Investigation of the effect of type and concentration of ionizable comonomer on the collapse behavior of N-isopropylacrylamide copolymer gels in water

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 molecular weight between crosslinks (M̄_c), effective crosslinking densities (ν_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.