Evolution process of polymethacrylate hydrogels investigated by rheological and dynamic light scattering techniques

Evolving gel formation processes of colloidal hydrogels derived from high solid-content polymethacrylate microemulsions have been studied using rheological and dynamic light scattering (DLS) techniques. The critical gel points, evaluated through oscillatory shear measurement and characterized by a single power-law exponent of G′ and G″ on the frequency (the power-law exponent, n = 0.36-0.65) were influenced more by the volume fraction of polymers than by the gelling temperature. Further topological fractal analysis and rheological studies suggested that reaction-limited cluster aggregation (RLCA) may dominate the aggregation behavior of polymer nanoparticles with the fractal dimension (df) = 1.8-2.2. The DLS results showed a fast relaxation mode with a single exponential decay followed by a slow one with a stretched exponential decay, which indicated that the gradual incorporation of polymer nanoparticles or clusters into the gel network leads to the maturity of the homogenous hydrogel network. Comparative studies on a series of polymethacrylate hydrogel systems with different hydrophilicities demonstrated that a slight hydrophilicity favors gel formation. The gelation mechanism was discussed based on the in-situ aggregation of nanoparticles as well as in terms of the colloidal instability of high solid-content nanolatex.