Ultrafiltration membranes.pdf

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2011 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimAdv.Mater.2011,XX, 15

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Ultrafiltration Membranes Composed of Highly Cross-Linked Cationic Polymer Gel: the Network Structure andSuperior Separation Performance

Dr. Q. Wang, Dr. S. Samitsu, Prof. I. IchinoseOrganic Nanomaterials CenterNational Institute for Materials Science11 Namiki, Tsukuba, Ibaraki 3050044, JapanE-mail: [email protected]

Prof. Dr. I. IchinoseJapan Science and Technology AgencyCREST, 5 Sanbancho, Chiyodaku, Tokyo 102-0075, Japan

DOI: 10.1002/adma.201100475

Hydrophilic polymer gels are attractive materials for a widerange of applications in the life sciences and bioengineering.The gels are environmentally responsive and have high potentialin numerous applications such as drug delivery, sensors, actua-tors, absorbents, and scaffolds. The transport of ions and mol-ecules through hydrated polymer gels has been investigated bymany researchers, as has the bioaffinity and mechanical prop-erties of these gels.[1] Poly(4-vinylpyridine) (P4VP) forms typical

hydrated polymer gels. This polymer is readily cross-linkedwith alkyl dihalide at room temperature via quaternization ofthe pyridine groups. Cross-linkers of various size and shapecan be used, resulting in P4VP gels with diverse structures andproperties. Thin films made of cross-linked poly(vinylpyridine)gels have been widely investigated as a means of applyingstimuli-evoked optical, impedance, and volume changes to sen-sors and biodevices.[2] P4VP has intrinsic selective permeabilityfor gases,[3] and the network structures of these gels can becontrolled by controlling the extent of cross-linking. Therefore,this polymer has also been studied for its utility in gas separa-tion, desalination, and pervaporation membranes.[4] However,like most polymer gels, P4VP gels are mechanically weak, and

the gel membranes, which are generally supported on a micro-filtration membrane, have thicknesses of a few to several tensof micrometers. In order to be practical, the separation layerneeds to be as thin as possible to realize high liquid/gas fluxes,which requires that the gels are mechanically stable. We herereport ultrafiltration membranes with thicknesses of severaltens to hundreds of nanometers, prepared from P4VP gels bymeans of a two-step cross-linking of the polymer chains. Thedetailed structure of the polymer network was characterizedusing spectroscopic techniques and permeation and rejectionexperiments. Furthermore, the gel membranes were demon-strated to distinguish between water-soluble proteins of dif-ferent molecular masses.

The methodology for preparing ultrathin ultrafiltra-

tion membranes of the cross-linked P4VP gel is shown in

Figure 1. First, P4VP was cross-linked with 1,3-dibromopropane(DBP) in dimethyl sulfoxide (DMSO), then the resulting weaklygelled solution was diluted with water (Figure 1a). To formP4VP gel membranes, we used an extremely flat sacrificial layerof metal hydroxide nanostrands. Without this layer, the gelreadily penetrates into the submicrometer pores of the microfil-tration membrane, resulting in decreased permeability to water.The nanostrands spontaneously form in dilute aqueous solution

of copper, zinc, or cadmium nitrate (or chloride) upon neutrali-zation of the solution with base (see Supporting Information).[5]In the present study, cadmium hydroxide nanostrands with adiameter of 1.9 nm and lengths of a few micrometers were fil-tered on a polycarbonate (PC) membrane filter (Figure 1b).[6]The resultant ultrathin nanofibrous layer guaranteed the uni-form flow of water required to form a P4VP gel layer with aconstant thickness. Subsequently, a certain volume of thediluted P4VP gel solution was filtered. At this stage, the P4VPgel layer was not mechanically stable and significantly swelledin water. Therefore, the membrane was immersed in ethanolto extract the water, then was further subjected to cross-linkingwith 1,3-dibromopropane (DBP) to fix the densified network ofthe polymer chains. Ethanol is a good solvent for P4VP, whilewater is a poor solvent. On the other hand, once P4VP is quater-nized, ethanol becomes a poor solvent and water becomes agood one. The final gel layer was more stable than the gel layerprior to cross-linking and swelling was significantly reduced.The effects of this two-step cross-linking process were con-firmed using bulk P4VP gels (Figure 1c). When a disc-shapedpiece of cross-linked P4VP gel was placed in water, its sizeapproximately doubled, whereas the disc shrunk by half if itwas immersed in ethanol. Following the second cross-linkingwith DBP, shrinkage upon immersion in ethanol was marginal,and the extent of swelling in water was halved compared to theoriginal disc.

The process by which P4VP gel membranes are formed was

investigated by scanning electron microscopy (SEM). Figure 2ashows an image of a nanostrand layer prepared on a PC mem-brane with 0.2-m pores. When a cross-linked P4VP gel solu-tion was filtered through this membrane, a thin layer of P4VPgel with a thickness of 300 15 nm formed on the nanostrandlayer (Figure 2b). After dissolving the nanostrands, the gellayer contracted to 135 30 nm upon treatment with ethanoland subsequent cross-linking (Figure 2c). The thickness of theP4VP gel membranes was easily tuned by choosing the appro-priate filtration volume of P4VP gel solution (see SupportingInformation). The SEM images shown in Figure 2b,c do notrepresent the morphology of wet P4VP gel membranes since

Qifeng Wang, Sadaki Samitsu, andIzumi Ichinose*


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2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv.Mater.2011,XX, 15wileyonlinelibrary.com

membrane has the potential to be used as an ultrafiltrationmembrane, which usually has pores in the range 1 to 100 nm.The permeation rate of water increases inversely to the thick-ness of the membrane and, in the case of ultrathin gel mem-branes, the permeation rate is very sensitive to changes in poresize distribution. This makes it easy to characterize the networkstructure of cross-linked hydrogels under certain conditions.

The pores in P4VP gel membranes were first investigated bystudying what size proteins could be retained by the mem-branes. The results are summarized in Table 1 for three mem-branes with different thicknesses: P4VP-100 (39 nm), P4VP-250(78 nm), and P4VP-500 (135 nm). When an aqueous solutionof cytochrome c (Cyt.c) was filtered in dead-end mode using aP4VP-100 membrane at pH 4.23, the protein concentration inthe permeate (2.16 mL) decreased to 4.7% (95.3% rejection) andthe concentration in the remaining feed (7.84 mL) increasedabout 27%. From this increase in the upper stream, we calcu-lated that 78.4% of the Cyt.c was rejected at the surface of themembrane. Furthermore, the filtration rate (flux) at a pressuredifference of 80 kPa was calculated to be 466 L m2 h1 based onfiltration time, the volume of the filtrate, and the effective mem-

brane area after considering the porosity of the substrate (10%).This value is one to two orders of magnitude greater than thoseof commercial ultrafiltration membranes with similar separa-tion properties. The separation (i.e., the rejection of moleculesabove a certain molecular size) properties of the membranesincreased with increasing pH of the test solution. In contrast,the rejection calculated from the upper stream decreased sig-nificantly at pH 7.52 and higher. This is because Cyt.c, whichhas an isoelectric point of 10.2, begins to electrostaticallyadsorb onto the outermost surface of the positively charged gelmembrane under neutral and alkaline conditions. The rejec-tion of Cyt.c by the P4VP-250 membrane was higher than 96%

the specimens had been dried in vacuum. Indeed, specimensprepared by a freeze-dry technique had double the thickness(310 10 nm, Figure 2d) of specimens dried at room tempera-ture (135 nm, Figure 2b). Freeze-dried gel membranes had veryflat and smooth cross sections.

Filtration experiments provide a tool for gaining insight intothe internal structures of cross-linked gels. When the pore sizes

of a hydrophilic gel are in the range of a few nanometers, the

Figure 1. a) Structure and photograph of cross-linked P4VP gel and photograph of the diluted solution. b) Scheme for the preparation of a two-stepcross-linked P4VP gel membrane. c) Photographs of disc-shaped P4VP gels and the expanded/shrunken gels in water and in ethanol, the P4VP gelfurther cross-linked with DBP, and the cross-linked gel expanded in water.

Figure 2. Cross-sectional SEM images of a cadmium hydroxide nano-strand layer (a), a P4VP gel layer formed on the nanostrand layer (b),two-step cross-linked P4VP gel membrane (c, d). Inset in (a) shows thesurface morphology of the nanostrand layer. The gel membranes wereprepared from 500 L of 0.23 mg mL1 P4VP gel solution. Image (d) wasobtained from a freeze-dried specimen. The scale bars are 200 nm.


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bromide) (DPX, 0.6 1.5 nm2), was 97.6%, as estimated fromthe permeate of P4VP-500 membranes.

Water permeation through a P4VP-500 membrane wasexamined in the range pH 2.010.0 (see Supporting Informa-tion) at a pressure difference of 80 kPa. The flux increasedwith pH from 220 L m2 h1 (pH 2.0) to 332 L m2 h1 (pH 7.0)to 580 L m2 h1 (pH 10.0), with a specific increase at pH 4.0(337 L m2 h1). Given the large flux increase at high pH, thepores in the polymer network should expand with increasing

pH, probably due to hydroxide ions. The specific increase at pH4.0 is likely explained by the protonation of free pyridine units,as reported by Mika et al.[10] Water flux increased inversely tothe membrane thickness. However, the flux observed for thethinnest membrane (P4VP-100) was about three times smallerthan the value expected from the thickness of the membrane.This is due to compression of the membrane by the appliedpressure. In contrast, water flux through P4VP-500 membranelinearly increased with applied pressure up to at least 80 kPaand compression gradually increased between 0.1 and 0.5 MPa(see Supporting Information). Therefore, we analyzed the fluxat 80 kPa using the HagenPoiseuille equation:

J=

gBr2p

8:*Jp (1)

where J is the volume flux (m3 m2 s1), is the surfaceporosity, rp is the average pore radius (m), is the viscosity(Pa s), is the thickness (m), is the tortuosity, and p is thepressure drop across the membrane (Pa).[11] P4VP-500 mem-brane had a water flux of 332 L m2 h1 at a pressure differ-ence (p) of 80 kPa at pH 7.0. Using the viscosity of water at20 C (1.002 103 Pa s), the thickness of freeze-dried mem-brane (310 nm), the presumed pore size (2.0 nm), and the tor-tuosity (1.25), we estimated the surface porosity of the mem-brane to be 28%.

between pH 2.28 and 9.48, as calculated from the concentrationof Cyt.c in the permeate. The much thicker P4VP-500 mem-brane rejected Cyt.c by more than 99% at pH values of 4.23 andlower, as calculated from the feed. This indicates that Cyt.c doesnot adsorb onto the membrane surface. The flux at pH 4.23 was305 L m2 h1, which is very high for an ultrafiltration mem-brane. Furthermore, P4VP gel membranes rejected more than95% of myoglobin (Mb) and hemoglobin (Hb), demonstratingthat these cross-linked gel membranes may be useful for con-

centrating proteins, at least within a certain pH range, withoutsignificant filtration loss. Since quaternized P4VP coatings havebeen used for forming permanent antibacterial surfaces,[7] themembranes described above demonstrate significant potentialfor medical applications.

The molecular dimensions of Cyt.c are 2.5 2.5 3.7 nm3.Therefore, the P4VP-100 membrane should have some poreslarger than this at pH 2.28, since the membrane rejectedless than 90% of Cyt.c. On the other hand, the pores in thethickest membrane (P4VP-500) are no more than 3.7 nm inwidth. To evaluate the pore size of P4VP-500, we examined therejection of a neutral -cyclodextrin derivative, mono-6-O-(p-toluenesulfonyl)--cyclodextrin (Ts--CD). Analysis of the per-meate showed that 35.4% was rejected by the membrane (Table 1);

since the molecular width of this cyclodextrin derivative is about1.7 nm (including the hydration water),[6b,8] this membraneshould have many pores of 1.52.0 nm diameter. Overall, thepolymer network of the cross-linked P4VP gel seems to havea pore size distribution in the range 2.0 1.0 nm. If the mem-brane charge density is higher than that of the solution, thesame charged ions are electrostatically repelled and permeationis suppressed by the membrane.[9] This was also observed forpositively charged P4VP gel membranes. For example, the rejec-tion of a cationic dye, meso-tetra(N-methyl-4-pyridyl)porphinetetratosylate salt (TMPyP, 1.7 1.7 nm2), was 99.0%, and that ofa relatively small organic compound, p-xylene-bis(N-pyridinium

Table 1. Separation performance of P4VP gel membranes.

pHb) P4VP-100a) P4VP-250a) P4VP-500a)

Flux [L m2 h1] Rejection [%] Flux [L m2 h1] Rejection [%] Flux [L m2 h1] Rejection [%]

Feed Permeate Feed Permeate Feed Permeate

Cyt.c 2.28 309 76.0 88.8 263 89.2 98.2 196 100.0 99.1

4.23 466 78.4 95.3 392 95.7 99.4 305 99.1 99.3

6.04 399 64.9 93.8 344 75.5 96.8 270 88.5 99.2

7.52 614 17.2 98.2 536 32.2 98.8 413 42.8 99.2

9.48 305 - 100.0 290 - 100.0 277 - 100.0

Mb 3.94 393 64.9 95.9 377 77.4 95.5 349 82.8 98.6

Hb 3.83 395 87.0 97.7 380 90.8 98.4 339 96.7 99.7

Ts--CD - - - - - - - 215 5.6 35.4

TMPyP - 638 13.3 79.9 364 51.4 96.9 202 95.2 99.0

DPX - - - - - - - 216 99.4 97.6

a)P4VP-100, P4VP-250, and P4VP-500 denote membranes prepared from 100 L, 250 L, and 500 L of P4VP gel solution (0.23 mg mL1). Their average thicknesses were

39 nm, 78 nm, and 135 nm, respectively. The effective membrane area was 2.84 cm2. The flux obtained at a pressure difference of 80 kPa was normalized with the 10%

porosity of PC membrane; b)The pH of protein solutions was adjusted with HCl and NaOH. The pH of organic dyes was not adjusted. Cyt.c (20 g mL1); Mb (20 g mL1);Hb (20 g mL1); Ts--CD, (0.1 mM); TMPyP (5 M); DPX (10 M).


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ethanol was used as the solvent for the cross-linker. Even thoughonly about 20% of the pyridine groups were cross-linked, the two-step cross-linked P4VP gel membranes were stable enough to beused for high-pressure filtration. Indeed, a pressure of at least2.6 MPa could be applied to a P4VP-500 membrane formed ona PC membrane filter (see Supporting Information). Almost all

the incorporated DBP seems to be used as cross-linker, since thebromo group was not detected by XPS. Since the degree of cross-linking is 17%, 1/6 of the pyridine groups are bound to otherpolymer chains. An average structural unit should therefore havea chain length composed of six ethylene units (1.4 nm) and havea half-length of two cross-linked pyridinium groups (0.6 nm). If18 ethylene units and three DBPs form a ring, the diameter iscalculated to be 2.5 nm. In this case, the pore size should be nomore than 2.0 nm, after subtracting the width of the alkyl chain(ca. 0.5 nm). In a similar way, 24 ethylene units and four cross-linkers give a 3.3 nm ring with a maximum pore size of 2.8 nm(see Supporting Information). The polymer network in P4VP gelmembranes should have such a cross-linked structure. As illus-trated in Figure 3c, the pores are composed of 10 to 25 ethylene

units and crosslinkers, and the pore sizes are distributed in therange 2.0 1.0 nm in the expanded conformation. The porosityshould be 28%, as suggested from the analysis of water permea-tion. These areas are drawn with hatched lines in Figure 3c. Thepores, which demonstrate effective separation performance,change size in response to pH and other external conditions.

We have demonstrated that mechanically stable ultrathinP4VP gel membranes are obtained by a two-step cross-linkingprocess. A nanofibrous sacrificial layer of cadmium hydroxidenanostrands is useful for the preparation of an ultrathin filtercake of the hydrophilic gel, due to the ultrafine nonwovenstructure of the nanostrand layer and the high and uniformwater permeability of the membrane. After removal of the

nanostrands and the subsequent second cross-linking, the gellayer became sufficiently stable to be subjected to high-pressurefiltration. An important point of this study is that evaluation ofthe rejection properties and water permeability of the mem-brane made it possible to elucidate the pore sizes and porosityof the hydrophilic gels. An understanding of the network struc-ture is indispensable for understanding the fundamental prop-erties of polymer gels, such as diffusion, thermal conductance,and mechanical properties. We should also stress that highlycross-linked P4VP gel membranes show excellent rejectionproperties for proteins and dye molecules and that the filtra-tion rate was one to two orders of magnitude higher than thatof commercial filtration membranes. Clearly, highly positivelycharged P4VP gel membranes demonstrate significant advan-

tages for the separation of small cationic molecules and will beuseful for a wide range of industrial applications.

Experimental Section

Materials: Poly(4-vinyl pyridine) (P4VP, Mw= 160 000), cytochromec (Cyt.c) from bovine heart, myoglobin (Mb) from equine heart, equinehemoglobin (Hb), meso-tetra(N-methyl-4-pyridyl)porphine tetratosylatesalt (TMPyP), and p-xylene-bis(N-pyridinium bromide) (DPX) werepurchased from Sigma-Aldrich. Mono-6-O-(p-toluenesulfonyl)--cyclodextrin (Ts--CD), 1,3-dibromopropane (DBP), CdCl22.5H2O,2-aminoethanol, and other chemicals were purchased from KantoChemical. All the chemicals were used as received without further

In our experimental conditions, the pyridine groups of P4VPwere highly quaternized with DBP. Figure 3a shows a N 1s XPSspectrum of a P4VP gel membrane transferred onto a silicon waferbefore the second cross-linking. The peaks located at 398.5 eVand 401.4 eV were assigned to the nitrogen of the neutralpyridine and that of the quaternized pyridine, respectively.[12]

Deconvolution confirmed that some pyridine groups coordi-nate to a cadmium ion (or form hydrogen bonds with partiallyhydrolyzed DBP), giving a binding energy of approximately399.8 eV.[13] The amount of quaternized pyridine was 23%, as cal-culated from the peak area. This ratio decreased to 17% after thesecond cross-linking (Figure 3b). Furthermore, the atomic ratioof carbon to nitrogen (C/N) increased from 8.43 to 9.53. Theoutermost membrane surface might be covered by relativelyhydrophobic polymer chains after the second cross-linking since

Figure 3. Nitrogen (N 1s) XPS spectra of P4VP gel membranes before(a) and after (b) the second cross-linking. c) Schematic illustration ofnetwork structure of cross-linked P4VP gel.


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purification. A track-etched polycarbonate (PC) membrane waspurchased from Whatman. Ultrapure water of 18.2 M produced by aMillipore direct-Q system was used for the experiments.

Preparation of P4vp Gel Membranes: Nanostrand solution (10 mL; seeSupporting Information) was suction filtered onto a PC membrane withan effective diameter of 1.9 cm. A given volume (100 L, 250 L, and500 L) of cross-linked P4VP solution (0.23 mg mL1) was diluted with

5 mL of water and filtered onto the PC membrane with the nanostrandlayer. Then, the nanostrand layer was dissolved by the passage of 10 mLof 0.01 M aqueous HCl. The remaining P4VP gel membrane was rinsedwith water, air dried, and contracted by slowly passing ethanol throughthe membrane for 30 min using a filtration funnel. Subsequently, a2 mM ethanolic solution of DBP was slowly passed though the membranefor 1 h, then the solution was kept on the membrane overnight byremoving the pressure difference. Finally, the DBP solution was passedthrough under reduced pressure, and the two-step cross-linked P4VP gelmembrane was rinsed with ethanol and water.

Characterization: P4VP gel membranes were characterized using ascanning electron microscope (SEM, Hitachi S-4800) after coating witha 2-nm-thick platinum layer.[14] X-ray photoelectron spectroscopy (XPS)measurements were carried out on a PHI Quantera SXM photoelectronspectrometer using a monochromatic Al KR X-ray source. The separationproperties were evaluated by filtering a given volume of protein or

dye solution. The changes in the concentrations of the feed and thepermeate were examined by UV-vis spectroscopy, allowing calculation ofthe percent rejection of the solute molecules.

Supporting Information

Supporting Information is available from the Wiley Online Library orfrom the author.

Acknowledgements

The authors thank J. P. Gong (Hokkaido University), W. Oppermann

(Clausthal University of Technology), and the 21

th

Polymer NetworkGroup Meeting (Goslar) for many helpful discussions.

Received: February 5, 2011Published online:

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