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http://www.e-polymers.org e-Polymers 2009, no. 080 ISSN 1618-7229

Alginate-based superabsorbent composed of carboxylic -amine interaction: preparation and characterization

Toshio Yoshimura, 1* Miyuki Matsunaga,1 Rumiko Fujioka1

1* Faculty of Human Environmental Science, Fukuoka Women’s University, 1-1-1 Kasumigaoka, Higashi-ku, Fukuoka, Japan; tel: +81-92-661-2411; fax: +81-92-661- 2415; e-mail: [email protected].

(Received: 28 August, 2008; published: 21 June 2009)

Abstract: Preparation of superabsorbent hydrogels based on alginate was investigated by mixing aqueous sodium alginate solution and aqueous multivalent amine (ethylenediamine, L-lysine, ) solution, followed by HCl addition. After precipitation in methanol, vacuum drying, and pulverization with mixer, granule products with acid-base interaction were obtained. The preparation condition was investigated by changing the feed amount of amine compound. Among those examined, the highest water absorbency was obtained with the sample for chitosan feed ratio to sodium alginate of 100 mol%, reaching 410 g/g in pure water, 100 g/g in 0.9 % solution, and 80 g/g in 3.5 % NaCl solution. Hydrogels in the present study exhibited good biodegradability irrespective of amine compound.

Introduction Superabsorbent hydrogels are loosely crosslinked polymers that have an ability to absorb and retain large amount of water and aqueous solutions. Thanks to their unique characteristics, they have been widely used in many fields such as hygiene products, additives for soil in agriculture and horticulture, and controlled release of drugs [1]. Nowadays, most of commercially available superabsorbent hydrogels are crosslinked sodium polyacrylates, which are resistant to attack of microorganisms. Major applications of superabsorbent hydrogels are in disposable goods, so they may result in environmental pollution. Thus, biodegradable superabsorbent hydrogels based on naturally occurring polymers have received increasing attention [1-7]. Alginate is abundant in the cell walls of marine brown algae and used extensively in dentistry, textiles, and food industry [8]. Chemically, it is an unbranched copolymer with homopolymeric blocks of (1-4)-linked -D-mannuronate and -L-guluronate, covalently linked together. Since alginate is biodegradable and its sodium salt is soluble in water, it is expected that it can be used as a substitute for polymers, without environmental problems. So far, alginate-based hydrogels have been prepared by various gelation techniques. Ionic were obtained in the presence of or other multivalent cations [8-13]. Covalently crosslinked hydrogels were prepared by use of crosslinker such as glutaraldehyde (GA) [14-16] or methylenebisacrylamide [17]. Semi-interpenetrating network based on sodium alginate and crosslinked poly(N-isopropylacrylamide) was investigated as a pH- and temperature-responsive [18-19].

1 Interaction between and amine is another candidate of crosslinking to prepare hydrogel from alginate. Thus, the present study deals with the preparation of alginate-based hydrogels with this concept by use of multivalent amine compounds (ethylenediamine, L-lysine, chitosan). Further, characterization of the products was carried out to clarify the relationship between preparation condition and water absorbency and biodegradability.

Results and discussion

Preparation and structure of hydrogels Hydrogels were prepared according to the procedure shown in Scheme 1. Clear mixed solution of sodium alginate and multivalent amines turned opaque when diluted hydrochloric acid was added to the mixture. Among amines examined, apparent gelation was observed most noticeably in the case of chitosan.

HCl R NH2 - + Alg COONa Alg COOH Alg COO H3 N R

Alg COOH ・・・H N R 2

Scheme 1. Preparation route of alginate based hydrogels composed of carboxylic acid-amine interaction.

IR spectra of sodium alginate, reaction products containing chitosan (feed amount: 40 mol% and 100 mol% to repeating unit of sodium alginate), and alginic acid are shown in Fig. 1.

Fig. 1. IR spectra of (a) sodium alginate, (b) reaction product of sodium alginate and chitosan 40 mol%, (c) that of sodium alginate and chitosan 100 mol%, (d) alginic acid.

Absorption band due to C=O stretching of carboxylate is observed at 1616 cm-1 in sodium alginate and reaction products. Absorption at 1739 cm-1 due to C=O

2 stretching of free carboxylic acid appears in the reaction products and alginic acid, which is absent in sodium alginate. These results indicate that not all carboxylic acid in the reaction product forms carboxylate with amine but some remains as free acid which may weakly interact with amino group. Fig. 2 exhibits SEM photograph of reaction products of sodium alginate and chitosan. After being pulverized with mixer, the reaction products became amorphous granule. No apparent difference was observed with feed amount of chitosan.

(a) (b)

Fig. 2. SEM photograph of (a) reaction product of sodium alginate and chitosan 40 mol% and (b) that of sodium alginate and chitosan 100 mol%. Magnification: 100.

Water absorbency Fig. 3 shows time dependences of absorbency of the reaction products, changing the feed amount of ethylenediamine to sodium alginate. When the feed ratio of ethylenediamine to repeating unit of sodium alginate was 10 mol%, the product gradually absorbed water, and the absorbency after 48 h treatment was ca. 100 g/g. As the feed ratio of ethylenediamine increased, water absorbency became higher, except for an ethylenediamine feed ratio of 100 mol%; its low absorbency may be due to excess crosslinking by ethylenediamine. The highest absorbency was obtained when the feed ratio was 40 mol%, reaching about 350 g/g.

500

) )

400 40mol% g/g g/g 300 20mol% 10mol% 200

100mol% Absorbency ( Absorbency ( Absorbency 100

0 0 1 3 24 48 Absorption Time (h)

Fig. 3. Absorbency of reaction products of sodium alginate and ethylenediamine. Figures in the graph indicate the feed amount of ethylenediamine to sodium alginate.

3 As shown in Fig. 4, reaction products with L-lysine exhibited similar absorption behaviour to those with ethylenediamine, that is, absorbency was highest when the feed ratio of L-lysine to sodium alginate was 40 mol%. However, the maximum absorbency was somewhat higher than those with ethylenediamine, about 400 g/g.

500

40mol% ) ) 10mol%

400 g/g g/g 300 20mol% 200

100mol% Absorbency ( Absorbency ( Absorbency 100

0 0 1 3 24 48 Absorption Time (h)

Fig. 4. Absorbency of reaction products of sodium alginate and L-lysine. Figures in the graph indicate the feed amount of L-lysine to sodium alginate.

In the case of reaction product of sodium alginate and chitosan, high water absorbency was obtained with the chitosan feed ratio of 40 mol% and 100 mol%, as shown in Fig. 5. This result is in contrast with those obtained with ethylenediamine or L-lysine; water absorbency was low when feed ratio of amine was 100 mol%, as shown in Fig. 3 and Fig. 4.

500 100mol% ) ) 40mol%

400 g/g g/g

300 20mol%

200 10mol%

Absorbency ( Absorbency Absorbency ( Absorbency 100

0 0 1 3 24 48 Absorption Time (h)

Fig. 5. Absorbency of the reaction products with chitosan. Figures in the graph indicate the feed amount of chitosan to sodium alginate.

Fig. 6 exhibits absorbency in aqueous NaCl solution of various superabsorbent hydrogels. Conventional superabsorbent hydrogel, crosslinked sodium polyacrylate showed high absorbency in pure water, but absorbency drastically decreased in aqueous NaCl solution. Low absorbency is explained for decrease of osmotic pressure generated from difference of ionic concentration between inside and outside the [20]. Similar tendency was observed for the reaction products in the present

4 study, and among them, the product with chitosan feed ratio of 100 mol% kept relatively high absorbency even in aqueous NaCl solution, as shown in Fig. 6. High absorbency in NaCl indicates high water affinity of the reaction products of sodium alginate and chitosan.

Pure water 500 0.9% NaCl aq.

3.5% NaCl aq. ) ) 400 410

400 400 400 g/g g/g 350 300

200

100

80 Absorbency ( Absorbency Absorbency ( Absorbency 100 80 60 55 50 30 3025 25 0 Crosslinked Hydrogel Hydrogel Hydrogel Hydrogel sodium crosslinked crosslinked crosslinked crosslinked by EDA by L-lysine by chitosan by chitosan (40mol%) (40mol%) (40mol%) (100mol%)

Fig. 6. Comparison of absorbency with various superabsorbent hydrogels.

As shown in Fig. 7, the reaction product with chitosan feed ratio of 100 mol% possesses no sodium carboxylate group but carboxylate with amine and carboxylic acid, where hydrophilic side group or crosslinking point are undistinguishable. The reason why this structure induces high water absorbency is not clear at present; elucidation of relationship between molecular structure and water absorbency is now under investigation.

Crosslinking Alginic acid point COONa COO-H3N+ - Chitosan COONa - COOH COO COONa COONa COO + + COO- H3N+ NH3 COONa NH3 NH2 COOH H2N COONa COONa COOH NH2 - COONa COONa COO +NH3 NH2 COONa COOH COOH COONa NH2 - COONa COO +NH3 COO- +NH3 COONa COO- +NH3 COONa COOH COONa NH2 COOH COOH COONa COONa NH2 NH2 COO- COONa +NH3

(a) (b)

Fig. 7. Idealized schematic diagram of (a) conventional superabsorbent hydrogel and (b) the reaction product of sodium alginate and chitosan 100 mol%.

5 Biodegradability Fig. 8 shows biodegradability of various materials measured by continuous BOD method. Cellulose was used as the standard material to check the activity of the activation sludge. The activated sludge degraded ca. 60 % of unmodified sodium alginate for 14 days. The reaction products with amine feed ratio of 40 mol% showed good biodegradability; their degradation speed was comparable with that of unmodified sodium alginate. As a consequence, hydrogels in the present study have proved to have good biodegradability together with high water absorbency.

100

Sodium alginate Cellulose ( ) crosslinked by chitosan ( ■ ) ▲ ▲ ▲ ▲ 80 ▲ ▲ ▲ Sodium alginate ▲ ▲ ■ ■ ■ ■ ■ crosslinked by ▲ ■ ■ L-lysine ( ▲ ) ■ ■ 60 ▲ ■ ■ Sodium 40 ▲ alginate ( ) ■ Sodium alginate ■▲

Biodegradability (%) Biodegradability crosslinked by EDA ( ) 20 ▲ 0■ ■ 0 2 4 6 8 10 12 14

Time (days)

Fig. 8. Biodegradability of various materials evaluated at 25 oC in activated sludge.

Conclusions Biodegradable hydrogels based on alginate were successfully prepared by simple procedure, i.e. mixing aqueous sodium alginate solution and multivalent amine solution, followed by HCl addition and precipitation in methanol. The highest water absorbency was obtained with the sample with chitosan feed ratio of 100 mol%, where hydrophilic side group and crosslinking point are indistinguishable. Hydrogels in the present study exhibited good biodegradability irrespective of amine compound. Thus, the present hydrogels are expected to be useful for biomedical and agricultural applications.

Experimental

Materials Sodium alginate, ethylenediamine, L-lysine (Sigma-Aldrich Japan Co. Ltd., Japan), and chitosan (degree of deacetylation: ca 90%, Tokyo Chemical Industry Co. Ltd., Japan) were commercially available and used without purification. Alginic acid for IR analysis was prepared by adding aqueous solution of sodium alginate to excess hydrochloric acid solution, followed by filtration and vacuum drying. Conventional superabsorbent hydrogel, crosslinked sodium polyacrylate (“Aqua-keep” SA 60) was kindly supplied from Sumitomo Seika Chemicals, Co., Ltd. Cellulose (power, Aldrich Chemical Company, Inc.) for the standard material of biodegradability test was purchased and used as received.

6 Preparation of superabsorbent hydrogels Superabsorbent hydrogels were prepared according to the Scheme 1. As an example, the procedure for preparation of superabsorbent hydrogel from sodium alginate and chitosan is described in detail. Sodium alginate (1.0 g, 5.1 mmol for repeating unit of sodium alginate) was dissolved in 99 g of water in 500 ml Erlenmeyer flask under stirring at room temperature. In a separate 300 ml Erlenmeyer flask, chitosan (2.0 g, 12.1 mmol for repeating unit of chitosan), 91.8 g of water, and 6.2 g of acetic acid (8.9 mmol) was added and the mixture was stirred at room temperature until clear solution was obtained. To the aqueous alginate solution mentioned above, 18.7 g of the aqueous chitosan solution (2.0 mmol for amino group in chitosan, 40 mol% for repeating unit of sodium alginate) was added slowly under stirring. Then, 7.3 g of diluted hydrochloric acid (conc. 1%, 2.0mmol) was added to the mixture under stirring. After stirring at room temperature overnight, the mixture was poured into 500 ml of methanol under stirring. The precipitated product was isolated by filtration, dried under reduced pressure, finely cut with mixer, and screened through a 16-mesh sieve to give off-white granule product. Hydrogels of different chitosan feed amount (10, 20, 100 mol% for repeating unit of sodium alginate) were prepared in the same way. Preparation of hydrogels of sodium alginate with ethylenediamine or L-lysine instead of chitosan was carried out in a similar way, except that amine was dissolved in pure water (concentration 10 %), not in aqueous solution of acetic acid.

Structural analysis IR spectra of sodium alginate, the reaction products, and alginic acid were recorded on a Horiba FT-710 spectrophotometer (KBr disk). SEM observation of the reaction products was conducted with 3D real surface view VE-9800 (Keyence, Japan).

Water absorbency Water absorbency of the product was measured by the tea-bag method (Japanese Industrial Standard, JIS K 7223). Nylon tea-bag whose size is 200 mm and 100 mm in length and width, were prepared by heat sealing, and the superabsorbent hydrogel sample (0.2 g) was charged in it. The tea-bag was immersed in water at 25 oC. After 1 h treatment in water, the tea-bag was picked up from the water, and excess water was drained for 5 min. The weight of tea-bag and hydrogel was then measured (Wt), and absorbency was calculated according to the following scheme; absorbency = (Wt – Wb – Wp) / Wp (1) where Wb is the weight of blank tea-bag after water treatment, and Wp is the weight of dry superabsorbent hydrogel sample. Again, the tea-bag was dipped for 2 h, and picked up for 5 min to evaluate absorbency (total treatment time: 3 h). Absorbency after 24 h and 48 h was evaluated in the same way. Further, absorbency in aqueous NaCl solution (concentration: 0.9%, and 3.5%) was investigated similarly. These concentrations were corresponding to those of physiological saline and seawater, respectively.

Biodegradability Biodegradability of the superabsorbent hydrogels was measured at 25 oC for 20 days with reference to JIS K 6950 (ISO 14851) in which the sample was placed in

7 activated sludge. The activated sludge was kindly supplied from Tataragawa sewage- treatment plant (Fukuoka, Japan) and used as received. The biodegradability was evaluated by monitoring the biological oxygen demand (BOD) using an OM3001 coulometer of Ohkura Electric Co.Ltd., Japan, which detected the consumption of the oxygen during the evaluation.

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