Effect of charge inhomogeneity of polyelectrolyte gels on their swelling behavior

The purpose of this paper is to discuss the role of ionic charges in the swelling of polyelectrolyte gels. According to the classical Floryʹs theory, osmotic pressure arising from mobile ions is regarded as a swelling force due to ionic charges; in other words, the gel was assumed to be surrounded by Donnan potential barriers. Although this assumption was criticized by Katchalsky et al., many studies have accepted it to account for the nature of the swelling or the phase transition in polyelectrolyte gels. Thus, we have tried to verify whether the ‘concept’ of osmotic pressure due to mobile ions is adequate or not. For this purpose, we have studied the swelling behavior of ionic gels with inhomogeneous charge distributions. A gel sample was obtained via inhomogeneous binding of anionic surfactants (e.g. sodium dodecylbenzene sulfate) to lightly crosslinked polymer of N-isopropylacrylamide (NIPA), while the other was based on the physical entrapment of poly(acrylic acid) within the NIPA chain network. Using both NIPA-based ionic gels, we observed a strong effect of the charge inhomogeneity on the degree of swelling. It is therefore indicated that our observations can no longer be explained in terms of the concept of osmotic pressure arising from mobile counterions within the gel phase.