Preparation of networks

A network is obtained by linking polymeric chains together into the form of a three-dimensional structure such as that shown in Figure 1.2. The process is generally known as cross linking, curing, or vulcanization (Coran, 1987, 2005; Akiba and Hashim, 1997; Ignatz-Hoover and To, 2004). The points of linking, called cross links or junctions, may be (1) randomly located along the chains, or (2) restricted to specific locations such as the chain ends or selected repeat units, particularly in the case of bioelastomers (Gosline, 1987; Lillie and Gosline, 1990; Erman and Mark, 1997). The required linking (cross linking or end linking) can be brought about by chemical reactions that form covalent bonds between chains, either randomly or controllably at chosen locations. The linking may also be accomplished, randomly, by the physical aggregation of units from two or more chains. The simplicity of the first technique and the permanence of the structures it provides are advantages for many applications. The main advantage of the second technique is the fact that the elastomer is frequently reprocessable, since the aggregation process is often reversible.

Random chemical reactions

The most important examples of cross linking using chemical reactions that attack the chains at random locations are given in the top part of Table 4.1.

Sulfur is used primarily with elastomers having numerous unsaturated groups, for example, those in the repeat units of the polyisoprenes and polybutadienes.