ORIGINAL PAPER Swelling behavior of sulfonated polyacrylamide nanocomposite hydrogels in electrolyte solutions: comparison of theoretical and experimental results Jamal Aalaie • Ebrahim Vasheghani-Farahani Received: 7 September 2011 / Accepted: 2 January 2012 / Published online: 17 February 2012 Ó Iran Polymer and Petrochemical Institute 2012 Abstract Nanocomposite hydrogels were prepared by cross-linking of aqueous solutions of sulfonated poly- acrylamide/sodium montmorillonite with chromium triac- etate as ionic cross-linker. The effect of montmorillonite content on equilibrium swelling in NaCl and CaCl 2 solu- tions, ultimate storage modulus and effective cross-link density was evaluated. The limiting storage modulus of the nanocomposite (NC) hydrogels dropped by increasing montmorillonite content up to 1,000 ppm, and then it increased by further montmorillonite loading. A mecha- nism is proposed for the formation of PAMPS/Na ? -MMT/ Cr 3? NC hydrogels. According to this mechanism, the drop in limiting storage modulus of the NC gels at low Na ? - MMT concentration is due to ionic interactions between the negative layers of sodium montmorillonite and Cr 3? , leading to decreased cross-link density. However, the increase of the limiting storage modulus of the NC gels at high clay concentration results from the strong interactions between the polyacrylamide chains and clay platelets. The equilibrium swelling ratio of the NC networks decreased with increase of montmorillonite content in both aqueous NaCl and CaCl 2 solutions. In addition, the experimental swelling data of these NC hydrogels were described by a modified Flory–Rehner theory. The modified model was sensitive to montmorillonite concentration and it described adequately the swelling data for NC gels in NaCl solutions. Nevertheless, theoretical predictions showed some deviations from experimental results for swelling of NC hydrogels in CaCl 2 solutions. Keywords Nanocomposite  Polymer gels  Sulfonated polyacrylamide  Swelling  Modeling  Electrolyte solutions Introduction In recent years, preparation of nanocomposite hydrogels has attracted more attention to overcome the weakness and limitations of the conventional hydrogels [1–3]. Nano- composite hydrogels have excellent properties, such as mechanical toughness, large deformability, high swelling/ deswelling rates, and high transparency. These remarkable improvements in properties of nanocomposite hydrogels were ascribed to their characteristic network structure [4, 5]. Numerous attempts have been made to develop theo- retical models to explain the swelling behavior of con- ventional hydrogels [6–8]. Yet, to the best of our knowledge, there has been no report on the theoretical description of NC hydrogel swelling, especially those which are based on sulfonated polyacrylamide. The most widely used theoretical model for explaining swelling behavior of conventional hydrogels is the classical swelling theory developed by Flory and Rehner [9], which describes the swelling behavior of gels where cross-links are introduced in the dry stage and assumed to be tetra- functional. Although Flory–Rehner theory has been criti- cized mainly on the assumption of additivity of mixing and elastic free energies in determining the changes in the free energy of the system, it is still being used as starting point of later theories. Dusek et al. [10] derived an equation for memory-term of gels prepared in a solvent. J. Aalaie  E. Vasheghani-Farahani (&) Chemical Engineering Department, Tarbiat Modares University, PO Box: 14155-143, Tehran, Iran e-mail: evf@modares.ac.ir J. Aalaie Polymer Science and Technology Division, Research Institute of Petroleum Industry (RIPI), PO Box: 14665-1137, Tehran, Iran Iran Polymer and Petrochemical Institute 123 Iran Polym J (2012) 21:175–183 DOI 10.1007/s13726-012-0016-3