Examination of the effects of computationally determined network topology on an analytical constitutive model for bimodal elastomers

Bimodal elastomer networks, so called due to their nominal bimodal molecular weight distribution of starting oligomers, are of continued interest due to the enhanced strength and toughness seen in certain mixtures. Researchers have suggested that the enhanced properties stem from the particular micromechanics of the networks formed by these systems at these optimal compositions. This work extends an existing analytical constitutive model for bimodal elastomer networks by incorporating aspects of network topology, including network connectivity patterns and realistic chain length distributions, determined through computational simulations of the formation of the network structure. These factors are included as functions of bimodal composition and are shown to affect the predicted mechanical, optical, and orientation responses of the network. The extended model elucidates how the naturally occurring doubled connection topology creates a micro-mechanism that lowers overall chain orientation in the lower molecular weight component and recreates experimentally observed optical response phenomena. Specifically, the model predicts that the presence of the stiff, contractile, doubled connections forces the rest of the network to conform more to the macroscopic stretch ratio while reducing the measured average orientation of the lower molecular weight component in the system; the effect diminishes as the composition-dependent population of doubled connections in the system decreases.