AN INVESTIGATION OF THE PROPERTIES OF NON-GAUSSIAN POLY(DIMETHYLSILOXANE) MODEL NETWORKS IN THE SWOLLEN STATE

Tetrafunctional poly(dimethylsiloxane) (PDMS) networks having unimodal and bimodal distributions of chain lengths between junction points were swollen by using the first three members of the linear phenylmethylsiloxane Me3SiO[PhMeSiO]ySiMe3 homologous series. The diluents were of low volatility, and the volume fractions of the polymer, ν2m, at equilibrium swelling for both the unimodal and bimodal PDMS elastomers were found to increase with increasing molar mass of the linear phenylmethylsiloxanes. The swelling behavior of the networks was interpreted by using the Flory-Rehner theory of equilibrium swelling-which gave interaction parameters χ1 in the range 0.063 ≤ χ1 ≤ 0.438. Uniaxial extension investigations of the bimodal networks in the unswollen and swollen states show characteristic non-Gaussian behavior-in particular large increases in modulus at high extensions, due to limited chain extensibility of the short network chains. The modulus values of the bimodal elastomers were found to decrease as a function of ν2m -and hence the molar mass of the phenylmethylsiloxane diluents-for the bimodal elastomer containing 90.4 mol % short chains (440 g mol-1). These findings may be contrasted with a PDMS bimodal elastomer containing 93.4 mol % short chains (880 g mol-1), where the modulus was found to increase upon swelling with the first member of the linear phenylmethylsiloxane series. The results of this study therefore demonstrate that the quantity of short network chains is a key parameter in determining whether or not this interesting-and possibly technologically useful-phenomenon is observed for non-Gaussian elastomeric materials. © 1990, American Chemical Society. All rights reserved.