Title

Computational studies of polymeric systems

Date of Completion

January 2009

Keywords

Physics, Condensed Matter|Nanotechnology|Engineering, Materials Science

Degree

Ph.D.

Abstract

Polymeric systems involving polyelectrolytes in surfaces and interfaces, semiflexible polyelectrolytes and biopolymers in solution, complex polymeric systems that had applications in nanotechnology were modeled using coarse grained molecular dynamics simulation. In the area of polyelectrolytes in surfaces and interfaces, the phenomena of polyelectrolyte adsorption at oppositely charge surface was investigated. Simulations found that short range van der Waals interaction was a major factor in determining morphology and thickness of the adsorbed layer. Hydrophobic polyelectrolytes adsorbed in hydrophobic surfaces tend to be the most effective in forming multi-layers because short range attraction enhances the adsorption process. Adsorbed polyelectrolytes could move freely along the surface which was in contrast to polyelectrolyte brushes. The morphologies of hydrophobic polyelectrolyte brushes were investigated and simulations found that brushes had different morphologies depending on the strength of the short range monomer-monomer attraction, electrostatic interaction and counterion condensation. Planar polyelectrolyte brushes formed: (1) vertically oriented cylindrical aggregates, (2) maze-like aggregate structures, or (3) thin polymeric layer covering a substrate. While, the spherical polyelectrolyte brushes could be in any of the previous morphologies or be in a micelle-like conformation with a dense core and charged corona. ^ In the area of biopolymers and semiflexible polyelectrolytes in solution, simulations demonstrated that the bending rigidity of these polymers was scale-dependent. The bond-bond correlation function describing a chain's orientational memory could be approximated by a sum of two exponential functions manifesting the existence of the two characteristic length scales. The existence of the two length scales challenged the current practice of describing chain stretching experiments using a single length scale. ^ In the field of nanotechnology, simulations of "nanoimprinting lithography" and "nanopropulsion engine" were performed. Nanoimprinting lithography simulations showed that the quality of the process depended on the elastic modulus of the mold, interfacial energy of the injected liquid, and the size of the master. For the nanopropulsion engine, simulations and scaling analysis established that the nozzle velocity was proportional to the chain's polymerization rate with the proportionality coefficient being determined by the nozzle geometry, the nozzle friction coefficient, and the dynamics of the polymer chains inside the nozzle. ^