4 - Polyelectrolytes, ion binding and gelation

Summary

Polyelectrolytes

General

The setting of AB cements is an example of gelation, and gelation is related to ion binding. A theoretical examination of the various phenomena associated with ion binding and gelation finds its clearest exposition in the field of polyelectrolytes. Moreover, this field may be wider than it seems at first.

Polyelectrolytes form the basis of those modern cements which are distinguished by their ability to adhere to reactive surfaces. At present the main use of such cements lies in the medical field, principally in dental surgery. They adhere permanently to biological surfaces where they have to withstand adverse conditions of wetness, chemical attack, the stress of biological activity, and chemical and biological changes within the substrate. Nevertheless, adhesive bonds are maintained.

Polyelectrolytes are polymers having a multiplicity of ionizable groups. In solution, they dissociate into polyions (or macroions) and small ions of the opposite charge, known as counterions. The polyelectrolytes of interest in this book are those where the polyion is an anion and the counterions are cations. Some typical anionic polyelectrolytes are depicted in Figure 4.1. Of principal interest are the homopolymers of acrylic acid and its copolymers with e.g. itaconic and maleic acids. These are used in the zinc polycarboxylate cement of Smith (1968) and the glass–ionomer cement of Wilson & Kent (1971). More recently, Wilson & Ellis (1989) and Ellis & Wilson (1990) have described cements based on polyphosphonic acids.