INFLUENCE OF THE FINAL EXTENT OF REACTION ON THE STRUCTURE OF MODEL POLYDIMETHYLSILOXANE NETWORKS OBTAINED BY THE END-LINKING HYDROSILATIONREACTION

Model silicone networks obtained by hydrosilation of vinyl terminated polydimethylsiloxane chains have been used extensively to verify molec ular theories of rubber elasticity. In these networks, the maximum ext ent of reaction obtained during cross linking considerably affects the ir final structure. The degree of completion of the hydrosilation reac tion depends on the concentration of reactive groups. A recursive appr oach and kinetic data from the literature are used to show that, for t his particular reaction, perfect networks, i.e. those with high concen tration of elastically active network chains, are only obtained when a relatively high concentration of reactive groups is present in the sy stem. This critical concentration corresponds in a bulk reaction to po lydimethylsiloxane chains with a number average molecular weight lower than 10,000.