Simulation of Mechanically-Assembled Monolayers Using Discontinuous Molecular Dynamics
Lawrence A. Strickland, Chemical and Biomolecular Engineering, North Carolina State University, College of Engineering I, Box 7905, 911 Partners Way, Raleigh, NC 27695 and Carol K. Hall, Department of Chemical and Biomolecular Engineering, North Carolina State University, College of Engineering 1, Box 7905, 911 Partners Way, Raleigh, NC 27695.
Although self-assembled monolayers allow for some adjustment of surface characteristics, their range is limited due to the relatively low surface density of the grafting sites and the accompanying defects that allow penetration of liquids into the monolayer. Mechanically-assisted monolayers (MAMs), which are polymer layers or “brushes” formed by depositing polymers on a pre-stretched surface and then releasing the stretch, can rectify the problems associated with low grafting-site surface densities by reaching higher surface coverage than those achievable solely through self-assembly. To this point, all of the research in the area of mechanically-assisted monolayers has been experimental. We simulate the formation of mechanically-assembled monolayers by using the discontinuous molecular dynamics (DMD) method. We investigate the formation of the brush and the effects of chain length and surface density. Monolayer thickness and structure is compared to previous results. The monolayer data which we collected by continual compression of the tethering surface matches well with data from brushes at constant surface density. Also we show that the thickness and structure of the brush can be manipulated by varying surface relaxation rate and solubility. We found that the slowest compression rates represent a quasi-equilibrium state that matches equilibrium data found at constant surface density more accurately than faster compression rates. Also fast compression creates brush deformations in poor solvents by preventing chains from reaching normal end-to-end length.