A controlled reaction schema for addition curing silicones leads both to significantly lower elastic modulus and lower viscous dissipation than for the chemically identical network prepared by the traditional reaction schema.

We investigated the linear dynamic response of a series of fully reacted unstoichiometric polydimethylsiloxane (PDMS) networks and of the two corresponding network fractions namely the sol and the washed network. The sol and the washed network were separated by a simple extraction process. This way it was possible to obtain rheological data from the washed network without interference from the sol fraction, and furthermore from the sol fraction without interference from the elastic washed network. When the stoichiometry increased towards perfectly reacted networks and beyond we observed harder networks both qualitatively and by rheology and the properties of the two fractions became more and more different. At the gel point, the sol fraction and the washed network have more or less identical properties which our data also shows. The storage and loss moduli, G ′ and G ′′ , were analysed with the gel equation as proposed by Winter and Chambon(1986;1987). We observed that one of the investigated samples which before the swelling experiment did not show any elastic response gave an elastic washed network after swelling, this was verified by analysis with the gel equation. We also calculated the weight fraction of the sol fraction by using the theory by Villar et al. (1996) and compared these with experimentally found values.

SynopsisA filament stretching rheometer was used for measuring the startup of uni-axial elongational flow followed by reversed bi-axial flow, both with a constant elongational strain rate. A narrow molecular mass distribution linear polyisoprene with a molecular weight of 483 kg/mole was subjected to the flow in the non-linear flow regime. This has allowed highly elastic measurements within the limit of pure orientational stress, as the time of the flow was considerably smaller than the Rouse time. A Doi-Edwards ͓J. Chem. Soc., Faraday Trans. 2 74, 1818-1832 ͑1978͔͒ type of constitutive model with the assumption of pure configurational stress was accurately able to predict the startup as well as the reversed flow behavior. This confirms that this commonly used theoretical picture for the flow of polymeric liquids is a correct physical principle to apply.

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