Surfactant-assisted synthesis of a transparent ionic nanocomposite hydrogel

Applied Clay Science 101 (2014) 335–338

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Surfactant-assisted synthesis of a transparent ionic nanocomposite hydrogel Huili Li, Renbao Gu, Shimei Xu ⁎, Adalati Abudurman, Jide Wang Key Laboratory of Oil & Gas Fine Chemicals, Ministry of Education, Xinjiang University, Urumqi, Xinjiang 830046, PR China

a r t i c l e

i n f o

Article history: Received 27 September 2013 Received in revised form 17 August 2014 Accepted 19 August 2014 Available online 22 September 2014 Keywords: Nanocomposites Polymers Hydrogel Ionic monomers

a b s t r a c t It was a challenge to prepare ionic nanocomposite hydrogels, because of the precipitation of the ionic monomer and clay complexes. In this work, a transparent ionic nanocomposite hydrogel cross-linked by Laponite XLG was successfully synthesized via the in-situ free radical polymerization of ionic monomer acrylic acid (AA) with assistance of sodium dodecyl sulfate (SDS). The addition of SDS can successfully avoid the aggregation of Laponite in the ionic monomer AA. It was a new approach to obtain ionic nanocomposite hydrogels with high transparency. The transparency of the hydrogel was changed reversibly with the temperature alternate changing. The elongation at break of the hydrogels was 2000% and the ultimate tensile strength was about 162 kPa. The facile method is important to the fabrication and potential applications of nanocomposite hydrogels. © 2014 Elsevier B.V. All rights reserved.

1. Introduction In recent years, owing to the unique and uniform three-dimensional network structure, Laponite polymer nanocomposite hydrogels had been widely investigated for the mechanical properties (Xiong et al., 2008), biological applications (Kokabi et al., 2007), stimuli-response (Wang and Chen, 2012) and so on. Typically, nanocomposite hydrogels, in which clay layers were regarded as multifunctional cross-linkers, were composed of nonionic monomers, such as acrylamide (AM) (Okay and Oppermann, 2007), N-isopropylacrylamide (NIPAm) (Shibayama et al., 2004), and N,N-dimethylacrylamide (DMAA) (Haraguchi et al., 2003). In the presence of ionic monomers, however, Laponite with strong negative charges on the surface and weak positive charges on the edge (Liu et al., 2006) tended to aggregate in the preparation process (Ruzicka et al., 2006). Preadsorption of nonionic monomers (i.e., NIPAm or AM) before further polymerization of ionic monomers (sodium methacrylate (SMA) (Hu et al., 2009) or 2-(dimethylamino) ethyl methacrylate (DMAEMA) (Zhu et al., 2010)) could avoid the aggregation of Laponite in certain degree. However, the dispersion problem still existed when the ionic segments increased to more than 1–15% of nonionic monomers. In the other case, supernatant of acid-activated Laponite XLS in acrylic acid (AA) was separated and used for the preparation of ionic nanocomposite hydrogel (Li et al., 2009). Till now, there had been limited success in the preparation of transparent ionic nanocomposite hydrogels.

In previous work, a transparent ionic nanocomposite hydrogel was successfully prepared via in-situ copolymerization of 2-acrylamido-2methylpropanesulfonic acid (AMPS) and AA in Laponite dispersion (Chen et al., 2013). This paper demonstrated a simple surfactantassisted method for the preparation of ionic nanocomposite hydrogel with excellent transparency and mechanical properties. With the assistance of anionic surfactants sodium dodecyl sulfate (SDS), Laponite could uniformly disperse in AA solution without any aggregation. The resulting hydrogels were synthesized via the in-situ free radical polymerization of AA in the presence of SDS and exfoliated clay layers. Besides, the resultant hydrogel showed a reversible change in transparency during polymerization. This work provided a simple method to prepare transparent ionic nanocomposite hydrogel. 2. Experimental part 2.1. Materials Laponite XLG (Mg5.34Li0.66Si8O20(OH)4Na0.66, Rockwood Co., U.S.), sodium dodecyl sulfate (SDS, Tianjin Sheng'ao Chemistry Reagent Factory, China), acrylic acid (AA, Tianjin Fuchen Chemistry Reagent Factory, China), and ammonium persulfate (APS, Xi'an Chemical Co., China). The reagents were of analytical grade and used as received without any further purification. All solutions were prepared in deionized water. 2.2. Preparation of Laponite PAA nanocomposite hydrogels

⁎ Corresponding author. Tel.: + 86 991 8583972; fax: + 86 991 8581018. E-mail address: [email protected] (S. Xu).

http://dx.doi.org/10.1016/j.clay.2014.08.024 0169-1317/© 2014 Elsevier B.V. All rights reserved.

To prepare the nanocomposite hydrogel, 0.32 g Laponite was first dispersed in 8 ml deionized water, then 0.55 g SDS (30 times as many

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H. Li et al. / Applied Clay Science 101 (2014) 335–338

as critical micelle concentration (CMC)) was mixed and stirred for 20 min. 3.2 g AA, which was added in an ice water bath to refrain from self-polymerization of AA, and 0.8 ml 10% initiator APS were put in under stirring for another 20 min, respectively. The reaction was carried out at 50 °C for 12 h in a glass tube (5 mm interior diameter × 150 mm length) till the nanocomposite hydrogel was obtained. To study the effect of SDS concentration on the mechanical properties of nanocomposite hydrogels, the content of SDS was changed to 0.5, 1 and 15 times as many as CMC, respectively. The as-prepared hydrogels were soaked in excess deionized water, which was exchanged everyday, for 1 week. Lastly, the hydrogels were dried at 80 °C until a constant mass was obtained. 2.3. Characterization The transparency changes of dispersion were monitored during the polymerization using a UV/vis 2450 spectrophotometer (Shimadzu, Japan) at wavelength 600 nm with a temperature controlling set. Fourier transform infrared (FTIR) spectra were carried out using IR spectrophotometer (Bruker Equinox 55). Transmission electron micrograph (TEM) was conducted with the HITACHI H-600 transmission electron microscope with an accelerating voltage of 100 kv. 2.4. Mechanical properties of nanocomposite hydrogels Tensile strength of the as-prepared hydrogel was performed using a H5KT testing system (Tinius Olsen, U.S.) at 25 °C. The hydrogel length between the jaws was 20 mm and the crosshead speed was 100 mm/min. 3. Results and discussion FTIR spectra were measured to make out characteristic structure of the Laponite PAA nanocomposite hydrogel (Fig. 1a). Comparing to FTIR spectra of Laponite, in which Si\O stretching and bending bands appeared at 1010 cm−1 and 660 cm−1, respectively, characteristic absorption peaks of Si\O in resultant hydrogel shifted to 1093 cm− 1 and 616 cm−1 accordingly. It was revealed that the dipolar interaction between Laponite and PAA retained in the spectrum of the Laponite PAA hydrogel. In addition, stretching vibrations of \COOH of AA segment emerged at 1718 cm− 1. The adsorptions at 1558 cm−1 and 1456 cm−1 were attributed to asymmetric and symmetric absorption peaks of \COO−. However, the characteristic peak of SDS could not be found. So SDS was washed away during the washing process. This indicated that SDS may play a dispersing role in the dispersion of Laponite in AA. Fig. 1b was the TEM micrograph of the Laponite PAA nanocomposite

Fig. 2. Changes of transparency during the polymerization of nanocomposite hydrogel (the content of SDS is 30 times as many as CMC); the inset images of (a) before polymerization; (b) after polymerization at 50 °C; (c) cool down to 25 °C after polymerization at 50 °C.

hydrogel. It demonstrated that Laponite was homogeneously dispersed in the polymer matrix, which contributed to high optical transparence. Changes in optical transmittance were the specific property of nanocomposite hydrogel (Haraguchi et al., 2005). However, a different change in transparency was observed in the work. Diversification of transparency during the polymerization of nanocomposite hydrogel was depicted in Fig. 2. The initial reaction solution was uniform but exhibited low transparence of 33% (Fig. 2a). The SDS formed micelles above CMC and aligned in the surrounding of the clay layers by electrostatic interaction that would scatter light. The transparency increased to 97% because of the micelle size of SDS shrank with increasing temperature to 50 °C (Hammouda, 2013). However, subsequent formation of Laponite-PAA brushes (Haraguchi et al., 2005) led to an abrupt drop of transparency at 3900 s. With the longer grafted polymer chains formed (Haraguchi et al., 2005), the transmittance increased in the further polymerization. However, the transmittance turned to a downward trend, and then a lower transparency of 6% was obtained with the increase of polymerization time at 50 °C (Fig. 2b). When the transparency became constant (dot b), the temperature was reduced to 25 °C (Fig. 2b) gradually. Interestingly, an excellent transparency of 98% was observed at 25 °C (Fig. 2c). From the results described above, a mechanism for the synthesis of nanocomposite hydrogel was proposed as depicted in Scheme 1.

Fig. 1. (a) FTIR spectra of Laponite and Laponite PAA nanocomposite hydrogel; (b) TEM image of Laponite PAA nanocomposite hydrogel. The concentration of SDS is 30 times as many as CMC.

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Scheme 1. Schematic representations of the model structures for the mechanism forming the nanocomposite hydrogel.

To confirm the reversibility of transmittance change of the asprepared hydrogels, the temperature was alternately changed between 40 and 50 °C, which have covered the entire range of the transmittance transition temperature, for several cycles as shown in Fig. 3. The lowest transmittance was observed at 50 °C, and the transmittance increased when the temperature was changed to 40 °C. The hydrogel showed a good reversibility of transparency with change in temperature. This phenomenon was possibly caused by different arrangements of SDS at different temperatures. It was confirmed by the fact that no transparency change would be observed if SDS was washed away after polymerization. More details will be discussed in further experiment. The tensile stress–strain curves of nanocomposite hydrogels with different values of CSDS (the content of SDS was about 0.5, 1, 15 and 30 times as many as CMC corresponding to lines a–d) were shown in Fig. 4. The elongation at break increased with gently increasing concentration of SDS. While the content of SDS increased from 0.5 to 30 as many as CMC, the elongation at break increased from 1031% to 2000% with tensile strength ranging from 108 to 162 kPa. The product showed higher tensile strength than the nonionic nanocomposite hydrogels containing AM about 117 kPa (Xiong et al., 2008). The mechanical property was closely dependent of the dispersion of clay in the polymer matrix.

Fig. 3. Reversible change in the transparency of hydrogels by alternating the temperature between 40 and 50 °C.

4. Conclusions In conclusion, the ionic monomer AA had been successfully polymerized into the nanocomposite hydrogels in the presence of SDS. The hydrogels demonstrated an excellent value of optical transmittance as high as 98%. The presence of SDS endowed the hydrogel a reversible transparency change upon temperature. The ionic nanocomposite hydrogel exhibited a good mechanical strength. The elongation at break of hydrogel increased while more SDS was added. The work provided a simple and effective way to fabricate both ionic nanocomposite hydrogels and stimuli-sensitive polymers.

Acknowledgment This work was financially supported by the Program for New Century Excellent Talents in University (NCET-11-1072) and National Natural Science Foundation of China (No. 51163015).

Fig. 4. Tensile stress–strain curves for the as-prepared nanocomposite hydrogels with different SDS contents (the concentration of SDS is 0.5, 1, 15 and 30 times as many as CMC corresponding to lines a–d.).

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