Physical gelation of supramolecular hydrogels cross-linked by metal-ligand interactions: Dynamic light scattering and microrheological studies

Microrheology based on diffusing-wave spectroscopy, dynamic light scattering (DLS), and macrorheology were performed on solutions of polyacrylamide-derivative associating polymer containing imidazole groups. By adding Ni ion, the physical gel is formed by metal-ligand bond. The physical gel point, where the percolation of a transient network is supposed to occur, was microrheologically determined. In the DLS measurement showing ergodicity at all the conditions studied, we observed three modes in the autocorrelation functions: fast, intermediate, and slow modes. The fast mode is the collective diffusion (gel mode), while the slow mode is attributed to the diffusion of clusters. For the intermediate mode, with increase in Ni ion concentration, the transition from diffusive mode to relaxation mode was observed near the microrheologically determined physical gel point. In postgel regime, intermediate mode can be attributed to the macroscopic relaxation since the activation energies are comparable with the relaxation energy estimated by macrorheology. (C) 2017 Elsevier Ltd. All rights reserved.