Poly(dimethylsiloxane) elastomers from aqueous emulsions: II. Mechanical properties

Stress-strain measurements in simple uniaxial extension were used to charac terize the mechanical properties of the elastomers prepared from poly(dimet hylsilxoxane) emulsions as described in the preceding paper. The studies we re carried out on the materials in the unswollen state, after they had rece ived different treatments, specifically no aging, aging in the dry state, o r aging in the wet (emulsion) state. Increase in silane crosslinker concentration was found to increase nominal stresses and moduli but to decrease extensibility, two changes that paralle l the observed decreases in soluble polymer fractions and extents of equili brium swelling reported earlier. The energy for rupture ("toughness") frequ ently stays roughly constant because the decreased extensibility at least p artially offsets the increased stresses. The changes in mechanical properti es are due both to increased crosslinking and to reinforcing Effects from s ilica generated from the silane, with the latter effect generally being the more important. The mechanical properties are much more affected by aging in the wet state, as opposed to aging in the dry state. For example, wet-aged elastomers gen erally had values of the nominal stress and modulus that were consistently lower than those for the unaged elastomers. Wet aging appears to be the res ult of the continual breaking and reforming of siloxane bonds in the aqueou s environment of the emulsion, as probably accelerated by the presence of t he tin catalyst.