In most barrier applications, both mechanical and diffusion properties of the material are important. In this thesis the evaluation of molecular mechanisms responsible for the enhanced elastic properties of Polymer Clay Nanocomposites (PCNs) and the molecular mechanisms of Oxygen diffusion in PolyButylene Terephthalate polymer are presented. Interaction energy between PCN constituents, conformational changes of polymer, interaction energy between Oxygen molecule and polymer, rate of Oxygen and Oxygen diffusion coefficient are evaluated. Molecular simulation studies of PolyButylene Terephthalate (PBT) clay nanocomposite and Nylon6 clay nanocomposite show that a higher crystallinity polymer such as PBT would require higher attractive and repulsive interactions with organic modifier in order to make significant change in the crystallinity of PBT in the nanocomposite and in turn enhance the elastic modulus and hardness. Molecular interactions energy between Oxygen molecule and polymer, change in polymer conformation caused by thermal energy assist the Oxygen molecule to diffuse through polymer. / NSF-EPSCoR FlexEM Grant

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Single-molecule stretching experiments reveal a distinct plateau region in force-extension curves of nucleic acids such as long double-stranded deoxyribonucleic acids (DNA) and ribonucleic acids (RNA). The dissertation comprises two parts. In the first part, we propose an approach to help analyze polymer force-extension curves that exhibit a distinct plateau region. When coupled to a bead-spring dynamic model, the approach qualitatively reproduces a variety of experimental force-extension curves of long double-stranded (ds) DNA and RNA, including torsionally constrained and unconstrained DNA, and negatively supercoiled DNA. In the plateau region of the force-extension curves, our molecular dynamics simulations show that the polymer separates into a mixture of slightly and highly stretched states without forming macroscopically distinct phases. In the second part, we hypothesize that, depending on the sequence composition, multiple distinct plateau regions can be seen in force-extension curves of long dsDNA fragments under physiological solvent conditions. We explore specific long double-stranded DNA sequences where we expect the phenomenon to occur, and to characterize the distribution of states along the polymer. Our molecular dynamics simulations show that multi-plateau regions are observed in the force-extension curves of specific long double-stranded DNA fragments. The formation of mixed states of slightly and highly stretched DNA, co-existing with macroscopically distinct phases in several segments in the plateau regions, is also predicted. / Doctor of Philosophy / Single-molecule stretching experiments reveal a distinct plateau region in force-extension curves of nucleic acids such as long double-stranded DNA and RNA. In this dissertation, we propose a simple bead-spring dynamic model that qualitatively reproduces a variety of experimental force-extension curves of long double-stranded DNA and RNA that exhibit a distinct plateau region. Based on the model, we make multiple predictions. In particular, we predict that multiple distinct plateau regions can be seen in force-extension curves of long composite double-stranded DNA fragments under physiological solvent conditions. We believe our findings should be of interest to both experimentalists and theoreticians. Experimentalists might find our model useful for routine analysis of force-extension curves of double-stranded DNA and RNA. Theoreticians may appreciate our general approach.

During high speed cutting processes, metals are subject to high strains and strain rates. The dynamic nature of the deformation during high speed cutting makes it difficult to detect atomic scale deformation mechanisms experimentally. Atomic scale plasticity behaviour is often studied using various micromachining techniques such as micropillar compression testing, nanoindentation, and nanoscratching. However, strain rates in micromachining experiments are far lower than those seen during high speed cutting. Atomistic simulations can be used to study high strain rate plasticity at nanometre length scales. In this thesis, we present results from molecular dynamics simulations of plasticity in nanostructures. Results from simulations of uniaxial strain of both bcc and fcc nanopillars are presented. We find that the outcomes of these uniaxial strain simulations depend sensitively on the initial configurations of the systems. In particular, the choice of crystallographic surfaces on the faces of the pillars and the means by which strain is implemented in the simulations can affect the simulation results. We find that the twinning anti-twinning asymmetry in bcc materials causes nanopillars to deform by dislocation glide in compression and by twinning in tension. This explains the compression tension asymmetry reported experimentally in bcc micropillars. We find that deformation is mediated by glide of shockley partials in fcc pillars for compressive and tensile strains. Simulations of pure shear of nanocrystalline Fe are also presented. We find a change in deformation mechanisms for this system when at high temperatures. At low temperatures, plasticity is mediated in part by dislocation glide and twinning. However, at temperatures above 1200K the deformation is dominated by grain boundary sliding, recrystallization, and amorphization.

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published_or_final_version / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

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