Densely crosslinked polycarbosiloxanes .2. Thermal and mechanical properties

The thermal and mechanical properties of two densely crosslinked polycarbosiloxane systems were investigated in relation to the molecular structure. The networks were prepared from functional branched prepolymers and crosslinked via a hydrosilylation curing reaction. The prepolymers having only vinyl functionalities (poly [phenylmethylvinyl] siloxanes) were crosslinked by using crosslinking agents with reactive silicon-hydrogen groups. In prepolymers having both silicon-vinyl and silicon-hydrogen groups (poly [phenylmethylvinylhydro)]siloxanes) crosslinking took place intermolecularly. The thermal and mechanical properties of the polymer networks were found to be dependent on the phenyl-Si-O-3/2 (branches) content in the prepolymer, the number of elastically effective crosslinks, the elastically effective network chain density and molecular weight between crosslinks, length of the chain segments introduced by the hydrosilylation crosslinking reaction, and the number of dangling ends. As a consequence of the dense crosslinking, the mechanical properties were also strongly dependent on the glass transition temperature. A tough-brittle transition was observed around the glass transition temperature of the polymer networks. The properties of the poly(phenylmethylvinylhydro)siloxane networks were found to be superior to those of the poly( phenylmethylvinyl) siloxane networks. (C) 1997 John Wiley & Sons, Inc.