Superabsorbent nanocomposite (alginate-g-PAMPS/MMT): Synthesis, characterization and swelling behavior

Abstract

A superabsorbent composite (alginate-g-PAMPS/MMT) was prepared by graft copolymerization from alginate, 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) and Na+ montmorillonite (MMT) in an inert atmosphere. Effects of polymerization variables on water absorbency, including the content of Na+ montmorillonite, sodium alginate, N,N′-methylenebisacrylamide and AMPS, were studied. The introduced montmorillonite formed a loose and porous surface and improved the water absorbency of the alginate-g-PAMPS/MMT superabsorbent composite. Swelling behaviors of the superabsorbent composites in various cationic salt solutions (NaCl, CaCl₂ and FeCl₃) and anionic salt solutions (NaCl and Na₂SO₄) were also systematically investigated. The superabsorbent composite was further characterized using Fourier transform infrared spectroscopy (FTIR), rheology, thermogravimetric analysis (TGA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) taking alginate-g-PAMPS as a reference.

Introduction

Superabsorbent hydrogels are mainly hydrophilic polymer networks that swell to a high degree in water or biological fluids due to an extremely high affinity to water (Fujioka et al., 2009, Ramazani-Harandi et al., 2006, Wang and Wang, 2009). Superabsorbents can absorb a large amount of water compared with general water absorbing materials in which the absorbed water is hardly removable even under some pressure. Because of their excellent characteristics, superabsorbents are widely used in health, agriculture and horticulture applications, which has resulted in considerable interest and research in this field (Sakiyama et al., 1993, Shiga et al., 1992, Shiga et al., 1993, Yoshida et al., 1989) since the first superabsorbent polymer was reported by the U.S. Department of Agriculture in 1976 (Fanta & Doane, 1976). Hydrogels that respond to external stimuli such as heat, pH, electric fields and chemical environments are often referred to as “intelligent” or “smart” hydrogels (Amalia et al., 2007, Peppas, 1986, Takashi et al., 2004). These responsive hydrogels have become an important area of research and development in the fields of medicine, pharmacy, and biotechnology (Andrade, 1976, Galaev and Mattiasson, 1993, Schacht, 2004). Most hydrogels are formed by copolymerization of different vinyl monomers containing hydrophilic side groups with natural polysaccharides as well as their derivatives (Pourjavadi et al., 2009, Pourjavadi et al., 2008, Pourjavadi et al., 2006). Because of their exceptional properties, i.e. biocompatibility, biodegradability, renewability, and non-toxicity, polysaccharides form the main part of the natural-based superabsorbent hydrogels. The higher production cast and low gel strength of these superabsorbents, however, severely restrict their application.

To overcome these limitations, low cost inorganic compounds can be used. The introduction of inorganic fillers to a polymer matrix increases its strength and stiffness. Among inorganic compounds, special attention has been paid to clay minerals for use in nanocomposites because of their small particle size and intercalation properties. Nanocomposites exhibit improved or even novel properties when compared to micro- and macro-composites. Strong interfacial interactions between the dispersed clay layers and the polymer matrix lead to enhanced mechanical, thermal and barrier properties over the virgin polymer. Montmorillonite (MMT) is a layered aluminum silicate with exchangeable cations and reactive OH groups on the surface. It is one of the most widely used layered silicates because its lamellar elements display high in plane strength, stiffness, and high aspect ratio (Mansoori, Atghia, Zamanloo, Imanzadeh, & Sirousazar, 2010). Alginate[degree of substitution (DS) = 0.5], also called algin or alginic acid, is a naturally occurring colloidal hydrophilic polysaccharide (Bhat & Aminabhavi, 2007) obtained from various species of brown seaweed (phaceophyceae) (Kulkarni et al., 2001, Lim et al., 1986, Medlin et al., 1997, Toti and Aminabhavi, 2002, Toti et al., 2002). It is a linear copolymer consisting mainly of a residue of β-1,4-linked d-mannuronic acid and α-1,4-linked l-guluronic, as shown in Scheme 1 (Painter and Aspinall, 1983, Rinaudo and Kamerling, 2007, Santacruz et al., 2002). It has a broad range of applications in pharmaceutical (Badwan et al., 1985, Renken and Hunkeler, 2007), biomedical (Yoshioka, Tsuru, Hayakawa, & Osaka, 2003) and agricultural areas (Lohmann, 1992, Tsuji, 1995). It also plays an important role as an adduct product (Martinsen, Skjak-Braek, & Smidsrod, 1989) in food due to its ability to form a highly viscous solution. Some workers reported that alginate can easily form a gel (Draget et al., 1989, Rhim, 2004) in the presence of a divalent cation as the calcium ion. Graft copolymerization of hydrophilic vinyl monomers onto alginate using efficient redox initiators is one of the best and most convenient methods of synthesizing these materials (Yadav, Mishra, Sand, & Behari, 2011) among many diverse approaches including the use of a catalyst such as AIBN (Abdel-Razik, 1997, Egboh et al., 1984). Further, high energy radiation (Cardona and George, 2002, Dargaville et al., 2003, Kabanov and Kudryavtsev, 2003) such as X-ray or gamma-ray is used for modifying the natural polysaccharides to impart desirable properties like swelling according to the needs of the particular final applications. Superabsorbents based on alginate and its graft copolymerization with methacrylamide have been reported (Pourjavadi, Amini-Fazl, & Hosseinzadeh, 2005). Further, 2-acrylamido-2-methylpropanoesulfonic acid (AMPS) is a hydrophilic vinyl monomer containing nonionic and anionic groups. Increasing the number of ionic groups in the superabsorbent is known to increase the swelling capacity, while the nonionic groups can improve the salt tolerance of the material. 2-Acrylamido-2-methylpropanesulfonic acid (AMPS) is also used in hydrogel preparation (Durmaz and Okay, 2000, Ozmen and Okay, 2005). AMPS-based hydrogels exhibit electroresponsive properties and can be used in electronics applications (Sayil and Okay, 2002, Zhang and Easteal, 2003). They can also be used, for example, in biochemistry applications for skin contact electrodes (Kim, Lee, & Kim, 2004) and as muscle actuators (gel robots) (El-Hag Ali, Abd El Rehim, Hegazy, & Ghobashy, 2006). The incorporation of MMT can not only reduce production cost, but also improve the properties (such as swelling ability, gel strength, mechanical and thermal stability) of superabsorbents. In this paper, we report the preparation of a superabsorbent by graft copolymerization of vinyl monomers along the chains of alginate in the presence of MMT.