Cure Terms Glossary

Gel

A gel is obtained when a dispersed phase combines with the dispersion medium to produce a semi-solid material. It can maintain a defined shape but may possess little in the way of stiffness. In molecular terms, it is a swollen network polymer. The network may be physically or chemically crosslinked.

A gel may be formed in one of two ways - either (i) by growing a network polymer from monomer(s) or lower molecular polymers (ii) by infusing a pre-existing polymer with solvating low molecular weight material. The gel may be an intermediate state or a product in its own right.

In the case of (i), increasing degrees of polymerisation result in the following sequence of changes,

monomer(s) & → liquid polymer → gel → solid polymer.

In the case of (ii), increasing levels of infusion by solvent would result the following sequence,

solid polymer → plasticised polymer → gel → polymer solution.

In a cure or crosslinking polymerisation, gelation occurs when an infinite molecular network first forms. That means there is at least one molecule which extends throughout the body of the material. At this point there are still many molecules of lower molecular weight present, and these provide the swelling medium for the initially-formed network.

Gels are also important products in their own right. Commercial examples are typically created products by infusing a polymer which is only partially soluble in the solvent used (e.g. by infusion into suitable block copolymers). A two-phase structure results with the regions of low solvation providing the network sites. Such gels can offer a myriad of applications - for example as soft structural components (e.g. in cosmetics) or in controlled release (e.g. in pharmaceuticals) or as precursors to other types of structure through sol exchange. Numerous applications exist for so-called hydrogels - i.e. gels where water is the solvation medium.

Gels can also be used to create microporous solids by removal of the sol component. The familiar silca gel, made by the dehydration of silicic acid hydrogels, is one such product. Silicic acid, formed in situ, undergoes spontaneous self condensation (Si—OH + HO—Si → Si—O—Si + H2O) to form the polymer component of the gel. The fully dehydrated product (silica gel) is actually a microporous silica - and is a solid, not a gel. Such densified products, formed by solvent removal from gels, are termed xerogels.