Molecular Dynamics Simulation of Structural Formation of Short Polymer Chains

Abstract

Computer simulation of the structural formation of polymer chains has recently become the focus of attention in physics, chemistry and material science. We aim at understanding the mechanisms of the structural for-.mation of polymer chains at the molecular level. To this end, we carry out the molecular dynamics simulations of 100 short polymer chains, each of which consists of 20 CH 2 groups, and analyze the formation process of the orientationally ordered structure.The united CH 2 groups interact via the bonded potentials (bond-stretching, bond-bending and torsional potentials) and the non-bonded potential (12-6 LennardJanes potential). The atomic force field used here is the DREIDING potential 1 ). We use the velocity version of the Verlet algorithm and apply the Nose-Hoover method in order to keep the temperature of the system constant. The integration time step and the relaxation constant for the heat bath variable are 0.001 ps and 0.1 ps, respectively. The cutoff distance for the Lennard-Janes potential is 10.5 A. The polymer chains are exposed to vacuum. The total momentum and the total angular momentum are taken to be zero in order to cancel overall translation and rotation of chains. At first, we prepare random configuration of short polymer chains at high temperature (T = 700 K) and then it is quenched to various low temperatures (T = 300, 320, ... , 460 K) 2 ).We show, in Fig. 1 At last they coalesce into a large cluster and a highly ordered monolayer structure is formed [ Fig. 1 (d)].In order to investigate the growth process of the global bond-orientational order, we calculate the global bond-orientational order parameter S, which is defined bywhere N and n are respectively the number of polymer chains and the number of CH 2 groups per polymer chain (N = 100, n = 20) and 1/Ji is the angle between the subbond vector of the m-th chain bf\ which is formed by connecting centers of two adjacent bonds i and i-1 of the m-th chain, and the director (c-axis) of the layer. The parameter S would take a value of 1.0, 0.0 or -0.5, respectively, for polymer chains whose sub-bonds are perfectly parallel, random or perpendicular to the director.We show the time dependence of S at T = 440 K in

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