In the Classroom
Synthesis of Colored Superabsorbent Polymer and Its Use To Demonstrate Convection Currents in Water by Heating Morio Takaki and Toshiyuki Itoh* Department of Chemistry, Faculty of Education, Okayama University, Okayama 700, Japan
Background Polymers with interesting properties offer new aspects of chemistry and are therefore important as teaching materials Figure 1. Experiment showing convection current of water by (1). Superabsorbent polymer is known to be a material capable heating. of absorbing thousands of times its weight in water, and hence has been used for such things as disposable diapers (2–4). Because its ability to absorb water is tremendous and is controlled by a salt or pH, it is valuable for several levels of chemistry instruction in schools (4–8). The material is a lightly cross-linked polyacrylic acid that has been partially neutralized and therefore possesses a buffer property. Polyacrylic acid gel that contains a plant pigment can be used as a pH indicator because it stabilizes the pigment by a and cross-linked in only a few places (2–4). Optimization buffer action and prevents the color from fading (6, 7). of the cross-linking level was, therefore, very important in The behavior of the polymer is very sensitive to the the present study because we wanted the colored polymer gel presence of salts (3, 4, 8). The absorbency is roughly ten times to have a similar density to water and to exhibit a convection less in 0.02% NaCl solution, and a block of the gel melts current of water by heating. We used very small amounts (ca. down completely like a slug when a small amount of salt is 0.09 mol% relative to acrylic acid) of the inexpensive watersprinkled on it (8). The water-absorbing action of the material soluble cross-linking agent N,N′-methylenebisacrylamide is believed to be due to high ionic strength inside the polymer, (MBAA) (2–4). (Scheme I). so that water will tend to diffuse into the polymer if the chains Initially, we used a commercial polyacrylic pigment (3a) are not too highly cross-linked (2–4 ). This phenomenon (Mr. Color [oily] from Gunze Co., Ltd.) to color the depends on the molar weight of the salt when the same salt polyacrylic acid. This resulted in a very brilliantly colored is used; this can be used in teaching to visually show the polymer effective for use as teaching material for the experiment differences in molar weight of a salt solution (8). to show the convection current of water created by heating Colored superabsorbent polymer gel can also demon(Fig. 1). The pigments we used are inexpensive and easily strate a convection current of water when heated because with available, but for proprietary reasons we were refused inforits high absorbent property the gel’s density is almost equal mation on their origin. We then tested three other common to that of water (5) (Fig. 1). types of pigments as colored reagents: Chicago Sky Blue 6B A cross-linked colored polymer gel must be used, rather (3b), basic fuchsin (pararosanilline chloride) (3c), and safrathan the usual colorless gel, to easily understand the experiment nin O (3,7-diamino-2,8-dimethyl-5-phenylphenazinium illustrated in Figure 1. Recently two papers in this Journal chloride) (3d). The color of the polymers from pigments 3b described the preparation of superabsorbent polymer and 3c gradually faded and the water layer became colored (polyacrylic acid) (2, 4). However, to the best of our knowlinstead, although we could use the polymers for the experiment edge, there is no published report on the preparation of a because they retained their color at least during the period colored polyacrylic acid. We describe here the synthesis of the experiment was performed (Fig. 1). Fortunately, the colored polyacrylic acids for use as a new teaching material. polymer derived from pigment 3d maintained its red color The polymerization of acrylic acid proceeds O O by a radical mechanism and requires a radical iniCO2Na CO2Na N N tiator (2, 3). The synthesis of polyacrylic acid H H CO2Na aq. NaOH CO2Na 2 CO2Na using VA-044 as a water-soluble initiator was CO2H O nO n R.T. Mr. Color (oily) (3a) recently reported (4 ). We used the couple of amN N 1 H H NMe 2 (NH4)2S2O8 - Me2N monium peroxodisulfate with N,N,N ′, N ′CO2Na CO2Na Toluene, R.T. 15 min. tetraethylenediamine (TMEDA) as a convenient 4 water-soluble initiator (3). To make the polymer insoluble in water, the chains must be crossNH2 linked. However, the ability of the polymer to N CH3 H3C NH2 OH OH NH2 swell decreases as the degree of cross-linking in- NaO3S NH2 H N N N NN N SO3Na 2 Cl creases. Each chain should be as long as possible OCH3 H3CO Cl *Corresponding author. Email: titoh@ cc.okayama-u.ac.jp. URL: http://sci.ed.okayama-u. ac.jp/sci/org/orindex.html.
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SO3Na
SO3Na
NH2
H2N
Chicago Sky Blue 6B ( 3b)
Basic Fuchsin (3c)
Scheme I
Journal of Chemical Education • Vol. 76 No. 1 January 1999 • JChemEd.chem.wisc.edu
Safranin O (3d)
In the Classroom
for several days in water. Commercial polyacrylic pigments 3a and safranin O (3d) are therefore recommended as colored agents for the present experiments. The procedure given here is written for microscale, but the reaction provides enough polymer for the experiment on convection water current and is easily scaled up if desired. Although we have not yet elucidated the mechanism of the coloring process, these results provide an interesting application of superabsorbent polymer in teaching chemistry, especially at the high school level. The most important thing is that making the colored polymer is great fun. Materials
(1)
acrylic acid (2.7 g)
(4)
toluene (40 mL) solution of the pigment (0.1g) and Span #83 (0.1g)
aq NaOH (1.07 g) in 10 mL of water ice bath magnetic stirrer
MBAA (5.2 mg) (2)
The pigments were purchased from the hobby craft division of Gunze Co., Ltd. (3-17 Kanda-nishikimachi, Chiyoda-ku, Tokyo 101, Japan); Chicago Sky Blue 6B, basic fuchsin, safranin O were purchased from Aldrich and used directly. CAUTION : Acrylic acid is toxic and will cause burns on prolonged contact with skin. The reaction must be carried out in a well-ventilated area.
TMEDA (0.8 mL) (5)
vigorous stirring at RT until the exothermic reaction ceases (ca. 15 min)
(6) (NH4)2S2O8 (0.1 g)
Procedure
(3)
In a 200-mL Erlenmeyer flask was placed a 10 mL of sodium hydroxide (1.07 g) aqueous solution and acrylic acid (2.7 g) was added dropwise at 0 °C with vigorous stirring with a magnetic stirrer. To this solution were added 5.2 mg of N,N′-methylenebisacrylamide (MBAA) and 0.1 g of ammonium peroxodisulfate ((NH4)2S2O4) at room temperature (R.T.). Forty milliliters of a solution of 0.1 g of the pigment (Mr. Color: Red) and 0.2 g of Span #83 (sorbitan sesquioleate, the surface-active agent) in toluene was added to the mixture. To the resulting suspension, 0.8 mL of N,N,N′,N′tetraethylenediamine (TMEDA) was added dropwise at room temperature while constant magnetic stirring was maintained. Exothermic polymerization immediately took place to form the polymer particles and the stirring was continued till the exothermic reaction ceased (ca. 15 min). After being cooled to room temperature, the liquid was poured off (into a waste bottle, not the sink). Then, 10 mL of acetone was added into the flask and the liquid was poured off; the process was repeated two more times. To prepare the polymer as a solid, the moist colored gel was placed onto a petri dish, then dried in an oven at 70 °C for at least three hours (longer was preferable) (ca. 3.5 g, yield >60%). This procedure is summarized in Figure 2. The solid polymer was ground to a powder in a mortar with a pestle and kept in a desiccator. This powder was very good material with which to demonstrate the superabsorbent property of the polymer. Demonstrating Water Convection Current by Heating To prepare the colored polymer pieces used to demonstrate the convection water current by heating, the polymer solids were removed from the oven after about 1 hour of drying and cut with scissors into small pieces. These were returned to the oven and dried for a further 3 hours at 70 °C. Pieces of the polymer weighing about 0.25 g were placed in a 1000-mL beaker containing about 500 mL of water prewarmed to 40 °C. A spirit lamp warmed the bottom of the beaker, causing a gentle water convection current that moved
colored polymer
polymer collected and and washed with acetone (10 mL) (two times)
polymer dried in an oven at 70 °C for at least 3 h (longer is preferable)
Figure 2. Procedure for synthesizing colored superabsorbent polymer.
the colored polymer gel to the surface; the gel moved down again as it cooled. This cycle continued as long as the temperature of the water was maintained at about 60–70 °C (Fig. 1). Acknowledgment This work was supported by a Grant-in-Aid from the Sasakawa Scientific Research Fund, Japan (The Nippon Foundation). Literature Cited 1. Direen, G. E.; Shakhashiri, B. Z. In Chemical Demonstrations: A Handbook for Teachers of Chemistry; Shakhashiri, B. Z., Ed.; University of Wisconsin Press Madison, WI, 1983; Vol 1, pp 205–249. 2. Buchholz, F. L. J. Chem. Educ. 1996, 73, 512. 3. Sugiyama, J. Kagaku to Kyoiku (Chem. Educ. Jpn.) 1994, 42, 260. 4. Garner, C. M.; Nething, M.; Nguyen P. J. Chem. Educ. 1997, 74, 95. 5. Takaki, M.; Itoh, T. Kagaku to Kyoiku (Chem. Educ. Jpn.) 1995, 43, 729. 6. Takaki, M.; Itoh, T. Kagaku to Kyoiku (Chem. Educ. Jpn.) 1996, 44, 131. 7. JETRO News (96-09-009-05), New Technol. Jpn. 1996, 24, 40. 8. Takaki, M.; Itoh, T. Kagaku to Kyoiku (Chem. Educ. Jpn.) 1996, 44, 664.
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