Development of dissociative force field for all-atomistic molecular dynamics calculation of fracture of polymers.

A dissociative force field for all-atomistic molecular dynamics calculations has been developed to investigate impact fracture of polymers accompanying dissociation of chemical bonds of polymer main chain. Energy of dimer molecules was evaluated as a function of both bond-length b and bond-angle θ by CASPT2 calculations, whose quality is enough to describe dissociation of chemical bonds. Because we found that the bond dissociation energy D decreases with increasing bond-angle, we employed the Morse-type function VBond (b, θ) = {D - VAngle (θ)}[1 - exp{-α(b - b0 ) - ß(b - b0 )2 }] where a quartic function VAngle (θ) = k1 (θ - θ0 ) + k2 (θ - θ0 )2 + k3 (θ - θ0 )3 + k4 (θ - θ0 )4 . This function reproduced well the CASPT2 potential energy surface in a wide range of b and θ. The parameters have been obtained for four popular glassy polymers, polyethylene, poly(methyl methacrylate), poly(styrene), and polycarbonate. © 2019 Wiley Periodicals, Inc.