11超滤膜英文文献

1、journal of membrane science 278 (2006) 261268high performanceultra?ltration composite membranesbasedonpoly(vinyl alcohol) hydrogel coating on crosslinkednano?brouspoly(vinyl alcohol) scaffoldxuefen wang, dufei fang,kyunghwan yoon, benjamin s. hsiao?, benjamin chu?department of chemistry, stony brook

2、 university, stony brook, ny 11794-3400, usareceived 2 june 2005; received in revised form 4 november 2005; accepted 4 november 2005available online 27 december 2005abstracta new kind of high ?ux ultra?ltration (uf) membrane basedon poly(vinyl alcohol) (pva) electrospun nano?brous scaffold support a

3、nd pvahydrogel coating has been demonstrated. the long-term performance of such membraneswas optimized by a variation of degreeof hydrolysis(i.e. 8899%)and molecular weight (e.g. 13,000 124,000g/mol) in the electrospun pva scaffold in conjunction with chemical crosslinking ofpva hydrogel coating. it

4、 was found that the electrospun scaffold fabricated by 96% hydrolyzed pva with relatively high molecular weight (e.g.85,000 124,000 g/mol) exhibited very good overall mechanical performance before andafter crosslinking. a pva hydrogel coating, fabricatedfrom the same material composition as the elec

5、trospun scaffold but different crosslinking conditions, was used as the anti-foulinglayer forhigh-throughput water ultra?ltration.results indicated that such unique hydrophilic nano?brous composite membranes exhibited a ?ux rate(130 l/m2h) signi?cantly higher than commercial uf membranesbut with sim

6、ilar ?ltration ef?ciency (rejection rate 99.5%).? 2005 elsevier b.v. all rights reserved.keywords:poly(vinylalcohol); nano?ber; hydrogel; ultra?ltration;oil/water emulsion1. introductionultra?ltration(uf) is animportant component in wastewatertreatment and in food industry 1,2 . with increasing conc

7、ernsand regulations in environment aswell asin food safety,the pro-cess of ultra?ltrationhas become more critical,whereby newtechnology development to provide faster and more ef?cientwater treatment is not only necessarybut also urgent. currently,conventional polymeric uf membranes are prepared main

8、ly bythe phaseimmersion process, typically generating anasymmet-ric porous structure with two major limitations:(1) relativelylow porosity and (2) fairly broad pore-size distribution3 5.as a result, these membranes suffer two de?ciencies: low ?uxrate due to the low porosity (i.e. limited permeabilit

9、y) and highfouling rate dueto the asymmetric pore-size distribution havingsmall pores on the surface 6 .?corresponding author. tel.: +1 631 632 7793; fax: +1 631 632 6518.?co-correspondingauthor. tel.: +1 631 632 7928.e-mail addresses: (b.s. hsiao), (

10、b. chu).foulingis an inevitableconsequence of pore blockagecaused by the solute adsorption on the membrane surface.solute adsorption often involves hydrophobicinteractionshydrophobic membranes havea high tendency to foul in watertreatments. however, many hydrophobic membranes remain themost useful m

11、edia for ultra?ltration dueto their superior perfor-mance in terms of mechanical, chemical and thermal stability.recently, thin ?lm composite (tfc) membranes involving sur-face coating of a hydrophilic layer on porous membranes havebeen shown to be very effective to reduce the foulingprob-lem in ult

12、ra?ltration.for example, nunes et al. 7 , pinnauand freeman 8,9, used commerciallyavailable hydrophilicpolyether- b-polyamide (pebax?) copolymers asthe top-coatingmaterials to improve thefouling performance of several porousuf membranes. mayes and co-workers synthesized a new typeof amphiphilicself-

13、organizingcomb copolymer,whichcanbe used as anti-foulingcoating as well as porous membranes10 . freeman and co-workers also developed a novel class ofhydrophilic/?uorinatedcopolymer that canundergo microphaseseparation and be used as non-porous coating materials on theporous membranes 11,12 . in the

14、se tfc membranes, the top0376-7388/$ see front matter ? 2005 elsevier b.v. all rights reserved.doi:10.1016/j.memsci.2005.11.009262x. wang et al. / journal of membrane science 278 (2006) 261268coating layer can be optimized for ultra?ltrationperformance,such ashigher ?ux rate, lower fouling rate with

15、 selective soluterejection capability;the porous membrane layer can be opti-mized for physical strength, compression resistance as wellas minimumresistance to the permeation ?ow 13,14 . sev-eral unique pathways have been demonstrated to improve theperformance of tfc membranes. for example, interfaci

16、al poly-merization has recently been usedto introduce the hydrophilicultra-thin top layer onto the porous hydrophobic support 15,16 .in this study, we demonstrate a new type of ultra?ltrationmembrane based on a different type of nanostructured poroussupport electrospun nano?brous scaffold in conjunc

17、tion witha very thin layer of hydrogel coating to minimize fouling. boththe nano?brous mid-layer support andthetop coating layer weremanufactured from crosslinked hydrophilic poly(vinylalcohol)(pva), where the degreeof hydrolysis andthe molecular weightof pva were simultaneously adjusted to partiall

18、y optimize the?ltration performance and the mechanical durability.electrospinning provides a simple and versatile way to fabri-cate very ?ne continuous polymer ?bers (from 10sto 100sof nanometers in diameter) that form a non-woven structurewith controllable pore-size openings (in the range of micron

19、s)17,18 . the membrane-like nano?brous scaffold exhibits goodtensile strength, high porosity, and thus lightweight.the highporosity is due to the entanglement of nano?bers, where all thepores (or the void space)arefully interconnected. recently, kimand co-workers have evaluated the physical properti

20、es of non-woven mats composed of nylon-6 nano?bers 19 . dependingon the ?ber diameter, the brunauer emmett teller(bet) sur-face areas of electrospun mats estimated was between 9 and51m2/g, while the porosity varied from 25 to 80% and the poresize was in the range of 2.7 0.17 ? m. electrospun non-wov

21、ennano?brous scaffold has a relativelysmall average pore sizein comparison with the commercial non-woven substrate basedon ?bers with micron size diameters, making them very goodcandidates for applications in ?ltration20 , catalyst support21 and biomedical engineering 22,23 . however, electrospunnan

22、o?brous scaffolds used as direct ?ltrationmedia would beextremely susceptible to fouling becauseof thevery large intrin-sic speci?c surface area. to minimize the foulingproblem, athin layer of hydrophilic non-porouscoating (e.g. hydrogel)should also beapplied onto tfc membranes. the coating layersho

23、uld inhibit the solute adsorption on the surface without sub-stantial hindrance of water permeation. however, the ?ux rateandtheselectivity in ?ltration aredirectly related to the effectivemesh size and the coating thickness of the hydrophilic coating24 , which must be optimized for different materi

24、al systems.in general, thethickness of the coating layer on the nano?brousscaffold can be slightly thicker than that in tfc asthe nano?-brous scaffold offers a higher permeation rate. the thicker toplayer can offer an option for better mechanical wear resistanceand thus prolonged foulingimprovement,

25、 if desired. in otherwords, the speci?cations can be more relaxed in meeting thestringent requirements for ultra?ltration.in this study, pva has been chosen as the base materialfor fabrication of both the porous nano?brous mid-layersup-port and the non-porous top coating layer. pva is often usedin u

26、ltra?ltrationbecause of its superb hydrophilicity,biocom-patibility,chemical and thermal stability. however, as pva iswater-soluble, it must becrosslinked to form water-resistant arti-cles. pva can becrosslinked through the reaction with hydroxygroups using awide range of chemicals 24,25 . typically

27、, pvais derived from the hydrolysis (or alcoholysis) of poly(vinylacetate), thus it is available with different degreesof hydrolysis,depending on the amount of acetategroups left in the backbone26 . recently, partially hydrolyzedpva (80 99%) has beendemonstrated by electrospinning in several laborat

28、ories 27,28 .in this work, one of our goals was to correlate the mechani-cal properties of electrospun pva scaffolds with the degree ofhydrolysis and molecular weight of pva in order to partiallyoptimize the long-term durability of ultra?ltrationmembranescontaining electrospun nano?brous support. an

29、other goal wasto evaluate the performance of nano?brous ultra?ltrationmem-branes based on electrospun pva scaffold and pva hydrogelcoating. the effective mesh size on the pva coating layer wasalso studied to yield optimal transport properties in water/oilemulsion separations.2. experimental2.1. mate

30、rialspva with various molecular weight and degree of hydrol-ysis are shown in table 1 (the samples were labeled ashmw,mmwand lmwto indicate the high, medium andlow molecular weights). these samples were purchased fromaldrichchemical, milwaukee, wi., except for sample 4 withweight average molecular w

31、eight (mw) of 78,000 g/mol (98%hydrolyzed) which was purchased from polysciences inc. tri-ton x-100, glutaraldehyde (ga) (50% aqueous solution) andhydrochloric acid (36.5% aqueoussolution) were alsopurchasedfromaldrich.pebax?1074 (polyeyleneoxide(peo)-block-polyamide 12 copolymer), used as a referen

32、ced coating layer,was kindly supplied by ato?na. the peo content in pebax1074was 55 wt.%, giving arelatively high hydrophilicity.all of thesematerials were usedas received without further puri?cation.2.2. electrospinning of pva solutionsthe chosen electrospinning procedures of pva were similarto tho

33、se described in the previous report 29 . aqueous pvasolutions were prepared by dissolving pva powder samples indistilled water at 40 95?c, depending on the degree of hydrol-table 1weight average molecular weight (mw) and degreeof hydrolysis (%) in chosenpva samplessample no.mw(g/mol)hydrolysis degre

34、e (%)185,000 124,000 (hmw)8889285,000 124,000 (hmw)96385,000 124,000 (hmw)9899478,000 (mmw)98513,000 23,000 (lmw)98x. wang et al. / journal of membrane science 278 (2006) 261268263ysis, with constant stirring for at least 6 h. for example, 10 wt.%pva (98 99% hydrolyzed pva with hmw) solution was pre

35、-pared by dissolving 1 g of pva powder in 9 ml distilled waterat 95?c under constant stirring for at least 6 h. when the solu-tion was cooled to room temperature, triton x-100 was addedinto the pva solution at a concentration of 0.6% (w/v). themixture was stirred further for 15min before electrospin

36、ning.triton x-100 surfactant was used to lower the surface tensionof the polymer solution 29,30 and to allow the electrospinningof pva to be carried out at a higher feeding rate (2.4 ml/h).in this study, typical parameters for the electrospinning of pvawere as follows. the pva solution feeding rate

37、was 2.4ml/h,the applied electric ?eld was 28 kv, and the distance betweenthe spinneret and collector was 10 cm.2.3. crosslinking of electrospun pva scaffoldscrosslinkingtreatments of electrospun pva nano?brousscaffolds have been demonstrated in our laboratory. detailedprocedures for optimization wer

38、e described elsewhere 29. thechosen optimal crosslinking conditions in our previous studywere asfollows. electrospun pva scaffolds werecrosslinked byimmersion in acetone solution with 30 mm ga and 0.01 n hclfor 24h. the crosslinked pva membrane was washed in waterseveral times and then dried in a ho

39、od before use.the density ofthe electrospun pva membrane was determined from an aver-ageof ?ve samples using the massdivided by the volume of thesample. the porosity of eachmembrane was calculated by usingporosity =1 -0 100(1)where is the density of electrospun membrane and 0is thedensity of bulk po

40、lymer.2.4. preparation of hydrophilic nano?brous compositemembrane for ultra?ltrationaqueous2.0wt.%pva(96%hydrolyzed,mw85,000 124,000) solution (ph 2, adjusted by hydrochloride acid) wasprepared for hydrogel coating on top of the electrospun scaffoldfor thefabrication of nano?brous composite membran

41、eswith thefollowingprocedures. an appropriate amount of ga was addedto the 2.0wt.% pva solution prior to coating. the crosslinkedelectrospun pva scaffold was ?rst placed on a commercialpoly(ethylene terephathalate) non-woven substrate(pet micro-?lter fo2413 from freudenburg non-wovens; the average ?

42、berdiameter in this substrate was about 10? m). the surface of thepva nano?brous support was rinsed with water andthen coatedwith pva coating solution. the composite membrane was sealed(to minimize the solvent evaporation) for 6h in order to allowfor the crosslinkingof the top layer. the resulting m

43、embranewas then dried at ambient temperature until constant mass wasachieved. the crosslinking degree of the pva hydrogel coat-ing layer was varied by changing the molar ratio of ga andpva repeat unit. for comparison, nano?brous composite mem-brane basedon electrospun pva support coatedwith hydrophi

44、liccopolymer pebax 1074 was also prepared using the followingprocedures. 1.0wt.% pebax 1074 solutions in 1-butanol wereprepared by re?uxingpebax in butanol for 24h. the coatingprocess of pebax was similar to that of pva hydrogel, exceptthat no crosslinking reaction was necessary.2.5. morphologicalch

45、aracterizationthe morphologyof electrospun pva nano?brous compos-ite membrane was examined by scanning electron microscopy(sem, leo 1550, leo, usa) after gold coating. sem cross-section images of composite membranes were obtained by frac-turing of samples in liquidnitrogen. the resolution of seminst

46、rument was about 25 nm with an operating voltage rangeof 1 30 kv.2.6. mechanical property evaluationthe mechanical properties of electrospun pva nano?broussupports andcomposite membraneswithout coating were deter-mined using an instron (model 4442) tensile machine at ambi-ent temperature. the sample

47、 gauge length was 10mm andthe cross-head speed was 2 mm/min.typical specimen sizesfor the mechanical evaluation were 20mm 5 mm 100? m(length width thickness).2.7. ultra?ltrationevaluationcross-?ow measurements were carried out by separating anoil/wateremulsion containing soybean oil (1350 ppm), non-

48、ionic surfactant (dow corning 193 ?uid, 150 ppm) and water.the chosen ?ltration conditions were as follows: the feed pres-sure (p) was 100 psi, temperature was 30 35?c and ?ltrationduration was 24h. the effective ?ltration area was 66.5 cm2.the quality of ?ltered water was evaluated by uv vis spec-t

49、roscopy (based on the absorbance at 230 nm). a calibrationcurve of waste oily water in the range of 0 100 ppm was usedto determine the organic concentration of ?ltered water. thepermeation ?ux was calculated by using thefollowingequation:j =qat(2)where j is the permeation ?ux (l/m2h), q is the perme

50、ationvolume of the testing solution; a is the effective area of thetestedmembrane (m2),andt is thesampling time (h). the totalorganic concentration (toc) rejection (r%) in the ?ltration ofoil/water emulsion is given byr% =1 -toc in permeatetoc in feed100(3)3. results and discussion3.1. mechanical pr

51、operty optimization of electrospun pvascaffoldsalthoughelectrospinningof pva with varying degree ofhydrolysis and molecular weight has been electrospun previ-264x. wang et al. / journal of membrane science 278 (2006) 261268fig. 1. sem images of electrospunpva membranes withvarious molec-ular weightb

52、ut similardegrees ofhydrolysis:(a)98%hydrolyzed,mw13,000 23,000 g/mol (electrospun from 24 wt.% solution); (b) 98% hydrolyzed,mw78,000 g/mol (from 11 wt.% solution);and (c) 9899%hydrolyzed, mw85,000 124,000 g/mol (from 9 wt.% solution).ously 27,28 , the mechanical properties of pva nano?brousscaffol

53、ds have yet to be reported. in order to optimize themechanical properties of these unique scaffolds for long-termoperational stability during ultra?ltration, the effects of hydrol-ysis and molecularweightwere systematicallyinvestigated.fig. 1 shows typical sem images of electrospun pva mem-branes pr

54、epared from three kinds of pva with asimilar degreehydrolysis (i.e. ca. 98%) and different molecular weights underan applied voltage of 28kv and the spinneret-to-collectordis-tance of 10cm. as seen in fig. 1, the optimal condition forfig. 2. stress-elongation curves of electrospun pva nano?brous sca

55、ffold beforecrosslinking with samples of different molecular weights but similar degrees ofhydrolysis(a) 98% hydrolyzed, lmw;(b) 98% hydrolyzed, mmw;and (c)9899%hydrolyzed, hmw.electrospinningof pva was achieved by using different pvaconcentrations, depending onthemolecular weight of thesample(the h

56、igher the molecular weight, the lower the concentration).all threescaffolds exhibited asimilar porosity and?ber diameter(between 100 and 300nm). the relations between the appliedstress and elongation ratio ( , where strain = 1 + ) of electro-spun pva scaffolds prior to the crosslinking reaction ares

57、hownin fig. 2. the corresponding values of tensile modulus, tensilestrength and elongation to break ratio of these electrospun pvascaffolds aswell asothers areillustrated in table 2. it was foundthat both elongation atbreak andtensile strength were increased,while the tensile modulus was decreasedwi

58、th increasing molec-ularweight.the electrospun scaffoldproduced frompvawith the highest molecular weight (mw85,000 124,000g/mol)exhibited thehighest stress(7.3 mpa) and thelargest elongationat break value (110%), when compared with scaffolds producedfrom pva of medium and low molecular weights.for e

59、lectrospun pva scaffolds, prepared by using the highmolecularweight pva samples (hmw)with three differentdegreesof hydrolysis (88, 96, and98 99%) but under the sameelectrospinning conditions (i.e., the solution concentration andapplied electric ?eld), the ?brous structure did not change sig-table 2t

60、ensile strength, tensile modulus and elongation to break ratio of electrospunpva scaffoldssampletensile modulus(mpa)tensilestrength (mpa)elongation atbreak (%)lmw(98%)1104.357mmw(98%)936.496hmw(98 99%)667.3120hmw(96%)407.6130hmw(88%)6.44.8174hmwa(98 99%)576.845hmwa(96%)4813.567acrosslinked sample.x.