Effects of network structure on elastomeric properties

General approach

Until recently, there was relatively little reliable quantitative information on the relationship of stress to structure, primarily because of the uncontrolled manner in which elastomeric networks were generally prepared (Flory, 1953; Treloar, 1975; Erman and Mark, 1997). Segments close together in space were linked irrespective of their locations along the chain trajectories, thus resulting in a highly random network structure in which the number and locations of the cross links were essentially unknown. Such a structure was shown in Figure 1.2. New synthetic techniques are now available, however, for the preparation of “model” polymer networks of known structure. More specifically, if networks are formed by end linking functionally terminated chains instead of haphazardly joining chain segments at random, then the nature of this very specific chemical reaction provides the desired structural information (Erman and Mark, 1997; Mark, 2004a; Mark et al., 2004). Thus, the functionality of the cross links is the same as that of the end-linking agent, and the molecular weight Mc between cross links and its distribution are the same as those of the starting chains prior to their being end linked.

An example is the reaction shown in Figure 10.1, in which hydroxyl-terminated chains of a polymer such as poly(dimethylsiloxane) (PDMS) are end linked using tetraethyl orthosilicate (alternatively called tetraethoxysilane).