Off-lattice simulation algorithms for athermal chain molecules under extreme confinement

Abstract

A new algorithmic approach is presented for the generation and successive equilibration of polymer configurations under conditions of extreme confinement where the inter-wall distance, in at least one dimension, approaches the diameter of the spherical monomers. It significantly improves on the Monte Carlo (MC) protocol described in Karayiannis and Laso (2008) [126]. The algorithm is designed to generate highly confined packings of freely-jointed chains of hard spheres of uniform size. Spatial confinement is achieved by including flat, parallel impenetrable walls in one or more dimensions of the simulation box. The present MC scheme allows the systematic study of the effect of chain length, polydispersity, volume fraction, bond tolerance (gap), cell aspect ratio and level of confinement on the short- and long-range structure of polymer chains near and far from the confining planes. In the present study we focus on the efficiency of the MC protocol in generating, equilibrating, and configurationally decorrelating chain assemblies with average lengths ranging from N = 12 to 1000 monomers and at volume fractions from dilute up to the maximally random jammed (MRJ) state. Starting from cubic amorphous cells filled with polymer chains, the MC algorithm is able to reach quasi 2-d (plate-like) and 1-d (tube-like) states under conditions of extreme confinement and/or cell aspect ratio where the inter-wall distance approaches the diameter of beads forming the chains. A comparison with corresponding bulk packings shows the similarities and differences produced by extreme spatial confinement.