Model networks of end-linked polydimethylsiloxane chains. I. Comparisons between experimental and theoretical values of the elastic modulus and the equilibrium degree of swelling

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Two samples of hydroxyl-terminated polydimethylsiloxane having molecular weights the order of 2×104 and 1×103 g mol−1 were separated into a total of six fractions of relatively narrow molecular weight distribution. Portions of one of the unfractionated polymers and each of the fractions were cross-linked by reacting the hydroxyl chain ends, in the undiluted state, with a tetrafunctional orthosilicate. The resulting networks of end-linked chains were studied with regard to their stress–strain isotherms in elongation at 25°C and their equilibrium swelling in benzene at room temperature. Values of the elastic modulus obtained from the isotherms support theoretical arguments that fluctuations in the network chain vectors reduce the value of the modulus to approximately one-half of the value predicted for affine deformations of chain vectors constrained in their fluctuations by cross-links firmly embedded in the network medium. Values of the equilibrium degree of swelling of the networks calculated on this basis are also in good agreement with experiment. The networks formed by end-linking relatively short chains have small values of the semiempirical constant 2C2 used as a measure of the departure of an observed stress–strain isotherm from the form predicted by theory. Although this observation is consistent with the suggestion that such end-linked networks have a much smaller number of interchain entanglements than do randomly cross-linked networks, other evidence and arguments unfortunately do not support this assumption.