Viscoelasticity of polydimethylsiloxane at the sol-gel threshold: structural effects

Six model polydimethylsiloxane (PDMS) networks were obtained by hydrosilation of a difunctional vinyl-terminated PDMS prepolymer with a SiH containing crosslinker. Viscoelastic experiments were performed in order to study the influence of molecular parameters on the dynamic properties around the sol-gel threshold. Critical parameters were determined close to and above the sol-gel threshold. Our results show, in agreement with those of Lusignan et al. (1999), that the gel growth mechanism is function of the ratio N/Ne, where N and Ne are the numbers of Kuhn monomers between branch points and between entanglements, respectively. By considering the literature results, three universality classes are pointed out for polymer gelation: the sol-gel transition is governed by critical percolation for N/Ne < 1, by Diffusion Limited Cluster Aggregation for 1 < N/Ne < 2, and by mean field theory for N/Ne > 2.