Modeling of amorphous polymer surfaces in computer simulation

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. Free surfaces are stabilized solely by van-der-Waals interactions whereas 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.

Hydrogen-Bond Structure at the Interfaces between Water/Poly(methyl methacrylate), Water/Poly(methacrylic acid), and Water/Poly(2-aminoethylmethacrylamide)

“…For the polymer substrate, it is observed that the density profiles fluctuated for all cases. To the best of our knowledge, the fluctuation of local density has been broadly observed in MD simulation for a polymer substrate, ,,− which was produced by the procedure that is similar to our work, especially for the all atomic model and a polymer with a complex functional group. ,, In addition, the procedures with slower compression and separation rate and a simulation with simulation annealing have been tested for producing the PMMA substrate with intrinsic property. However, those results still show the characteristic of the layering structure.…”

The Soft-Confined Method for Creating Molecular Models of Amorphous Polymer Surfaces

“…After the first atomistic modeling of amorphous glassy polymers done by Theodorou and Suter, two different methods for producing amorphous polymer surfaces were developed and tested: a “free” polymer surface and a confined polymer surface . The free polymer surface was created by arbitrarily elongating one of the box dimensions to a long coordinate and equilibrating the periodic system in isometric–isothermal (NVT) conditions, which were used by many others. − Unfortunately, this method can produce high and uncontrollable roughness, nonuniform density distributions, and a high surface energy that does not adequately represent the real system.…”

“…In such cases, the rings fold unnaturally at the confined border due to the hard wall repulsions, which results in atomic clashes or higher potential energies. In addition, the confined polymer surface method can cause high density of the interface, as was observed for a PP film where the density at the surface was twice as high as that in the middle . The free polymer surface method does not have this drawback, but the large and uncontrollable roughness can be a serious problem to study polymer surfaces, and also may not be the best representation of the real system.…”

Simulated Glass Transition of Poly(ethylene oxide) Bulk and Film: A Comparative Study

“…molecular simulations with CG models can study those phenomena covering much wider time and space scales than those based on AA models. However, it should be noted that the simulated T g 's based on CG models can not be directly compared to the corresponding experimental data, and thus, it is difficult to evaluate the effects of subtle structure on T g s. Note that various atomistic molecular modeling methods have previously been developed and validated for representing polymer films. − With the great progress in computer techniques, now it is moderately feasible to study glass transition of polymer films using AA molecular simulations. It also has to be mentioned that most simulations of glass transition of polymer films are carried out on PS and other hydrocarbons.…”