Browsing by Author "Richard Spontak, Committee Member"

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Design and Interfacial Activity of Copolymers with Controlled Monomer Sequence Distributions
(2004-01-13) Semler, James Joseph; Jan Genzer, Committee Chair; Carol Hall, Committee Member; Harold Ade, Committee Member; Peter Kilpatrick, Committee Member; Richard Spontak, Committee Member
We study the bulk and interfacial behavior of A-B copolymers. Emphasis is placed upon addressing the role of the monomer sequence distribution in A-B copolymers as it pertains to the copolymer's mobility in confined geometries and its ability to recognize chemical patterns on surfaces. Monte Carlo simulations are used to study the ability of block (A-b-B) and alternating (A-alt-B) copolymers to recognize chemical patterns on flat, impenetrable surfaces comprising two distinct chemical sites, C and D. The copolymer adsorption is driven by the repulsion between A and B segments along the copolymer chain and the attraction between B segments and D sites on the surface. The principle parameters that govern the ability of A-b-B and A-alt-B copolymers to recognize surface patterns are: the strength of the interaction between B segments and D surface sites, the A-B monomer sequence distribution, and the size and spatial distribution of adsorbing D sites. Our simulations reveal that both A-b-B and A-alt-B copolymers are capable of recognizing surface patterns and increasing the B-D attraction enhances the partitioning of A and B segments at the surface. Commensurability between the copolymer's monomer sequence distribution and the size and spatial distribution of the surface heterogeneities is also found to affect the ability of A-b-B and A-alt-B copolymers to recognize surface chemical patterns. When the adsorbing domain size exceeds the size of the copolymer's parallel component to the radius of gyration, A-b-B copolymers are found to transfer the surface pattern into the bulk with high fidelity. A-alt-B copolymers, however, are able to replicate the surface pattern into the bulk material when heterogeneous domain sizes are much smaller. We introduce a novel 'coloring' scheme to synthesize polystyrene-polybromostyrene (PS-co-PBrS) copolymers with statistically random (r-(PS-co-PBrS)) and random-blocky (b-(PS-co-PBrS)) monomer sequence distributions. Our results show that r-(PS-co-PBrS) and b-(PS-co-PBrS) copolymers with equivalent bromine content possess different intrinsic viscosities and radii of gyration. We attribute this behavior to the ability of b-(PS-co-PBrS) coils to form globular structures in toluene where PBrS forms a dense core and PS remains predominantly in a loose corona. This behavior is in contrast to that of r (PS co-PBrS) coils where both the PBrS and PS are homogeneously distributed. The interfacial behavior of the random and blocky copolymers is also found to differ. Specifically, thin films of r-(PS-co-PBrS) deposited on top of flat silica substrates covered with a semifluorinated self-assembled monolayer are found to dewet at a faster rate than b-(PS-co-PBrS) of comparable thickness at the same T−Tg, where Tg is the bulk glass transition temperature of the PS-co-PBrS copolymer. To our knowledge, this is the first experimental evidence that supports claims from computational studies arguing that the sequence distribution of random copolymers affects the chain's mobility on a surface. Molecular insights into the 'coloring' reaction are provided by Monte Carlo simulations of the experimental reaction scheme. The probability of chemically altering expanded homopolymer coils is found to be equal for all units along the length of the chain. In contrast, 'coloring' of collapsed homopolymer coils reveals that the probability of modification is widely distributed. These results further support our claim that copolymers with random and random-blocky monomer sequence distributions can be synthesized by 'coloring' expanded and collapsed homopolymer coils, respectively.
Electrospinning Yarn Formation and Coating.
(2010-11-22) Sahbaee Bagherzadeh, Arash; William Oxenham, Committee Chair; Behnam Pourdeyhimi, Committee Chair; Saad Khan, Committee Member; Gregory Parsons, Committee Member; Richard Spontak, Committee Member
The Growth and Characterization of Alkylphosphonic Acid Self-Assembled Nanofibers
(2006-12-06) Salmon, Michael Edward; Phillip Russell, Committee Chair; Dieter Griffis, Committee Member; John Mackenzie, Committee Member; Richard Spontak, Committee Member
The focus of this research was to investigate the formation and properties of novel Self-Assembled Nanofibers (SANs) created by the treatment of Al with solutions of short chain-length alkylphosphonic acids (APAs) in ethanol. A special emphasis was placed on the creation of APA SANs isolated from the immersed Al source and development of analysis techniques for artifact reduced characterization of as-grown individual SANs. Novel immersion growth techniques were devised for the reproducible creation of supported and unsupported isolated methylphosphonic acid (C1), propylphosphonic acid (C3), and pentylphosphonic acid (C5) SANs on Si3N4 and Al coated ProtoChipsTM DuraSiNTM Si3N4 meshes respectively. Additionally, a novel biased immersion growth technique was developed, increasing growth rates as well as allowing for APA SAN deposition onto a variety of substrates including Au microelectrodes. A combination of complimentary analysis techniques including: Atomic force microscopy (AFM), Scanning Transmission Electron Microscopy (STEM), Energy Dispersive Spectrometry (EDS), X-Ray Photoelectron Spectroscopy (XPS), and Electron Energy Loss Spectroscopy (EELS) were utilized to characterize the morphology, composition and chemistry of isolated individual APA SANs. STEM and AFM revealed individual APA SANs are actually composed of layered fibril bundles. Qualitative compositional analysis showed APA SANs were primarily composed of O, C, P, and Al with P:Al ratios determined to be between 1.5 and 4.2. Quantitative XPS and EELS analysis provided further evidence that the detected Al was non-metallic and likely oxidized. STEM with EELS was utilized to definitively correlate the presence of Al, P, O, and C to a 5 nm region of several overlapping unsupported C1 SANs. Thermal analysis of APA SANs on Al as well as isolated on Si3N4 revealed a nearly 5X increase in thermal stability as compared to the ˜ 100C-120C melting points of pure APAs. AFM nanoindentation and nanoscratching were utilized to investigate the mechanical response of individual APA SANs. Evidence of cracking and layering were observed in good agreement with the STEM fibril observations. The reduced elastic modulus, E*, or stiffness, was estimated utilizing a Hertzian mechanics analysis of AFM nanoindentation data and determined to range from ˜ 10GPa to 1 GPa varying inversely with chain-length. Electric Force Microscopy (EFM) of C1 SANs revealed no evidence of conductivity as compared to a control sample consisting of Focused Ion Beam (FIB) deposited Pt nanowires on Si3N4. Additionally, Current-Voltage (IV) measurements were made on individual APA SANs deposited on Au microelectrodes again with no evidence of conductivity.
Hyaluronic Acid-based Nanofibers via Electrospinning
(2006-12-11) Young, Denice Shanette; Wendy Krause, Committee Co-Chair; Maurice Balik, Committee Co-Chair; Richard Spontak, Committee Member
Electrospinning is a novel technology that uses an electric field to form fibrous materials from a polymer solution. Unlike traditional spinning techniques, electrospinning can produce fibers, on the order of 100 nm, that can be utilized in applications where nanoscale fibers are necessary for specific applications, including tissue engineering and filtration. Outside of a smaller fiber diameter, electrospun nanofibers are also advantageous for biomedical applications because they have a larger surface area and pore size which promotes cell growth. A number of polymers have been electrospun successfully, including polyethylene (PEO) and polyvinyl chloride (PVC), which are two the most investigated electrospun materials. For the purpose of this study, hyaluronic acid (HA), a widely used biopolymer found in the extracellular matrix, was the chosen polymer to investigate the successful production of HA nanofibers for use in tissue engineering. Few studies have been conducted on electrospinning HA. Indeed, when this project was initiated, no investigations on electrospinning HA had been published. The goal of this research was to produce continuous fibrous strands of HA to be used as a mesh or scaffolding material. The high viscosity and surface tension of HA make it challenging to electrospin, as both are important parameters in successful production of nanofibers. To promote HA fiber formation by electrospinning, the effects of salt (NaCl), which is used to reduce the viscosity of aqueous HA solutions; molecular weight of the HA; and an additional biocompatible polymer (e.g., PEO) were investigated.
Investigating Adsorption of Synthetic Nanoparticles and Biological Species using Surface-grafted Molecular and Macromolecular Gradient Assemblies
(2005-08-16) Bhat, Rajendra R; Jan Genzer, Committee Chair; John van Zanten, Committee Member; Daniel Feldheim, Committee Member; Richard Spontak, Committee Member
We utilize novel surface-grafted molecular and macromolecular gradient assemblies to investigate: 1) dispersion of nanoparticles in organic matrices tethered to a substrate, and 2) adsorption of proteins and adhesion of cells to synthetic polymeric surfaces. Application of gradient surfaces facilitates unambiguous identification and analysis of the key parameters governing these complex, multivariate phenomena. First, we demonstrate the formation of and control over the two-dimensional assemblies of nanoparticles bound to a flat substrate via self-assembled monolayer adhesive coating. A molecular gradient template formed by vapor transport of organosilane is used to tune the number of surface-bound particles. Number density of particles is shown to be directly proportional to the surface concentration of organosilane species comprising the monolayer coating. The gradient geometry is further utilized to elucidate the inverse relationship between surface concentration and degree of ionization of organosilane species required to achieve a given particle number density. We also form a new class of nanocomposite materials by dispersing nanoparticles in surface-anchored polymer assemblies. In order to systematically probe the influence of various polymer properties on the structure of resulting nanocomposite, we employ novel architectures of surface-grafted polymers that offer either 1) unidirectional variation of polymer molecular weight (so-called linear gradient) or 2) bidirectional, simultaneous variation of molecular weight and grafting density (called as orthogonal gradient). The number of particles in the polymer brush/particle hybrid is found to increase with increasing polymer molecular weight due to an increase in the number of sites, to which particles can bind. For a given grafting density of polymer brush, larger particles predominantly reside near the brush-air interface. In contrast, smaller nanoparticles penetrate deeper into the polymer brush, thus forming a three-dimensional structure. Upon increasing grafting density of the chains, the number of attached particles exhibits different trends depending on the particle size. For larger particles, a continuous increase in particle loading is detected as a function of increasing grafting density. In contrast, polymer brushes containing smaller particles exhibit a maximum in particle concentration at some intermediate value of grafting density. We rationalize the latter behavior in terms of competition between enthalpic gain upon particle attachment to the polymer chains and entropic penalty induced by the insertion of particles in the dense brush. We also demonstrate that polymer brushes that respond to changes in environmental conditions (temperature in particular) can be harnessed to further tune nanoparticles loading in polymer brush-particle composites. Our experimental results concur very well with theories describing organization of nanoparticles in polymer brushes. Finally, we apply gradients in molecular weight and grafting density of protein-repelling polymer to tailor protein adsorption and cell adhesion to surfaces. Protein adsorption is shown to decrease with increasing surface coverage of polymer, which can be achieved by increasing molecular weight and/or grafting density of tethered polymer. Polymer gradient substrates are utilized to tailor the amount of adsorbed fibronectin (FN), which in turn regulates adhesion of osteoblast precursor cells. Cells are well attached and spread in a polygonal fashion on parts of the gradient with high FN coverage (least polymer coverage) whereas the cells are poorly anchored and elongated in areas of the gradient that are fully decorated by grafted polymer.
Molecular Models for Templated Mesoporous materials: Mimetic Simulation and Gas Adsorption
(2006-04-06) Bhattacharya, Supriyo; Keith E. Gubbins, Committee Chair; Richard Spontak, Committee Member; Orlin D Velev, Committee Member; Carol K Hall, Committee Member
The complex structures of the Templated Mesoporous Materials (TMMs) are difficult to capture using experiments. On the other hand, detailed structural information is required in order to study the confinement effects and predict material properties. We therefore present a methodology to prepare realistic molecular models of the TMMs using molecular simulations. Mimetic simulations are used to simulate the synthesis of the TMMs resulting in mesoscale models of the materials. Using this technique, we have developed models for SBA-15 and the Mesostructured Cellular Foams (MCF). The mimetic simulations also allow us to study the phase diagrams of the surfactants involved in the synthesis. We have investigated the ternary phase diagrams (surfactant-oil-water and surfactant-silica-water) of model triblock surfactants and have highlighted the effects of oil on the ordered structures. The simulation results for the effect of oil are in partial agreement with the experiments. Next, we devise a technique to convert the mesoscale TMM models into atomistic ones. The method has been demonstrated by preparing atomistic models for SBA-15. The physical properties of the models (pore size distribution, surface area, TEM and AFM images) are compared to the experimental ones. The porosities and the surface areas of the models are in quantitative agreement with those of the experimental SBA-15, whereas the pore size distribution and TEM results agree qualitatively with the experiments. We also present new methods for characterizing model structures including a fast technique for computing pore size distributions. The results from our new technique show speed increases of several orders of magnitude compared to the existing method. Finally we simulate the adsorption of Argon inside the model SBA-15 using Grand canonical Monte Carlo simulations. The adsorption isotherm from the model is in semi-quantitative agreement with that of an experimental SBA-15. The adsorption behavior of several different pore models are investigated, which provides new light on the roles of surface roughness and micropores in determining adsorption properties. We conclude by saying that the pore models developed in this work may be used in studying phase transitions, adsorption, diffusion and reactions inside nanopores, and in preparing new mesoporous material models such as the CMK carbons.
Study of Particle Formation using Supercritical CO2 as an Antisolvent
(2007-04-03) Chang, Alan An-Lei; Michael Dykstra, Committee Member; George Roberts, Committee Member; Ruben G. Carbonell, Committee Chair; Richard Spontak, Committee Member; Robert Kelly, Committee Member
Particle design using supercritical CO2 has been of great interest in the pharmaceutical, microelectronic, catalytic, and related industries over the past 10 years. There have been numerous papers and patents published on the processes studied in this work. The solubility of most drug compounds in carbon dioxide is very low, making it a very attractive antisolvent for particle formation at suitable ranges of temperatures and pressures. This thesis explores the use of different CO2 antisolvent precipitation system designs for the formulation of small crystalline drug particles of a given size, morphology, and uniformity, using the precipitation of acetaminophen from ethanol as an example. In order to understand the precipitation process, the equilibrium concentration of acetaminophen in CO2 and CO2 plus ethanol were measured at a range of temperatures and pressures in a high-pressure extraction system. This information is important in understanding the supersaturation of the drug at various precipitation conditions. Several antisolvent processes were tested in order to determine their effectiveness in controlling the precipitation of acetaminophen from ethanol. The first system involved the use of Solvent Enhanced Dispersion by Supercritical Fluids (SEDS) patented by Hanna and York (WO9501221, 1994). This process uses a coaxial nozzle design where the solvent with the solute of interest is injected in the inner tube and the supercritical CO2 is injected in the outer tube. The two streams mix at nearly constant pressure and temperature in a small volume region of the nozzle before expanding through the nozzle tip into a chamber maintained at a fixed temperature and pressure. The fast mixing process rapidly expands the solvent with CO2 in order to induce phase split of the solid drug particles. The chamber pressure is maintained constant and nearly equal to the pressure in the nozzle. This process was studied because it was claimed that SEDS gave the best control of system parameters. However, the thermodynamic, hydrodynamic and kinetic mechanisms resulting in particle formation are still not well understood. The effects of the nozzle dimensions and vessel dimensions on system performance had not been studies previously. In addition, little work has been published on the effects of variables such as liquid solvent and CO2 flow rates, solute concentration, temperature, and pressure on particle size and morphology. A Design of Experiments (DOE) analysis was used to identify the more important process parameters that control particle size and morphology. DOE is a useful statistical tool to reduce the number of experiments necessary to find the most important variables at an early stage of experimentation. With DOE, a 512 full factorial run was reduced to 32 runs by confounding primary variables with higher order interactions (Example: concentration + temperature). The results of these experiments indicated that the most important factors in determining particle size and morphology are the concentration of acetaminophen in the solvent, the nozzle geometry (length of the mixing zone), pressure and temperature. These parameters were singled out for more detailed experiments aimed at determining the influence of these variables on particle size and morphology. A key feature of the experiments described in this thesis is the use of on-line monitoring of the acetaminophen concentration at the exit to the capture vessel in order to determine how the supersaturation of the solute varied with time during the process. In this way it was possible to determine the nozzle effectiveness in particle precipitation. In addition, the experiment performed in this thesis recognized that the SEDS process is in essence a batch process and it studied the effect of transients in co-solvent concentrations in the particle capture vessel on particle size and morphology. In addition to SEDS, the precipitation of acetaminophen from ethanol was carried out using a Precipitation with Compressed Antisolvent (PCA) process, which is very similar to SEDS without the coaxial configuration. This system is simple to install and has been widely studied. The parameters that were important from the SEDS experiments were studied in the PCA to characterize their effects on particle size and morphology for this system. These results were compared to those obtained using the SEDS process. Both SEDS and PCA yielded equal particle size and morphology if designed properly. The major feature of this work was the emphasis on the design of effective nozzles for the PCA application. Similar to the SEDS results, a good mixing volume along with adequate residence time for micromixing are the best nozzle designs.
Supercritical CO2 Aided Processing of Thin Polymer Films Studied Using the Quartz Crystal Microbalance
(2006-11-22) Hussain, Yazan Ahed; Christine Grant, Committee Chair; Ruben Carbonell, Committee Member; Saad Khan, Committee Member; Richard Spontak, Committee Member
Fundamental and applied aspects of the interactions between carbon dioxide (CO₂) and different polymer systems were investigated to demonstrate the effect and performance of CO₂ during polymer processing. From a fundamental perspective, the sorption of CO₂ into a non-soluble polymer and its dependence on the different system variables were examined. Another fundamental study investigated the dissolution of a fluorinated polymer in CO₂ at different conditions. Finally, the application of supercritical CO₂ for the impregnation of additives into two different polymers was evaluated. In all these studies, the quartz crystal microbalance (QCM) was used as the primary analytical technique. In the first part of this work, the sorption of CO₂ into poly(methyl methacrylate), PMMA, was investigated. The effect of several parameters, including pressure, temperature, film thickness, and polymer state, on the equilibrium and kinetics of the sorption process was studied. The uptake isotherms of CO₂ into PMMA were estimated from the QCM frequency change. This uptake was found to decrease with temperature and to depend on the film thickness. The presence of hysteresis in the sorption-desorpotion isotherms clearly marked the glass transition which was found to be in good agreement with previously reported values. This glass transition also affected the sorption kinetic. In the glassy state, two-stage sorption curves were observed, whereas in the rubbery stage, Fickian diffusion was evident. The results from this study were utilized to examine the reliability of Sauerbrey equation for mass calculation. By measuring the change in QCM resistance, it was found that both the thickness and the amount of CO₂ dissolved in the polymer can affect the QCM response. However, it was demonstrated that Sauerbrey equation was still applicable for films up to ˜1 μm thick. In the next part, the dissolution of poly(dihydroperfluorooctyl methacrylate-r-tetrahydropyranyl methacrylate); PFOMA, a copolymer was studied. The dissolution process consisted of two stages: CO₂ sorption and polymer dissolution. The measured frequency was utilized to determine mass changes for both processes. In the sorption stage, the solubility of CO₂ into PFOMA was measured at different temperatures and pressures. The solubility was found to depend on both the CO₂ density and the temperature. Polymer dissolution started at pressures between 1100 and 1600 psi, depending on the temperature. The dissolution rate was found to increase as the CO₂ density increases, but has a possible dependence on the temperature. Finally, the fraction of undissolved polymer after 1 hour of CO₂ exposure was estimated. This fraction increased linearly from 20 to more than 90% with CO₂ density. The last part in this work examined the impregnation of ibuprofen (IBU) into two biocompatible polymers: PMMA and poly(vinyl pyrrolidone), PVP. For PMMA, the amount of impregnated IBU decreased as the CO₂ density increased. The solubility parameter approach provided a possible explanation for this behavior based on the interactions among PMMA, IBU, and CO₂. High partitioning coefficients of IBU between PMMA and CO₂ were estimated, indicating a thermodynamically driven impregnation mechanism. A linear increase in the IBU uptake with the initial polymer mass was observed. This behavior could indicate uniform distribution of IBU in the polymer sample. The impregnation rate was found to have a strong dependence on the temperature. Pressure, on the other hand, did not seem to have significant effect. For the impregnation of IBU into PVP, the frequency response was significantly larger than the PMMA case. This unusual behavior can indicate that the PVP films physical properties (e.g., viscoelastic nature of the film or in the film-substrate adhesion) are affected by IBU which might add a non-gravimetric contribution to the frequency change.
Surface-Grafted Polymer and Copolymer Assemblies with Gradient in Molecular Weight and Composition
(2005-12-29) Tomlinson, Michael Ralph; Sergei Sheiko, Committee Member; Richard Spontak, Committee Member; Jan Genzer, Committee Chair; Chris Gorman, Committee Member; Ken Caster, Committee Member
The chief goal of this Ph.D. dissertation was to develop methodologies facilitating the formation of assemblies comprising grafted polymers on surfaces with gradually varying length (or, alternatively, molecular weight). Our additional goals accomplished include expansion of these methodologies to incorporate multiple monomer systems and block copolymer assemblies. Lastly we demonstrate the utility of these gradient assemblies to study some complex phenomena of scientific interest. Surface-grafted polymer gradients represent important tools in the combinatorial study of tethered polymer layers. This approach can lead to rapid screening of properties and development of new or more efficient technologies involving tethered polymer films. The areas/technologies of interest are cited throughout the work and span organic electronic materials, responsive surfaces, nonfouling coatings, drug delivery applications, and manipulation of matter on 'small scales' leading to developments in nanotechnology. In Chapter 3, I describe procedures, methods, and several evolutions of a gradient chamber designed to create homopolymer gradient assemblies. Chapter 3 also includes studies using polymer gradients to understand polymer surface growth kinetics and an introduction to the concept of orthogonal gradient samples. I include, as supplemental information, my studies involving Atom Transfer Radical Polymerization (ATRP) simulations using a step time based Fortran program. Chapter 4 introduces the concept of block-copolymer gradients and describes my progress and major accomplishments in achieving my goal of formation of tethered copolymer gradient assemblies. I also discuss several studies involving these tethered copolymers. I use a combinatorial approach to create 'step' multiblocks on one sample surface in order to study the efficiency and characteristics of growth of these multiblock layers. I describe how I was able to produce surface-grafted diblock copolymer gradients and how I was able to chemically modify these layers. A major portion of Chapter 4 is devoted to describing a study in which a tethered diblock copolymer gradient was subjected to two selective-solvent exposure procedures designed to collapse the top and bottom blocks, respectively. I was able to study, combinatorially, the formation of and characteristics of micellar and bicontinuous structures formed via these solvent exposure techniques. Clear AFM images, ellipsometric thickness, and wettability measurements made on this sample reveal a possible relationship between surface morphology and the rearrangement of the diblock surface. I also introduce a triangular triblock triple gradient, which I successfully created. Such gradients will yield a wealth of information when applied to the study of tethered triblock copolymers.