British Polymer Journal 22 (1990) 155-159

Measurement of Transport Properties of Poly( methylmet hacrylate-co-

methacrylic acid) Ion-Containing Membranes

Oya $anli

Department of Chemistry, Faculty of Arts and Science, Gazi University, Ankara, Turkey

&

Leyla Aras*

Department of Chemistry, Faculty of Arts and Science, Middle East Technical University, Ankara, Turkey

(Received 11 January 1989; revised version received 17 April 1989; accepted 24 April 1989)

Abstract: Several membranes prepared from poly(methylmethacry1ate-co- methacrylic acid) and its Li' and Zn2+ ionomers were tested for NaCI, creatinine and urea permeability. The permeabilities of the membranes were explained on the basis of pore contents determined from their scanning electron microscope micrographs. All the membranes showed higher permeabilities during the first 2 hours of experimentation. Introduction of Zn2 + ions into the copolymer as crosslinking agent did not have much effect on the membrane properties but the properties of the copolymer were modified.

Key words: polymethylmethacrylate membrane, poly(methylmethacry1ate-co- methacrylic acid) membrane, ionically crosslinked poly(methy1methacrylate-co- methacrylic acid) membrane, permeability coefficient.

1 INTRODUCTION The exact nature of the ion bonding is not clear but it has been found that the amount of bound ions

In this paper we report investigation of the increases with the extent of neutralization and with transport properties of poly(methylmethacry1ate- the charge on the ion. It is believed that small valence co-methacrylic acid) [poly(MMA-co-MA)] and its counterions (e.g. Li +, Na +) are generally surrounded ionomer membranes. Poly(acry1ic acid) [PAA] by an electric field created by the charged polymer based membranes are of particular interest because molecules, while higher valence counterions (Cu2 +, of their hydrophilicity and because the carboxylic Zn2 +) are located more specifically at the carboxylic groups can dissociate to give a charged character to sites.5 the membrane material. The introduction of Experiments on the membrane properties of PAA charged groups affects the potential to form ionic prepared by wet and dry techniques have been bonds' and modifies the properties of the materials reported. Habert et aL6 prepared by a wet technique profoundly, i.e. increases the Tg,2 viscosity and the ionically crosslinked PAA membrane for possible modulus.* application in dialysis by casting a film of PAA

neutralized with NaOH, followed by immersion in * To whom all correspondence should be addressed. solutions of the appropriate metal salts. Habert and

155 British Polymer Journal 0007-1641/89/$03~50 0 1989 Society of Chemical Industry. Printed in Great Britain

156 Oya Tanli, Leyla Aras

Huang7 developed a dry technique by casting a solution containing the metal salt and unneutralized PAA and subjecting the cast film to a heat treatment to promote ionic crosslinking. Dickson et a/.* used ionically crosslinked PAA membranes prepared by a dry technique in reverse osmosis.

Sakai and Tanzova' reported that the membrane properties of poly(methylmethacry1ate) [PMMA] depended on the stereospecificity of the material. It was found that NaCl permeability of the membrane was the same as that of cellulosic membranes.

To attain a membrane of appropriate solute (NaC1, urea, creatinine) permeability we used a copolymer of methylmethacrylate and methacrylic acid. The effect of the counterion was also studied for Li+ and Zn2+ ionomers of the copolymer.

2 EXPERIMENTAL

2.1 Membrane preparations

Poly(MMA-co-MA) prepared anionically with a ratio of carboxyl to ester group of 1 :2 was supplied by Rohm Pharma. Its molecular weight was 135 000. Membrane 1. 2.5ml of an 8wt% solution of the copolymer in ethyl alcohol (Merck) was cast on a glass plate (9 cm x 9 cm) and after a gelation period of 1Omin it was immersed in deionized water (non- solvent) and the film was removed. Membrane 2. Membrane 2 was prepared in the same way as membrane 1 except its 'gelation period' was the time required to obtain dryness of the film. Membrane 3. The same procedure as for the preparation of membrane 1 was followed but the membrane was kept in deionized water for 2 days.

Magnetic stirrer

PMMA sheet Fig. 1. Dialyser for measuring solute permeability. C,, the initial concentration in the left-hand compartment; C't, concentration at time t in the left-hand compartment; 0, 0 concentration in right-hand compartment; C"t, concentration

at time t in right-hand compartment.

2.2 Permeability measurements

All experiments were performed at 23°C using a glass cell as illustrated in Fig. 1. The effective membrane area was 9.07 cm2 and the wet thickness was 4 x 10- cm. The left compartment of the cell was filled with 90 ml of an aqueous solution of NaC1, urea or creatinine. An equal volume of water was poured into the right side. Both compartments were stirred magnetically at a constant rate. The con- centrations of the substrates transported from left to right across the membrane were measured. NaCl was determined by electric conductometry (EM 776), urea and creatinine by visible spectrometry (Met- rohm Herrisov).

The scanning electron microscope (JEOL) micro- graphs of the membranes were taken after sticking the membranes on Jeol stubs, followed by freeze drying and coating with gold. The magnification was in the range 100&20000.

3 RESULTS AND DISCUSSION Membranes 4 and 5. The copolymer was completely neutralized with Lie1 and ZnC1, by the addition of ,.,redetermined amounts of each salt. stirring mag-

The permeability coefficients for all the membranes were calculated using the following relation~hip:~

(1) hetically for 2 days. Membranes 4 and 5 of &e L?

membrane 1. Membrane 6. PMMA, prepared free radically, with a molecular weight of 150000 was also used as a mem- brane material and was prepared as membrane 1.

AC 1 J , = P 2 - and Zn2 + containing copolymers were prepared as

where J,, AC and A are solute flux (mol/cm2 s), concentration difference (mol/cm3) across the mem- brane, and membrane thickness, respectively. The

TABLE 1. NaCl permeabilities of the membranes ~

Time (min) ~

Na CI (g /I itre)

Membrane 1 Membrane 2 Membrane 3 Membrane 4 Membrane 5 Membrane 6

0 8.00 8.00 8.00 8.00 8.00 8.00 60 7.20 7.50 7.00 7.80 7.10 7.8 132 6.90 7.00 6.85 7.00 6.60 7.1 5 1 80 6.60 6.90 6.50 6.80 6.40 7.0 240 6.20 6.80 6.40 6.70 6.20 6.8 300 6.1 0 6.70 6.20 6.50 6.1 0 6.5 360 6.00 6.50 6.00

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Poly(methylmethacry1ate-co-methacrylic acid) membranes 157

Coefficient P, was calculated from the following equation:

(2) 2.303iu"

c,- ( 1 + , co ::) cl' P, =- - --log,, (1 +;)A

where u', u", A , C , and C," are the volumes of the concentrate and diluen t compartments, effective membrane area, initial concentration of the concen- trate, and the concentration of the diluent at sampling time t , respectively.

3.1 Sodium chloride permeability

The results of the NaCl permeability of membranes 1-6 are reported in Table 1. The lower permeability of membrane 2 compared with membrane 1 can be explained by examining their SEM micrographs [lo00 magnification], which show that membrane 2 has a nonporous structure while membrane 1 is highly porous (Fig. 2).

Table 1 shows that the permeability of mernbranc 3 compared with membrane 1 is slightly higher in the first 3 h but is lower thereafter. The permeability of membrane 6 is lower than that i f membrane 1 throughout the experiment.

Comparing the NaCl permeabilities of mem- branes 1, 4 and 5 shows that membrane 5 which contains Zn2 + ions has the highest permeability and membrane 4 which contains Li+ ions has the lowest permeability.

SEM micrographs at 20 000 magnification for membranes 1, 4 and 5 are shown in Fig. 3. These micrographs show the different sizes and distri- butions of the pores in the membranes. It is apparent that there is a wide distribution of pores in the copolymer membrane, whereas in the ion- containing membranes the pores are much less in number.

From Table 1, the Zn2+ containing copolymer membrane has higher NaCl permeability than the Li + containing copolymer membrane. This may be ascribed to the size and charge difference of the two cations.

NaCl permeability coefficients calculated from experimental results are given in Table 2.

TABLE 2. NaCl permeability coefficients of the membranes

Membrane Permeability coefficient

1 2 3

8.850 x 1 0-7 7.867 x 9.320 x 1 0-7

4 7.867 x 1 0-7 5 10.930 x 1 0 - 7

(b)

Fig. 2. SEM micrographs of the membranes a t x loo0 6 6.560 x magnification. (a) Membrane 1; (b) membrane 2.

BRITISH POLYMER JOURNAL VOL. 22, NO. 2,1990

158 Oya Sanli, Leyla Aras

Fig. 3. SEM micrographs of the membranes at 20000 magnification. (Top) Membrane 1; (bottom left) membrane 4; (bottom right) membrane 5.

A number of experiments were done to compare the permeability of membrane 1 and membrane 6 and the results (Table 1) show that PMMA has lower permeability then poly(MMA-co-MA).

3.2 Crea tinine permeability

A series of experiments were conducted with 1.13 g/litre creatinine (BDH) solutions for time periods ranging from 60 to 360min. At predeter-

mined times 0.2 ml samples were taken; 5.8 ml water, 2 ml saturated picric acid (Riedel-De-Honeag) and 2 ml of 0.75 N NaOH were added. The absorbance of the coloured complex at 550 nm was measured after 20min. The decrease in the concentration of creatinine in the concentrate compartment is given in Table 3 and the permeability coefficients for membrane 1 and membrane 5 are given in Table 4. Both types of membrane show similar permeabilities for creatinine.

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Poly(merhylmethacrylate-co-methacrylic acid) membranes 159

TABLE 3. Creatinine permeability of poly( P M MA- co-MA) and Zn2+ ion-containing copolymer

Time (min) Creatinine (g/litre)

Membrane 1 Membrane 5

0 1.13 1.13 60 1.02 0.96

120 0.82 0.96 180 0.58 0.58 240 0.48 0.51 300 0.34 0.36

TABLE 4. Urea and creatinine permeability coefficients

Membrane Permeability coefficient

Creat i n i ne Urea

1 2.1 38 x 1 0-6 2.435 x 1 0-6 5 2.968 x 1 0 - 6 2.1 45 x 1 0-6

3.3 Urea permeability

Urea permeabilities of membranes 1 and 5 were tested using 0*8g/litre urea solutions for the same time periods as used for creatinine permeability. To l m l of samples taken from the concentrate com- partment, 10 ml p-dimethylaminobenzaldehyde solu- tion [2 g p-dimethylaminobenzaldehyde in 100 ml

TABLE 5. Urea permeability of poly( MMA-co-MA) and Zn2+ ion-containing membranes

Time (min) Urea (g/litre)

Membrane 1 Membrane 5

0 0.80 60 0.71

120 0.56 180 0.33 240 0.1 4 31 5 0.05

0.80 0.71 0.58 0.46 0.33 0.1 3

of 95% ethylalcohol (Merck)] were added. After dilution to 25 ml, absorbance values were recorded at 420 nm. The change in urea concentration and the permeability coefficients for membranes 1 and 5 are shown in Tables 5 and 4 respectively.

The permeability of membrane 1 is similar to that of membrane 5 up to 60min, but for longer permeation times membrane 1 had higher permeability.

4 CONCLUSION

Poly(MMA-co-MA) membranes freshly prepared after a period of gelation are permeable to NaCI, urea and creatininine solutions.

Depending on the character of the bound cation in the copolymer, the permeability may be altered. The comparative data show that Zn2 + containing membranes have similar permeabilities to the poly(MMA-co-MA) membranes without Zn. Since many properties (mechanical, T,, etc.) of a polymer may be improved by the addition of ions, Zn2 + ion- containing membranes may be preferable to poly (MMA-co-MA) membranes under certain conditions.

ACKNOWLEDGEMENTS

This work is partially supported by the Middle East Technical University, Ankara, Turkey, Project AFP- 87-01-03-12, and the Turkish National Technical Research Council (TBTAK) by TUMKA, which is greatly acknowledged.

REFERENCES

1 Holiday, L., Ionic Polymers, Hastead Press, New York, 1975. 2 Yeo, S. C. & Eisenberg, A. J., J. Appl. Polym. Sci., 21 (1977) 875. 3 Lopez, M., Kipling, B. & Yeager, H. L., A n d . Chem., 48 ( I 976) I 12G2. 4 Yeager, H. L. & Kipling, B. J., Phys. Chem., 83 (1979) 1836-9. 5 Miller, M. L., The Structure of Polymers, Reinhold, New York, 1966,

6 Habert, A. C., Huang, R. Y. M. & Burns, C. M., J. Appl. Polym. Sci., 24

7 Habert,A. C. & Huang, R. Y. M.,J. Appl. Polym. Sci., 24(1979) 801-9. 8 Dickson, J. M., Loyd, D. R. & Huang, R. Y., J. Appl. Polym. Sci., 24

9 Sakai, Y. & Tanzova, H., J. Appl. Polym. Sci., 22 (1978) 1805-15.

Chapter 12.

(1979) 489-501.

(1979) 1341-51.

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