poly(acrylic acid-co-acrylamide) super-absorbent hydrogel nanocomposites

Colloids and Surfaces A: Physicochem. Eng. Aspects 401 (2012) 97–106

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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Preparation and swelling properties of graphene oxide/poly(acrylic acid-co-acrylamide) super-absorbent hydrogel nanocomposites Yiwan Huang a , Ming Zeng a,b,∗ , Jie Ren a , Jing Wang a , Liren Fan a , Qingyu Xu c,d,∗∗ a

Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences, Wuhan 430074, PR China State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 430074, PR China Hubei Research Institute of Chemistry, Wuhan 430074, PR China d Haiso Technology Co. Ltd., Wuhan 430074, PR China b c

a r t i c l e

i n f o

Article history: Received 13 November 2011 Received in revised form 8 March 2012 Accepted 13 March 2012 Available online 21 March 2012 Keywords: Graphene oxide Super-absorbent hydrogels Swelling kinetics pH sensitivity

a b s t r a c t A series of novel graphene oxide (GO)/poly(acrylic acid-co-acrylamide) super-absorbent hydrogel nanocomposites were prepared by in situ radical solution polymerization. The effects of GO content on the chemical structure, morphology and miscibility of the hydrogels were studied. The swelling behaviors, swelling kinetics and pH-responsive behaviors of the hydrogels were also evaluated. Owing to the hydrogen bonds and possible covalent bonds between GO and polymer chains, relatively lower content (<0.30 wt%) of GO could be dispersed well in the polymer matrix and enhanced the intermolecular interactions between the components effectively. On the contrary, an excessive amount of GO might form large agglomerates and weakened the interfacial interactions, resulting in the micro-phase separation between the components. Furthermore, the swelling capacities and swelling rates of hydrogels went up with increasing GO loadings to 0.30 wt% and then decreased with further increasing GO loadings. It is worth noting that the hydrogel only containing 0.10 wt% GO exhibited significant improvement of swelling capacity in neutral medium, and could also retain relatively higher swelling capacities to a certain degree at acidic and basic solutions. Therefore, the as-prepared GO-based super-absorbent hydrogels might have potential applications in many areas, such as biomedical engineering, construction engineering and hygienic products. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Graphene, a two-dimensional monolayer of sp2 -bonded carbon atoms, has attracted increasing attention [1] owing to its excellent electrical and thermal conductivities [2,3], great mechanical strength [4], high specific surface area [5] and potentially low production cost [6]. It has shown potential applications in fabricating electronic devices [7], sustainable energy storage and conversion devices [8], sensors [9], and nanocomposites [10,11]. Graphene oxide (GO) is also a two-dimensional carbon material with similar one-atom thickness but with a large number of hydrophilic oxygenated functional groups including hydroxyl ( OH), epoxy ( C O C ), carbonyl ( C O) and carboxyl ( COOH)

∗ Corresponding author at: Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences, Wuhan 430074, PR China. Tel.: +86 15623351996. ∗∗ Corresponding author at: Hubei Research Institute of Chemistry, Wuhan 430074, PR China. Tel.: +86 13886007392. E-mail addresses: [email protected], [email protected], [email protected] (M. Zeng), [email protected] (Q. Xu). 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2012.03.031

groups [12]. These groups make GO sheets hydrophilic and dramatically improve their miscibility with polymer matrix. Super-absorbent polymers have been extensively studied and widely applied in various fields, such as hygienic products [13], drug delivery system [14,15], and agriculture [16] as well as waste-water treatment [17,18]. Generally, super-absorbents are moderately crosslinked hydrophilic polymer networks which can absorb large quantities of water or other aqueous fluids [19]. Some nanofillers have been introduced into the polymer matrix. Bao et al. [20] and Wang and Wang [21] have successfully synthesized clay-based super-absorbent hydrogels to improve the swelling capacities as well as to reduce the production cost. However, low swelling properties in saline solutions still limit their applications. Carbon nanotubes [22] as new nanomaterials are also dispersed into the polymer networks to reinforce the hydrogels. Unfortunately, there are also some defects, such as high production cost and complex synthesis process. GO-based nanocomposites have been significantly focused on and demonstrated to have exceptional performances in many aspects [23–25]. Recently, there are only a few investigations devoted to GO-based composite hydrogels [26–29]. Bai et al. [26] have prepared a novel GO/poly(vinyl alcohol) composite

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hydrogel, which can be utilized for selective drug release at physiological pH. Sun and Wu [29] have synthesized a GO/poly(Nisopropylacrylamide) interpenetrating hydrogel, exhibiting thermal and pH responses, which may have potential applications as carriers for controlled drug delivery. However, no investigation has been reported on GO-based super-absorbent hydrogels until now. In fact, GO is a perfect candidate for fabricating novel superabsorbent hydrogels because of abundant hydrophilic groups on the surface [2–5]. It is easy for GO to form a good interfacial interaction with the hydrophilic matrix through chemical or hydrogen bonds. In addition, the thermal behavior and mechanical strength of the hydrogels may be enhanced owing to the outstanding thermal resistance and mechanical properties of GO sheets [4]. Moreover, it is expected that the swelling properties of the hydrogel nanocomposites may be improved due to the numerous hydrophilic groups on the surface of GO sheets. In this manuscript, we prepared a series of graphene oxide/poly(acrylic acid-co-acrylamide) (GO/P(AA-co-AM)) superabsorbent hydrogel nanocomposites via in situ radical solution polymerization. The effects of GO content on the chemical structure, morphology and miscibility of the super-absorbent hydrogels were studied by Fourier transform infrared spectroscopy (FTIR), wide-angle X-ray diffraction (WXRD), dynamic mechanical thermal analysis (DMA), field emission scanning electron microscopy (FESEM) and optical microscopy (OM). Additionally, the swelling behaviors, swelling kinetics and pH-responsive behaviors of the hydrogels were also evaluated in details. A basic understanding of the swelling behavior for super-absorbent hydrogel nanocomposites in the solutions with different pH values (1–13) is essential for a successful research and application of the new materials. 2. Materials and methods 2.1. Materials Natural graphite flake with an average particle size of 23 ␮m and a purity of 99.99% was obtained from Qingdao Guyu Graphite Co. Ltd. Acrylic acid (AA, Chengdu Kelong Chemical Reagent Co. Ltd., Chengdu, China) was distilled under reduced pressure before use. Concentrated sulfuric (H2 SO4 , Wuhan Huasong Fine Chemical Co. Ltd., Wuhan, China), sodium nitrate (NaNO3 , Tianjin Tailande Chemical Reagent Factory, Tianjin, China), potassium permanganate (KMnO4 , Tianjin Tailande Chemical Reagent Factory, Tianjin, China), hydrogen peroxide solution (H2 O2 , 30%, Tianjin Tianli Chemical Reagent Co. Ltd., Tianjin, China), hydrochloric acid solution (HCl, 36–38%, Kaifeng Dongda Chemical Reagent Co. Ltd., Kaifeng, China), sodium hydroxide (NaOH, Tianjin Dalu Chemical Reagent Co. Ltd., Tianjin, China), acrylamide (AM, Amresco Inc., Solon, Ohio, USA), N,N -methylenebisacrylamide (NMBA, Wuhan Huashun Biotechnology Co. Ltd., Wuhan, China) and ammonium pursulfate (APS, Sinopharm Chemical Reagent Co. Ltd., Shanghai, China) were used as received. All reagents were analytical grade and all aqueous solutions were prepared with deionized water. 2.2. Synthesis of graphite oxide Graphite oxide was synthesized through a modified Hummers method [30] from nature graphite flake. 2 g of graphite and 1 g of NaNO3 were dissolved in 46 mL of concentrated H2 SO4 under an ice bath. After about 15 min of stirring, 6 g of KMnO4 was gradually added into the suspension with stirring as slowly as possible in order to control the reaction temperature below 20 ◦ C. The suspension was stirred for 2 h, and then maintained at 35 ◦ C for 30 min. 92 mL of deionized water was slowly poured into the suspension, resulting in a quick increase in temperature, and the temperature

should be controlled lower than 98 ◦ C. After 15 min, the suspension was then further diluted to approximately 280 mL with warm deionized water. 20 mL of 30% H2 O2 was added for the purpose of removing the residual KMnO4 and MnO2 to change the color into luminous yellow. Then, the suspension was filtered and washed with warm 5% HCl aqueous solution and deionized water, respectively, until no sulfates were detected and the pH of the filtrate was adjusted to 7. The sample of graphite oxide was dried under vacuum at 50 ◦ C to a constant weight, and then milled to an ideal particle size. 2.3. Preparation of GO/P(AA-co-AM) super-absorbent hydrogel nanocomposites A series of GO/P(AA-co-AM) super-absorbent hydrogel nanocomposites were prepared by in situ free radical solution polymerization according to the following procedure [31]. An appropriate amount of sodium hydroxide solution was added slowly to 5 g of AA in a beaker cooled under an ice bath, in order to achieve a neutralization degree of 60 mol%. Then, the mixed solution of 0.83 g of AM, 1.2 mL of NMBA (1 mg/mL), 2.9 mL of APS (2 mg/mL) and 17.5 mL of deionized water were gradually added into the above solution with stirring continually. By changing the weight ratios (0.05, 0.10, 0.15, 0.30, and 0.50 wt%) of graphite oxide in the blends, a certain amount of graphite oxide was dispersed into the mixed solution, followed by ultrasonic treatment for 10 min to obtain a homogenous dispersion. The water bath was heated to 70 ◦ C with vigorous stirring, and the viscosity of the mixed solution increased sharply after about 30 min. Then the reactor was kept at 70 ◦ C for 3 h to complete the polymerization. Finally, the resulting product GO/P(AA-co-AM) was cut into small pieces and then vacuum-dried at 70 ◦ C to a constant weight. The P(AA-co-AM) super-absorbent hydrogel was also prepared according to the above procedure without the addition of GO. All products were milled and sieved to 35–60 mesh. 2.4. Swelling properties of super-absorbent hydrogels A fixed weight (ca. 0.10 g) of dry super-absorbent hydrogel was immersed into adequate deionized water or 0.9% (w/v) NaCl solution (physiological saline) at room temperature for 60 min to reach the swelling equilibrium. The same procedure was conducted for the solutions with different pH values (1–13) which were adjusted by using aqueous HCl and NaOH solution. The swollen samples were filtered through a 120-mesh screen carefully to remove the unabsorbed water on the surface of samples. The swelling ratio (g/g) of the super-absorbent hydrogel was calculated as follows: W=

Ws − Wd Wd

(1)

where W was the swelling ratio at time t, Ws and Wd were the weights of the swollen sample and the dry sample, respectively. W was calculated as grams of water per gram of sample. Parallel measurements in triple were carried out for every sample. Swelling kinetics of super-absorbent hydrogels was evaluated by the previously reported method [21]. 2.5. Characterization 2.5.1. Fourier transform infrared spectroscopy (FTIR) The spectroscopic analyses of samples were carried out using a Fourier transform infrared spectrometer (Nicolet 6700, Thermo Scientific, USA). Spectra in the wavenumber range of 4000–400 cm−1 were collected over 36 scans with a resolution of 2 cm−1 . The test specimens were vacuum-dried and then prepared by the KBr-disk method.

Y. Huang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 401 (2012) 97–106

Fig. 1. FTIR spectra of pristine graphite (a) and graphite oxide (b).

2.5.2. Wide-angle X-ray diffraction (WXRD) Wide-angle X-ray diffraction (WXRD) was recorded on an X-ray diffractometer (X’Pert pro MPD, Philips), by using Cu K␣ radiation ( = 15.405) at 40 kV and 30 mA with a scan rate of 4◦ min−1 . The diffraction angle ranged from 3 to 40◦ . 2.5.3. Scanning electron microscopy (SEM) and optical microscopy (OM) The dried hydrogel samples were fractured after immersing them in liquid nitrogen for the purpose of analyzing the internal morphologies (transversal view) of the samples. The samples were gold-coated in a JUC-500 Magnetron Sputtering Device (JEOL, Tokyo, Japan), and a Field Emission Scanning Electron Microscope (FESEM, INSPECT F, FEI, the Netherlands) was used for the observation of the cross section of the samples. The surface morphologies of pristine graphite and graphite oxide were also observed by FESEM. The surface morphologies of the hydrogel nanocomposites were observed by optical microscopy (OM, DM2500 P, Leica, Germany). 2.5.4. Dynamic mechanical thermal analysis (DMA) The thermo-mechanical properties of dried super-absorbent hydrogels were evaluated using a dynamic mechanical analyzer (DMA/SDTA861e , Mettler Toledo, Switzerland) with the shear mode. The sample cylinders (diameter: 10 mm, thickness: ca. 1 mm) were measured with a vibration frequency of 1 Hz and 10 ␮m amplitude under air atmosphere, and with a heating rate of 5 ◦ C/min from 30 to 350 ◦ C. 3. Results and discussion 3.1. Structure and morphology of graphite oxide Graphite oxide was synthesized from nature graphite flake by a modified Hummers method [30]. Many investigations [27–29] confirmed that the oxygen atoms in GO presented in form of COOH, C O, OH and C O C groups. The hydrophilic oxygenated functional groups on the surface or at the edge of GO sheets played a critical role in improving the compatibility between GO and the polymer matrix. In addition, the swelling properties of GO/P(AA-co-AM) super-absorbent hydrogel nanocomposites in deionized water or aqueous solutions were also closely related to these hydrophilic groups. Fig. 1 showed the FTIR spectra of graphite and graphite oxide. The spectrum in Fig. 1a represented the vibrations that resulted from the graphite. As shown in Fig. 1b, the FTIR spectrum of graphite oxide appeared a broad absorption peak

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Fig. 2. WXRD patterns of pristine graphite (a), graphite oxide (b) and superabsorbent hydrogel containing 0.10 wt% GO (c). The interlayer spacing is represented by d.

at 3000–3500 cm−1 , which was related to the OH groups, and the absorption peaks at 1720, 1225 and 1060 cm−1 revealed the presence of COOH, C O C , and C OH groups, respectively. Besides, the spectrum also showed an absorption peak of C C group at 1615 cm−1 corresponding to the remaining sp2 character of graphite. The spectrum of graphite oxide was in good agreement with previous work, which proved the successful oxidation of graphite to graphite oxide [27–29]. Fig. 2 showed the WXRD patterns of graphite and graphite oxide. Compared with pristine graphite (Fig. 2a), the inter-layer spacing of graphite oxide (Fig. 2b) was increased obviously from 0.336 to 0.817 nm, indicating weakening of the inter-layer van der Waals interactions. Therefore, it was relatively easier for graphite oxide to be exfoliated to single or a few layers of GO by following ultrasonic treatment. The surface morphologies of graphite and graphite oxide were observed by FESEM in Fig. 3. Fig. 3A showed that the surface of pristine graphite was very flat and smooth, and each layer connected together closely due to the strong inter-layer van der Waals interactions. Unlike graphite, graphite oxide (Fig. 3B) exhibited a rough surface and appeared a large number of irregular wrinkles on the surfaces, which might result from numerous functional groups introduced by the oxidation process. Moreover, the distance between two layers became larger than pristine graphite, which supported the conclusion of WXRD. All of the above results demonstrated that the oxidation conversion process from graphite to graphite oxide was successful. Meanwhile, the process was also very essential for the effective exfoliation of graphite oxide to single or a few layers of GO sheets by ultrasonic treatment, resulting in a good dispersion of GO in the polymer matrix during the in situ reaction process. 3.2. Chemical structure of super-absorbent hydrogel nanocomposites The FTIR spectra of P(AA-co-AM) and GO/P(AA-co-AM) superabsorbent hydrogel nanocomposites were shown in Fig. 4. In the spectra of the nanocomposites, compared with the pure hydrogel, some absorption peaks changed in intensity and new absorption peaks appeared due to the presence of hydrogen bonds and possible covalent bonds between GO sheets and polymer chains. The absorption peaks at 2920, 2850 and 1456 cm−1 were the stretching and bending vibrations of CH2 groups, respectively. The

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Fig. 3. FESEM images with different magnifications of pristine graphite (A) and graphite oxide (B).

Fig. 4. FTIR spectra of P(AA-co-AM) (a) and GO/P(AA-co-AM) super-absorbent hydrogels containing 0.05 (b), 0.10 (c) and 0.30 (d) wt%.

absorption peak at 3000–3500 cm−1 corresponding to the OH and NH2 groups could be observed in each one. As GO content increased, the peak became stronger and broader obviously, mainly because a great number of OH groups on the surface of GO sheets enhanced the characteristic absorption. The characteristic absorption peaks of C O and C N groups at 1580 and 1406 cm−1 shifted to lower wavenumbers with increasing GO content, indicating that the strong hydrogen bonds existed between GO sheets and polymer chains. Meanwhile, the characteristic absorption peaks of COOH groups (1705 cm−1 ) (Fig. 4a) became broader obviously (Fig. 4c and d) and a new absorption peak appeared at 1595 cm−1 (Fig. 4c and d), suggesting that the ester groups ( COOR) formed in the GO/P(AA-co-AM) networks. This esterification reaction might take place between OH groups of GO sheets and COOH groups of polymer chains, and this reaction between GO and polymers had also been proved in other GO/polymer composite hydrogels [28,29]. Fig. 5 showed the schematic illustration of the preparation of GO/P(AA-co-AM) super-absorbent hydrogel nanocomposites by in situ radical solution polymerization. It revealed that possible

Fig. 5. Schematic illustration of the preparation of GO/P(AA-co-AM) super-absorbent hydrogel nanocomposites by in situ radical solution polymeration.

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Fig. 6. FESEM images with different magnifications of P(AA-co-AM) super-absorbent hydrogel (A) and GO/P(AA-co-AM) super-absorbent hydrogels containing 0.10 (B) and 0.50 wt% (C) GO.

covalent bonds formed between GO sheets and polymer networks during the in situ reaction process. 3.3. Morphologies of super-absorbent hydrogel nanocomposites Fig. 6 showed the FESEM images of dried P(AA-co-AM) and GO/P(AA-co-AM) super-absorbent hydrogel nanocomposites. As seen in Fig. 6A, the cross-section of P(AA-co-AM) hydrogel was extremely smooth, which might be intimately related to its crosslinking polymer networks. Furthermore, the cross-section morphology of GO/P(AA-co-AM) hydrogel containing 0.10 wt% GO sheets in Fig. 6B showed that the surface was also very compact, indicating that GO sheets were dispersed homogeneously in the polymer matrix and there were interfacial interactions between GO and the matrix. Fig. 2 showed the WXRD patterns of graphite oxide and GO/P(AA-co-AM) super-absorbent hydrogel nanocomposite containing 0.10 wt% GO sheets. It was clear that a crystalline peak of graphite oxide (Fig. 2b) was observed at 2 = 10.8◦ , which disappeared in the nanocomposite (Fig. 2c), indicating the complete exfoliation of graphite oxide into the individual GO sheets through ultrasonic treatment, and the good dispersion level of GO in the polymer matrix. The WXRD results were in accordance with the FESEM observation. However, when the amount of GO sheets was increased to 0.50 wt% in Fig. 6C, the cross-section morphology of the hydrogel was very different. It was apparent that the micro-phase separation between GO and the matrix was observed on a large scale, leading to a relatively loose structure of the hydrogel, which might result from the aggregation of GO sheets in the matrix during the polymerization.

The OM images of the direct surfaces of the dried hydrogels were shown in Fig. 7. Obviously, the surfaces of the nanocomposites were very rough and tight compared with the pure hydrogel. Moreover, the surfaces of the nanocomposites containing a small amount of GO sheets were relatively uniform, indicating that GO sheets did not form large agglomerates in the matrix. However, as the amount of GO sheets was further increased to 0.30 and 0.50 wt%, the apparent aggregations of GO sheets appeared in Fig. 7E and F. Thus, the OM results were in agreement with the FESEM morphologies as shown in Fig. 6. Therefore, owing to the hydrogen bonds and possible covalent bonds between GO sheets and polymer chains, an appropriate amount of GO sheets could be dispersed well in the matrix and enhanced effectively the intermolecular interactions between the components. On the contrary, an excessive amount of GO sheets might form large agglomerates and weakened the interfacial interactions, resulting in the micro-phase separation between the components. 3.4. Thermo-mechanical behavior of super-absorbent hydrogel nanocomposites Figs. 8 and 9 showed temperature dependence of the loss factor (tan ı) and the storage modulus of dried P(AA-co-AM) and GO/P(AA-co-AM) hydrogels, respectively. Tan ı was the ratio of energy dissipated as heat to the maximum energy stored in the sample. From Fig. 8, there were two relaxation peaks (␣- and ␤relaxation) for pure and composite hydrogels. The ␤-relaxation peak was broad and relatively low, which might be owing to the movements of the hydroxyl groups related to the water molecules

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Fig. 7. Optical microscope images of P(AA-co-AM) super-absorbent hydrogel (A) and GO/P(AA-co-AM) super-absorbent hydrogels containing 0.05 (B), 0.10 (C), 0.15 (D), 0.30 (E) and 0.50 (F) wt% GO.

Fig. 8. Temperature dependence of the loss factor, tan ı of P(AA-co-AM) superabsorbent hydrogel (a) and GO/P(AA-co-AM) super-absorbent hydrogels containing 0.05 (b), 0.10 (c), 0.15 (d), 0.30 (e) and 0.50 (f) wt% GO.

Fig. 9. Temperature dependence of the storage modulus, M of P(AA-co-AM) superabsorbent hydrogel (a) and GO/P(AA-co-AM) super-absorbent hydrogels containing 0.05 (b), 0.10 (c), 0.15 (d), 0.30 (e) and 0.50 (f) wt% GO.

contained in the polymer networks. This result was in agreement with the previous study [32]. This phenomenon might be due to the presence of a small amount of water molecules absorbed from humid air by the samples. Meanwhile, as the temperature increased from 30 to 100 ◦ C, the storage modulus for pure and composite hydrogels in Fig. 9 decreased slightly also mainly owing to the presence of water molecules that acted as a plasticizer to decrease the stiffness of polymer chains. With the temperature going up, as the water molecules in the polymer networks converted into water vapor to escape to the air continuously, the storage modulus increased to a maximum value, which also might be due to the effects of the water molecules as a plasticizer. Generally, the maximum of the tan ı peak (␣-relaxation peak) reflected the glass transition temperature (Tg ) of the polymers as well as their composites, and the width and intensity of the tan ı peak might be also used to evaluate the chemical structure as well as the interaction between fillers and the matrix [32–34]. Apparently, the ␣-relaxation peaks corresponding to the Tg s of the P(AA-co-AM) and GO/P(AA-co-AM) nanocomposites containing an appropriate amount of GO sheets appeared at relatively higher temperatures (>220 ◦ C), as seen in Fig. 8. It was noted that the nanocomposites containing 0.10 and 0.15 wt% GO sheets had higher Tg s (283 and 270 ◦ C) than that of the pure hydrogel, indicating that hydrogen bonds and possible covalent bonds between GO sheets and polymer chains limited the polymer segment motions to result in relatively higher Tg s [35]. In addition, the relatively higher Tg s of the nanocomposites were also due to the improvement of the damping properties of the polymer matrix with the addition of GO sheets. However, as the amount of GO sheets further increased, the Tg went down sharply and the reason might be explained as follows. When the nanocomposites were filled with a small amount of GO sheets, GO could form both abundant hydrogen bonds and possible covalent bonds with polymer chains because of the sufficient functional oxygenated groups on the surface of GO, thus, the Tg s of the composites could be improved obviously. On the contrary, when the amount of GO sheets increased more, the increased GO sheets tended to aggregate together, as a result, the interactions between GO sheets and the polymer matrix would become weak. Moreover, the increased GO sheets might also restrict the formation of covalent bonds of the polymer networks during the in situ polymerization. Therefore, the ␣-relaxation of the polymer chains appeared at relatively lower temperature, and the Tg s went down.

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Fig. 10. Swelling capacities of P(AA-co-AM) super-absorbent hydrogel and GO/P(AA-co-AM) super-absorbent hydrogels with various GO loadings in deionized water and in 0.9% NaCl solution.

Furthermore, the addition of GO sheets could broaden the ␣relaxation peak. All nanocomposites except the one containing 0.50 wt% GO sheets had relatively broader ␣-relaxation peaks in comparison with the pure hydrogel. The broader width of the ␣relaxation peak revealed multi-transition and molecular motion, implying that the inner chemical structures of the composite hydrogels were changed with the addition of GO sheets. Whereas, there existed the clear phase separation between the components for the composite hydrogel containing 0.50 wt% GO sheets, resulting in the relatively lower Tg and two distinguished ␣-relaxation peaks related to two components. In Fig. 9, the storage modulus of the pure hydrogel at glassy state was lower than the nanocomposites except the one containing 0.05 wt% GO sheets. This implied that the GO sheets could enhance the mechanical properties of the hydrogels. The mechanism of the enhancement might be due to the exceptional mechanical strength of GO sheets [4] and the good interactions between GO sheets and polymer chains. It was worth noting that the nanocomposite containing 0.10 wt% GO sheets possessed the highest Tg as well as the highest storage modulus at rubbery state, which might be owing to the hydrogen bonds and possible covalent bonds formed between GO sheets and polymer chains. Obviously, these results were in accord with the FTIR, FESEM and OM results. 3.5. Swelling behaviors of super-absorbent hydrogel nanocomposites The swelling capacity has been studied by many literatures related to super-absorbent hydrogels to evaluate the water-holding capacity [20,21]. The swelling capacities of P(AA-co-AM) and GO/P(AA-co-AM) super-absorbent hydrogel nanocomposites containing different amounts of GO sheets in deionized water and 0.9% NaCl solution were shown in Fig. 10. The swelling ratios of the hydrogels in 0.9% NaCl solution declined sharply comparing to the values measured in deionized water. In fact, this phenomenon was commonly observed in the swelling of polyelectrolyte hydrogels [21,36,37], and the reason may be explained as follows. The charge screening effect caused by counter ions (Na+ ) in salt solution could induce a clear decline of electrostatic repulsions, leading to a decrease of the osmotic pressure between hydrogel networks and the external solution. It could be observed that the swelling ratio of hydrogels went up with increasing GO loadings to 0.10 wt% and then decreased with further increasing GO loadings. Compared with the swelling capacity of pure P(AA-co-AM), 757 g/g in deionized water and

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Fig. 11. Swelling rates of P(AA-co-AM) and GO/P(AA-co-AM) super-absorbent hydrogels in deionized water.

42 g/g in 0.9% NaCl solution, the nanocomposites containing only 0.10 wt% GO sheets acquired the highest swelling capacity, 1094 g/g in deionized water and 75 g/g in 0.9% NaCl solution. The significant improvement of the swelling capacities of the nanocomposites containing extremely low GO content might be mainly due to the fact that the GO sheets containing plenty of functional groups, such as COOH, C O, OH and C O C groups on the surface, could dramatically increase the density of the hydrophilic groups of the polymer networks. Besides, the GO sheets dispersed homogeneously in the matrix might influence the microstructure of the polymer networks because of the exceptional nanostructure character of GO, which also could influence the swelling capacities. Furthermore, there might be some synergetic intermolecular interactions between GO sheets and the polymer networks for holding water or other aqueous fluids, leading to the enhancement of the swelling capacities. However, as the amount of the GO sheets further increased to 0.30 and 0.50 wt%, the swelling capacities of the nanocomposites decreased obviously. This might be owing to the fact that the excessive GO sheets tended to aggregate and weakened the synergetic interactions between GO sheets and the polymer networks, which have been confirmed by the FESEM and OM morphologies presented in Figs. 6 and 7, respectively. And this result was also in agreement with the thermo-mechanical properties shown in Figs. 8 and 9. GO sheets in the matrix could also affect the rate of the water diffusion in the process of swelling. Therefore, the swelling kinetics of the nanocomposites was very essential to be discussed. Figs. 11 and 12 showed the swelling rates and kinetics of P(AA-co-AM) and GO/P(AA-co-AM) super-absorbent hydrogel nanocomposites containing different amounts of GO sheets in deionized water, respectively. As shown in Fig. 11, it could be seen that the swelling rates of both pure and composite hydrogels were quick at the initial stage and then began to level off. The swelling equilibrium could be achieved within about 25 min. Moreover, the swelling rates of the nanocomposites containing a small amount (<0.30 wt%) of GO sheets were a little higher than the pure hydrogel at the initial stage. This might be attributed to the fact that a great number of hydrophilic groups on the surface of GO sheets fastened the water diffusion in the polymer networks. Besides, GO sheets also could introduce more carboxylic groups ( COO− ) into the polymer networks, as a result, the osmotic pressure of the nanocomposites increased dramatically during the swelling process. In contrast, an apparent decrease in swelling rates appeared at the initial stage when the amount of the GO

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Fig. 12. Pseudo-second-order kinetics of P(AA-co-AM) and GO/P(AA-co-AM) superabsorbent hydrogels in deionized water.

Fig. 13. Swelling ratios of P(AA-co-AM) and GO/P(AA-co-AM) super-absorbent hydrogels at different pHs.

sheets was more than 0.30 wt%. This might be explained that the excessive GO sheets formed agglomerates and the synergetic interactions between GO sheets and the polymer networks became weak, thus, the swelling rates went down. Furthermore, we also discussed the swelling kinetics of the pure and composite hydrogels in deionized water by a pseudo-secondorder model [21,38,39]. And the swelling rate at any time was expressed as follows:

a small amount (<0.30 wt%) of GO sheets could swell more quickly than pure hydrogel at the initial stage, it took more time for the nanocomposites to reach the swelling equilibrium. The nanocomposites containing only 0.10 wt% GO sheets possessed the highest theoretical and experimental swelling capacity and the lowest swelling rate constant (K). Thus, the pseudo-second-order model was suitable to discuss the swelling kinetics of the GO/P(AA-co-AM) super-absorbent hydrogel nanocomposites.

dW = K(We − W )2 dt

3.6. pH-sensitive behaviors of super-absorbent hydrogel nanocomposites

(2)

Integration of Eq. (2) for the boundary conditions t = 0 − t and W = 0 − Wt gave t 1 = Kt + We − W We

(3)

And then Eq. (3) could be rearranged to obtain a linear form, t 1 t + = W We K · We2

(4)

where W was the swelling ratio of the hydrogel at time t, We was the swelling ratio of the hydrogel at equilibrium and K was the rate constant. Obviously, the plot of t/W versus t gave a straight line with a slop of 1/We and an intercept of 1/(K · We2 ). Therefore, the swelling capacity (We ) and the swelling rate constant (K) could be evaluated by the slop and intercept, respectively. As seen in Fig. 12, the plot of t/W versus t showed a very straight line, suggesting that the experimental data were well fitted by the pseudo-second-order model. From the fitting curves, the swelling properties could be evaluated efficiently, i.e. the swelling capacities of the nanocomposites from the slop of the line and the relative swelling rates of the composite hydrogels from the intercept of the line. Table 1 summarized the data of the swelling rate constant (K) and the swelling capacities (We ) of all super-absorbent hydrogels. According to Table 1, the theoretical data of swelling capacities were close to the experimental data for all hydrogels. This result revealed that although the nanocomposites containing

Both P(AA-co-AM) and GO/P(AA-co-AM) super-absorbent hydrogel nanocomposites contained a great number of carboxylic groups ( COOH) which could be converted into COO− . In addition, the degree of ionization of COOH was different at various pH solutions. Therefore, the hydrogels exhibited different swelling behaviors at a wide range of pH values. Fig. 13 and Table 2 presented the swelling capacities of the pure hydrogel and the nanocomposites containing 0.10 and 0.50 wt% GO sheets at various pHs ranging from 1 to 13. Because the swelling capacities of the hydrogels could be strongly affected by the ionic strength of the medium, no additional ions (through buffer solution) were added to the medium for setting pH. In this section, NaOH (0.1 mol/L) and HCl (0.1 mol/L) solutions were diluted with deionized water to obtain the desired basic and acidic pHs, respectively. The similar swelling tendency was observed for both pure P(AAco-AM) and GO/P(AA-co-AM) super-absorbent hydrogels in the solutions with different pH values (1–13) shown in Fig. 13. It could be seen that all hydrogels exhibited low swelling capacities at very acidic medium (pH < 3). This might be due to the fact that most of the carboxylic groups ( COOH) in the polymer networks and GO sheets were protonated. As a result, numerous hydrogen bonds were formed among COOH, OH, C O, C O C and NH2 groups and the networks of the hydrogels tended to shrink, leading to the limited swelling capacities. As the pH value of the medium increased from 3 to 7, a great number of COOH groups

Table 1 Fitting swelling kinetic parameters of P(AA-co-AM) and GO/P(AA-co-AM) super-absorbent hydrogels in deionized water. GO content (wt%)

0

0.05

0.10

0.15

0.30

0.50

K ×10−5 g/(g/s) We (fitting data, g/g) We (experimental data, g/g)

1.56 781.25 757.41

0.50 1030.54 965.19

0.33 1192.10 1093.79

0.47 1044.88 976.63

1.28 666.67 745.98

1.68 378.78 393.71

Y. Huang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 401 (2012) 97–106

105

Table 2 Swelling ratios (g/g) of P(AA-co-AM) and GO/P(AA-co-AM) super-absorbent hydrogels at different pHs. pH

1

2

3

4

5

7

9

10

11

12

13

P(AA-co-AM) GO/P(AA-co-AM) (0.10 wt% GO) GO/P(AA-co-AM) (0.50 wt% GO)

5.07 4.75 3.17

59.64 84.69 37.63

287.43 366.86 232.49

437.43 614.38 222.39

575.85 802.98 304.68

757.41 1093.79 393.71

635.36 925.14 353.96

459.98 645.21 273.57

344.71 586.30 234.30

247.51 285.39 164.95

78.00 99.75 70.95

were ionized and converted into COO− so that many hydrogen bonds were broken and the electrostatic repulsion of anionic groups increased dramatically in the networks of the hydrogels. Hence, the swelling capacities of the hydrogels went up obviously. The maximum swelling capacities were observed at pH 7, which might be attributed to the fact that the hydrogels could swell more easily in the high osmotic pressure because of the lowest ionic strength of external solution. However, as the pH value further increased, the swelling capacities of the hydrogels declined sharply. This might be owing to the fact that the osmotic pressure of external solution was higher at high pHs. Besides, at very basic medium, the screening effect of the counter ions (Na+ ) shielded the charge of COO− and made the repulsive electrostatic interactions become weak, thus, the swelling capacities decreased continually. Fortunately, it was apparent that the swelling capacity of the nanocomposite containing 0.10 wt% GO sheets was higher than that of both the pure hydrogel and the nanocomposites containing 0.50 wt% GO sheets at various pH solutions. This indicated that the GO sheets uniformly dispersed in the polymer matrix could improve the swelling properties dramatically. This improvement might be due to the fact that a small amount of GO sheets could be dispersed homogeneously in the matrix to form intermolecular interactions with polymer networks by hydrogen bonds as well as possible covalent bonds. On the other hand, plenty of hydrophilic oxygenated groups on the surface of GO sheets could affect the swelling properties at acidic or basic solutions as well. However, when the amount of the GO sheets was excessive (0.50 wt%), the swelling capacity might be lower even compared with pure hydrogel because of the poor dispersion of the GO sheets as well as the micro-phase separation between GO sheets and the polymer matrix, which was confirmed by the FESEM and DMA measurements. 4. Conclusions We prepared a series of novel GO/P(AA-co-AM) super-absorbent hydrogels by a facile strategy of in situ radical solution polymerization successfully. Owing to the hydrogen bonds and possible covalent bonds between GO and polymer chains, relatively lower content (<0.30 wt%) of GO could be dispersed well in the polymer matrix and enhanced the intermolecular interactions between the components effectively. On the contrary, an excessive amount of GO sheets might form large agglomerates and weakened the interfacial interactions, resulting in the micro-phase separation between the components. Interestingly, the nanocomposites containing 0.10 wt% GO sheets possessed the highest Tg as well as the highest storage modulus at rubbery state. Furthermore, the GO content significantly influenced the swelling behaviors and swelling kinetics of the super-absorbent hydrogel nanocomposites. The swelling capacities and swelling rates of hydrogels went up with increasing GO content (<0.30 wt%) and then decreased with further increasing GO loadings. The pseudo-second-order model was suitable to discuss the swelling kinetics of the GO/P(AA-co-AM) super-absorbent hydrogel nanocomposites. It was worth mentioning that the hydrogels only containing 0.10 wt% GO sheets showed outstanding swelling capacities (1094 g/g in deionized water and 75 g/g in 0.9% NaCl solution), which were much higher than the pure hydrogel. Moreover, the GO/P(AA-co-AM) super-absorbent

hydrogels exhibited pH-sensitive behavior and could also retain relatively higher swelling capacities to a certain degree at acidic and basic solutions. Therefore, the as-prepared GO-based superabsorbent hydrogels containing extremely low GO content might have potential applications in many areas, such as biomedical engineering, construction engineering and hygienic products. Acknowledgements The authors acknowledge the SRF for ROCS, State Education Ministry, PR China, the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (Contract Grant No.: CUGL090223), Hubei Provincial Department of Education (XD2010037), Opening Project of Teaching Laboratory of China University of Geosciences (Wuhan) (SKJ2011118), and the grant of the Opening Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University) (KF201106). This work is partially supported by National High-Tech R&D Program (863 program) for the 12th Five-Year Plan, Ministry of Science and Technology, PR China (SQ2010AA1000690005). References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669. [2] A.K. Geim, K.S. Novoselov, The rise of graphene, Nat. Mater. 6 (2007) 183–191. [3] A.A. Balandin, S. Ghosh, W.Z. Bao, I. Calizo, D. Teweldebrhan, F. Miao, C.N. Lau, Superior thermal conductivity of single-layer graphene, Nano Lett. 8 (2008) 902–907. [4] C. Lee, X.D. Wei, J.W. Kysar, J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene, Science 321 (2008) 385–388. [5] M.D. Stoller, S.J. Park, Y.W. Zhu, J.H. An, R.S. Ruoff, Graphene-based ultracapacitors, Nano Lett. 8 (2008) 3498–3502. [6] S. Park, R.S. Ruoff, Chemical methods for the production of graphenes, Nat. Nanotechnol. 4 (2009) 217–224. [7] Q. Wu, Y. Xu, Z. Yao, A. Liu, G. Shi, Supercapacitors based on flexible graphene/polyaniline nanofiber composite films, ACS Nano 4 (2010) 1963–1970. [8] N. Yang, J. Zhai, D. Wang, Y. Chen, L. Jiang, Two-dimensional graphene bridges enhanced photoinduced charge transport in dye-sensitized solar cells, ACS Nano 4 (2010) 887–894. [9] P.K. Ang, W. Chen, A.T.S. Wee, K.P. Loh, Solution-gated epitaxial graphene as pH sensor, J. Am. Chem. Soc. 130 (2008) 14392–14393. [10] M.A. Rafiee, J. Rafiee, I. Srivastava, Z. Wang, H. Song, Z. Yu, N. Koratkar, Fracture and fatigue in graphene nanocomposites, Small 6 (2010) 179–183. [11] Y. Zhan, F. Meng, X. Yang, X. Liu, Magnetite-graphene nanosheets (GNs)/poly(arylene ether nitrile) (PEN): Fabrication and characterization of a multifunctional nanocomposite film, Colloid Surf. A: Physicochem. Eng. Aspects 390 (2011) 112–119. [12] D.R. Dreyer, S. Park, C.W. Bielawski, R.S. Ruoff, The chemistry of graphene oxide, Chem. Soc. Rev. 39 (2010) 228–240. [13] K. Kosemund, H. Schlatter, J.L. Ochsenhirt, E.L. Krause, D.S. Marsman, G.N. Erasala, Safety evaluation of superabsorbent baby diapers, Regul. Toxicol. Pharm. 53 (2009) 81–89. [14] C. Chang, B. Duan, J. Cai, L. Zhang, Superabsorbent hydrogels based on cellulose for smart swelling and controllable delivery, Eur. Polym. J. 46 (2010) 92–100. [15] N.N. Reddy, K. Varaprasad, S. Ravindra, G.V.S. Reddy, K.M.S. Reddy, K.M.M. Reddy, K.M. Raju, Evaluation of blood compatibility and drug release studies of gelatin based magnetic hydrogel nanocomposites, Colloid Surf. A: Physicochem. Eng. Aspects 385 (2011) 20–27. [16] R. Liang, H. Yuan, G. Xi, Q. Zhou, Synthesis of wheat straw-g-poly(acrylic acid) superabsorbent composites and release of urea from it, Carbohydr. Polym. 77 (2009) 181–187. [17] L. Wang, J. Zhang, A. Wang, Removal of methylene blue from aqueous solution using chitosan-g-poly(acrylic acid)/montmorillonite superadsorbent nanocomposite, Colloid Surf. A: Physicochem. Eng. Aspects 322 (2008) 47–53. ˜ F. Santiago-Gutiérrez, J.L. Morán-Quiroz, S.L. Hernandez[18] E. Orozco-Guareno, Olmos, V. Soto, W. Cruz, R. Manríquez, S. Gomez-Salazar, Removal of Cu(II)

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