Molecular dynamics study of EPDM- Part II: applicability of time superposition principles

Abstract Ethylene—Propylene—Diene Monomer (EPDM), as the main material in the current sealing strip industry, its aging seriously affects the sealing performance. Various superposition principles regarding factors such as time, temperature, stress, and strain have been widely used in EPDM aging life prediction. However, their applicability has not been effectively verified. Based on the assumption of the relationship between long-term viscosity and free volume in the WLF equation, the effects of temperature, stress, strain, and their coupling are investigated in this manuscript on the microstructure-free volume of EPDM through molecular dynamics (MD) simulations, and the applicability of these equivalence principles is also demonstrated for EPDM from the microscopic domain. Based on our previous research on MD modelling of EPDM, a crosslinked EPDM molecular model is established with the optimal polymerization degree and crosslinking density. The free volume is obtained by simulating the movement of EPDM segments using the probe method. The probe radius is determined to be 1.55 Å by testing the influence of the probe radius on the free volume and analyzing the distribution of C atoms in EPDM using the mean square displacement (MSD) and radial distribution function (RDF). The free volume under different temperatures, stresses, and strains is precisely measured, and the time shift factors under a single factor and two factors are calculated. The results show that the time-temperature and time-strain superposition principles are all applicable to EPDM. The temperature change has almost no effect on the strain-free volume relationship, which proves the applicability of the time-temperature-strain superposition principle in EPDM. The formula for the recommended selection range of reference and target conditions for the above superposition principles is inferred from the simulation results.