Investigation of the Effect of Type and Concentration of Ionizable Comonomer on the Collapse Behavior of N-Isopropylacrylamide Copolymer Gels in Water CANDAN ERBI ˙ L, SEVAL ARAS, NURSELI ˙ UYANIK I ˙ stanbul Technical University, Chemistry Department, 80626 Maslak, I ˙ stanbul, Turkey Received 6 June 1998; accepted 26 October 1998 ABSTRACT: This work was done to investigate the effect of three different ionizable components (acrylic acid, AA; itaconic acid, IA; maleic acid, MA) on the volume phase transitions and swelling equilibria of thermoshrinking type N-isopropylacrylamide (NIPAAM) gels in water. NIPAAM copolymer gels were synthesized by free radical crosslinking copolymerization of NIPAAM with each of AA, IA, and MA, the difference being both between configurations and carboxyl group numbers, and pK values, in the presence of N,N'-methylene-bis-acrylamide (MBAAM). The influence of comonomer concentrations (1, 5, and 10 mol %), MBAAM content (0.0096, 0.0193, and 0.0288 g), and comonomer type (AA, IA, and MA) on the external views, the percentages of equilibrium mass, and volume swellings [S %(m), S % (v)], the number-average molec- ular weight between crosslinks (M  c ), effective crosslinking densities ( e ) , the change of the collapse temperatures, and swelling ratios in the swelling–shrinking process of the gels were examined. It was observed that phase transition temperature and swelling degree in the case of MA having a cis configuration and higher pK value are larger than those of the samples containing IA and AA. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1847–1855, 1999 Keywords: copolymers; gels; N-Isopropylacrylamide; acrylic acid; itaconic acid; ma- leic acid INTRODUCTION It is known that there are four intermoleculer forces (van der Waals, hydrogen bonding, hydro- phobic, and electrostatic interactions) that are responsible for the phase transitions in polymer networks that can reversibly swell or shrink in response to surrounding conditions such as tem- perature, pH, and solvent composition. 1–6 The gels having these properties have been considered as canditade materials for controlled release, sep- arations, artificial organs, and sensor applica- tions. 7–10 Thermoshrinking-type volume phase transi- tion (collapse) behaviors of poly (N-isopropylac- rylamide) (PNIPAAM) gels have been attracting a great deal of attention because of their technical importance and scientific interest. 11–15 Above the collapse temperature, which is analogous to the coil– globule transition in the dilute polymer so- lutions with lower critical solution temperature (LCST) of PNIPAAM, the hydrogen bonds be- tween the amide groups of hydrophobic NIPAAM and surrounding water molecules are destroyed, and the hydrophobic interaction contributed to the volume phase transition of thermoshrinking polymers and gels is promoted. The temperature-sensitive networks contain- ing ionizable functional groups exhibit pH sensi- Correspondence to: C. Erbil Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 37, 1847–1855 (1999) © 1999 John Wiley & Sons, Inc. CCC 0887-624X/99/121847-09 1847