An optimized fully-atomistic procedure to generate glassy polymer films for molecular dynamics simulations

Abstract

An explicit-atom wall-compression procedure has been optimized in order to generate glassy polymer films containing both surface-like and bulk-like chains for use in molecular dynamics (MD) simulations. All steps are carried out at the fully-atomistic level, which avoids the complexities of coarse-graining and back-mapping. One dimension of an isotropic bulk MD box is extended and walls are placed at both ends of the enlarged simulation box. The walls progressively advance towards the polymer at a slow rate until the resulting film regains a bulk-like density in its core. Following local relaxation at moderate and high temperatures, the walls are pushed back to the edges of the simulation box and the free-standing glassy film is allowed to relax. The 6FDA-6FpDA, 6FDA-6FmDA and 6FDA-DAM polyimides were chosen as test cases. The effects of the chain lengths, the rate of wall compression, the temperatures as well as the conditions of the simulations were assessed by monitoring the densities, conformations and configurations of the chains at each step. The optimized procedure leads to surface chains being aligned and flattened with respect to the interfaces, while the bulk chains in the centre of the models remain unaffected. Molecular models of glassy polymer films containing ~80,000 atoms with an isotropic core of volume ~(60 × 100 × 100) Å3 and two interfacial regions, each of volume ~(20 × 100 × 100) Å3, were created. As found before for simpler models, the interfacial width is of the order of the average radius of gyration of the chains.