Modelling of amorphous polymer surfaces in computer simulation

TL;DR

This paper investigates surface effects in amorphous polymers using molecular dynamics simulations, comparing free and confined surfaces and analyzing their structural and density profiles.

Contribution

It introduces two methods for preparing polymer surfaces in simulations and compares their effects on surface structure and properties.

Findings

Free surfaces show monomer migration to the surface.

Density and pressure profiles depend on surface preparation.

Confined surfaces exhibit density and pressure oscillations.

Abstract

We study surface effects in amorphous polymer systems by means of computer simulation. In the framework of molecular dynamics, we present two different methods to prepare such surfaces. {\em Free} surfaces are stabilized solely by van--der--Waals interactions whereas {\em confined} surfaces emerge in the presence of repelling plates. The two models are compared in various computer simulations. For free surfaces, we analyze the migration of end--monomers to the surface. The buildup of density and pressure profiles from zero to their bulk values depends on the surface preparation method. In the case of confined surfaces, we find density and pressure oszillations next to the repelling plates. We investigate the influence of surfaces on the coordination number, on the orientation of single bonds, and on polymer end--to--end vectors. Furthermore, different statistical methods to determine location and width of the surface region for systems of various chain lengths are discussed and applied. We introduce a ``height function'' and show that this method allows to determine average surface profiles only by scanning the outermost layer of monomers.