Networks, bimodal short-chain model

To have greater control of the chain topology, it was decided to use model networks. PDMS networks were chosen because of the extensive work by Mark and his coworkers on these systems. The-networks were prepared by crosslinking divinyl terminated PDMS chains with a tetra silane in the presence of a platinum catalyst. All networks were prepared in bulk. By choosing the appropriate prepolymers, it was possible to prepare unimodal and bimodal networks. For the bimodal networks, the short chains had M = 770 and the long chains had M = 22,500. In both cases M /M 1.8. 

One of the most interesting types of model networks is the bimodal. These networks consist of very short chains intimately end-linked with the much longer chains that are representative of elastomeric materials.237,251,253,268-278 Such a network is shown in Figure 4.11,269 where the short chains are arbitrarily drawn thicker than the long ones. 

The long and short chains in the bimodal PDMS networks were assumed to have values of n of 20 and 250, respectively (9-16). The long chains were modeled as Gaussian chains, whereas the distribution function for the short chains was determined from Monte Carlo calculations as already described. The entropy of the bimodal network was then taken to be the sum of... 

In the case of such noncrystalUzable, unfilled elastomers, the mechanism for network rupture has been elucidated to a great extent by studies of model networks similar to those already described. For example, values of the modulus of bimodal networks formed by end-linking mixtures of very short and relatively long chains as illustrated in Fig. 6.4 were used to test the weakest-link theory [7] in which rupture was thought to be initiated by the shortest chains (because of their very limited extensibility). It was observed that increasing the number of very short chains did not significantly decrease the ultimate properties. The reason [85] is the very nonaffine... 

Figure 18(a) presents normalized proton DQ buildup functions of end-linked polydimethylsiloxane model networks prepared by mixing and subsequent cross-linking of very short and rather long chains. Such bimodal networks are known to consist of clusters of highly cross-linked regions embedded in a long-chain elastomer matrix. The buildup curves as well as the cross-link density distributions (i.e., the distributions of D s) derived from these show clear bimodal... 

---