Browsing by Author "Wendy E. Krause, Committee Chair"

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    Fiber Lubrication: A Molecular Dynamics Simulation Study
    (2009-05-27) Liu, Hongyi; Melissa A. Pasquinelli, Committee Member; Harvey Charlton, Committee Member; Orlando J. Rojas, Committee Co-Chair; Wendy E. Krause, Committee Chair
    LIU, HONGYI. Fiber Lubrication: A Molecular Dynamics Simulation Study. (Under the direction of Wendy E. Krause and Orlando J. Rojas). Molecular and mesoscopic level description of friction and lubrication remains a challenge because of difficulties in the phenomenological understanding of to the behaviors of solid-liquid interfaces during sliding. Fortunately, there is the computational simulation approach opens an opportunity to predict and analyze interfacial phenomena, which were studied with molecular dynamics (MD) and mesoscopic dynamics (MesoDyn) simulations. Polypropylene (PP) and cellulose are two of most common polymers in textile fibers. Confined amorphous surface layers of PP and cellulose were built successfully with xenon crystals which were used to compact the polymers. The physical and surface properties of the PP and cellulose surface layers were investigated by MD simulations, including the density, cohesive energy, volumetric thermal expansion, and contact angle with water. The topology method was employed to predict the properties of poly(alkylene glycol) (PAG) diblock copolymers and Pluronic triblock copolymers used as lubricants on surfaces. Density, zero shear viscosity, shear module, cohesive energy and solubility parameter were predicted with each block copolymer. Molecular dynamics simulations were used to study the interaction energy per unit contact area of block copolymer melts with PP and cellulose surfaces. The interaction energy is defined as the ratio of interfacial interaction energy to the contact area. Both poly(proplene oxide) (PPO) and poly(ethylene oxide) (PEO) segments provided a lipophilic character to both PP and cellulose surfaces. The PPO/PEO ratio and the molecular weight were found to impact the interaction energy on both PP and cellulose surfaces. In aqueous solutions, the interaction energy is complicated due to the presence of water and the cross interactions between the multiple molecular components. The polymer-water-surface (PWS) calculation method was proposed to calculate such complex systems. In a contrast with a vacuum condition, the presence of water increases the attractive interaction energy of the diblock copolymer on the cellulose surface, compared with that on the PP surface. Water decreases the interaction energy of the triblock copolymer on the cellulose surface, compared with that on the PP surface. MesoDyn was adopted to investigate the self-assembled morphology of the triblock copolymer, in aqueous solution, confined and sheared at solid-liquid interfaces. In a bulk aqueous solution, when the polymer concentration reached 10% v/v, micelles were observed with PPO blocks in the core and PEO blocks in the shell of the micelles. At the concentrations of 25% and 50%, worm-like micelles and irregular cylinders were observed in solutions, respectively. The micelles were formed faster in aqueous solutions confined by cellulose surfaces than that in the bulk. The formed micelles were broken under shearing, which led to a depletion of polymers at the interfaces. During the shearing on the PP surfaces, the polymers were adsorbed on the surfaces protecting the PP surfaces. This simulation study in the fiber lubrication was in good agreement with the experimental results and so provided an approach to visualize the polymer configuration at the liquid-solid interface, predict the lubricant-surface systems, and theoretically guide the experiments of designing new/efficient lubricants for fibers.
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    Investigation of Synthetic and Natural Lubricants
    (2008-08-19) Liang, Jing; Sam M. Hudson, Committee Member; Jan Genzer, Committee Member; Alan E. Tonelli, Committee Co-Chair; Wendy E. Krause, Committee Chair
    Nanoindenter-based scratch tests are simple, rapid, and reliable method to evaluate the tribological properties of materials at the microscale. It not only works well on dry polymer surfaces, but also can evaluate thin fluid film lubrication on polymer surfaces. By using different experimental tips (different shape and radius) on the same polymer surfaces, different coefficient of friction results are obtained. Because of the different molecular structures, different polymer surfaces show different tribological properties. Furthermore, the relationship between the adsorption of lubricant and the lubricity of lubricant has been studied. Lubrication is an extremely complex phenomenon and it can be influenced dramatically by the lubricant, including the lubricants'molecular weight, molecular structure (both its chemical structure and its geometry (et al., di-block versus tri-block) and chain length, the solvent, the substrates, including the substrates' roughness and chemistry, the number of lubrication layers, the sliding velocity, and applied load. In addition, the lubrication properties of synovial fluid and its components (sodium hyaluronate (HA), proteins, and phospholipids) have also been studied. The concentration and molecular weight of sodium hyaluronate both have great effect on its lubricity. The concentrations of gamma-globulins and albumin also have effect on their lubricating properties. With just one type of protein or with excessive proteins, the solutions looses lubricity. Addition of HA can improve the lubricating properties of some protein solutions. DPPC (dipalmitoyl phosphatidylcholine) also shows some lubricity for polyethylene. However, without HA, the concentration of DPPC has no effect on its lubricity at the microscale. Addition of HA can improve the lubricity of DPPC solution when the concentration of DPPC is high. Depending on the concentration, every component has some lubricity, but working synergistically at the concentrations found in typical, healthy synovial fluid can greatly improve their lubricity. Indeed, the Synovial fluid model, which has gamma-globulins at 7 mg⁄mL, albumin at 11 mg⁄mL, HA (MW: 2.0 MDa) at 3 mg⁄mL, showed the best lubricity.