Coarse-grained polyethylene: Including cross terms in bonded interactions and introducing anisotropy into the model for the orthorhombic crystal

Our previous paper (E.A. Zubova, I.A. Strelnikov, N.K. Balabaev, A.V. Savin, M.A. Mazo, and L.I. Manevich, Polym. Sci., Ser. A 59 (1) (2017)) addressed the simplest coarse-grained model of polyethylene and alkanes. The CH₂ group in this united-atom model is replaced with a bead. In the framework of the model, nonbonded interactions are described by the Lennard-Jones potential (6–12), whereas the potential for bonded interactions accounts for the bonds between beads and for coarse grained angles and dihedral angles but not for the cross terms between them. We found the area of geometrical parameters of the model where all the three known crystalline phases of polyethylene are stable at low temperatures. We parametrized the force field of the model using the dynamic properties of the system, namely, the inelastic neutron scattering spectrum of the orthorhombic phase of polyethylene. However, the simplest model underestimates the value of the elastic modulus along the chains of the crystal by a factor of two. The derived setting angle of the molecules also differs appreciably from the experimental data. Moreover, the acoustic dispersion curves for the modes with the wave vector directed along the chain axis deviate from the experimental data at low frequencies. In the present study, we included the cross terms into the bonded interactions. This made it possible to reproduce the experimental elastic modulus along the chains of the crystal and to decrease the frequency range for the optical skeletal dispersion curve to proper values. As for the beads, we separated force centers for the bonded and nonbonded interactions, which enabled us to reproduce the optical skeletal curve and to bring the anisotropy of interchain interactions in line with the experiment. However, the model fails to reproduce the balance of interactions between the neighboring chains in the crystal. For this, a different form of the potential energy of van der Waals interactions seems to be needed.