Browsing by Author "Richard Kotek, Committee Member"

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Nanostructuring Polymers with Cyclodextrins
(2005-09-26) Uyar, Tamer; Richard Kotek, Committee Member; C. Maurice Balik, Committee Member; Alan E. Tonelli, Committee Chair; Juan P. Hinestroza, Committee Member
The formation of polymer-cyclodextrin inclusion compounds of polycarbonate (PC), poly(methylmethacrylate) (PMMA) and poly(vinylacetate) (PVAc) guests with host γ-cyclodextrin (γ-CD) have been successfully achieved. Coalesced bulk polymer samples were obtained by removal of γ-CD from their inclusion compounds (ICs). Spectroscopic findings indicated that the chain conformations of the bulk polymers were altered when they were included inside the CD channels and extended chain conformations were retained when coalesced from their ICs. Significant improvements were observed in the thermal transitions for the coalesced polymers, with glass transitions shifted to higher temperatures. Thermal studies reveal that the thermal stabilities of coalesced polymers increased slightly compared to the corresponding as-received polymers and degradation products of the polymers are affected once the polymers chains are included inside the γ-CD-IC cavities. A procedure for the formation of intimate blends of binary and ternary polymer systems; PC/PMMA, PC/PVAc, PMMA/PVAc and PC/PMMA/PVAc was studied. PC/PMMA, PC/PVAc, PMMA/PVAc and PC/PMMA/PVAc were included in γ-CD channels and were then simultaneously coalesced from their common γ-CD-ICs to obtain intimately mixed blends. It was observed that the ratios of polymers in coalesced blends were significantly different than the starting ratios, and PC was found to be preferentially included in γ-CD channels when compared to PMMA or PVAc. Physical mixtures of polymer blends were also prepared by co-precipitation and solution casting methods for comparison. The analysis indicates that the ternary and the binary blends of these polymers achieved by coalescence from their common γ-CD-IC results in a homogeneous polymer blends, possibly with improved properties, whereas co-precipitation and solution cast methods produced phase separated polymer blends. It was shown that coalescence of two or more normally immiscible polymers from their common CD-ICs appears to be an applicable method for obtaining well-mixed, intimate blends. The solid complex of guest styrene included inside the channels of host γ-cyclodextrin (styrene/γ-CDchannel-IC) was formed in order to perform polymerization of styrene in a confined environment (γ-CD channels). Modeling of polystyrenes (PS) with various stereosequences in the narrow cylindrical channels corresponding to those found in γ-CD ICs has been conducted. It was calculated that only isotactic PS stereoisomers can fit into the γ-CD cavity. Thus, based on the modeling of stereoisomeric PSs in narrow γ-CD channels, it was suggested that polystyrene with unusual microstructures might be produced via constrained polymerization of styrene monomer in its γ-CD-IC crystals. The in situ polymerization of styrene inside the narrow channels of its γ-CD-IC crystals was performed in aqueous media. Alternatively, the solid-state polymerization of styrene/ϒ-CD IC has also been carried out by radiation polymerization. It was found that most styrene monomer migrates from the γ-CD channels and polymerizes outside of the channels. Yet, a rotaxaned structure has been obtained where some CD molecules entrapped along the PS chains after the polymerization.
Performance of Compliant Electrodes in Electro Active Polymer (EAP) Actuators
(2004-05-19) Akbay, Mehmet Cuneyt; Richard Kotek, Committee Member; John F. Muth, Committee Co-Chair; Dr. Tushar K. Ghosh, Committee Chair
Dielectric elastomer (DE) actuators, based on the field-induced deformation of elastomeric polymers with compliant electrodes, can produce a large strain response. To accommodate the high strain during actuation the electrodes around the DE should also deform without imposing any restraints while maintaining their conductivity. The electrodes have to be compatible with the DE in their mechanical properties. In addition to mechanical properties such as elastic moduli, hysterisis, etc., other properties such as conductivity, percolation, are also of importance. Therefore the compliant electrode is a key feature of the DE actuator technology. Many types of compliant electrodes used in conjunction with DE actuators have been reported in the literature. Among them are particle included polymers, metals, and conductive polymers. Particle included polymer based electrodes generally consist of carbon or silver as particles and a polymer medium such as silicone. Conductive polymers such as polypyrrole can be used as compliant electrode as well. In this work an effort has been made to characterize various compliant electrodes on dielectric elastomer EAPs under different process conditions. Characterization of the electrodes includes their response to applied voltage, their conductivity values under different test conditions and their topography. Three different types of compliant electrode have been characterized. These were rubber electrodes, grease electrodes and polypyrrole electrodes. The results showed that the crack formation was related with the amount of polymer carrier for grease and rubber electrodes. Both rubber and grease electrodes, which were prepared with Nusil CF19-2186, showed the worst results in terms of uniformity of the electrodes and areal strain rates to the applied voltage. For rubber electrodes, electrodes, which were prepared with Sylgard 186 and Sylgard 184, showed similar results in terms of uniformity of the electrode. Generally Sylgard 184 rubber electrodes showed higher areal strain rates to the applied voltage than that of Sylgard 186 electrodes. Higher conductivity values were achieved with Sylgard 186 rubber electrodes comparing to Sylgard 184 rubber electrodes. For grease electrodes, electrodes, which were prepared with Sylgard 186 and Sylgard 184 did not show similar results in terms of uniformity and areal strain. Higher conductivity values were observed with Sylgard 186 grease electrodes comparing to Sylgard 184 grease electrodes. Both Sylgard 184 and Sylgard 186 rubber electrodes lost its conductivity at 100%-100% nominal strain rate. It was observed that increasing the number of polymerizing process, thus number of rinsing, enable to remove black pyrrole particles more efficiently for polypyrrole electrodes. Relatively higher conductivity values and lower areal strain values were achieved with Ppy electrodes comparing to rubber and grease electrodes. Ppy electrodes lost its conductivity at 100%-100% nominal strain ratio.
Structural Study of Bombyx mori Silk Fibroin during Processing for Regeneration
(2005-02-28) Ha, Sung-Won; Maurice C. Balik, Committee Member; Clay Clark, Committee Member; Samuel M. Hudson, Committee Chair; Richard Kotek, Committee Member; Alan E. Tonelli, Committee Member
Bombyx mori silk fibroin has excellent mechanical properties combined with flexibility, tissue compatibility, and high oxygen permeability in the wet condition. This important material should be dissolved and regenerated to be utilized as useful forms such as gel, film, fiber, powder, or non-woven. However, it has long been a problem that the regenerated fibroin materials show poor mechanical properties and brittleness. These problems were technically solved by improving a fiber processing method reported here. The regenerated fibroin fibers showed much better mechanical properties compared to the original silk fibers. This improved technique for the fiber processing of Bombyx mori silk fibroin may be used as a model system for other semi-crystalline fiber forming proteins, becoming available through biotechnology. The physical and chemical properties of the regenerated fibers were characterized by SinTech® tensile testing, X-ray diffraction, solid state 13C NMR spectroscopy, and SEM. Unlike synthetic polymers, the molecular weight distribution of Bombyx mori silk fibroin is mono-disperse because silk fibroin is synthesized from DNA template. Genetic studies have revealed the entire amino acid sequence of Bombyx mori silk fibroin. It is known that the crystalline silk II structure is composed of hexa-amino acid sequences, GAGAGS. However, in the amino acid sequence of Bombyx mori silk fibroin heavy chain, there are present 11 chemically irregular but evolutionarily conserved sequences with about 31 amino acid residues (irregular GT~GT sequences). The structure and role of these irregular sequences have remained unknown. One of the most frequently appearing irregular sequences was synthesized by a peptide synthesizer. The three-dimensional structure of this irregular silk peptide was studied by the high resolution two-dimensional NMR technique. The three-dimensional structure of this peptide shows that it makes a turn or loop structure (distorted Ω shape), which means the proceeding backbone direction is changed 180° by this sequence. This may facilitate the β-sheet formation of the crystal forming building blocks, GAGAGS/GY~GY sequences, in fibroin heavy chain. It may also facilitate the solubilization of the fibroin heavy chain within the silk gland.
Synthesis of a Fiber-Reactive Chitosan Derivative and Its Application to Cotton Fabric as an Antimicrobial Finish and a Dyeing-Improving Agent
(2003-03-31) Lim, Sang-Hoon; Suzanne T. Purrington, Committee Member; David Hinks, Committee Member; Richard Kotek, Committee Member; Samuel M. Hudson, Committee Chair
The purpose of this research has been to develop a textile finish based on chitosan that is a biopolymer. A fiber-reactive chitosan derivative was synthesized from chitosan with a low molecular weight and a high degree of deacetylation. The synthesis was composed of two steps. As a first step, a water-soluble chitosan derivative was prepared by introducing quaternary ammonium salt groups on the amino groups of chitosan. The derivative was further modified by introducing functional groups (acrylamidomethyl) on the primary alcohol groups of the chitosan backbone, which can form covalent bonds with cotton. The fiber-reactive chitosan derivative (NMA-HTCC) itself showed complete bacterial reduction against Staphylococcus aureus and Escherichia coli at the concentration of 10 ppm. The NMA-HTCC was applied to cotton fabrics by a pad-batch method in the presence of an alkaline catalyst. The 1% NMA-HTCC treated cotton showed 100% bacterial reduction against S. aureus. The fabric maintained over 99% of bacterial reduction even after 50 home launderings. The NMA-HTCC cotton was dyed with direct and reactive dyes without addition of salt. The color yield was higher than that of untreated cotton, which required a large amount salt for dyeing. The NMA-HTCC cotton showed better washfastness than untreated cotton, but the lightfastness was inferior to that of untreated cotton. The antimicrobial activity of the NMA-HTCC cotton was considerablely decreased after dyeing due to the blocking of the cationic groups of the NMA-HTCC by dye molecules.