Gelation studies on acrylic acid‐based hydrogels via in situ photo‐crosslinking and rheology

The gelation characteristics of acrylic acid (AA)‐based hydrogels were investigated using real time in situ photocrosslinking and rheological measurements. The gel point and gelation times were established using Winter–Chambon criteria. Frequency independence of tan δ was observed in all cases, such that G′ and G″ scaled as ～ωⁿ. The Flory–Stockmayer theory was used alongside other indices in order to probe the gelation and the post‐gelation characteristics of the critical gels and the fully formed hydrogels. Network relaxation exponents (n) were influenced by the concentrations of AA and methylene bis‐acrylamide. The gel stiffness (S) decreased with an increase in the concentration of the monomer and of the concentration of the crosslinker, while network branching decreased (lower fractal dimensions) at the gel point. The conversion at the gel point was found to be iso‐conversion with respect to the intensity of the UV irradiance used in the photocrosslinking reactions (1–20 mW/cm²). Thus, network clusters and the crosslinking reaction mechanism were the same irrespective of radiation intensity, although the rates of the reactions were affected. Having sufficient amounts of reactive species at the time of cure drive the crosslinking reactions beyond the gel point to greater crosslink density and smaller mesh sizes. The effects of auto‐acceleration and free‐volume were observed and shown to have key effects on the gelation mechanisms and the branching topographies of the network, when the concentration of the known polyacrylamide medium were not controlled. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46691.