In the Laboratory
Synthesis of a Superabsorbent Polymer Charles M. Garner, Matthew Nething, and Phuc Nguyen Department of Chemistry, Baylor University, Waco, TX 76798 Syntheses of polymers make especially popular and instructive experiments in the sophomore organic laboratory, probably because of the “real world” relevance of these materials. However, practically all such experiments deal with polymers that are of interest for strictly mechanical properties (strength, etc.). Polymers with additional interesting properties (e.g., electrical conductivity) (1) bring new aspects of chemistry and applications to such experiments. For example, certain polymeric materials are capable of absorbing up to thousands of times their weight of water. Over the last 15 years, these materials have been widely used in personal products, especially disposable diapers. Unfortunately, procedures for preparing such materials are almost strictly confined to the patent literature, and to our knowledge no experiments using this chemistry have been published.1 For several years, our sophomore organic students have been preparing a superabsorbent polymer based on polyacrylic acid, using a procedure adapted from a patent (2). A monograph describing many of the general synthetic aspects has also appeared (3). The superabsorbent material is a lightly cross-linked polyacrylic acid that has been partially neutralized. It is shown schematically in Figure 1. The cross-linking is required to render the material insoluble in water. When exposed to distilled water, the material will absorb 500–3000 times its weight in water, yielding a clear, colorless jelly. The behavior of the polymer is very sensitive to the presence of salts and also to pH. The absorbency is roughly ten times less in 0.2% NaCl solution, and is reduced to only about five times by weight if a small amount of acid is added. How can this behavior be accounted for? One way to picture the water-absorbing action of the material is to consider that the ionic strength inside the polymer is very high, and so water will tend to diffuse into the polymer (if the chains are not too highly cross-linked) to decrease the ionic strength. This is similar to a salt solution separated from distilled water by a permeable membrane: the distilled water will flow into the salt solution. Eventually, the chains will be unable to move apart further or the difference in osmotic gradient approaches zero, and the absorbing process stops. This picture explains why these polymers expand much more in distilled water than in salt solutions, and also why the addition of acid reverses the process. (In both cases the difference in osmotic gradient is affected.) Acrylic acid is polymerized by a radical mechanism and requires a radical initiator. We have used two watersoluble initiators. In both cases, we were able to accomplish the polymerization without removing the inhibitor (200 ppm of 4-methoxyphenol). Initially, we used a redox reaction between ascorbic acid and hydrogen peroxide, which is effective even at 0 °C. However, this system is very sensitive to the presence of oxygen and requires thorough purging with argon or nitrogen. Oxygen interferes with the polymerization by reacting with radical sites (R? + O2 gives the less reactive radical ROO?). A more convenient thermal initiator, which is water-soluble and functions at reasonably low temperatures, is VA-044 (see Fig. 2).2 When this initiator is heated above about 40–45 °C, it decomposes to give ni-
trogen gas and two radicals are produced, which initiate the polymerization. With VA-044 initiation, the polymerization is much more tolerant of air, probably because radicals are generated slowly over a much longer period of time (decomposition half-life at 44 °C is 10 h), more efficiently consuming the oxygen present. In either case, a very low concentration of initiator (0.04 mol% relative to acrylic acid) is used to promote the formation of long chains. To render the polymer insoluble in water, the chains must be cross-linked. However, the ability of the polymer to swell decreases as the degree of cross-linking increases (see Fig. 3). So it is desired that each chain be as long as possible and cross-linked in only a few places. We use very small amounts (ca. 0.08 mol% relative to acrylic acid) of the inexpensive water-soluble cross-linking agent N,N9-methylenebisacrylamide, or MBA (Fig. 2). CO2H
CO2H
CO2Na CO2H
CO2Na CO2Na
CO2H
CO2Na CO2H
CO2Na
CO2Na CO2Na CO2H
CO2H
regions of high ionic strength, diluted by diffusion of water into the polymer network.
CO2Na
CO2Na
Figure 1. Idealized structure of cross-linked, neutralized polyacrylate.
H O
H
H
O N H
H
N CH2
H
N
N H
CH3
CH3
N CH2
CH3
N
• 2 HCl
N CH3
N
H
N,N'-methylenebisacrylamide (MBA)
VA-044
Figure 2. Cross-linker and initiator structures for superabsorbent polymer synthesis.
Figure 3. Absorbency vs. mole percent cross-linker relative to acrylic acid (using redox initiation).
Vol. 74 No. 1 January 1997 • Journal of Chemical Education
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In the Laboratory
Because it contains two reactive double bonds, it can become incorporated into two different chains as the polymerization proceeds, providing a cross-link. Although MBA has an aminal functionality, it appears to be quite resistant to hydrolysis under the conditions used in this experiment. The procedure given here is written for a microscale, but the reaction is easily scaled up. However, on a sufficiently large scale (several grams of acrylic acid) the considerable exothermicity of the polymerization (18.5 kcal/ mol) will become evident and could be problematic. As written, the procedure uses 0.08 mol% of MBA, and students typically obtain 200–250 mg of dry polymer with an average absorbency of about 900 times its weight in distilled water. An interesting extension of the experiment is to study how the amount of cross-linker affects the absorbency of the polymer. Figure 3 shows our results of such a study using the ascorbic acid/H2O2 initiation system. Absorbency increases significantly with lower cross-linker levels, but the gel exhibits less strength. VA-044 appears to give material of somewhat higher absorbency than does the redox initiator. Interestingly, we have observed that the superabsorbent polymer will absorb large volumes of hydrogen peroxide solutions, which suggests possible applications in reactions (e.g., controlled oxygen generation). Experimental Procedure CAUTION : Acrylic acid is toxic and will cause burns on prolonged contact with skin. Wash well with water immediately if you get the acid solution on yourself. MBA may contain traces of acrylamide, which is a cancer suspect agent. Wear gloves when preparing the solution for polymerization.
Procedure In a 5-mL conical vial place a stir bar, 1.0 mL (2.78 mmol) of a 20% solution of acrylic acid in water, 35 µL (0.0023 mmol) of a 1% aqueous N,N9-methylenebisacrylamide solution in water, and 35 µL (0.0011 mmol) of a 1% aqueous VA-044 solution.3 Cap the vial with a septum cap and stir the solution briefly to make sure all components are well mixed. Then place the vial in an oven at 50 °C, leaving it at least overnight. NOTE: The presence of oxygen will increase the time required to gel (become immobile), though it is not usually necessary to deoxygenate the solution. Exposure to larger amounts of oxygen (i.e., larger reaction vessels) may inhibit the polymerization excessively. A simple solution is to evacuate the vessel using aspirator vacuum before heating. However, never evacuate a flat-bottom, thin-walled vessel. Modifications To Use Redox Initiation In place of VA-044, add 50 µL (0.0011 mmol) of 0.39% aqueous ascorbic acid.3 Cap the vial with a septum cap and cool it for several minutes in ice water. Then deoxygenate the solution (in the hood) by bubbling nitrogen or argon through a needle into the solution for at least 10 min, using a second needle to vent the gas. While maintaining good magnetic stirring, add 25 µL of 0.15% aqueous H2O2 (0.0011 mmol) by syringe through the septum. Maintain stirring at 0 °C until the solution gels (which should be less than 2 min if degassed well). Then leave the vial at room temperature at least overnight.
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Workup CAUTION: Do not touch the gel without gloves on because it may still contain some acrylic acid. The contents of the vial (which should be a very stiff gel) are transferred to a beaker containing 4 mL of 0.5 M NaOH. Stainless steel scissors are used to cut the gel into many small pieces (about 3 mm across) and achieve contact with the base solution. Be careful not to splash the base solution and be certain that safety glasses are worn! When all of the aqueous solution has been absorbed into the gel (about 10 min), 10 mL of methanol is added to the beaker and the contents are mixed until the gel ceases to contract (perhaps 10 min). The liquid is poured off (into a waste bottle, not the sink) and the process is repeated two more times (or until no further shrinkage is apparent). The moist gel is placed onto a watch glass and dried in an oven at 80–85 °C until dry (at least one hour, longer is OK). Analysis The resulting solid is cooled and weighed. A piece weighing about 10 mg (exact weight recorded) is selected and placed in a labeled beaker containing 50 mL of distilled water. Then a second piece of similar weight (record) is placed in a labeled beaker containing 50 mL of 0.2% NaCl solution. Both beakers are sealed with plastic wrap to prevent evaporation and left at least overnight in the lab drawer. During the next lab period, the results are described in the student’s notebook and each piece of gel is weighed.4 This is best done by (i) pouring off as much excess water as possible, (ii) weighing the beaker + gel, then (iii) dumping out the gel into a separate container (not into the sink!) and reweighing the beaker. After the weights of the swollen polymers have been determined, the gel that had absorbed distilled water is placed into a beaker with more distilled water, and 1 mL of 1 M HCl is added. The gel is observed intermittently for the remainder of the lab period. (CAUTION: the acidified polymer is extremely sticky!) In the lab write-up, each student calculates how many times its own weight of distilled water and of dilute salt water the polymer absorbed. Notes 1. An article describing a large-scale preparation of a superabsorbent polymer, with interesting background material, appeared in the Journal while the present paper was in press (Buchholz, F. L. J. Chem. Educ. 1996, 73, 512). 2. The initiator VA-044 is available from Wako Pure Chemical Industries, 1600 Bellwood Road, Richmond, VA 23237; phone (804) 271-7677. 3. The initiator and crosslinker solutions are preferably prepared just before use, but may be stored overnight in the refrigerator. 4. The swollen gel has a refractive index nearly identical to that of water and will be extremely difficult to see in the solution. It is easier to see if you move it around a bit.
Literature Cited 1. Sherman, B. C.; Euler, W. B.; Forcé, R. R. J. Chem. Educ. 1994, 71, A94–A96. 2. Brandt, K. A.; Goldman, S. A.; Inglin, T. A. U.S. Patent 32 649, 1988. 3. Kinney, A. B.; Scranton, A. B. In Superabsorbent Polymers: Science and Technology; Buchholz, F. L.; Pappas, N. A., Eds.; ACS Symposium Series 573, 1993; Chapter 1.
Journal of Chemical Education • Vol. 74 No. 1 January 1997
