Simulation of Mechanically Assembled Monolayers and Polymers in Good Solvent Using Discontinuous Molecular Dynamics

We present the results of discontinuous molecular dynamics simulations of mechanically assembled monolayers in good solvent. Polymers of chain lengths 5-100 were end-grafted to surfaces at low density and then compressed laterally at varying rates. Data for brush thickness and end-monomer density were collected as a function of surface density; they were shown to correspond well with theoretical predictions and simulation results performed at constant surface density. Brush thickness for all chain lengths could be controlled by judicious choice of the compression rate. Defects in the brush layer were dependent on chain length; it was shown that quick compression for relatively short chains allowed the layer no time to relax into coil form. Quick compression on long chain systems led to entanglement in the brush layer since the longer-chained system was not being afforded the long relaxation time required to form a fully relaxed brush. Hysteresis effects were examined by allowing the brush to relax to a lower surface density, and it was shown that higher surface compression/relaxation rates led to an increase in disparity between brush thickness found during the compression and relaxation stages; in large part, this disparity was due to inadequate equilibration time. Last, results from nonuniform compression in good solvent show negligible effects on monolayer height and structure.