Molecular dynamics simulation of polymer liquid and glass. II. Short range order and orientation correlation

Abstract

Molecular dynamics simulation has been performed with an assembly of n-alkane-like chains subject to potentials restricting bond lengths, bond angles, and trans–gauche dihedral angles and interacting with neighboring chains according to a truncated Lennard-Jones potential. The system, on stepwise cooling under constant pressure, undergoes a transition exhibiting characteristics of glass transition observable with laboratory polymers. The short range order in the system was monitored by evaluating the radial distribution function gk(r), where r is the distance between two short subchains, each comprising k consecutive CH2 units. The orientation correlations between neighboring subchains were monitored by evaluating the function Sk (r), giving the average cosine square of the angle between the end-to-end vectors of two neighboring subchains of k CH2 units separated by a scalar distance r. Integration of Sk(r), weighted by gk(r), gives the correlation volume. Both the short range order and the orientation correlation are seen to improve steadily as the temperature is lowered. This increased order is brought about as a result of increased density, reduced thermal motion, and more extended chain conformation. Extrapolation of the orientation correlation against temperature predicts an isotropic–nematic transition temperature Tc, as suggested by the Landau–deGennes theory. Isothermal annealing just below the predicted Tc indeed produces a highly ordered structure.