Facile equilibration of fully-entangled semiflexible bead-spring polymer melts

Equilibrating model entangled polymer melts is challenging because their longest relaxation times τ_max scale as (N/N_e)^3.4, where N is their degree of polymerization and N_e is their entanglement length. The well-known double-bridging hybrid (DBH) algorithm reduces these times to τ_max ∼ (N/N_e) by performing periodic chain-topology switching Monte Carlo (MC) moves during a molecular dynamics (MD) simulation. For semiflexible chains, however, the high energy barriers associated with these MC moves make the prefactors to this (N/N_e) scaling prohibitively large. Here we overcome this issue by combining DBH with the use of core-softened pair potentials. By beginning with soft pair and bond interactions, and slowly stiffening them until they reach their final functional forms while keeping the equilibrium bond length constant, we are able to equilibrate 400K-monomer systems with N > 20 N_e and chain stiffnesses all the way up to the isotropic-nematic transition, using single-cluster-node simulations that last no more than ∼250 hours. We use this new algorithm to develop improved expressions for Kremer-Grest melts' chain-stiffness-dependent N_e and Kuhn length l_K.