Definition of the concept of polymer gel

The current ideas concerning polymer gels are outlined. The following definition of gel is proposed: a gel is a solid composed of at least two components, one of which (polymer) forms a three-dimensional network by virtue of covalent or noncovalent bonding (chemical and physical gels, respectively) in the medium of the other component (liquid), wherein the minimum amount of the liquid is sufficient for ensuring the elastic properties of the gel, although it may exceed tens to hundreds of times the amount of the polymer. It is noted that, at a high network density or high polymer-chain rigidity, the formation of fragile gels is possible. A general feature of physical gels is the existence of the yield point. Earlier results, which suggest the existence of two yield points in polymer gels, as found in studies of gelatin hydrogels over a wide concentration range, are suveyed.