Effect of degree of cross-linking on spatial inhomogeneity in charged gels. 3. Ionization effect

The effect of degree of ionization, f, on the spatial inhomogeneities in weakly charged polymer gels has been investigated by light scattering (LS) and small-angle neutron scattering. An anomalous cross-link density (CD) dependence of the scattered intensity, I(q), was observed in a poor solvent (above an inversion temperature, T-inv), where q is the scattering vector. That is, a gel became more homogeneous with increasing CD in a poor solvent (above T-inv) though it became more heterogeneous with increasing CD in a good solvent (below T-inv). LS data indicated the following three features of the gels: (1) an increase of T-inv, with increasing f (2) a decrease of the critical degree of polymerization between cross-linking points, N-c, with increasing f, where N-c is the critical value of N at which I(q) diverges with decreasing N (similar to 1/CD), and (3) a decrease of I(q) with f. The influence of f on the anomaly as well as on the microscopic and macroscopic phase separation was discussed in terms of the Rabin-Panyukov (RP) theory proposed for weakly charged gels.