Browsing by Author "John H. van Zanten, Committee Member"

Now showing 1 - 6 of 6

Engineered Deposition of Functional Coatings from Micro- and Nanoparticles using Convective Assembly
(2006-05-10) Prevo, Brian Geoffrey; John H. van Zanten, Committee Member; Stefan Zauscher, Committee Member; Saad Khan, Committee Member; Orlin D. Velev, Committee Chair
The potential technological applications of micro- and nanoparticle coatings necessitate the development of rapid, inexpensive and easily controlled deposition procedures. We have developed a technique for making structured thin films from micro- and nanoparticles by dragging on a substrate a liquid meniscus at constant velocity. The advantages of this technique are improved process speed, efficiency and reduced material consumption relative to standard dip coating techniques. The governing mechanism of the deposition process was found to be convective assembly at high volume fractions. Uniform, large area coatings (square centimeters in area) can be deposited in minutes at rates approaching 100 microns per second from microliters of suspension. Operational 'phase' diagrams were constructed from coating data, relating the coating layer thickness and particle packing symmetry to the process parameters: deposition speed, particle volume fraction, and solvent evaporation rate. Varying these parameters provided the means to control and tune nanocoating structure and properties. We found the most potent parameter to be the deposition speed. The deposition process was well modeled by a simple macroscopic species balance taken around the thin film drying site. We have successfully applied this deposition technique to a wide variety of colloidal systems including: latex and silica microspheres, gold nanoparticles, ferritin proteins, and living yeast cells. Conductive coatings from metal nanoparticles exhibited tunable optical and electronic properties simply by virtue of the adjusting deposition speed. The antireflective (AR) capabilities of silica nanoparticle coatings on glass and silicon substrates can also be facilely tuned using this deposition process. These AR coatings demonstrably improved the photovoltaic efficiency of solar cells. We have also investigated the use of compressed carbon dioxide as a replacement solvent for colloidal coating deposition. We achieved rapid sedimentation of uniform, conformal nanoparticle coatings using liquid and supercritical carbon dioxide (primarily as an antisolvent). These results show potential for fabricating conformal coatings of self cleaning, technologically relevant materials by simple self-assembly techniques.
Fumed oxide-based nanocomposite polymer electrolytes for rechargeable lithium batteries
(2003-03-18) Zhou, Jian; Peter S. Fedkiw, Committee Chair; Saad A. Khan, Committee Member; Daniel L. Feldheim, Committee Member; John H. van Zanten, Committee Member
Rechargeable lithium batteries are promising power sources for portable electronic devices, implantable medical devices, and electric vehicles due to their high-energy density, low self-discharge rate, and environmentally benign materials of construction. However, the high reactivity of lithium metal limits the choice of electrolytes and impedes the commercialization of rechargeable lithium batteries. One way to tackle this problem is to develop electrolytes that are kinetically stable with lithium. Composite polymer electrolytes (CPEs) based on fumed oxides presented in this work are promising candidates for rechargeable lithium batteries. The effects of fumed oxides (SiO2, Al2O3, TiO2) and binary mixtures of oxides (SiO2/Al2O3) on ionic conductivity of CPEs based on poly(ethylene oxide) (PEO) oligomers (Mw =250, 200, 1000, and 2000) + lithium bis(trifluromethylsulfonyl)imide [LiN(CF3SO2)2] (LiTFSI) (Li:O=1:20) are studied using electrochemical impedance spectroscopy (EIS), differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy. Fillers show similar effect on conductivity in all systems: no distinguishable effect is found with filler type, and addition of filler decreases conductivity at temperatures above the melting point (Tm) but increases conductivity at temperatures below. The insulating nature of fillers and stiffening of the polymer solvent (as evidenced by FTIR and DSC data) in the presence of fillers cause a decrease in conductivity at temperatures above Tm, which remains constant upon addition of fillers. The increase in conductivity at temperatures below Tm can be attributed to faster ion transport along the filler surface. Addition of fumed oxides increases electrolyte viscosity (and elasticity) and the extent of enhancement varies with filler type: fumed silica shows the strongest and titania the least. Elastic modulus, yield stress, and normalized viscosity of gel-type composite electrolytes decrease with increasing oligomer Mw when electrolytes are amorphous. The reduction in structure strength may be ascribed to the enhanced interactions between surface hydroxyl groups on fumed oxides and polyether oxygens. Thus, the number of accessible ?OH groups is reduced for interactions among fumed oxide particles, which dictates the strength of solid-like structure. The interfacial stability between electrolyte and lithium is enhanced in the presence of fumed silica. The enhancement in interfacial stability is seen as a decrease in interfacial resistance and cell polarization, and an increase in lithium cycleability and cell capacity. The improved interfacial stability between CPE and lithium is attributed to less lithium corrosion (fillers scavenge water impurities that corrode lithium) and dendrite formation (electrolyte elasiticty inhibits dendrite formation). The extent of the enhancing effect of fumed silica depends on its surface chemistry, with the largest effect seen with hydrophilic fumed silica, which has the largest scavenging capacity and highest elasticity. The effect on cycle capacity is reported of cathode material (metal oxide, carbon, and current collector) in lithium/metal oxide cells cycled with fumed silica-based composite electrolytes. Cells with composite electrolytes show higher capacity and less cell polarization than those with filler-free electrolyte. Among the three active materials studied (LiCoO2, V6O13, and LixMnO2), V6O13 cathodes deliver the highest capacity and LixMnO2 cathodes render the best capacity retention. Discharge capacity of Li/LiCoO2 cells is affected greatly by cathode carbon type and discharge capacity increases with decreasing carbon particle size. Current collector materials also play a significant role in cell cycling performance: Li/V6O13 cells deliver increased capacity using Ni foil and carbon fiber current collectors in comparison to an Al foil. In summary, fumed oxide-based nanocomposite electrolytes are promising candidates for lithium battery applications with high room-temperature conductivity, good mechanical strength, stable interface between lithium metal and electrolytes, and reasonable capacity and capacity retention with optimized cathode compositions.
Molecular Dynamics Simulations of Micellization in Model Surfactant/CO2 Systems
(2003-04-22) Li, Zhengmin; Carol K. Hall, Committee Chair; Peter K. Kilpatrik, Committee Member; John H. van Zanten, Committee Member
Discontinuous molecular dynamics simulations are performed on surfactant (H[subscript n] T[subscript m])/solvent systems modeled as a mixture of single-sphere solvent molecules and freely-jointed surfactant chains composed of n slightly solvent-philic head spheres (H) and m solvent-philic tail spheres (T), all of the same size. We use a square-well potential to account for the head-head, head-solvent, tail-tail and tail-solvent interactions and a hard sphere potential for the head-tail and solvent-solvent interactions. We first simulate homopolymer/supercritical CO₂ (scCO₂) systems to establish the appropriate interaction parameters for a surfactant/scCO₂ system. Next we simulate surfactant/scCO₂ systems and explore the effect of the surfactant mole fraction, packing fraction and temperature on the phase behavior of a surfactant/scCO₂ system. The transition from the two-phase region to the one-phase region is located by monitoring the contrast structure factor of the equilibrated surfactant/scCO₂ system and the micelle to unimer transition is located by monitoring the micelle size distribution of the equilibrated surfactant/scCO₂ system. The phase diagram for the surfactant/scCO₂ system and the density dependence of the critical micelle concentration are in qualitative agreement with experimental observations. The phase behavior of a surfactant/scCO₂ system can be directly related to the solubilities of the corresponding homopolymers that serve as the head and tail block for the surfactant. The location of the micelle-unimer transition is strongly affected by the head-solvent attraction but only weakly affected by the tail-tail and tail-solvent attractions. Both micellization and phase separation upon decreasing the temperature are found in our simulations.
Nanostructred Polymeric Membranes for Selective CO2 Removal from Light Gas Mixtures
(2004-06-27) Patel, Nikunj Pragjibhai; Steve D. Smith, Committee Member; Saad A. Khan, Committee Member; Richard J. Spontak, Committee Chair; John H. van Zanten, Committee Member
Two primary materials strategies have been developed to produce nanostructured polymer membranes for selective CO2 removal from mixed light-gas streams. In one approach, a microphase-ordered poly(styrene-b-ethylene oxide-b-styrene) (SEOS) triblock copolymer and its miscible blends with poly(ethylene glycol) (PEG) differing in molecular weight have been investigated to establish structure-transport property relationships. These membranes exhibit high CO2/H2 selectivity due to the affinity of CO2 for the ether moiety in the copolymer/homopolymer backbone. Crystalline regions in the EO microphase or introduced by relatively high-molecular-weight PEG serve as impermeable barriers to penetrating gas molecules and therefore compromise membrane performance. This drawback can be overcome through the physical addition of low-molecular-weight PEG, which behaves as a diluent. Upon PEO crystal melting at elevated temperatures, the CO2/H2 selectivity undergoes an abrupt increase consistent with the hypothesis that only amorphous regions can participate in penetrant transport. An alternative approach to near-equilibrium block copolymer/homopolymer blends is the introduction of a B-compatible homopolymer into a swollen ABA triblock or higher-order multiblock copolymer. The resultant "mesoblends" are reproducible, nonequilibrium blends that do not undergo the same morphological transitions induced in the near-equilibrium blend analogues. This procedure has been adopted here to generate novel morphologies in the SEOS triblock copolymer and a poly(amide-b-ethylene glycol) (AEG) multiblock copolymer with PEG homopolymers. Solvent quality, solution concentration and temperature have a profound impact on PEG solubility within the copolymer. Incorporation of amorphous PEG into the AEG copolymer is found to enhance CO2 permeability, as well as CO2/H2 selectivity. The second approach examined here relies on chemically crosslinked PEG diacrylate (PEGda) oligomers differing in molecular weight, as well as their nanocomposites prepared with up to 10 wt% methacrylate-functionalized fumed silica (FS) or an organically-modified nanoclay. The mechanical, thermal and morphological characteristics of these membranes have been probed by dynamic rheology, thermal gravimetric analysis (TGA) and transmission electron microscopy (TEM), respectively. These PEGda membranes exhibit exceptionally high acid-gas selectivity coupled with high gas permeabilities that tend to increase with increasing oligomer molecular weight. Addition of FS results in improved mechanical properties without deteriorating transport properties. Temperature-dependent permeation studies demonstrate Arrhenius behavior with considerably lower activation energy of permeation for CO2. The polarity of the matrix, represented by PEGda oligomer molecular weight, and the transmembrane pressure allow systematic tuning of CO2/H2 selectivity and CO2 permeability. Crosslinked poly(propylene glycol) diacrylate (PPGda) membranes with various additives have also been synthesized due to their reportedly higher CO2 solubility. Gas transport and rheological properties are extremely sensitive to the molecular weight of oligomer, as in the case of the corresponding PEGda membranes. The major difference between these two membranes is the higher CO2 permeability, but lower CO2/H2 selectivity, in the PPGda membranes. Gas transport properties vary according to the rule of mixtures in PPGda/PEGda membranes blended prior to chemical crosslinking.
Self-Assembled Thin Films: Peptides in Hybrid Bilayers and Mixed Organosilanes on Silica
(2008-01-19) Smith, Matthew Brian; Robert M. Kelly, Committee Member; Jan Genzer, Committee Co-Chair; Peter K. Kilpatrick, Committee Chair; Peter S. Fedkiw, Committee Member; John H. van Zanten, Committee Member
Simulation of Polyglutamine Aggregation With An Intermediate Resolution Protein Model
(2006-04-07) Marchut, Alexander Joseph; Carol K. Hall, Committee Chair; Robert M. Kelly, Committee Member; John Cavanagh, Committee Member; John H. van Zanten, Committee Member
The pathological manifestation of nine hereditary neurodegenerative diseases including Huntington's disease is the presence within the brain of aggregates of disease-specific proteins that contain polyglutamine tracts longer than a critical length. The molecular level mechanisms by which these proteins aggregate are still unclear. In an effort to shed light on this important phenomenon, we are investigating the aggregation of model fibril-forming peptides using molecular-level computer simulation. A simplified model of polyglutamine, the protein that is known to form fibrils (ordered aggregates of proteins in beta-sheet conformations) in the brains of victims of Huntington's disease, has been developed. This model accounts for the most important types of intra- and inter-molecular interactions - hydrogen bonding and hydrophobic interactions - while allowing the folding process to be simulated in a reasonable time frame. The model utilizes discontinuous potentials such as hard spheres and square wells in order to take advantage of discontinuous molecular dynamics (DMD), a fast simulation technique that is very computationally efficient. DMD is used to examine the folding and aggregation of systems of model polyglutamine peptides ranging in size from isolated peptides to 96 peptides. In our simulations we observe the spontaneous formation of aggregates and annular structures that are made up of beta sheets starting from random configurations of random coils. The effect of chain length on the behavior of our model peptides was examined by simulating the folding of isolated polyglutamine peptides 16, 32, and 48 residues long and the folding and aggregation of systems of twenty-four model polyglutamine peptides 16, 32, 36, 40, and 48 residues long. In our multi-peptide simulations we observed that the optimal temperature for the formation of beta sheets increases with chain length up to 36 glutamine residues but not beyond. Our finding of this critical chain length of 36 glutamine residues is interesting because a critical chain length of 37 glutamine residues has been observed experimentally.