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 V_Bond (b, θ) = {D - V_Angle (θ)}[1 - exp{-α(b - b₀ ) - β(b - b₀ )² }] where a quartic function V_Angle (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.