Reactive Bead-Spring Models for Associative Polymer Networks Far From Equilibrium

Associating polymers form dynamic networks of reversible bonds that rearrange their network topology. This makes them promising candidates for adaptive and reprocessable network materials; however, the dynamic bonding produces a hierarchy of chain relaxation modes that give rise to complex flow behavior. Understanding these dynamics requires models that can capture the feedback between dynamic bonding and polymer relaxation in nonequilibrium conditions, but most existing models for dynamic bonding use Monte Carlo algorithms with reaction rates that lose validity under flow. Here we share a new reactive molecular dynamics model that combines a Tersoff bond order potential for associative bond chemistry and a bead-spring model for entangled polymers. The resulting coarse-grained model captures the essential physics of chain dynamics, chain entanglement, and coordinated dynamic bonding and can be tuned to capture a variety of associative bond kinetics. The many-body Tersoff Hamilitonian for dynamic bonding remains valid in nonequilibrium flow conditions. We use this model to simulate canonical Kremer-Grest bead-spring melts with binary associative bonds of varying cohesive strength. We measure the gelation transition with increasing association strength and identify a gel-point at an associative bond strength ~ 1kT. We also assess how chain dynamics and network viscoelasticity change as the degree of gelation increases.