Bimodal networks can enhance the mechanical properties of elastomers. These special materials are synthesized by end-linking short and long precursor chains whose molecular weights differ by a factor of 10 or more. These materials 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. Erman and Mark (Macromolecules, 31, 3099, 1998) have performed theoretical calculations on such networks and found that those with longest chains on the order of 100,000 g/mol showed higher toughness than their bimodal counterparts. However, little experimental work has been done on trimodal systems to date, and it has focused on networks whose chains were 20,000 g/mol or less.

We have tested such multimodal networks by synthesizing PDMS 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 different stoichiometric ratios via a hydrosilylation reaction. The resulting model 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.

We cut samples from these networks and measured stress-strain curves until fracture. 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.

The deformation of these materials at the polymer segment length scale is revealed in solid state deuterium NMR spectra. Deuterated PDMS chains crosslinked with protonated PDMS chains of different size form bimodal and trimodal networks that allow the deuterium NMR to focus on either the short or long chains. We will present deuterium NMR spectra of selectively labeled multimodal networks under uniaxial elongation.