Torsion Studies of Thermoelasticity and Stress–Strain Isotherms of Unimodal, Bimodal, and Filled Networks of Poly(dimethylsiloxane)

Abstract

End-linking reactions were used to convert chains of poly(dimethylsiloxane) (PDMS) into (i) unfilled networks having either unimodal or bimodal chain-length distributions, or (ii) filled networks in which either a separately-prepared zeolite had been blended into the polymer prior to end linking, or silica was precipitated into the networks afterwards. The resulting elastomers were studied in torsion, the aspect of particular interest in the case of the unfilled networks being the fraction Me/M of the total couple which is due to changes in conformational energy. The values of Me/M were found to be independent of crosslink density, extent of torsional strain, and extent of any elongation imposed during the torsion. The values for the bimodal networks, however, were much smaller than those for the unimodal ones, presumably because of the non-Gaussian nature of the short chains. None of the unimodal or bimodal unfilled networks showed any upturn in modulus at high torsional deformations similar to the upturns shown by bimodal networks in elongation, biaxial extension, and shear. Neither did the filled networks, but they did show significant increases in modulus in general, thus demonstrating reinforcement at least in this regard.