Preparation of pH-responsive membranes with amphiphilic copolymers by surface segregation method☆

Yanlei Su,Yuan Liu,Xueting Zhao,Yafei Li,Zhongyi Jiang*

Key Laboratory for Green Chemical Technology,School of Chemical Engineering and Technology,Tianjin University,Tianjin 300072,China

Collaborative Innovation Center of Chemical Science and Engineering(Tianjin),Tianjin University,Tianjin 300072,China

Keywords:Amphiphilic copolymers pH-responsive Surface segregation

ABSTRACT Novel pH-responsive membranes were prepared by blending pH-responsive amphiphilic copolymers with polyet hersulfone(PES)via a nonsolvent-induced phase separation(NIPS)technique.The amphiphilic copolymers bearing Pluronic F127 and poly(methacrylic acid)(PMAA)segments,abbreviated as PMAA n–F127–PMAA n,were synthesized by free radical polymerization.The physical and chemical properties of the blend membranes were evaluated by scanning electron microscopy(SEM),Fourier transform infrared(FTIR)spectrum,water contact angle,Zeta potential and X-ray photoelectron spectroscopy(XPS).The enrichment of hydrophilic PMAA segments on the membrane surfaces was attributed to surface segregation during the membrane preparation process.The blend membranes had significant pH-responsive properties due to the conformational changes of surface-segregated PMAA segments under different pH values of feed solutions.Fluxes of the blend membranes were larger at low pH values of feed solutions than that at high pH values.The pH-responsive ability of the membranes was enhanced with the increase of the degree of PMAA near-surface coverage.

1.Introduction

Stimuli-responsive membranes are able to exhibit switchable permeability and selectivity in response to external triggers,such as pH value,ionic strength,temperature,light,electric/magnetic field and chemicals[1–7].Among the triggers,the adjustment of pH of feed solutions is a green,low-energy and simple method to induce the change of membrane permeability and selectivity[4,6].Therefore,there has been an increasing interest in the preparation and development of pH-responsive membranes,ranging from basic researches to practical applications.The pH-responsive behaviors of membranes can be realized by introducing pH-responsive polymers or copolymers bearing polyelectrolyte segments on the membrane surfaces,such as poly(N,N-dimethylaminoethyl methacrylate)(PDMAEMA),poly(acrylic acid)(PAA),poly(methacrylic acid)(PMAA),and poly(4-vinylpyridine)(P4VP)[4–7].These polyelectrolyte segments can exhibit conformational transitions under different pH values of feed solutions[1,8],leading to the pH-adjustable pore size,cut-off molecular mass,and permeate flux of membranes.

Coating and grafting are conventional approaches to introduce pH-responsive polyelectrolyte polymers on the membrane surfaces.A coating method,such as dipping-coating and spin-coating,is easily operated,but has the disadvantages of membrane pore blockage and the functional polymers' easy detachment from membrane matrix due to the weak adhesion force[9–11].The grafting method is typically preformed with the covalent bond to obtain a stable linkage,but harsh preparation conditions are usually required,such as plasma,UV light,and γ ray irradiation-induced grafting polymerization[2,12,13].Moreover,both the coating and grafting methods required an additional post-treatment process,increasing the membrane manufacture cost.Therefore,an in situ method is regarded as the promising and attractive method to generate pH-responsive segments on the membrane surfaces.

As a facile and simple method of membrane surface modification,surface segregation can not only overcome the disadvantages of coating and grafting methods,but also generate functional polymer brushes through in-situ and three-dimensional modification[11,14–19].In this method,an amphiphilic comb or block copolymer,consisting of hydrophobic backbones and hydrophilic side chains,is employed as the surface segregation additive.Hydrophilic blocks can spontaneously aggregate to the membrane surfaces due to the entropic driving force during the membrane preparation process,constructing the functional layers on the membrane surfaces and internal pore surfaces[11,14–20].Meanwhile,the hydrophobic blocks can tightly entangle with membrane matrix,ensuring the stable existence of surface-segregated functional brushes on the membrane surfaces[11,14–19].Pluronic F127,an amphiphilic copolymer,has been widely utilized as a surface segregation additive,which endows membrane with both anti-fouling and self-healing properties[14,15].However,few researches are carried out to prepare pH-responsive membranes by the surface segregation method.

In the present study,the pH-responsive amphiphilic copolymers bearing Pluronic F127 and poly(methacrylic acid)(PMAA)segments,abbreviated as PMAAn–F127–PMAAn,were synthesized by free radical polymerization.PMAAn–F127–PMAAnwas then incorporated into the PES matrix to prepare PES/PMAAn–F127–PMAAnblend membranes by the surface segregation method.The membranes were systematically characterized by scanning electron microscopy(SEM),Fourier transform infrared(FTIR)spectrum,water contact angle,Zeta potential and X-ray photoelectron spectroscopy(XPS).The relevant surface segregation behavior and the mechanism were analyzed.The pH-responsive properties of the blend membranes were investigated through permeate experiments.

2.Experimental Section

2.1.Materials

PES(Ultrason E6020P,BASF Co.,Germany)was dried at 110°C for 12 h before using.Pluronic F127(PEO98–PPO65–PEO98,Mn=12600),was purchased from Sigma Chemical Co.(USA).Methacrylic acid(MAA),N,N-dimethyl form amide(DMF),ammonium cerium(IV)nitrate,nitric acid(65%,by mass),sodium hydroxide,hydrochloric acid,disodium phosphate and citric acid were all purchased from Kewei Chemical Reagent Co.(Tianjin,China).Bovine serum albumin(BSA)was purchased from Institute of Hematology,Chinese Academy of Medical Science(Tianjin,China).The deionized water was used throughout the experiments.

2.2.Synthesis and characterization of amphiphilic copolymers PMAAn–F127–PMAAn

The amphiphilic copolymers bearing Pluronic F127 and poly(methacrylic acid)(PMAA)segments,were synthesized via free radical polymerization using a cerium ion redox system as initiator in aqueous acidic medium under nitrogen atmosphere[21,22].Table 1 showed the formulations of chemical synthesis.All the polymerization reactions were carried out under a nitrogen atmosphere at 45°C and initiated by Ce(IV)/HNO3solution(2.0 mmol Ce salt dissolved in 20 ml,1.0 mol·L?1nitric acid solution).After a polymerization reaction for 8 h,the reaction mixtures were terminated by exposing to air.The synthesized copolymers were purified by dialyzing with a dialysis bag in water at room temperature for 3 days.After evaporation in a rotary evaporator at 50°C,the product copolymers were further dried in a freeze dryer to remove the residual water.At last,the amphiphilic PMAAn–F127–PMAAncopolymers were obtained,where n represented the polymerization degree of PMAA blocks.

Table 1 Synthesis composition,polymerization degree(n)and molecular mass(M n)of the PMAA n–F127–PMAA n copolymers

FTIR spectra of PMAAn–F127–PMAAncopolymers were measured by a FTIR spectrometer(VERTEX 70,Bruker Co.,Germany)using the KBr pellet method with air as the background.Chemical compositions of the synthesized PMAAn–F127–PMAAncopolymers were analyzed by1H nuclear magnetic resonance(NMR,INOVA-500,Varian Inc.,USA)using deuterated chloroform as solvent and tetramethylsilane as the internal standard.

2.3.Preparation and characterization of membranes

The pH-responsive blend membranes were prepared by the NIPS technique,utilizing PES as the membrane material,the as-synthesized PMAAn–F127–PMAAncopolymers as the additives,DMF as the solvent and water as the nonsolvent coagulation bath.The detailed components of casting solutions were presented in Table 2.The casting solutions were agitated for 5 h at 60°C.Then the mixtures were kept static to achieve absolute release of bubbles.After cooling to room temperature,the casting solutions were cast on the glass substrates with a steel knife.Subsequently,the glass plates were immersed in the nonsolvent coagulation bath.After peeling off from the glass plates,the membranes were rinsed with water to remove residual solvent DMF.Then,the asfabricated membranes were preserved in water prior to utilization and denoted as PES/PMAAn–F127–PMAAn.PES/F127 membrane was fabricated as the control membrane following the above method.In order to distinguish the control membrane,PES/PMAAn–F127–PMAAnmembranes were named as blend membranes.

Table 2 Composition of casting solutions of the control and blend membranes

The cross-section and top surface morphologies of as-prepared membranes were observed by SEM(Nova Nanosem 430,FEI Co.,USA).The membrane samples freeze-dried with a vacuum freeze dryer(FD-1C-50,Boyikang Co.,China)were fractured in liquid nitrogen and then sputtered with gold for generating electric conductivity.

Membrane porosity was determined by the method of dry–wet mass.The wet mass of membrane was measured after wiping the excess water.The dry mass of membrane was measured after drying in a freeze dryer.The porosity of membrane was calculated as follows:

where ε is the membrane porosity,Ww(g)is the wet membrane mass,Wd(g)is the dry membrane mass,ρw(g·cm?3)is the water density,A(cm2)is the membrane area and δ(cm)is the membrane thickness.

The membrane mean pore size rm(nm)was determined by the Guerout–Elford–Ferry equation:

where η is the water viscosity(8.9 × 10?4Pa·s),Q is the volume of the permeation of water per unit time(m3·s?1),and ΔP is the operation pressure(Pa).

Surface functional groups of the blend membranes were measured using an attenuated total reflection-Fourier transform infrared spectrometer(ATR-FTIR,VERTEX 70,Bruker Co.,Germany),equipped with both horizontal attenuated total reflectance accessories.The near surface compositions of the fabricated membranes were analyzed by XPS(Perkin Elmer Phi 1600 ESCA system)using Mg Kα(1254.0 eV)as the radiation source.Survey scans were taken in the range of 0–1100 eV at a take-off angle of 90°.

Water contact angles of membrane surfaces were measured by a contact angle goniometer(JC2000C Contact Angle Meter,Power each Co.,Shanghai,China).To get a reliable value,six measurements at different spots on each surface were carried out and an average value was calculated.Zeta potentials were measured through tangential flow streaming potential measurement by an electrokinetic analyzer(Anton Paar KG,Austria)in 0.001 mol·L?1KCl solution with a pH value of 7.0.Zeta potential values were calculated using the Helmholtz–Smoluchowski equation.

2.4.Evaluations of membrane pH-responsive properties

The water fluxes of the control and blend membranes at different pH values were measured by a dead-end stirred cell filtration apparatus equipped with a nitrogen gas cylinder and solution reservoir.All membranes were fixed in the filtration test cell(Model 8200,Millipore Co.,USA)with a volume capacity of 200 ml and an inner diameter of 62 mm.The effective area of each membrane was 28.7 cm2.The pH values of aqueous solutions from 2.0 to 10.0 were adjusted with 0.1 mol·L?1HCl or 0.1 mol·L?1NaOH solutions.Firstly,each membrane sample was compacted at 0.15 MPa for 30 min to obtain a stable flux.Then the operation pressure was decreased to 0.10 MPa for flux measurement.The flux J(L·m?2·h?1)was calculated by the following equation:

where V(L)is the volume of permeated water,A(m2)is the membrane effective area and Δt(h)is the permeation time.The reversibility of pH-responsive fluxes was tested with the alternative feed solutions of pH 2.0 and 10.0 for several cycles.To evaluate the pH-responsive ability,pH-responsive coefficient(α)was defined and calculated by the following expression:

where JpH2.0and JpH10.0were the water fluxes of pH 2.0 and pH 10.0,respectively.

BSA rejection experiments at varied pH values were carried out by the above mentioned dead-end filtration apparatus.Feed solutions were prepared by dissolving BSA in buffer solutions at different pH values(BSA concentration is 1.0 g·L?1).After 20 min of BSA filtration,the feed and permeate solutions were collected and the rejection ratio(R)of BSA was calculated according to the following equation:

where Cpand Cf(g·L?1)are the concentrations of BSA in the permeate and feed solutions,respectively.The concentrations of BSA were measured by a UV–vis spectrophotometer(UV-2800,Hitachi Co.,Japan)at a wavelength of 278 nm.

3.Results and Discussion

3.1.Characterization of amphiphilic copolymers PMAAn–F127–PMAAn

The synthesis of amphiphilic copolymers PMAAn–F127–PMAAnwas carried out through free radical polymerization using a cerium ion redox system as initiator in aqueous acidic medium[21,22].Each F127 molecule has two carbon atoms linked with hydroxyl groups at the end of polymer chains,which were able to be activated by the redox reaction between cerium ions(IV)and CH–OH groups of F127[18].The obtained free radicals on those carbon atoms were transferred from F127 to MAA monomer,then the polymerization propagated.It was possible to control the polymerization degrees of PMAA blocks of the synthesized copolymers at different amounts of the added MAA monomers.

FTIR spectra of amphiphilic copolymers F127 and PMAAn–F127–PMAAnwere given in Fig.1.The peak at 1108 cm?1corresponding to the C–O–C functional group was observed from the spectra of both F127 and PMAAn–F127–PMAAncopolymers.Compared with the spectrum of F127,a new peak at 1724 cm?1occurred in the spectra of PMAAn–F127–PMAAncopolymers,which was assigned to the carbonyl functional groups of PMAA segments,indicating the existence of PMAA blocks in the synthesized copolymers.The signal intensity of the peak at 1724 cm?1was increased with an increase of MAA content in the formulations for chemical synthesis.

Fig.1.FTIR spectra of F127,PMAA2.1–F127–PMAA2.1 and PMAA8.1–F127–PMAA8.1 copolymers.

Detailed chemical compositions of PMAAn–F127–PMAAncopolymers were further determined by1H NMR.The1H NMR spectra of F127 and PMAAn–F127–PMAAncopolymers were presented in Fig.2.The proton signal at δ=1.11 was attributed to the protons of the–CH3group from both PPO and PMAA.The characteristic peak at δ=3.38 was assigned to the proton of the–O–CH–group from PPO.The characteristic peak at δ=3.61 was assigned to the protons of the–O–CH2–group from both PEO and PPO[18,23].The polymerization degree of PMAA blocks(n)in the PMAAn–F127–PMAAncopolymers was an important parameter to determine their properties.There was one–O–CH–group in each PO unit,and one–CH3group in each PO and MAA unit.The polymerization degree of PMAA blocks in the PMAAn–F127–PMAAncopolymers was obtained from the signal intensity ratio of–O–CH– protons of PPO(Ia)and the total methyl protons of PPO and PMAA(Ib+c)according to the following expression:

where Iais the intensity of the proton peak for the–O–CH–group in PPO blocks,and Ib+cis the intensity of the proton peaks of total methyl protons from PMAA and PPO blocks.nPOdenoting the polymerization degree of PO units in F127 was 65.The calculated values of n and molecular structures of PMAAn–F127–PMAAncopolymers were given in Table 1.These results indicated the effective grafting polymerization of MAA on F127.Most of the added MAA monomers in the synthesis process were reacted and grafted on F127.

Fig.2.1H NMR spectra of F127,PMAA2.1–F127–PMAA2.1,PMAA4.2–F127–PMAA4.2 and PMAA8.1–F127–PMAA8.1 copolymers.

Based on n values,the molecular mass(Mn)of PMAAn–F127–PMAAncopolymers was calculated by the following equation:

where MF127is the molecular mass of F127,and 86 is the molecular mass of MAA monomer.The calculated molecular mass(Mn)of PMAAn–F127–PMAAncopolymers was also listed in Table.1.The as synthesized PMAAn–F127–PMAAncopolymers were used to prepare the pH-responsive membranes.

3.2.Characterization of membranes

To characterize the cross-sectional and top surface morphologies of the control(1#membrane)and blend membranes(2#,3#and 4#membranes),SEM was utilized and the images were given in Fig.3.All the membranes exhibited the typical asymmetric structures,which included a top dense layer,a porous sublayer and fully developed macropores at the bottom.There was no evident morphological change between the control and blend membranes,indicating that the bulk PES retained excellent membrane-forming ability after incorporating PMAAn–F127–PMAAncopolymers.In addition,it was clear that there were no cracks and big holes on the membrane top surfaces.

ATR-FTIR was employed to investigate the functional groups on the control and blend membrane surfaces,and the results were depicted in Fig.4.Compared with the control membrane(1#membrane),the blend membranes(2#and 4#membranes)showed very similar FTIR spectra.Peaks at around 1151 and 1486 cm?1in reference to symmetric stretching of O=S=O and C=C stretching of aromatic nature were the typical characteristic peaks of PES.Beside the PES bands,a new weak peak at 1724 cm?1corresponding to C=O stretching vibration appeared in the spectra of the blend membranes,which indicated the existence of PMAAn–F127–PMAAncopolymers on the membrane surfaces.In addition,the corresponding intensity of the carbonyl peak at 1724 cm?1increased with an increasing PMAA polymerization degree of amphiphilic copolymers PMAAn–F127–PMAAn.

Water contact angle is a convenient and common method to assess the hydrophilicity and wetting characteristics of the membrane surface[6,24].In Table 3,the control PES membrane(1#membrane)possessed a contact angle of 55.6°± 1.9°,while the water contact angles of the blend membranes(2#,3#and 4#membranes)showed relatively lower values.With an increase of the PMAA polymerization degree,the water contact angles of the blend membranes were decreased from 50.3°± 1.8°to 39.4°± 2.1°.The results indicated that the introduction of PMAAn–F127–PMAAncopolymers effectively enhanced the hydrophilicity of membrane surfaces.

The surface charge of the control and blend membranes was evaluated by streaming potential measurement and the results were given in Table 3.Zeta potential values of the blend membranes revealed more negative charge than that of the control membrane.PMAA segments on the membrane surfaces were dissociated and negatively charged in aqueous solution of pH 7.0[1–3,5].With an increase of the PMAA polymerization degree,the Zeta potential of the blend membranes shifted from(?37.1±1.0)mV to a more negative value(?49.2±1.0)mV,which was an indirect reflection of more negatively charged PMAA functional groups appearing on the membrane surfaces.

3.3.Surface segregation behavior and mechanism

Fig.3.SEM cross-section and top surface morphologies of the membranes:(a)and(b)1#membrane;(c)and(d)2#membrane;(e)and(f)3#membrane;and(g)and(h)4#membrane.

Fig.4.FTIR spectra of the control membrane(1#membrane)and blend membranes(2#and 4#membranes).

The near-surface compositions of the control and blend membranes were determined from XPS analysis.C,O and S elements appeared in the wide-scan XPS spectra for all fabricated membranes.Fig.5 showed the high-resolution XPS spectra of C1s peaks for all as fabricated membranes.The peaks were deconvoluted using a sum of Lorentzian–Gaussian functions.To obtain quantitative data,the high-resolution XPS spectra of C1s peak were curve- fitted with several peaks representing different chemical environments.The C1s spectrum obtained from the control PES membrane(1#membrane)was curve- fitted with two component peaks:one peak with the binding energy at around 284.9 eV was identified for carbon atom in C–C species,another peak with the binding energy at around 286.2 eV was assigned to carbon atom in C–O species[5,14,15,18].In comparison,a new component peak at the binding energy of around 288.9 eV for carbon atom in C=O species appeared in the high-resolution C1s spectra of the blend membranes(2#,3#and 4#membranes)[5,11,18].Since PMAA segments were the only source of C=O species,the results of XPS spectra further indicated that the PMAA segments existed on the membrane surfaces.

The experimental value of the carbon molar ratio in C=O species on the membrane surface(Ae)based on XPS data was calculated by the following equation:

where AC–C,AC–Oand AC=Oare the fitted peak areas of carbon atoms in C–C,C–O and C=O species,respectively.The calculated Aevalues of the blend membranes were presented in Table 3.It could be seen that Aevalues were increased with an increase of PMAA polymerization degree.

The theoretical value of the carbon molar ratio in C=O species(At)was calculated according to the composition of the casting solution for membrane fabrication,assuming that all the PMAAn–F127–PMAAncopolymers were completely entrapped into the membranes.Atwas calculated by the following equation:

Table 3 The theoretical(A t)and experimental(A e)values of the carbon molar ratio in C=O species,the degree of surface enrichment(E)and PMAA near-surface coverage(ΦPMAA),water contact angles,zeta potentials,membrane porosity(ε)and mean pore size(r m)of the control and blend membranes

Fig.5.High-resolution XPS spectra of C1s of the control membrane(1#membrane)and blend membranes(2#,3#and 4#membranes).

where CC=Ois the carbon molar number in C=O species from PMAA blocks,and Ctis the total carbon molar number in PES and PMAAn–F127–PMAAnaccording to the composition of the casting solution in Table 2.Atvalues were calculated and given in Table 3.The values of Aewere remarkably higher than that of At,which was the typical consequence of surface segregation during the membrane preparation process[14–19].The thermodynamic incompatibility between the hydrophilic PMAA segments and hydrophobic PES matrix as well as the hydrogen bonding interactions between the amphiphilic PMAAn–F127–PMAAncopolymers and water were favorable for the hydrophilic PMAA segments to migrate toward the interface of membrane and water phase[5,8,11,14,15].Meanwhile,the hydrophobic PPO segments in the PMAAn–F127–PMAAncopolymers were entangled with the PES membrane matrix[14,15,25].As a result,the hydrophilic PMAA segments were tightly anchored on the membrane surfaces.

The degree of surface enrichment(E)was calculated according to the following expression:

The corresponding values of E of the blend membranes were also listed in Table 3.For example,the E value for 4#membrane was 8.93,which indicated that the PMAA content on the membrane surface was 8.93 times higher than that in the membrane matrix.Therefore,surface segregation was a facile and simple method to introduce polyelectrolyte segments on the membrane surfaces.

The degree of PMAA near-surface coverage(ΦPMAA)was introduced to evaluate the surface modification,which was calculated using the following expression:

where the factor 1/4 accounts for the fact that there is 1 C=O carbon atom in total4 carbon atoms of each MAA unit.If the membrane surface were totally covered with PMAA segments,the ΦPMAAvalue would be 100%.The calculated ΦPMAAvalues for the blend membranes(2#,3#and 4#membranes)according to XPS analysis were also showed in Table 3.It was found that ΦPMAAvalues were increased from 10.92%to 31.44%with an increase of PMAA polymerization degree,indicating that the coverage of PMAA segments on membrane surfaces was improved to a relatively high level.The increase of hydrophilic PMAA segments on the membrane surfaces would improve the hydrophilicity and surface charge of membrane,which was consistent with the water contact angle and zeta potential results.

3.4.pH-responsive properties of membranes

Fig.6.Water fluxes of the control membrane(1#membrane)and blend membranes(2#,3#and 4#membranes)as a function of pH values of feed solutions.

The existence of PMAA segments on the membrane surfaces and pore walls exhibited conformational transitions in response to pH values of feed solutions,endowing the membranes with pH-responsive properties.Water fluxes of the control and blend membranes were investigated as a function of pH values of feed solutions and the results were showed in Fig.6.The water fluxes of the control membrane(1#membrane)were relatively steady at(180.8±2.8)L·m?2·h?1when pH values increased from 2.0 to 10.0.However,water fluxes of blend membranes(2#,3#and 4#membranes)showed pH-responsive behavior.Water fluxes of 2#,3#and 4#membranes were 192.1,218.5 and 238.6 L·m?2·h?1at pH 2.0,which were decreased to 140.4,127.2 and 117.6 L·m?2·h?1at pH 10.0,respectively.The rapid decrease of water fluxes occurred at pH values ranging from pH 4.0 to 6.0.Under feed solutions with different pH values,the pH-responsive fluxes were interpreted by the conformational transitions of surface-segregated PMAA segments on the membrane surfaces and pore surfaces[1–3,5].PMAA is a weak polyelectrolyte acid with the acid dissociation constant p Kaof 5.5[1–3,5].When the pH values are higher than 5.5,the carboxyl groups of the PMAA chains were dissociated and negatively charged.Therefore,the PMAA chains were stretched due to the strong electrostatic repulsion between PMAA segments.The extension of the dissociated PMAA chains under higher pH value conditions would shrink the pores to a certain extent.According to Hagen–Poiseuille's Law,the water flux of a porous membrane was determined by the fourth power of the pore diameter[5].Therefore,the water fluxes of blend membranes became low when pH values were higher than 5.5.On the contrary,when pH values of feed solutions were lower than 5.5,the carboxyl groups of the PMAA segments were protonated.Since the electrostatic repulsion was decreased,the PMAA chains exhibited contracted conformation.The shrinkage of neutral PMAA chains on both membrane surfaces and pore walls resulted in the enlarged pore diameters,so that the water fluxes of the blend membranes increased at lower pH values of feed solutions.

The calculated α values were 1.37,1.72 and 2.03 for 2#,3#and 4#membranes,respectively,which had a relationship with the degree of PMAA segment surface coverage.According to XPS analysis,the degree of PMAA near-surface coverage was increased with an increase ofPMAA polymerization degree in PMAAn–F127–PMAAncopolymers.The higher the degree of PMAA segment surface coverage was,the stronger the pH-responsive ability of the blend membranes was.4#membrane had the largest α value of about 2.03,which meant that the water flux at pH 2.0 was 2.03 times higher than that at pH 10.0.

For the asymmetric membrane,the molecular sieving property was significantly influenced by the effective pore diameter of the skin layer.BSA rejection ratio of the membrane was usually utilized as a parameter to probe the effective pore size[4,26].Fig.7 showed the BSA rejection ratios under pH values from 4.0 to 8.0 for the control and blend membranes(1#and 4#membranes).1#membrane possessed relatively stable BSA rejection ratios of above 95.0%.While BSA rejection ratios for 4#membrane were increased from 85.5%to 98.3%with an increase of pH values,which indicated the decrease of the effective pore size due to the extension of PMAA chains under higher pH values[1–3,5].In addition,the rejection results were consistent with the pH-dependent fluxes of the membranes,and the separation performance of the membranes was in the ultra filtration range.

Fig.7.BSA rejection ratios of the control membrane(1#membrane)and blend membranes(4#membrane)as a function of pH values of feed solutions.

3.5.Reversible and durable pH-responsive properties

Reversibility and durability of pH-responsive membranes were important for practical applications.2#and 4#membranes were used to evaluate the reversible and durable pH-responsive properties.As shown in Fig.8,the water fluxes for 2#membrane reversibly changed from(141.4 ± 3.4)to(193.6 ± 3.5)L·m?2·h?1,and 4#membrane had a reversible flux from(116.7± 3.5)to(233.4± 5.1)L·m?2·h?1.The nearly reversible flux changes indicated the reversible change of pore size under alternatively acidic and alkaline feed solutions[1–3,5].In the acidic feed solution,the PMAA segments exhibited a compacted conformation,so the effective pore size of the membranes was increased substantially.In the alkaline feed solution,the PMAA segments presented a highly extended conformation,leading to the decrease of membrane effective pore size[1–3,5].

Fig.8.Reversible changes of water fluxes for the blend membranes(2#and 4#membranes)with alternate pH values of 10.0 and 2.0 of feed solutions.

The calculated α values for the blend membranes kept steady values of about 1.37±0.06(2#membrane)and 2.00±0.06(4#membrane)during several cycles,respectively,which suggested that the pH-responsive abilities of the blend membranes were significantly durable.The durable pH-responsive ability was ascribed to that the reversible ionization and protonation of the surface-segregated PMAA segments resulted in the reversibly extended and contracted conformation of PMAA chains.The hydrophobic PPO segments tightly entangling with the hydrophobic PES matrix ensured the stable existence of surfacesegregated PMAA segments on the membrane surface[14,15,25].The reversible and durable pH-responsive properties made the membranes promising potential for practical applications,such as water treatment and bioseparation[4,6].

4.Conclusions

Amphiphilic copolymers,PMAAn–F127–PMAAn,were successfully synthesized by free radical polymerization.The pH-responsive membranes were fabricated by blending PMAAn–F127–PMAAnwith PES through the NIPS technique.The membrane surfaces were covered with abundant hydrophilic PMAA segments due to surface segregation during the membrane preparation process.The degree of PMAA nearsurface coverage was increased with an increase of PMAA polymerization degree.Due to the conformational variation of PMAA segments under different pH values,the resultant membranes exhibited significant,reversible and durable pH-dependent flux.The pH-responsive ability of the membranes was elevated at high degrees of PMAA nearsurface coverage.

Nomenclature

A membrane area,m2

Cfsolute concentrations of feed solution

Cpsolute concentrations of permeate solution

E the degree of surface enrichment,%

F127 PEO98–PPO65–PEO98

J water flux,L·m?2·h?1

ΔP operation pressure,MPa

R rejections,%

Δt operation time,h

V volume of permeated water,L

α pH-responsive coefficient

Φ the degree of surface coverage,%