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Ion exchange membranes blended by cellulose cuoxam with alginate
j o u r n a l of MEMBRANE SCIENCE ELSEVIER
Journal of Membrane Science 124 (1997) 195-201
Ion exchange membranes blended by cellulose cuoxam with alginate Lina Zhang *, Daochun Zhou, Hao Wang, Shuyao Cheng Department of Chemistry, Wuhan University, Wuhan 430072, China
Received 1 April 1996; revised 17 July 1996; accepted 19 July 1996
Abstract Ion exchange membranes were prepared by coagulating a blend of 6.4 wt% cellulose cuoxam (I) and 3 wt% aqueous alginate solution (II). The tensile strength of both the dry and wet blend membranes was obviously higher than that of membranes made of pure alginate. The value of ion exchange capacity was 1.25 meq/g dry membrane for blend membrane in which I / I I was 1:1.4 by weight, and the ion exchange membrane has excellent reproducibility. IR, SEM, X-ray diffraction, AAS and DTA results showed that the blend exhibited a certain level of miscibility, and the strong interaction between the cellulose and alginic acid was caused by occurring hydrogen bonding of their intermolecules. The mechanical properties of alginic acid membranes in water were significantly improved by blending with cellulose cuoxam, thus the blend membranes can be used as ion exchange membranes in water swollen state. Keywords: Ion exchange membrane; Cellulose; Alginate; Blend; Scanning electron micrography
1. Introduction Nowadays, ion exchange membranes have been used to remove a large amount of toxic heavy metals from effluents, and also have many other practical uses. Equilibration experiments revealed that alginate resin adsorbs metal ions more extensively than Ambeflite IRC-50 [1]. Membranes prepared from polyvinylchloride and alginic acid crosslinked with formaldehyde were used to study the transport behavior of ferric, aluminum and cupric ions and the selective permeability of ferric ion [2]. Moreover, an alginic acid membrane was developed for the vapor permeation membrane separation technique in order
* Corresponding author.
to separate ethanol-water mixtures [3]. However, the mechanical strength of the alginic acid membrane is very weak, so that its application is limited. Freddi et al. reported [4] that the mechanical properties of silk fibroin films were improved by blending with cellulose. In previous work [5,6], the miscibilities of regenerated cellulose membranes blended with cellulose c u o x a m / z i n c o x e n e as well as cellulose cuoxam/casein were studied. The experimental resuits showed that the mechanical properties were significantly improved. It is worth noting that utilizing natural cellulose and alginate as raw material can protect the environment because of their biodegradability. This has prompted us to study ion exchange membrane blended with cellulose cuoxam and alginate. Alginic acid belongs to a heteropolysaccharide that consists of [3-(1--* 4)-o-mannuronic acid and
0376-7388/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S0376-7 3 8 8(96)00227-X
L. Zhang et al./ Journal of Membrane Science 124 (1997) 195-201
196 COOH
H
H
H
0 OH
HO
H
H [M]
C00H H
HH//~0~ 0~
H
H
O HO H H
[G]
F_O
H [M]
CNAL6-3, CNAL6-4, CNAL6-5, CNAL6-6, CNAL6-7, CNAL6-8 and CNAL6-9. The membranes CN6 and AL3 were prepared from pure cellulose cuoxam (cuprophane) and aqueous sodium alginate solution, respectively.
Scheme 1.
2.2. Characterization of membranes et-(1 ~ 4)-L-glucuronic acid as depicted in Scheme 1 [7]. However, our recent work [8] showed that some microporous membranes were formed by mixing cellulose cuoxam with gelatin, which was dissolved in coagulation bath and removed from the membrane. In this work, first the conditions of forming blend membranes and bonding between cellulose and alginic acid groups were investigated, then the mechanical property and ion exchange capacity were measured.
2. Experimental
2.1. Preparation of membranes The linters used were supplied by Chemical Fiber Manufacture of Hubei. The viscosity average molecular weight Mn, which was determined by viscosity method, is equal to 1.96 x 105. The 6.4 wt% cellulose solution in cuoxam (I) was prepared according to our previous work [9]. Sodium alginate was purchased from Branched Factory of Shanghai Chemical Reagent Company. The preparation method of the blend membrane was as follows: a mixture of I and I I (3 wt% aqueous sodium alginate solution) was stirred, then filtered and degassed. The mixture of the solutions was spread over a glass plate to a depth of 0.3 mm and coagulated in a coagulation bath of 5 wt% CaC12 aqueous solution. The membranes were regenerated in dilute H2SO 4 to get blend membranes of cellulose/alginic acid, then washed at least 2 h with running water and plasticized with 5 wt% glycerine. The blend membranes were dried on the glass plate in air at 50°C. By changing the weight ratio of I to I ! such as 1:0.2, 1:0.5, 1:0.7, 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4 and 1:1.5, we prepared a series of the regenerated cellulose blend membranes coded as CNAL6-1, CNAL6-2,
Scanning electron micrographs (SEM) were taken with a Hitachi X-650 SEM. The membranes were coated with gold, subsequently their surfaces were observed and photographed. X-ray diffractions were measured with an X-ray diffractometer Rigaku Dmax-rA. The X-ray diffraction patterns with Cu K ot at 40 kV and 25 mA were recorded in the region of 20 = 5-45 °. The degree of crystallinity (X c) was calculated according to the usual method [10]. IR spectra of the membranes were recorded with a Nicolet F r - I R spectrometer. The content of copper, calcium and sodium in the membranes was determined by atomic absorption spectrometry (AAS, Hitachi 180/80). For the determination of these elements, the membranes were treated as follows: 0.1 g of the membrane plus 5 ml concentrated HNO 3 and 0.5 ml H202 were heated to about 100°C to get a carbonization product, then kept at 500°C for 2 h. After cooling, 6 ml concentrated HNO 3 and 0.5 ml H202 were added, subsequently the mixture was heated again to get a clear solution for the AAS determination. Differential thermal analysis (DTA) was carried out using a Shimadzu DT-40 thermal analyzer. The membrane was cut to pieces with 2 mm length (5 mg), then analyzed under nitrogen atmosphere from 30°C to 450°C at a heating rate of 20°C/min. The tensile strength (o-b) of dry and wet membranes was measured by an electronic strength tester (XLD-0.1) according to the Chinese standard method (GB 4456-84) at a tensile rate of 100 m m / m i n . The wet membrane was measured immediately after immersing in water for 10 min.
2.3. Ion exchange capacity The active exchange capacity is defined experimentally as the number of exchange sites within the membrane structure which are titrable while leaving
L. Zhang et al. / Journal of Membrane Science 124 (1997) 195-201
the membrane intact. The membrane was equilibrated with 1 M HC1, rinsed free of diffusible acid, blotted quickly and eluted with an excess of CuSO 4 of about 0.01 M. The eluent was titrated in conventional fashion with standard base [11]. The ion exchange capacity (A R) of the dry membrane was determined by the following equation NV AR
~
- -
W
where N (mol/l) is the concentration of the standard base; V (ml), the volume of the consumed base; W (g), the weight of the dry membrane.
197
3. Results and discussion 3.1. Structure analysis
Fig. 1 shows SEM of the membrane prepared from pure sodium alginate (AL3) and blend membranes of CNAL6-1, CNAL6-2 and CNAL6-9. The AL3 membrane displays a coniferous architecture (A). The blend membrane CNAL6-1 has a smooth surface (B) similar to the membrane from pure cellulose cuoxam [6], indicating a good miscibility between cellulose and a small amount of alginate.
Fig. 1. SEM of pure alginic acid membrane(A) and blend membranesof CNAL6-1(B), CNAL6-2(C) and CNAL6-9(D).
L. Zhang et al. / Journal of Membrane Science 124 (1997) 195-201
198
However, as shown in Fig. I(C) the membrane CNAL6-2 displays a short stick architecture. It suggests that the alginic acid was dispersed uniformly in cellulose according to the mode of microphase separation, and displayed a certain level of miscibility. This microphase separation is just the state that a good blend requires. The surfaces of A, B and C in Fig. 1 implies that a strong interaction between cellulose and alginic acid changed the alginic acid architecture. When the weight ratio of I / I I reaches 1:1.5, as shown in Fig. I(D) the membrane CNAL6-9 shows a very rough surface resulting from macroscopic phase separation in the blend. The X-ray diffractograms of the membranes are described in Fig. 2. The degree of crystallinity (X c) has the order: CN6 < CNAL6-9 < CNAL6-1 < CNAL6-2 < CNAL6-8 < AL3 and gives values of 40, 41, 45, 49, 52 and about 60%, respectively. It is clear that the X c values of the blend membranes are higher than that of membrane CN6, and increase with increasing II content except for CNAL6-9, suggesting the certain level of miscibility for CNAL6-1, CNAL6-2 and CNAL6-8 as seen from Fig. 1. The IR spectra of the membranes indicated that the blend membranes of CNAL6-2 and CNAL6-4 have the characteristic absorption peaks of cellulose at 897, 1640 and for alginic acid at 819 cm -1. It is interesting that the peak at 1600 cm- 1 ( C O 0 - of the alginate) disappeared, and a new peak at 1730 cm -1, which is absent in both cellulose and alginate, appeared. It implies that there are hydrogen bonds between groups of cellulose and alginic acid resulting in strong interaction of intermolecules, which broke the coniferous architecture of alginic as shown in Fig. I(B and C).
_ ~_
~
~
_A_L3
CNAL6--9
i
i
i
|
5
15
25
35
20/deg Fig. 2. X-ray diffraetograms o f t h e m e m b r a n e s .
Table 1 shows experimental values of Cu, Ca and Na content of the membrane AL3 and CNAL6-4 determined by atomic absorption spectrometry. The Cu 2+ molar mass per g membrane after ion exchange was calculated to be 0.54 mmol. On the other hand, the H + molar mass per g membrane obtained by titration was 0.97 mmol, i.e., almost twice that of Cu 2+, indicating that nearly all the H + in the blend membrane (after equilibrating with HC1) was exchanged to Cu 2+. The data of AL3 show that a small amount of alginate-calcium complex as calcium bridge exists in the alginic acid membrane [7], and
Table 1 Analytic results by AAS and A R values (experimental and calculated) Membrane
AL3 a AL3 b CNAL6-4 a CNAL6-4 b " Before ion exchange. b After ion exchange.
/
45
Element content by AAS (mmol g - 1 )
A R (meq. g - 1)
Cu
Ca
Na
Experimental (by titration)
0 _ 0 0.54
0.39 _ 0.04 0.01
0.07 _ 0.07 0.02
m
Calculated m
4.37
5.68
0.97
1.81
199
L. Zhang et al. / Journal of Membrane Science 124 (1997) 195-201
that the calcium can not be substituted by Cu 2+. Hence, the experimental A R value of membrane AL3 is lower than that calculated. It is worth noting that the value of ion exchange capacity for CNAL6-4 membrane is only almost half of the calculated value. One of the causes is considered to be strong bonds forming between - C O O - groups of alginic acid and - O H groups of cellulose in the blend membrane. Fig. 3 shows the DTA thermograms of the membranes. The endothermic peak below 100°C represents desorption of water in cellulose. The endothermic peak occurring in the range of ca. 200-300°C suggests that some chain scission and loss of water molecules coming from the primary - O H groups of cellulose occurs [12]. The endothermic peaks shift to the lower temperature with increasing content of alginate in the blend membrane, and are 276, 261, 235 and 216°C for the membranes CN6, CNAL6-2, CNAL6-4 and CNAL6-9, respectively. It implies that owing to the adding of alginate, the hydrogen bonds between cellulose molecules are broken by the strong interaction between alginic acid and cellulose molecules. In addition, the blend membranes were highly transparent except for CNAL6-9 (1:1.5) and it implies that the I / I I blend is miscible and in good agreement with the results of SEM.
.•
C9
EXO
600
7~ 450
o
• 300
150
0
0
,
210
40
60
80
100
WAL/% Fig. 4. Effect of the percent content of alginate (WAL) on the tensile strengths (o-b) of the dry (O) and wet (O) membranes.
3.2. Mechanical properties The effect of the percent content of alginate (WAL) on the mechanical properties of the blend membranes are described in Fig. 4. The tensile strength (o-b) of both the dry and wet membranes decreases with increasing alginate content. The decrease of the tensile strength of wet membranes is slower than that of the dry membranes. It shows that the membrane AL3 prepared by using pure alginate has such a low tensile strength that it can hardly be used as water swollen membrane. However, the tensile strength of the blend membrane of alginate with cellulose was obviously improved resulting in extensive applica1.5 O
~
I
I
200
400
A
T J.O
ENDO
.~0.5
0 600
T/'C Fig. 3. DTA curves of the membranes of CN6 (A), CNAL6-2(B), CNAL6-4 (C) and CNAL6-9 (D).
L 10
20
30
w~/~ Fig. 5. Effect of the percent content of alginate (WAL) on ion exchange capacity (A R) of the blend membranes.
200
L. Zhang et al. / Journal of Membrane Science 124 (1997) 195-201 1.3
L12 <1.1 S
1.0
I
1
I
0
©
0
I
I
I
2 3 4 5 Reproduction times
I
I
6
7
Fig. 6. Effect of reproduction times of the membranes of CNAL6-5 (O) and CNAL6-8 ( 0 ) on A R.
acid molecules exist in the blend membrane, and result in breaking the coniferous architecture of alginic acid. The mechanical properties of alginic acid membranes were obviously improved by blending with cellulose, thus the blend membrane can be used as ion exchange membrane in the water swollen state. The blend membranes have sufficient tensile strength, good ion exchange capacity and excellent reproducibility. In addition, the biodegradability of both the cellulose and the alginate is beneficial to environmental protection. This work provides a simple approach to obtaining modified nature polymer materials.
Acknowledgements tion. It is important that the production of the ion exchange membrane blended with cellulose and alginate is simpler than that of membranes made of polyvinylchloride and alginic acid reacted with formaldehyde [2].
This project was supported by the State Economy and Trade Commission of China.
References 3.3. I o n e x c h a n g e c a p a c i t i e s
Fig. 5 shows the dependence of ion exchange capacity on alginate content ( W ~ ) of the blend membranes. The ion exchange capacity increases with increase of alginate content, and A R value of the membrane CNAL6-8, which has sufficient tensile strength, is given to be 1.25 m e q . / g dry membrane. The A R values of the membranes CNAL6-6, CNAL6-7 and CNAL6-8 show that the blend membranes can be used as good ion exchange membranes as same as that reported in literature [13-15]. It is worth pointing out that after the membranes had been used many times, the ion exchange capacity of the reproduced membranes remained unchanged (Fig. 6). It suggests that these ion exchange blend membranes have an excellent reproducibility.
4. Conclusion The ion exchange membranes were satisfactorily prepared by blending cellulose cuoxam (I) with aqueous sodium alginate solution (II) when the w e i g h t ratio o f I / I I w a s l a r g e r t h a n 1 / 1 . 4 . T h e s t r o n g h y d r o g e n b o n d s b e t w e e n c e l l u l o s e a n d alginic
[1] M. Seno and T. Yamabe, Absorption of the ferric ion by weakly acidic resins. Preferential uptake of the ferric ion by alginic acid, Bull. Chem. Soc. Jpn., 33 (1960) 590. [2] M. Seno, T. Saito and T. Yamabe, Transport behavior of ferric, aluminum and cupric ions across the alginate and the other membranes, Bull. Chem. Soc. Jpn., 33 (1960) 563. [3] T. Uragami and M. Saito, Studies on syntheses and permeabilities of special polymer membranes. 68. Analysis of permeation and separation characteristics and new technique for separation of aqueous alcoholic solutions through alginic acid membranes, Separation Sci. Tech., 24 (718) (1989) 541. [4] G. Freddi, M. Romano, M. Rosaria and M. Tsukada, Silk fibroin/cellulose blend films; preparation, structure and physical properties, J. Appl. Polym. Sci., 56 (1995) 1537. [5] L. Zhang, G. Yang and W. Fang, Regenerated cellulose membrane from cuoxam/zincoxene, J. Membrane Sci., 56 (1991) 207. [6] L. Zhang, G. Yang and L. Xiao, Blend membranes of cellulose cuoxam/casein, J. Membrane Sci., 103 (1995) 65. [7] M. Nakamura, Water Soluble Polymer, Bessatsu Kagaku, Kogyo, 1991. [8] G. Yang and L. Zhang, Regenerated cellulose microporous membranes by mixing cellulose cuoxam with a water soluble polymer, J. Membrane Sci., 114 (1996) 149. [9] L. Zhang, G. Yang, S. Yan and H. Liu, Regenerated cellulose film with water resistance, Chinese Pat. CN 1091 144A (C1.C08L97/02), 1994. [10] J.F. Rabek, Experimental Methods in Polymer Chemistry: X-ray Diffraction Analysis, Wiley-Interscience, Chichester, 1980, p. 488.
L. Zhang et al. / Journal of Membrane Science 124 (1997) 195-201 [11] H.P. Gregor, H. Jacobson, R.C. Shair and D.M. Wetstone, Interpolymer ion-selective membranes. I. Preparation and characterization of polysterenesulfonic acid-dynel membranes, J. Phys. Chem., 61 (1957) 141. [12] S.R. Shukla and A.R. Athalye, Mechanical and thermal properties of glycidyl methacrylate grafted cotton cellulose, J. Appl. Polym. Sci., 57 (1995) 983. [13] K. Osamu, Method of making microporous ion-exchange membranes from cellulose, Jpn. Kokai Tokyo Koho JP 63 23 934.
201
[14] H. Matsuyama, M. Teramoto and K. Iwai, Development of a new functional cation-exchange membrane and its application to facilitated transport of CO 2, J. Membrane Sci., 93 (1994) 237. [15] Y. Mizutani, R. Yamane, H. Ihara and H. Motomura, Studies of ion exchange membranes. XVI. The preparation of ion exchange membranes by the "paste method", Bull. Chem. Soc. Jpn., 36 (1963) 361.
Journal of Membrane Science 124 (1997) 195-201
Ion exchange membranes blended by cellulose cuoxam with alginate Lina Zhang *, Daochun Zhou, Hao Wang, Shuyao Cheng Department of Chemistry, Wuhan University, Wuhan 430072, China
Received 1 April 1996; revised 17 July 1996; accepted 19 July 1996
Abstract Ion exchange membranes were prepared by coagulating a blend of 6.4 wt% cellulose cuoxam (I) and 3 wt% aqueous alginate solution (II). The tensile strength of both the dry and wet blend membranes was obviously higher than that of membranes made of pure alginate. The value of ion exchange capacity was 1.25 meq/g dry membrane for blend membrane in which I / I I was 1:1.4 by weight, and the ion exchange membrane has excellent reproducibility. IR, SEM, X-ray diffraction, AAS and DTA results showed that the blend exhibited a certain level of miscibility, and the strong interaction between the cellulose and alginic acid was caused by occurring hydrogen bonding of their intermolecules. The mechanical properties of alginic acid membranes in water were significantly improved by blending with cellulose cuoxam, thus the blend membranes can be used as ion exchange membranes in water swollen state. Keywords: Ion exchange membrane; Cellulose; Alginate; Blend; Scanning electron micrography
1. Introduction Nowadays, ion exchange membranes have been used to remove a large amount of toxic heavy metals from effluents, and also have many other practical uses. Equilibration experiments revealed that alginate resin adsorbs metal ions more extensively than Ambeflite IRC-50 [1]. Membranes prepared from polyvinylchloride and alginic acid crosslinked with formaldehyde were used to study the transport behavior of ferric, aluminum and cupric ions and the selective permeability of ferric ion [2]. Moreover, an alginic acid membrane was developed for the vapor permeation membrane separation technique in order
* Corresponding author.
to separate ethanol-water mixtures [3]. However, the mechanical strength of the alginic acid membrane is very weak, so that its application is limited. Freddi et al. reported [4] that the mechanical properties of silk fibroin films were improved by blending with cellulose. In previous work [5,6], the miscibilities of regenerated cellulose membranes blended with cellulose c u o x a m / z i n c o x e n e as well as cellulose cuoxam/casein were studied. The experimental resuits showed that the mechanical properties were significantly improved. It is worth noting that utilizing natural cellulose and alginate as raw material can protect the environment because of their biodegradability. This has prompted us to study ion exchange membrane blended with cellulose cuoxam and alginate. Alginic acid belongs to a heteropolysaccharide that consists of [3-(1--* 4)-o-mannuronic acid and
0376-7388/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S0376-7 3 8 8(96)00227-X
L. Zhang et al./ Journal of Membrane Science 124 (1997) 195-201
196 COOH
H
H
H
0 OH
HO
H
H [M]
C00H H
HH//~0~ 0~
H
H
O HO H H
[G]
F_O
H [M]
CNAL6-3, CNAL6-4, CNAL6-5, CNAL6-6, CNAL6-7, CNAL6-8 and CNAL6-9. The membranes CN6 and AL3 were prepared from pure cellulose cuoxam (cuprophane) and aqueous sodium alginate solution, respectively.
Scheme 1.
2.2. Characterization of membranes et-(1 ~ 4)-L-glucuronic acid as depicted in Scheme 1 [7]. However, our recent work [8] showed that some microporous membranes were formed by mixing cellulose cuoxam with gelatin, which was dissolved in coagulation bath and removed from the membrane. In this work, first the conditions of forming blend membranes and bonding between cellulose and alginic acid groups were investigated, then the mechanical property and ion exchange capacity were measured.
2. Experimental
2.1. Preparation of membranes The linters used were supplied by Chemical Fiber Manufacture of Hubei. The viscosity average molecular weight Mn, which was determined by viscosity method, is equal to 1.96 x 105. The 6.4 wt% cellulose solution in cuoxam (I) was prepared according to our previous work [9]. Sodium alginate was purchased from Branched Factory of Shanghai Chemical Reagent Company. The preparation method of the blend membrane was as follows: a mixture of I and I I (3 wt% aqueous sodium alginate solution) was stirred, then filtered and degassed. The mixture of the solutions was spread over a glass plate to a depth of 0.3 mm and coagulated in a coagulation bath of 5 wt% CaC12 aqueous solution. The membranes were regenerated in dilute H2SO 4 to get blend membranes of cellulose/alginic acid, then washed at least 2 h with running water and plasticized with 5 wt% glycerine. The blend membranes were dried on the glass plate in air at 50°C. By changing the weight ratio of I to I ! such as 1:0.2, 1:0.5, 1:0.7, 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4 and 1:1.5, we prepared a series of the regenerated cellulose blend membranes coded as CNAL6-1, CNAL6-2,
Scanning electron micrographs (SEM) were taken with a Hitachi X-650 SEM. The membranes were coated with gold, subsequently their surfaces were observed and photographed. X-ray diffractions were measured with an X-ray diffractometer Rigaku Dmax-rA. The X-ray diffraction patterns with Cu K ot at 40 kV and 25 mA were recorded in the region of 20 = 5-45 °. The degree of crystallinity (X c) was calculated according to the usual method [10]. IR spectra of the membranes were recorded with a Nicolet F r - I R spectrometer. The content of copper, calcium and sodium in the membranes was determined by atomic absorption spectrometry (AAS, Hitachi 180/80). For the determination of these elements, the membranes were treated as follows: 0.1 g of the membrane plus 5 ml concentrated HNO 3 and 0.5 ml H202 were heated to about 100°C to get a carbonization product, then kept at 500°C for 2 h. After cooling, 6 ml concentrated HNO 3 and 0.5 ml H202 were added, subsequently the mixture was heated again to get a clear solution for the AAS determination. Differential thermal analysis (DTA) was carried out using a Shimadzu DT-40 thermal analyzer. The membrane was cut to pieces with 2 mm length (5 mg), then analyzed under nitrogen atmosphere from 30°C to 450°C at a heating rate of 20°C/min. The tensile strength (o-b) of dry and wet membranes was measured by an electronic strength tester (XLD-0.1) according to the Chinese standard method (GB 4456-84) at a tensile rate of 100 m m / m i n . The wet membrane was measured immediately after immersing in water for 10 min.
2.3. Ion exchange capacity The active exchange capacity is defined experimentally as the number of exchange sites within the membrane structure which are titrable while leaving
L. Zhang et al. / Journal of Membrane Science 124 (1997) 195-201
the membrane intact. The membrane was equilibrated with 1 M HC1, rinsed free of diffusible acid, blotted quickly and eluted with an excess of CuSO 4 of about 0.01 M. The eluent was titrated in conventional fashion with standard base [11]. The ion exchange capacity (A R) of the dry membrane was determined by the following equation NV AR
~
- -
W
where N (mol/l) is the concentration of the standard base; V (ml), the volume of the consumed base; W (g), the weight of the dry membrane.
197
3. Results and discussion 3.1. Structure analysis
Fig. 1 shows SEM of the membrane prepared from pure sodium alginate (AL3) and blend membranes of CNAL6-1, CNAL6-2 and CNAL6-9. The AL3 membrane displays a coniferous architecture (A). The blend membrane CNAL6-1 has a smooth surface (B) similar to the membrane from pure cellulose cuoxam [6], indicating a good miscibility between cellulose and a small amount of alginate.
Fig. 1. SEM of pure alginic acid membrane(A) and blend membranesof CNAL6-1(B), CNAL6-2(C) and CNAL6-9(D).
L. Zhang et al. / Journal of Membrane Science 124 (1997) 195-201
198
However, as shown in Fig. I(C) the membrane CNAL6-2 displays a short stick architecture. It suggests that the alginic acid was dispersed uniformly in cellulose according to the mode of microphase separation, and displayed a certain level of miscibility. This microphase separation is just the state that a good blend requires. The surfaces of A, B and C in Fig. 1 implies that a strong interaction between cellulose and alginic acid changed the alginic acid architecture. When the weight ratio of I / I I reaches 1:1.5, as shown in Fig. I(D) the membrane CNAL6-9 shows a very rough surface resulting from macroscopic phase separation in the blend. The X-ray diffractograms of the membranes are described in Fig. 2. The degree of crystallinity (X c) has the order: CN6 < CNAL6-9 < CNAL6-1 < CNAL6-2 < CNAL6-8 < AL3 and gives values of 40, 41, 45, 49, 52 and about 60%, respectively. It is clear that the X c values of the blend membranes are higher than that of membrane CN6, and increase with increasing II content except for CNAL6-9, suggesting the certain level of miscibility for CNAL6-1, CNAL6-2 and CNAL6-8 as seen from Fig. 1. The IR spectra of the membranes indicated that the blend membranes of CNAL6-2 and CNAL6-4 have the characteristic absorption peaks of cellulose at 897, 1640 and for alginic acid at 819 cm -1. It is interesting that the peak at 1600 cm- 1 ( C O 0 - of the alginate) disappeared, and a new peak at 1730 cm -1, which is absent in both cellulose and alginate, appeared. It implies that there are hydrogen bonds between groups of cellulose and alginic acid resulting in strong interaction of intermolecules, which broke the coniferous architecture of alginic as shown in Fig. I(B and C).
_ ~_
~
~
_A_L3
CNAL6--9
i
i
i
|
5
15
25
35
20/deg Fig. 2. X-ray diffraetograms o f t h e m e m b r a n e s .
Table 1 shows experimental values of Cu, Ca and Na content of the membrane AL3 and CNAL6-4 determined by atomic absorption spectrometry. The Cu 2+ molar mass per g membrane after ion exchange was calculated to be 0.54 mmol. On the other hand, the H + molar mass per g membrane obtained by titration was 0.97 mmol, i.e., almost twice that of Cu 2+, indicating that nearly all the H + in the blend membrane (after equilibrating with HC1) was exchanged to Cu 2+. The data of AL3 show that a small amount of alginate-calcium complex as calcium bridge exists in the alginic acid membrane [7], and
Table 1 Analytic results by AAS and A R values (experimental and calculated) Membrane
AL3 a AL3 b CNAL6-4 a CNAL6-4 b " Before ion exchange. b After ion exchange.
/
45
Element content by AAS (mmol g - 1 )
A R (meq. g - 1)
Cu
Ca
Na
Experimental (by titration)
0 _ 0 0.54
0.39 _ 0.04 0.01
0.07 _ 0.07 0.02
m
Calculated m
4.37
5.68
0.97
1.81
199
L. Zhang et al. / Journal of Membrane Science 124 (1997) 195-201
that the calcium can not be substituted by Cu 2+. Hence, the experimental A R value of membrane AL3 is lower than that calculated. It is worth noting that the value of ion exchange capacity for CNAL6-4 membrane is only almost half of the calculated value. One of the causes is considered to be strong bonds forming between - C O O - groups of alginic acid and - O H groups of cellulose in the blend membrane. Fig. 3 shows the DTA thermograms of the membranes. The endothermic peak below 100°C represents desorption of water in cellulose. The endothermic peak occurring in the range of ca. 200-300°C suggests that some chain scission and loss of water molecules coming from the primary - O H groups of cellulose occurs [12]. The endothermic peaks shift to the lower temperature with increasing content of alginate in the blend membrane, and are 276, 261, 235 and 216°C for the membranes CN6, CNAL6-2, CNAL6-4 and CNAL6-9, respectively. It implies that owing to the adding of alginate, the hydrogen bonds between cellulose molecules are broken by the strong interaction between alginic acid and cellulose molecules. In addition, the blend membranes were highly transparent except for CNAL6-9 (1:1.5) and it implies that the I / I I blend is miscible and in good agreement with the results of SEM.
.•
C9
EXO
600
7~ 450
o
• 300
150
0
0
,
210
40
60
80
100
WAL/% Fig. 4. Effect of the percent content of alginate (WAL) on the tensile strengths (o-b) of the dry (O) and wet (O) membranes.
3.2. Mechanical properties The effect of the percent content of alginate (WAL) on the mechanical properties of the blend membranes are described in Fig. 4. The tensile strength (o-b) of both the dry and wet membranes decreases with increasing alginate content. The decrease of the tensile strength of wet membranes is slower than that of the dry membranes. It shows that the membrane AL3 prepared by using pure alginate has such a low tensile strength that it can hardly be used as water swollen membrane. However, the tensile strength of the blend membrane of alginate with cellulose was obviously improved resulting in extensive applica1.5 O
~
I
I
200
400
A
T J.O
ENDO
.~0.5
0 600
T/'C Fig. 3. DTA curves of the membranes of CN6 (A), CNAL6-2(B), CNAL6-4 (C) and CNAL6-9 (D).
L 10
20
30
w~/~ Fig. 5. Effect of the percent content of alginate (WAL) on ion exchange capacity (A R) of the blend membranes.
200
L. Zhang et al. / Journal of Membrane Science 124 (1997) 195-201 1.3
L12 <1.1 S
1.0
I
1
I
0
©
0
I
I
I
2 3 4 5 Reproduction times
I
I
6
7
Fig. 6. Effect of reproduction times of the membranes of CNAL6-5 (O) and CNAL6-8 ( 0 ) on A R.
acid molecules exist in the blend membrane, and result in breaking the coniferous architecture of alginic acid. The mechanical properties of alginic acid membranes were obviously improved by blending with cellulose, thus the blend membrane can be used as ion exchange membrane in the water swollen state. The blend membranes have sufficient tensile strength, good ion exchange capacity and excellent reproducibility. In addition, the biodegradability of both the cellulose and the alginate is beneficial to environmental protection. This work provides a simple approach to obtaining modified nature polymer materials.
Acknowledgements tion. It is important that the production of the ion exchange membrane blended with cellulose and alginate is simpler than that of membranes made of polyvinylchloride and alginic acid reacted with formaldehyde [2].
This project was supported by the State Economy and Trade Commission of China.
References 3.3. I o n e x c h a n g e c a p a c i t i e s
Fig. 5 shows the dependence of ion exchange capacity on alginate content ( W ~ ) of the blend membranes. The ion exchange capacity increases with increase of alginate content, and A R value of the membrane CNAL6-8, which has sufficient tensile strength, is given to be 1.25 m e q . / g dry membrane. The A R values of the membranes CNAL6-6, CNAL6-7 and CNAL6-8 show that the blend membranes can be used as good ion exchange membranes as same as that reported in literature [13-15]. It is worth pointing out that after the membranes had been used many times, the ion exchange capacity of the reproduced membranes remained unchanged (Fig. 6). It suggests that these ion exchange blend membranes have an excellent reproducibility.
4. Conclusion The ion exchange membranes were satisfactorily prepared by blending cellulose cuoxam (I) with aqueous sodium alginate solution (II) when the w e i g h t ratio o f I / I I w a s l a r g e r t h a n 1 / 1 . 4 . T h e s t r o n g h y d r o g e n b o n d s b e t w e e n c e l l u l o s e a n d alginic
[1] M. Seno and T. Yamabe, Absorption of the ferric ion by weakly acidic resins. Preferential uptake of the ferric ion by alginic acid, Bull. Chem. Soc. Jpn., 33 (1960) 590. [2] M. Seno, T. Saito and T. Yamabe, Transport behavior of ferric, aluminum and cupric ions across the alginate and the other membranes, Bull. Chem. Soc. Jpn., 33 (1960) 563. [3] T. Uragami and M. Saito, Studies on syntheses and permeabilities of special polymer membranes. 68. Analysis of permeation and separation characteristics and new technique for separation of aqueous alcoholic solutions through alginic acid membranes, Separation Sci. Tech., 24 (718) (1989) 541. [4] G. Freddi, M. Romano, M. Rosaria and M. Tsukada, Silk fibroin/cellulose blend films; preparation, structure and physical properties, J. Appl. Polym. Sci., 56 (1995) 1537. [5] L. Zhang, G. Yang and W. Fang, Regenerated cellulose membrane from cuoxam/zincoxene, J. Membrane Sci., 56 (1991) 207. [6] L. Zhang, G. Yang and L. Xiao, Blend membranes of cellulose cuoxam/casein, J. Membrane Sci., 103 (1995) 65. [7] M. Nakamura, Water Soluble Polymer, Bessatsu Kagaku, Kogyo, 1991. [8] G. Yang and L. Zhang, Regenerated cellulose microporous membranes by mixing cellulose cuoxam with a water soluble polymer, J. Membrane Sci., 114 (1996) 149. [9] L. Zhang, G. Yang, S. Yan and H. Liu, Regenerated cellulose film with water resistance, Chinese Pat. CN 1091 144A (C1.C08L97/02), 1994. [10] J.F. Rabek, Experimental Methods in Polymer Chemistry: X-ray Diffraction Analysis, Wiley-Interscience, Chichester, 1980, p. 488.
L. Zhang et al. / Journal of Membrane Science 124 (1997) 195-201 [11] H.P. Gregor, H. Jacobson, R.C. Shair and D.M. Wetstone, Interpolymer ion-selective membranes. I. Preparation and characterization of polysterenesulfonic acid-dynel membranes, J. Phys. Chem., 61 (1957) 141. [12] S.R. Shukla and A.R. Athalye, Mechanical and thermal properties of glycidyl methacrylate grafted cotton cellulose, J. Appl. Polym. Sci., 57 (1995) 983. [13] K. Osamu, Method of making microporous ion-exchange membranes from cellulose, Jpn. Kokai Tokyo Koho JP 63 23 934.
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[14] H. Matsuyama, M. Teramoto and K. Iwai, Development of a new functional cation-exchange membrane and its application to facilitated transport of CO 2, J. Membrane Sci., 93 (1994) 237. [15] Y. Mizutani, R. Yamane, H. Ihara and H. Motomura, Studies of ion exchange membranes. XVI. The preparation of ion exchange membranes by the "paste method", Bull. Chem. Soc. Jpn., 36 (1963) 361.