pH effect on molecular size exclusion of polyacrylonitrile ultrafiltration membranes having carboxylic acid groups

j o u r n a l of MEMBRANE SCIENCE ELSEVIER

Journal of Membrane Science 123 (1997) 185-195

pH effect on molecular size exclusion of polyacrylonitrile ultrafiltration membranes having carboxylic acid groups Mangala S. Oak, Takaomi Kobayashi *, Hong Ying Wang, Takahiro Fukaya, Nobuyuki Fujii Department of Chemistry, Nagaoka University of Technology, Kamitomioka, Nagaoka 940-21, Japan Received 2 May 1996; revised 11 July 1996; accepted 11 July 1996

Abstract Polyacrylonitrile copolymer membranes with filtration performance controlled by pH were prepared by phase inversion technique. The pH sensitive molecular sieve effect on transport of macromolecular solute was compared in both copolymer membranes having acrylic acid (AA) and methacrylic acid (MA) segments. The water permeation rate through the membranes was remarkably decreased by the surrounding pH change from acid to alkali condition. The molecular weight cut-off data obtained by the dextran permeation at pH 4, 6 and 10 indicated that pore size of the membrane is significantly reduced in the alkali condition. Without and with 0.1 M NaC1 in the permeate solution, different molecular sieve effects were observed in the copolymer membrane with the AA segments, whereas the membrane with MA segments made no significant change. Evidence was presented that the carboxylic acid segmental conformation is associated with the pH sensitive molecular sieve effect of the copolymer membranes.

Keywords: Ultrafiltration membranes; pH; Filtration; Molecular size exclusion; Carboxylic acid

1. Introduction There is importance in studies on polymeric organic assemblies having functional groups environmentally controlled by pH, temperature, and light. By the various external stimuli, the change of conformation, charge condition of the polymeric chains and hydrophilic or hydrophobic nature can be easily switched o n / o f f and influences permeation behavior

* Corresponding author. Tel.: +81-2-5846600; fax: +81-258466507.

of solute molecule and release control of substances through the polymeric membranes [1,2]. Because of potential uses in designing and making sustained drug releasing devices and model as artificial cells [3-5], considerable efforts have been made to develop such functional membranes. Okahata et al. [2] reported a porous Nylon-capsule membrane with surface grafted polyelectrolytes of poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMA), which can be used for making pH sensitive capsules. In this case, the permeability through the capsule membrane is reversibly regulated by pH changes of the outer medium. Kokufuta et al. reported the preparation of

0376-7388/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. Pll S0376-73 88(96)00214- 1

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a polyelectrolyte coated with pH sensitive polystyrene microcapsules and the effect of pH on the permeability of the polyelectrolyte coated capsule membranes [3]. Further, coated polyamide capsules with lipid membranes were studied by Kono and coworkers [4,5]. With lipid membranes having amphiphilic polypeptide, they achieved pH dependent release of NaC1 from the capsule membrane. To improve pH dependent control, interpenetrating networks of poly(vinyl alcohol) and PAA have been made by Gudeman and Peppas for membrane applications [6,7]. In the pH responsive hydrogel, the mesh size of the gels increases, as the surrounding pH increases. In another paper on pH responsive synthetic membranes, porous polycarbonate membranes with straight pores were used as the graft substrate by the PAA modification [8]. Furthermore, pH responsive ultrafiltration (UF) membranes with carboxylic acid or pyridine groups were recently developed, pH changes affecting carboxylated polysulfone membranes were studied in relation to membrane porosity at different pH [9]. It was shown that acid treatment led to a slight decrease in pore size, while base treatment led to large increase in the size. For copolymer membrane with both vinyl pyridine and acrylonitrile, protein purification at various pHs was reported by Masawaki et al. [ 10]. Works have been reported by the authors on transport performance of ionic and nonionic macromolecule solutes across polyacrylonitrile (PAN) membranes with ionic groups. The UF performance through negatively charged PAN membranes is due to formal charge of the ionized groups on porous membrane surface [11], conformation change of the polyionic segments [12-14] and hydrophilic or hydrophobic nature [15]. In these membranes, it is evident that the PAN segments act only as membrane formation sites. The charged segments in the membranes perform as functional sites to control the filtration behavior of the membranes. So, if AA and MA groups having functionality environmentally controlled by pH, are introduced to the PAN segments, the permeation behavior of the resultant membrane would reflect the change in permeation characteristics of the membrane having molecular sieve effect. In the present work, PAN copolymers with AA and MA groups were prepared and used for UF membrane materials. The pH sensitive perfor-

mance was discussed by observing the filtration properties such as the molecular sieve effect on macromolecular solute transport.

2. Experimental 2.1. M a t e r i a l s

All chemicals used were of reagent grade unless mentioned otherwise. Deionized water was used throughout the experiments. Whenever required, dimethylsulfoxide (DMSO), acrylic acid (AA), methacrylic acid (MA) and acrylonitrile (AN) were distilled under reduced pressure prior to use. Dextrans (Pharmacia) with a molecular weight (MW) of 1 X 10 4, 4 X 10 4, 7 X 10 4, 5 X 10 5 and 2 X 10 6 were used without further purification for determining the molecular weight cut-off curves of UF membranes. 2.2. P o l y m e r s y n t h e s e s

Two types of PAN copolymers (Scheme 1) with AA and MA groups, abbreviated as P(AN-co-AA) and P(AN-co-MA), respectively, were prepared by radical copolymerization in DMSO solution in the presence of azobis(isobutylonitrile) (AIBN). The details for P(AN-co-AA) are as follows: 30.4 g (566 mmol) of distilled AN, 7.51 g (104 mmol) of AA, 110.5 g of DMSO and 0.22 g of AIBN were taken in a round bottomed flask with 500 ml capacity. Copolymerization was carried out at 60°C for 6 h in nitrogen flow. After the reaction, the viscous reaction mixture was poured into a large quantity of water to precipitate the crude copolymer. The crude polymer was then well washed in water and was finally soaked in methanol. The white copolymer was dried in vacuo (55% conversion). The P(AN-coMA) copolymer was synthesized analogously with reaction mixture of AN (566 mmol) and MA (110 mmol) in DMSO (110.5 g) containing AIBN (0.22 CH3 I

CN

COOH

P(AN-co-AA)

ON P(AN co MA)

Scheme 1.

COOH X+Y-1

M.S. Oak et al. / Journal of Membrane Science 123 (1997) 185-195

g). The P(AN-co-MA) was obtained by 62% yield. PAN homopolymer was prepared in the same manner of the P(AN-co-AA) and P(AN-co-MA) by using AN (566 mmol) in DMSO (100 g) in the presence of AIBN (0.22 g) with 70% yield. The content of AA and MA group in each copolymer were estimated as y = 0.145 and 0.137 for P(AN-co-AA) and P(AN-coMA) membranes, respectively, by using IR measurements of the copolymer ( y means molar fraction of AA or MA and x + y = 1). The FT-IR spectra of the copolymer were recorded with KBr on a Shimadzu FTIR 8200 infrared spectrophotometer. The COOH contents in the copolymer were estimated from the IR spectra by using mixtures of PAN and PAA or PMA. Here, data of IR peak height appeared near 1720 cm -l and 2250 cm -1 for - C = O stretching and - C N stretching, respectively [16], were employed for the evaluation of COOH content in the copolymers. For the MW estimation of the resultant copolymers, viscosities of the copolymer-dimethylformamide solutions were measured at 25°C using an Ubbelohde viscometer. The MW values obtained were calculated as 7 X l 0 4 and 7.5 X 104 for P(ANco-AA) and P(AN-co-MA), respectively, in accord to Ref. [17]. 2.3. Casting o f copolymer membranes

UF membranes of the PAN copolymers were prepared by phase inversion method, which we reported previously [18,19]. For the casting process, the copolymer obtained was dissolved in DMSO with 9 wt% concentration at 55°C. The polymer solution was cast about 100 Ixm thickness on flat glass plate (25 X 30 c m 2) warmed at 50°C. The membrane was obtained by coagulation into deionized water and stored in water for three days to effectively remove residual DMSO left in the membrane. The ion exchange capacity of the membranes was estimated by titration of P(AN-co-AA) with 0.01 M NaOH and P(AN-co-MA) with 0.005 M NaOH and estimated as 2.2 and 2.4 ( m m o l / g membrane), respectively. 2.4. Permeation experiments

UF experiments were carried out similarly to previous reports [12,18]. The obtained membrane

187

sheet was cut into disks with 43 mm diameter before use. All filtration experiments were carried out at 25°C and 760 mm H20 applied pressure. Volume flux was calculated from permeate volume (m s ) per filtration time (s) per unit area (m 2) of the membrane. For 0.1 wt% of dextran, aqueous solution permeations were taken place under the pressure driven condition. Experimentally obtained rejection, R, of dextran permeation through the UF membranes is defined as R = ((C r - C p ) / C f ) × 100. The dextran concentrations of feed (C r) and permeate (Cp) were analyzed colorimetrically [20]. The absorbances at 485 nm were measured on a Shimadzu double beam UV-190 spectrophotometer. All pH measurements were made using a Horiba F-11 pH meter, pH was adjusted with dilute HC1 and dilute NaOH. Before the filtrations at pH 4, 6 and 10, the membranes were soaked in the medium overnight.

2.5. Membrane characterizations

Morphology of the cross-section of the membrane was observed with a scanning electron micrograph (SEM) JXA-733 (Jeol). A sample of wet membrane was freeze-dried under vacuum. The cross-section of the membrane was obtained by fracturing it at liquid nitrogen temperature. The fractured sample was gold-coated using a SPM-112 (Anelva) sputter gun. Fluorescence probe uptake was measured. In order to uptake the 8-anilino-l-naphthalene sodium sulfonate (ANS) probe to the copolymer membrane, a 50 txl of ANS (15 ml) solution was used. By equilibrating the weighed membrane in the solution overnight at the pH required, fluorescence of ANS adsorbed on the wet membrane at 365 nm excitation was measured on a Hitachi YR-G(FT-MH) spectrofluorimeter. The wet sample was mounted at 45 ° to the excitation beam. The ANS taken into the weighed membrane was estimated at the absorbance maximum of 350 nm by the absorbance decrease of the ANS solution after the immersion. Absorption spectra of ANS aqueous solution was recorded with a Shimadzu UV-190 double beam spectrophotometer. Here, the molecular extinction coefficient, E = 5000 M -] cm - l [21], at 350 nm was used for ANS concentration determination.

M.S. Oak et al. / Journal of Membrane Science 123 (1997) 185-195

188

3. Results and discussion

3.1. pH dependence of membrane performance Water permeation through the copolymer membranes and PAN membrane was examined at different pH. Fig. 1 shows plots of water volume flux against bulk pH. When pH of the water permeate is varied from acid to alkaline conditions, each volume flux for the P(AN-co-AA) and P(AN-co-MA) membranes show slight decrease until about pH 8 and then a significant decrease in pH 8-10 region. But, the value of volume flux for the PAN membrane without COOH groups remains almost constant in the whole pH region at 3 × 10 5 m 3 / m 2 s. It is also noticeable that the slope of the curve for the P(ANco-AA) membrane becomes sharper than that of the P(AN-co-MA) in pH 8 - 1 0 region; The volume flux of the P(AN-co-MA) begins to gradually change over pH 6, while the change of the volume flux for the P(AN-co-AA) membrane does in the range of pH 9-9.5. Fig. 2 shows scanning electron micrograph (SEM) photographs of the cross section for both P(AN-co-AA) and P(AN-co-MA) membranes treated at different pH. The morphology of the copolymer membranes shows an asymmetric structure consist-

10 -4

g 10 .5

!1.

0 \

\ \,

>

i 10-6

I

I

I

I

I

I

4

5

6

7

9

10

11

pH Fig. 1. Volume flux of water permeate through the copolymer membranes and PAN membrane at various pHs under applied pressure of 760 mm H20. (D) P(AN-co-AA),( • ) P(AN-co-MA), (A) PAN.

ing of a top thin layer supported by microvoids with fingerlike structure, as similar as in a typical UF membrane [22]. The presence of the fingerlike macrovoids implies that instantaneous coagulation of the copolymer in water medium occurs [22,23]. In comparison with SEM photographs, it is noted that both copolymer membranes with COOH groups have similar morphology with that of the PAN membrane, as previously reported [14]. Hence, PAN segments in the copolymer coagulate strongly with each other without disruption by COOH groups. It is shown by these micrographs at different pHs that no significant change of the membrane morphology in the cross section is observed in the pH 4 - 6 region. In contrast to this pH region, in the P(AN-coAA) membrane at pH 10, the fingerlike layer appears slightly swelled although the basic structure for the fingerlike macrovoids is observed without remarkable change. Relative to the P(AN-co-AA) membrane, the fingerlike macrovoids observed in the P(AN-co-MA) membrane appear to be broken in the alkali condition, because of the irregular fingerlike macrovoids. In this type of asymmetric membrane, it is known that permeation rate of solute solution is determined by properties of the top surface thin layer. In addition, separations in UF processes are effected by molecular sieve property of the upstream surface of the membrane [22]. That is, pore size in the top surface of the asymmetric membrane is mainly responsible for the molecular size exclusion and the fingerlike macrovoid layer acts only as a support. In order to get an insight of the pore size in the top surface layer, various MW dextrans in a dilute concentration were permeated through the membranes. Fig. 3 shows molecular weight cutoff (MWCO) curves of (a) P(AN-co-AA) and (b) P(AN-co-MA) membranes at pH 4, 6 and 10. The plots of rejection versus the MW of dextran show that dextran with 1 X 104 MW is not effectively cutoff by the membrane and that with 2 X 10 6 MW is almost cutoff by the membrane. The Stokes radius of dextrans for 1 X 10 4 and 2 X 10 6 of MW is 23.3 and 372 ,~, respectively [24,25], and, therefore, these membranes have a pore size in the MW range of the dextran used. It was shown that, as pH changes from acid to basic condition, the MWCO curves shift to a low molecular side of dextran. This means that the pore

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M.S. Oak et al. / Journal of Membrane Science 123 (1997) 185-195

estimated are listed in Table 1. In addition, the pore radius (%) of the top surface for the molecular sieve effect of the membranes is calculated according to sieve slit model [28] and also shown in the table. When the bulk pH changes from acidic to alkali region, the MWCO data mean that the pore size of the copolymer membranes is reduced. It is well known that the PAA or PMA segments in aqueous solutions dissociate as follows [29]:

size of the membrane decreases in pH 10 compared to that in pH 4. At pH 10 for the P(AN-co-MA) membrane, diffuse cutoff data are mainly due to a large cutoff of 1 X 10 4 MW dextran by the membrane. It is noted in the MWCO curves that the degree of the shift between pH 6 and 10 is much larger than that between pH 6 and 4. This phenomenon corresponds well to the tendencies of the water permeation data, as shown in Fig. 1. In general, the MWCO of UF membranes is defined as MW at 90% rejection in the MWCO curves [26,27]. As shown in Fig. 4, the data of Fig. 3 is replotted in the log-normal probability. From the relationship at the 90% rejection, the MWCO values a) P(AN-co-AA) pH 4

-COOH ~ -COO- + H + The p K a value of PAA and PMA is 4.28 and 4.67, respectively [30]. Therefore, dissociation of the AA and MA segments in the copolymer affects the pH 6

20 ~tm

b) P(AN-co-MA) pH 4

pH 10

20 B m

pH6

20 ~tm 20 txm Fig. 2. SEM photographsof the cross section of the copolymermembranes.

20 ~tm

pH 10

20 Ixm

190

M.S. Oak et al. / Journal of Membrane Science 12311997) 185-195 O

1(10 a) P(AN-co-AA) 811

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Fig. 4. Plots of the log-normal probability of rejection versus molecular weight (MW) of dextrans. P(AN-co-AA) ( - - ) for pH 4 (zx), 6 ( D ) and 10 (©), P(AN-co-MA) ( - - - ) for pH 4 (A), 6 (11) and 10 ( 0 ) and PAN ( - - - ) for pH 4 (0).

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filtration performance of the copolymer membranes. The change of molecular sieve effect of the both copolymer membranes can be explained in terms of the dissociation of COOH groups in the copolymer membranes. At the alkali condition of pH 10, COOH groups in the copolymer dissociate to C O O - and H +. As we reported previously [12,13,19], the charged segments in the charged PAN UF membranes vary the pore size of the top surface of the asymmetric membrane by segmental repulsion between the ionic segments. Thus, charged C O O segments in the P(AN-co-AA) and P(AN-co-MA) membranes take an extended chain conformation at pH 10, owing to electrostatic repulsion between the neighboring C O O - groups. The expanded conformation of AA and MA segments reduces the pore size of the membrane, especially in the top skin surface layer. In most pH responsive membranes, the change

in permeation behavior occurs under pH conditions around the p K a for COOH segments, because COOH segments on the membrane surface are exposed to the bulk solution medium; PAA and PMA segments were grafted on the porous membrane surface [2,8]. But, in the cases of P(AN-co-AA) and P(AN-co-MA) membranes, the data of the permeation rate, SEM morphology, and molecular cutoff confirmed that the ionized chain effect on the filtration performance appears over pH 9 and does not occur around the p K a region of PAA and PMA. This implies that the change of chain conformation for ionized AA and MA segments stimulated by pH occurs above p K a. Table 1 MWCO estimated from cut-off data for various MWs of dextran permeation and apparent pore radius of the top surface of the membranes at different pHs MWCO a pH:

4

rp (fi~) b 6

10

4

6

10

P(AN-co-AA) 2.8X106 2.81<106 1.11<105 400 400 90 P(AN-co-MA) 4.1>1106 2.81<106 1.21<106 650 400 330 PAN 2.5>1105 2.51<105 1 2 0 120 a MWCO value was estimated from MW of dextran cut-ff by the 90% of the rejection in the MWCO curves. b Average pore radius of the upstream of the membrane was estimated by sieve slit model [27].

191

M.S. Oak et al. / Journal o f M e m b r a n e Science 123 (1997) 1 8 5 - 1 9 5

100

As the copolymer cast solution was solidified in the water medium, AA and MA segments coagulate with PAN segments. Since PAN segments are covalently bound to AA and MA segments, AA and MA conformation change by the ionization at alkali pH is much more limited. This is supported by the SEM photograph that the morphology at pH 6 is similar to that at pH 4, indicating that the dissociation of COOH groups between both pHs has little influence on the membrane structure. In addition, the nonpolar medium effect [31,32] of PAN segment may influence the dissociation of COOH groups in the membranes. Hence, the effect of the ionized C O O - segments on the filtration properties of the copolymer membranes appears in higher pH region.

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3.2. Salt effect on the filtration properties

60

Fig. 5 shows volume flux of 0.1 M NaC1 aqueous solution at various pHs. For the P(AN-co-AA) membrane, at pH 4, the volume flux of the saline solution is almost equal to that of saline free solution. As well as salt free solution in Fig. 1, the flux values of the saline solution are decreased when pH increases from 4 to 10. However, at pH 10, there is a relatively large effect of salt addition on the permeability of

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the P(AN-co-AA) membrane. The value for the salt solution is higher than that for the salt free solution. The salt effect is the same as that observed in charged PAN UF membranes with styrenesulfonic acid group [12] and quaternized ammonium group [15]. The data of the permeation rate observed in the P(AN-co-AA) membrane at pH 10 implies that AA segments are in negatively ionized C O 0 - form and show the conformation change of the polymeric chains from the expanded form to the shrunk one by the salt addition. The salt effect is due to the polyion charges being shielded by the counterions with the increase in salt concentration [29]. As a result, the ionized AA segments behave as well as nonionized segments due to the electrostatic shielding. But, for the P(AN-co-MA) membrane, there is little salt effect on the volume flux for water permeation. This means that the MA chain conformation with charges

M.S. Oak et al. / Journal of Membrane Science 123 (1997) 185-195

192

does not change significantly by the salt addition at pH 10. The corresponding dextran rejection curves are shown in Fig. 6. The curves shift to a low MW side relative to that observed in salt free condition (dash line) at pH 10. This means that the pore is shrunk by the salt addition at the pH. At pH 4 the COOH group has no charges and little salt effect on the MWCO curves is observed as well as the P(ANco-MA) membranes. For PMA, it is known that the dependence on the dissociation to the ionized species is more complex due to conformational transition of PMA segments [29]. We think that, in acidic condition, the MA segments in the P(AN-co-MA), form a tighter chain conformation than AA segments in the P(AN-co-AA). Consequently, relative to the P(ANco-AA) membrane, the effect of salt on the filtration behavior is diminished because of the hydrophobic aggregation effect with MA segment having methyl group and PAN segments. It is probable, even at pH 10, that ionized C O 0 groups in the MA segments are not in the extremely extended conformation [30], as suggested by little salt effect on the MWCO curve a t p H 10.

3.3. Fluorescence probe uptake N-Arylaminonaphthalenesulfonate probes are well known to be taken into hydrophobic regions in proteins and emit characteristic fluorescence from the environment in which the probe is present [33-36]. The extreme sensitivity of the ANS fluorescence to the environment makes it suitable for use as probe. Hence, we used ANS, in the present work, as a fluorescent probe to get a qualitative understanding of the copolymer environment in different pHs. Fig. 7 shows fluorescence spectra of ANS uptake to the

Table 2 ANS uptake in the membranes at pH 4 and 10 ANS uptake ( m o l / g dry membrane)

)tmax (nm)

pH:

4

10

4

10

P(AN-coAA) P(AN-co-MA) PAN

8 . 2 × 1 0 -5 8.2× 10 -5 3.0)<10 4

0.5 × 10 -5 0.4× 10 5 2.5)<10-4

438 440 466

_ a - " 466

a The value of )krnax could not be estimated from fluorescence spectra with very weak intensity.

(a)

=

~.

I

400

(f) xSO

I

500

600

Wavelength(nm) Fig. 7. Fluorescence spectra of ANS uptake into the copolymer membranes. Excitation of the probe uptake was carried out by 355 nm wavelength irradiation to the membrane. PAN membrane at pH 4 (a) and 10 (b), P(AN-co-AA) (c) and P(AN-co-MA) (d) at pH 4, and P(AN-co-AA) (e) and P(AN-co-MA) (f) at pH 10.

copolymer membranes and the PAN membrane at pH 4 and 10. Table 2 lists ANS amounts taken into the membrane and fluorescence maximum (Amax) observed in fluorescence spectra at pH 4 and 10. For the PAN membrane, the uptake ANS emits with •~-. . . . of 466 nm at both pHs. As reported [34], an intense blue fluorescence of ANS is observed in less polar solvents, whereas a weak green fluorescence is emitted in aqueous solutions. In addition, it was reported that the optimum maximum is at 515 nm in water, 484 in ethylene glycol, 476 nm in methanol, 468 nm in ethanol, 466 nm in n-propanol and 464 nm in n-octanol [33]. Therefore, the data of the P A N / A N S indicate that ANS probe is located in or in the vicinity of the nonpolar environment of the PAN membrane, because the Areax appears in blue region relative to that in water; The A.... = 466 nm (Umax = 2.15 X 10 4 c m - 1 ) corresponds to a polarity (20.7-24.6) for alcohol and dioxane-water mediums. Also, the strong emission intensity supports the nonpolar medium of the PAN microenvironment, in which ANS is uptake. Relative to the P A N / A N S system, emission intensities of the ANS taken in P(AN-co-AA) and P(AN-co-MA) membranes become weak in both pHs. As shown in Table 2, it is noted that amounts of the ANS uptake in the copolymer membranes at the

M.S. Oak et al. / Journal of Membrane Science 123 (1997) 185-195

alkali condition are much smaller than that at the acidic condition, pH 4. Therefore, the fluorescence intensity at pH 10 was too weak to estimate Areax values of the probe for the copolymer systems. Obviously, the very weak emission intensities are due to less amounts of the probe taken in the copolymer, because of charges of the copolymer membranes in the alkali condition. In copolymer membrane/ANS systems, the weak emission intensity at pH 4 is possibly due to the probe being in the vicinity of COOH groups rather than PAN segments. In the P(AN-co-AA) and P(ANco-MA)/ANS systems, the Amaxappears near 438 nm and 440 nm at pH 4, respectively. It is noted that, in the P(AN-co-MA) membrane having nondissociated COOH groups at the acidic pH, the emission maximum is slightly shifted toward the red region compared to that of the P(AN-co-AA). However, we noted that both ANS data in copolymer membrane/ANS system do not obey the relationship to the solvent dependence of the spectrum as seen in the reference [21,37]; The Areax emission energies (derived from the fluorescence spectra) for the copolymer/ANS systems deviated from a series of dioxane-water and alcohols. The abnormal emission behavior of copolymer/ANS system may be due to hydrogen bonding interaction [21] between - N H segments of ANS and COOH segments of the copolymer, because the uptake ANS is near the undissociated COOH group in the copolymer membrane. We checked ANS emissions in formic acid, showing that the Areax = 439 rim. Also, it is observed that ANS emission solution in PAA and PMA in the acidic form appears in 434 and 440 nm. Apparently, these values of ANS in carboxylic acid mediums show blue shift relative to ANS data obtained in alcohols and dioxane-water systems. As a result, the ANS uptake to the copolymer membrane has some problems for the estimation of polarity in the copolymer environment at different pHs. Further, in these systems, we cannot control the location of ANS probe in the copolymer membrane, because the uptake amount of the probe strongly depends on the charge condition of the copolymers. Thus, the uptake experiments gave little information on the microenvironment and no direct observation of the pH dependence of the membrane transition by the pH changes.

193

However, as shown in Table 2, the amounts of ANS taken into the membrane at each pH appear to give some information on charges in the membranes. In P(AN-co-AA) and P(AN-co-MA) membranes the values of the ANS uptake at pH 10 ape one-order smaller than those at pH 4. The difference is due to the carboxylic acid groups being in charged condition at pH l0 or uncharged at pH 4. Note that PAN membranes without COOH groups have no pH effect on the ANS uptake. Therefore, the decrease of the uptake at pH 10 for copolymer membranes is caused by pH effect of COOH groups binding to the copolymer membrane. At the alkali condition, the negative charges, C O 0 - , of the copolymer act as barrier to the ANS uptake due to electrostatic repulsion between the membrane and the probe with SO 3 group. The observed amounts in ANS emission peak intensity are also influenced by uptake amounts of the probe. The weak emission intensity observed in the P(AN-co-AA) and P(AN-co-MA) at pH 10 may be due to very few amounts of ANS uptake to the membrane environment with charged nature. As a result, at pH 10, the probe molecules cannot be taken in the hydrophobic part of the copolymer membrane and the observed emission peak is low in relation to that at pH 4. It is reasonable that PAN membrane has larger ANS uptake amounts than that of the copolymer membranes. The strong emission intensity in PAN membrane at pH 4 and 10 is due to the result of high uptake amounts to the nonpolar part of the membrane by hydrophobic interaction. This is supported by the red shift of the emission peak for the PAN membranes.

4. Conclusions pH sensitive UF membranes were made of P(ANco-AA) and P(AN-co-MA) copolymers by phase inversion method in DMSO/water medium. The filtration performance of both copolymer membranes was compared in water permeation and molecular sieve effect at different pH. The copolymer membranes with AA and MA segments showed significant filtration changes in different pH as follows. As pH varied from acid to alkali, the permeation rate was reduced and rejection of dextran by the membrane increased. Salt effect on the filtration performance

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strongly indicated that the COOH segmental conformation at pH 10 influences the cutoff property of the membrane. Evidence was shown that the dissociation of carboxylic acid segment in the copolymers causes the pH sensitive molecular sieve effect.

Acknowledgements This work was partially supported by the Grantin-Aid for the Ministry of Education, Science and Culture, Japan. One of us (M.S.O) would like to express her thanks to the Japan Society for the Promotion of Science for the fellowship and grant in aid and also the Institute Centre, Advanced Technology India, for the encouragement. The authors are grateful to Mr. Matsutaro Ono and Mr. Masaki Shibata for their valuable help in this work.

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