Browsing by Author "Dr. Saad Khan, Committee Member"
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- Applications of functional polymer brushes for nanoparticle uptake and prevention of protein adsorption(2010-04-21) Arifuzzaman, Shafi Mahmud; Dr. Jan Genzer, Committee Member; Dr. Saad Khan, Committee Member; Dr. Orlin D. Velev, Committee Member; Dr. Orlando Rojas, Committee MemberThe central theme of this Ph.D. dissertation is to develop novel multifunctional polymer coatings for understanding partition of proteins and nanoparticles on polymers grafted to flat surfaces (so-called brushes). Systematic investigation of the adsorption phenomena is accomplished by utilizing surface-anchored assemblies comprising grafted polymers with variation in physical properties (i.e., length or/and grafting density) and chemical functionality. The chemical composition of the brush is tailored by either “chemical coloring†of a parent homopolymer brush with selective chemical moieties or by sequential growth of two chemically dissimilar polymer blocks. We present preparation of two types of tailor-made, surface-grafted copolymers: 1) those composed of hydrophilic and hydrophobic blocks (so-called amphiphilic polymer brushes), and 2) those comprising of anionic and cationic polymer segments (so-called polyampholyte brushes). We describe the organization of functionality in the grafted polymer brushes and the partitioning of proteins and nanoparticles using a battery of complementary analytical probes. Specifically, we address how varying the molecular weight, grafting density, and chemical composition of the brush affects adsorbtion and desorbtion of model proteins and gold nanoparticles. Our observations indicate densely-populated responsive amphiphilic polymers are very efficient in suppressing protein adsorption. In addition, we have established that the length of poly(ethylene glycol) spacers attached to a parent homopolymer brush is a key factor governing uptake of gold nanoparticles. Both grafting density and molecular weight of the coating are important in controlling the kinetics and thermodynamics of protein adsorption on surfaces. Our findings and methodologies can lead to the development of next generation environmentally friendly antifouling surfaces and will find application in medical devices, antifouling coatings and anti reflection finishes.
- Chemically Crosslinked Polymer Electrolyte Membranes from Fluorinated Liquid Precursors for Application in Fuel Cells(2010-07-29) Yadav, Rameshwar; Dr. Peter S. Fedkiw, Committee Chair; Dr. Joseph M. DeSimone, Committee Co-Chair; Dr. Saad Khan, Committee Member; Dr. Xiangwu Zhang, Committee MemberChemical crosslinking of polymer electrolyte membrane (PEM) liquid precursors has the ability to support variable acid loading and create intricate structures that are highly effective in suppressing methanol crossover while maintaining reasonable conductivity in PEMs for direct methanol fuel cells (DMFCs). PEM fabrication from photocuring of liquid precursors is another advantage over traditional methods of melt-processing and solvent-casting in which high cost and sophisticated equipment are employed. Linear-chain PEMs have certain shortcomings for application in DMFCs because of methanol crossover from the anode to the cathode, limited conductivity and high cost from processing steps and conditions. In our approach, photocuring of liquid precursors produces chemically crosslinked PEMs with good mechanical strength and dimensional stability. Styrene functionalization of perfluoropolyethers diols of six different molecular weights (MWs) between 1000-4000 g mol-1 yields crosslinkers that afford crosslink density corresponding to their MWs. Copolymerization of the fluorinated liquid crosslinker and a styrene sulfonate ester co-monomer via UV-light initiated free-radical bulk polymerization produces chemically crosslinked PEMs. Conversion of the ester into corresponding sulfonic acid through base/alcohol hydrolysis and ion-exchange in acid solution confers these PEMs excellent proton conductivity. The variable crosslink density from different MWs of crosslinker provides wide window of compostions to form PEMs with ion-exchnage capacity (IEC) varying from 0.5 to 1.85 meq g-1. The higher end of acid loading is two times that of benchmark Nafion membrane at good mechanical strength and dimensional stability in these crosslinked PEMs. Such high-acid loading in linear-chain PEMs leads to dissolution in polar solvents. Combining low MW (1000 g mol-1) and high MW (4000 g mol-1) crosslinkers in these PEMs improves the mechanical strength further. The fluorinated chain in crosslinked structure from perfluoropolyethers provides thermal stability up to 260 °C that is sufficient for most practical applications of DMFCs. Due to IEC of 1.85 meq g-1, these PEMs have shown conductivities of 220 to 340 mS cm-1 at 100% relative humidity and at 25 to 60 °C, respectively, that are 3-fold higher than that of Nafion 117 conductivity. For methanol crossover reduction, the nature of crosslinked structure has been exploited to obtain PEMs with good methanol barrier and proton conduction properties. An objective of this research was to optimize the composition of PEMs derived from crosslinker with six different MWs and comonomer resulting in low methanol permeability and reasonable conductivity. This combination of low permeability and reasonable conductivity has resulted in the selectivity of 1.36 ï‚´ 105 S cm-1/cm2 sec-1 that is more than three times that of Nafion 117 selectivity. Depending on the crosslinker MW and composition, these crosslinked PEMs have conductivities in the range of 10-4 to 10-1 S cm-1 and methanol permeabilities in the range of 10-9 to 10-6 cm2 sec-1. Under identical acid content, incorporation of low MW crosslinkers reduces the methanol permeability further by increasing the crosslink density. Nafion 117 has conductivity of 1.04  0.02 ï‚´ï€ 10-1 S cm-1 and methanol permeability of 2.19 ï‚±ï€ ï€°ï€®ï€¶ï€³ ï‚´ 10-6 cm2 sec-1 in liquid water and at room temperature. In addition to crosslinked PEMs in acidic form, photocuring of mixture of liquid precursor and 4-vinylbenzyl trimethyl chloride dissolved in a solvent (3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-1-octanol) yields a novel crosslinked alkaline anion exchnage membrane (AEMs). Without any optimization in composition, these crosslinked AEMs in hydroxyl form have shown good conductivity of 45 mS cm-1 in liquid water and at room temperature for IEC of 1.43 meq g-1. Analogous to crosslinked acidic PEMs, excellent opportunity exists to exploit the crosslinking approach and optimize the composition and processing condition to achieve maximum anionic ion-exchnage capacity, high conductivity and low methanol permeability in these crosslinked AEMs for application in alkaline fuel cells.
- Mechanisms in Bicomponent Fiber Spinning During Melt Blown Process(2009-02-23) Zapletalova, Terezie; Dr. Saad Khan, Committee Member; Dr. Stephen Michielsen, Committee Member; Dr. Eunkyoung Shim, Committee Member; Dr. Jan Genzer, Committee Co-Chair; Dr. Behnam Pourdeyhimi, Committee ChairZAPLETALOVA, TEREZIE. Mechanisms in Bicomponent Fiber Spinning During Melt Blown Process (Under the direction of Dr. Behnam Pourdeyhimi and Dr. Jan Genzer) A series of melt blown bicomponent webs made with various grades of polypropylene and polyethylene were evaluated in terms of influence of process and polymer properties on fiber morphology and web mechanical properties. This study utilized full quadratic model with continuous factors of temperature and component ratio. We were able to obtain the effects these parameters on fiber and substrate properties with high statistical significance. Overall fiber crystallinity content as well as crystallinity content of each separate component can be well explained linked with a high degree of confidence to the processing temperature, component ratio, and several polymer component properties, including, enthalpy of melting, zero shear viscosity, activation energy of flow, and highest crystallization rate temperature. Fabric mechanical properties can also be correlated with the same structural parameters. Bicomponent fiber diameter could not be satisfactorily explained in terms of the same parameters, which indicates that there is another mechanism not described by the studied process and polymer data that largely influences substrate fiber diameter.
- Modification of Polymer Membranes: A Study of Crosslinking and In-Situ Growth of Palladium-Containing Nanoparticles in Polymer Matrices.(2008-08-21) Aberg, Christopher Mark; Dr. Richard J. Spontak, Committee Chair; Dr. C. Maury Balik, Committee Member; Dr. Orlin Velev, Committee Member; Dr. Saad Khan, Committee MemberA crucial step in obtaining pure hydrogen is separating it from other compounds—mainly CO2—that often accompany hydrogen in industrial chemical reactions. Advanced membrane technology may prove to be the key to the successful, economical production of molecular hydrogen for the eventual consumer market. Size-sieving glassy polymer membranes can separate H2 on the basis of its small size. Alternatively, reverse-selective rubbery polymers can expedite the passage and, hence, removal of CO2 due to its relatively high solubility in such membranes alone or in conjunction with dissociative chemical reactions. Transition-metal membranes and their alloys can adsorb H2 molecules, dissociate the molecules into H atoms for transport through interstitial sites, and subsequently recombine the H atoms to form molecular H2 again on the opposite membrane side. Microporous amorphous silica and zeolite membranes comprising thin films on a multilayer porous support exhibit good sorption selectivity and high diffusion mobilities for H2, leading to high H2 fluxes. Finally, carbon-based membranes, including carbon nanotubes, may be viable for H2 separation on the basis of selective surface flow and molecular sieving. One approach to achieve higher gas selectivity is to cross-link polymer membranes, thus restricting the ability of gases of various sizes to readily permeate at an unimpeded rate. Cross-linking can occur through a number of means: UV and ion irradiation, plasma treatment, or chemical and thermal techniques. In this study, a chemical technique has been chosen to cross-link the polyimide Matrimid®. Polyimides are well-established as gas-separation membranes due to their intrinsically low free-volume and correspondingly high H2 selectivity relative to other gases such as CO2. Prior studies have established that H2⁄CO2 selectivity can be improved by cross-linking polyimides with diamines differing in spacer length. In this first set of work, we follow the evolution of macroscopic and microscopic properties of a commercial polyimide over long cross-linking times (tx) with 1,3-diaminopropane. According to spectroscopic analysis, the cross-linking reaction saturates after ˜24 h, whereas tensile, nanoindentation and stress-relaxation tests reveal that the material stiffens, and possesses a long relaxation time that increases, with increasing tx. Although differential scanning calorimetry shows that the glass transition temperature decreases systematically with increasing tx, permeation studies indicate that the permeabilities of H2 and CO2 decrease, while the H2⁄CO2 selectivity increases markedly, with increasing tx. At long tx, the polyimide becomes impermeable to CO2, suggesting that it could be used as a barrier material. Alternatively, polymer nanocomposites continue to receive considerable attention as multifunctional hybrid materials, with most nanocomposites fabricated by physical dispersion of surface-functionalized nanoscale objects. In the second study, we explore the viability of growing Pd-containing nanoparticles from Na2PdCl4 in two different polymers —hyper-cross-linked polystyrene (HPS) and an aromatic polyimide (PIm). In HPS, single Pd-containing nanoparticles possessing a relatively narrow size distribution (ca. 1-4 nm) are observed to form upon reduction of the divalent PdCl4-2 ions and cluster more readily if the reducing agent is introduced as a liquid. Single nanoparticles with a broad size distribution ranging from ˜2 to 16 nm develop in PIm, which simultaneously undergoes chemical cross-linking during ion reduction. The conditions yielding Pd incorporation in PIm are explored through the use of instrumental neutron activation analysis. Such Pd-containing hybrid materials hold promise in molecular catalysis and gas separations. Results from these studies give prospect that these materials, with a great deal of future research, could be developed for H2 separations applications.
- A Rotating Disk Study of the Mechanisms of Calcite Dissolution in the Presence of Environmentally Benign Polyaspartic Acid(2002-09-04) Burns, Kathie Lee; Dr. John van Zanten, Committee Member; Dr. Saad Khan, Committee Member; Dr. Christine S. Grant, Committee ChairA rotating disk technique was used to investigate the mechanisms of calcite (CaCO3) dissolution using environmentally benign polyaspartic acid (PASP) under controlled hydrodynamic conditions. Additional techniques including scanning electron microscopy and dynamic light scattering explored the specific role of PASP in the dissolution process. Using this approach, rates of dissolution were evaluated as a function of pH, rotating speed, polymer concentration and molecular weight. In this research, it was determined that PASP is an effective dissolving agent for calcite mineral over a range of pHs (3.5-10.0), rotating speeds (150-1500 rpm), PASP concentrations (0.001-0.1M) and PASP molecular weights (3,000 and 10,000 Mw). An enhancement factor, ηenh, was developed to quantify the effect of PASP on dissolution behavior. It is defined as the rate of dissolution in PASP over the rate in water. Maximum enhancement was observed at pHs in the range of 4-5 for high concentrations and low molecular weights of PASP. Results demonstrate that dissolution is governed primarily by interfacial phenomena, including adsorption and surface reaction, at high pHs (>7), while limited chiefly by mass transport at low pHs (<7). Dissolution at high pHs proceeds via a surface complexation mechanism involving the chelation of calcium by PASP. At the high pHs, dissolution is inhibited by small amounts of PASP (0.001-0.01M) and enhanced by large quantities (0.1M) of PASP. In contrast, at low pHs, dissolution occurs predominantly by acid attack, or the reaction of hydrogen ion with calcite. At low pHs, PASP enhances dissolution over the entire concentration range (0.001-0.1M). For the two molecular weights studied, the lower molecular weight (3,000) is the most efficient dissolving agent at low pH, while both molecular weights dissolve calcite at comparable rates at high pH. Finally, a model was developed based on fundamental calcite and sequestration chemistry to predict the dissolution kinetics of calcite in the presence of PASP at pHs above 7. The model agrees closely with experimental dissolution rates at pH 10 and shows that the water reaction with calcite dominates dissolution at low PASP concentrations while the PASP ligand reaction with calcite is the primary interfacial reaction at high PASP concentrations.
- Solid-State 13C-NMR Investigation of Di- and Tri-block Biodegradable Copolymers Isolated In Their Inclusion Compounds with Cyclodextrins(2003-04-16) Porbeni, Francis Ebikefe; Dr. Saad Khan, Committee Member; Dr. Alan E. Tonelli, Committee Chair; Dr. Jeffrey L. White, Committee Member; Dr. Mohan Srinivasarao, Committee Member; Dr. Hawthorne Davis, Committee MemberWe have synthesized and characterized two biodegradable copolymers: poly(e-caprolactone)-poly(L-lactide), (PCL-b-PLLA) diblock copolymer, and poly(e-caprolactone)-poly(propylene glycol), (PCL-PPG-PCL) triblock copolymer. The lengths of each block in the diblock were determined to be: PCL = 92 and PLLA = 84. While in the tri-block, each PCL block length was found to be 45, and the PPG was 60. Inclusion compounds (ICs) of these copolymers with alpha- and gamma- cyclodextrins (CDs) were formed and characterized. The solid crystals of the ICs were found to be in the channel packing mode, allowing the copolymers to be isolated as straight chains within the cylindrical channels of the CDs. We have investigated the conformations and dynamics of the isolated di- and triblock copolymer chains entrapped within the channels of alpha- and gamma-cyclodextrins (CD) using solid-state 13C-NMR techniques. The PCL block chains isolated within the cavities of alpha-CD, adopt a conformation similar to that of the bulk semi-crystalline di- and tri-block copolymers. While in gamma-CD/IC, an upfield shift by approximately 1 ppm was observed for the PCL block chains in the triblock. The spin-lattice relaxation time T1 (C) measurements confirmed the semi-crystalline morphology of both of these copolymers. A dramatic difference was observed in the mobility of the polymer chains in the bulk compared to the ICs. In the absence of intermolecular interactions, the isolated chains experienced much rapid mobility relative to their behavior in the bulk. This result reflects on the role of cooperative interactions between the polymer chains in both the bulk di- and tri-block copolymers. The length scale of proton spin diffusion was probed using T1(1H) and T1rho (1H). The T1(1H) in the bulk copolymers averaged to a single value, while the proton T1 indicated that the copolymers were phase-separated. In the ICs, exchange of proton magnetization through spin diffusion was observed between the polymer chains and the CDs, but it was not totally efficient. 2D heteronuclear correlation method was also employed to monitor the nature of proton communication in these samples. Intra-block exchange of proton magnetization was observed in the bulk copolymers at short mixing times. In the ICs, intra-block 1H-1H spin communication was observed for the isolated chains, in spite of the physical closeness between the isolated chains and CD molecules efficient proton spin diffusion was not observed.
- Study of Morphological, Mechanical and Electrical properties of Electrospun Poly(lactic acid) Nanofibers incorporated with Multiwalled Carbon Nanotubes as a Function of Thermal Bonding.(2009-08-11) Ramaswamy, Sangeetha; Dr. Saad Khan, Committee Member; Dr. Russell Gorga, Committee Chair; Dr. Laura Clarke, Committee Co-ChairRAMASWAMY SANGEETHA. Study of Morphological, Mechanical and Electrical properties of Electrospun Poly (lactic acid) Nanofibers incorporated with Multiwalled Carbon Nanotubes as a Function of Thermal Bonding. (Under the direction of Dr. Russell E. Gorga and Dr Laura A. Clarke). This work aims at enhancing the properties of electrospun poly (lactic) acid (PLA) nanofibers incorporated with various loading of multiwalled carbon nanotubes (MWNT) using the process of thermal bonding. Thermal bonding of the electrospun fibers improved the fiber-fiber bonds. Tensile strength of the nano-composites increased as a function of the bonding temperature. The modulus increased at lower bonding temperatures as the mat became more coherent and reduced at higher bonding temperatures as the stiffness of the mat decreased. A cold crystallization phenomenon was seen in the PLA nanofibers when bonded near the Tm, which lead to a sharp increase in tensile strength as well as modulus. The effect of thermal bonding on the electrical properties of the nanocomposites was also studied. It was found that the electrical conductance was the highest when the fibers were bonded close to their melting point. The percolative behavior of the nanocomposite mats bonded near the melting point was compared to the as-spun ones and it was found that improvement in the fiber-fiber bonds improved the connectivity of the mat and lowered the percolation threshold and raised the overall conductivity of the mat.
- Surface Modification of Polypropylene Nonwovens to Improve Adhesion to Elastomers(2009-02-25) Paul, Shreya; Dr. Saad Khan, Committee Member; Dr. Jan Genzer, Committee Member; Dr. Behnam Pourdeyhimi, Committee Chair; Dr. Eunkyoung Shim, Committee Co-ChairThis study addresses how one can use blending, grafting and UV radiation techniques to make polar or hydrophilic polypropylene (PP) to overcome the shortcomings of the inert (hydrophobic) nature of the PP surface. For this purpose a functional monomer, glycidyl methacrylate (GMA), was chosen as the modifier. Moreover, the similarity and differences in results between the different techniques are reported. For both blending and grafting techniques a range of weight percentage of GMA was added to the base polymer PP and it was observed that even low amounts of GMA (0.5 wt %) was sufficient to modify the surface property of the PP and produce enhanced adhesion to elastomeric polymers such as thermoplastic polyurethane (TPU) and Pebax. Increased adhesion up to 300% has been reported in this work. The physical modification of PP using UV irradiation also proved effective and has improved the hydrophilicity of PP. The laminated samples of modified PP and elastomeric polymers were subjected to barrier and moisture transport tests. Under specific conditions of lamination, these composites have proved to be an effective barrier to water but at the same time can provide comfort property by maintaining the moisture vapor transfer through them.
