Viscoelastic properties of a cross-linked polysiloxane near the sol-gel transition

Experimental results are presented concerning the zero frequency and dynami c properties of a cross-linked polysiloxane near gelation threshold. It is prepared by hydrosilation of a difunctional vinyl-terminated polydimethylsi loxane prepolymer and a seven-functional poly(dimethylsiloxane-co-methylhyd rogenosiloxane) prepolymer. The zero frequency and dynamic properties durin g cross linking have been studied by rheological experiments in the framewo rk of the linear viscoelasticity. The material at the gelation threshold fo llows a power law viscoelastic behavior, G* proportional to omega (u) with u = 0.77. The zero frequency exponents s and t (eta proportional to epsilon (s); G proportional to epsilon (t)) were determined from the theological d ata (s = 0.76 and t = 2.54). This study shows that the zero frequency and d ynamic exponents obey to the scaling relation u = t/(s+t) and are not unive rsal. Above the gelation threshold, a theological master curve could be bui lt showing that the viscoelastic properties of the gel are governed by the molecular structure of the prepolymers and the self-similarity of the mass distribution of clusters. In this way, this system follows the percolation theory. Assuming that the nonreacted prepolymers swell the network, the mol ecular weight between crosslinks was estimated. (C) 2001 The Society of Rhe ology.