Research Article

Received: 21 July 2008, Revised: 18 September 2008, Accepted: 29 September 2008, Published online in Wiley InterScience: 19 November 2008

(www.interscience.wiley.com) DOI: 10.1002/pat.1322

Synthesis and properties of a novel doublenetwork nanocomposite hydrogel

Jiantao Lina, Shimei Xua, Xiaomei Shia, Shun Fenga and Jide Wanga*

A novel polyacrylamide/polyacrylic acid (PAAm/PAA

Polym. Adv

) double network (DN) nanocomposite (NC) hydrogel had beensynthesized by two-step solution polymerization. The PAAm network was crosslinked by inorganic clay while the PAAnetwork was crosslinked by a chemical crosslinker. The chemical structure of the network was confirmed by Fouriertransform infrared (FTIR), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The swelling andmechanical strength properties of PAAm/PAA hydrogels were examined. The results showed that a DN hydrogelachieved both a high swelling capacity of 1219g/g in deionized water and 124g/g in 0.9wt% NaCl solution and highcompressive stress of 21.5 kPa in a high water content of 99.58%. Copyright � 2008 John Wiley & Sons, Ltd.

Keywords: hydrogel; crosslinking; copolymerization; nanocomposite

* Correspondence to: J. Wang, Key Laboratory of Oil & Gas Fine Chemicals,

Ministry of Education, Xinjiang University, Urumqi, Xinjiang 830046, P.R. China.

E-mail: awangjd@xju.edu.cn

a J. Lin, S. Xu, X. Shi, S. Feng, J. Wang

Key Laboratory of Oil & Gas Fine Chemicals, Ministry of Education, Xinjiang

University, Urumqi, Xinjiang 830046, P.R. China

Contract/grant sponsor: Natural Science Foundation of China; contract/grant

number: 20506020, 50367005. 6

INTRODUCTION

Hydrogel is a kind of three-dimensional crosslinked polymericmaterial and used as the water-filled inner core of networks, andattracts much attention as functional soft materials. However, thelow fracture toughness of chemically crosslinked hydrogelsrestricts their application in biomedicine,[1] organ engineering,[2]

and other fields.[3–5] Many efforts have beenmade to increase themechanical strength of hydrogels by the introduction ofinorganic nanoclay into organic polymer networks.[6–9] Chisholmet al. prepared nanocomposite (NC) hydrogels derived fromsulfonated poly(butylene terephthalate), in which clay plateletorientation increased with the increasing –SO3Na content. Itsuggested that the presence of strong matrix–clay interactionsenhanced the mechanical property. Instead of the traditionalchemically crosslinked hydrogels, Haraguchi and his cow-orkers[10–13] create a novel class of NC hydrogel with a uniqueorganic(polymer)/inorganic(clay) network structure, in which aninorganic clay laponite is exfoliated and uniformly dispersed inan aqueous media as an inorganic crosslinker. The NC gel, withexcellent properties, such as mechanical toughness, largedeformability, high swelling/deswelling rates, and high transpar-ency, swells to 110 times its original weight and can be stretchedto nearly 15 times its original length.[10] Zhu et al. successfullysynthesized a series of high clay content NC hydrogels composedof poly(N-isopropylacrylamide) and laponite modified by tetra-sodium pyrophosphate. It is found that these hydrogels showedsurprisingmechanical properties and complicated the deswellingbehavior.[14]

Different from the NC hydrogels, independently crosslinkeddouble network (DN) hydrogels were synthesized to enhance themechanical properties by Gong’s group.[15–17] The DN hydrogelconsists of two polymeric networks: one made of highlycrosslinked polymers, and the other made of loosely crosslinkedpolymers. Excellent mechanical performance was achievedwhen the DN hydrogels were composed of poly(2-acrylamide-

. Technol. 2009, 20 645–649 Copyright �

2-methyl-propane sulfonic acid) and poly(acrylamide). Thefracture stress was 20 times larger than those of individualsingle network hydrogels.[18] However, superabsorbent abilitywas not taken into consideration in the case of NC hydrogels orDN hydrogels.In this paper, a novel DN NC hydrogel with superabsorbent

ability was developed by combining the NC hydrogels with DNhydrogels through a two-step solution polymerization. Firstly, the1st network was made of PAAm with laponite as the crosslinker,while the 2nd network was made of chemically crosslinked PAA.High swelling and mechanical strength were anticipated in theresulting DN NC hydrogel. The Fourier transform infrared (FTIR)was utilized to analyze the chemical composition of the networks,while X-ray diffraction (XRD) and transmission electron micro-scopy (TEM) were used to observe their structural properties,furthermore confirming the conformation of the first network.The swelling behavior and compressive strength of the DN NChydrogels were also investigated.

EXPERIMENTAL

Materials

Acrylic acid (AA) (Tianjin Fuchen Chemistry Reagent Factory,analytical grade), sodium hydroxide (NaOH) (Beijing ChemicalFactory, analytical grade), acrylamide (AAm) (Tianjin Yongda

2008 John Wiley & Sons, Ltd.

45

Figure 1. FTIR spectra of (a) laponite, (b) the first network, and (c) double

network nanocomposite hydrogel.

J. LIN ET AL.

646

Chem. Co, analytical grade), N, N0-methylene bisacrylamide(MBAM) (Shanghai Chemical Co.), sodium pyrophosphate (SPP),laponite XLG (Rockwood Co., U.S., Mg5.34Li0.66Si8O20(OH)4Na0.66),sodium hydrogen sulphite (SHS) (Tianjin Chemistry ReagentFactory, analytical grade) and ammonium persulfate (APS) (Xi’anChemical Co.) were used as received.

Preparation of double network nanocomposite hydrogel

Hydrogels were prepared by a two-step free-radical crosslinkingcopolymerization. In the first step, a transparent aqueous solutionwas prepared by mixing water (10ml), clay (0–1.7 g), and AAm(0.4 g). 7.68% SPP (relate to clay mass) was added to deduce theviscosity when the mass ratio of laponite to water was more than4.5%. APS (0.02 g) and SHS (0.02 g) were subsequently added toinitiate the polymerization under magnetic stirring at roomtemperature. Free-radical polymerization was carried out for 2 hrbefore the second step. Then 4.5 g AA, neutralized previously by25wt% NaOH solution to 75mol% neutralization degree in icebath, was added into the resulting polyacrylamide–clay NCsolution above to get a clear solution under stirring, followed byMBAM (varied at 0.030, 0.050, and 0.067wt% based on the weightof AA) and 2.5wt% APS aqueous solutions (based on the weightof AA), respectively. Afterwards the solution was poured into atube (interior size 18mm diameter, 100mm length), and heatedslowly to 558C and kept for 2 hr. The resulting products were driedand the weight was recorded as w0 (g), then immersed in excessdeionized water overnight to remove the solubles in thehydrogels, dehydrated with absolute ethanol and dried at808C to a constant weight w1 (g). The yield (h) was calculatedaccording to the equation h(%)¼w1/w0� 100%. The driedproduct wasmilled to 20–40mesh for the swelling measurement.

Swelling measurement

The swelling ability was measured by a filtration method.A 0.1 gsample was immersed in 250ml deionized water or 100ml0.9wt% NaCl solution for 12 hr to reach swelling equilibrium. Theswollen sample was filtered by a 100 mesh nylon gauze. Theswelling ratio Q (g/g) was calculated as:

Q ¼ M�M0M0

where M (g) represents the mass of the swollen hydrogel afterswelling equilibrium and M0 (g) is the mass of the dry hydrogel.

Mechanical measurement

The swollen sample was cut into a cylindrical shape (30mmØ� 10mm length).The mechanical performance of the hydrogelwas evaluated by using homemade equipment according toRamazani–Harandi’s work.[19] All mechanical tests were per-formed at room temperature.

Characterization methods

FTIR measurements were performed on a BRUKER EQINOX55 FTIRspectrometer. All spectra in the range 400–4000 cm�1 with2 cm�1 spectral resolution were obtained from compressed KBrpellets in which the samples were evenly dispersed. XRD werecarried out using a Rigaku D/max 2000 H X-ray diffractometer(40 kV, 30mA) equipped with Cu Ka radiation (l¼ 0.154 nm) anda curved graphite crystal monochromator at a scanning rate of0.58/min. TEM observation was conducted on a HITTACHI H-600

www.interscience.wiley.com/journal/pat Copyright � 2008

transmission election microscope at an acceleration voltageof 75 kV.

RESULTS AND DISCUSSION

FTIR

FTIR spectroscopy was carried out to characterize the chemicalstructure of the DN. The IR spectrum of laponite (Fig. 1a) presentstwo sharp absorption bands at 1010 and 1645 cm�1, and a broadband at 3464 cm�1 ascribed to Si–O stretching and O–H bending,and O–H stretching modes, respectively. A new IR adsorptionband at 1670 cm�1 appears at the spectrum of the first network(Fig. 1b.), which can be attributed to the carbonyl stretching whilethe band at 1454 cm�1 correspons to the C–N stretching of theAAm. This suggests that the first network consists of AAm andlaponite. Besides 1671 and 1010 cm�1 ascribed to –C––Ostretching and Si–O stretching in the polyacrylamide/polyacrylicacid (PAAm/PAA) DN hydrogel (Fig. 1c), a new adsorption at1708 cm�1, was assigned to stretching vibrations of –COOH ofAA. The peaks of –COO� asymmetric and symmetric stretchingwere found at 1407 and 1561 cm�1 (Fig. 1c). As a result, it can beconcluded that the DN was formed.

XRD

There is no doubt that AA can form a polymer network in thepresence of the chemical crosslinker MBAM. To simplify theanalysis, we tried to characterize the DN by only testifying theformation of the first network composed of PAAm with inorganicclay as a crosslinker. XRD measurements were performed toreveal the dispersion of clay platelets in the first network. Clayshowed a diffractive peak at 2u¼ 6.778 corresponding to thespacing between clay platelets of 1.30 nm (Fig. 2a). Accordingly,the peak shifted towards a lower angle when the first networkwas formed (Fig. 2b) and showed a diffraction peak at 2u¼ 4.29,which corresponded to a d-spacing of 2.06 nm. Therefore, it canbe concluded that clay was well-dispersed in the polyacrylamide,and the clay was successfully intercalated.

John Wiley & Sons, Ltd. Polym. Adv. Technol. 2009, 20 645–649

Figure 3. TEM of the first network.Figure 2. XRD of (a) laponite and (b) the first network.

NOVEL DOUBLE NETWORK NANOCOMPOSITE HYDROGEL

TEM

TEM was further performed to confirm the XRD result. In Fig. 3well-dispersed platelets morphology of clay is observed in arandomway, which is similar to the Haraguchi’ report. In addition,it was observed in our experiment the NC hydrogel formed whenthe clay content increased to some degree in the first step. Itrevealed that laponite acted as a crosslinker by polar interactionwith PAAm chains.[13] The TEM results corresponded well with the

Scheme 1. Proposed formation of doub

Polym. Adv. Technol. 2009, 20 645–649 Copyright � 2008 John

XRD patterns. Therefore, it can be concluded that the firstnetwork was formed by loosely crosslinking in the first step,which in turn confirmed the formation of a DN.Therefore, based on the above-mentioned results of experi-

ments, the reaction process is predicted to be as follows: theinorganic clay XLG with a lamellar crystal structure was swollenwhen dispersed in water and gradually cleaved into discretedisklike particles (Scheme 1a, b). Monolayer XLG clay particles areanisotropic platelets 30 nm in diameter and 1 nm thickness.[13]

le network nanocomposite hydrogels.

Wiley & Sons, Ltd. www.interscience.wiley.com/journal/pat

647

Table 1. The hydrogel strength and swelling ratio of hydrogel under different laponite contents and different MBAM contents

Hydrogel composition Hydrogel strength Swelling capacity (g/g)

YieldLaponite (%) MBAM (%) Fracture (kPa) Water content (%) Deionized water 0.9wt%NaCl solution (%)

0 0.03 —a — 1047 110 75.90 0.05 — — 889.6 105 77.92 0.067 6.7 99.82 1132 114 80.95 0.030 10.6 99.17 1159 128 83.55 0.050 21.5 99.58 1219 124 79.65 0.067 7.1 99.84 1038 113 92.7

10 0.067 13.3 99.98 1061 110 85.42 0 —b — — — —5 0 — — — — —

aHydrogels are too fragile to be measured.b Hydrogels cannot be formed in the experimental conditions.

J. LIN ET AL.

648

Acrylamide was then added (Scheme 1c), the first network PAAmwas formed after the polymerization of acrylamide by partiallycrosslinking by clay XLG (Scheme 1d).[10] When AA was addedsubsequently (Scheme 1e), PAA interpenetrated to the PAAmnetwork and formed a tight continuous network due to thechemical crosslinking and physical entanglement (Scheme 1f).

Swelling capacity and mechanical strength

The crosslinker dose is one of the most important factors for theswelling behavior and mechanical strength of hydrogels bydeciding the network size. In our work, laponite was used as thefirst crosslinker in the first network while MBAM was used as thesecond one in the subsequent fabrication of the organic network.The effects of their dose on the swelling capacity and mechanicalstrength are shown in Table 1.When theMBAM content was fixed at 0.067%, it was found that

the swelling ratio decreased slightly with the increasing contentof laponite from 2 to 10%. The main reason may be that thecrosslinking density increased with increasing dose of laponite,which reduced the swelling capacity. Besides, the effect of thedose of MBAM in the second network is presented in Table 1 interms of swelling capacity. The results revealed that when theMBAM dose increased from 0.030 to 0.067%, the swellingcapacity increased at first and then decreased, and it reached amaximum value of 1219 g/g when the MBAM content was0.050%. In comparison, it can be concluded that laponite was notan effective crosslinker as MBAM since it only affected thenetwork size to a limited degree even at a wide range of laponitecontent.Table 1 also showed ultimate compressive stress data of DN NC

hydrogels with different laponite and MBAM contents. Thehydrogels with high clay content showed a higher resistance tocompression (13.3 kPa) even under a higher water content of99.98%. This suggested that the mechanical performances wasmore related to the structure of the hydrogels. When the MBAMcontent was 0.030 and 0.050% without the laponite, thehydrogels are too fragile to measure the strength. So thenanoparticle clay acted as an effective reinforcing agent by

www.interscience.wiley.com/journal/pat Copyright � 2008

forming an organic/inorganic network structure.[13] When thecontent of laponite was fixed to 5%, the mechanical strength ofthe DN hydrogel increased from 10.6 to 21.5 kPa when the MBAMcontent increased from 0.03 to 0.05%, which conformed to thetheory that high crosslinking could strengthen the mechanicalstrength. However, the hydrogel strength decreased rapidly to7.1 kPa under a higher MBAM content range of 0.067%. It waspossibly caused by the nonhomogeneous distribution of thechemical crosslinking points. It was also found that the normalhydrogels can not be formed when no MBAM was added at the2nd step (Table 1). It can be explained as follows: the first networkwas a loosely crosslinked polymer in experimental conditions, sothe MBAM content is very pivotal for the hydrogel formation.Compared to amphoteric semi-IPN NC hydrogels based on theintercalation of cationic polyacrylamide into bentonite, whichshowed a compressive stress of 11.2 kPa under a water content of99.6%,[20] the mechanical strength of the DN NC hydrogel wasobviously improved. In comparison, the yield of the hydrogel withtwo crosslinkers was higher than the hydrogel with only onecrosslinker. The reason was that a higher crosslinking density ofthe former reduced the solubles. This phenomenon alsoconfirmed the crosslinking function of nanoclay and MBAM.

CONCLUSION

A novel DN NC hydrogel consisting of the first network of PAAmusing laponite as a crosslinker and the second network of PAAusing MBAM as a crosslinker was synthesized by two-stepsolution polymerization. The resulting superabsorbent compositeshowed an excellent swelling capacity of 1219 g/g in distilledwater, 124 g/g in 0.9 wt% NaCl solution and 21.5 kPa highcompressive stress in a high water content when 5wt% laponiteand 0.05wt% MBAM were used during the polymerizationrespectively. In addition, entanglement effect and contents oftwo kinds of crosslinkers were two important factors that wouldinfluence the swelling capacity and mechanical properties of theDN NC hydrogel.

John Wiley & Sons, Ltd. Polym. Adv. Technol. 2009, 20 645–649

NOVEL DOUBLE NETWORK NANOCOMPOSITE HYDROGEL

REFERENCES

[1] C. H. Bamford, K. G. Al-Lamee, Polymer 1996, 37, 4885–4889.[2] Y. Ding, Y. Hu, L. Zhang, Y. Chen, X. Jiang, Biomacromolecules 2006, 7,

1766–1772.[3] X. J. Loh, K. B. Colin Sng, J. Li, Biomaterials 2008, 29, 3185–3194.[4] S. M. Ong, C. Zhang, Y. C. Toh, S. H. Kim, H. L. Foo, Biomaterials 2008,

29, 3237–3244.[5] Q. Tang, J. Wu, H. Sun, S. Fan, D. Hu, Carbohydr. Polym. 2008, 73,

473–481.[6] Y. Xiang, Z. Peng, D. Chen, Eur. Polym. J. 2006, 42, 2125–2132.[7] J. Mu, S. Zheng, J. Colloid Interface Sci. 2007, 307, 377–385.[8] B. Strachotov, A. Strachota, M. Uchman, M. Slouf, J. Brus, Polymer

2007, 48, 1471–1482.[9] B. J. Chisholm, R. B. Moore, G. Barber, F. Khouri, A. Hempstead,

Macromolecules 2002, 35, 5508–5516.[10] K. Haraguchi, T. Takehisa, Adv. Mater. 2002, 14, 1120–1124.

Polym. Adv. Technol. 2009, 20 645–649 Copyright � 2008 John

[11] M. Shibayama, J. Suda, T. Karino, S. Okabe, T. Takehisa, Macromol-ecules 2004, 37, 9606–9612.

[12] K. Haraguchi, H. J. Li, Polymer 2005, 117, 6658–6662.[13] K. Haraguchi, H. J. Li, K. Matsuda, T. Takehisa, E. Elliott,Macromolecules

2005, 38, 3482–3490.[14] Y. Y. Liu, Y. H. Shao, L. Jian, Biomaterials 2006, 27, 4016–4024.[15] S. S. Jang, W. A. Goddard, M. Y. S. Kalani, D. Myung, C. W. Frank, J. Phys.

Chem. B 2007, 111, 14440–14440.[16] D. Myung, W. Koh, J. Ko, Y. Hu, M. Carrasco, Polymer 2007, 48,

5376–5387.[17] L. Weng, A. Gouldstone, Y. Wu, W. Chen, Biomaterials 2008, 29,

2153–2163.[18] T. Karino, Y. Okumura, K. Ito, M. Shibayama, Adv. Mater. 2004, 37,

6177–6182.[19] M. J. Ramazani-Harandi, M. J. Zohuriaan-Mehr, A. A. Yousefi, A.

Ershad-Langroudi, K. Kabiri, Polym. Test. 2006, 25, 470–474.[20] S. Xu, S. Zhang, J. Yang, Mater. Lett. 2008, 62, 3999–4002.

Wiley & Sons, Ltd. www.interscience.wiley.com/journal/pat

649