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.