Properties of post‐cured siloxane networks

Post‐curing of vinyl‐addition cured siloxane networks can take place due to the presence of excess silicon hydride. The reaction of these groups, which leads to increased crosslink density especially at elevated temperatures, can be retarded by treatment with ammonia gas or eliminated by exposure to ethylene gas. There is an interesting correspondence between post‐curing behavior and the gelation process. In both cases, two curing regimes are observed. For low extents of reaction, the mobility of chains is high, even though the post‐curing experiment is performed on samples which have already gelled, and the cure is controlled by reaction kinetics, not by the diffusion of the reactants. At high degrees of cure or post‐cure, both cases behave as crosslinked networks to reduce the probability of interaction between reactants, and the cure is diffusion‐controlled. However, the post‐cured gels do not exhibit ideal network behavior since the last crosslinks formed during post‐curing have a much greater effect on the physical properties than those formed initially. This may be due to the bimodal distribution of chain lengths between crosslinks that is formed during post‐curing.