Quantifying Chemical Composition and Cross-link Effects on EPDM Elastomer Viscoelasticity with Molecular Dynamics

A Wang and F Vargas-Lara and JM Younker and KA Iyer and KR Shull and S Keten, MACROMOLECULES, 54, 6780-6789 (2021).

DOI: 10.1021/acs.macromol.1c00162

To understand the effect of chemical composition, cross-link density, and microstructure on the linear and nonlinear viscoelasticity of ethylene propylene diene monomer (EPDM) rubber, we carried out high- frequency oscillatory shear molecular dynamics simulations at varying shear strain rates. Sweeping through different EPDM compositions with varying ethylene, propylene, and diene ratios, a positive correlation was observed between the ratio of the propylene monomer and the complex shear modulus of EPDM in the high-frequency glassy regime. For small deformations in this regime, we found that the simplest measure of local molecular stiffness, namely, the Debye-Waller factor, is predictive of the complex shear modulus and loss modulus of 20 unique systems with distinct compositions and cross-link densities. Polymer design parameters that reduce the Debye-Waller factor, including cross-linking or increased propylene content generally, result in higher moduli. Remarkably, large-amplitude oscillatory shear simulations revealed that dissipation becomes strongly influenced by polymer entanglements, which results in divergent optimal compositions for small-strain vs large- strain applications of EPDM. Utilizing time-temperature superposition and varying strain rates in simulations, we were able to capture rheological properties over 6 orders of magnitude in frequency. The data was captured well using a Rouse model superposed with a stretched exponential function, which was used to predict key constants that determine the mechanical behavior in these regimes. Our findings establish a chemistry-specific molecular simulation approach for capturing the constitutive behavior of elastomers and pave the way for multiscale analyses linking composition and microstructure to performance.

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