Weak and Strong Gels and the Emergence of the Amorphous Solid State
Jack F. Douglas
Gels are amorphous solids whose macroscopic viscoelastic response derives from contraints in the material that serve to localize the constituent molecules or particles about their average positions in space. These contraints may either be local in nature, as in chemical cross- linking or direct physical associations, or non-local as in case of topological entanglement interactions between highly extended crumpled fiber or sheet structures. Either of these interactions, or both combined, can lead to gelation or amorphous solidification. While gels are often considered to be inherently non-equilibrium materials, and correspondingly termed soft glassy matter, this is not generally the case.For example, the formation of vulcanized rubbers by cross-linking macromolecules can be exactly described as a second order phase transition from an equilibrium fluid to an equilibrium solid state and there is much evidence that amorphous solidification also arises in diverse other materials (worm-like micelle-forming systems, supramolecular fiber-forming gels, etc.) that are formed by supramolecular self-assembly. As equilibrium, or near equilibrium systems, such gels can be expected to exhibit universal linear and non-linear viscoelastic properties, especially under near the critical conditions at which the gel state emerges.