Technical Note Improved mechanical and swelling behavior of the composite hydrogels prepared by ionic monomer and acid-activated Laponite Peng Li a , Nam Hoon Kim b , David Hui c , Kyong Yop Rhee d , Joong Hee Lee a,b, ⁎ a BIN Fusion Research Team, Department of Polymer and Nano Engineering, Chonbuk National University, Jeonju, Jeonbuk, 561-756, South Korea b Department of Hydrogen and Fuel Cell Engineering, Chonbuk National University, Jeonju, Jeonbuk, 561-756, South Korea c Dept of Mechanical Engineering, University of New Orleans, New Orleans, LA 70148, USA d Department of Mechanical Engineering, Kyonghee University, Seochun-dong 1, Yongin-si, Kyongki-do, 446-701, South Korea abstract article info Article history: Received 12 June 2009 Received in revised form 11 October 2009 Accepted 14 October 2009 Available online 24 October 2009 Keywords: Acid-activation Hydrogel X-ray diffraction (XRD) Swelling Rheological properties A composite hydrogel (CH) with much improved mechanical and swelling properties was prepared using an ionic monomer and acid-activated Laponite XLS which was used as a cross-linking agent. Addition of acid- activated clay solved the gelation problem when ionic monomers were added to clay mineral dispersions. Reaction of Laponite XLS with sulfuric acid yielded amorphous silica. A dispersion of the acid-activated Laponite and the monomers was used to synthesize composite hydrogels by in-situ polymerization. The FT- IR spectra and rheological results of the composite hydrogels demonstrated the formation of a network. The equilibrium swelling ratios of composite hydrogels (> 6000 g/g) were more than 18 times larger than traditional organic cross-linked hydrogels. The moduli G′ and G″ in the observed frequency range were about 4 and 10 times larger than those of organic cross-linked hydrogel (OR gel). The improvement in both the equilibrium swelling ratio and mechanical strength was attributed to the homogeneous cross-linked network structure. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Hydrogels consisting of three-dimensional polymer networks filled with water have attracted considerable attention as functional soft materials. Hydrogels were used in many fields, such as soft con- tact lenses, superabsorbent polymeric gels, carriers for proteins and nucleic acids in gel electrophoresis, amendments in greening and agriculture, medical and food products, etc. (Matsumoto et al., 2004; Akashi et al., 2002; Hellweg et al., 2000). Most conventional hydrogels are cross-linked by organic compounds. However, an inhomogeneous randomly cross-linked network often makes the hydrogel weak and fragile, which limit its application seriously (Kim et al., 1995). Several attempts were made to design hydrogels with improved mechanical properties (Okumurs and Ito, 2001; Gong et al., 2003). Recently, three new hydrogels with excellent mechanical properties (Tanaka et al., 2005) were prepared: a ‘topological gel (TP gel)’ using figure-of-eight cross-linkers (Okumurs and Ito, 2001), a ‘nanocompo- site gel (NC gel)’ using a Hectorite as cross-linker (Haraguchi and Takehisa, 2002), and a ‘double network gel (DN gel)’ by interpene- trating polymer networks (IPN) (Gong et al., 2003). Compared with the other two gels, the NC gel had the advantage of simplicity of synthesis. However, it is very difficult to prepare NC hydrogels con- taining ionic groups. When ionic monomers are added, the viscosity of the clay mineral dispersion increases significantly due to the aggregation of the clay mineral particles which inhibits polymeriza- tion. Application of ionic monomers in hydrogels limited the weak mechanical stability. It is a challenge to prepare NC hydrogels with ionic monomers. In order to solve the problem, quaternary ammonium ions con- taining two double bonds were added as co-monomers or intercalat- ing agents (Zhang et al., 2005; Xu et al., 2007). However, anionic NC gels cannot be prepared in this way. Reaction of Laponite with acids yields amorphous silica with high specific surface area, pore volume and surface charge density (Van Rompaey et al., 2002; Madejova, 2003; Komadel and Madejová, 2006). Another possibility is to prepare hydrogels by cross-linking with acid-activated clay minerals. Due to the large specific surface area and high surface charge density, sufficient amounts of the initiator can be adsorbed so that a network structure forms without an organic cross-linker. 2. Experimental 2.1. Materials Acrylic acid (AA) (TCI, Japan), N,N′-methylene-bis-acrylamide (Sigma, reagent grade), N,N,N′,N′-tetramethylenediamine (TEMED) Applied Clay Science 46 (2009) 414–417 ⁎ Corresponding author. Department of Polymer and Nano Engineering, Chonbuk National University, Duckjin-dong 1Ga 664-14, Jeonju, Jeonbuk, 561-756, South Korea. Tel.: +82 63 270 2342; fax: +82 63 270 2341. E-mail address: jhl@chonbuk.ac.kr (J.H. Lee). 0169-1317/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2009.10.007 Contents lists available at ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/clay