聚苯并咪唑PBI的制备及性能研究

Beyond polyimide:Crosslinked polybenzimidazole membranes fororganic solvent nanofiltration(OSN)in harsh environmentsIrina B.Valtcheva,Santosh C.Kumbharkar,Jeong F.Kim,Yogesh Bhole,Andrew G.Livingston nDepartment of Chemical Engineering,Imperial College,Exhibition Road,South Kensington Campus,London SW72AZ,UKa r t i c l e i n f oArticle history:Received3November2013Accepted27December2013Available online18January2014Keywords:Organic solvent nanofiltration(OSN)Polybenzimidazole(PBI)CrosslinkingExtreme pHAcid/basefiltrationa b s t r a c tIn this work,we report a new class of organic solvent nanofiltration(OSN)membranes fabricated frompolybenzimidazole(PBI)which exhibit superior chemical stability compared to other well-known polymericmembranes such as polyimide.Integrally skinned asymmetric PBI membranes were prepared and crosslinkedusing either an aliphatic or an aromatic bifunctional crosslinker.Three batches of membranes with the samecomposition were prepared and crosslinked with each crosslinker.The membrane performance showedexcellent reproducibility in organic solvents and water in terms offlux and retention profile.Also,bothmembranes showed good tolerance towards extreme pH conditions.To critically assess their chemical stabilitythe membranes were tested in realistic chemical process conditions that employ different types of acids andbases,e.g.concentrated dichloroacetic acid in acetonitrile,piperidine in N,N-dimethylformamide(DMF)andmonoethanolamine in water.The membranes modified with aliphatic crosslinker could not retain theirproperties when DMF was used as the organic solvent.This was found to be due to dissolution of PBI in DMFrather than degradation due to pH exposure.On the other hand,it was shown that the membrane modifiedwith the aromatic crosslinker exhibits superior stability and higher permeances in comparison to themembrane crosslinked with the aliphatic crosslinker.The results obtained show that membranes fabricatedfrom crosslinked PBI were stable and have the potential for applications in chemically harsh conditions foundin processes ranging from pharmaceutical to petrochemical industries.&2014Elsevier B.V.All rights reserved.1.IntroductionSeparations of solutes in organic solvents are widely employed inpharmaceutical and chemical industries.Typical techniques used forfractional separation are distillation,adsorption andflash chromato-graphy.These processes usually involve the utilisation of fresh solventsand energy.In particular,flash chromatography is relatively cumber-some to scale-up due to its aspect ratio limitation[1].These operationsresult in increased cost of goods,and raise environmental concerns.Asa result,cost effective alternatives are desired in order to minimise theplant footprint,waste production and energy anicsolvent nanofiltration(OSN)is a membrane-based separation processwhich can provide the necessary molecular discrimination[2],and hasalready shown great potential in terms of savings[1,3]and applic-ability[4–6]to industrial processes.Nanofiltration(NF)is already astate-of-the-art process for water purification and treatment[7,8].However,the available NF and reverse osmosis(RO)membranes are inmost cases not stable in harsh and corrosive environments typicallyrequired for OSN[2].The solvent and pH resistance of the membraneis one of the main challenges for implementing OSN in relevantprocesses.Inorganic materials are ideal for such conditions since theydo not easily dissolve or deform in organic solvents.On the otherhand,they are difficult to fabricate,handle and are more expensive ascompared to organic membranes[2,9].For this reason the majority ofreported OSN membranes are made out of polymeric materials.The most exhaustively studied polymer for application in OSN ispolyimide(PI)[2,10,11].The usual solvents in which PI dissolvesare polar aprotic ones,such as N,N-dimethylformamide(DMF)anddimethyl sulfoxide(DMSO)and these are typically used in membranefabrication.For a PI membrane to be stable in applications using thesesolvents,it needs to be chemically crosslinked with diamines[10,11].PI is stable to some extent in weak acids and low concentrations ofbases but is not recommended for use in inorganic acids and degradeswhen exposed to high concentrations of organic and inorganic bases[12].Apart from solvent stability,tailored membrane performance is adesirable feature,e.g.varying the molecular weight cut-off(MWCO)1of PI membranes[13].A few polymeric membranes have been reported in literaturewhich can withstand either acids or bases in aqueous feed streams[14–16].However,these membranes are based on sulfonatedpolyether ether ketone(SPEEK)or polysulfone(PSf)which willnot withstand many organic solvents.Hence,there is a need forContents lists available at ScienceDirectjournal homepage:/locate/memsciJournal of Membrane Science0376-7388/$-see front matter&2014Elsevier B.V.All rights reserved./10.1016/j.memsci.2013.12.069n Corresponding author.Tel.:þ442075945582;fax:þ442075945639.E-mail address:a.livingston@(A.G.Livingston).1Molecular weight cut-off(MWCO)is defined as the lowest molecular weightof a solute rejected at90%by the membrane.Journal of Membrane Science457(2014)62–72alternative membrane materials which can withstand both organic solvents and acidic/basic conditions.Surface crosslinked chitosan/polyacrylonitrile composite NF membranes have been shown to maintain their stability under basic pH and in several important organic solvents [17].However,the acid/base resistance was only demonstrated in aqueous media.Polybenzimidazole (PBI,Fig.1)has been studied extensively for reverse osmosis [18–21].More recently PBI has gained much atten-tion for applications in gas separation [22],aqueous NF [23],forward osmosis [24]and fuel cells [25,26]due to its outstanding properties (thermal,mechanical and chemical stability in corrosive environ-ments)[27].In addition,PBI has the advantage of possessing excellent stability towards acids and bases [12].PBI dissolves in polar aprotic solvents,such as N ,N -dimethylaceta-mide (DMAc),N -methylpyrrolidone (NMP)and DMSO,from which dope solutions can be prepared and cast.Uncrosslinked PBI hollow fibre membranes were shown to be effective in the removal of chro-mate from alkaline wastewater [28].PBI flat sheet membranes cross-linked with α,α0-dichloro-p-xylene were reported for the concentra-tion and separation of cephalexin from aqueous solutions over a pH range from 2to 10[23].Similar to polyimide membranes,unmodi fied PBI membranes cannot be used for polar aprotic solvents.Several chemical modi fication procedures have been reported in literature including treatment of PBI membranes with α,α0-dichloro-p-xylene [23],divinyl sulphone [29]and 1,4-dibromobutane [30].In this study,we report on the fabrication and performance of crosslinked PBI NF membranes for applications in organic solvents containing acids or bases.The conditions of two pharmaceutical and one chemical process have been chosen to demonstrate the solvent and acid/base resistance of the prepared membranes.Also,the batch-to-batch reproducibility of crosslinked PBI membranes has been evaluated.The membranes were characterised with FTIR,contact angle measurement and SEM.2.Experimental 2.1.MaterialsCelazole s S26polybenzimidazole (PBI,MW ¼27,000g mol À1)solution was purchased from PBI Performance Products Inc.(USA).The solution contains 26wt%polymer solids and 1.5wt%lithium chloride (stabiliser)dissolved in DMAc.Non-woven polypropylenefabric Novatexx 2471was from Freudenberg Filtration Technolo-gies (Germany).All solvents such as DMAc,propan-2-ol (IPA),acetonitrile (MeCN),and DMF were HPLC grade and were used as received from VWR (UK).The chemicals for crosslinking were α,α0-dibromo-p-xylene (DBX)and 1,4-dibromobutane (DBB)from VWR (UK)and Sigma Aldrich (UK),respectively.Polyethylene glycol (PEG)of three different molecular weights was purchased from VWR (UK)and Sigma Aldrich (UK).Dichloroacetic acid (DCA)and piperidine (PIP)were both from Sigma Aldrich (UK);monoetha-nolamine (MEA)was from VWR (UK).De-ionised water was produced by passing tap water through an RO filtration unit.2.2.Fabrication of integrally skinned (IS)asymmetric PBI membranes and chemical crosslinkingCelazole s S26was diluted with DMAc to 17wt%polymer con-centration and stirred continuously at 2170.51C until a homo-geneous dope solution was obtained.The polymer solution was then left overnight to remove air bubbles.Membranes were cast on polypropylene non-woven using a bench top laboratory casting machine with an adjustable knife set at 250m m (Elcometer,UK).Following this,the membranes were immersed in a water precipita-tion bath at 22711C for 24h,and subsequently washed with IPA to remove residual solvent and water.The viscosity of the dope solutions was measured using a rotary viscometer Model 2020from Cannon Instrument Company (USA)with a spindle size 16suitable for high viscosities.All viscosities were recorded at 10rpm spindle speed and at 211C.Membranes were crosslinked by immersion in a solution contain-ing 3wt%DBX or 10wt%DBB in acetonitrile (Fig.1).The reaction was carried out at 801C for 24h under constant stirring and re flux.After crosslinking,the membranes were first immersed in IPA to remove residual reagents and later on impregnated with PEG 400by immer-sion in a PEG 400/IPA (1:1)solution for 4h to preserve the pore structure and allow dry storage.After obtaining the final membrane,the thickness was recorded using a digital micrometre purchased from Mitutoyo (UK).The stated membrane thicknesses include the impregnated polymer film and the polypropylene non-woven and are the mean between six measured points in different parts of the membrane sheet.2.3.Selection of model solutionsTypically,a range of polystyrene (PS)[31]or polyethylene glycol (PEG)oligomers [32]is used to determine the rejection performance of OSN membranes.The main advantage of both methods is that the rejection of a range of homologous solutes can be analysed simulta-neously,and an MWCO curve can be obtained for each membrane.Also,both types of oligomers are soluble in a wide variety of organic solvents.However,PS solutes are more expensive and not soluble in water due to their hydrophobic character.Since the application of crosslinked PBI membranes is intended for but not limited to iterative synthesis of therapeutic drugs [5,33,34],which closely resemble PEG compounds,PEGs in different MW were chosen as the marker solutes in the current study.Three solvent systems,tested in this work,were chosen based on three commercial processes which employ acidic or basic conditions in the production.The first one mirrors one of the reaction condi-tions used in oligonucleotide synthesis,which is 3wt%DCA in MeCN [35].The second system replicates one of the reaction conditions used in peptide synthesis –20wt%piperidine in DMF [36].The third filtration solution –20wt%MEA in DI water –is representative of liquids used in carbon capture and storage (CCS)[37].Fig.1.(a)Chemical crosslinking mechanism of 2,2-(m-phenylene)-5,5-bibenzimida-zole (PBI)with a)α,α0-dibromo-p-xylene (DBX)and (b)1,4-dibromobutane (DBB).I.B.Valtcheva et al./Journal of Membrane Science 457(2014)62–72632.4.Membrane characterisation2.4.1.Fourier transform-infrared spectroscopy (FT-IR)Infrared spectra were recorded on a Perkin Elmer-Spectrum 100.The samples were fixed on a zinc/selenium diamond plate with the separating layer facing the beam.Prior to FT-IR measure-ments the membrane samples were immersed in water followed by IPA to remove traces of residual chemicals and PEG 400.2.4.2.Soak test for stabilityThe stability of crosslinked PBI membranes was compared with the stability of commercially available polyimide membranes (Table 2).Small pieces of DuraMem 200[38](a crosslinked polyimide membrane from Evonik-MET)and lab prepared DBX and DBB crosslinked PBI membranes were cut and washed with DI water and IPA to remove any traces of conditioning chemicals.The pieces were then dried in vacuum and their weight was measured and recorded.Five soak solutions were prepared:20wt%DCA/acetonitrile,20wt%PIP/DMF,20wt%MEA/water,20wt%NaOH/water and 20wt%HCl/water.Two pieces from two batches of membranes with the same composition were soaked in each solution to evaluate reproducibility.Along with these tests,one of each five solutions was left blank.This test was carried out for two months at 201C,after which the membrane pieces were withdrawn from the solutions,washed with water and vacuum dried.Then the weight of the sample was recorded and compared to the initial value.2.4.3.Contact angleThe contact angle of the membranes was obtained with an Easy-Drop Instrument (Kruess,Germany).Water drops of constant size were deposited on the membrane surface using amicropipette.The digital image from the camera was then analysed using a built-in drop shape analysis tool.The reported contact angle for each membrane is an average of 10drop measurements.2.5.Membrane performance and analysisThe filtration experiments were all carried out in cross flow cells connected in series (Fig.2),each holding membrane discs with an effective area of 14cm 2.Pressure and temperature were kept constant at 10bar and 301C,respectively,throughout the experiment.The pump flow rate was set at 40L h À1for all experiments.The permeance B is de fined as the volumetric flow rate of solution per unit membrane area per unit pressure drop and can be calculated using the given equation B ¼V ΔðL m À2h À1bar À1Þð1Þwhere V is the collected permeate volume,A is the effective membrane area,t is the time and Δp is the applied trans-membrane pressure.Feed and permeate samples were taken at different time intervals for rejection R i determination.Rejection was calculated using Eq.(2),where C p,i and C f,i represent the concentration of solute i in the permeate and the feed,respectively.R i ¼1ÀC p ;if ;i100ð%Þð2ÞA solute mixture containing polyethylene glycols (PEG)of threedifferent molecular weights (400,2000and 8000g mol À1)was used to determine the rejection properties of the membranes.The PEGs were dissolved in MeCN,DMF and DI water at a concentration of 1g L À1for each MW (referred to as standard solution from here on).Collected samples containing PEGs,DCA and MEA were analysed using an Agilent HPLC coupled to an evaporative light scattering detector (ELSD,Varian-385).The ELSD evaporator was set at 401C and the nebuliser at 551C.Nitrogen gas was supplied to the detector at a flow rate of 1.5SLM (standard L m À1).A reverse phase column (C18-300,250mm Â4.6mm,ACE Hichrom)was used and the mobile phases were methanol and DI water buffered with 0.1M ammonium acetate.The HPLC pump flowrate was set at 1ml min À1and the column temperature was set at 301C.The concentration of PIP was analysed using an Agilent GC with an HP-5column (5%phenyl methyl siloxane;capillary:30m Â0.530mm Â1.50m m)coupled to a flame ionisation detector (FID).The temperature ramp was from 401C (hold for 1min)until 2001C with an increase of 101C min À1.Table 1Summary of crosslinked PBI membranes prepared from eight different dope solutions of the same composition.Four membranes were crosslinked with DBX and the other four with DBB.Membranes with entry nos.1–6were used for reproducibility evaluation.Entries 7and 8were used to demonstrate the chemical resistance of crosslinked PBI membranes.Entry no.Membrane codeComposition Viscosity (cP)at 211CCrosslinking Thickness(μm)1M1.1–DBX 17wt%PB Iin DMAc 77203wt%DBX 262.5752M1.2–DBX 80903wt%DBX 242.7743M1.3–DBX 68503wt%DBX 251.7744M2.1–DBB 17wt%PBI in DMAc 810010wt%DBB 228.0745M2.2–DBB 750010wt%DBB 230.7746M2.3–DBB 710010wt%DBB 215.2787M1–DBX 17wt%PBI in DMAc 75003wt%DBX 250.8758M2–DBB769010wt%DBB241.377Table 2Percentage weight loss of membrane samples which were inserted in five different acidic/basic solutions.Membrane CodeWeight loss (%)a 20wt%DCA/MeCN20wt%PIP/DMF 20wt%MEA/water (pH ¼12)20wt%HCl/water (pH ¼0)20wt%NaOH/water (pH ¼14)DuraMem 200170397222728173157M1–DBX 070170271170070M2–DBB070372270470170aThe error indicates the standard deviation in %from the average weight loss value of two membrane samples.I.B.Valtcheva et al./Journal of Membrane Science 457(2014)62–72642.6.Viscosity and molar volume of solvent solutionsThe kinematic viscosity of the solvents and solvent mixtures used was determined using a BS/U/M2miniature viscometer at 301C.Three consecutive measurements were taken (with coef fi-cient of variation less than 1%)and the mean was used to calculate the kinematic viscosity.By multiplying with the corresponding density,the dynamic viscosity for each liquid was obtained.The molar volume V m for pure solvents was taken from literature [39]and the molar volume V m ,mix of acidic and basic solutions was calculated according to the equation V m ;mix ¼∑i x i M iρmixðmol cm À3Þð3Þwhere x i and M i are the mole fraction and molecular weight of component i ,respectively,and ρmix is the measured density of the mixture at 301C.2.7.Scanning electron microscopy (SEM)Scanning electron micrographs of membrane surface and cross-section were recorded using a JEOL 5610LV.The samples were first washed with IPA in order to remove PEG 400.The surface samples were prepared by cutting small squares and pasting them onto SEM sample holders covered with carbon tape.For the preparation of cross-section samples,small pieces of membrane were snapped under liquid nitrogen and pasted vertically onto SEM stubs.Finally,the samples were sputtered with gold under argon atmosphere (Emitech K550coater).The SEM was operated at an acceleration voltage of 10kV.2.8.Experimental sequenceEight membranes were cast from eight different dope solutions by diluting the commercial PBI solution to 17wt%of polymer solids.Four membranes were crosslinked with DBX and the other four with DBB.The procedure is described in detail in Section 2.2,and Table 1summarises the composition and physical properties of the eight membranes.Two membrane discs were cut out of each of the eight batches to eliminate errors resulting from defective membrane coupons.First,the reproducibility of crosslinked PBI membranes was determined in different solvents –acetonitrile,DMF and DI water using entries 1–6listed in Table 1.The pure solvent permeance was established for each of the membranes after washing out PEG 400preservative.The steady state value was taken as such after three consecutive hourly measurements were within 5%of each other.Then the standard PEG/solvent mixture was filtered for 24h and rejection was taken at that point.Secondly,the filtrations of acidic and basic solvent mixtures were performed as solvent swap experiments (Fig.3).The evalua-tion of the membrane stability was done by first establishing the membrane performance with solutions containing only the PEG marker solutes for 24h.Then,without removing the membranes from the test cells they were exposed for 24h to the acidic or basic conditions.Next,by filtering pure respective solvents the rig was cleaned from the previous mixtures.The last step involved filtra-tion of the initial solvent containing the PEG markers.The chemical stability of the membranes was evaluated by comparing membrane performance before and after acid/base exposure.The composition and physical properties of the two membranes M1–DBX and M2–DBB can be found in Table 1entry nos.7and 8.DueFig.2.(a)Schematic representation of the cross flow filtration system with pressure gauge P ,flow meter F and temperature control T .The solid lines represent the feed/retentate flow and the dashed lines –the permeate flow;(b)top and cross-section view of a cross flow cell with (1)feed inlet,(2)retentate outlet,(3)permeate outlet,(4)o-rings,(5)membrane disc,and (6)sintereddisc.Fig.3.Filtration sequences followed for evaluating the chemical stability of crosslinked PBI membranes:Sequence 1was used for one set of M1–DBX and M2–DBB membranes and sequence 2was used for a new set of the membranes.I.B.Valtcheva et al./Journal of Membrane Science 457(2014)62–7265to failure of M2–DBB after completing the first filtration sequence (Fig.3–Sequence 1),fresh membrane discs were taken to evaluate their performance upon exposure to MEA (Fig.3–Sequence 2).All results are reported as the mean between the tested samples and the error values represent the one standard deviation from the mean.3.Results and discussion 3.1.Membrane characterisation3.1.1.FT-IRPrior to filtration tests,small membrane samples were used to con firm that the crosslinking was successful.Fig.4shows the IR spectra of uncrosslinked,DBX and DBB crosslinked membranes.The characteristic peaks of PBI were con firmed with spectra of PBI published in literature [40].The peak at 3415cm À1can be attributed to non-hydrogen bonded N –H stretching and the peak at 3145cm À1to hydrogen bonded N –H stretching.Unfortunately,these signals are not useful because O –H stretching is registered as a broad peak in the same area,overlapping the N –H stretches.The peaks in the finger-print area (at 1612,1590,1443,1286,801and 705cm À1)are all attributed to benzene and imidazole rings and their conjugation.Two distinct peaks at 2920cm À1and 2850cm À1were observed in the spectra of the crosslinked membranes which are attributed to C –H (the terminal C of the crosslinker)and C –N (the link between the crosslinker and the polymer backbone)stretches,con firming the crosslinking between the imidazole rings.3.1.2.Soak test for stabilityTo assess the stability of crosslinked PBI membranes,the mem-branes were soaked in different chemical solutions and the observed stability was compared to that of commercially available PI mem-branes (DuraMem 200).Table 2summarises the percentage weight loss of the three membranes after withdrawing them from the corresponding solutions.Polyimide membrane degraded in MEA,PIP,HCl and NaOH.The DCA soak test showed localised swelling of polyimide membrane surface but did not result in membrane disintegration.The two types of crosslinked PBI membranes were found to be stable in all five test solutions based on visible observation and weight loss measurement,except for M2–DBB exposed to HCl/water and PIP/DMF.The PIP/DMF solution turned a slight yellow colour due to dissolution of PBI in DMF.As the five blank solutions remained clear for the tested period,it can be concluded that any visible observation was due to the degradationof the membrane pieces.From Table 2it can be concluded that DBX crosslinked PBI membranes,compared to DuraMem 200and DBB crosslinked membranes,showed superior chemical stability in the pH range from 0to 14.3.1.3.Contact angle measurementsThe contact angles of crosslinked PBI membranes were measured and compared with uncrosslinked PBI samples.It can be seen in Table 3that the contact angles of crosslinked PBI membranes decreased compared to uncrosslinked PBI,suggesting that the mem-branes became more hydrophilic after the crosslinking treatment.The lowest contact angle was measured for DBX treated PBI membranes,decreasing by more than 40%from the value of untreated PBI.This is in agreement with the observed higher permeances of M1–DBX in all tested solvents compared to M2–DBB (described in Section 3.2).Such behaviour has previously been reported by Soroko et al.[41]for TiO 2doped PI membranes.The higher TiO 2content resulted in more hydrophilic surfaces and an increase in ethanol flux.Bhanushali et al.[42]tested various polar and non-polar solvents with commer-cial hydrophilic and hydrophobic membranes.They concluded that hydrophilic membranes provide higher fluxes with polar as opposed to non-polar solvents.3.2.Membrane performance3.2.1.Reproducibility of crosslinked PBI membranesCrosslinked PBI membranes (entry nos.1–6in Table 1)were tested for their reproducibility in the three solvents of interest –acetonitrile,DMF and DI water.Six membrane discs from each batch were tested in each solvent and a new set of discs was used for each solvent.First,the pure solvent permeance of the membranes was measured after 24h of operation and the data is summarised in Table 4.Comparing the pure solvent permeances shown in Table 4,a correlation is evident between solvent viscosity,molar volume,surface tension (Table 5)and the respective permeance.Consistent with the trend reported in literature [43,44],solvent transport through cross-linked PBI membranes increased with decreasing viscosity,molar volume and surface tension (Table 5).Hence,the highest permeance,observed for acetonitrile,can be attributed to the lowest viscosity.On the other hand the lowest permeance was obtained for DMF due to its higher viscosity and higher molar volume.Water has the highestTable 3Contact angles for unmodi fied and modi fied PBI membranes.Membrane Contact angle (deg )a Uncrosslinked PBI 60.572.3M1–DBX 35.375.4M2–DBB55.072.3aThe error indicates the standard deviation between 10drop measurements on the same membrane.Table 4Pure solvent permeances for DBX and DBB crosslinked membranes measured after establishing steady state.MembranePure solvent permeance (L m À2h À1bar À1)a AcetonitrileDMF DI water DBX crosslinked PBI 37777711277DBB crosslinked PBI1777170472aThe error given in the table is the standard deviation from the average calculated value of the total of three membrane batches (entry nos.1–6in Table 1).Fig.4.FT-IR spectra for uncrosslinked,DBB and DBX crosslinked PBI membrane samples.I.B.Valtcheva et al./Journal of Membrane Science 457(2014)62–7266viscosity and surface tension,which should result in the lowest permeance.However,due to its small molar volume,water had an intermediate flux,slightly higher than DMF (Table 4).After obtaining the pure solvent permeance,the pure solvent was replaced by a solution containing the PEG markers.The flux was monitored over time and PEG rejection was measured after 24h.The results are shown in Fig.5.In all filtrations DBB membranes showed a lower permeance than DBX membranes.This was likely due to the more hydrophilic surface of M1–DBX (Table 3)which enhanced the permeance of polar solvents.In Fig.5(a),it is interesting to note that filtration of PEG/acetonitrile through DBX crosslinked membranes led to around 50%decrease in permeance (from 21to 11L m À2h À1bar À1)whereas filtration of PEG/DMF and PEG/DI water resulted in no signi ficant permeance loss (Fig.5(a)).On the other hand,for the membranes crosslinked with DBB no impact of the solvent/solutesystem was observed (Fig.5(b)).It is speculated that such trend comes from the initial swelling of DBX membranes by MeCN,slowly compensated by pressure compaction.Furthermore,Fig.5(c)and (d)show the rejection of PEG markers in the three different solvents.It can be noted that DBX crosslinked membranes had the same rejection pro file in all tested solvents,while DBB crosslinked ones appeared to have a higher rejection of PEGs when water is the solvent and lower in the case of MeCN and DMF.Overall,the error bars in Fig.5indicate that crosslinked PBI membranes have a consistent performance from batch to batch and it can be concluded that all demonstrated effects are reproducible.3.2.2.Filtration under acidic/basic conditionsTo assess the chemical stability membranes M1–DBX and M2–DBB (entry nos.7and 8in Table 1)were used.The experimental solvent sequences,shown in Fig.3,were used to investigate the resistance in acidic and basic environments.The permeance results collected from the filtrations are shown in Fig.6.Similar to the solvent permeance data in Fig.5(a),M1–DBX membranes had higher permeances than M2–DBB membranes (Fig.5(b))in all tested solvents.The solvent/solute permeances followed the same trend as seen in Fig.5and increased in the order:PEG/DMF o PEG/DI water o PEG/acetonitrile.A steady state flux was reached after 24h filtration of PEG/solvent solutions in each case.After washing the rig with fresh solvents to ensure removal of PEGs,the corresponding acidic or basic solution was loaded and circulated for 24h.The permeances for acidic and basic solutions are shown in Fig.6in the middle panel of each graph.In all three cases,an instant dropTable 5Solvents and solvent mixtures used in the study and their characteristics.Solvent mixtureViscosity a (cP)Molar volume b (cm 3mol À1)Surface tension b (mN m À1)Pure Acetonitrile0.3452.728.033wt%DCA/acetonitrile 0.3652.8–Pure DMF0.7877.433.9020wt%PIP/DMF 0.8281.4–DI water0.8318.171.6720wt%MEA/DI water1.2721.2–aMeasured viscosity at 301C.bValues for pure solvents taken from [39]at 301C and for mixtures calculated using Eq.(3)at 301C.Fig.5.Average solvent permeance pro files over time of (a)PBI membranes crosslinked with DBX and (b)PBI membranes crosslinked with DBB;average rejection of PEGs after 24h for (c)PBI membranes crosslinked with DBX and (d)PBI membranes crosslinked with DBB.The error bars represent one standard deviation from the average value of the measurement,where each point summarises the average value from tests on M1.1–DBX,M1.2–DBX,M1.3–DBX panels (a)and (c)and M2.1–DBB,M2.2–DBB,M2.3–DBB panels (b)and (d).I.B.Valtcheva et al./Journal of Membrane Science 457(2014)62–7267。