Dependence of the elastomeric properties of bimodal networks on the lengths and amounts of the short chains

Bimodal elastomeric networks were synthesized by tetrafunctionally end-linking very short and relatively long hydroxyl-terminated chains of poly(dimethylsiloxane). Decrease in the molecular weight of the short chains (from 880 to 660 to 460 g mol^?1) generally results in significant increases in ultimate strength and energy for rupture. Decrease in the number of short chains, however, causes these two quantities to go through a maximum. Too many short chains gives essentially a brittle thermoset, and this precludes any upturn in the modulus from the limited extensibility of the short chains. Having too few short chains makes their limited extensibility irrelevant since the entire macroscopic deformation can then be taken up by the long chains present in the network.