Bimodal networks can enhance the mechanical properties of elastomers. These special materials are synthesized by end-linking short and long precursor chains with narrow polydispersities whose molecular weights differ by a factor of 10 or more. Such elastomers are able to withstand higher loads than unimodal networks without the usual decrease in extensibility seen in networks made from shorter chains. In addition, the mechanical properties for bimodal networks are improved without the addition of filler particles often used to strengthen elastomers. Trimodal networks, those containing three distributions of chain lengths, are also of interest, especially when the longest component chain approaches 100,000 g/mol.

Multimodal PDMS networks were synthesized from telechelic chains of varying molecular weight distributions with relatively low polydispersity (< 1.3) using established methods. The chains were endcapped with vinyl groups. Shorter chains were then cross-linked with longer chains at optimal stoichiometric ratios via a hydrosilylation reaction. The resulting bimodal and trimodal elastomeric networks have relatively few dangling chains and defects. The longer chains in these networks varied from 10,000 g/mol up to nearly 100,000 g/mol.

Networks of varying compositions and precursor chain lengths were measured for maximum extensibility, engineering stress, toughness, Young's modulus, and swelling in toluene. Toughness values of multimodal networks show the greatest enhancement when the molecular weights of the shortest and longest chains are most widely separated. The material properties are compared to those obtained by simulations of coarse-grained flexible-chain networks performed using Monte Carlo techniques and to previous theoretical work.

We also tested the threshold tearing energies for multimodal networks using “pure shear” style test pieces. These pre-cut samples were uniaxially extended at sufficiently slow rates to obtain stress and strain measurements independent of strain rate. We compare the energies required to tear unimodal, bimodal, and trimodal networks.

Polymer segment orientation in these materials is revealed in solid state deuterium NMR spectra. Deuterated PDMS chains crosslinked with protonated PDMS chains of different size form bimodal networks that allow the deuterium NMR to focus on either the short or long chain segment orientation. We synthesized deuterated PDMS chains of about 5000 g/mol and 80,000 g/mol to probe the segmental motion of each component in the bimodal networks. We will present deuterium NMR of these selectively labeled networks and compare the results with both simulations and mechanical testing of similarly constituted networks.