Contents lists available at ScienceDirect

Carbohydrate Polymers

journal homepage: www.elsevier.com/locate/carbpol

Super water absorbing polymeric gel from chitosan, citric acid and urea:Synthesis and mechanism of water absorption

Abathodharanan Narayanan, Ravishankar Kartik, Elanchezhian Sangeetha,Raghavachari Dhamodharan⁎,1

Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India

A R T I C L E I N F O

Keywords:SuperabsorbentMacroporous polymerWater retention in soilChitosanHydrothermal synthesis

A B S T R A C T

A new superabsorbent with maximum water absorption capacity of ∼1250 g/g is prepared by hydrothermalsynthesis from sustainable and biodegradable resources such as chitosan, citric acid and urea (denoted as‘CHCAUR’). CHCAUR is characterized extensively by various analytical techniques such as PXRD, SSNMR, FTIR,and TGA. Pure and saline water absorption study showed that CHCAUR could be a better adsorbent compared tothe super absorbent polymer (SAP) used in commercial diaper material. The mechanism of water absorption isshown to arise out of a combination of electrostatic attraction of water to the ionic crosslinks and the presence ofmacropores as well as undulated surface due to the formation of nanofibrous bundles. When applied to soilCHCAUR was found to decrease water evaporation rate significantly.

1. Introduction

Super absorbing polymers (SAPs) absorb and retain very largequantity of liquid up to several hundred times their own weight(Omidian, Rocca, & Park, 2005) and when the liquid being absorbed iswater they find application in fields such as agriculture, personal hy-giene, etc. There are two main classes of SAPs: synthetic (petrochemicalbased) and natural (biopolymers). The SAPs used in commercial ap-plications are manufactured from non-renewable petrochemical re-sources and especially from acrylic acid and acrylamide based mono-mers (Krul et al., 2000). Every year, several million metric tons ofsynthetic SAP materials are produced and used for different applica-tions. The World’s consumption of SAP is many orders of magnitudemore for personal hygiene and health care than any other application(“Super Absorbent Polymer (SAP) Market: Global Industry Analysis andOpportunity Assessment 2015 – 2020,” n.d.). Hydrogels, a special formof SAPs, exhibit water absorption capacity in the vicinity of 100%, swellin water and retain a significant fraction of water (> 20%) within theirstructure without dissolving (Buchholz & Graham, 1998). The flex-ibility of hydrogels is comparable to natural tissue due to the flexiblemacromolecular component and water content and therefore is gainingimportance as scaffolds in tissue engineering.

The disposal of SAPs after use in landfill causes pollution in theenvironment as they do not biodegrade within reasonable period innature (Zohuriaan-Mehr et al., 2008) and hence there continues to be

interest in developing SAPs that are part synthetic and part natural. Inthis context, the synthesis of SAP material through graft copolymer-ization of synthetic monomers with biopolymers such as cellulose,chitosan, starch, alginates and their derivatives as one of the in-gredients has been reported. These materials by design are not fullybiodegradable, since the grafts such as poly(acrylate) or poly(acryla-mide) or poly(acrylic acid) are not biodegradable (Liu et al., 2017; Liu,Miao, Wang, & Yin, 2009; Omidian, Rocca, & Park, 2006; Ye, Tang,Hong, & Hui, 2016). Among the biopolymers, the preparation of graftcopolymers of chitosan with acrylic acid, acrylates, acrylonitrile andacrylamides acquired significance in applications demanding highwater uptake capacity (Fernández-Gutiérrez et al., 2016; Huacai, Wan,& Dengke, 2006; Mahdavinia, Pourjavadi, Hosseinzadeh, & Zohuriaan,2004; Zhang, Wang, & Wang, 2007). Water absorption capacity in therange 150–650 g/g has been reported. It was shown that chitosan-starch citrate crosslinked polymer exhibited high water uptake in wateras well as saline medium (Salam, Pawlak, Venditti, & El-Tahlawy,2010).

The most important challenge in the area of biodegradable SAPs isto synthesize or fabricate fully biodegradable polymer based super-absorbing materials that would rapidly and reversibly absorb waterwith good mechanical strength. For this purpose biopolymers such asstarch, cellulose and cellulose derivatives, chitosan, and marine poly-saccharides would be most ideal. Among the biopolymers, chitosan ismore promising for application in super water absorbency in view of

https://doi.org/10.1016/j.carbpol.2018.03.028Received 25 October 2017; Received in revised form 13 February 2018; Accepted 12 March 2018

⁎ Corresponding author.

1 This work constitutes a part of patent application No. 2505/CHE/2015.E-mail address: damo@iitm.ac.in (R. Dhamodharan).

Carbohydrate Polymers 191 (2018) 152–160

Available online 14 March 20180144-8617/ © 2018 Published by Elsevier Ltd.

T

several important properties such as biodegradability (Salam et al.,2010; Vårum, Myhr, Hjerde, & Smidsrød, 1997), biocompatibility(Hirano and Noishik., 1985) and bioactivity (Domard, Domard, & Lyon,2002). Chitosan is reported to be haemostatic (Malette, Quigley,Gaines, Johnson, & Rainer, 1983), fungi- and bacteria- static (Strand,Vandvik, Vårum, & Østgaard, 2001). It is also known to support cellproliferation and tissue regeneration (Boucard et al., 2007;Montembault et al., 2006). The preparation of a multi-membranestructured material based on chitosan and sodium alginate without anexternal crosslinker was reported (Ladet, David, & Domard, 2008). Thepresence of an amino group and a primary as well as secondary hy-droxyl groups per repeat unit of chitosan makes it a hydrophilicpolymer (Hamed, Özogul, & Regenstein, 2016; Martínez, Blanco,Davidenko, & Cameron, 2015; Zohuriaan and Kabiri, 2008). An inter-esting recent example demonstrates that a mixture of polysachcharide(comprising of carrageenan and calcium alginate) functions as a fastswelling composite absorbent for saline water with high mechanicalstrength. (Ye et al., 2016). The preparation of superabsorbing materialsfrom polysaccharides and proteins (Ichikawa & Nakajima, 1996) as wellas shellfish waste (Dutkiewicz, 2002; Ichikawa & Nakajima, 1996) havebeen reviewed.

The use of chitosan or chitosan based materials as SAP demands thatthe physicochemical properties of chitosan be studied to take advantageof the delicate balance between the hydrophobic (-NHCOCH3) andhydrophilic (eOH and eNH2) groups (Schatz, Viton, Delair, Pichot, &Domard, 2003; Sorlier, Denuzire, Viton, Domard, & Denuzie, 2001) andoptimize the physical microstructure (Spagnol et al., 2012; Vunain,Mishra, & Mamba, 2016; Zhang, Zeng, & Cheng, 2016). Porous filmsand three dimensional scaffolds of chitosan were prepared by freezedrying its solution in aqueous acetic acid. This resulted in pore size andpore thickness larger than several hundreds of micrometers (Anithaet al., 2014; Esquerdo, Cadaval, Dotto, & Pinto, 2014; Kim et al., 2011;Madihally & Matthew, 1999). Such films and scaffolds exhibited rela-tively less and slow water uptake due to low specific surface area that inturn results in less exposure of water to hydrophilic groups present inthe backbone.

The method of direct crosslinking of polysaccharides and morespecially cellulose derivatives with divinyl sulphone/epichlorohydrin/glyoxal/POCl3/citric acid is well known (Demitri et al., 2008; Sanninoet al., 2003). An interesting recent development in this area is thesynthesis of SAP using citric acid as one of the monomers (Kim, Koo,Kim, Hwang, & Im, 2017). An important recent development as far asusing chitosan as a superabsorbent, is the successful preparation ofnanoporous chitosan films from dioxane, DMSO and aqueous aceticacid, a ternary solvent system, which is noteworthy in many aspects(Wang, Lou, Zhao, & Song, 2016).

Recently, we reported on the synthesis ad characterization of a SAPbased on chitosan, EDTA, and urea (CHEDUR). The maximum waterabsorption capacity of this material was 500 g/g (Narayanan &Dhamodharan, 2015). The high water absorption of CHEDUR wasproposed to result from the presence of macro pores in the pre-dominantly chitosan matrix as well as due to the presence of oligomersof EDTA and urea that were present in smaller quantity and possiblyfunctioned as crosslinks between the chitosan molecules. This study leftus with few unanswered questions such as: can natural multifunctionalacids replace EDTA? the precise mechanism of high water uptake, themechanical properties of the gels (whether they can be tuned), biode-gradability of the product, exploring specific applications. In this work,the synthetic methodology adopted for the preparation of a SAP usingchitosan, urea and citric acid is reported (CHCAUR). The structure ofCHCAUR is elucidated by extensive analytical tools, its water absorp-tion capacity and water retention in soil are also presented in thispaper.

2. Materials and methods

2.1. Materials

Raw chitosan (Mn=48700 Da; 80% deacetylated as determined byFTIR; (Muzzarelli, Tanfani, & Scarpini, 1980)) was purchased from M/s.Matsyafed, Kochi. Citric acid (GR grade), 25% v/v ammonia solution(GR grade), urea (GR grade), and methanol (GR grade) were purchasedfrom M/s. Vijaya Scientific Company, Chennai and used without furtherpurification. Water absorbing material from “Huggies” [Kimbery-ClarkLever Ltd.] commercial baby diaper was used.

2.2. Synthetic methodology for the preparation of CHCAUR and controlreactions

The new material was prepared by reacting hydrothermally chit-osan with citric acid and urea in the weight ratio of 1:2:2 and denotedas CHCAUR. The reaction was performed in a cylindrical stainless steelreactor of volume 900mL that was closed permanently at the bottom.The required quantity of powdered chitosan, citric acid and urea wereadded to the reactor through the top. Then the desired amount of water(66mL/g of chitosan) was added and the mixture was stirred well for10min at room temperature. During this process the solution turnedvery thick forming a highly viscous mass. The pH was found to be be-tween 5 and 6. It was then sealed securely with a circular SS flange anda Viton ‘O’ ring that was placed in between the lid at the top and bottompart of the reactor and tightened. The reactor was then placed in aprogrammable hot air oven and heated. For all the preparations, thesame heating program was used: heating from 35 °C to 100 °C at 5 °Cper min followed by isothermal heating at 100 °C for 650min. After thisduration, the reactor was allowed to cool to room temperature underambient conditions. The product obtained was a gel (pH=7 to 7.5irrespective of the composition of the gel) and this was rinsed with basicmethanol solution (pH 8). This step was repeated several times to re-move the unreacted substances and other byproducts that could bepresent in the gel, as confirmed by analytical methods such as PXRD,SSNMR and FTIR. The gel thus obtained was dried in a hot air ovenmaintained at 50 °C overnight to remove the residual solvent. Theproduct was powdered well using agate mortar and packed in airtightbottle for further studies. The water to chitosan ratio was varied be-tween 200mL/g to 50mL/g to prepare gels of different water absorp-tion capacity. The batch size varied between 3–10 g of chitosan. Thepreparations at the 10 g scale were carried out thirty times and werefound to be reproducible as assessed by PXRD, SSNMR and FTIR.

Three control reactions were performed to understand the forma-tion of CHCAUR. The first one was the reaction between chitosan andcitric acid and the product obtained was denoted as CHCA. The secondwas the reaction between chitosan and urea and the product obtained isdenoted as CHUR. The third control reaction was performed betweencitric acid and urea and the product was denoted as CAUR. For all thesecontrol reactions the reaction composition and conditions were similarto that used in the preparation of CHCAUR and mentioned above.

3. Characterization

Gel permeation chromatography of chitosan was performed usingWaters GPC (two ultra-hydrogel 250 SS columns 30 cm×7.8mm;0.1 N sodium nitrate in 20mL glacial acetic acid and 1000mL water asthe mobile phase, 0.8 mL/min flow rate) equipped with a RI detector.Narrow molecular weight PEG standards were used for calibration.Powder x-ray diffraction pattern was recorded using Bruker D8Advance diffractometer equipped with Cu anode (Cu Kα source of thewavelength of 1.5406 Å) between 5–60° (2&z.Theta;). Differentialscanning calorimetry was performed using TA Instruments Q200 MDSC.The thermal decomposition of all the materials was studied using TAInstruments Q500 Hi-Res TGA. Around 5mg of sample was taken in a Pt

A. Narayanan et al. Carbohydrate Polymers 191 (2018) 152–160

153

pan and heated from 35 to 950 °C at 10 °C per min heating ramp withthe sample purged by nitrogen flow at the rate of 60mL per min. FTIRwas carried out using JASCO 4100 FTIR spectrometer (JASCO, Japan).3–5mg of the sample was mixed with 100mg of dry KBr followed bygrinding to a very fine powder. This solid mixture was then pressed intoa transparent pellet using a hydraulic pelletizer. The spectrum for thispellet was recorded after baseline correction. The wave number regionselected for recording is 4000–400 cm−1. NMR spectroscopic mea-surements in solution were carried out with Bruker spectrometers(operating at 400MHz as well as 500MHz for proton in suitable sol-vents). Solid state NMR measurements were carried out using BrukerAvance III spectrometer (Bruker) operating at 400MHz for 1H and100MHz for 13C. The NMR in the solid state (CPMAS) was taken underthe following conditions: spinning rate 10,000 Hz, repetitiontime=5 s, contact time= 2000 μsec, number of scans= 10000. HRMASS spectrum for unit mass was recorded using Q-Tof micromassmass spectrometer. Low vacuum SEM images were taken using Quanta200 Model SEM with Low Field Detector (acceleration voltage= 30 kV;chamber pressure 100 Pa).

3.1. Water absorption studies

The water/saline absorption study was carried out as described inour earlier work (Narayanan & Dhamodharan, 2015). Around 20mg ofwell powdered (particle size less than 53 μm) and dried sample wasexactly weighted (w1) and taken in a filter cone. This was placed on topof a 100mL beaker filled with distilled water/saline water such that thesample was well immersed inside the liquid. After the desired timeintervals, the filter cone was removed from the beaker and allowed todrain liquid for 5min. After this, the tip of the filter cone was gentlytouched once with dry tissue paper and the weight of swollen gel wasdetermined (w2). The water/saline water absorbed per gram of materialwas calculated from the formula (w2-w1)/w1. This was repeated till thewater/saline water absorption capacity reached saturation.

Water retention in soil was carried out at 35 °C for dry soil samplesloaded with 4 wt% of CHCAUR. Prior to the experiment, the soil samplewas powdered and sieved to get particles of size less than 100 μm. Thiswas dried at 80 °C overnight to remove volatile matter. 19.2 g of driedsoil, 0.8 g of CHCAUR and 30 g of water were taken in a porous pot andmixed well. The weight of this pot containing the mixture was taken(w1). This was kept in a hot air oven maintained precisely at35 °C ± 0.1 °C. After every half an hour time the weight was taken

(w2). The initial water content of the soil was calculated as [30/(19.2+0.8+ 30)]*100=60%. The water content of the soil aftereach half an hour was calculated as [60-(w1-w2)*100].

4. Results and discussion

A crosslinked gel (CHCAUR) is formed when chitosan, citric acidand urea were heated in aqueous medium, inside a closed container, at100 °C. The crosslinking of the gel could arise due to the protonation ofchitosan by citric acid (being a trifunctional acid) resulting in physicalcrosslinks through ionic bonds. In addition, the reaction of chitosanwith urea as well as the product of the reaction between urea and citricacid could also be additional reasons for the formation of a crosslinkednetwork of chitosan. It may be noted that the reaction between citricacid and urea, resulting in the formation of an intermediate product(Paleckiene, Sviklas, & Šlinkšiene, 2005)] and the reaction betweenurea with alcohol in presence of an acid, resulting in the formation ofcarbamate bond (NH3-CO-O-R) (Lei, Wang, & Li, 2009)], have beenreported. Due to the above reactions the viscosity of the medium in-creases and gelation occurs. This phenomenon is noticed visibly whenthe reaction was performed in a semi-transparent polypropylene bottle.During the initial 2–3 h of heating the reaction mixture was a thickmass, which becomes highly viscous liquid in 4–5 h. On further heating,gas evolution is seen by way of frothing in the reaction mixture. After650min of reaction, thick gel was formed with plenty of entrapped gasbubbles as shown in Fig. 1.

The formation of highly porous gel could be due to the evolution ofammonia and carbon dioxide gases as urea is known to react with citricacid forming urea citrate adduct, which decomposes to ammonia andcarbon dioxide (Shaw & Bordeaux, 1955). The formation of carbondioxide during the decomposition of citrate salts under pressure is alsoknown. This interpretation is supported by the increase in the pH of thereaction to 7.2 after the completion of reaction and the conspicuoussmell of ammonia noticed upon opening the reaction vessel to atmo-sphere after the reaction.

To understand the reaction between citric acid and urea the productCAUR formed in one of the control reactions was investigated by massspectrometry. This showed the presence of a mixture of citric acid-ureaadducts of varying molecular weights with m/z values of 400, 592, 783,975… etc., with 192m/z (citric acid) being the difference betweensuccessive peaks (Supplementary Material Fig. S1). In the light of theabove observation and the literature on the formation of urea-citric acid

Fig. 1. Process flow chart representing the preparation of CHCAUR.

A. Narayanan et al. Carbohydrate Polymers 191 (2018) 152–160

154

adducts as well as the known reaction between urea and a hydroxylfunctional group, the following scheme is proposed for CAUR oligomerformation.

The reaction of alcohol with urea to form carbamate as well as or-ganic carbonates (on further reaction with one more alcoholic group)with ammonia evolution has been reported (Shaikh & Sivaram, 1996).Accordingly in the scheme proposed above, the tertiary hydroxyl groupof citric acid reacts with the eNH2 group of urea and forms carbamatebond with liberation of ammonia as shown in the above scheme [Molarmass 235 and molecular formula C7H9NO8]. The carbamate loses onemolecule of ammonia and carbon dioxide upon reaction with a mole-cule of citric acid to form the product with molar mass and m/z value of400 and molecular formula C12H20N2O13 as shown in the scheme. Theaddition of one more molecule of urea and citric acid and the elim-ination of a molecule each of ammonia and carbon dioxide results in thestructure with molar mass and m/z value of 591.1. This chemical re-action process repeats resulting in the formation of species with molarmasses of 783.2 and 973.2 and so on. The formation of the speciesshown in the above scheme is not only supported by mass spectrometrybut also reinforced by NMR spectroscopic data (Supplementary Mate-rial Figs. S2 to S12). The increase of pH and the formation of ammoniagas during the reaction provide additional proof for the proposed re-action pathways. CAUR was found to be highly hygroscopic and theabove structures support this observation.

The structure of the crosslinked polymer formed in the above re-action (CHCAUR) was analyzed by powder x-ray diffraction method.The diffraction pattern of CHCAUR as well as CHCA, CHUR (products

obtained in the control reactions CHCA − chitosan and citric acid,CHUR-chitosan and urea under same conditions of CHCAUR prepara-tion.) and raw chitosan are shown in Fig. 2. Raw chitosan powdershows two distinct peaks at 2&z.Theta; values of 9.31 and 20.35

emerging due to the reflections from (020) and (110) planes as shownin Fig. 2 (i). There is significant decrease in the intensity of the (020)plane in CHCAUR as well as CHCA as seen in Figs. 2 (ii) & (iv). Such adecrease is not noticed in CHUR (Fig. 2 (iii)), which suggests the de-struction of higher order crystallinity in chitosan. The PXRD of CAUR(highly viscous liquid) reveals its amorphous nature (SupplementaryMaterial Fig. S13). In fact, differential scanning calorimetric analysis(Supplementary Material Fig. S14) of CAUR confirmed its amorphousnature as evident from the glass transition temperature of around−32 °C (at the heating rate of 10 °C/min).

The solid state NMR spectrum of CHCAUR, CHCA, and CHUR areshown in Fig. 3. The peak values in ppm are given in Table.1. The peakassignments was done based on literature reference for chitosan (Huanget al., 2015; Tabeta, Division, & Ogawa, 1987).

The solid state NMR spectrum of CHCAUR shows all the char-acteristic peaks of chitosan as indicated in Table 1 and in addition three

A. Narayanan et al. Carbohydrate Polymers 191 (2018) 152–160

155

peaks at 45.54 ppm (from secondary carbon atoms of citric acid),161.21 ppm (carbonyl group of urea) and multiple peaks between177–182 ppm with peak maximum at 178.54 ppm (from the eCOO− of

citric acid in different chemical environments). This confirms the for-mation of adduct involving chitosan, citric acid and urea and that theproduct is stable in the basic methanol medium used to isolate it.

Fig. 2. PXRD patterns of (i) raw chitosan, (ii) CHCA, (iii) CHUR, (iv) CHCAUR.

Fig. 3. Solid state NMR spectrum of (i) CHCA, (ii) CHUR and (iii) CHCAUR.

A. Narayanan et al. Carbohydrate Polymers 191 (2018) 152–160

156

The SSNMR of CHCA shows seven peaks at 23.2 ppm (-CH3),57.3,60.9,74.9,87.3,104.7 ppm, respectively (corresponding to C2,C6,C3&C5,C4,C1 carbon atoms of chitosan repeat unit shown above), and174.03 ppm (-NH-COeCH3), which matches well with the values re-ported in the literature for chitosan (Heux, Brugnerotto, Desbrières,Versali, & Rinaudo, 2000). It is to be noted that even though CHCA ismost likely to be protonated by citric acid during the course of thereaction, no characteristic peaks of pure citric acid (that is 44.3 ppm forsecondary carbon atom) and 76.9 ppm (tertiary carbon atom) was ob-served suggesting that the concentration of citric acid may not beadequate enough to be detected by SSNMR. This could be due to the useof basic methanol treatment used to precipitate the product, which mayhave dissolved the citric acid present as physical crosslinks in CHCA. Inthe case of the other control reactions, the SSNMR of CHUR shows allthe characteristic peaks of chitosan and one additional peak at160.15 ppm, which confirms the presence of urea.

The FT-IR spectrum of CHCAUR, CHCA, CHUR, and raw chitosanare shown in Fig. S15 of the Supplementary Material. The FTIR spectraof CHCAUR and CHCA showed the presence of all the peaks char-acteristic of chitosan, which suggested that the backbone of chitosanremained intact following the reactions. The FTIR of CHUR showed anincrease in intensity of the peak at 1560 cm−1 and a new peak at1115 cm−1, which is attributed to reaction between urea and chitosanin acid medium to form a urethane bond (NH3-COeOeR; bending vi-bration of CeOeC urethane bond linked to chitosan) (Lei et al., 2009).Chitosan repeat unit has one primary alcohol group at C6 and onesecondary alcohol group at C3 position, which can react with ureaunder acid medium to form urethane bonds with liberation of am-monia, which might also be the reason for the increase in pH from 5 to7.5 after the reaction resulting in ammonia evolution. The increase inthe intensity of 1560 cm−1 peak could be due to the formation of ad-ditional amide bonds linked to chitosan. The peak at 1115 cm−1 isabsent in CHCA, which confirms the reaction of urea with chitosan inpresence of acid. The formation of CHCAUR is also confirmed bythermogravimetric analysis (Supplementary Material Fig. S16).

The absorption of water by CHCAUR as well as the SAP materialused in Huggies diaper with time was carried out and the results arepresented in Fig. 4. The water absorption versus time plot for 0.1%sodium chloride solution is shown in Supplementary Material Fig. S17.The presence of ions in water affects the water absorption capacity ofSAPs. This can be quantified by an equation f= 1− (swelling saline/swelling in distilled water) (Guilherme et al., 2015). Accordingly f valuewas calculated for CHCAUR and found to be 0.639 when 0.1% sodiumchloride solution was used. For commercial diaper material this value is0.293 suggesting that CHCAUR could be a substitute for commercialSAPs used in diapers. The study shows that the water absorption ca-pacity of CHCAUR was about eight times compared to the SAP used incommercial diaper and about 2.5 times that reported earlier by us forCHEDUR (Narayanan & Dhamodharan, 2015). The higher water ab-sorption capacity of CHCAUR might arise due to the presence of phy-sical crosslinks of CAUR connecting the chitosan molecules as shownbelow.

Table 1SSNMR peak assignments for CHCA, citric acid, urea, CHUR and CHCAUR

Peak 2 and 10 of CHCAUR (highlighted) are assigned to the eCH2eCH2e and eCOO− group of citric acid incorporated into chitosan polymer chainrespectively.

Fig. 4. Water absorption versus time for CHCAUR, commercial diaper material andCHCAUR after extraction with sodium hydroxide.

A. Narayanan et al. Carbohydrate Polymers 191 (2018) 152–160

157

This structure provides a number of ionic sites by way of chargedoxygen and nitrogen atoms in a rigid polymer backbone structure,which should also enable higher water absorption by electrostatic hy-drogen bonding in addition to the porous structure of CHCAUR.

The higher water absorption capacity could also arise out of themacro porous structure of CHCAUR. Hence the morphology of CHCAURwas assessed by SEM. This confirmed the presence of macro poressurrounded by a fibrous network of chitosan molecules forming anundulated surface as shown in Fig. 5. The extent of water absorptionarising out of the presence of physical crosslinks consisting of electro-valent bonds as well as that arising out of the morphology could also bequantitated. Thus CHCAUR was extracted with sodium hydroxide in 1:1water: methanol followed by repeated rinsing with methanol and dried.The powder formed in this process confirmed the removal of CAUR

from CHCAUR (Supplementary Material Figs. S18 and S19) suggestingthat CAUR is present as physical ionic crosslinks. The water absorptioncapacity of CHCAUR decreased from 1250 g/g to 500 g/g upon thisstep, which suggested that 40% of water absorption arises from mor-phology related effect and the rest was from the physical crosslinksconsisting of CAUR. The SEM image of CHCAUR after extraction withsodium hydroxide suggested that there was no significant change inmorphology after the extraction of CAUR (Supplementary Material Fig.S20).

The SEM picture (insert No.7 in Fig. 5) was processed by ImageJsoftware. This software is used for calculating the mean pore diameter,standard deviation and% porosity. The results showed that theCHCAUR gel mean pore diameter was 0.359mm and the standard de-viation was 0.134. The porosity was calculated to be 27.61%.

The water absorption capacity of CHCAUR could be varied by finetuning the reaction parameters. One such is the quantity of water usedin the preparation of CHCAUR. Upon increasing the quantity of waterwith respect to the mass of chitosan (200mL/g), the water absorptioncapacity could be reduced thus enabling the formation of CHCAURsuitable for the preparation of hydrogel.

The application of CHCAUR in water conservation was tested byperforming moisture retention of soil loaded with CHCAUR. The use ofsuper absorbents in agriculture as well as horticulture for water con-servation is known for many years, and many extensive studies havebeen reported (Dabhi, Bhatt, & Pandit, 2013). Chitosan, the majorcomponent of CHCAUR, is used in agriculture in seed protection, asanti-microbial agent, and as disease resistant additive to soil (ElHadrami, Adam, El Hadrami, & Daayf, 2010). Further, with 11%

Fig. 5. SEM images of CHCAUR at different magnifications− (1) 1mm (2) 500 μm (3) 200 μm (4) 100 μm (5) 50 μm (6) 10 μm (7) 3mm. The image generated by ImageJ software and gelporosity histogram are shown in (8) and (9), respectively.

A. Narayanan et al. Carbohydrate Polymers 191 (2018) 152–160

158

nitrogen content, CHCAUR can also serve as a multipurpose material inagriculture and especially as controlled releasing agent of micro andmacro nutrients to soil. Therefore a detailed study was performed onthe effect of addition of 4% by weight CHCAUR to soil on the waterretention capacity at 35 °C. The result is presented in Fig. 6. The initialmoisture content was 30% for soil with 4% CHCAUR as well as control(soil without CHCAUR). After 1 h of heating, the soil with CHCAURretained 25% of moisture while the control could retain only 10%. After5 h heating, the control retained only 2% moisture while it took nearly9 h of heating before the soil with CHCAUR could reach that stage. Thisexperiment shows that the water evaporation rate from soil is sloweddown considerably in the presence of CHCAUR even at a low dosage of4%.

These experiments suggested that CHCAUR could find application inhousehold potted plant growth wherein daily watering is required. Itsapplication at a predefined level with soil could reduce the number ofwatering cycles thereby conserving water. This was observed by us forthe growth of chilly plant (data not shown) wherein a 5% loading ofCHCAUR to soil reduced the frequency of watering to once in everythree days.

5. Conclusion

We report the preparation and characterization of a new superwater absorbent material denoted as CHCAUR, from renewable sourcessuch as chitosan, citric acid and urea by the hydrothermal synthesis.CHCAUR was found to absorb a maximum of around 1250 g/g of dis-tilled water and 210 g/g of 0.1% sodium chloride solution, which issuperior to the absorption of SAP used in Huggies brand baby diapermaterial. The water absorption is shown to arise out of the physicalcrosslinks consisting of electrovalent bonds, macro pores and morpho-logical features. CHCAUR is observed to contain a very large proportionof chitosan and a small proportion of CAUR and therefore it is likely tobiodegrade in soil to a very large extent as observed in preliminaryexperiments (results not presented here). CHCAUR when blended withsoil even in low quantity delays the water evaporation from soil.CHCAUR can be a promising multipurpose soil additive, in view of thecitric acid component, to supply micro and macro nutrients to soil withcontrolled release of water.

Acknowledgments

The authors wish to thank the Department of Materials andMetallurgical Engineering for the electron microscopy facilities andespecially Prof. S. Sankaran. Abathodharanan Narayanan thanks IITMadras for enabling him to register for Ph.D. This work was madepossible due to the support of IIT Madras.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in theonline version, at https://doi.org/10.1016/j.carbpol.2018.03.028.

References

Anitha, A., Sowmya, S., Kumar, P. T. S., Deepthi, S., Chennazhi, K. P., Ehrlich, H., ...Jayakumar, R. (2014). Chitin and chitosan in selected biomedical applications.Progress in Polymer Science, 39(9), 1644–1667. http://dx.doi.org/10.1016/j.progpolymsci.2014.02.008.

Boucard, N., Viton, C., Agay, D., Mari, E., Roger, T., Chancerelle, Y., & Domard, A. (2007).The use of physical hydrogels of chitosan for skin regeneration following third-degreeburns. Biomaterials, 28(24), 3478–3488. http://dx.doi.org/10.1016/j.biomaterials.2007.04.021.

Buchholz, F., & Graham, A. (1998). Modern superabsorbent polymer technology. NEWYORK: Wiley − VCHhttp://dx.doi.org/10.1002/cite.330700824.

Dabhi, R., Bhatt, N., & Pandit, B. (2013). Super absorbent polymers – An innovative watersaving technique for optimizing crop yield International Journal of InnovativeResearch in Science. Engineering and Technology, 2(10), 5333–5340.

Demitri, C., Del Sole, R., Scalera, F., Sannino, A., Vasapollo, G., Maffezzoli, A., ... Nicolais,L. (2008). Novel superabsorbent cellulose-based hydrogels crosslinked with citric acid.Wiley InterScience.

Domard, A., Domard, M., & Lyon, B. (2002). Chitosan: Structure –Properties relationshipand biomedical applications. Polymeric Biomaterials. http://dx.doi.org/10.1201/9780203904671.

Dutkiewicz, J. K. (2002). Superabsorbent materials from shellfish waste – A review.Journal of Biomedical Materials Research, 63(3), 373–381. http://dx.doi.org/10.1002/jbm.10231.

El Hadrami, A., Adam, L. R., El Hadrami, I., & Daayf, F. (2010). Chitosan in plant pro-tection. Marine Drugs, 8(4), 968–987. http://dx.doi.org/10.3390/md8040968.

Esquerdo, V. M., Cadaval, T. R. S., Dotto, G. L., & Pinto, L. A. A. (2014). Chitosan scaffoldas an alternative adsorbent for the removal of hazardous food dyes from aqueoussolutions. Journal of Colloid and Interface Science, 424, 7–15. http://dx.doi.org/10.1016/j.jcis.2014.02.028.

Fernández-Gutiérrez, M., Fusco, S., Mayol, L., San Román, J., Borzacchiello, A., &Ambrosio, L. (2016). Stimuli-responsive chitosan/poly (N-isopropylacrylamide)semi-interpenetrating polymer networks: Effect of pH and temperature on theirrheological and swelling properties. Journal of Materials Science: Materials in Medicine,27(6), http://dx.doi.org/10.1007/s10856-016-5719-0.

Guilherme, M. R., Aouada, F. A., Fajardo, A. R., Martins, A. F., Paulino, A. T., ... Davi, M.F. T., Muniz, E. C. (2015). Superabsorbent hydrogels based on polysaccharides forapplication in agriculture as soil conditioner and nutrient carrier: A review. EuropeanPolymer Journal, 72, 365–385. http://dx.doi.org/10.1016/j.eurpolymj.2015.04.017.

Hamed, I., Özogul, F., & Regenstein, J. M. (2016). Industrial applications of crustaceanby-products (chitin, chitosan, and chitooligosaccharides): A review. Trends in FoodScience and Technology, 48, 40–50. http://dx.doi.org/10.1016/j.tifs.2015.11.007.

Heux, L., Brugnerotto, J., Desbrières, J., Versali, M. F., & Rinaudo, M. (2000). Solid stateNMR for determination of degree of acetylation of chitin and chitosan.Biomacromolecules, 1(4), 746–751. http://dx.doi.org/10.1021/bm000070y.

Hirano, S., & Noishik, Y. (1985). The blood compatibility of chitosan and. Journal ofBiomedical Materials Research, 19, 413–417.

Huacai, G., Wan, P., & Dengke, L. (2006). Graft copolymerization of chitosan with acrylicacid under microwave irradiation and its water absorbency. Carbohydrate Polymers,66(3), 372–378. http://dx.doi.org/10.1016/j.carbpol.2006.03.017.

Huang, Y., He, M., Lu, A., Zhou, W., Stoyanov, S. D., Pelan, E. G., & Zhang, L. (2015).Hydrophobic modification of chitin whisker and its potential application in struc-turing oil. Langmuir, 31(5), 1641–1648. http://dx.doi.org/10.1021/la504576p.

Ichikawa, T., & Nakajima, T. (1996). Superabsorptive polymers (from natural poly-saccharides and peptides). Polymeric materials encyclopedia. Boca Raton (Florida:Salamone8051–8059.

Kim, J., Cai, Z., Lee, H. S., Choi, G. S., Lee, D. H., & Jo, C. (2011). Preparation andcharacterization of a Bacterial cellulose/Chitosan composite for potential biomedicalapplication. Journal of Polymer Research, 18(4), 739–744. http://dx.doi.org/10.1007/s10965-010-9470-9.

Kim, H. J., Koo, J. M., Kim, S. H., Hwang, S. Y., & Im, S. S. (2017). Synthesis of superabsorbent polymer using citric acid as a bio-based monomer. Polymer Degradation andStability, 144, 128–136. http://dx.doi.org/10.1016/j.polymdegradstab.2017.07.031.

Krul, L. P., Nareiko, E. I., Matusevich, Y. I., Yakimtsova, L. B., Matusevich, V., & Seeber,W. (2000). Water super absorbents based on copolymers of acrylamide with sodiumacrylate. Polymer Bulletin, 45, 159–165. http://dx.doi.org/10.1007/PL00006832.

Ladet, S., David, L., & Domard, A. (2008). Multi-membrane hydrogels. Nature, 452(7183),76–79. http://dx.doi.org/10.1038/nature06619.

Lei, C., Wang, Q., & Li, L. (2009). Effect of interactions between poly(vinyl alcohol) andurea on the water solubility of poly(vinyl alcohol). Journal of Applied Polymer Science,114(1), 517–523. http://dx.doi.org/10.1002/app.30504.

Liu, Z., Miao, Y., Wang, Z., & Yin, G. (2009). Synthesis and characterization of a novelsuper-absorbent based on chemically modified pulverized wheat straw and acrylicacid. Carbohydrate Polymers, 77(1), 131–135. http://dx.doi.org/10.1016/j.carbpol.2008.12.019.

Liu, T. G., Wang, Y. T., Guo, J., Liu, T. B., Wang, X., & Li, B. (2017). One-step synthesis ofcorn starch urea based acrylate superabsorbents. Journal of Applied Polymer Science,134(32), 1–10. http://dx.doi.org/10.1002/app.45175.

Madihally, S. V., & Matthew, H. W. T. (1999). Porous chitosan sca!olds for tissue

Fig. 6. Moisture content in soil versus time, with and without CHCAUR (4 wt%) heated at35 °C.

A. Narayanan et al. Carbohydrate Polymers 191 (2018) 152–160

159

engineering. Biomaterials, 20(November 1998), 1133–1142.Mahdavinia, G. R., Pourjavadi, A., Hosseinzadeh, H., & Zohuriaan, M. J. (2004). Modified

chitosan 4. Superabsorbent hydrogels from poly(acrylic acid-co-acrylamide) graftedchitosan with salt- and pH-responsiveness properties. European Polymer Journal,40(7), 1399–1407. http://dx.doi.org/10.1016/j.eurpolymj.2004.01.039.

Malette, W. G., Quigley, H. J., Gaines, R. D., Johnson, N. D., & Rainer, W. G. (1983).Chitosan: A new hemostatic. Annals of Thoracic Surgery, 36(1), 55–58. http://dx.doi.org/10.1016/S0003-4975(10)60649-2.

Martínez, A., Blanco, M. D., Davidenko, N., & Cameron, R. E. (2015). Tailoring chitosan/collagen scaffolds for tissue engineering: Effect of composition and different cross-linking agents on scaffold properties. Carbohydrate Polymers, 132, 606–619.

Montembault, A., Tahiri, K., Korwin-Zmijowska, C., Chevalier, X., Corvol, M. T., &Domard, A. (2006). A material decoy of biological media based on chitosan physicalhydrogels: Application to cartilage tissue engineering. Biochimie, 88(5), 551–564.http://dx.doi.org/10.1016/j.biochi.2006.03.002.

Muzzarelli, R. A. A., Tanfani, F., & Scarpini, G. (1980). The degree of acetylation of chitinsby gas chromatography infrared spectrometry chitin/chitosan, Vol. 2, 299–306.

Narayanan, A., & Dhamodharan, R. (2015). Super water-absorbing new material fromchitosan, EDTA and urea. Carbohydrate Polymers, 134, 337–343. http://dx.doi.org/10.1016/j.carbpol.2015.08.010.

Omidian, H., Rocca, J. G., & Park, K. (2005). Advances in superporous hydrogels. Journalof Controlled Release, 102(1), 3–12. http://dx.doi.org/10.1016/j.jconrel.2004.09.028.

Omidian, H., Rocca, J. G., & Park, K. (2006). Elastic, superporous hydrogel hybrids ofpolyacrylamide and sodium alginate. Macromolecular Bioscience, 6(9), 703–710.http://dx.doi.org/10.1002/mabi.200600062.

Paleckiene, R., Sviklas, A., & Šlinkšiene, R. (2005). Reaction of urea with citric acid.Russian Journal of Applied Chemistry, 78(10), 1651–1655.

Salam, A., Pawlak, J. J., Venditti, R. A., & El-Tahlawy, K. (2010). Synthesis and char-acterization of starch citrate-chitosan foam with superior water and saline absor-bance properties. Biomacromolecules, 11(6), 1453–1459. http://dx.doi.org/10.1021/bm1000235.

Sannino, A., Esposito, A., De Rosa, A., Cozzolino, A., Ambrosio, L., & Nicolais, L. (2003).Biomedical application of a superabsorbent hydrogel for body water elimination inthe treatment of edemas. Journal of Biomedical Materials Research. Part A, 67(3),1016–1024. http://dx.doi.org/10.1002/jbm.a.10149.

Schatz, C., Viton, C., Delair, T., Pichot, C., & Domard, A. (2003). Typical physicochemicalbehaviors of chitosan in aqueous solution. Biomacromolecules, 4(3), 641–648. http://dx.doi.org/10.1021/bm025724c.

Shaikh, A.-A. G., & Sivaram, S. (1996). Organic carbonates. Chemical Reviews, 96(3),951–976. http://dx.doi.org/10.1021/cr950067i.

Shaw, W. H. R., & Bordeaux, J. J. (1955). The decomposition of urea in aqueous media.Journal of the American Chemical Society, 77(1953), 4729–4733. http://dx.doi.org/10.

1021/ja01623a011.Sorlier, P., Denuzire, A., Viton, C., Domard, A., & Denuzie, A. (2001). Relation between

the degree of acetylation and the electrostatic properties of chitin and chitosan re-lation between the degree of acetylation and the electrostatic properties of chitin andchitosan. Society, 2(3), 765–772. http://dx.doi.org/10.1021/bm015531+.

Spagnol, C., Rodrigues, F. H. A., Pereira, A. G. B., Fajardo, A. R., Rubira, A. F., & Muniz, E.C. (2012). Superabsorbent hydrogel composite made of cellulose nanofibrils andchitosan-graft-poly(acrylic acid). Carbohydrate Polymers, 87(3), 2038–2045. http://dx.doi.org/10.1016/j.carbpol.2011.10.017.

Strand, S. P., Vandvik, M. S., Vårum, K. M., & Østgaard, K. (2001). Screening of chitosansand conditions for bacterial flocculation. Biomacromolecules, 2(1), 126–133. http://dx.doi.org/10.1021/bm005601x.

Super Absorbent Polymer (SAP) Market: Global Industry analysis and opportunity as-sessment 2015–2020. (n.d.). Retrieved December 16, 2017, from https://www.futuremarketinsights.com/reports/super-absorbent-polymer-market.

Tabeta, R., Division, B., & Ogawa, K. (1987). High-resolution solid-state 13C NMR studyof chitosan and its salts with acids. Macromolecules, 20, 2424–2430.

Vårum, K. M., Myhr, M. M., Hjerde, R. J. N., & Smidsrød, O. (1997). In vitro degradationrates of partially N-acetylated chitosans in human serum. Carbohydrate Research,299(1), 99–101.

Vunain, E., Mishra, A. K., & Mamba, B. B. (2016). Dendrimers, mesoporous silicas andchitosan-based nanosorbents for the removal of heavy-metal ions: A review.International Journal of Biological Macromolecules, 86, 570–586. http://dx.doi.org/10.1016/j.ijbiomac.2016.02.005.

Wang, X., Lou, T., Zhao, W., & Song, G. (2016). Preparation of pure chitosan film usingternary solvents and its super absorbency. Carbohydrate Polymers, 153, 253–257.http://dx.doi.org/10.1016/j.carbpol.2016.07.081.

Ye, Z., Tang, M., Hong, X., & Hui, K. S. (2016). Sustainable composite super absorbentsmade from polysaccharides. Materials Letters, 183, 394–396. http://dx.doi.org/10.1016/j.matlet.2016.07.067.

Zhang, J., Wang, L., & Wang, A. (2007). Preparation and properties of chitosan- g –poly(acrylic acid)/montmorillonite superabsorbent nanocomposite via in situ intercalativepolymerization. 2497–2502.

Zhang, L., Zeng, Y., & Cheng, Z. (2016). Removal of heavy metal ions using chitosan andmodified chitosan: A review. Journal of Molecular Liquids, 214, 175–191. http://dx.doi.org/10.1016/j.molliq.2015.12.013.

Zohuriaan, M. J., & Kabiri, K. (2008). Superabsorbent polymer materials: A review.Iranian Polymer Journal, 17(6), 451–477.

Zohuriaan-Mehr, M. J. M., Kabiri, K., Zohuriaan, M. J., Kabiri, K., Zohuriaan-Mehr, M. J.M., Kabiri, K., ... Kabiri, K. (2008). Superabsorbent polymer materials: A review.Iranian Polymer Journal, 17(6), 451–477. https://doi.org/http://journal.ippi.ac.ir.

A. Narayanan et al. Carbohydrate Polymers 191 (2018) 152–160

160

本文献由“学霸图书馆-文献云下载”收集自网络，仅供学习交流使用。

学霸图书馆（www.xuebalib.com）是一个“整合众多图书馆数据库资源，

提供一站式文献检索和下载服务”的24 小时在线不限IP

图书馆。

图书馆致力于便利、促进学习与科研，提供最强文献下载服务。

图书馆导航：

图书馆首页 文献云下载 图书馆入口 外文数据库大全 疑难文献辅助工具