DENSELY CROSS-LINKED POLYCARBOSILOXANES .2. THERMAL AND MECHANICAL-PROPERTIES

The thermal and mechanical properties of two densely crosslinked polyc arbosiloxane systems were investigated in relation to the molecular st ructure. The networks were prepared from functional branched prepolyme rs and crosslinked via a hydrosilylation curing reaction. The prepolym ers having only vinyl functionalities (poly [phenylmethylvinyl] siloxa nes) were crosslinked by using crosslinking agents with reactive silic on-hydrogen groups. In prepolymers having both silicon-vinyl and silic on-hydrogen groups (poly [phenylmethylvinylhydro)]siloxanes) crosslink ing 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 e ffective crosslinks, the elastically effective network chain density a nd molecular weight between crosslinks, length of the chain segments i ntroduced by the hydrosilylation crosslinking reaction, and the number of dangling ends. As a consequence of the dense crosslinking, the mec hanical properties were also strongly dependent on the glass transitio n temperature. A tough-brittle transition was observed around the glas s transition temperature of the polymer networks. The properties of th e poly(phenylmethylvinylhydro)siloxane networks were found to be super ior to those of the poly( phenylmethylvinyl) siloxane networks. (C) 19 97 John Wiley & Sons, Inc.