Composite membranes based on polyelectrolyte complexes

Journal of Membrane Science, 62 (1991) 131-143 Elsevier Science Publishers B V , Amsterdam

131

Composite membranes based on polyelectrolyte complexes L.E. Bromberg* “Natural Resources”Sc~ence

and Engineering

Centre, Moscow (USSR)

(Received October 16,1989, accepted m revised form December 12,199O)

Abstract Composite membranes (CMs) were obtained by lmpregnatmg Mtilpore mtrocellulose acetate ultrafilters with polyelectrolyte complexes (PECs ) prepared from poly (ethyleneimme ) , poly (ethylenepiperazme) and polyacryhc acid (PAA) CMs have shown good stablhty m aqueous salt solutions of pH 13-115 and lonlc strength up to 15 M The cause of CM stablhty m aqueous solutions lies m the ion bmdmg of polyelectrolytes dispersed m the mtrocellulose acetate framework pores The carboxyl group of PAA, ionized at pH > 6 5, created a transmembrane potential at a tenfold difference m KC1 concentration The sign of the potential corresponded to the preferential permeability of KC cations through CM as against Cl- anions The dependence of transmembrane potent& difference on KCl, NaCl and LlCl salt concentration differences at pH 9 0 was close to Nernstian, which confirms the usablhty of CMs m electrochemical testmg of salts Keywords

composite membranes, membrane preparation, blomedlcal use

Introduction

The increasing interest m new materials based on polyelectrolyte complexes (PECs) is due to the use of PECs as flocculants, colloidal suspension stabllizers, sorbents, ion-exchange materials and membranes [ 11. In Refs [ 21 and [ 3 1, the development and features of strongpolyelectrolytebased PEC membranes have been described. These membranes are highly permeable and can be made stronger by impregnating filter paper with the aqueous polyelectrolyte solution [ 21. In Refs. [ 4]- [ 61, PEC hydrogel membranes have been obtained from weak polyelectrolytes: polyacrylic acid (PAA), poly (ethylenepiperazine ) (PEPP) and poly (ethyleneimine ) (PEI ). The use of weak polyelectrolytes gives PECs with widely varying composltion Such PEC membranes, free from inorgamc salts, have high hydraulic *Present address Department of Matenals Research, The Weizmann Institute of Science, P 0 Box 26, Rehovot 76100 (Israel)

0376-7388/91/$03 50 0 1991 Elsevler Science Publishers B V All rights reserved

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permeability, are diffusely permeable to uncharged molecules, and are also selective to amino acids. For medical use, it is essential that materials modified with PEC hydrogels are thromboresistant and do not traumatize blood formula elements [ 71 However, when in contact with salt solutions, such polyelectrolyte membranes lose their strength due to marked swelling and dispersion. Moreover, air-dry PEC films are very brittle. Therefore there is a certain mterest m composite ion-exchange membranes obtained by impregnating strong polymeric supports with PEC solutions. The present paper deals with the method of obtammg such membranes and with some of their properties. Experimental

MaterraLs Prehmmary experiments were aimed at choosing the optimal composition of PEC and polymeric support. The criteria for good composite membranes (CMs) were stability in salt solutions, hydraulic permeability and conductlvity. PECs of readily available polyelectrolytes were used: polyacryhc acid (PAA ) , poly (ethylenepiperazine ) (PEPP) and poly (ethyleneimme ) (PEI) PECs are not contaminated with inorgamc salts when obtained from the volatile casting electrolyte solvent. PEC films were obtained by casting 30% formic acid solutions of PAA (weight-average molecular mass M, = 5 x 105), PEPP (M, = 3 x 104) and branched PEI (M, = 5 x lo4 ) on polyethylene. The polydispersity index of the polyelectrolytes was 1.8-2.2. The total content of polyelectrolytes in solution was lo-15% (by mass). Films were air-dried, kept at 145” C for 1 hr and washed with distilled water until the wash water had a neutral pH value. When forming a membrane from a mixture of polyelectrolytes, water and formic acid, evaporation of the solvent leads to a gradual increase in concentration of the solution until a transparent film of a polyelectrolytic mixture forms. Even vacuum-dried films contained a significant amount of formic acid. Thus a polymer was formed without reaction to form a PEC having begun This polymer consisted of a mixture of polyaminoformate and free polyacryhc acid After the film was placed in water it swelled greatly, and after a certain period of time its specific volume decreased to almost its initial volume. Judgmg by IR spectroscopic data, a PEC was formed due to hydrolysis of polyaminoformate and to formation of salt bonds between the ammo and carboxy1 groups of the polymer components. During the reaction, formic acid was extracted from the PEC film into an aqueous environment and removed. The course of the reaction determmed the low degree of swelhng of the film m water. Thus membranes were formed using PEC by way of reaction among polyelectrolytes, which took place in a pre-formed polymeric body. The general reaction can be represented by Scheme 1

133

--l------AH

T------T--dH

~__,____~~~~~~~____~~

AH -

BHCOOH

BHCOOH

6

6

0 BH

0 BH

t

BHCOOH

X

HCOOH

0 BH

_l______1____.___L___ ___L______L______i_. Scheme 1

Here z is an anion of polyacrylic acid

’ (-cH2-x’d,H),

is a protonated group of the polybase poly (ethylenepiperazme ) , CH,-CH, \ N-CH,-CH, or poly (ethyleneimme ) ( -NH-CH2-N > n ’ CH2-CHB ’ CH,-) n However, PEC membranes prepared according to Scheme 1 were unstable m aqueous solutions at pH <3 and pH > 10, in which they dispersed The number of bonds among polyelectrolyte chains could be increased by a reaction of intracomplex amidatlon, as in Scheme 2. ;H

coo

0

coo

coo

0

0

yH

p

t=o I

yH

2=0

c=o

T’ I

I

_ .

Scheme 2

The intermacromolecular reaction of amidation occurring m the sohd phase is characterized by the phenomenon of “kinetic arrest”, i.e. a sharp drop m the reaction rate on reaching a certain extent of conversion, and depending on the temperature [ 81. The conversion of the interchain salt bonds to amide occurs m the temperature interval 145-240” C [8,9]. It was proved that when the reaction shown in Scheme 2 was carried out, it led to partial covalent binding of polyelectrolytes and essentially increased PEC film stability in the pH interval 1-14. The number of covalent bonds in the membranes depended on PEC composition and temperature, and, in general, was below 510% of the total amount of the functional groups in the polybases. The processes during preparation of PEC films have been described m detail [1,4,8-lo].

Composite membranes (CMs ) were obtained from Mlllipore ultrafilters based on nitrocellulose acetate. The ultrafilters were kept m 30% formic acid solutions containing up to 40% (by mass ) of PAA-PEPP-PEI mixture for 2448 hr After that the impregnated membranes were air-dried, impregnated again,

134

washed with distilled water and dried at 60°C After impregnating and drymg, the weight gain was 200-250%. Apparatus

PECs and CMs were equilibrium titrated usmg a Beckman-@71 ion-meter. CM or PEC samples of a known dry weight were placed in air-tight measuring bottles containing 100 ml of NaOH or HCl solution at a known imtial pH The solutions were previously saturated with NP. The solutions, in sealed bottles, were stirred, and their pH was regularly measured. The pH value became constant in 5-15 days. The ion-exchange capacity of the membranes and the volume concentration of fixed ions were determmed as described in the monograph [ 111 The deviation of ion-exchange capacity of similarly prepared membranes swollen under the same conditions was less than 10%. Infrared spectra of the dry membranes were recorded with a Bruker IFS-48 spectrometer at room temperature. The number of salt bonds in the non-crosslinked initial PEC was determmed from the absorbance D (1715) at 1715 cm-l, which corresponds to vibrations of carbonyl in the non-ionized carboxyl group, and the absorbance D(1410) at 1410 cm-‘, which corresponds to antisymmetric vibrations of the carboxylate amon Calibrating spectra were prepared with a known ratio of COO- and COOH groups, assuming the equality of the absorption coefficients of the corresponding absorption band of PAA m PEC and that of poly (acrylic acid) partially neutrahzed by NaOH The number of salt bonds, qs, was calculated accordmg to the equation [9] qs=D(1715)/2.2

[0(1410)

-0.1 D(1715)]

In the crosslinked PEC, the number of covalent crosslinks (amide bonds) was evaluated by analyzing the change m the absorbance of the bands at 1410 and 1715 cm-’ (the stretching modes of, respectively, the carboxylate anions and non-ionized carboxyl groups) m relation to the unchanged band at 1450 cm-’ characterlzmg the vibrations of the CH, group m the initial PEC and after thermal treatment The sum of the fractions of all the bonds stabilizing the membrane (considering that the free carboxyl group may form hydrogen bonds ) was taken as unity. The calculation was based on the equations [ 81: qs= [D(1450)/D(1410)]/[D’(1450)/0’(1410)] qc= [D(1450)/D(1715)]/[D’(1450)/0’(1715)]

Q*=l-G-3, where D and D’ are the absorbances for the initial and heated PEC at the corresponding wavenumbers; qs is the fraction of the remaining interchain salt bonds not converted to the amide; qc 1s the fraction of free carboxyl groups, and qa is the fraction of the amide bonds (crosslinks).

135

:

L--J,, \

I

11

10

[?/

-,

-++---f

‘1 ‘\ \ ‘\ \;, s

y’ /II

,,,J---q /’

,’

,’

,I’

,1’

/’

I

Ir

/’ ,’

13

Fig 1 Design of the test apparatus ( 1) membrane, (2 ) poly (tetrafluoroethylene ) cell, (3 ) metal casing, (4) Pt electrodes, (5) Ag/AgCl electrodes, (6) stirrer, (7) pH-meter/mlll~voltmeter, (8) meter, (9) capacitance bridge, (10) null mdlcator, (11) slgnal generator, (12) DC bndge, (13) recorder, (14) computer

The membrane stability in aqueous solutions was estimated by measuring the membrane dry mass before and after the experiment. Hydraulic permeabrhty and swelling were measured as described earlier [ 12,131 Resistance and capacity of the membranes reached stable values after 12-15 hr of swelling. Electrochemical properties of the membranes were studied usmg the apparatus shown schematically in Fig. 1 The membrane (1)) with operating area L&=3.14 cm’, was placed m a cell with donor and acceptor sections of equal volume V= 15 ml. The cell was equipped with flat Pt electrodes with an area much greater than S, The distance between the electrodes was 6 cm. The cell was placed within an earthed, thermostatted ( t = 20” C ) casing (3 ). The electrolyte was stirred with mechamcal stirrers (6). Electrrcal capacitance and conductlvlty of the cell with an without the membrane and through the applied frequency range (f= 60 Hz-100 kHz) were measured with a unit conslstmg of an E8-2 capacitance bridge (9), an F582 null detector (10) and a GZ-33 signal generator (11). Direct current resistance (Rconst)of samples was measured with an R4833 DC bridge (12) with correctron for the cell and elec-

136

trolyte resistances. The potential difference across the membrane was measured with an EV-74 pH-meter/millivoltmeter (7) using Ag/AgCl electrodes (5) Ion concentrations m the donor and acceptor sections were measured by regular collection of output samples as well as by constant testing with an ionmeter or a spectrophotometer (8) connected to a recorder (13). All calculations were carried out using a computer (14). All the units shown m Fig. 1 except the PC (Olivetti M-24) were made in the USSR. The swelled membrane thickness L, was measured with a Karl Frank-16257 micrometer. Results and discussion

Composite membranes obtained by impregnating Millipore nitrocellulose acetate ultrafilters with PAA, PEPP and PEI polyelectrolyte complexes showed good stability m NaCl, KC1 and LlCl aqueous solutions with pH = 1.3-115 and ionic strength up to 1.5 M Electrical conductivity and diffusion permeability to urea [ 131 remained constant for 10 days or more after the beginning of swelhng. The degree of elution of PEC from filter pores was evaluated gravlmetrlcally and also by elemental analysis, and reached lo-15% after 21-30 days. On the CM surface and m its pores there was observed a layer of swelled PEC tightly bound to the nitrocellulose acetate matrix CM hydraulic permeability showed dependence on ultrafilter pore size and varied from lo-’ ( m2kg-‘-set-‘) at d,,,, 4.0 pm to lo-” (m2-kg-‘-set-l) at d,,,=O.O5 pm for equal specific amounts of electrolytes in membranes. A marked dependence of electrical conductivity on the specific amount of PEC m the membranes was observed. However, although after lmpregnatmg and drying the weight gain was 200-250% (see “Materials”), the deviation of the electrical conductivity of equally swollen membranes was under 10%. Figure 2 shows typical potentiometric titration curves for Millipore VSWP

01

02

03 01

Fig

2

‘I’ypml

M HCL

04 (cm3)

potentlometnc tltratlon curves for M&pore VSWP ultrafilters washed with water

( 1) and for the same ultrafilters impregnated with polyelectrolyte complex (2 )

137

/

1600

1600 Wavenumber

1400 (cm-‘)

Fig 3 Multiple total mternal reflection (MTIR) IR spectra of M&pore VSWP mtrocellulose acetate filters ( 1) and of the same ultrafllters impregnated with poly (ethylenelmme ) (2 )

ultrafilters pre-washed with distilled water at 60” C (1) and for the same ultrafilters impregnated with PEC (2) consisting of PAA and PEPP in equal proportions. The pH of the solution (volume 100 ml) in equilibrium with the membrane was measured. PEC composition (Z) is characterized hereafter by the ratio between the concentration of PAA carboxyl groups and the total number of PEPP and PEI ammo groups undergoing the complex-forming reaction. The Titration curve 1 (Fig. 2) shows a distinct buffer area at pH 5.5-6.5. The cation exchange capacity of the ultrafilter was ca. 4 x 10T6 M, which agrees with published data [ 141. The existence of ion-exchange groups, mainly carboxyl, determines the adsorption features of ultrafilters [ 151. However, the total ion-exchange capacity of a membrane (Fig. 2) was ca. 2 x 10e2 M, hence the PEC ion-exchange capacity was much greater than that of the filter (cf curves 1 and 2) and thus determined the ion-exchange properties. The significant concentration of ion-exchange groups on the surface of Millipore filters led to the conclusion that mteraction of filters and PEC components forms ionic bonds. Indeed, when filters were processed by immersing them in aqueous PEI solution, with subsequent CM vacuum drying, multiple total mternal reflection (MTIR) spectra of the membranes showed an increase m absorption band mtenslty at 1550 and 1430 cm-l (Fig. 3). Bands at these wavenumbers correspond to asymmetric and symmetric stretching vibrations and the C-O group m the carboxylate anion. Similar spectral changes took

138

place on treating the filters with 1 M NaOH solution, and seemed to indicate salt bond formation between impurity carboxyl groups on the mtrocellulose acetate surface and polybase ammo groups. Judging from the MTIR-IR spectroscopic data, there was no substantial mteractlon between the filters and PAA. Salt bond formation between mtrocellulose acetate and PEI does not seem to explain the phenomenon of high PEC stability m the ultrafilter pores, as each of the polyelectrolytes individually impregnating the ultrafilter could be totally washed out by water. Therefore the cause of CM stability m aqueous solution lies w&h ion bonding of the polyelectrolytes making up the PEC dispersed in the mtrocellulose acetate framework pores. The structure of the swollen PEC films based on weak polyelectrolytes may be represented m terms of alternating sequences of ionic mtercham bonds and “defective” loop-like regions incorporating the uncoupled umts of both chains [ 1,8] (Scheme 3)

/ Two-than

sequence

Scheme 3

The relative number of interchain ionic bonds, which determines the ionexchange capacity of the PEC and the CM, can be controlled by changing the PEC composition and the pH and ionic strength of the medmm m which the membrane is swelling The state of the ionogenic groups in the swollen membranes was monitored by potentlometrlc titration to analyze the binding of OH- and H+ ions by the PEC samples. The titration method was proposed [8,16] for studying the binding of low molecular weight salts. With such an approach, the carboxyl groups located m the defective regions of the PEC must bind OH ions, i.e. become ionized according to the process of independent adsorption. At the same time, the rupture of mterchain salt bonds accompamed by uptake of OH- ions takes place in a more alkaline medium according to the process of cooperative adsorption [ 81. Figure 4 shows curves for the pH of the solution (volume 100 ml) in equilibrium with PEC hydrogels of different composition containing PAA and PEPP. It is natural to suppose that the plateau at pH =6-7 corresponds to complete titration of the free carboxyl groups of PAA m loops. The plateau becomes broader with the Increase of PAA concentration m PEC. Aqueous PEC stability was observed over a wide range of PEC compositions: 1
139

sP” 6-

21 0

10

30 20 01 M NOOH (cm’)

40

Fig 4 Typical potentlometrx tltratlon curves for PEC hydrogels of PAA and PEPP, contalmng covalent PAA-PEI bonds [9,10], vs gel composltlon ,Z Composltlon Z= PAA PEPP (1) Z= 1, (2) Z=2, (3) C=4, (4) C=6

Fig 5 Plot of DC conductlvlty G,_,,,t (circles) and full admittance (at f= 60 Hz) Y60 (crosses) of PEC hydrogel vs its composltlon C=PA PB, where PA = polyacryhc acid concentration, PB = polybase concentration PEPP PEI = 3 1 as per 1 mol of PEPP Hydrogel membranes swollen m 0 01 M NaCl solution at pH 7 0 The dashed hne shows the plot for the specific amount of free water (crystalhzed by coohng to - 50” C ) , I& The I& values were taken from Ref [ 17 ]

wrth those observed for PEC membranes based on poly(vmylbenzyltrlmethylammonium ) and poly (styrenesulphonate) [ 3 ] Membranes with a non-stolchrometric composition of polyelectrolytes behaved as ion-exchange membranes with a fixed charge density of 3 M for the anion and cation exchangers, whereas neutral membranes behaved as weak (charge z 0 06 M) amon exchangers with KC1 or CaCl,. The authors of Ref [3] assumed that

140

possible reasons for those results are slight deviations from stoichiometry in the reaction mixture or burial of some of the less bulky sulphonate groups (with associated microions) m the gelatlon step. In our experiments the ionexchange capacity of CMs containing PECs with fixed composition of Z depended mainly on the specific amount of PEC. Variability of the CM properties has advantages (it allows use of CM m different situations) and drawbacks (it requires strict standardization of membrane preparation methods). It can be supposed that with varying PEC composition the size of the defect loops formed by disengaged polyelectrolyte umts changes. It seems that it is the disengaged polyelectrolyte units included in defective PEC sections that determine the value of swelling for these systems. Figure 5 shows a plot of electrical conductivity of PEC hydrogels vs. their composition Z The dashed line shows the plot for the specific amount of free water Hf (specific content of water which crystallized when cooled down to -50°C) [17,18]. At a twoor three-fold PAA surplus m PEC, there is a maximum amount of mtra- and mtermolecular hydrogen bonds between protonated PAA COOH groups not formmg salt bonds, while the amount of free water [ 171, and subsequently the electrical conductivity of PEC hydrogels, both have minimum values. Reference [17] shows that m this area of Z the hydrauhc permeability and PEC diffusion permeability for urea also have mnnmum values. At Z> 3 the amount of free water and the PEC electrical conductivity rise; however, mechanical strength and stabihty decline [ 51. With the increase of PAA concentration m PEC, the initial value of modulus of elasticity of PEC (E,,) changed to an extreme At X=1, 2, 3, 4 and 6 the value of E,, of the PEC films was ca. 10,125,130,100 and 40 MPa respectively [ 51. IR spectroscopy proved that this relationship represents the process of formation and destruction of structures stablhzed by hydrogen bonds of redundant PAA macromolecules [ 8,9], as shown m Scheme 4. Polybose

0

coo

0

coo

PAA

I c=o

0 BH

0 BH

I B

\ Regm

stobtllzed

by

hydrogen

bonds

Scheme 4

The polyelectrolyte PAA-PEI complex contains far fewer uncoupled units m the loop-hke defects as compared with the PAA-PEPP complex. In fact, the number of carboxyl groups in the defects of the former is = 25% (connected with the high structural complementary of the chains of PAA and PEI formmg the PEC) whereas in the PAA-PEPP complex this number is ~70% [8] Therefore, on adding PEI to a polyelectrolyte PAA-PEPP complex there is an increase m concentration of ionic bonds m the complex. This is accompanied

141

Fig 6 (a) Degree of dlssoclatlon ((u) of excess PAA COOH groups m PEC vs pH ( 1) , degree of swelling of composite membranes vs pH (2) (b) Potential difference (do) vs pH for composite membranes at a tenfold difference m KC1 concentration See explanations m the text

by a rise in PEC stability but a decrease in electrical conductivity and permeability For instance, with the molar ratio PEPP - PEI = 3 1 the electrical conductivity of the PEC was almost 5 times higher than that with a ratio PEPP PEI = 0.3 10. Judging by the usage of CMs, we consider stoichlometric PECs to be the best to use in preparing CMs from the viewpoint of their electrical conductances and accompanymg stability. As the titration curves show (Fig. 4)) at pH > 6.5 free carboxyl groups of PEC iomze and give an excess effectrve negative charge of the CM. By effective charge we mean, as in Ref [ 191, that associated with the PEC specimen bathed m its liquid chemical environment. It is this effective charge that is reflected m measurable macroscopic properties such as ion-exchange capacity, reslstance, transport and transmembrane potential. Consequently CMs must have distinct pH-dependent features Figure 6 plots against pH the CM characteristics of optimal composition based on Milhpore VMWP ultrafilters (pores of 0.05 ,um diameter) It is clear that all the S-like curves are very similar With the increase m the degree of dissociation ((x) of excess PAA COOH groups (curve 1 m Fig. 6a), PEC hydrogen bonds are broken, which leads to expansion of the hydrogel structure and increase m the degree of membrane swelling q% (mass ratio of water to sohd, curve 2) Carboxy1 group iomzation at pH > 6.5 created transmembrane potentials at a ten-

142

Fig 7 Transmembrane potential dfierence for CM at pH =9 0 vs the concentration ratio of l1 salts on different sides of the membrane, log( C&/C&),with C,= lob5 M Total ion-exchange capacity of the CM was ca 0 1 M The solutions on either side of the membrane contained 10 n-A4 trls(hydroxymethyl)ammomethane

fold difference m KC1 concentration (Fig. 6b). At pH > 7.5 the transmembrane potential difference (A@), near 58 mV, indicates an ideal CM catlomcamomc selectivity under these conditions. The sign of the potential corresponded to a preferential permeability of K+ cations through CM as against Cl- amens (the 402 value m the dilute solution was more positive than in the concentrated solution ) . Measured values of d@ directly reflect CM charge density In general, the higher the charge density, the closer d@ should approach the value attained for the hmit of an ideal semipermeable membrane where colon transport becomes small [ 191. It should be noted that a similar result was achieved in experiments with non-stolchlometric PECs of strong polyelectrolytes with surplus polyanion [ 3 1. The dependence of transmembrane potential difference on log (CJC,) (C, and C, are KCl, NaCl and LiCl salt concentrations on erther srde of the membrane) at pH 9.0 was close to Nernstran (Fig. 7) which confirms the usability of CM m electrochemical testing of salts. Composite membranes are easy to obtain using polyelectrolyte complexes and they possess pH-dependent ion-exchange selectivity. Therefore, they can be expected to be of practical use in the future. Acknowledgements

The author would like to thank Drs. A.R. Rudman, N.A Vengerova, B.A. Gorehk, N.M. Kochergmsky and B.S. Eltsefon for their comprehensive help.

143

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8

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15 16 17

18 19

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