Coarse-grained molecular simulation of the role of curing rates on the structure and strength of polyurea

We present a reactive coarse-grained model that can more accurately simulate the interplay between step-growth polymerization and phase segregation in elastomeric copolymers such as polyurea. The coarse-grained force fields were calibrated by the iterative Boltzmann inversion method to reproduce the target structural distributions sampled from atomistic simulations of partially-cured, oligomeric prepolymers. Using this coarse-grained model, polymerization simulations were performed at simulated curing rate spanning five decades to draw connections between the rate of polymerization, the microstructure, and the high-strain rate mechanical properties of the cured elastomer. Analysis of the simulated polyurea systems suggests that curing at faster reaction rates leads to hard domains with smaller diameter ligaments that are more strongly interconnected with bridging soft segments than those formed at slow curing rates. Simulated uniaxial compression of the cured systems indicates that the key process-induced microstructural variation influencing the hardening behavior of the elastomer is the fraction of soft segments that bridge across hard domain ligaments, which can lead to a substantial increase in the flow stress at large compressive strains.