Computer Simulation of the Formation of Mechanically-Assembled Monolayers and Heteropolymers with Adjustable Monomer Sequences

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Title: Computer Simulation of the Formation of Mechanically-Assembled Monolayers and Heteropolymers with Adjustable Monomer Sequences
Author: Strickland, Lawrence Anderson
Advisors: Carol K. Hall, Committee Chair
Jan Genzer, Committee Co-Chair
Orlin D. Velev, Committee Member
Keith E. Gubbins, Committee Member
Maurice Balik, Committee Member
Abstract: This thesis describes a computational investigation, using discontinuous molecular dynamics simulation, of the formation and properties of two types of self assembled polymer structures: mechanically-assembled monolayers (MAMs) and heteropolymers with adjustable monomer sequences (HAMs). MAMs are described in part 1 and HAMS are described in part 2. MAMs in good solvent were created by grafting polymers composed of 5 to 100 hard-sphere monomers to surfaces at low density and then compressing the surface laterally at varying rates. Data for brush thickness and end-monomer density were collected as a function of surface density; this data corresponded well with theoretical predictions and simulation results performed by others. Brush thickness for all chain lengths could be controlled by judicious choice of the compression rate. Defects in the brush layer depended on chain length; we showed that quick compression for short chains allowed the layer no time to relax into coil form. Quick compression for long chains increased entanglement and allowed the chains no time to form a fully-relaxed brush. After compressing the systems to high surface densities, the brushes were allowed to relax to a lower surface density. It was shown that higher compression/relaxation rates led to an increase in disparity between the brush thicknesses found during the compression and relaxation stages; this disparity was largely due to inadequate equilibration time. Last, compressing non-uniformly in the x- and y-directions showed negligible effects on monolayer height and structure. We also investigated the effect of poor-solvent conditions on MAM formation. Square-well chains composed of 20 to 100 units end-grafted to a hard surface at low density were compressed laterally at varying rates. Brush thickness depended on the interplay between solvent quality and the substrate compression rate. Brushes formed in poor solvents at fast compression rates were thinner and exhibited more heterogeneity in coverage than brushes formed in good solvents at slow compression rates. End-monomer trapping increased with increasing compression rate and/or decreasing solvent quality. By varying relaxation/compression rate, we could modify the effects of brush thickness hysteresis. Finally, we suggested a general blueprint for efficient formation of defect-free monolayers. We investigated the formation of heteropolymers with adjustable monomer sequences (HAMs) by simulating a “coloring†reaction performed on A-type homopolymers of length ranging from 100 to 300 units. The transformation of selected A-type monomers to B-type monomers along the macromolecule led to A1-x-co-Bx random copolymers, where x is the mole fraction of B. We showed that for a fixed A-B interaction, the distribution of A and B units in A1-x-co-Bx could be tuned by adjusting both the degree of “coloring†and the solubility of the A and B segments with respect to the implicit solvent. In general, increasing the solubility of the A-type homopolymer or the degree of coloring led to a decrease in blockiness in the co-monomer distribution. Decreasing the solubility of the B species increased the blockiness of the final A1 xBx copolymer. Last, we investigated the effect of tethering the polymers to an impenetrable, surface during the coloring process. Polymers of length 50 to 300 were tethered at various surface densities and then “colored†over a range of temperature and monomer solubilities, resulting in A1-x-co-Bx HAMs. We showed that while blockiness can be adjusted over a small range by varying temperature and solubility, we could significantly affect blockiness by varying chain length and surface density. We showed that for systems of long chains in poor solvents, blocky copolymers formed due to chain-chain overlap; in good solvents, blocky copolymers formed due to chain extension. We observed that for a given change in surface density, blockiness increased more for systems of long chains than for shorter chains.
Date: 2009-12-07
Degree: PhD
Discipline: Chemical Engineering

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