Received: 3 November 2007, Revised: 14 June 2008, Accepted: 20 November 2008, Published online in Wiley InterScience: 29 December 2008 Synthesis and properties of poly(acrylamide-co-acrylic acid)/ polyacrylamide superporous IPN hydrogels Haiyong Ao a , Miaoliang Huang a , Jihuai Wu a * , Jianming Lin a , Qunwei Tang a and Hui Sun a Poly(acrylamide-co-acrylic acid)/polyacrylamide [P(AM-co-AA)/PAM] hydrogel with superporous and interpenetrating network (IPN) structure was prepared by a prepolymerization reaction and a synchronous polymerization reaction and frothing process. Scanning electron microscope (SEM) images show that the resultant hydrogel possesses abundant interconnected pores. DSC indicates that the porous structure enhances the swelling ratio and reduces the interaction between water and the hydrogel. In contrast, the IPN by PAM decreases water absorbency and enhances water retentivity. It is found that a superporous stucture in the hydrogel increases the equilibrium swelling ratio and decreases the compressive strength of the hydrogel. On the other hand, the increase in AM oligomer (oligo-AM) amount decreases the equilibrium swelling ratio and improves the compressive strength of the hydrogel. Therefore, the two-steps synthesis method can be used to construct a hydrogel with superporous and IPN structure. The swelling and mechanical properties of the hydrogel can be improved effectively. Copyright ß 2008 John Wiley & Sons, Ltd. Keywords: polyacrylamide; poly(acrylic acid); superporous hydrogel; interpenetrating networks; swelling ratio; mechanical property INTRODUCTION About three decades ago, superabsorbent polymers (SAPs) were introduced into the agriculture and diaper making industry,to utilize the excellent water holding property of SAPs. In 1994, superporous hydrogels (SPHs) were introduced as a different category of water-absorbent polymer systems. SPHs, just like SAPs, are structurally crosslinked hydrophilic polymers, besides they have superporous structure and can absorb considerable amounts of water or aqueous fluids (10–1000 times of their original weight or volume) in relatively short periods of time. [1–4] Unique physical properties of SPHs such as high water affinity, high thermal and mechanical stability, biocompatibility, etc. provided them with a variety of industrial applications. [5,6] Although large pores provide SPHs with fast and high water absorption and retention, the mechanical strength of the SPHs is too weak to sustain their functions for a long time. Their porous structures are destroyed easily even under a small pressure. In order to improve the mechanical strength of SPHs, some crosslinked hydrophilic polymers such as Ac-Di-Sol 1[7] and Carbopol 1[8] were used to prepare the SPH composites (SPHCs). But the prepared methods are moderately complicated and the mechanical strength of the SPHCs is still lower. The interpene- trating network (IPN) technique has been believed to be an important method to improve the mechanical properties of hydrogels. [9–11] Furthermore, the two-step synthesization pro- cedure is a simple way to prepare IPN hydrogels. [12,13] In this paper, a poly(acrylamide-co-acrylic acid)/polyacrylamide [P(AM-co-AA)/PAM] hydrogel with superporous and INP structure was prepared by a simple two-steps method. PAM and PAA have high water affinity and commercial availability. However, since PAM and PAA do not have enough mechanical strength in the swollen state, an INP structure is constructed to improve their mechanical strength. The synthesis course is shown in Eq. (1). Oligo-AM was formed in the prepolymerization reaction (step I); then the mixed solution of AM and AA monomers was introduced into the system; a P(AM-co-AA)/PAM hydrogel with superporous and IPN structure was formed in the synchronous polymerization and frothing process (step II). AM ! prepolymerization oligo  AM ! introducing AMþAA oligoAMþAAþAM ! polymerizationþfrothing PðAM  co  AAÞ=PAM (1) (www.interscience.wiley.com) DOI: 10.1002/pat.1361 Research Article * Correspondence to: J. Wu, The Key Laboratory for Functional Materials of Fujian Higher Education, Institute of Materials Physical Chemistry, Huaqiao University, Quanzhou 362011, China. E-mail: jhwu@hqu.edu.cn a H. Ao, M. Huang, J. Wu, J. Lin, Q. Tang, H. Sun The Key Laboratory for Functional Materials of Fujian Higher Education, Institute of Materials Physical Chemistry, Huaqiao University, Quanzhou 362011, China Contract/grant sponsor: National Natural Science Foundation of China; contract/grant numbers: 50572030; 50372022. Contract/grant sponsor: Key Scientific Technology Program of Fujian, China; contract/grant numbers: 2005HZ01-4; 2007HZ0001-3. Contract/grant sponsor: Specialized Research Fund for the Doctoral Program of Higher Education; contract/grant number: 20060385001. Polym. Adv. Technol. 2009, 20 1044–1049 Copyright ß 2008 John Wiley & Sons, Ltd. 1044