VTechWorks Repository :: Browsing by Author "Riffle, Judy S."

Browsing by Author "Riffle, Judy S."

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Adhesion of novel high performance polymers to carbon fibers: fiber surface treatment, characterization, and microbond single fiber pull-out test
Heisey, Cheryl L. (Virginia Tech, 1993-11-05)
The adhesion of carbon fibers to several high performance polymers, including a phosphorus-containing bismaleimide, a cyanate ester resin, and a pyridine-containing thermoplastic, was evaluated using the microbond single fiber pull-out test. The objective was to determine the chemical and mechanical properties of the fiber and the polymer which affect the fiber/polymer adhesion in a given composite system. Fiber/matrix adhesion is of interest since the degree of adhesion and the nature of the fiber/matrix interphase has a major influence on the mechanical properties of a composite. The surface chemical composition, topography, tensile strength, and surface energy of untreated AU-4 and commercially surface treated AS-4 carbon fibers were evaluated using x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), single fiber tensile tests, and dynamic contact angle analysis. The commercial surface treatment which converted the AU-4 to the AS-4 fiber oxidized the carbon fiber surface. The surface of the AS-4 carbon fiber was further modified using air, oxygen, ammonia, and ethylene plasmas. The AS-4 fiber tow was also characterized following exposure to the aqueous poly(amic acid) solution used to disperse the matrix powder during aqueous suspension prepregging of thermoplastic matrix composites. The air and oxygen plasma treatments significantly oxidized and roughened the surface of the AS-4 carbon fibers. In addition, the air and oxygen plasma increased the the polar component of the AS-4 fiber surface energy. The ammonia plasma increased the concentration of nitrogen on the fiber surface, without significantly altering the fiber topography (at a nlagnification of 50,000X). The atomic oxygen present in the air and oxygen plasma treatments is capable of reacting with both the edge and basal planes in the carbon fiber structure. As a result, the oxygen-containing plasmas progressively ablated the organic material in the carbon fiber surface. Energetic species in the ammonia plasma cleaned the fiber surface and reacted with the carbon fiber surface, increasing the concentration of amine groups in the fiber surface. The ethylene plasma deposited a layer of plasma polymerized polymer on the carbon fiber surface. The AS-4 carbon fibers were coated with poly(amic acid) when the tow was wet with the aqueous suspension prepregging solution. The carbon fiber adhesion of bis(3-maleimido phenoxy) triphenylphosphine oxide was compared to that of Ciba-Geigy's Matrimid 5292 A/B bismaleimide system. With both bismaleimides, the carbon fiber adhesion increased significantly when the fiber received an oxidative commercial surface treatment or was exposed to an air or ammonia plasma prior to bonding. In contrast, the poly(pyridine-bis A) microbond pull-out test results showed that the carbon fiber adhesion of poly(pyridine-bis A) was not affected by the fiber surface chemical composition, fiber surface energy, or topography.
Advanced Polymeric Membranes and Multi-Layered Films for Gas Separation and Capacitors
Shaver, Andrew Thomas (Virginia Tech, 2016-06-30)
The following studies describe the synthesis and properties of a family of poly(arylene ether ketone)s which are well known to have good thermal stability, mechanical durability, and other film properties. These poly(arylene ether ketone)s were functionalized with fluorine, oxidized, blended, and crosslinked to increase performance with focus on materials for polymeric capacitors and gas separation membranes. There is a need for polymeric capacitors with improved energy storage density and thermal stability. In this work, the affect of polymer molecular structure and symmetry on Tg, breakdown strength, and relative permittivity was investigated. A systematic series of four amorphous poly(arylene ether ketone)s was compared. Two of the polymers had symmetric bisphenols while the remaining two had asymmetric bisphenols. Two contained trifluoromethyl groups while the other two had methyl groups. The symmetric polymers had Tg's of approximately 160 °C while the asymmetric polymers showed higher Tg's near 180 °C. The symmetric polymers had breakdown strengths near 380 kV/mm at 150 °C. The asymmetric counterparts had breakdown strengths near 520 kV/mm even at 175 °C, with the fluorinated polymers performing slightly better in both cases. The non-fluorinated polymers had higher relative permittivities than the fluorinated materials, with the asymmetric polymers being better in both cases. Two amorphous, high glass transition, crosslinkable poly(arylene ether)s for gas purification membranes have been studied. The polymers were polymerized via step growth and contained tetramethyl bisphenol F and either 4,4'-difluorobenzophenone or 4,4'-dichlorodiphenylsulfone. The benzylic methylene group in tetramethyl bisphenol F can undergo oxidation reactions and crosslinking with UV light. The polymers were oxidized under two different conditions, one by chemical treatment using oxone and KBr and one by elevated thermal treatment in air. Thermogravimetric analysis, 1H-NMR and attenuated total reflectance Fourier transform infrared spectroscopy revealed the progress of the thermal oxidation reactions. Both polymers produced tough, ductile films and gas transport properties of the non-crosslinked linear polymers and crosslinked polymer was compared. Crosslinking was performed by irradiating polymer films for one hour on each side in air under a 100W high intensity, long-wave UV lamp equipped with a 365-nm light filter. The O2 permeability of tetramethyl bisphenol F containing non-crosslinked poly(arylene ether ketone) was 2.8 Barrer, with an O2/N2 selectivity of 5.4. Following UV crosslinking, the O2 permeability decreased to 1.8 Barrer, and the O2/N2 selectivity increased to 6.2. Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) is a commercial polymer that is utilized for gas separation membranes. It has a relatively high free volume with high gas permeabilities but suffers from low gas selectivities. In this study, PPO polymers with number average molecular weights of 2000, 6000, 17,000, 19,000 and 22,000 were synthesized and blended with a poly(arylene ether ketone) synthesized from bisphenol A and difluorobenzophenone (BPA-PAEK) to make UV-crosslinkable films. The ketone and benzylic methylene groups on the BPA-PAEK and the PPO polymers respectively formed crosslinks upon exposure to broad wavelength UV light. The crosslinked blends had increased selectivities over their linear counterparts. DSC thermograms showed that the blends with all but the lowest molecular weight PPO had two Tg's, thus suggesting that two phases were present, one high in PBA-PAEK and the other high in PPO composition. The PBA-PAEK blend with the 2000 Mn PPO showed only one Tg between the two control polymers. Despite the immiscibility of these films, the gel fractions after UV exposure were high. Gel fractions as a function of the amount of the 22,000 Mn PPO were explored and did not show any significant change. UV spectroscopy of the individual components and the blends showed that more broad wavelength light was transmitted through the PPO component, so it was reasoned that films that was high in PPO composition crosslinked to deeper depths. The O2/N2 permeabilities and selectivities were measured for the linear and crosslinked films. Between the 33/67, 67/33, and 90/10 22k PPO/BPA PAEK crosslinked blended films, the 90/10 PPO/BPA PAEK gained the most selectivity and maintained a larger amount of its permeability. In comparison to commercial gas separation polymers, the non-crosslinked 33/67 22,000 Mn PPO/BPA PAEK blend outperformed polysulfone and cellulose acetate with a 2.45 degree of acetylation. Overall, we were able to blend a small amount of BPA PAEK with the commercially used PPO to create a mechanically robust crosslinked polymer film.
Ammonium Bisphosphonate Polymeric Magnetic Nanocomplexes for Platinum Anticancer Drug Delivery and Imaging with Potential Hyperthermia and Temperature-Dependent Drug Release
Zhang, Rui; Fellows, Benjamin; Pothayee, Nikorn; Hu, Nan; Pothayee, Nipon; Jo, Ami; Bohórquez, Ana C.; Rinaldi, Carlos; Mefford, Olin Thompson; Davis, Richey M.; Riffle, Judy S. (Hindawi, 2018-08-05)
Novel magnetite-ammonium bisphosphonate graft ionic copolymer nanocomplexes (MGICs) have been developed for potential drug delivery, magnetic resonance imaging, and hyperthermia applications. The complexes displayed relatively uniform sizes with narrow size distributions upon self-assembly in aqueous media, and their sizes were stable under simulated physiological conditions for at least 7 days. The anticancer drugs, cisplatin and carboplatin, were loaded into the complexes, and sustained release of both drugs was observed. The transverse NMR relaxivities (s) of the complexes were 244 s−1 (mM Fe)−1 which is fast compared to either the commercial T2-weighted MRI agent Feridex IV® or our previously reported magnetite-block ionomer complexes. Phantom MRI images of the complexes demonstrated excellent negative contrast effects of such complexes. Thus, the bisphosphonate-bearing MGICs could be promising candidates for dual drug delivery and magnetic resonance imaging. Moreover, the bisphosphonate MGICs generate heat under an alternating magnetic field of 30 kA·m−1 at 206 kHz. The temperature of the MGIC dispersion in deionized water increased from 37 to 41°C after exposure to the magnetic field for 10 minutes, corresponding to a specific absorption rate of 77.0 W·g−1. This suggests their potential as hyperthermia treatment agents as well as the possibility of temperature-dependent drug release, making MGICs more versatile in potential drug delivery applications.
Anisotropic Morphologies and Properties in Perfluorosulfonate Ionomer-Based Materials
Park, Jong Keun (Virginia Tech, 2009-12-09)
The overall goal of this investigation was to elucidate specific structure-property relationships in perfluorosulfonate ionomers (PFSIs)-related materials. The project can be broken into two primary foci. First, we explored the current state of understanding related to morphology-property relationships in PFSIs with specific attention to the nano-scale organization of the ionic and crystalline domains. Specifically, the effect of uniaxial orientation on the structure and transport properties of Nafion® membranes was examined. Small angle X-ray scattering (SAXS) experiments on dry membranes that were uniaxially elongated showed a strong anisotropic morphology which was shown to persist over the swelling process without a significant relaxation. Herman's order parameters for the ionomer peak were strongly influenced by uniaxial deformation, which supports the presence of cylindrical rather than spherical morphology for ionic domains. Comparison of the water diffusion coefficients between unoriented and oriented samples revealed that uniaxial deformation of Nafion® membranes essentially enhances transport ability in one direction (i.e., the parallel to draw direction) and suppresses in the other two directions (i.e., two orthogonal directions relative to the stretching direction). Based on 1-dimensional analyses of oriented SAXS patterns at the azimuthal angle 90o, three recent models (lamellar model, semicrystalline rod-like model and fringed-micelle model) for the morphology of PFSIs were critically evaluated. The loss of meridional scattering, different orientation behavior of the crystalline and ionic domains, and inherent chain stiffness precludes the possibility of a chain-folded lamellar morphology. While the inter-aggregate dimensions remain constant at high draw ratios, the inter-crystalline spacings decrease significantly. Coupled with the distinctly different orientation behavior, these observations preclude the existence of crystallites solely within rod-like aggregates. While the worm-like ionic channel model was able to explain the behavior of SAXS and wide angle X-ray scattering (WAXS) relatively well, this model also had limitations such as (1) crystalline domains directly linked to the ionic domain (and thus a lack of amorphous domains) and (2) a presence of only a single ionic channel between two neighboring crystallites. Second, electroactive materials, specifically ionic polymer-metal composites (IPMCs) that undergo bending motions with the stimulus of a relatively weak electric field were fabricated. To understand the role of the nanoscale morphology of the membrane matrix in affecting the actuation behavior of IPMC systems, we evaluated actuation performance of IPMCs subjected to uniaxial orientation. The PFSI nanostructure altered by uniaxial orientation mimicked the fibrillar structure of biological muscle tissue and yielded a new anisotropic actuation response. It was evident that IPMCs cut from films oriented perpendicular to the draw direction yielded displacement values that were significantly greater than that of unoriented IPMCs. In contrast, IPMCs cut from films oriented parallel to the draw direction appeared to resist bending and yield displacement values that were much less than that of the unoriented IPMC. This anisotropic actuation behavior was attributed to the contribution of the nanoscale morphology to the bulk bending modulus. Overall, this study clearly demonstrated, for the first time, the importance of the nanoscale morphology in affecting/controlling the actuation behavior in IPMC systems.
Antibacterial efficacy of core-shell nanostructures encapsulating gentamicin against an in vivo intracellular Salmonella model
Ranjan, Ashish; Pothayee, Nikorn; Seleem, Mohamed N.; Tyler, Ronald D.; Brenseke, Bonnie; Sriranganathan, Nammalwar; Riffle, Judy S.; Kasimanickam, Ramanathan K. (Dove Medical Press, 2009-01-01)
Pluronic based core-shell nanostructures encapsulating gentamicin were designed in this study. Block copolymers of (PAA(+/-)Na-b-(PEO-b-PPO-b-PEO)-b-PAA(+/-)Na) were blended with PAA(-) Na(+) and complexed with the polycationic antibiotic gentamicin to form nanostructures. Synthesized nanostructures had a hydrodynamic diameter of 210 nm, zeta potentials of -0.7 (+/-0.2), and incorporated approximately 20% by weight of gentamicin. Nanostructures upon co-incubation with J774A.1 macrophage cells showed no adverse toxicity in vitro. Nanostructures administered in vivo either at multiple dosage of 5 microg g(-1) or single dosage of 15 microg g(-1) in AJ-646 mice infected with Salmonella resulted in significant reduction of viable bacteria in the liver and spleen. Histopathological evaluation for concentration-dependent toxicity at a dosage of 15 microg g(-1) revealed mineralized deposits in 50% kidney tissues of free gentamicin-treated mice which in contrast was absent in nanostructure-treated mice. Thus, encapsulation of gentamicin in nanostructures may reduce toxicity and improve in vivo bacterial clearance.
Approaches towards therapeutic development against chronic brucellosis in a mouse model
Jain, Neeta (Virginia Tech, 2012-01-19)
Brucellosis is the most common zoonotic disease worldwide. The intracellular localization of Brucella hinders the action of drugs that poorly cross cell membrane barriers. Additionally, when the immune response fails to clear the infection, chronic brucellosis ensues that becomes more challenging to treat with antibiotics. Therefore, two approaches, intracellular drug delivery and immunostimulation, have been explored in this dissertation, with an aim to develop a better therapeutic against Brucella infection in mice. First, to overcome the cell membrane barriers, drug loaded nanoparticles were tested to treat B. melitensis infection in mice. Gentamicin loaded block-ionomer complexes (BICs) and magnetite block-ionomer complexes (MBICs) were tested in vitro and along with clusters of MBICs (MBIClusters) were tested in vivo as tools to deliver gentamicin intracellularly. While these complexes showed very high efficacy compared to free gentamicin against Brucella in macrophage cell culture, they failed to show similar efficacies in mice. Histopathological examination of kidneys from mice treated with MBICs or MBIClusters showed deposition of brown pigment-laden macrophages in peri-renal adipose tissue and the pigment was confirmed as MBICs or MBIClusters based on special staining for iron. Additionally, it was found that doxycycline-gentamicin (DG) treatment results in better clearance of Brucella from infected mice compared to doxycycline alone. Secondly, two vaccine candidates, irradiated B. neotomae (IBN) and outer membrane vesicles (OMVs), were tested as immunostimulants to treat chronic B. melitensis infection in mice in combination with antibiotics. The non-ionic block co-polymer Pluronic P85, when mixed with OMVs as an adjuvant showed significantly higher protection against B. melitensis challenge in vaccinated mice compared to those vaccinated with OMVs alone. When tested as immunostimulants, there was no additive effect of vaccines and antibiotics on Brucella clearance from mice. However, IBN enhanced the production of IFN-γ while OMVs were associated with enhanced antibody production. This enhancement in the immune system resulted in the control of Brucella growth after the end of treatment. When given without antibiotics, vaccine alone failed to clear any Brucella from infected mice. The use of these vaccine candidates in combination with antibiotics shows a potential to prevent relapses in cases of brucellosis.
Bioresorbable Electrospun Tissue Scaffolds of Poly(ethylene glycol-b-lactide) Copolymers for Bone Tissue Engineering
Badami, Anand Shreyans (Virginia Tech, 2004-10-01)
Poly(Î±-hydroxy esters) are a class of biocompatible resorbable polyesters including poly(lactic acid) (PLA) and poly(glycolic acid) (PGA) that are FDA-approved for clinical use. Preliminary tissue culture studies have demonstrated that these poly(Î±-hydroxy esters) support bone tissue development both in vitro and in vivo, but biocompatibility issues still exist. Tissue scaffolds fabricated from these materials by current methods have biocompatibility limitations because they are chemically and topographically inert to cells. The chemical composition of these scaffolds does not influence cell behavior (i.e. proliferation, differentiation) and their surface topography is on a scale length larger than a cell, which is too large to affect cell adhesion or orientation. It is hypothesized that poly(Î±-hydroxy ester) tissue scaffolds can be made more bioactive by (1) incorporating poly(ethylene glycol) (PEG) into the polymer interface to promote osteoblastic differentiation and (2) controlling topography to direct cell behavior. The novel processing technique of electrospinning allows the fabrication of nanofiber scaffolds with topographical features the size of focal adhesion contacts capable of influencing cell behavior. Thus, the overall objective of this research project is to characterize electrospun PEG-PLA diblock copolymers as substrates for bone tissue engineering. To accomplish this, PEG-PLLA and PEG-PDLLA diblock copolymers were synthesized with target molecular weights of 42,000 g/mol (PEG:2000, PLLA or PDLLA:40,000). Next, these two polymers and commercially available PLLA and PDLLA were electrospun to form scaffolds with fibers of diameters 0.14 to 2.1 Î¼m. Finally, cell culture studies were performed to characterize cell morphology, proliferation, and osteoblastic differentiation. Results indicate electrospun fiber scaffolds limit cell spreading and persist in cell culture for two weeks. Analysis of cells cultured over 14 days revealed that there were no differences in cell density between polymers with and without PEG. Cell density increased with fiber diameter, indicating that fiber diameter affects cell adhesion and proliferation and suggesting that cells may migrate into scaffolds with large diameter fibers. In contrast to cell density, ALP activity, an indicator of osteoblastic differentiation, was unaffected by fiber diameter.
Block and Graft Copolymers Containing Carboxylate or Phosphonate Anions
Hu, Nan (Virginia Tech, 2014-11-06)
This dissertation focuses on synthesis and characterization of graft and block copolymers containing carboxylate or phosphonate anions that are potential candidates for biomedical applications such as drug delivery and dental adhesives. Ammonium bisdiethylphosphonate (meth)acrylate and acrylamide phosphonate monomers were synthesized based on aza-Michael addition reactions. Free radical copolymerizations of these monomers with an acrylate-functional poly(ethylene oxide) (PEO) macromonomer produced graft copolymers. Quantitative deprotection of the alkylphosphonate groups afforded graft copolymers with zwitterionic ammonium bisphosphonate or anionic phosphonate backbones and PEO grafts. The zwitterionic copolymers spontaneously assembled into aggregates in aqueous media. The anionic copolymers formed aggregates in DMF and DMSO, while only small amounts of aggregates were present in copolymer/methanol or copolymer/water solutions. Binding capabilities of the acrylamide phosphonic acids were investigated through interactions with hydroxyapatite. Previously our group has prepared poly(ethylene oxide)-b-poly(acrylic acid) (PEO-b-PAA) copolymers and used these polymers as carriers for both MRI imaging agents and cationic drugs. To enhance the capabilities of those carriers in tracking and crosslinking, we have designed, synthesized and characterized amine functionalized PEO-b-PAA copolymers. First, heterobifunctional poly(ethylene oxide) (PEO) with three different molecular weights were synthesized. Modification on one of these afforded a PEO macroinitiator with a bromide on one end and a protected amine on the other end. ATRP polymerization of tert-butyl acrylate (tBuA) in the presence of this initiator and a copper (I) bromide (CuBr) catalyst yielded a diblock copolymer. The copolymer was deprotected by reaction with trifluoroacetic acid (TFA) and formed an amine terminated H2N-PEO-b-PAA. Recently our group has utilized the novel ammonium bisdiethylphosphonate (meth)acrylate and acrylamide phosphonate copolymers to incorporate Carboplatin. The resulting complexes exhibited excellent anticancer activity against MCF-7 breast cancer cells which might be related to ligand exchange of the dicarboxylate group of Carboplatin with the phosphonic acid moieties in the copolymer. Hence, complexation of small-molecule phosphonic acids with Carboplatin was investigated. Three compounds, vinylphosphonic acid, 3-hydroxypropyl ammonium bisphosphonic acid and 2-hydroxyethyl ammonium phosphonic acid were complexed with Carboplatin under acidic and neutral conditions. Covalent bonding of these acids to carboplatin was only observed under acidic pH. The covalently bonded percentage was 17%, 37% and 34%, respectively. More in-depth investigation was of great importance to further understand this complexation behavior.
Characterization of PF Resol/Isocyanate Hybrid Adhesives
Riedlinger, Darren Andrew (Virginia Tech, 2008-02-01)
Water-based resol phenol formaldehyde, PF, and organic polymeric methylenebis(phenylisocyanate), pMDI, are the two primary choices for the manufacture of exterior grade wood-based composites. This work addresses simple physical blends of pMDI dispersed in PF as a possible hybrid wood adhesive. Part one of this study examined the morphology of hybrid blends prepared using commercially available PF and pMDI. It was found that the blend components rapidly reacted such that the dispersed pMDI droplets became encased in a polymeric membrane. The phase separation created during liquid/liquid blending appeared to have been preserved in the cured, solid-state. However, substantial interdiffusion and copolymerization between blend components also appeared to have occurred according to measured cure rates, dynamic mechanical analysis, and atomic force microscopy. In the second part of this study a series of PF resins was synthesized employing the so-called "split-cook" method, and by using a range of formaldehyde/phenol and NaOH/phenol mole ratios. These neat PF resins were subjected to the following analyses: 1) steady-state flow viscometry, 2) free formaldehyde titration, 3) non-volatile solids determination, 4) size exclusion chromatography, 5) quantitative solution-state ¹³C nuclear magnetic resonance, NMR, 6) differential scanning calorimetry, 7) parallel-plate oscillatory cure rheology, and 8) dielectric spectroscopy. The neat PF analytical results were unremarkable with one exception; NMR revealed that the formaldehyde/phenol mole ratio in one resin substantially differed from the target mole ratio. The neat PF resins were subsequently used to prepare of series of PF/pMDI blends in a ratio of 75 parts PF solids to 25 parts pMDI solids. The resulting PF/pMDI blends were subjected to the following analyses: 1) differential scanning calorimetry, 2) parallel-plate oscillatory cure rheology, and 3) dielectric spectroscopy. Similar to what was inferred in part one of this study, both differential scanning calorimetry (DSC) and oscillation cure rheology demonstrated that cure of the PF continuous phase was substantially altered and accelerated by pMDI. However within actual wood bondlines, dielectric analysis detected little variation in cure speed between any of the formulations, both hybrid and neat PF. Furthermore, the modulated DSC curing experiments detected some latent reactivity in the hybrid system, both during initial isothermal curing and subsequent thermal scanning. The latent reactivity may suggest that a significant diffusion barrier existed between blend components, preventing complete reaction of hybrid blends even after thermal scanning up to 200 °C. Part three of this work examined the bonded wood mode-I fracture performance of hybrid resins as a function of the resol formaldehyde/phenol ratio and also the alkali content. A moderate increase in unweathered fracture toughness was observed for hybrid formulations relative to neat PF. Following accelerated weathering, the durability of the hybrid blends was promising: weathered hybrid toughness was equivalent to that of weathered neat PF. While the resol F/P ratio and alkali content both influenced hybrid fracture toughness, statistical modeling revealed interaction between these variables that complicated result interpretation: the influence of hybrid alkali content depended heavily on each formulation's specific F/P ratio, and vice versa.
Characterization of Structure-Property Relationships in Hydrophilic-Hydrophobic Multiblock Copolymers for Use in Proton Exchange Membrane Fuel Cells
Lane, Ozma Redd (Virginia Tech, 2011-11-18)
Proton exchange membrane fuels cells (PEMFCs) are one of the primary alternatives to internal combustion engines. The key component is the proton exchange membrane, or PEM, which should meet a number of requirements, including good proton conductivity under partially humidified conditions. A number of alternative PEMs have been synthesized by copolymerizing various aromatic comonomers, but the smaller ion channels prohibit rapid proton transport under partially hydrated conditions. One solution has been to synthesize multiblock copolymers from hydrophilic and hydrophobic oligomers to ensure sufficient ion channel size. Four multiblock systems were synthesized from hydrophobic and hydrophilic oligomers and were characterized in this thesis. The first multiblock system incorporated a partially fluorinated monomer into the hydrophobic block, to improve phase separation and performance under partially humidified conditions. The second study was focused on phase separation and structure-property relationships as a function of casting conditions of a biphenol-based multiblock series. The third study featured a novel hydroquinone-based hydrophilic oligomer in the multiblock copolymer, which showed the promise of a higher ionic density, degree of phase separation and proton conductivity values. The fourth study in this thesis entailed the comparison of a block copolymer produced with two distinct synthetic routes: the multiblock synthesis from separate oligomers as previously published in the literature, and a segmented route seeking to achieve comparable structure-property relationships with the same monomers, but using a simpler synthetic route. The two block copolymer series were found to be comparable in their structure-property relationships.
Characterization of Structure-Property Relationships of Poly(urethane-urea)s for Fiber Applications
O'Sickey, Matthew J. (Virginia Tech, 2002-01-18)
Poly(urethane)s and poly(urethane-urea)s (PUU) are nearly ubiquitous, having been in existence since before the Second World War. Spandex, a poly(urethane-urea) elastomeric fiber, is found in nearly all articles of apparel as well as in an increasing array of other consumer items. The technology and chemistry of spandex is largely unchanged since its inception in the late 1950s, with the majority of spandex employing poly(tetramethylene ether glycol) as soft segments. Recent developments in catalyst technology have resulted in the production of ultra-low monol content poly(propylene glycol) (PPG), which is nearly difunctional (f=1.95+). This enhancement in difunctionality has potentially enabled the use of PPG as a spandex soft segment with potential spandex processing, performance, and economic benefits. PPG-based spandex elastomers were evaluated in both film and fiber form for the purpose of investigating morphological, orientational, mechanical, and thermal properties with the goal of understanding relationships between chemistry, morphology and properties. Key variables of interest were soft segment molecular weight (MW), molecular weight distribution (MWD), and composition, and hard segment content and composition. Of those, the influence of the molecular weight distribution of the polyol used for soft segments was of foremost interest and had previously been largely neglected during the course of poly(urethane) and poly(urethane-urea) research. It was found that over the range of PUU compositions suitable for production of spandex, that hard segment content and composition had little effect upon the morphology and thermal and mechanical properties. Appreciable trends as functions of soft segment molecular weight were observed. The soft segment MWD was adjusted through the addition of a low molecular weight homolog of PPG, tri(propylene glycol) (TPG), decreasing the number average soft segment MW. The results of these experiments were contrary to those for variation of soft segment molecular weight. It was determined that the low MW portion of the polyol MWD contributes to the building of hard segments in addition to or in lieu of soft segments. Incorporation of TPG in the PUUs resulted in larger, presumably less cohesive, hard domains and increased hard segment content. The TPG containing materials had enhanced tensile properties, less permanent set, and less residual orientation after deformation. These materials proved quite comparable to those using PTMEG soft segments. Comparison of film and fiber PUUs revealed only minor differences, implying that the trends and conclusions drawn from the study of films with spandex-like compositions would also hold for fibers. The key difference between films and fibers is that fibers maintain some residual ordering and orientation due to drawing of the fibers during processing. Of the processing variables investigated, none impacted the morphology as determined from small angle x-ray scattering. It was concluded, that of the various compositional variables germane to spandex, the polyol MW and MWD played key roles in development of morphology, and hence properties. The role of polyol MWD had been woefully neglected during the development of spandex previously, and was observed to be a critical variable.
Chemical and Physical Modifications of Semicrystalline Gels to Achieve Controlled Heterogeneity
Anderson, Lindsey J. (Virginia Tech, 2019-02-07)
Sulfonated polyaromatic hydrocarbon membranes have emerged as desirable candidates for proton exchange membranes (PEMs) due to their excellent mechanical properties, high thermal and chemical stability, and low cost. Specifically, sulfonated multiblock copolymers are attractive because their phase-separated morphologies aide in facile proton transport. In this work, the functionalization of semicrystalline gels of poly(ether ether ketone) (PEEK) is explored as a novel post-polymerization method to prepared blocky copolymers, and the effect of copolymer architecture on membrane physical properties, structure, and performance is extensively investigated. First, the blocky sulfonation of PEEK was explored to prepare blocky copolymers (SPEEK) with densely sulfonated domains and unfunctionalized, crystallizable domains. Compared to random SPEEK ionomers at similar ion content, blocky SPEEK exhibited enhanced crystallizability, decreased melting point depression, and faster crystallization kinetics. Phase separation between the hydrophilic sulfonated blocks and hydrophobic PEEK blocks, aided by polymer crystallization, resulted in enhanced water uptake, superior proton conductivity, and more closely associated ionic domains than random SPEEK. Furthermore, the random and blocky bromination of PEEK was investigated to prepare PEEK derivatives (BrPEEK) with reactive aryl-bromides. Spectroscopic evidence revealed long domains of unfunctionalized homopolymer for blocky BrPEEK, and this translated to an increased degree of crystallinity, higher melting temperature, and more rapid crystallization kinetics than random BrPEEK at similar degrees of bromination. The subsequent sulfonation of blocky BrPEEK resulted in a hydrophilic-hydrophobic blocky copolymer with clear multi-phase behavior. The phase-separated morphology contributed to decreased water uptake and areal swelling compared to random SPEEK and resulted in considerably higher proton conductivity at much lower hydration levels. Moreover, Ullmann coupling introduced superacidic perfluorosulfonic acid side chains to the BrPEEK backbone, which yielded membranes with less water content and less dimensional swelling than random SPEEK. Superior proton transport than random SPEEK was observed due to the superacid side chain and wider hydrophilic channels within the membranes, resulting in more continuous pathways for proton transport. Overall, this work provided a novel platform for the preparation of functionalized PEEK membranes using a simple post-polymerization functionalization procedure. The established methods produced blocky-type copolymers with properties reminiscent of multiblock copolymers prepared by direct polymerization from monomers/oligomers.
Chemical Modification of Alginates in Organic Media
Pawar, Siddhesh Nitin (Virginia Tech, 2013-07-25)
Alginates are (1and4) linked linear copolysaccharides composed of B-D-mannuronic acid (M) and its C-5 epimer, a-L-guluronic acid (G). Several strategies to synthesize organically modified alginate derivatives have been reported, but almost all chemistries are performed in either aqueous or aqueous-organic media. The ability to react alginates homogeneously in organic solvents would open up access to a wide range of new chemistries and derivatives. However, past attempts have been restricted by the absence of methods for alginate dissolution in organic media. We therefore report a strategy to solubilize tetrabutylammonium (TBA) salts of alginic acid in polar aprotic solvents containing tetrabutylammonium fluoride (TBAF). Acylation of TBA-alginate was performed in DMSO/TBAF to get products with DSacetyl up to ~ 1.0. We further report that by using appropriate solvent conditions, placement of acyl groups can be controlled to achieve either random or M-selective substitution. Alginate acetates synthesized in an M-selective fashion were used to study the ability of these derivatives to form Ca-crosslinked hydrogels. Detailed structure-property analyses were performed to identify acetylation reaction conditions and product properties that may be ideal for hydrogel formation. Furthermore, alginate esters were synthesized via modification of carboxylate groups on the backbone. These derivatives dissolved in polar aprotic solvents without the need to add TBAF. A proof of concept study showed their utility in the solubility enhancement of the poorly water soluble flavonoid naringenin.
The Chemistry of Dimethacrylate-Styrene Networks and Development of Flame Retardant, Halogen-Free Fiber Reinforced Vinyl Ester Composites
Rosario, Astrid Christa (Virginia Tech, 2004-08-02)
One of the major classes of polymer matrix resins under consideration for structural composite applications in the infrastructure and construction industries is vinyl ester resin. Vinyl ester resin is comprised of low molecular weight poly(hydroxyether) oligomers with methacrylate endgroups diluted with styrene monomer. The methacrylate endgroups cure with styrene via free radical copolymerization to yield thermoset networks. The copolymerization behavior of these networks was monitored by Fourier Transform Infrared Spectroscopy (FTIR) at various cure conditions. Reactions of the carbon-carbon double bonds of the methacrylate (943 cm-1) and styrene (910 cm-1) were followed independently. Oligomers possessing number average molecular weights of 700 g/mole were studied with systematically increasing levels of styrene. The Mortimer-Tidwell reactivity ratios indicated that at low conversion more styrene was incorporated into the network at lower cure temperatures. The experimental vinyl ester-styrene network compositions deviated significantly from those predicted by the Meyer-Lowry integrated copolymer equation at higher conversion, implying that the reactivity ratios for these networks may change with conversion. The kinetic data were used to provide additional insight into the physical and mechanical properties of these materials. In addition to establishing the copolymerization kinetics of these materials, the development of halogen free fiber reinforced vinyl ester composites exhibiting good flame properties was of interest. Flame retardant vinyl ester resins are used by many industries for applications requiring good thermal resistance. The current flame-retardant technology is dependent on brominated vinyl esters, which generate high levels of smoke and carbon monoxide. A series of halogen free binder systems has been developed and dispersed in the vinyl ester to improve flame retardance. The binder approach enables the vinyl ester resin to maintain its low temperature viscosity so that fabrication of composites via Vacuum Assisted Resin Transfer Molding (VARTM) is possible. The first binder system investigated was a polycaprolactone layered silicate nanocomposite, which was prepared via intercalative polymerization. Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD) data indicated a mixed morphology of exfoliated and intercalated structures. The mechanical properties and the normalized peak heat release rates were comparable to the neat vinyl ester resin. Alternative binder systems possessing inherent flame retardance were also investigated. A series of binders comprised of novolac, bisphenol A diphosphate, and montmorillonite clay were developed and dispersed into the vinyl ester matrix. Cone calorimetry showed reductions in the peak heat release rate comparable to the brominated resin.
Cobalt Nanoparticle-Macromolecular Complexes and Their Conversion to Oxidatively Stable Entities
Baranauskas, Victor Vincent (Virginia Tech, 2005-04-22)
The goal of the research presented in this dissertation was to synthesize novel macromolecular materials that would afford oxidative stability to magnetic cobalt nanoparticles under ambient conditions. The cobalt nanoparticles were formed via the thermolysis of Co2(CO)8 in concentrated solutions of toluene containing the macromolecular dispersion stabilizers. The copolymers were designed to encapsulate the nanoparticles with a number of thin protective coatings to prevent their undesirable oxidation under ambient condtions. Cobalt nanoparticles encased with an organic glass were synthesized by stabilizing cobalt nanoparticles with poly(methyl methacrylate-co-2-vinylpyridine-g-dimethylsiloxane) whereas nanoparticles encapsulated with triazine networks were formed via the thermal treatment of cobalt particles complexed with poly(styrene-b-4-vinylphenylcyanate). Cobalt nanoparticles coated with a combination of carbonaceous and silica char were obtained by pyrolyzing cobalt particles stabilized with poly (4-vinylphenoxyphthalonitrile-co-4-vinylphenoxytriethoxysilane-g-dimethylsiloxane) graft copolymers. Moreover, cobalt nanoparticles encapsulated with either phthalonitrile networks or graphitic char were prepared via the thermal treatment of nanoparticles stabilized with poly(styrene-b-4-vinylphenoxyphthalonitrile). Oxidatively-stable, magnetic cobalt nanoparticle complexes may be prepared by heating cobalt nanoparticles encapsulated in poly(styrene-b-4-vinylphenoxyphthalonitrile) block copolymers at elevated temperatures. The block copolymers were synthesized through the sequential anionic polymerization of styrene and tert-butyldimethylsilyloxystyrene. The silyl ether protecting groups on the second block were hydrolyzed under acidic conditions to afford poly(styrene-b-4-vinylphenol), and the pendent phenols of the diblock copolymer were chemically modified with 4-nitrophthalonitrile to afford poly(styrene-b-4-vinylphenoxyphthalonitrile). Stable suspensions of ~8-10 nm diameter cobalt metal nanoparticles were formed by thermolysis of dicobalt octacarbonyl in solutions of toluene containing poly(styrene-b-4-vinylphenoxyphthalonitrile). The cobalt-polymer nanoparticle complexes were pyrolyzed under argon to afford highly magnetic cobalt nanoparticles encased in graphitic coatings. Magnetic susceptibility measurements indicate that the cobalt-graphitic particles are oxidatively-stable and retain their high saturation magnetizations (~ 95-100 emu g-1) for at least a year under ambient conditions.
A comparison of ultraviolet, thermal, and microwave polymerized acrylamide terminated polydimethylsiloxane
Hall, Grace Louise (Virginia Tech, 1993-10-05)
A novel oligomer was synthesized for the purpose of investigating the effects of ultraviolet, thermal, and microwave polymerization. The synthesis involved an anionic ring-opening equilibration reaction to produce poly(dimethylsiloxane) which was then endcapped with an acid chloride to result in a material which was linear, flexible, had a functionality of four, and a strong dipole moment. Acrylamide terminated poly(dimethylsiloxane) was the end product and was cured with ultraviolet radiation, thermal energy, and microwaves. Characterization of the cured materials demonstrated interesting findings. Microwave cured materials resulted in higher degrees of cure than thermal or ultraviolet cured samples, despite using the best match for the heating rate-sample temperature profiles for each cure process. Several characterization techniques were employed and the procedures and results may be useful for others interested in finding the most efficient cure mode for their material.
Complexation of Block Copolysiloxanes with Cobalt Nanoparticles
Vadala, Michael Lawrence (Virginia Tech, 2003-04-12)
Poly(dimethylsiloxane-b-methylvinylsiloxane) (PDMS-b-PMVS) diblock copolymers were synthesized via anionic living polymerization with controlled molecular weights and narrow molecular weight distributions. Targeted molecular weights agreed well with experimental values determined by 1H NMR, 29Si NMR, and GPC. Morphologies were investigated by DSC to analyze glass transition temperatures. Only one Tg was observed for each PDMS-b-PMVS block copolymer suggesting that the blocks were miscible in bulk. Tg's ranged from approximately -126 to -128 °C and were between the Tg's of the PDMS (-123 °C) and PMVS (-137 °C) homopolymers. The PMVS blocks were functionalized with trimethoxysilethyl or triethoxysilethyl pendent groups via hydrosilations to yield poly(dimethylsiloxane-b-[poly(methylvinyl)-co-(methyl-(2-trimethoxysilethyl)siloxane)] (PDMS-b-[PMVS-co-PMTMS]) or poly(dimethylsiloxane-b-[poly(methylvinyl)-co-(methyl-(2-triethoxysilethyl)siloxane)] (PDMS-b-[PMVS-co-PMTES]) copolymers, respectively. The PMVS blocks were either derivatized with the functional groups or half of the repeat units were functionalized. The fully hydrosilated materials were diblock copolymers, and the materials that were 50% hydrosilated had a random sequence of methylvinylsiloxy units and methyl-(trialkoxysilethyl)siloxy units. The PDMS-b-[PMVS-co-PMTES] block copolymers had Tg's ranging from -124 to -126 °C and only one Tg was observed. Surface tension measurements suggested that PDMS-b-[PMVS-co-PMTES] copolymers formed aggregates in toluene. Stable suspensions of superparamagnetic cobalt nanoparticles were prepared in toluene in the presence of PDMS-b-[PMVS-co-PMTMS] or PDMS-b-[PMVS-co-PMTES] copolymers via thermolysis of Co2(CO)8. It is hypothesized that the block copolymers functioned as micellar templates for the cobalt nanoparticles. TEM micrographs showed non-aggregated cobalt nanoparticles coated with copolymers that had mean particle diameters ranging from ≥10-15 nm. Specific saturation magnetizations of these cobalt-copolymer complexes ranged from 90-110 emu g-1 Co, comparable to literature values for this particle size.
Compression Creep Rupture of an E-glass/Vinyl Ester Composite Subjected to Combined Mechanical and Fire Loading Conditions
Boyd, Steven Earl (Virginia Tech, 2006-11-20)
Polymer matrix composites are seeing increasing use in structural systems (e.g. ships, bridges) and require a quantitative basis for describing their performance under combined mechanical load and fire. Although much work has been performed to characterize the flammability, fire resistance and toxicity of these composite systems, an understanding of the structural response of sandwich type structures and laminate panels under combined mechanical and thermal loads (simulating fire conditions) is still largely unavailable. Therefore a research effort to develop a model to describe the structural response of these glass/vinyl esters systems under fire loading conditions is relevant to the continuing and future application of polymer matrix composites aboard naval ships. The main goal of the effort presented here is to develop analytical models and finite element analysis methods and tools to predict limit states such as local compression failures due to micro-buckling, residual strength and times to failure for composite laminates at temperatures in the vicinity of the glass transition where failure is controlled by viscoelastic effects. Given the importance of compression loading to a structure subject to fire exposure, the goals of this work are succinctly stated as the:(a)Characterization of the non-linear viscoelastic and viscoplastic response of the E-glass/vinyl ester composite above Tg. (b)Description of the laminate compression mechanics as a function of stress and temperature including viscoelasticity.(c)Viscoelastic stress analysis of a laminated panel ([0/+45/90/-45/0]S) using classical lamination theory (CLT). Three manuscripts constitute this dissertation which is representative of the three steps listed above. First, a detailed characterization of the nonlinear thermoviscoelastic response of Vetrotex 324/Derakane 510A - 40 through Tg was conducted using the Time - Temperature - Stress - Superposition Principle (TTSSP) and Zapas - Crissman model. Second, the modeling approach and viscoelastic relaxation mechanism is validated by substituting the shear relaxation modulus into a compression strength model to predict lifetimes for isothermal and one sided heating of unidirectional laminates. Finally, viscoelastic stress analysis using CLT is performed for a general laminated panel to predict lifetimes under one sided heating. Results indicate that when temperatures remain in the vicinity of Tg, the laminate behavior is controlled by thermoviscoelasticity.
Cooperative Electrostatic Polymer-Antibiotic Nanoplexes
Vadala, Timothy Patrick (Virginia Tech, 2010-05-24)
Many pathogenic bacteria can enter phagocytic cells and replicate in them, and these intracellular bacteria are difficult to treat because the recommended antibiotics do not transport into the cells efficiently. Examples include food-borne bacteria such as Salmonella and Listeria as well as more toxic bacteria such as Brucella and the Mycobacteria that lead to tuberculosis. Current treatments utilize aminoglycoside antibiotics that are polar and positively charged and such drugs do not enter the cells in sufficient concentrations to eradicate the intracellular infections. We have developed core-shell polymeric drug delivery vehicles containing gentamicin to potentially overcome this challenge. Pentablock and diblock copolymers comprised of amphiphilic nonionic polyether blocks and anionic poly(sodium acrylate) blocks have been complexed with the cationic aminoglycoside gentamicin. The electrostatic interaction between the anionic polyacrylates and the cationic aminoglycosides form the cores of the nanoplexes, while the amphiphilic nature of the polyethers stabilize their dispersion in physiological media. The amphiphilic nature of the polyethers in the outer shell aid in interaction of the nanoplexes with extra- and intra-cellular components and help to protect the electrostatic core from any physiological media. This thesis investigates the electrostatic cooperativity between the anionic polyacrylates and cationic aminoglycosides and evaluated the release rates of gentamicin as a function of pH.
Copolymerizing Acrylonitrile and Methyl Acrylate by RAFT for Melt Processing Applications: A Synthetic Investigation of the Effects of Chain Transfer Agent, Initiator, Temperature, and Solvent
Beck, Susan Ashley (Virginia Tech, 2014-06-23)
Statistical copolymers of acrylonitrile (AN) and methyl acrylate (MA) were successfully prepared and characterized using reversible addition-fragmentation chain transfer (RAFT) copolymerization. A typical copolymer was charged with 15 wt. % MA content. This thesis describes a systematic variation of the RAFT copolymerization variables to optimize this system. In particular, the effects of chain transfer agent, initiator, temperature, and solvent on the copolymer properties were studied.

Browsing by Author "Riffle, Judy S."

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