preparation, swelling, and stimuli-responsive characteristics ......the gel strength greatly...

Research Article

1602

Received: 3 October 2009, Revised: 24 November 2009, Accepted: 8 December 2009, Published online in Wiley Online Library: 8 January 2010

(wileyonlinelibrary.com) DOI: 10.1002/pat.1647

Preparation, swelling, and stimuli-responsivecharacteristics of superabsorbentnanocomposites based on carboxymethylcellulose and rectorite

Wenbo Wanga,b and Aiqin Wanga*

Utilization of naturally available raw materials for th

Polym. Adv

e fabrication of eco-friendly functional materials has long beendesired. In this work, a series of superabsorbent nanocomposites were prepared by radical solution copolymerizationof sodium carboxymethyl cellulose (CMC), partially neutralized acrylic acid (NaA), and rectorite (REC) in the presenceof initiator ammonium persulfate (APS) and crosslinker N,N’-methylene-bis-acrylamide (MBA). The optimal reactionvariables including themass ratio of acrylic acid (AA) to CMC,MBA concentration, and REC contentwere explored. FTIRspectra confirmed that NaA had been grafted onto CMC and REC participated in polymerization. REC was exfoliatedand uniformly dispersed in the CMC-g-PNaA matrix without agglomeration as shown by XRD, TEM, and SEM analysis.The thermal stability, swelling capabilities, and rate of the nanocomposites were improved after introducing REC, andthe gel strength greatly depended on the concentration of crosslinker MBA. The nanocomposite showed excellentresponsive properties and reversible On–Off switching characteristics in various saline, pH, and hydrophilic organicsolvent/water solutions, which provided great possibility to extend the application domain of the superabsorbentnanocomposites such as drug delivery system. Copyright � 2010 John Wiley & Sons, Ltd.

Keywords: carboxymethyl cellulose; rectorite; superabsorbent nanocomposite; swelling; stimuli-responsive

* Correspondence to: A. Wang, Center for Eco-material and Green Chemistry,Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou,730000, P.R. China.E-mail: [email protected]

a W. Wang, A. Wang

Center for Eco-material and Green Chemistry, Lanzhou Institute of Chemical

Physics, Chinese Academy of Sciences, Lanzhou, 730000, P.R. China

b W. Wang

Graduate University of the Chinese Academy of Sciences, Beijing, 100049, P.R.

China

Contract/grant sponsor: Western Action Project of CAS; contract/grant num-

ber: KGCX2-YW-501.

Contract/grant sponsor: 863 Project of the Ministry of Science and Technology,

P.R. China; contract/grant number: 2006AA03Z0454, 2006AA100215.

INTRODUCTION

Organic/inorganic nanocomposites are functional materialsderived from the nanoscale modification of fully organicpolymers by the incorporation of inorganic nano- or micro-particles.[1] Polymer/clay hybrids are representative nanocompo-sites because they frequently exhibit excellent performancesuperior to those of the components, cost-efficient character-istics, and extended applications.[2,3] Superabsorbents arefunctional materials with a unique 3-D hydrophilic networkstructure and highly water-swellable property, which have raisedcontinuous interest and have found extensive application inmany fields such as hygienic products,[4] agriculture,[5,6] waste-water treatment,[7,8] coal dewatering,[9] catalyst supports,[10]

communication cable,[11] and drug-delivery systems,[12] etc.However, these applications encounter some limitations becausethe superabsorbents are mainly petroleum-based syntheticpolymers with high production costs and poor environmentalcharacteristics.[13] Thus, new types of multi-component super-absorbents by introducing naturally available raw materials asadditives have long been desired.Recently, polysaccharide-based materials have attracted

considerable interests because they show low-cost, renewable,biodegradable, non-toxic, and biocompatible character-istics.[14–16] The composites or nanocomposites of polysacchar-ides with abundant, cost-efficient, and eco-friendly inorganic clayminerals also exhibited outstanding hybrid performance andhave been honored as one of the material families ‘‘in greening

. Technol. 2011, 22 1602–1611 Copyright � 2

the 21st century materials world’’.[17] Thus, considerable effortshave been made to develop novel superabsorbent by thecompounding of polysaccharide with clay minerals.[18–21] Amongnumerous polysaccharides, cellulose is potentially useful becauseit is the most abundant natural polymer with excellentbiodegradability and biocompatibility. However, the poorsolubility and reactivity of cellulose make it difficult to directlymodify and produce other useful materials. Carboxymethylcellulose (CMC) is a representative cellulose derivative withcarboxymethyl groups (-CH2-COONa) bound to some of thehydroxyl groups on the cellulose backbone; it can be easilyproduced by the alkali-catalyzed reaction of cellulose with

010 John Wiley & Sons, Ltd.

STIMULI-RESPONSIVE NANOCOMPOSITE BASED ON CMC AND REC

chloroacetic acid. The polar carboxyl groups render the cellulosesoluble, chemically reactive, and strongly hydrophilic, and so theapplication of CMC in superabsorbent fields becomes attractiveand promising.[22]

Rectorite (REC) is a particular claymineral. It is a sort of regularlyinterstratified clay mineral with alternate pairs of a non-expansible dioctahedral mica-like layer and an expansibledioctahedral smectite-like layer existing in a 1:1 ratio.[23] REChas a water-swelling property similar to that of montmorillonite,and it is possible to formmonolithic REC layers with the thicknessof a single REC platelet, about 2 nm, and to prepare nano-composites by exfoliation and intercalation techniques.[24] It isexpected that the type and number of hydrophilic groups, as wellas the network structure of the superabsorbents, could beimproved by the nanoscale composite of CMC with REC clay.Based on the above background, in this work, the series of

sodium carboxymethyl cellulose-g-poly(sodium acrylate)/rector-ite (CMC-g-PNaA/REC) superabsorbent nanocomposites wereprepared by the nanoscale combination of CMC and REC. Thestructure and morphologies of the developed nanocompositeswere characterized by Fourier Transform Infrared Spectroscopy(FTIR), X-ray Diffraction (XRD), Transmission Electron Microscopy(TEM), Scanning Electron Microscopy (SEM), and Thermogravi-metric Analysis (TGA) techniques. In addition, the swellingproperties and stimuli-responsive behaviors of the nanocompo-sites were evaluated systematically.

1

EXPERIMENTAL

Materials

Sodium carboxymethyl cellulose (CMC, CP, 300� 800mPa � S(25 g/L, 258C)) was from Sinopharm Chemical Reagent Co., Ltd,China. Acrylic acid (AA, chemically pure, Shanghai ShanpuChemical Factory, Shanghai, China) was distilled under reducedpressure before use. REC micropowder (Mingliu Colloidal Co.,Hubei, China) was milled and passed through a 320-mesh screenprior to use. Ammonium persulfate (APS, Analytical grade, Xi’anChemical Reagent Factory, China) and N,N’methylene-bis-acrylamide (MBA, Chemically Pure, Shanghai ChemicalReagent Corporation, China) were used as received. The otherreagents used were of analytical grade and all the solutions wereprepared with distilled water.

Preparation of CMC-g-PNaA/REC nanocomposites

CMC (1.20 g) was dissolved in 30ml distilled water in a 250mlfour-necked flask equipped with a mechanical stirrer, athermometer, a reflux condenser, and a nitrogen line. Theresultant solution was heated in an oil bath at 608C and purgedwith N2 for 30min to remove oxygen. Then, an aqueous solution(5ml) of the initiator APS (0.1008 g) was charged to the flaskunder continuous stirring and kept at 608C for 10min. Aftercooling to 508C, a solution of 7.2 g of AA neutralized by 8ml8.8mol/L NaOH solution, crosslinker MBA (21.6mg), and RECpowder (0.45 g) was added, and the temperature was slowlyraised to 708C, and maintained for 3 hr to complete polymeri-zation. Continuous purging of nitrogen was used throughout thereaction period. The gel products were dried to a constant massat 708C and ground and passed through 40–80 mesh sieve(180� 380mm).

Polym. Adv. Technol. 2011, 22 1602–1611 Copyright � 2010 John Wil

Measurements of equilibrium water absorption andswelling kinetics

0.05 g of superabsorbent particle was immersed in an aqueoussolution at room temperature for 4 hr to reach swellingequilibrium. The swollen gels were filtered using a 100-meshsieve and then drained for 10min until no free water remained.After weighing the swollen gels, the equilibriumwater absorption(Qeq, g/g) was derived from the mass change of samples beforeand after swelling.Swelling kinetics of superabsorbents in aqueous solutions was

measured as follows: 0.05 g samples were placed in 200mlaqueous solution for set intervals. The swollen gels were filteredby a sieve and then the water absorption at a given time wascalculated according to the mass changes of sample before andafter swelling. In all cases three parallel samples were used andthe averages are reported in this paper.

Evaluation of stimuli-responsivity

NaCl, MgCl2, and AlCl3 solutions with various molar concen-trations (0.01, 0.1, 1, 10, 20, 30, and 100mmol/L) and mixtures ofmethanol, ethanol, and acetone/water with various volumeconcentrations (5, 10, 30, 40, 50, 60, 70, 80, and 90 v/v%) wereprepared by dissolving calculated amounts of the correspondingsolutes in distilled water. Buffer solutions with various pHs wereprepared by combining KH2PO4, K2HPO4, H3PO4, and NaOHsolution. Ionic strengths of all the buffer solutions were adjustedto 0.1M with NaCl solution. The pH values were determined bya pH meter (DELTA-320). The equilibrium water absorption (Qeq)in each solution was determined by a method similar to that indistilled water. The saline-, pH-, and hydrophilic organicsolvent-reversibility of the nanocomposite were investigated interms of its swelling and deswelling in each media. Typically, thesample particle (0.05 g, 180� 380mm) was placed in a 100meshsieve and adequately contacted with 100mmol/L salinesolutions, pH 2.0 buffer solution, or 90 v/v% hydrophilic organicsolvents/water mixtures until equilibrium was reached. Then, theswollen samples were soaked in 0.01mmol/L saline solutions, pH7.2 buffer solutions, or 5 v/v% hydrophilic organic solvents/watermixtures for set time intervals. The swollen samples were filtered,weighed, and the water absorption of nanocomposite at a givenmoment was calculated according to themass change of samplesbefore and after swelling. The time interval is 15min for eachcycle, and the same procedure was repeated for five cycles. Afterevery measurement, each solution was renewed.

Determination of gel strength

The gel strength of the swollen nanocomposites was determinedwith the Physica MCR 301 rheometer (Germany) at 258Caccording to a previously developed method.[25] The testedsamples were fully swollen in 0.9wt% NaCl solution. The strainamplitude was chosen as 0.5% and the angular frequency v wasdefined in the range of 0.1� 100 rad/sec. The results were theaverage of at least three measurements.

Characterizations

FTIR spectra were recorded on a Nicolet NEXUS FTIR spec-trometer in 4000–400 cm�1 region using KBr pellets. XRDanalyses were performed using a X-ray power diffractometerwith Cu anode (PAN analytical Co. X’pert PRO), running at 40 kV

ey & Sons, Ltd. wileyonlinelibrary.com/journal/pat

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W. WANG AND A. WANG

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and 30mA, scanning from 38 to 108 at 38/min. TEM wasperformed on a JEM1200EX (Japan) instrument, and thepowdered specimen was placed on the copper grids aftersonicating a suspension in ethanol and drying for 30min. Themorphologies of the samples were examined using a JSM-5600LVSEM instrument (JEOL) after coating the sample with gold film.Thermal analysis of samples was carried out on a Perkin–ElmerTGA-7 thermogravimetric analyzer (Perkin–Elmer Cetus Instru-ments, Norwalk, CT), within a temperature range of 25–8008C at aheating rate of 108C/min, using dry nitrogen purge at a flow rateof 50ml/min.

RESULTS AND DISCUSSION

FTIR spectra analysis

FTIR spectra of REC, CMC, CMC-g-PNaA, CMC-g-PNaA/REC(5wt%), and the physical mixture of CMC-g-PNaA with REC (m/m¼ 18:1) are shown in Fig. 1. The characteristic absorption bandsof CMC at 1061, 1115, and 1159 cm�1 (stretching vibrations ofC–OH groups) were obviously weakened after reaction(Fig. 1(b–d)). New bands at 1714 cm�1 (C¼O stretching of–COOH), at 1576 and 1572 cm�1 (asymmetric stretching of –COO–

groups), and at 1455 and 1410 cm�1 (symmetric stretching of–COO– groups) appear in the spectra of CMC-g-PNaA andCMC-g-PNaA/REC (Fig. 1 (c, d)). This revealed that NaA had beengrafted onto CMC macromolecular chains. The strong absorptionof CMC at 1613 cm�1 (asymmetric stretching vibration of–COONa groups) can be observed in Fig. 1(b), but it overlappedwith the absorption bands of –COO– groups of grafted PNaAchains and appeared as a broad band (Fig. 1 (c, d)). The –O–Hstretching vibration of REC at 3638 cm�1 and the –OH bendingvibration at 1634 cm�1 disappeared in the spectrum of thenanocomposite (Fig. 1 (a, d)), but they can be clearly observed inthe spectrum of the physical mixture of CMC-g-PNaA with REC

Figure 1. FTIR spectra of (a) REC, (b) CMC, (c) CMC-g-PNaA, (d) CMC-g-PN

(m/m¼ 18). This figure is available in color online at wileyonlinelibrary.com/

wileyonlinelibrary.com/journal/pat Copyright � 2010 John Wiley

(Fig. 1(e)). The absorption bands of REC at 1049 cm�1 (Si–O–Siasymmetrical stretching) and 1022 cm�1 (––Si–O asymmetricalstretching), at 938 cm�1 (Si–O–Si symmetrical stretching) and915 cm�1 (––Si–O symmetrical stretching), as well as at about 546and 489 cm�1 (Si–O(H) bending vibration) appear in thespectrum of CMC-g-PNaA/REC with a weakened intensity(Fig. 1(d)), but can be clearly observed in the spectrum of thephysical mixture of CMC-g-PNaA and REC with considerableintensity (Fig. 1(e)). This information indicates that RECparticipated in the graft copolymerization reaction through itsactive silanol groups.[26]

XRD analysis

REC showed a strong (001) reflection at 2u¼ 3.608 with a basalspacing (d) of 2.46 nm (Fig. 2a), but no REC reflections could beobserved in the XRD patterns of the nanocomposites, even if thecontent of REC reached 15wt% (Fig. 2(c, d)). Thus, REC may beexfoliated and the REC platelets thoroughly dispersed in thepolymer matrix.[27] A similar observation was made for super-absorbent nanocomposites based on starch grafted PAA-co-AM/MMT.[28]

TEM and SEM analysis

TEM observation (Fig. 3) revealed good dispersion of REC inCMC-g-PNaA/REC (5wt%). Due to the correspondingly greaterinterlayer spacing of REC, the rectorite particles exfoliated duringthe polymerization reaction and distributed in the polymericmatrix in the form of exfoliated platelets. This result is inaccordance with the XRD analysis.The SEM micrographs of CMC-g-PNaA and the CMC-g-PNaA/

REC nanocomposites with 5, 10, and 20wt% of REC are shown inFig. 4. CMC-g-PNaA only exhibits a smooth and dense surface;while CMC-g-PNaA/REC nanocomposites show a correspondinglycoarser surface (Fig. 4(b)). This implies that the incorporation of

aA/REC (5wt%), and (e) the physical mixture of CMC-g-PNaA with REC

journal/pat

& Sons, Ltd. Polym. Adv. Technol. 2011, 22 1602–1611

Figure 3. TEM image of superabsorbent nanocomposite CMC-g-PNaA/

REC (5wt%).Figure 2. XRD patterns of (a) REC, (b) CMC-g-PNaA/REC (5wt%), and (c

CMC-g-PNaA/REC (15wt%). This figure is available in color online a

wileyonlinelibrary.com/journal/pat

Figure 4. Micrographs of the superabsorbents (a) CMC-g-PNaA, (b) CMC

(20wt%).

Polym. Adv. Technol. 2011, 22 1602–1611 Copyright � 2010 Joh


)

t

REC improved the surface structure of superabsorbent. Also, itwas observed that the REC is almost embedded and dispersedwithin the CMC-g-PNaA matrix (Fig. 4 (c)), with a homogeneouscomposition and without the flocculation of REC particles. Thisobservation is consistent with the results of XRD and TEM.

Thermal stability

Figure 5 depicts the TGA curves of CMC-g-PNaA hydrogel andCMC-g-PNaA/REC (5wt%) nanocomposite. It is obvious that REC

-g-P

n Wil

has great influence on the thermal stability; the weight-loss ratewas slowed after introducing REC. The weight loss about 21wt%below 3198C for CMC-g-PNaA and about 18wt% below 3278C forCMC-g-PNaA/REC (5wt%) can be ascribed to the removal ofabsorbed water, the dehydration of saccharide rings, and thebreaking of C–O–C bonds in the chain of CMC.[29] The weight lossof about 17wt% (319� 4318C) for CMC-g-PNaA and about16wt% (327� 4328C) for CMC-g-PNaA/REC (5wt%) can beattributed to the elimination of water molecule derived fromthe two neighboring carboxylic groups of the grafted PNaA

NaA/REC (5wt%), (c) CMC-g-PNaA/REC (10wt%), and (d) CMC-g-PNaA/REC


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Figure 5. TGA curves of CMC-g-PNaA and CMC-g-PNaA/REC (5wt%)

superabsorbent nanocomposite. This figure is available in color online

at wileyonlinelibrary.com/journal/pat

W. WANG AND A. WANG

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chains due to the formation of anhydride.[30] The weight lossesabout 24wt% (431� 5088C) for CMC-g-PNaA and about 18wt%(432� 5128C) for CMC-g-PNaA/REC (5wt%) are due to main-chain scission and the destruction of the crosslinked networkstructure. In comparison with CMC-g-PNaA matrix, CMC-g-PNaA/REC nanocomposite showed slower weight-loss rate and lessertotal weight loss, which indicated that the incorporation of RECenhanced the thermal stability of superabsorbent.

Effect of mass ratio of AA to CMC on water absorption

The water absorption (Fig. 6) increased as the mass ratio of AA toCMC is enhanced, reaching amaximum at 7, and then decreasing.Because the dosage of initiator APS is definite in the reactionsystem, the number of free radicals generated by thedecomposition of APS is constant. Thus, the dosage of monomermay directly determine the reaction rate as well as the amount ofhydrophilic groups in polymer network. Enhancing the mass ratioof AA to CMC, more monomer can participate in the grafting

Figure 6. Effect of the mass ratio of AA to CMC on water absorption. APS

concentration, 6.07mmol/L; MBA concentration, 3.59mmol/L; Neutraliz-

ation degree of AA, 70%; REC content, 5wt%; Reaction temperature, 708C.This figure is available in color online at wileyonlinelibrary.com/journal/

pat


reaction in the vicinity of active sites of CMC. As a result, thehydrophilicity of the polymeric network was improved due to thegrafting of PNaA chains. Moreover, the osmotic pressuredifference derived from the dissociation of –COONa groupsand the repulsive interaction derived from the negative –COO–

groups also increased with increase in the dosage of AA, which isalso responsible for the enhancement of water absorption.However, a high ratio of AA in the reaction system causes ashrinkage of the water absorption. This is because a highermonomer concentration will increase the number of free radicalsgenerated during the chain transfer reaction process, whichresults in an increase in the crosslink density and a decrease in thewater absorption.

Effect of MBA concentration on water absorption and gelstrength

As shown in Fig. 7, the crosslinker concentration exhibits morenotable effects on water absorption than others. It is obviousthat water absorption rapidly decreases with an increase inthe crosslinker concentration from 3.59 to 10.74mmol/L. Apower law relation between the equilibrium water absorption(Qeq, g/g) and the concentration of crosslinker MBA (CMBA, mol/L)

Figure 7. Effects of crosslinker concentration on water absorption indistilled water and 0.9wt% NaCl solution. APS concentration, 6.07mmol/

L; Mass ratio of AA to CMC, 7; Neutralization degree of AA, 70%; REC

content, 5wt%; Reaction temperature, 708C. The inset illustrations are theplots of Ln(Qeq) against Ln(1/CMBA). This figure is available in color onlineat wileyonlinelibrary.com/journal/pat



was expressed according to Eqn (1) and its logarithmic form(Eqn (2)).[31]

Qeq ¼ kCMBAð�nÞ (1)

LnðQeqÞ ¼ Lnk þ nLnð1=CMBAÞ (2)

k and n are constant for an individual superabsorbent. The plotsof Ln(Qeq) against Ln(1/CMBA) give perfect straight lines with goodlinear correlation coefficient (R> 0.9939), and the k and n valuescan be calculated through the slope and intercept of thestraight line. For CMC-g-PNaA/REC nanocomposite, the effect ofcrosslinker concentration on equilibrium water absorptionfollows the relation Qeq¼ 5.08Cð�0:8461ÞMBA in distilled water andQeq¼ 10.87Cð�0:3063ÞMBA in 0.9wt% NaCl solution.Figure 8 showed the dependence of storage modulus (G’) of

the swollen superabsorbents on angular frequency(v¼ 0.1�100 rad/sec; strain g ¼ 0.5%) and the concentration ofcrosslinker MBA. The storagemodulus (G’) of the nanocompositesincreased with increase in the angular frequency and concen-tration of crosslinker MBA. The maximum G’ of the nanocompo-site reached 2340 Pa (g ¼ 0.5%, v¼ 0.1 rad/sec) and 2980 Pa(g ¼ 0.5%, v¼ 100 rad/sec) when the crosslinker concentration is

Figure 8. Dependence of the storage modulus (G’) versus angular

frequency v (a) and versus MBA concentration (b) at a constant strain

(0.5%) for the nanocomposites swollen in 0.9wt% NaCl solution. Thisfigure is available in color online at wileyonlinelibrary.com/journal/

pat


10.78mmol/L, and the G’ also reached 632 Pa (g ¼ 0.5%,v¼ 0.1 rad/sec) and 789 Pa (g ¼ 0.5%, v¼ 100 rad/sec) even ata low concentration of crosslinker (3.59mmol/L).

Effect of REC content on water absorption

As depicted in Fig. 9, the water absorption of the nanocompositecontaining 5wt% of REC increased by 62% in contrast to theREC-free sample. The greater improvement in the waterabsorption can be attributed to the following: REC can participatein the polymerization reaction through the active silanol groupson its surface or edges of the platelet, which improve the networkstructure and surface morphologies of the superabsorbent. Also,the incorporation of rigid REC platelets may prevent enlacementof polymer chain, weaken the hydrogen-bonding interactionamong hydrophilic groups, and decrease the physical cross-linking degree. These factors facilitate the improvement in waterabsorption. However, the addition of excessive amounts of REC(> 5wt%) into polymeric network induced the reduction in waterabsorption. This is because the excess of REC platelets mayphysically fill in the network voids. On the one hand, the amountof hydrophilic groups such as –COOH and –COO� in unit volumewas reduced and the hydrophilicity of nanocomposite wasdecreased[32]; on the other hand, the physically filled RECplatelets in polymer network plugged up some network voidsused for holding water. These factors lead to the shrinkage ofwater absorption of the nanocomposite. The similar tendencycan also be observed in Starch-graft-acrylic acid/Na-MMT.[28] Asdiscussed above, REC may exist in polymer network by chemicalbonding and physical filling as shown in Scheme 1.

Swelling kinetics

Figure 10 represents the swelling kinetics curves of thesuperabsorbent nanocomposites with various amounts of RECin distilled water. It can be seen that the swelling rate of thenanocomposites is faster within 1200 sec and then reached aplateau. In this section, the swelling kinetics can be expressed by

Figure 9. Effect of REC content on water absorption in distilled water

and 0.9wt% NaCl solution. APS concentration, 6.07mmol/L; Mass ratio ofAA to CMC, 7; MBA concentration, 3.59mmol/L; Neutralization degree of

AA, 70%; Reaction temperature, 708C. This figure is available in coloronline at wileyonlinelibrary.com/journal/pat


1607

Scheme 1. Schematic network structure of the superabsorbent nano-

composite.

W. WANG AND A. WANG

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Schott’s pseudo second order kinetics model (Eqn (3)).[33]

t=Qt ¼ 1=Kis þ ð1=Q1Þt (3)

Qt is the water absorption at a given swelling time t (s);Q1 (g/g) isthe power parameter, denoting the theoretical equilibrium water

Figure 10. (a) Swelling kinetic curves of the superabsorbents with various co

This figure is available in color online at wileyonlinelibrary.com/journal/pat


absorption; Kis is the initial swelling rate constant (g/g�s). Asshown in Fig. 10(b), the plots of t/Qt versus t exhibited a perfectstraight line with a good linear correlation coefficient, whichindicates that the swelling kinetics of the nanocomposites followthe pseudo second order model. By fitting experimental datausing Eqn (3), the values of Kis and Q1 can be calculated throughthe slope and intercept of the above lines and are listed inTable 1. According to the obtained values of Kis and Q1 for eachsuperabsorbent, it can be concluded that the initial swelling ratedecreased in the order: CMC-g-PNaA/REC (5wt%)>CMC-g-PNaA/REC (10wt%)>CMC-g-PNaA/REC (20wt%)> CMC-g-PNaA.PNaA. This indicates that the incorporation of moderate amountof REC contributes to improvement in the swelling rate.

Saline-responsive behaviors

The osmotic pressure difference between the interior gel networkand exterior swelling media is a driving force for the swelling ofsuperabsorbent, and so the phase transition occurs after contactwith the electrolyte solution. As shown in Fig. 11, the waterabsorption remains almost constant at low concentration ofsaline, but it rapidly decreases after reaching a critical con-centration (about 0.5mmol/L), and the decrease in the tendencyfollows the order of Naþ

Table 1. Swelling kinetic parameters of the superabsorbents

SamplesQ1(g/g)

Kis(g/g � sec) R

CMC-g-PNaA 380 2.8688 0.9999CMC-g-PNaA/REC (5wt%) 613 5.2482 0.9997CMC-g-PNaA/REC (10wt%) 598 4.6294 0.9998CMC-g-PNaA/REC (20wt%) 395 3.6757 0.9997


the screening effects of cations on negative charges of polymerchains. However, the relatively strong screen effects andadditional complexing interaction of Mg2þ or Al3þ cations withthe –COO� groups make the water absorption decrease fasterthan monovalent cation. For evaluating the reversibility of salinestimuli-responsive behavior, the swelling–deswelling behavior ofthe nanocomposite was circularly measured between 0.01 and100mmol/L of saline solution. It was observed that thenanocomposite showed better On–Off switching properties inNaCl solution, but this property was decreased in MgCl2 solutionand disappeared in AlCl3 solution. For evaluating the effect of

Figure 11. (a) Swelling curves of CMC-g-PNaA/REC (5wt%) nanocom-posite in various saline solutions, and (b) the reversible On–Off switching

swelling behaviors of the nanocomposites between 0.01 and 100mmol/L

of NaCl or MgCl2 solutions. This figure is available in color online at



multivalent cations on –COOH and –COO– groups, the FTIRspectra of the nanocomposite after swelling in NaCl, MgCl2, andAlCl3 solutions were obtained and are shown in Fig. 12. Thecharacteristic absorption bands of –COOH and –COO� groupsshow no obvious change after swelling in NaCl solution,indicating no interaction between –COO– and Naþ. After swellingin MgCl2 solution, the characteristic absorption bands of –COOHand –COO– groups shifted to 1708 and 1562 cm�1, respectively,which implied that Mg2þ was complexed with –COO� groups.Different from the situation in NaCl and MgCl2 solutions, thecharacteristic absorption band of –COOH shifted to 1717 cm�1

with increased intensity and the absorption of –COO– groupshifted from 1572 to 1617 cm�1 after swelling in AlCl3 solution.The –COO– groups are not only complexed by Al3þ, but alsoprotonated by the acidic AlCl3 solution. In this process, a portionof the –COO� groups are transformed into –COOH groups, whichincreases the physical crosslinking degree of the hydrophilicnetwork. These lead to the collapse of gel network in AlCl3solution and the collapsed nanocomposite hydrogel is notrecovered. In MgCl2 solution, Mg

2þ has correspondingly weakcomplexation with –COO� groups, and so the gel network canre-expand to some degree and exhibit weak reversibility.[34] InNaCl solution, the On–Off switching swelling behavior of thenanocomposite is reversible.

pH-responsive behaviors

Figure 13 shows the variation of the water absorption forCMC-g-PNaA/REC nanocomposite with pH of the external buffersolution (0.1mol/L). It is evident that the water absorption islower at pH< 4, but sharply increases until a plateau is reached at

Figure 12. FTIR spectra of the nanocomposite after swelling in NaCl,

MgCl2, and AlCl3 solutions. This figure is available in color online at



1609

Figure 13. Variation of swelling ratio for CMC-g-PNaA/REC (5wt%)

nanocomposite at the buffer solution of various pH values and the On–off

switching behavior as reversible pulsatile swelling (pH 7.2) and deswelling

(pH 2.0) of the nanocomposite. This figure is available in color online atwileyonlinelibrary.com/journal/pat

Figure 14. (a) The swelling curves of the CMC-g-PNaA/REC (5wt%)

nanocomposite in various hydrophilic organic solvents/water mixture,and (b) the reversible On–Off swelling behaviors of the nanocomposites

between 5 v/v% and 90 v/v% hydrophilic organic solvents/water mixture

solutions. This figure is available in color online at wileyonlinelibrary.com/

journal/pat

W. WANG AND A. WANG

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pH> 4. As an anionic superabsorbent, the nanocompositecontains numerous hydrophilic –COO– and –COOH groups thatcan interconvert. In acid medium (pH< 4), most of –COO– groupstransform into –COOH groups. Consequently, the hydrogenbonding interaction among hydrophilic groups and thecorrelative physical crosslinking degree is increased, and thewater absorption is reduced. As pH increases beyond 4, a contraryprocess occurs. The hydrogen bonding interaction is broken andthe electrostatic repulsion among negatively charged polymerchains is increased, and so the polymer network tends to swellmore. The evident change of water absorption with altering ofthe pH of external buffer solution confirmed the excellentpH-responsive characteristic of CMC-g-PNaA/REC nanocompo-site. For evaluating the pH reversibility of the nanocomposite, theswelling–deswelling behavior was investigated in 0.1mol/Lbuffer solution of phosphate at pH 2 and 7.2 (Fig. 13(b)). Thenanocomposite exhibited higher swelling capability at pH 7.2, butthe swollen gel rapidly shrinks at pH 2 due to the protonation of–COO– groups. The intriguing On–Off switching behavior wasobserved. After five On–Off cycles, the nanocomposite stillexhibited responsivity. This suggested pH reversibility and renderthe nanocomposite suitable candidates for controlled drugdelivery systems.


Hydrophilic organic solvent-responsive behaviors

Besides external saline and pH, the nanocomposite showedexcellent phase transition and responsivity to hydrophilic organicsolvents (Fig. 14(a)). It can be noticed that the water absorption isalmost constant at low concentration of hydrophilic organicsolvent, but sharply decreases after a critical concentration(40 v/v% for methanol and ethanol, 30 v/v% for acetone), and thetendency in acetone/water solution is more obvious than inmethanol or ethanol. This behavior can be attributed to thechange in dielectric constant of solution due to the addition oforganic solvents.[35] For evaluating the reversibility of the gelphase transition, the swelling–deswelling cycles were madebetween 5 v/v% and 90 v/v% of methanol, ethanol, and acetone/water mixtures (Fig. 14(b)). Interestingly, the nanocompositeshowed excellent On–Off switching behavior and betterreversibility.

CONCLUSIONS

As a part of the endeavor to minimize the consumption ofpetroleum resource and the environmental impact resulting from



industrial polymers and to extend the application of super-absorbents, the novel superabsorbent nanocomposites wereprepared by radical solution polymerization using CMC as matrixand REC as an inorganic component. FTIR, XRD, TEM, SEM, andTGA analyses indicated that (i) the PNaA chains had been graftedonto CMC main chains and REC participated in the polymeri-zation reaction through active –OH groups; (ii) REC has beenexfoliated after polymerization and equably embedded withinthe CMC-g-PNaA matrix by the nanoscale platelets with noflocculation; and (iii) the thermal stability of the superabsorbentwas enhanced due to the introduction of REC. The incorporationof REC improved the water absorption and swelling rate of thesuperabsorbent. The superabsorbent nanocomposite exhibitedexcellent saline-, pH-, and hydrophilic organic solvent-responsivecharacteristics and better reversibility can also be observed foreach responsivity cycle. Besides the water absorption properties,the additional responsivity of the nanocomposite contributes toextend the application of superabsorbent in other fields such asdrug delivery carrier. Thus, superabsorbent nanocompositesbased on renewable and biodegradable CMC and abundant RECshowed enhanced swelling capability, swelling rate and excellentsaline-, pH, and organic solvent-responsivity, which can be usedas potential water-manageable materials for agriculture orsuitable candidate for the controlled release carrier of drugs.

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preparation, swelling, and stimuli-responsive characteristics ......the gel strength greatly...

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