A Conductive Hydrogel by Poly(Sodium Acrylate)/Montmorillonite Superabsorbent Composite Yiming Xie, Jihuai Wu*, Jianming Lin, Yuelin Wei and Jinfeng Zhong Institute of Materials Physical Chemistry, Huaqiao University, Quanzhou, Fujian 362021, China Received: 8 March 2006 Accepted: 12 June 2006 SUMMARY Poly(sodium acrylate)/montmorillonite superabsorbent composite was synthesised by an inverse suspension polymerisation method. A novel hydrogel electrolyte with conductivity of 100 ms cm -1 was prepared by immersing the superabsorbent composite in 1 M KCl aqueous solution. The conductivity of the hydrogel electrolyte was investigated, and it was found that the hydrogel electrolyte exhibits liquid-like ionic conductivity. The conductivity depends mainly on the ionic intensity of the solution in which it is immersed and on the water absorbency of the superabsorbent. 1. INTRODUCTION Hydrogel electrolytes have recently acquired special interest, both from the fundamental science and the application perspectives. This is because of their importance as electrolytes possessing liquid-like ionic conductivity while preserving the dimensional stability of a solid state. They are of potential use in fuel cells, electric double layer capacitors, dye sensitive solar cells and rechargeable lithium batteries 1-4. On the other hand, they are also natural and artificial biologically active systems, and being biocompatible, hydrogels have found numerous applications in pharmaceutics and in medicine production 5. Hydrogels are mixtures consisting of a substantial amount of water and highly swollen, cross-linked hydrophilic polymer networks. The polymer network occupies only a small fraction of the volume and it simply provides a structural framework that prevents the fluid from flowing away. Compared with ordinary hydrogels, superabsorbents are polymers with molecular chains *Fax: 0086-595-22693999; E-mail: jhwu@hqu.edu.cn Rapra Technology, 2007 that are lightly crosslinked but have many hydrophilic groups on them. This means that hundreds or thousands of times more water is absorbed and the absorbed water flows away with more difficulty 6-8, so hydrogel electrolytes made from superabsorbents possess better electric properties. In order to enhance hydrogels strength and reduce their cost, mineral particles, such as montmorillonite ultrafine powders can be added 7-8. Here, a novel hydrogel electrolyte based on poly(sodium acrylate)/ montmorillonite superabsorbent composite was prepared, and the conductivity investigated. 2. EXPERIMENTAL 2.1 Preparation of Superabsorbent Composite Poly(sodium acrylate)/montmorillonite superabsorbent composite was synthesised by an inverse suspension polymerisation method. Acrylic acid monomer (15.78 g) was purified by active carbon absorbing and decompression pumping. Sodium acrylate monomer was prepared by partially neutralising acrylic acid monomer with a sodium hydroxide solution in an ice-bath. Montmorillonite ultrafine powder (1.50 g) was dissolved in 20 ml distilled water of with ultrasonic vibration for 5 min. The polymerisation was carried out in a three-necked flanged flask fitted with a stirrer, an efficient reflux condenser and nitrogen purging. Hexahydrobenzene (80 ml) together with dispersing agent (0.78 g) and auxiliary dispersing agent (0.16 g) was poured into the reactor, stirred and heated to 40 ºC. Then sodium acrylate monomer solution, montmorillonite solution, crosslinker (N, N - methylene-bisacrylamide) (0.002 g) and initiator (potassium persulphate) (0.1 g) were added to the reaction system in turn. Once all the compounds were added, the mixture was heated to 70 ºC and continuously stirred for another 90 min to complete the polymerisation reaction. 2.2 Measurement of Conductivity and Water Absorbency A powdered superabsorbent composite (0.5 g) was immersed in an electrolyte 29
Yiming Xie, Jihuai Wu, Jianming Lin, Yuelin Wei and Jinfeng Zhong solution (1000 ml) for 90 min at ambient temperature to reach swelling equilibrium, which resulted in the absorption of water by the superabsorbent and the formation of a hydrogel electrolyte. The unabsorbed water was removed by filtering with an 80-mesh stainless steel screen and the filtered swollen sample was hung up to drain for 25 min. The conductivity of the hydrogel was measured by placing a 30g sample in a cylinder and inserting a pocket conductivity meter (HANNA8733) into the hydrogel. The water absorbency (Q H2O ) of the superabsorbent composite was determined by weighing the swollen sample after centrifuging at 4000 r/min for 20 min. The Q H2O of the sample was calculated according to the following equation: QHO 2 = Wt( swollen sample) Wt( dried sample) Wt(dried sample) Obviously, with an increase in the molecular concentration, the ionic intensity of the solution increases, leading to an increase in the conductivity of the hydrogel. On the other hand, at a fixed molecular concentration, the ionic concentrations for NaCl, Na 2 and Na 3 are all different. The ionic intensity is I Na3PO4 > I Na2SO4 > I NaCl both in aqueous solution and in hydrogels, and the conductivity of the hydrogels is in the corresponding order: κ Na3PO4 > κ Na2SO4. Figure 2 shows the conductivities of hydrogels obtained by immersing a superabsorbent in LiCl, NaCl and KCl solutions. The conductivities were in the order: κ KCl > κ LiCl at the same molecular concentration and ionic intensity in the solution, which is in accordance with the three salts in aqueous solution 9. The reason is that although the radius of the three ions is R Li+ < R Na+ < R K+, the radii of the hydrated ions are in the order R Li+ > R Na+ > R K+. The larger hydrated ions mean a slower movement rate and a lower hydrogel conductivity. Hydrogels were made using the same cation but different anions from those in the electrolyte, i.e. KCl, KBr and KI. The relationship between the conductivity of the hydrogel and the concentration of the solution was measured and is shown in Figure 3. At the same molecular and ionic Figure 1. Influence of concentration of NaCl, Na 2 and Na 3 on conductivity of hydrogel where Q H2O is the water absorbency of the superabsorbent composite, Wt (swollen sample) is the weight of the swollen sample, and Wt (dried sample) is the weight of the dried sample. 3. RESULTS AND DISCUSSION 3.1 Influence of Electrolyte Solution on the Conductivity of the Hydrogel Figure 1 shows the conductivities of the hydrogels made by immersing a superabsorbent in sodium chloride (NaCl), sodium sulfate (Na 2 ) and sodium phosphate (Na 3 ) solutions. The conductivity increased with the concentration of the electrolyte solution with all three kinds of salt. At the same molecular concentrations, the conductivity of the hydrogels was in the order: κ Na3PO4 > κ Na2SO4. As is well known, the conductivity of a solution depends on its ionic intensity defined by I = 1/2 m i z 2, where m is i i the concentration of the electrolyte s ion, and Z i is its electric charge. Figure 2. Influence of concentration of LiCl, NaCl and KCl on the conductivity of the hydrogel 30
Figure 3. Influence of concentration of KCl, KBr and KI on the conductivity of the hydrogel Figure 4. Influence of ph of solution on the conductivity of hydrogel Figure 5. Influence of temperature on the conductivity of hydrogel concentration, the conductivity of the hydrogel immersed in KBr solution was slightly higher than that of the ones immersed in KCl and KI solutions, and the conductivities of the hydrogels immersed in KCl and KI solutions were approximately the same. Compared with Figure 2, the influence of the anion radius on the conductivity was less than that of the cation. This finding is similar to the one concerning the conductivity of the three salts in water solution 9. 3.2 Influence of ph and Temperature on the Conductivity The conductivity is affected by the ph of the aqueous solution adsorbed. Hydrogels with different ph values (adjusted by adding NaOH or HCl to distilled water) were made, and the conductivity was measured and shown in Figure 4. The conductivity decreased when the ph of the solution was increased from 1 to 4, but it increased in the ph range from 8 to 14, when the concentration of Na + and OH - ions increased, and it reached a minimum value at a solution ph of 4~8. Figure 5 shows the influence of temperature on the conductivity. The conductivity increased with the temperature. The reason is that with the elevation of temperature, the viscosities of the hydrogels decrease and the movement of ions in the hydrogel accelerates. 3.3 Relationship Between Conductivity of Hydrogel and Water Absorbency of Superabsorbent In order to know the relation between the conductivity of a hydrogel and the water absorbency of the superabsorbent, the latter (0.5 g) was immersed in 1000 ml distilled water, and the hydrogel was taken out at various times. The water absorbency of the superabsorbent and the conductivity of the hydrogel were measured. Meanwhile the conductivity 31
Yiming Xie, Jihuai Wu, Jianming Lin, Yuelin Wei and Jinfeng Zhong Figure 6. The relation between water absorbency of superabsorbent and conductivity of hydrogel of the filtrate was also measured, and the results are shown in Figure 6. With an increase in the water absorbency of the superabsorbent, the conductivity of the hydrogels decreases, and the conductivity of the filtrate increases. Eventually the conductivities of the hydrogel and the filtrate converge. Since there are very few ions in the distilled water, the conductivity of both the hydrogel and the filtrate comes mainly from the Na + and OH - ions in the superabsorbent. When a superabsorbent is immersed in distilled water, the water molecules gradually penetrate the polymer network, the water absorbency of the superabsorbent improves gradually, and the ion concentration in the hydrogel network decreases, which leads to a decrease in the conductivity of the hydrogels. Meanwhile, the Na + and OH - ions in the superabsorbent diffuse from the interior to the exterior of the polymer network, causing an increase in the ionic concentration and conductivity of the filtrate. After a period of time (80 mins in our experiment) for penetrating and diffusing, the ion concentrations inside the network (hydrogel) and outside it (filtrate) become equal, and the conductivities of the hydrogel and filtrate become very similar. 3.4 Influence of Superabsorbent Preparation Conditions on the Conductivity of the Hydrogel In order to understand the influence of the preparation condition used to make the superabsorbent on the conductivity of the hydrogel, a series of superabsorbents were synthesised under different conditions, varying such parameters as the amount of montmorillonite, crosslinker and NaOH. The water absorbency of the superabsorbent and the water content of hydrogel were controlled by putting a 0.5 g sample in distilled water for 90 min, which allowed the superabsorbent to swell adequately, whereupon the water absorbency of the superabsorbent and the conductivity of the hydrogels were measured. The conductivity of the hydrogel is shown in Table 1 for various amounts of NaOH and various water absorbency values of the superabsorbent. The same property is shown in Table 2 for various amounts of montmorillonite and various values of the water absorbency of the superabsorbent. Table 3 shows the same property for various amounts of crosslinker and various values of the water absorbency of the superabsorbent. According to the three Tables the conductivity of the hydrogel depends mainly on the water absorbency of the superabsorbent, instead of on the amount of montmorillonite, crosslinker or NaOH. It is well known that the conductivity is affected by the ion intensity of the hydrogels, more than the composition of the superabsorbent. The water absorbency Table 1. The conductivity (ms/cm) of gel versus. water absorbency of superabsorbent and amount of NaOH NaOH (%) Water 58 59 60 61 62 absorbency (g/g) 100 1.45 1.44 1.45 1.47 1.49 200 0.66 0.67 0.66 0.68 0.69 300 0.54 0.54 0.55 0.57 0.57 400 0.43 0.44 0.45 0.44 0.46 500 0.38 0.38 0.39 0.41 0.40 Table 2. The conductivity (ms/cm) of gel versuss the water absorbency and amount of montmorillonite amount Montmorillonite (%) Water absorbency (g/g) None 3.3 10.0 16.7 23.3 30.0 100 1.43 1.44 1.46 1.47 1.48 1.50 200 0.62 0.65 0.66 0.66 0.67 0.68 300 0.50 0.53 0.55 0.54 0.56 0.58 400 0.42 0.44 0.46 0.45 0.47 0.48 500 0.35 0.38 0.40 0.39 0.38 0.42 32
Table 3. Conductivity (ms/cm) of gel versuss. the water absorbency of the superabsorbent and amount of crosslinker Crosslinker (%) 0.002 0.003 0.007 0.020 0.027 Water absorbency (g/g) 100 1.45 1.40 1.45 1.47 1.49 200 0.66 0.67 0.66 0.65 0.68 300 0.53 0.52 0.54 0.53 0.53 400 0.43 0.44 0.42 0.44 0.42 500 0.35 0.38 0.36 0.38 0.37 of the superabsorbent has a major effect on the ion intensity of the hydrogels. Consequently, the water absorbency of the superabsorbent affects the conductivity of the hydrogel to a similar extent. 4. CONCLUSIONS 1. A novel conductive hydrogel was prepared by immersing poly(sodium acrylate)/ montmorillonite superabsorbent in a 1 M KCl aqueous solution. Poly(sodium acrylate)/ montmorillonite superabsorbent composite was synthesised by an inverse suspension polymerisation method. 2. The hydrogel exhibits liquidlike ionic conductivity, and the conductivity of the hydrogel electrolyte depends mainly on the ionic intensity, the ph and the temperature of the solution in which it is immersed. 3. The water absorbency of the superabsorbent has a great impact on the conductivity of the hydrogel. But the conditions of preparation of the superabsorbent also have a small influence on the conductivity of the hydrogel electrolyte. REFERENCES 1. P. Pissis and A. Kyritsis, Solid State Ionics, 97 (1997), 105. 2. X-G. Sun, G. Liu, J-B. Xie, Y-B. Han and J.B. Kerr, Solid State Ionics, 175 (2004), 713. 3. H. Wada, S. Nohara, N. Furukawa and C. Iwakura, Electrochimica Acta, 49 (2004), 4871. 4. A. Lewandowski, M. Zajder, E. Frackowiak and F. Beguin, Electrochim. Acta, 46 (2001), 2777. 5. A.A. Konsta, D. Daoukaki, P. Pissis and K. Vartzeli, Solid State Ionics, 125 (1999), 235. 6. J-H. Wu, Y-L. Wei, J-M. Lin and S- B. Lin, Polymer, 44 (2003), 6513. 7. J-H. Wu, J-M. Lin and M. Zhou, Macromol. Rapid Commun., 21 (2000), 1032. 8. J-M. Lin, J-H. Wu, Z-F. Yang and M- L. Pu, Macromol. Rapid Commun., 22 (2001), 422. 9. W.J. Moore, Physical Chemistry, 5th ed, London, Longman Group Limited, 1976. ACKNOWLEDGEMENTS The authors thank jointly the support by the National Natural Science Foundation of China (No. 59772034 and No. 50572030) and the Key Scientific Technology Program of Fujian, China (No. 2002H002 and No. 2004HZ01-3). 33