solution BSA solutions with different pH values permeating through the membranes are studied and the results are shown in Fig. 7. As can be seen, the BSA rejection decreases with increasing P(AA-co-BA) microgels content in the blend membranes. According to the FESEM results (Fig. 3), the lower rejection ratio should be ascribed to the larger pore size of the membrane. Carefully observing Fig. 7, it can be noted that the rejection variation with pH value shows a noticeable different between the pristine membrane and the blend membrane. The pristine PVDF membrane M00 has the same separation characteristics for BSA solutions with different pH values. However, the blend membranes show a gradual decrease of BSA rejection as the pH of the feed solution decreases.

Furthermore, the decline tendency becomes larger with increasing microgels content in blend membranes. For example, the BSA rejections for M05 are 89.8%, 89.0% and 84.8% at pH 9, 7.4 and 2 respectively, while those for M25 are 82.1%, 76.7% and 25.0% under the same conditions. Further observing Fig. 7, it can be also found that the rejection variation with microgels content is greatly affected by pH conditions. At pH 7.4 and 9, the BSA rejections of the blend membranes show no significant difference; however, at pH 2, it shows a significant decrease with increasing microgels content in blend membranes. Separation capability of a pH-sensitive membrane is a collective effect of membrane pore size and electrostatic interaction. In one hand, varying pH can lead to conformational change of P(AA-coBA) microgels and thus change the pore size of the membrane. In this case, the membrane pores are small at pH 9 due to the swelling of the P(AA-co-BA) microgels in the skin layer of the blend membranes, which favors a high rejection ratio for membranes. As the solution pH decreases, the membrane pore size becomes larger due to the collapse of P(AA-co-BA) microgels, which will result in a less steric hindrance and thus favors a low rejection. On the other hand, electrostatic interactions between BSA and P(AA-co-BA) microgels in the membrane also influences the rejection. It is well known that the pI of BSA is about 4.9, where a BSA molecule carries no net change [40]. The BSA molecules carry a negative charge at a pH above their pI. Meanwhile, P(AA-co-BA) microgels are negatively charged (ACOO group) at a high pH(Fig. 4). Hence, amino groups of BSA and carboxyl groups of P(AA-co-BA) microgel tend to repel each other due to the electrostatic repulsion of the negatively charged ions. Thus, electrostatic repulsion between the negatively charged BSA and completely dissociated P(AA-co-BA) microgels in the membrane make the BSA difficultly pass through the membrane. As a result of small pores and electrostatic repulsions, a high rejection ratio above 82% is obtained at pH 9 for all the blend membranes. At pH 7.4, a decreased electrostatic repulsion as well as an enlarged pore causes a slightly lower rejection of membranes. At pH 2, a largely decreased electrostatic interaction and highly enlarged membrane pores lead to a drastic decline of rejection with increasing microgels content in blend membranes. The above results suggest that the electrostatic interaction as well as the membrane pore size dominates the BSA separation. The adjustable ability of the pore size by changing microgels conformation is further investigated in the BSA filtration. Based on the BSA rejection data, the pore sizes of the membranes are calculated according to Eq. (6) and shown in Table 3. The pore size of the membrane increases with increasing microgels content. Furthermore, the pore sizes at pH 2 are about 1.0, 1.1, 1.2, 1.4, 1.8 and 2.4 times large as that at pH 9 for M00, M05, M10, M15, M20 and M25 respectively, indicating the pore size adjusted by microgels increases with increasing microgels content. For a same

Fig. 8. A Schematic representation of the response mechanism of pH-sensitive PVDF/P(AA-co-BA) membranes.

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Fig. 9. The permeability (A) and total fouling ratio (B) of the prisitine membrane and the blend membranes.

blend membrane, the size change of the membrane pore on filtrating BSA filtration is not consistent with that on filtrating water. An important point for this difference is possibly because the pore sizes are obtained from different calculating methods. In addition, the difference of electrostatic and electroviscous effect on BSA solution and water filtration is possible another important factor [41]. In any case, we can confirm that pH-sensitive filtration by blend membranes can be realized by blending P(AA-co-BA) microgels in PVDF membranes. On the basis of the above results, the concept of pH-sensitive water flux and BSA rejection is schematically illustrated in Fig. 8. When pH is lower than pKa of PAA, the protonated P(AA-co-BA) microgels shrink and then opens the membrane pores, which accelerates the water flow rate. At the same time, a larger pore size of blend membranes leads to a low BSA rejection. When ambient pH is higher than pKa of PAA, the deprotonated P(AA-co-BA) microgels swell and consequently the membrane pores partly closes, the water flux is thus low and the rejection is high. The permeation of BSA molecule through the blend membrane is also restricted by the electrostatic interaction due to the chemical valve behavior of the pH-sensitive P(AA-co-BA) microgels under different pH conditions. Thus, the P(AA-co-BA) microgels can act as a sensor of pH regulating the membrane filtration by varying pH values. 3.6. Antifouling performance assessment The antifouling performance of the membrane is investigated by selecting BSA as a representative foulant and the results are shown in Fig. 9. As can be seen, all the JBSAs are smaller than Jw1s. In general, the flux decrease is caused by the concentration polarization or membrane fouling. Since the concentration polarization could be neglected as the high molecular weight BSA solution is cross-flow filtration driven by a diaphragm pump, the flux decline is possibly caused by a combination of reversible and irreversible membrane fouling [31,42]. For further differentiating the fouling, the fouled membranes are washed by water. As shown, the Jw2 is still smaller than Jw1 after washing. Since the flux can be recovered for a reversible membrane fouling, this result further indicates the pore channels are blocked to some extent by the irreversible membrane fouling. The antifouling performance is comparably evaluated by relative flux recovery ratio (FR). The FR in Fig. 9A follows an ascending order with increasing microgels in blends membranes, confirming that the BSA adsorption and deposition are suppressed by incorporating hydrophilic P(AA-co-BA) microgels into the membrane. To further quantitatively evaluate the reversible and irreversible fouling of the membranes, fouling resistance ratio is calculated and the results are shown in Fig. 9B. The total fouling ratio Rt shows

an decreasing tendency from M00 to M25. The lower the values of Rt, the better antifouling performance of the membranes [43]. However, the irreversible fouling cannot be eliminated completely. Furthermore, the reversible fouling ratio Rr increases while the irreversible fouling ratio Rir decreases with increasing microgels in blend membranes, which clearly indicates that more BSA adsorption and deposition can be removed by a water rinse when more microgels are incorporated into the membrane. A strong electrostatic repulsion between negatively charged P(AA-co-BA) microgels and negatively charged BSA causes a minimal adsorption of BSA at pH 7.4, which is possibly responsible for the easy cleaning. Thus, the antifouling performance of the blend membranes can be improved by increasing P(AA-co-BA) microgels additives. 4. Conclusion P(AA-co-BA) microgels were facilely used to confer pH-sensitive characteristics on PVDF membrane by phase-separation of a mixture of PVDF/P(AA-co-BA) microgels/DMF in aqueous solution. As a pore-forming agent, P(AA-co-BA) microgel induced the well evolution of finger-like pore structures and surface pores in blend membranes. As a hydrophilic modifier, P(AA-co-BA) microgels improved the hydrophilicity of membrane surface and changed the surface chemistry composition. These changes were resulted from P(AA-co-BA) microgels addition. With increasing P(AA-co-BA) microgels content in blend membranes, the roved performances such as porosity, surface hydrophilicity, water flux and antifouling property were achieved. As a pH-sensitive additive, P(AA-co-BA) microgel imparted pH-sensitive filtration to the membrane. At pH range of 2–9, these novel mixed matrix membranes had great potential for selectively permeating and rejecting some solutes in water treatment. Acknowledgments The authors acknowledge financially sponsored of this work by National Natural Science Foundation of China (No. 51003076, 21101113), Science and Technology Commission Foundation of Tianjin (No. 10JCZDJC22000) and Universities of Science and Technology Development Fund Planning Project of Tianjin (No. 20120308). References [1] M.A.C. Stuart, W.T.S. Huck, J. Genzer, M. Muller, C. Ober, M. Stamm, G.B. Sukhorukov, I. Szleifer, V.V. Tsukruk, M. Urban, F. Winnik, S. Zauscher, I. Luzinov, S. Minko, Nat. Mater. 9 (2010) 101–113. [2] D. Wandera, S.R. Wickramasinghe, S.M. Husson, J. Membr. Sci. 357 (2010) 6– 35.

66 [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]

Y. He et al. / Reactive & Functional Polymers 74 (2014) 58–66 H. Meng, J. Intell. Mater. Syst. Struct. 21 (2010) 859–885. D. Roy, J.N. Cambre, B.S. Sumerlin, Prog. Polym. Sci. 35 (2010) 278–301. P. Brown, C.P. Butts, J. Eastoe, Soft Matter 9 (2013) 2365–2374. I.-K. Park, K. Singha, R.B. Arote, Y.-J. Choi, W.J. Kim, C.-S. Cho, Macromol. Rapid Commun. 31 (2010) 1122–1133. C. Zhao, S. Nie, M. Tang, S. Sun, Prog. Polym. Sci. 36 (2011) 1499–1520. G. Jeon, S.Y. Yang, J.K. Kim, J. Mater. Chem. 22 (2012) 14814–14834. C.D.H. Alarcon, S. Pennadam, C. Alexander, Chem. Soc. Rev. 34 (2005) 276–285. M. Ulbricht, Polymer 47 (2006) 2217–2262. S. Belfer, React. Funct. Polym. 54 (2003) 155–165. L. Ying, E.T. Kang, K.G. Neoh, J. Membr. Sci. 224 (2003) 93–106. M. Ulbricht, H. Yang, Chem. Mater. 17 (2005) 2622–2631. L. Ferro, O. Scialdone, A. Galia, J. Supercrit. Fluids 66 (2012) 241–250. K.C. Khulbe, C. Feng, T. Matsuura, J. Appl. Polym. Sci. 115 (2010) 855–895. M. Sßahin, H. Görçay, E. Kır, Y. S ß ahin, React. Funct. Polym. 69 (2009) 673–680. C. Zhao, J. Xue, F. Ran, S. Sun, Prog. Mater. Sci. 58 (2013) 76–150. F. Liu, N.A. Hashim, Y. Liu, M.R.M. Abed, K. Li, J. Membr. Sci. 375 (2011) 1–27. G.R. Guillen, Y.J. Pan, M.H. Li, E.M.V. Hoek, Ind. Eng. Chem. Res. 50 (2011) 3798–3817. C. Mbareck, Q.T. Nguyen, O.T. Alaoui, D. Barillier, J. Hazard. Mater. 171 (2009) 93–101. C. Charbonneau, T. Nicolai, C. Chassenieux, O. Colombani, M. Miriam de Souza Lima, React. Funct. Polym. 73 (2013) 965–968. K.Y. Yoon, Z. Li, B.M. Neilson, W. Lee, C. Huh, S.L. Bryant, C.W. Bielawski, K.P. Johnston, Macromolecules 45 (2012) 5157–5166. X. Zhai, J. Meng, R. Li, L. Ni, Y. Zhang, Desalination 274 (2011) 136–143. S. Wall, Curr Opi Colloid Interface Sci 15 (2010) 119–124. A. Mansourizadeh, A.F. Ismail, J. Membr. Sci. 348 (2010) 260–267.

[26] X. Chen, S. Bi, C. Shi, Y. He, L. Zhao, L. Chen, Colloid. Polym. Sci. (2013) 1–10. [27] E. Yuliwati, A.F. Ismail, T. Matsuura, M.A. Kassim, M.S. Abdullah, Desalination 283 (2011) 206–213. [28] T. Xiang, Q. Zhou, K. Li, L. Li, F. Su, B. Qian, C. Zhao, Sep. Sci. Technol. 45 (2010) 2017–2027. [29] S. Ikeda, K. Nishinari, Biomacromolecules 1 (2000) 757–763. [30] Y. Hao, A. Moriya, T. Maruyama, Y. Ohmukai, H. Matsuyama, J. Membr. Sci. 376 (2011) 247–253. [31] J.-H. Li, X.-S. Shao, Q. Zhou, M.-Z. Li, Q.-Q. Zhang, Appl. Surf. Sci. 265 (2013) 663–670. [32] X. Chen, C. Shi, Z. Wang, Y. He, S. Bi, X. Feng, L. Chen, Polym. Compos. 34 (2013) 457–467. [33] Z. Zhenxin, T. Matsuura, J. Colloid Interface Sci. 147 (1991) 307–315. [34] T.-H. Young, L.-W. Chen, Desalination 103 (1995) 233–247. [35] P. Wang, J. Ma, Z. Wang, F. Shi, Q. Liu, Langmuir 28 (2012) 4776–4786. [36] W. Chen, Y. Su, J. Peng, X. Zhao, Z. Jiang, Y. Dong, Y. Zhang, Y. Liang, J. Liu, Environ. Sci. Technol. 45 (2011) 6545–6552. [37] S.S. Madaeni, A.H. Taheri, Chem. Eng. Technol. 34 (2011) 1328–1334. [38] R.M. Boom, I.M. Wienk, T. van den Boomgaard, C.A. Smolders, J. Membr. Sci. 73 (1992) 277–292. [39] H.F. Mark, J.I. Kroschwitz, Encyclopedia of Polymer Science and Technology, John Wiley & Sons, 2003. [40] N. Akkilic, M. Mustafaev, V. Chegel, Macromol. Symp. 269 (2008) 138–144. [41] A. Bentien, T. Okada, S. Kjelstrup, J. Phys. Chem. C 117 (2012) 1582–1588. [42] R. Bian, K. Yamamoto, Y. Watanabe, Desalination 131 (2000) 225–236. [43] M. Hashino, K. Hirami, T. Ishigami, Y. Ohmukai, T. Maruyama, N. Kubota, H. Matsuyama, J. Membr. Sci. 384 (2011) 157–165.