Physical gelation of hydrophobically modified polyelectrolytes. 1. Homogeneous gelation of alkylated poly[acrylamide-co-sodium acrylate]

Stiff, viscoelastic hydrogels have been fabricated from hydrophobically modified poly(acrylamide-co-sodium acrylate) (HRAM). These materials are macroscopically homogeneous but microscopically heterogeneous, containing hydrophobic aggregates that bridge polymer chains in a three-dimensional network. The charge level at which the dynamic storage modulus is maximized for a given polymer concentration corresponds closely with the charge level giving a maximum in hydrophobicity of the aggregates, suggesting a link between the mechanical stability of these aggregates and their ability to exclude water effectively. The gel point, where the system first exhibits viscoelastic behavior, may also be identified from discontinuities in the hydrophobicity (as measured by the fluorescence spectra of solubilized pyrene) and equivalent conductivity with polymer concentration. The effects of various aspects of polymer architecture on hydrogel properties have been investigated. The dynamic storage modulus of the network ex...