pH-response of chitosan, k-carrageenan, carboxymethyl cellulose sodium salt complex hydrogels Tetsu Mitsumata a, * , Yuuki Suemitsu a , Keiko Fujii b , Tomoyuki Fujii c , Takashi Taniguchi a , Kiyohito Koyama a a Department of Polymer Science and Engineering, Faculty of Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa 992-8510, Japan b Faculty of Education, Yamagata University, Yamagata 992-8560, Japan c Department of Applied Biological Chemistry, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan Received 14 May 2003; received in revised form 29 August 2003; accepted 2 September 2003 Abstract We synthesized the polyelectrolyte complex hydrogel consisting of chitosan, k-carrageenan, and carboxymethyl cellulose sodium salt (NaCMC) and investigated the swelling properties of the gel varying with carrageenan/NaCMC compositions. In a lower composition of carrageenan, heterogeneous gels were obtained indicating strong electrostatic interactions among these polyelectrolytes. Oppositely, in a higher composition of carrageenan, a gelation did not occur. It was cleared that a homogeneity of the gel strongly depended on the carrageenan/NaCMC composition. The degree of swelling at the equilibrium decreased proportional to the carrageenan composition. The gels showed a maximum degree of swelling in the range of pH 11 – 12. The maximum degree of swelling discontinuously decreased with increasing the NaCMC composition and was independent of the composition at a higher composition of NaCMC. Swelling properties in pure water and in alkaline solutions were also affected by salt concentrations of each polyelectrolyte aqueous solutions. q 2003 Elsevier Ltd. All rights reserved. Keywords: Hydrogels; Polyelectrolytes; pH response 1. Introduction The stimuli-responsive properties of polymer gels have been widely investigated and explored for industrial and biomedical fields. It is well known that the polyelectrolyte gels exhibit the volume phase transition [1] and the bending phenomena with response to electric field that is called chemomechanical behavior [2]. Amphiphilic polymer gels composed of n-stearyl acrylate and acrylic acid undergo the volume phase transition depending on temperature and alcohol/water composition. This characteristic is based on the amorphous – crystalline transition and it has been utilized as a shape memory material [3] and solvent-driven chemical motor applications [4,5]. Temperature responsive hydrogels such as poly(N-isopropylacrylamide) (PNIPA) gel are famous for their remarkable phase transition and are employed in drug delivery systems [6] and gel display [7]. The polymer gel containing magnetic substance shows the noncontinuous magnetoelastic behavior [8] with response to the magnetic field. It has been cleared that the modulus of the magnetic gel changed being accompanied by the magnetic field [9–11]. pH-sensitive polymer gels are typical stimuli-responsive gels and they have great potential to be applied for the drug delivery systems because the pH varies at each organs or the diseased part of human body. It has been reported that polypeptides gel [12], and synthetic polypeptides gel [13] showed swelling behavior in response to ambient pH owing to conformational transitions. Poly(N-isopropylacrylamide- co-methacrylic acid) gel membrane showed pH response with a very narrow range of pH 4.9 – 5.2 [14]. Poly(hydroxy- ethyl methacrylate-co-acrylic acid) or poly(acrylamide-co- acrylic acid) hydrogels showed swelling behavior at approximately pH 7.4 and was examined as an enteric delivery of drugs such as indomethacin [15]. One of the authors reported that the polyelectrolyte complex gel consisting of chitosan, and k-carrageenan showed swelling behavior with a narrow range of pH around 10.5 [16]. They explained the swelling behavior of the 0032-3861/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2003.09.001 Polymer 44 (2003) 7103–7111 www.elsevier.com/locate/polymer * Corresponding author. Tel.: þ 81-238-26-3078; fax: þ81-238-26-3101. E-mail address: tetsu@yz.yamagata-u.ac.jp (T. Mitsumata).