Multiscale Simulation of EPDM Vulcanization: Coupling Atomistic Molecular Dynamics with Continuum Chemorheology

Accurate prediction of the cure state in ethylene-propylene-diene monomer (EPDM) rubber injection molding remains a critical challenge due to the inability of macroscopic empirical models to capture structural transformations under severe non-isothermal gradients. To overcome this limitation, this work proposes a bidirectional multiscale simulation framework that couples Molecular Dynamics (MD) with continuum mechanics via 3D Finite Element Method (FEM). At the nanoscale, MD trajectory analysis quantifies the Mean Squared Displacement (MSD) and Fractional Free Volume (FFV). Integrating the thermal expansion of the free volume into the Doolittle theory yields a kinetic model governed exclusively by steric hindrance, bypassing empirical fitting parameters. Topological arrest and macroscopic fluidity collapse are defined by critical chain confinement limits (MSD∗≤2.5 AA2 and dMSD/dt≤10−4 nm2/ps), dynamically determining the gelation point αgel(T) within the Castro-Macosko chemorheological equation. Macroscopically, an implicit 3D FEM solver iteratively updates local Arrhenius reaction rates and non-Newtonian viscous dissipation, simultaneously resolving cavity filling dynamics, exothermic crosslinking kinetics, and asymptotic viscosity divergence (η→∞ ). Thermokinetic predictions were experimentally validated through industrial injection molding trials and post-molding hardness measurements under ASTM D1418-18 and ASTM D2240 standards. The proposed model establishes optimal processing windows that effectively mitigate undercuring in the core and thermal reversion at the surface.