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Super water-absorbing new material from chitosan, EDTA and urea
Carbohydrate Polymers 134 (2015) 337–343
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Super water-absorbing new material from chitosan, EDTA and urea夽 Abathodharanan Narayanan, Raghavachari Dhamodharan ∗ Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India
a r t i c l e
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Article history: Received 7 May 2015 Received in revised form 5 August 2015 Accepted 6 August 2015 Available online 11 August 2015 Keywords: Chitosan Super water absorbing polymer Hydrogel Biodegradable Renewable
a b s t r a c t A new, super water-absorbing, material is synthesized by the reaction between chitosan, EDTA and urea and named as CHEDUR. CHEDUR is probably formed through the crosslinking of chitosan molecules (CH) with the EDTA–urea (EDUR) adduct that is formed during the reaction. CHEDUR as well as the other products formed in control reactions are characterized extensively. CHEDUR exhibits a very high water uptake capacity when compared with chitosan, chitosan–EDTA adduct, as well as a commercial diaper material. A systematic study was done to find the optimum composition as well as reaction conditions for maximum water absorbing capacity. CHEDUR can play a vital role in applications that demand the rapid absorption and slow release of water such as agriculture, as a three in one new material for the slow release of urea, water and other metal ions that can be attached through the EDTA component. The other potential advantage of CHEDUR is that it can be expected to degrade in soil based on its chitosan backbone. The new material with rapid and high water uptake could also find potential applications as biodegradable active ingredient of the diaper material. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Increasing global population from 1.6 billion to 7 billion during the last 120 years had placed a demand on higher production of agricultural products. This in turn has led to rapid and high turnout based production strategies, which is reflected by the depletion of soil quality as well as by the deteriorating quality and quantity of ground water. To compensate for the loss of fertility of soil, macro-nutrients (nitrogen, potassium, phosphorous, calcium, etc.,) and micro-nutrients (boron, iron, copper, nickel, zinc, etc.,) (Pandey & Singh, 2010) are provided during cultivation through the use of chemicals such as EDTA chelates (Mg, Fe, Mn, Ca; sold by Akzo Nobel under the trade name Rexolin) and urea. Urea is the most used nitrogen fertilizer with nitrogen content just over 45% by weight. Urea, under high temperature and high humid climatic conditions, in the presence of urease enzyme in soil, decomposes fast, liberating ammonia gas into the atmosphere (Mori, 1927). In addition, the release of excess urea due to rundown of water into natural resources such as lakes and ponds also causes pollution. The decomposition is fast, in more acidic soils. Therefore, it will be more economical and eco-friendly if urea is made available to soil from a material in which it is linked with a
夽 A part of this work is the subject matter of Indian Patent Application 2505/CHE/2014. ∗ Corresponding author. E-mail address: [email protected] (R. Dhamodharan). http://dx.doi.org/10.1016/j.carbpol.2015.08.010 0144-8617/© 2015 Elsevier Ltd. All rights reserved.
fairly stable yet bio-degradable matrix. The literature reports few different methods of controlled release of urea into the soil. The controlled release of urea using naturally occurring biopolymers can offer some advantage to address these issues (He et al., 2007; Wu & Liu, 2008; Wu, Liu, & Rui, 2008). The use of chitosan, a linear random copolymer consisting of 2-amino-2-deoxy--1,4-d-glucopyranose (major) and 2-acetylamino-2-deoxy--1,4-d-glucopyranose (minor), in agriculture, is reported. Chitosan is derived from chitin by the deacetylation of the N-acetyl group using hot alkaline sodium hydroxide (Muzzarelli, 1973; Pillai, Paul, & Sharma, 2009). Chitosan is used as a plant growth enhancer as well as to prevent disease attack on seeds (Zeng, Luo, & Tu, 2012). It has been reported that foliar application of chitosan results in lesser transpiration thus reducing the application of water in agriculture (Valenta, Christen, & Bernkop-Schnürch, 1998a). The recommended dosage for foliar application of chitosan is between 100 or 125 ppm, during the early stage of the growth of okra plant, to achieve maximum yield (Bittelli, Flury, Campbell, & Nichols, 2001). Chitosan is reported to be active against viruses, bacteria and other pests (El Hadrami, Adam, El Hadrami, & Daayf, 2010; Mondal, Malek, Puteh, Ismail, & Ashrafuzzaman, 2012). The use of chitosan coated urea for the controlled release of the fertilizer to soil is also reported (Chen, Lu, Zhang, & Deng, 2006; Chen, Lu, Zhang, Wu, & Deng, 2005; Liu et al., 2012; Teixeira et al., 1990). Chitosan microspheres are used widely for the controlled release of drugs such as, antibiotics, antiinflammatory, anti-hypertensive and anti-cancer agents, peptides and proteins (Khan, Alam, Rahman, Noor, & Khan, 2009). The use of
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chitosan in the removal of metal ions such as Zn(II), Cu(II), Cr(VI), Fe(II) (Mohanasrinivasan et al., 2013) and Fe(III) (Burke, Yilmaz, & Hasirci, 2000) through chelation has been established. The covalent bonding of EDTA to chitosan has been reported (Loretz & Bernkop-Schnuerch, 2006). Chitosan–EDTA conjugate is used in pharmaceutical field as topical application gel (Valenta, Christen, & Bernkop-Schnürch, 1998b). It has also been shown that chitosan–EDTA conjugates, prepared by using EDTA to crosslink, improves the flexibility of chitosan films and decreases the hardness thus facilitating better controlled and sustained delivery system (Singh, Suri, Tiwary, & Rana, 2012a). Chitosan–EDTA derivatives have been developed as promising candidates for the treatment of bacterial and fungal infections (El-Sharif & Hussain, 2011b). Further, it was shown to display quick swelling properties in water and basic aqueous solutions (Bernkop-Schnurch & Krajicek, 1998). The use of chitosan based materials as high water absorbing polymers has not been reported. In commercial diaper products, crosslinked sodium poly(acrylate) is the most popular material that is used (for example in baby diapers). The other materials used include copolymers (of acrylamide; of ethylene-maleic anhydride; of vinyl alcohol), crosslinked polymers [poly(ethylene oxide); carboxymethylcellulose] and graft copolymers [poly(acrylonitrile) grafted to starch]. The retention of water by a water absorbing polymer depends critically on the ionic strength and especially the concentration of cations, especially if it is a polyelectrolyte. For example, crosslinked sodium poly(acrylate) can absorb up to 800 g of distilled water per g while it is reduced to 50 g/g if tap water (containing sodium, calcium cations) is used. Water absorbing polymers find application in diapers, bed liners, sanitary napkin, water retention agent (in horticulture), water-penetration blocker and control of water spill. If they can be designed using biopolymers that are known to degrade in nature, this would help in water conservation and significant improvement in the environment. Biopolymer based super absorbing polymers may also reduce water runoff and soil erosion. The typical water absorbing polymer used in soil is crosslinked acrylic acid-acrylamide copolymers neutralized with potassium. This polymer is not biodegradable and would remain in the soil for centuries without adding much value. Based on the literature reports it is clear that the use of chitosan–EDTA–urea conjugate material has not been reported in any form. Chitosan, a natural biopolymer, EDTA (a well known chelating agent for metal ions) and urea (fertilizer), when linked suitably, can be promising towards multiple purposes in agriculture as well as materials science. Towards this objective, the anchoring of EDTA and urea to chitosan was undertaken. The new material consisting of chitosan–(EDTA–Urea) crosslinks could lead to potential applications in terms of holding water and nutrients and releasing it in a controlled manner to the soil. The results from the synthesis, structure elucidation of the new material and the water holding studies are reported in this paper.
2. Materials and methods 2.1. Materials Raw chitosan [80 mol% deacetylated by FTIR (Muzzarelli, Tanfani, Scarpini, & Laterza, 1980), 76 mol% by solid state NMR], glacial acetic acid (GR grade), 25% v/v ammonia solution (GR grade), EDTA (GR grade), urea (GR grade) and methanol (GR grade) were purchased from R.K. Scientific Company, Chennai and were used without further purification. Water absorbing material from “Huggies” [Kimbery-Clark Lever Ltd.] commercial baby diaper was used as received.
2.2. Methods Synthesis of chitosan–EDTA–urea–adduct (CHEDUR) The required quantity of acetic acid (1–5% v/v; suitable for dissolving chitosan) was taken in a 500 ml poly(propylene) bottle with an air-tight lid. To this, calculated amount of urea and EDTA (urea/EDTA mole ratio 4.9) were added and shaken well. This was followed by the addition of the required quantity of chitosan, such that the weight ratio of chitosan to urea–EDTA mixture was 1:1–1:2. The mixture was shaken manually for 5 min. It turned viscous due to the dissolution of chitosan in acetic acid. The poly(propylene) bottle was then placed in a programmable air oven. The reaction mixture was heated from room temperature to 100 ◦ C at a heating rate of 5 ◦ C per min. The heating was continued for further period of 650 min at 100 ◦ C (urea is soluble in this reaction medium, at room temperature itself. EDTA is sparingly soluble in this mixture at room temperature, but totally soluble at the reaction temperature). It was then allowed to cool to room temperature (35 ◦ C). This product was observed to be brown in colour and was either a loose gel or solution, depending on the weight ratio of the reactants and the concentration of acetic acid. It was poured in to excess methanol with sufficient NH4 OH solution to maintain pH in the range of 8–9. At this pH, the mixture was allowed to stand for 30 min and then it was filtered using suction. The gel was rinsed with methanol to remove soluble components and then allowed to stand in suction for 30 min. The product was dried at 50 ◦ C for 5 h in an air oven and then powdered well. This new material was denoted as ‘CHEDUR’. Yield: 27 g for 15 g of starting material chitosan (44%; the expected yield for 100% reaction is 48.4 g, if one EDTA reacts with one amino group of every repeat unit in chitosan, which is 80% deacetylated, and one urea adds to one EDTA). 2.3. Characterization NMR, in solution, was carried out using Avance spectrometer (Bruker; operating at 500 MHz for 1 H and 125 MHz for 13 C). The solvent used for solution spectra was either HCl/D2 O or D2 O. NMR (13 C solid state) was carried out using a Avance III spectrometer (Bruker; 400 MHz for 1 H 100 MHz for 13 C). Spectral data in the solid state were gathered with the following parameters: sample packed in 4 mm diameter zirconia tube (MAS; spinning rate 10,000 kHz), number of scans = 1024. FTIR was carried out using JASCO 4100 FTIR spectrometer (JASCO, Japan). The solid samples required for the analysis was prepared in the pellet form by mixing 3–5 mg of the sample with 100 mg of dry KBr. ESI Mass spectra were recorded using Q-Tof micro Mass spectrometer (UK). MADLI-MASS was performed using UltrafleXtreme (Bruker). The thermogravimetric analysis (TGA) was carried out with TA Instruments Q500 Hi-Res TGA. The samples were heated at 10 ◦ C min−1 under flowing N2 atmosphere. X-ray diffraction patterns of all the materials were recorded with a Bruker D8 Advance diffractometer equipped ˚ SEM with Cu anode and a Cu K␣ source of wavelength of 1.5406 A. images were obtained using Quanta 450 scanning electron microscope. Elemental analysis was done with Perkin Elmer CHNS/O 2400 elemental analyzer. Surface area measurements were done with Micromeritics ASAP2020, using nitrogen adsorption at liquid nitrogen temperature and applying BET equation for surface area calculation. Prior to the analysis, the sample was degassed at 100 ◦ C for 12 h to remove all the pre-adsorbed gases. 2.4. Water absorption studies Prior to the experiment, all the samples used in the water absorption studies were dried at 50 ◦ C for 5 h. In a typical experiment, 20 mg of sample was weighed (w1 ) into a filter cone. This was immersed into a 100 ml beaker containing 80 ml of distilled
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Fig. 1. FT-IR spectrum of EDUR, CHEDUR and CHI-blank.
water. The weight gain of the sample due to water absorption was measured at different time intervals. After the desired time interval, the filter cone was removed from the beaker and the excess water was allowed to drip down for 5 min. Then the tip of the filter cone was gently swiped one time with dry tissue paper. The weight (w2 ) of the material was noted. The water uptake per g of material is calculated from the formula (w2 − w1 )/w1 g of water per g of material. 3. Results and discussion The reaction between chitosan (which consists of an amino group in every repeat unit), urea and EDTA can be expected to produce a crosslinked material, if the amino groups of the chitosan repeat units react with the carboxylic acid groups of EDTA through condensation. Despite few reports on the formation of conjugate material between chitosan and EDTA, there is no information in the published literature about the nature of the bond formed and the chemistry of this reaction (Bernkop-Schnurch & Krajicek, 1998; ElSharif & Hussain, 2011a; Loretz & Bernkop-Schnuerch, 2006; Singh, Suri, Tiwary, & Rana, 2012b). Hence, it can be surmised that the formation of distinct chemical bonds between chitosan and EDTA has not been reported. However, the crosslinking of chitosan through the interaction of its amino groups with EDTA is possible, since EDTA is a tetra functional acid. The protonation of urea by organic acids resulting in compounds of varying stoichiometry and structure has also been reported (Paleckiene, Sviklas, & Slinksiene, 2005; Radell, Brodman, & Domanski, 1967). Thus, the reaction between urea and citric acid is reported to result in the formation of urea citrate (with a molar ratio 1:1). The reaction of urea with dibasic acid such as HOOC (CH2)n COOH resulted in the formation of a salt when n = 0 and hydrogen bonded complexes for n = 2–5. In all the cases, the stoichiometry was reported to be 2H2 N CO NH2 . HOOC (CH2)n COOH. Based on the above facts, it was expected that a mixture of EDTA and urea could form a hydrogen bonded complex in which at least some of the carboxylic acids groups should be available for crosslinking with chitosan. In the absence of specific details from earlier literature, the expectations were checked by performing reaction between EDTA and urea (control reactions, the details of which are given in Supplementary Data). It was observed that
EDTA and urea react with each other under the reaction conditions employed in this work. The FTIR spectrum of EDTA-urea (EDUR) product is shown in Fig. 1. This exhibited prominent peaks (cm−1 ) at 3437 (a peak at 3442 is observed in urea), 3372 (new), 3200 (new), 3022 (a peak at 3018 is observed in EDTA), 1674 (the urea amide carbonyl is observed at 1678), 1624 (also observed in urea), 1456 (observed in urea), and 1400 (observed at 1395 in EDTA blank), 1320 (EDTA blank), 1167 (new), 1090 (observed in EDTA blank), 911 (EDTA blank), 855 (new), 785 (EDTA) and 700 (EDTA blank). The presence of some of the prominent peaks from EDTA (see the characterization data pertinent to EDTA blank or control reaction as well as unreacted EDTA in Supplementary Data) and urea (see the characterization data pertinent to urea blank or control reaction as well as unreacted urea in Supplementary Data) as well as new peaks in the product confirmed that a reaction had taken place between urea and EDTA. This was not surprising considering that organic acids are known to react with urea (Paleckiene et al., 2005). The proton and 13 C NMR, ESI-MS, MALDI-MASS, PXRD, DSC and TGA of EDUR (Supplementary Data Figs. S1–S9) confirmed, without any ambiguity, that EDTA and urea react to form a methanol soluble product that is amorphous in the solid state. The MALDI-MASS (Supplementary Data Figs. S5 and S6) of the methanol soluble part of EDUR exhibited prominent peaks at m/z values of 644, 727, 781, 1117 and 1491 corresponding to (EDTA)2 urea, (EDTA–urea)2 Na+, (EDTA–urea)2 (urea)NH3 , (urea–EDTA)4 (urea)Na+. The ESI-MS of the product obtained in the control reactions, with and without acetic acid (Supplementary Data Figs. S3 and S4), suggests the formation of oligomers of EDTA and urea with a degree of polymerization between 10 and 18. The variable temperature proton NMR of EDUR (Supplementary Data Fig. S10) indicated that the distinct signals from EDUR, in the range 3–4 ppm, moved to higher chemical shift value at higher temperature (as opposed to merging of signals) and did not merge into fewer signals suggesting that these oligomers may not be EDTA–urea association complexes with weak bonds. Based on the spectroscopic data (especially the ESI MS) and other characterization data the structure of the amorphous methanol soluble product of the reaction between EDTA and urea (EDUR) appears to be an oligomer of EDTA and urea with an “n” value ranging from 10 to 19 (based on the m/z peaks observed at 855, 811 as well as 459), although its exact chemical structure could not be confirmed.
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Fig. 2. Solid state NMR spectrum of CHEDUR (top) and CHI-blank (control).
Although chitosan and EDTA as well as chitosan and urea did not react significantly under the experimental conditions reported here (data shown in the Supplementary Data), a mixture of chitosan with EDTA and urea was observed to react, possibly due to the electrostatic interaction between chitosan and the EDTAUrea adduct. The new material obtained (CHEDUR) was observed to be crosslinked (gel formation and insoluble in a number of common organic solvents, water and acetic acid). It was insoluble in water and hence the 1 H and 13 C NMR of CHEDUR gel was taken in D2 O/HCl mixture. The 1 H NMR spectrum of the gel showed the peaks at 4 ppm (OOC CH2 N ), 3.42 ppm ( N CH2 CH2 N ), 3.3 ppm (H attached to C2 of chitosan) and 2 ppm ( NHCOCH3 ) that are also observed in the case of EDUR (methanol soluble). The solid state NMR spectrum of the new material, CHEDUR, as well as the product obtained in the control reaction [by subjecting chitosan to identical reaction conditions and workup but in the absence of EDTA and urea (CHI-blank)] is shown in Fig. 2. CHEDUR exhibits peaks at 22.5 ( NHCO CH3 ), 49 (from EDTA part), 51.3, 55, 58 (from EDTA), 59.8, 74, 82.5, 83.6, 95.2, 103 (all from chitosan), 160.2 (from H2 N CO NH2 ), 169.1 (from H2 N CO NH3 + ), 171.1 ( CO OH from EDTA), 173.1 ( NH CO CH3 from chitosan), 177.8 ( CO O− from EDTA) ppm, respectively. The peaks are assigned by comparing the spectrum with those of the solid state NMR spectra of chitosan-blank as well as that of CHUR (formed by the reaction between chitosan and urea; Supplementary Data Fig. S11). The new peak at 177.8 ppm in CHEDUR most likely arises from the carboxylate carbonyl of EDTA that in turn is formed due to the protonation of amine groups of chitosan by the carboxylic acid group of EDTA. The peak at 171.1 ppm arises from the EDTA carbonyl groups of free carboxylic acid groups. The new peak at 169.1 ppm could arise out of the urea carbonyl carbons adjacent to the protonated NH2 groups in urea (with the carboxylic acid group in EDTA). The other new peaks at 49 and 58 ppm confirm the presence of EDTA moiety in CHEDUR. The peaks from chitosan backbone are also seen in CHEDUR at 51.3 (C2), 55 (C3), 59.8 (C6), 74 (C5), 82.5, 83.6 (C4), 95.2 (C1 with NHCOCH3 in C2) and 103 (C-1 with NH2 in C2) (106 ppm), respectively (Saito, Tabeta, & Ogawa, 1987). The extent of crosslinking could be determined by solid state NMR since the area under the respective carbons (with the exception of carbonyl carbons) appeared to be quantitative as shown in Fig. 2 (see control experiments CHI-blank and CHUR; Supplementary Data Fig. S11). Based on this data and by comparing the area under the peaks between 48 to 65 ppm (from C2 and C6 of chitosan as well as from EDTA) and 92 to 112 ppm (arising only from the anomeric carbon) the degree of crosslinking is found to be 0.16. The FT-IR spectrum of CHEDUR along with EDUR and chitosan is presented in Fig. 1 while those of the raw materials used in the reaction (EDTA and urea) are presented in Supplementary Data as Fig. S12. The major peaks observed in the case of CHEDUR are
assigned to the following vibrations: the broad band appearing between 3200 and 3500 cm−1 in the case of chitosan is due to the combined absorption due to the stretching of –OH and NH bonds (Zhang et al., 2011). This turns broader in the case of CHEDUR compared to chitosan, which could be due to the presence of EDTA and urea (EDTA shows a broad peak around 3000 cm−1 and urea shows a broad peak between 3400 cm−1 and 3300 cm−1 ; groups that can hydrogen bond extensively). The C H stretching band seen at 2855 cm−1 for chitosan is reduced in intensity in the case of CHEDUR. The prominent carbonyl stretching peaks associated with EDTA (observed at 1700 cm−1 ) and urea (observed at ∼1680 cm−1 ) are seen to disappear although there is some absorption in this region. A new peak is observed for CHEDUR at 1640 cm−1 that can be attributed to the EDUR (amide I of amide carbonyl moiety) present in CHEDUR, while the amide I band specific to –NHCOCH3 of chitosan and observed at 1655 cm−1 is seen to reduce in intensity. The amide II band observed at 1567 cm−1 in the case of chitosan is shifted to 1560 cm−1 in the case of CHEDUR suggesting that the N H could be interacting with EDUR. The peak observed at 1400 cm−1 for CHEDUR as well as EDUR arises from specific C-H bending of the CH2 group (present in EDTA). EDTA and urea show very specific and sharp absorptions at 800 cm−1 that is seen to vanish in the case of CHEDUR. The FT-IR spectrum thus confirms that the product CHEDUR is formed as a result of reaction between chitosan and EDUR (EDTA and urea conjugate). The FTIR spectrum of the materials (CHI-blank, CHED and CHUR) obtained in the control experiments are shown in the Supplementary Data as Fig. S13. The TGA data for CHEDUR and the raw materials used in its preparation are compared in Fig. 3. From this figure it can be inferred that urea (being the least stable among the raw materials) decomposes between 150 and 210 ◦ C and loses almost 80% of its weight while EDTA is stable up to 200 ◦ C and loses around 70% of its weight in the temperature region 230–320 ◦ C. Chitosan is observed to be stable up to 270 ◦ C (ignoring the loss of moisture of around 5% up to 150 ◦ C) and then it loses nearly 35% of its weight between the temperature range 270–320 ◦ C. In comparison, CHEDUR loses mass in two steps, while all the raw materials lose weight in a single step up to 350 ◦ C (if the initial moisture loss up to 150 ◦ C is ignored). The weight loss between 150 and 260 ◦ C, in the first step, arises predominantly from urea (87% of urea decomposes) and to a significant extent from EDTA (22.4% of EDTA decomposes) and the weight loss between 250 and 350 ◦ C, in the second step arises principally from chitosan and EDTA. The residue, left after heating to 900 ◦ C in nitrogen atmosphere for CHEDUR, is 28%. Since chitosan, EDTA and urea leave 37.5%, 10.8% and 0.6% residue, respectively, at 900 ◦ C, it follows that EDTA and urea are incorporated in CHEDUR. The TGA of the products obtained in the control experiments is shown in the Supplementary Data as Fig. S9. The powder XRD (PXRD) pattern of CHEDUR and chitosan are shown in Fig. 4 (the PXRD patterns of EDTA and urea are given in the Supplementary Data Fig. S14). The prominent diffraction peaks for chitosan are observed at 2Â values of 9.43 [d = 9.37 A˚ and (0 2 0)], 20.3 [d = 4.37 A˚ and (1 1 0)] and 29.58 [d = 3.02 A˚ and (1 0 4) of calcite present in chitosan]. The peak widths are observed to be higher than that of EDTA and urea (and are also broad at the base) suggesting that chitosan is semi-crystalline. The observed PXRD patterns are consistent with that reported in the literature for chitin nanofibers [anti parallel crystal pattern of alpha chitin (Kumirska et al., 2010)] and calcite [which exhibits the most prominent diffraction peaks at 2Â = 29 (1 0 4)]. In comparison, CHEDUR exhibits diffraction peaks at 2Â values of 10.44 [d = 8.47 A˚ and (0 2 0)], 20.12 [d = 4.41 A˚ and (1 1 0)], 26.72 and 29.58 [d = 3.02 A˚ and (1 0 4), of calcite present in chitosan]. The peak widths are also larger compared to chitosan and suggest that the new product is less crystalline and essentially amorphous, in comparison with chitosan (as well as EDTA and urea). The shift in the 2Â value for (1 1 0) and (0 2 0) planes in
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Fig. 3. TGA (left) and DTG (right) of urea, EDTA, CHEDUR and chitosan.
CHEDUR to a higher value (versus chitosan), indicate the decrease in inter planar distance, possibly due to crosslinking (as the EDTA is a tetra-functional molecule that can crosslink the chitosan with a free amine group in every repeat unit). It can also be inferred from the PXRD of CHEDUR that small molecular reactants (such as EDTA, urea) are absent. This further confirms that the product is not just a physical mixture of chitosan, EDTA and urea but chemically combined. The PXRD analysis of the materials obtained in the control experiments is shown in the Supplementary Data as Fig. S15. Under the reaction conditions employed in this work, no significant and measurable reaction could be observed between chitosan and EDTA as well as chitosan and urea, as confirmed by the results associated with the characterization of the products of the reaction (details presented in the Supplementary Data). The PXRD of chitosan–EDTA conjugate (Supplementary Data; impure due to the presence of unreacted EDTA) suggests the formation of a material similar to CHEDUR. However the PXRD of chitosan–urea reaction (Supplementary Data) results in the formation of a more crystalline material than chitosan. The details of the water absorption of the new materials were studied as described in Section 2. The new material was observed
to form a transparent gel when exposed to water (Supplementary Data Fig. S16). With a view to prepare a material of high water absorption property, the reaction condition was varied to obtain different materials. For this purpose the following variables were optimized: (i) mole ratio of EDTA/chitosan (EDTA/chitosan mole ratio was varied between 0.4 and 10); (ii) mole ratio of EDTA/urea (EDTA/urea mole ratio was varied between 0.137 and 1); and the (iii) concentration and volume of the dissolution medium i.e., acetic acid between 1 and 5% v/v. The results are summarized in Table 1. The maximum water absorption of 570 ± 20 g/g was obtained with the CHI: EDTA: Urea mole ratio of 1:1.11:5.4. The above mole ratio was kept constant and the reaction medium concentration and volume was varied. The results thus obtained are shown in Table 1. Based on these studies, 66 ml of 1% v/v acetic acid per g of chitosan was found to be the most suited for maximum water absorption. The very high water uptake of CHEDUR can be attributed to a combination of factors: the osmotic interaction of water with the ionic bonds present between chitosan and EDUR (similar to what is observed with the sodium salts of crosslinked poly(acrylic acid)) and the change in the morphology from the relatively lower amorphous content to mostly amorphous state (CHEDUR) upon which the glucosamine repeat units with four hydroxyl groups are
Fig. 4. PXRD pattern of CHEDUR and chitosan.
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Table 1 Effect of preparation variables on the water absorption of CHEDUR. CODE
RDAN 42 RDAN 43 RDAN 45 RDAN 48 RDAN 53 RDAN 56 RDAN78 CHI blank a
Composition (CHI:ED:UR) Mole ratioa
EDTA:urea Mole ratio
1: 1.11: 8.1 1: 1.66: 8.1 1: 10: 10 1: 2.0: 9.8 1: 1.11: 5.4 1: 0.4: 1.8 1:1.11:5.4 1:0:0
0.137 0.206 1.00 0.206 0.206 0.222 0.206 0
Acetic acid solution
CHN
% v/v
v/g of chitosan
%C
%H
%N
5 5 5 5 5 1 1 1
100 100 100 100 66 66 66 66
35.1 37.7 35.9 32.1 38.1 38.5 37.7 40.16
6.7 6.3 4.5 5.7 6.2 6.4 6.2 6.7
15.4 11.8 11.6 12.9 11.9 11.1 11.8 6.7
Water uptake (g/g)
47 64 104 176 120 9.2 570 0
The mole ratio for chitosan was taken on the basis of the number of glucosamine repeat units (mass of the sample/molar mass of the glucosamine repeat unit).
available for water absorption in contrast to the crystalline morphology where all the functional groups are buried inside the crystal structure. In contrast, the water uptake in typical diaper material [crosslinked sodium poly(acrylate)]is attributed to the strong electrostatic interaction between the water and the anion present in the crosslinked polymer, in addition to the chemical potential driven migration of the sodium from the polymer matrix to the water phase upon which the osmosis of water is facilitated. The high water uptake is not due to high surface area and capillary action as can be seen from the results shown in the Supplementary Data as Table S1. The results from the control experiments confirm that chitosan, EDTA and urea are necessary for the high water uptake. The nitrogen content of the new material was determined by elemental analysis. This suggests that the extent of water uptake increases as the nitrogen content increases from blank chitosan to chitosan–urea to chitosan–EDTA and finally CHEDUR as given in the Supplementary Data Table S1. The water (distilled) uptake of CHEDUR was compared with a commercial diaper material, and a mixture containing 1:1 weight ratio of CHEDUR and commercial diaper material. For this study, the water absorbing part of ‘Huggies’ baby diaper was used. The results thus obtained are plotted and shown in Fig. 5. It can be inferred from this figure that the rate of water absorption of CHEDUR was the same as that of commercial diaper material in the initial five minutes. In the case of the commercial diaper material the maximum water absorption capacity of 238 g/g is reached within 10 min, following which no further absorption is observed over a 1 h study
Fig. 5. Water uptake versus time for CHEDUR, commercial diaper and a 1:1 mixture.
period. However, CHEDUR shows water absorption over a period of 30 min with a maximum water uptake of 570 g/g, which is nearly 2.3 times that of the commercial diaper material used. The 1:1 mixture (by weight) of commercial diaper material and CHEDUR, follows the expected water uptake rate. The rate of saline water uptake was investigated for CHEDUR as well as the commercial diaper material and the results are shown in Fig. 6. It is clear from this study that CHEDUR is a better water absorber at lower concentration of saline, while it is equivalent to the commercial diaper material at higher saline content.
Fig. 6. Saline water uptake versus time for CHEDUR and commercial diaper.
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The hydrogel formed upon the absorption of water by the new material exhibits fibrous morphology with the diameter of fibre bundles being ∼250 nm as shown by the SEM picture in the Supplementary Data as Fig. S17. The new material can, in principle, function as a matrix for the slow release of urea as chitosan is known to be biodegradable. It can also be modified by a simple ion exchange reaction to introduce essential micronutrients such as metal ions that can be released to soil on biodegradation. The scope of this work is under detailed investigation. 4. Conclusions A super water-absorbing hydrogel is synthesized from chitosan, a carbohydrate polymer, EDTA and urea. The structure of the crosslinked polymer is characterized by solid state NMR, FTIR, TGA and PXRD. The new polymer is observed to be essentially amorphous in contrast to chitosan, which is semi-crystalline. The high water uptake of this material is attributed to the presence of ionic bonds between chitosan and EDUR as well as due to the change in morphology upon reaction. The new material can play a vital role in applications that demand the rapid absorption and slow release of water, such as agriculture where it can also offer an additional advantage of biodegradation in soil in view of its chitosan backbone. In principle, it can function as a three in one new material for the slow release of urea, water and essential metal ions but this aspect has to be tested thoroughly both in the lab and the field. The new material with rapid and high water uptake could also find potential applications as biodegradable active ingredient of the diaper material. Acknowledgements The first author thanks IIT Madras for supporting his pursuit of research. The last author thanks IIT Madras for financial support and grant of sabbatical leave that enabled the possibility to explore new directions. The Department of Chemistry, IIT Madras is thanked for providing the necessary infrastructure. We thank Shasun Research Centre, Chennai for the solid state NMR spectra. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbpol.2015.08.010 References Bernkop-Schnurch, A., & Krajicek, M. E. (1998). Mucoadhesive polymers as platforms for peroral peptide delivery and absorption: Synthesis and evaluation of different chitosan–EDTA conjugates. Journal of Controlled Release, 50(Copyright (C) 2014 American Chemical Society (ACS). All Rights Reserved), 215–223. Bittelli, M., Flury, M., Campbell, G. S., & Nichols, E. J. (2001). Reduction of transpiration through foliar application of chitosan. Agricultural and Forest Meteorology, 107(3), 167–175. Burke, A., Yilmaz, E., & Hasirci, N. (2000). Evaluation of chitosan as a potential medical iron(III) ion adsorbent. Turkish Journal of Medical Sciences, 30(Copyright (C) 2014 American Chemical Society (ACS). All Rights Reserved), 341–348. Chen, Q., Lu, W.-J., Zhang, W.-Q., Wu, J.-H., & Deng, X. (2005). Nutrient release characteristic of chitosan coated urea and change of penetrability of its coating layer. Huadong Ligong Daxue Xuebao, Ziran Kexueban, 31(Copyright (C) 2014 American Chemical Society ACS. All Rights Reserved), 764–767. Chen, Q., Lu, W., Zhang, W., & Deng, X. (2006). Effect of temperature on nitrogen release characteristic of chitosan-coated urea. Zhiwu Yingyang Yu Feiliao Xuebao, 12(Copyright (C) 2014 American Chemical Society ACS. All Rights Reserved), 727–731. El Hadrami, A., Adam, L. R., El Hadrami, I., & Daayf, F. (2010). Chitosan in plant protection. Marine Drugs, 8(Copyright (C) 2014 American Chemical Society ACS. All Rights Reserved), 968–987.
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Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol
Super water-absorbing new material from chitosan, EDTA and urea夽 Abathodharanan Narayanan, Raghavachari Dhamodharan ∗ Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India
a r t i c l e
i n f o
Article history: Received 7 May 2015 Received in revised form 5 August 2015 Accepted 6 August 2015 Available online 11 August 2015 Keywords: Chitosan Super water absorbing polymer Hydrogel Biodegradable Renewable
a b s t r a c t A new, super water-absorbing, material is synthesized by the reaction between chitosan, EDTA and urea and named as CHEDUR. CHEDUR is probably formed through the crosslinking of chitosan molecules (CH) with the EDTA–urea (EDUR) adduct that is formed during the reaction. CHEDUR as well as the other products formed in control reactions are characterized extensively. CHEDUR exhibits a very high water uptake capacity when compared with chitosan, chitosan–EDTA adduct, as well as a commercial diaper material. A systematic study was done to find the optimum composition as well as reaction conditions for maximum water absorbing capacity. CHEDUR can play a vital role in applications that demand the rapid absorption and slow release of water such as agriculture, as a three in one new material for the slow release of urea, water and other metal ions that can be attached through the EDTA component. The other potential advantage of CHEDUR is that it can be expected to degrade in soil based on its chitosan backbone. The new material with rapid and high water uptake could also find potential applications as biodegradable active ingredient of the diaper material. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Increasing global population from 1.6 billion to 7 billion during the last 120 years had placed a demand on higher production of agricultural products. This in turn has led to rapid and high turnout based production strategies, which is reflected by the depletion of soil quality as well as by the deteriorating quality and quantity of ground water. To compensate for the loss of fertility of soil, macro-nutrients (nitrogen, potassium, phosphorous, calcium, etc.,) and micro-nutrients (boron, iron, copper, nickel, zinc, etc.,) (Pandey & Singh, 2010) are provided during cultivation through the use of chemicals such as EDTA chelates (Mg, Fe, Mn, Ca; sold by Akzo Nobel under the trade name Rexolin) and urea. Urea is the most used nitrogen fertilizer with nitrogen content just over 45% by weight. Urea, under high temperature and high humid climatic conditions, in the presence of urease enzyme in soil, decomposes fast, liberating ammonia gas into the atmosphere (Mori, 1927). In addition, the release of excess urea due to rundown of water into natural resources such as lakes and ponds also causes pollution. The decomposition is fast, in more acidic soils. Therefore, it will be more economical and eco-friendly if urea is made available to soil from a material in which it is linked with a
夽 A part of this work is the subject matter of Indian Patent Application 2505/CHE/2014. ∗ Corresponding author. E-mail address: [email protected] (R. Dhamodharan). http://dx.doi.org/10.1016/j.carbpol.2015.08.010 0144-8617/© 2015 Elsevier Ltd. All rights reserved.
fairly stable yet bio-degradable matrix. The literature reports few different methods of controlled release of urea into the soil. The controlled release of urea using naturally occurring biopolymers can offer some advantage to address these issues (He et al., 2007; Wu & Liu, 2008; Wu, Liu, & Rui, 2008). The use of chitosan, a linear random copolymer consisting of 2-amino-2-deoxy--1,4-d-glucopyranose (major) and 2-acetylamino-2-deoxy--1,4-d-glucopyranose (minor), in agriculture, is reported. Chitosan is derived from chitin by the deacetylation of the N-acetyl group using hot alkaline sodium hydroxide (Muzzarelli, 1973; Pillai, Paul, & Sharma, 2009). Chitosan is used as a plant growth enhancer as well as to prevent disease attack on seeds (Zeng, Luo, & Tu, 2012). It has been reported that foliar application of chitosan results in lesser transpiration thus reducing the application of water in agriculture (Valenta, Christen, & Bernkop-Schnürch, 1998a). The recommended dosage for foliar application of chitosan is between 100 or 125 ppm, during the early stage of the growth of okra plant, to achieve maximum yield (Bittelli, Flury, Campbell, & Nichols, 2001). Chitosan is reported to be active against viruses, bacteria and other pests (El Hadrami, Adam, El Hadrami, & Daayf, 2010; Mondal, Malek, Puteh, Ismail, & Ashrafuzzaman, 2012). The use of chitosan coated urea for the controlled release of the fertilizer to soil is also reported (Chen, Lu, Zhang, & Deng, 2006; Chen, Lu, Zhang, Wu, & Deng, 2005; Liu et al., 2012; Teixeira et al., 1990). Chitosan microspheres are used widely for the controlled release of drugs such as, antibiotics, antiinflammatory, anti-hypertensive and anti-cancer agents, peptides and proteins (Khan, Alam, Rahman, Noor, & Khan, 2009). The use of
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chitosan in the removal of metal ions such as Zn(II), Cu(II), Cr(VI), Fe(II) (Mohanasrinivasan et al., 2013) and Fe(III) (Burke, Yilmaz, & Hasirci, 2000) through chelation has been established. The covalent bonding of EDTA to chitosan has been reported (Loretz & Bernkop-Schnuerch, 2006). Chitosan–EDTA conjugate is used in pharmaceutical field as topical application gel (Valenta, Christen, & Bernkop-Schnürch, 1998b). It has also been shown that chitosan–EDTA conjugates, prepared by using EDTA to crosslink, improves the flexibility of chitosan films and decreases the hardness thus facilitating better controlled and sustained delivery system (Singh, Suri, Tiwary, & Rana, 2012a). Chitosan–EDTA derivatives have been developed as promising candidates for the treatment of bacterial and fungal infections (El-Sharif & Hussain, 2011b). Further, it was shown to display quick swelling properties in water and basic aqueous solutions (Bernkop-Schnurch & Krajicek, 1998). The use of chitosan based materials as high water absorbing polymers has not been reported. In commercial diaper products, crosslinked sodium poly(acrylate) is the most popular material that is used (for example in baby diapers). The other materials used include copolymers (of acrylamide; of ethylene-maleic anhydride; of vinyl alcohol), crosslinked polymers [poly(ethylene oxide); carboxymethylcellulose] and graft copolymers [poly(acrylonitrile) grafted to starch]. The retention of water by a water absorbing polymer depends critically on the ionic strength and especially the concentration of cations, especially if it is a polyelectrolyte. For example, crosslinked sodium poly(acrylate) can absorb up to 800 g of distilled water per g while it is reduced to 50 g/g if tap water (containing sodium, calcium cations) is used. Water absorbing polymers find application in diapers, bed liners, sanitary napkin, water retention agent (in horticulture), water-penetration blocker and control of water spill. If they can be designed using biopolymers that are known to degrade in nature, this would help in water conservation and significant improvement in the environment. Biopolymer based super absorbing polymers may also reduce water runoff and soil erosion. The typical water absorbing polymer used in soil is crosslinked acrylic acid-acrylamide copolymers neutralized with potassium. This polymer is not biodegradable and would remain in the soil for centuries without adding much value. Based on the literature reports it is clear that the use of chitosan–EDTA–urea conjugate material has not been reported in any form. Chitosan, a natural biopolymer, EDTA (a well known chelating agent for metal ions) and urea (fertilizer), when linked suitably, can be promising towards multiple purposes in agriculture as well as materials science. Towards this objective, the anchoring of EDTA and urea to chitosan was undertaken. The new material consisting of chitosan–(EDTA–Urea) crosslinks could lead to potential applications in terms of holding water and nutrients and releasing it in a controlled manner to the soil. The results from the synthesis, structure elucidation of the new material and the water holding studies are reported in this paper.
2. Materials and methods 2.1. Materials Raw chitosan [80 mol% deacetylated by FTIR (Muzzarelli, Tanfani, Scarpini, & Laterza, 1980), 76 mol% by solid state NMR], glacial acetic acid (GR grade), 25% v/v ammonia solution (GR grade), EDTA (GR grade), urea (GR grade) and methanol (GR grade) were purchased from R.K. Scientific Company, Chennai and were used without further purification. Water absorbing material from “Huggies” [Kimbery-Clark Lever Ltd.] commercial baby diaper was used as received.
2.2. Methods Synthesis of chitosan–EDTA–urea–adduct (CHEDUR) The required quantity of acetic acid (1–5% v/v; suitable for dissolving chitosan) was taken in a 500 ml poly(propylene) bottle with an air-tight lid. To this, calculated amount of urea and EDTA (urea/EDTA mole ratio 4.9) were added and shaken well. This was followed by the addition of the required quantity of chitosan, such that the weight ratio of chitosan to urea–EDTA mixture was 1:1–1:2. The mixture was shaken manually for 5 min. It turned viscous due to the dissolution of chitosan in acetic acid. The poly(propylene) bottle was then placed in a programmable air oven. The reaction mixture was heated from room temperature to 100 ◦ C at a heating rate of 5 ◦ C per min. The heating was continued for further period of 650 min at 100 ◦ C (urea is soluble in this reaction medium, at room temperature itself. EDTA is sparingly soluble in this mixture at room temperature, but totally soluble at the reaction temperature). It was then allowed to cool to room temperature (35 ◦ C). This product was observed to be brown in colour and was either a loose gel or solution, depending on the weight ratio of the reactants and the concentration of acetic acid. It was poured in to excess methanol with sufficient NH4 OH solution to maintain pH in the range of 8–9. At this pH, the mixture was allowed to stand for 30 min and then it was filtered using suction. The gel was rinsed with methanol to remove soluble components and then allowed to stand in suction for 30 min. The product was dried at 50 ◦ C for 5 h in an air oven and then powdered well. This new material was denoted as ‘CHEDUR’. Yield: 27 g for 15 g of starting material chitosan (44%; the expected yield for 100% reaction is 48.4 g, if one EDTA reacts with one amino group of every repeat unit in chitosan, which is 80% deacetylated, and one urea adds to one EDTA). 2.3. Characterization NMR, in solution, was carried out using Avance spectrometer (Bruker; operating at 500 MHz for 1 H and 125 MHz for 13 C). The solvent used for solution spectra was either HCl/D2 O or D2 O. NMR (13 C solid state) was carried out using a Avance III spectrometer (Bruker; 400 MHz for 1 H 100 MHz for 13 C). Spectral data in the solid state were gathered with the following parameters: sample packed in 4 mm diameter zirconia tube (MAS; spinning rate 10,000 kHz), number of scans = 1024. FTIR was carried out using JASCO 4100 FTIR spectrometer (JASCO, Japan). The solid samples required for the analysis was prepared in the pellet form by mixing 3–5 mg of the sample with 100 mg of dry KBr. ESI Mass spectra were recorded using Q-Tof micro Mass spectrometer (UK). MADLI-MASS was performed using UltrafleXtreme (Bruker). The thermogravimetric analysis (TGA) was carried out with TA Instruments Q500 Hi-Res TGA. The samples were heated at 10 ◦ C min−1 under flowing N2 atmosphere. X-ray diffraction patterns of all the materials were recorded with a Bruker D8 Advance diffractometer equipped ˚ SEM with Cu anode and a Cu K␣ source of wavelength of 1.5406 A. images were obtained using Quanta 450 scanning electron microscope. Elemental analysis was done with Perkin Elmer CHNS/O 2400 elemental analyzer. Surface area measurements were done with Micromeritics ASAP2020, using nitrogen adsorption at liquid nitrogen temperature and applying BET equation for surface area calculation. Prior to the analysis, the sample was degassed at 100 ◦ C for 12 h to remove all the pre-adsorbed gases. 2.4. Water absorption studies Prior to the experiment, all the samples used in the water absorption studies were dried at 50 ◦ C for 5 h. In a typical experiment, 20 mg of sample was weighed (w1 ) into a filter cone. This was immersed into a 100 ml beaker containing 80 ml of distilled
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Fig. 1. FT-IR spectrum of EDUR, CHEDUR and CHI-blank.
water. The weight gain of the sample due to water absorption was measured at different time intervals. After the desired time interval, the filter cone was removed from the beaker and the excess water was allowed to drip down for 5 min. Then the tip of the filter cone was gently swiped one time with dry tissue paper. The weight (w2 ) of the material was noted. The water uptake per g of material is calculated from the formula (w2 − w1 )/w1 g of water per g of material. 3. Results and discussion The reaction between chitosan (which consists of an amino group in every repeat unit), urea and EDTA can be expected to produce a crosslinked material, if the amino groups of the chitosan repeat units react with the carboxylic acid groups of EDTA through condensation. Despite few reports on the formation of conjugate material between chitosan and EDTA, there is no information in the published literature about the nature of the bond formed and the chemistry of this reaction (Bernkop-Schnurch & Krajicek, 1998; ElSharif & Hussain, 2011a; Loretz & Bernkop-Schnuerch, 2006; Singh, Suri, Tiwary, & Rana, 2012b). Hence, it can be surmised that the formation of distinct chemical bonds between chitosan and EDTA has not been reported. However, the crosslinking of chitosan through the interaction of its amino groups with EDTA is possible, since EDTA is a tetra functional acid. The protonation of urea by organic acids resulting in compounds of varying stoichiometry and structure has also been reported (Paleckiene, Sviklas, & Slinksiene, 2005; Radell, Brodman, & Domanski, 1967). Thus, the reaction between urea and citric acid is reported to result in the formation of urea citrate (with a molar ratio 1:1). The reaction of urea with dibasic acid such as HOOC (CH2)n COOH resulted in the formation of a salt when n = 0 and hydrogen bonded complexes for n = 2–5. In all the cases, the stoichiometry was reported to be 2H2 N CO NH2 . HOOC (CH2)n COOH. Based on the above facts, it was expected that a mixture of EDTA and urea could form a hydrogen bonded complex in which at least some of the carboxylic acids groups should be available for crosslinking with chitosan. In the absence of specific details from earlier literature, the expectations were checked by performing reaction between EDTA and urea (control reactions, the details of which are given in Supplementary Data). It was observed that
EDTA and urea react with each other under the reaction conditions employed in this work. The FTIR spectrum of EDTA-urea (EDUR) product is shown in Fig. 1. This exhibited prominent peaks (cm−1 ) at 3437 (a peak at 3442 is observed in urea), 3372 (new), 3200 (new), 3022 (a peak at 3018 is observed in EDTA), 1674 (the urea amide carbonyl is observed at 1678), 1624 (also observed in urea), 1456 (observed in urea), and 1400 (observed at 1395 in EDTA blank), 1320 (EDTA blank), 1167 (new), 1090 (observed in EDTA blank), 911 (EDTA blank), 855 (new), 785 (EDTA) and 700 (EDTA blank). The presence of some of the prominent peaks from EDTA (see the characterization data pertinent to EDTA blank or control reaction as well as unreacted EDTA in Supplementary Data) and urea (see the characterization data pertinent to urea blank or control reaction as well as unreacted urea in Supplementary Data) as well as new peaks in the product confirmed that a reaction had taken place between urea and EDTA. This was not surprising considering that organic acids are known to react with urea (Paleckiene et al., 2005). The proton and 13 C NMR, ESI-MS, MALDI-MASS, PXRD, DSC and TGA of EDUR (Supplementary Data Figs. S1–S9) confirmed, without any ambiguity, that EDTA and urea react to form a methanol soluble product that is amorphous in the solid state. The MALDI-MASS (Supplementary Data Figs. S5 and S6) of the methanol soluble part of EDUR exhibited prominent peaks at m/z values of 644, 727, 781, 1117 and 1491 corresponding to (EDTA)2 urea, (EDTA–urea)2 Na+, (EDTA–urea)2 (urea)NH3 , (urea–EDTA)4 (urea)Na+. The ESI-MS of the product obtained in the control reactions, with and without acetic acid (Supplementary Data Figs. S3 and S4), suggests the formation of oligomers of EDTA and urea with a degree of polymerization between 10 and 18. The variable temperature proton NMR of EDUR (Supplementary Data Fig. S10) indicated that the distinct signals from EDUR, in the range 3–4 ppm, moved to higher chemical shift value at higher temperature (as opposed to merging of signals) and did not merge into fewer signals suggesting that these oligomers may not be EDTA–urea association complexes with weak bonds. Based on the spectroscopic data (especially the ESI MS) and other characterization data the structure of the amorphous methanol soluble product of the reaction between EDTA and urea (EDUR) appears to be an oligomer of EDTA and urea with an “n” value ranging from 10 to 19 (based on the m/z peaks observed at 855, 811 as well as 459), although its exact chemical structure could not be confirmed.
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Fig. 2. Solid state NMR spectrum of CHEDUR (top) and CHI-blank (control).
Although chitosan and EDTA as well as chitosan and urea did not react significantly under the experimental conditions reported here (data shown in the Supplementary Data), a mixture of chitosan with EDTA and urea was observed to react, possibly due to the electrostatic interaction between chitosan and the EDTAUrea adduct. The new material obtained (CHEDUR) was observed to be crosslinked (gel formation and insoluble in a number of common organic solvents, water and acetic acid). It was insoluble in water and hence the 1 H and 13 C NMR of CHEDUR gel was taken in D2 O/HCl mixture. The 1 H NMR spectrum of the gel showed the peaks at 4 ppm (OOC CH2 N ), 3.42 ppm ( N CH2 CH2 N ), 3.3 ppm (H attached to C2 of chitosan) and 2 ppm ( NHCOCH3 ) that are also observed in the case of EDUR (methanol soluble). The solid state NMR spectrum of the new material, CHEDUR, as well as the product obtained in the control reaction [by subjecting chitosan to identical reaction conditions and workup but in the absence of EDTA and urea (CHI-blank)] is shown in Fig. 2. CHEDUR exhibits peaks at 22.5 ( NHCO CH3 ), 49 (from EDTA part), 51.3, 55, 58 (from EDTA), 59.8, 74, 82.5, 83.6, 95.2, 103 (all from chitosan), 160.2 (from H2 N CO NH2 ), 169.1 (from H2 N CO NH3 + ), 171.1 ( CO OH from EDTA), 173.1 ( NH CO CH3 from chitosan), 177.8 ( CO O− from EDTA) ppm, respectively. The peaks are assigned by comparing the spectrum with those of the solid state NMR spectra of chitosan-blank as well as that of CHUR (formed by the reaction between chitosan and urea; Supplementary Data Fig. S11). The new peak at 177.8 ppm in CHEDUR most likely arises from the carboxylate carbonyl of EDTA that in turn is formed due to the protonation of amine groups of chitosan by the carboxylic acid group of EDTA. The peak at 171.1 ppm arises from the EDTA carbonyl groups of free carboxylic acid groups. The new peak at 169.1 ppm could arise out of the urea carbonyl carbons adjacent to the protonated NH2 groups in urea (with the carboxylic acid group in EDTA). The other new peaks at 49 and 58 ppm confirm the presence of EDTA moiety in CHEDUR. The peaks from chitosan backbone are also seen in CHEDUR at 51.3 (C2), 55 (C3), 59.8 (C6), 74 (C5), 82.5, 83.6 (C4), 95.2 (C1 with NHCOCH3 in C2) and 103 (C-1 with NH2 in C2) (106 ppm), respectively (Saito, Tabeta, & Ogawa, 1987). The extent of crosslinking could be determined by solid state NMR since the area under the respective carbons (with the exception of carbonyl carbons) appeared to be quantitative as shown in Fig. 2 (see control experiments CHI-blank and CHUR; Supplementary Data Fig. S11). Based on this data and by comparing the area under the peaks between 48 to 65 ppm (from C2 and C6 of chitosan as well as from EDTA) and 92 to 112 ppm (arising only from the anomeric carbon) the degree of crosslinking is found to be 0.16. The FT-IR spectrum of CHEDUR along with EDUR and chitosan is presented in Fig. 1 while those of the raw materials used in the reaction (EDTA and urea) are presented in Supplementary Data as Fig. S12. The major peaks observed in the case of CHEDUR are
assigned to the following vibrations: the broad band appearing between 3200 and 3500 cm−1 in the case of chitosan is due to the combined absorption due to the stretching of –OH and NH bonds (Zhang et al., 2011). This turns broader in the case of CHEDUR compared to chitosan, which could be due to the presence of EDTA and urea (EDTA shows a broad peak around 3000 cm−1 and urea shows a broad peak between 3400 cm−1 and 3300 cm−1 ; groups that can hydrogen bond extensively). The C H stretching band seen at 2855 cm−1 for chitosan is reduced in intensity in the case of CHEDUR. The prominent carbonyl stretching peaks associated with EDTA (observed at 1700 cm−1 ) and urea (observed at ∼1680 cm−1 ) are seen to disappear although there is some absorption in this region. A new peak is observed for CHEDUR at 1640 cm−1 that can be attributed to the EDUR (amide I of amide carbonyl moiety) present in CHEDUR, while the amide I band specific to –NHCOCH3 of chitosan and observed at 1655 cm−1 is seen to reduce in intensity. The amide II band observed at 1567 cm−1 in the case of chitosan is shifted to 1560 cm−1 in the case of CHEDUR suggesting that the N H could be interacting with EDUR. The peak observed at 1400 cm−1 for CHEDUR as well as EDUR arises from specific C-H bending of the CH2 group (present in EDTA). EDTA and urea show very specific and sharp absorptions at 800 cm−1 that is seen to vanish in the case of CHEDUR. The FT-IR spectrum thus confirms that the product CHEDUR is formed as a result of reaction between chitosan and EDUR (EDTA and urea conjugate). The FTIR spectrum of the materials (CHI-blank, CHED and CHUR) obtained in the control experiments are shown in the Supplementary Data as Fig. S13. The TGA data for CHEDUR and the raw materials used in its preparation are compared in Fig. 3. From this figure it can be inferred that urea (being the least stable among the raw materials) decomposes between 150 and 210 ◦ C and loses almost 80% of its weight while EDTA is stable up to 200 ◦ C and loses around 70% of its weight in the temperature region 230–320 ◦ C. Chitosan is observed to be stable up to 270 ◦ C (ignoring the loss of moisture of around 5% up to 150 ◦ C) and then it loses nearly 35% of its weight between the temperature range 270–320 ◦ C. In comparison, CHEDUR loses mass in two steps, while all the raw materials lose weight in a single step up to 350 ◦ C (if the initial moisture loss up to 150 ◦ C is ignored). The weight loss between 150 and 260 ◦ C, in the first step, arises predominantly from urea (87% of urea decomposes) and to a significant extent from EDTA (22.4% of EDTA decomposes) and the weight loss between 250 and 350 ◦ C, in the second step arises principally from chitosan and EDTA. The residue, left after heating to 900 ◦ C in nitrogen atmosphere for CHEDUR, is 28%. Since chitosan, EDTA and urea leave 37.5%, 10.8% and 0.6% residue, respectively, at 900 ◦ C, it follows that EDTA and urea are incorporated in CHEDUR. The TGA of the products obtained in the control experiments is shown in the Supplementary Data as Fig. S9. The powder XRD (PXRD) pattern of CHEDUR and chitosan are shown in Fig. 4 (the PXRD patterns of EDTA and urea are given in the Supplementary Data Fig. S14). The prominent diffraction peaks for chitosan are observed at 2Â values of 9.43 [d = 9.37 A˚ and (0 2 0)], 20.3 [d = 4.37 A˚ and (1 1 0)] and 29.58 [d = 3.02 A˚ and (1 0 4) of calcite present in chitosan]. The peak widths are observed to be higher than that of EDTA and urea (and are also broad at the base) suggesting that chitosan is semi-crystalline. The observed PXRD patterns are consistent with that reported in the literature for chitin nanofibers [anti parallel crystal pattern of alpha chitin (Kumirska et al., 2010)] and calcite [which exhibits the most prominent diffraction peaks at 2Â = 29 (1 0 4)]. In comparison, CHEDUR exhibits diffraction peaks at 2Â values of 10.44 [d = 8.47 A˚ and (0 2 0)], 20.12 [d = 4.41 A˚ and (1 1 0)], 26.72 and 29.58 [d = 3.02 A˚ and (1 0 4), of calcite present in chitosan]. The peak widths are also larger compared to chitosan and suggest that the new product is less crystalline and essentially amorphous, in comparison with chitosan (as well as EDTA and urea). The shift in the 2Â value for (1 1 0) and (0 2 0) planes in
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341
Fig. 3. TGA (left) and DTG (right) of urea, EDTA, CHEDUR and chitosan.
CHEDUR to a higher value (versus chitosan), indicate the decrease in inter planar distance, possibly due to crosslinking (as the EDTA is a tetra-functional molecule that can crosslink the chitosan with a free amine group in every repeat unit). It can also be inferred from the PXRD of CHEDUR that small molecular reactants (such as EDTA, urea) are absent. This further confirms that the product is not just a physical mixture of chitosan, EDTA and urea but chemically combined. The PXRD analysis of the materials obtained in the control experiments is shown in the Supplementary Data as Fig. S15. Under the reaction conditions employed in this work, no significant and measurable reaction could be observed between chitosan and EDTA as well as chitosan and urea, as confirmed by the results associated with the characterization of the products of the reaction (details presented in the Supplementary Data). The PXRD of chitosan–EDTA conjugate (Supplementary Data; impure due to the presence of unreacted EDTA) suggests the formation of a material similar to CHEDUR. However the PXRD of chitosan–urea reaction (Supplementary Data) results in the formation of a more crystalline material than chitosan. The details of the water absorption of the new materials were studied as described in Section 2. The new material was observed
to form a transparent gel when exposed to water (Supplementary Data Fig. S16). With a view to prepare a material of high water absorption property, the reaction condition was varied to obtain different materials. For this purpose the following variables were optimized: (i) mole ratio of EDTA/chitosan (EDTA/chitosan mole ratio was varied between 0.4 and 10); (ii) mole ratio of EDTA/urea (EDTA/urea mole ratio was varied between 0.137 and 1); and the (iii) concentration and volume of the dissolution medium i.e., acetic acid between 1 and 5% v/v. The results are summarized in Table 1. The maximum water absorption of 570 ± 20 g/g was obtained with the CHI: EDTA: Urea mole ratio of 1:1.11:5.4. The above mole ratio was kept constant and the reaction medium concentration and volume was varied. The results thus obtained are shown in Table 1. Based on these studies, 66 ml of 1% v/v acetic acid per g of chitosan was found to be the most suited for maximum water absorption. The very high water uptake of CHEDUR can be attributed to a combination of factors: the osmotic interaction of water with the ionic bonds present between chitosan and EDUR (similar to what is observed with the sodium salts of crosslinked poly(acrylic acid)) and the change in the morphology from the relatively lower amorphous content to mostly amorphous state (CHEDUR) upon which the glucosamine repeat units with four hydroxyl groups are
Fig. 4. PXRD pattern of CHEDUR and chitosan.
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Table 1 Effect of preparation variables on the water absorption of CHEDUR. CODE
RDAN 42 RDAN 43 RDAN 45 RDAN 48 RDAN 53 RDAN 56 RDAN78 CHI blank a
Composition (CHI:ED:UR) Mole ratioa
EDTA:urea Mole ratio
1: 1.11: 8.1 1: 1.66: 8.1 1: 10: 10 1: 2.0: 9.8 1: 1.11: 5.4 1: 0.4: 1.8 1:1.11:5.4 1:0:0
0.137 0.206 1.00 0.206 0.206 0.222 0.206 0
Acetic acid solution
CHN
% v/v
v/g of chitosan
%C
%H
%N
5 5 5 5 5 1 1 1
100 100 100 100 66 66 66 66
35.1 37.7 35.9 32.1 38.1 38.5 37.7 40.16
6.7 6.3 4.5 5.7 6.2 6.4 6.2 6.7
15.4 11.8 11.6 12.9 11.9 11.1 11.8 6.7
Water uptake (g/g)
47 64 104 176 120 9.2 570 0
The mole ratio for chitosan was taken on the basis of the number of glucosamine repeat units (mass of the sample/molar mass of the glucosamine repeat unit).
available for water absorption in contrast to the crystalline morphology where all the functional groups are buried inside the crystal structure. In contrast, the water uptake in typical diaper material [crosslinked sodium poly(acrylate)]is attributed to the strong electrostatic interaction between the water and the anion present in the crosslinked polymer, in addition to the chemical potential driven migration of the sodium from the polymer matrix to the water phase upon which the osmosis of water is facilitated. The high water uptake is not due to high surface area and capillary action as can be seen from the results shown in the Supplementary Data as Table S1. The results from the control experiments confirm that chitosan, EDTA and urea are necessary for the high water uptake. The nitrogen content of the new material was determined by elemental analysis. This suggests that the extent of water uptake increases as the nitrogen content increases from blank chitosan to chitosan–urea to chitosan–EDTA and finally CHEDUR as given in the Supplementary Data Table S1. The water (distilled) uptake of CHEDUR was compared with a commercial diaper material, and a mixture containing 1:1 weight ratio of CHEDUR and commercial diaper material. For this study, the water absorbing part of ‘Huggies’ baby diaper was used. The results thus obtained are plotted and shown in Fig. 5. It can be inferred from this figure that the rate of water absorption of CHEDUR was the same as that of commercial diaper material in the initial five minutes. In the case of the commercial diaper material the maximum water absorption capacity of 238 g/g is reached within 10 min, following which no further absorption is observed over a 1 h study
Fig. 5. Water uptake versus time for CHEDUR, commercial diaper and a 1:1 mixture.
period. However, CHEDUR shows water absorption over a period of 30 min with a maximum water uptake of 570 g/g, which is nearly 2.3 times that of the commercial diaper material used. The 1:1 mixture (by weight) of commercial diaper material and CHEDUR, follows the expected water uptake rate. The rate of saline water uptake was investigated for CHEDUR as well as the commercial diaper material and the results are shown in Fig. 6. It is clear from this study that CHEDUR is a better water absorber at lower concentration of saline, while it is equivalent to the commercial diaper material at higher saline content.
Fig. 6. Saline water uptake versus time for CHEDUR and commercial diaper.
A. Narayanan, R. Dhamodharan / Carbohydrate Polymers 134 (2015) 337–343
The hydrogel formed upon the absorption of water by the new material exhibits fibrous morphology with the diameter of fibre bundles being ∼250 nm as shown by the SEM picture in the Supplementary Data as Fig. S17. The new material can, in principle, function as a matrix for the slow release of urea as chitosan is known to be biodegradable. It can also be modified by a simple ion exchange reaction to introduce essential micronutrients such as metal ions that can be released to soil on biodegradation. The scope of this work is under detailed investigation. 4. Conclusions A super water-absorbing hydrogel is synthesized from chitosan, a carbohydrate polymer, EDTA and urea. The structure of the crosslinked polymer is characterized by solid state NMR, FTIR, TGA and PXRD. The new polymer is observed to be essentially amorphous in contrast to chitosan, which is semi-crystalline. The high water uptake of this material is attributed to the presence of ionic bonds between chitosan and EDUR as well as due to the change in morphology upon reaction. The new material can play a vital role in applications that demand the rapid absorption and slow release of water, such as agriculture where it can also offer an additional advantage of biodegradation in soil in view of its chitosan backbone. In principle, it can function as a three in one new material for the slow release of urea, water and essential metal ions but this aspect has to be tested thoroughly both in the lab and the field. The new material with rapid and high water uptake could also find potential applications as biodegradable active ingredient of the diaper material. Acknowledgements The first author thanks IIT Madras for supporting his pursuit of research. The last author thanks IIT Madras for financial support and grant of sabbatical leave that enabled the possibility to explore new directions. The Department of Chemistry, IIT Madras is thanked for providing the necessary infrastructure. We thank Shasun Research Centre, Chennai for the solid state NMR spectra. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbpol.2015.08.010 References Bernkop-Schnurch, A., & Krajicek, M. E. (1998). Mucoadhesive polymers as platforms for peroral peptide delivery and absorption: Synthesis and evaluation of different chitosan–EDTA conjugates. Journal of Controlled Release, 50(Copyright (C) 2014 American Chemical Society (ACS). All Rights Reserved), 215–223. Bittelli, M., Flury, M., Campbell, G. S., & Nichols, E. J. (2001). Reduction of transpiration through foliar application of chitosan. Agricultural and Forest Meteorology, 107(3), 167–175. Burke, A., Yilmaz, E., & Hasirci, N. (2000). Evaluation of chitosan as a potential medical iron(III) ion adsorbent. Turkish Journal of Medical Sciences, 30(Copyright (C) 2014 American Chemical Society (ACS). All Rights Reserved), 341–348. Chen, Q., Lu, W.-J., Zhang, W.-Q., Wu, J.-H., & Deng, X. (2005). Nutrient release characteristic of chitosan coated urea and change of penetrability of its coating layer. Huadong Ligong Daxue Xuebao, Ziran Kexueban, 31(Copyright (C) 2014 American Chemical Society ACS. All Rights Reserved), 764–767. Chen, Q., Lu, W., Zhang, W., & Deng, X. (2006). Effect of temperature on nitrogen release characteristic of chitosan-coated urea. Zhiwu Yingyang Yu Feiliao Xuebao, 12(Copyright (C) 2014 American Chemical Society ACS. All Rights Reserved), 727–731. El Hadrami, A., Adam, L. R., El Hadrami, I., & Daayf, F. (2010). Chitosan in plant protection. Marine Drugs, 8(Copyright (C) 2014 American Chemical Society ACS. All Rights Reserved), 968–987.
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