Synthesis and Characterization of Tailored Photoactive Macromolecules

Type of Document Dissertation
Author Trenor, Scott Russell
Author's Email Address strenor@vt.edu
URN etd-04262004-150913
Title Synthesis and Characterization of Tailored Photoactive Macromolecules
Degree PhD
Department Macromolecular Science and Engineering
Advisory Committee
Advisor Name Title
Brian J. Love Committee Co-Chair
Timothy E. Long Committee Co-Chair
David A. Dillard Committee Member
Judy S. Riffle Committee Member
Thomas C. Ward Committee Member
Keywords

* Coumarin
* Polymer Photochemistry
* Adhesives
* Alkyl Acrylates
* Cinnamates

Date of Defense 2004-04-16
Availability unrestricted
Abstract

Coumarin and cinnamate derivatives were positioned as either polymer chain ends or side groups to synthesize photoactive macromolecules and gain the ability to reversibly control molecular weight and crosslink density using UV light. The cinnamates and coumarins were reacted onto the polymers via multiple reaction pathways. Polymers were functionalized with coumarin or cinnamate groups via an esterification reaction between hydroxyl functionalities and an acid chloride derivatized coumarin group. In addition to the esterification reaction, cinnamates were also coupled to polymers via a ring opening reaction between a hydroxyl functionalized cinnamate derivative and a maleic anhydride repeat unit copolymerized into the polymer. Both functional groups undergo a [2p + 2p] photodimerization reaction (coumarin groups in the UVA and cinnamate groups in the UVB), which was utilized to crosslink and chain-extend macromolecules. Coumarin dimers possess the additional ability to photocleave and thus reverse when irradiated at 254 nm.

The coumarin reversible photodimerization reaction was utilized to reversibly increase the molecular weight and molecular weight distribution of coumarin-functionalized PEG monols and diols. For example, the number average molecular weight of the coumarin-functionalized PEG diol doubled and the

molecular weight distribution increased from 1.08 to 2.75 when exposed to 110 J cm-2 of UVA irradiation. Subsequent photocleavage (UVC irradiation, 2 J cm-2) of the chain-extended PEGs, cleaved coumarin dimers decreasing the molecular weight and molecular weight distribution to their original values.

A number of poly(alkyl acrylate) and poly(methyl acrylate) systems were functionalized with coumarin groups to study the effect of the glass transition temperature and alkyl ester side group composition on the photodimerization reaction and subsequent crosslinking. The glass transition temperature (Tg) acted as an on/off switch for the photodimerization reaction. While the absolute difference between Tg and irradiance temperature did not affect the rate or extent of photodimerization reaction, polymers with a Tg greater than the irradiance temperature displayed less reaction than those with a Tg lower than the irradiance temperature. The final extent of conversion was controlled by a complex combination of factors including alkyl ester side chain steric bulkiness. Coumarin-functionalized alkyl acrylates based on ethylhexyl acrylate were tested as detachable PSAs. A 98% decrease in the adhesive peel strength was observed after exposure to UVA irradiation.

Cinnamate groups were utilized in the design and synthesis of UV-curable hot melt pressure sensitive adhesives (PSAs). The cinnamate groups were attached to the PSAs to provide a method to increase molecular weight and add a small amount of crosslinking leading to an increase the adhesive strength of the PSAs. Broadband UV irradiation from a laboratory scale industrial lamp increased the peel strength of the adhesives. Postcure of the irradiated cinnamate-functionalized UV-curable hot melt PSAs was reduced compared to photoinitiated free-radical photocurable UV-curable hot melt PSAs.


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Second-Order Nonlinear Optical Characteristics of Nanoscale Self-Assembled Multilayer Organic Films

Type of Document Dissertation
Author Neyman, Patrick J
Author's Email Address pneyman@vt.edu
URN etd-07072004-141443
Title Second-Order Nonlinear Optical Characteristics of Nanoscale Self-Assembled Multilayer Organic Films
Degree PhD
Department Macromolecular Science and Engineering
Advisory Committee
Advisor Name Title
James R Heflin Committee Chair
Guy Indebetouw Committee Member
Harry W. Gibson Committee Member
Herve Marand Committee Member
Richey M. Davis Committee Member
Keywords

* polymer
* thin film
* nonlinear optics
* nanotechnology
* chromophore
* second harmonic generation
* self assembly

Date of Defense 2004-06-16
Availability unrestricted
Abstract

Ionically self-assembled monolayer (ISAM) films are typically an assemblage of oppositely charged polymers built layer by layer through Coulombic attraction utilizing an environmentally friendly process to form ordered structures that are uniform, molecularly smooth and physically robust. ISAM films have been shown to be capable of the noncentrosymmetric order requisite for a second-order nonlinear optical response with excellent temporal and thermal stability. However, such films fabricated with a nonlinear optical (NLO) polyanion result in significant cancellation of the chromophore orientations. This cancellation occurs by two mechanisms: competitive orientation due to the ionic bonding of the polymer chromophore with the subsequent polycation layer, and random orientation of the chromophores within the bulk of each polyanion layer. A reduction in film thickness accompanied by an increase in net polar ordering is one possible avenue to obtain the second-order susceptibility chi(2) necessary for practical application in electro-optic devices. In this thesis, we discuss the structural characteristics of ISAM films and explore a novel approach to obtain the desired characteristics for nonlinear optical response. This approach involves a hybrid covalent / ionic self-assembly technique which affords improved net dipole alignment and concentration of monomer chromophores in the film. This technique yields a substantial increase in chi(2) due to the preferential chromophore orientation being locked in place by a covalent bond to the preceding polycation layer. The films fabricated in this manner yield a chi(2) that substantially exceeds that of any known polymer-polymer ISAM film. This covalent-hybrid ionically self-assembled multilayer (CHISAM) technique is demonstrated to result in films suitable for electro-optic devices, with measured electro-optic coefficient (14 pm/V) comparable to that of the inorganic crystal lithium niobate (30 pm/V). Thermal and temporal stability are important properties of electro-optic device implementation, and are demonstrated for CHISAM films. CHISAM films have remained stable at room temperature for more than 420 days, and suffered no loss of chi(2) when held at 80 C for 36 hours, followed by 150 C for 24 hours. Studies are also presented that demonstrate the ability to produce ISAM chi(2) films that are nearly one micron thick, and exhibit no evidence of a thickness limitation to the polar order. Analytical considerations for second-order NLO characterization of thick films are addressed in detail. The effect of absorption of the second harmonic wavelength and resonant enhancement of chi(2) are investigated, and it is demonstrated that accurate determination of chi(2) may be made for thick films and for films that absorb the second harmonic. The temporal and thermal stability of a variety of ISAM and CHISAM NLO films are examined in detail. In some cases, a decrease in the NLO response is observed at elevated temperature that is completely restored upon cooling. Studies are presented that suggest this effect is a result of thermally induced trans-to-cis isomerization of azo linkages in the NLO chromophores.


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HIGH PERFORMANCE DISULFONATED POLY(ARYLENE SULFONE) CO- AND TERPOLYMERS FOR PROTON EXCHANGE MEMBRANES FOR FUEL CELL AND TRANSDUCER APPLICATIONS: SYNTH

Type of Document Dissertation
Author Wiles, Kenton Broyhill
Author's Email Address kwiles4@yahoo.com
URN etd-04212005-215145
Title HIGH PERFORMANCE DISULFONATED POLY(ARYLENE SULFONE) CO- AND TERPOLYMERS FOR PROTON EXCHANGE MEMBRANES FOR FUEL CELL AND TRANSDUCER APPLICATIONS: SYNTHESIS, CHARACTERIZATION AND FABRICATION OF ION CONDUCTING MEMBRANES
Degree PhD
Department Macromolecular and Science Engineering
Advisory Committee
Advisor Name Title
James E. McGrath Committee Chair
Garth L. Wilkes Committee Co-Chair
Donald G. Baird Committee Member
Judy S. Riffle Committee Member
Timothy E. Long Committee Member
Keywords

* Poly(arylene thioether sulfone)
* Transducer
* Proton Exchange Membrane
* Phosphine Oxide
* Fluorine
* Membrane Electrode Assembly
* Poly(arylene ether sulfone)
* Fuel Cell
* Nafion

Date of Defense 2005-04-15
Availability unrestricted
Abstract

The results described in this dissertation have demonstrated several alternative proton exchange membranes (PEM) for hydrogen-air and direct methanol fuel cells (DMFC) that perform as well or better than the state of the art Nafion perfluorosulfonic acid membrane. Direct aromatic nucleophilic substitution polycondensations of disodium 3,3„S-disulfonate-4,4„S-difluorodiphenylsulfone (SDFDPS), 4,4„S-difluorodiphenylsulfone (DFDPS) (or their chlorinated analogs, SDCDPS, DCDPS) and 4,4„S-thiobisbenzenethiol (TBBT) in the presence of potassium carbonate were investigated. Electrophilic aromatic substitution was employed to synthesize the SDFDPS or SDCDPS comonomers in high yields and purity. High molecular weight disulfonated poly(arylene thioether sulfone) (PATS) copolymers were easily obtained using the SDFDPS monomers, but in general, slower rates and a lower molecular weight copolymer was obtained using the analogous chlorinated monomers. Tough and ductile membranes were solution cast from N,N-dimethylacetamide for both series of copolymers. The degrees of disulfonation (20-50%, PATS 20-50) were controlled by varying the ratio of disulfonated to unsulfonated comonomers. Composite membranes were prepared by homogeneous solution blending the copolymers with phosphotungstic acid (PTA) in dimethylacetamide (DMAc). The composite PATS membranes exhibited moderate PTA molecule water extraction after acidification treatments performed at either room or boiling temperatures. The membranes containing HPA showed improved conductivity at high temperatures (120 ¢XC) and low relative humidities when compared to the pure copolymers.

Molecular weight of the copolymers plays a critical role in the overall copolymer physical behavior. It is well known that molecular weight has an enormous impact on practically all of the physical properties of polymeric systems. This dissertation discusses the influence of molecular weight on the characteristics of a specific family of PEM PATS copolymers. This study elucidated that the lower molecular weight materials did indeed behave differently than the higher molecular weight copolymers. Specifically, the water uptake and permeability to methanol decreased with increasing molecular weight. Furthermore, the fully hydrated mechanical properties also improved with molecular weight.

The synthesis and fabrication of 45 mole percent disulfonated poly(arylene ether phenyl phosphine oxide diphenyl sulfone) terpolymer-heteropolyacid (HPA) composite membranes and membrane electrode assemblies were chosen for detailed investigation. A series of 45 mole percent disulfonated biphenol-based poly(arylene ether phenyl phosphine oxide diphenyl sulfone) terpolymers (BPSH45-PPO) were also synthesized by nucleophilic aromatic substitution polymerizations. The level of disulfonation was constant at 45 mole percent providing a compromise between high conductivity at low humidity and reasonable mechanical properties in liquid water. The amounts of 4,4¡¦-difluorodiphenyl phenyl phosphine oxide comonomer incorporated into the terpolymer backbone were precisely controlled from 0-50 mole percent relative to the 4,4¡¦-dihalodiphenyl sulfone. Phosphine oxide moieties were employed to enhance the interactions with the PTA relative to the pure copolymer. The composite BPSH45-PPO membranes exhibited lower HPA molecule water extraction after acidification at room and boiling temperatures, which was ascribed to the strong hydrogen and polar interactions between the phosphine oxide moiety and functional groups on the HPA. The membranes containing HPA displayed improved conductivity at high temperatures and low relative humidities when compared to the pure terpolymer samples. The increase of proton conductivity was attributed to the water retention characteristics of the HPA molecules, which allowed enhanced mobility of the protons even at lower humidification levels, providing superior hydrogen-air fuel cell performance.

The effect of hexafluoroisopropylidene bisphenol (6FBP) incorporation into 45 mole percent disulfonated poly(arylene ether sulfone) copolymers was investigated. This novel series of directly disulfonated poly(arylene ether sulfone) copolymers with various mole ratios of the 6FBP were synthesized in high molecular weight. The levels of fluorination within the statistically random copolymer architecture were varied from 0-100 mole percent using 6FBP and the correct stoichiometric amount of 4,4¡¦-biphenol. The 6FBP monomer was introduced to decrease the water swelling and improve bonding characteristics with Nafion-bonded electrodes. Indeed, water uptake decreased with increasing incorporation of the 6FBP monomer into the terpolymer. This suggested that the hydrophobic fluorinated material aided in water repulsion of the system. Proton conductivity decreased slightly as the amount of fluorination increased, which was interpreted to be due to the decrease in the ion-exchange capacity. High temperature hydrogen/air fuel cell experiments indicated better Nafion-bonded electrode adhesion for the partially fluorinated materials, as depicted by high temperature (120 ¢XC) and low humidity (50% RH) hydrogen-air fuel cell performance.

Investigations into polymeric electromechanical transducers were based on poly(arylene sulfone) ion-exchange membranes bonded between two conductive metal layer electrodes. Imposed deformations and small electric fields allowed similar explorations of both sensing and actuation applications. These copolymers produced larger sensitivities than the benchmark Nafion systems, which was interpreted as being due to their higher hydrated moduli. Methodologies for better defining the morphology of the electrodes were identified to enhance the capacitance and effective interfacial area of the conductive electrodes. The new procedures afforded major improvements to performance and transduction. Transducer actuation at lower frequencies was improved by employing a new direct assembly electrode fabrication technique that suggested a strong correlation between the capacitance and charge motion.


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Preparation and Functionalization of Macromolecule-Metal and Metal Oxide Nanocomplexes for Biomedical Applications

Type of Document Dissertation
Author Vadala, Michael Lawrence
URN etd-04212006-123212
Title Preparation and Functionalization of Macromolecule-Metal and Metal Oxide Nanocomplexes for Biomedical Applications
Degree PhD
Department Macromolecular and Science Engineering
Advisory Committee
Advisor Name Title
Dr. Judy S. Riffle Committee Chair
Dr. Alan Esker Committee Member
Dr. James E. McGrath Committee Member
Dr. Richey M. Davis Committee Member
Dr. Timothy E. Long Committee Member
Keywords

* polysiloxane
* phthalonitrile
* cobalt
* poly(ethylene oxide)
* nanoparticle

Date of Defense 2006-04-18
Availability unrestricted
Abstract

Preparation and Functionalization of Macromolecule-Metal and Metal Oxide Nanocomplexes For Biomedical Applications

Michael L. Vadala

Abstract

Copolymer-cobalt complexes have been formed by thermolysis of dicobalt octacarbonyl in solutions of copolysiloxanes. The copolysiloxane-cobalt complexes formed from toluene solutions of PDMS-b-[PMVS-co-PMTMS] block copolymers were annealed at 600-700 °C under nitrogen to form protective siliceous shells around the nanoparticles. Magnetic measurements after aging for several months in both air and in water suggest that the ceramic coatings do protect the cobalt against oxidation. However, after mechanical grinding, oxidation occurs. The specific saturation magnetization of the siliceous-cobalt nanoparticles increased substantially as a function of annealing temperature, and they have high magnetic moments for particles of this size of 60 emu g-1 Co after heat-treatment at temperatures above 600 °C.

The siliceous-cobalt nanoparticles can be re-functionalized with aminopropyltrimethoxysilane by condensing the coupling agent onto the nanoparticle surfaces in anhydrous, refluxing toluene. The concentration of primary amine obtained on the surfaces is in reasonable agreement with the charged concentrations. The surface amine groups can initiate L-lactide and the biodegradable polymer, poly(L-lactide), can be polymerized directly from the surface. The protected cobalt surface can also be re-functionalized with poly(dimethylsiloxane) and poly(ethylene oxide-co-propylene oxide)

providing increased versatility for reacting polymers and functional groups onto the siliceous-cobalt nanoparticles.

Phthalonitrile containing graft copolysiloxanes were synthesized and investigated as enhanced oxygen impermeable shell precursors for cobalt nanoparticles. The siloxane provided a silica precursor whereas the phthalonitrile provided a graphitic precursor. After pyrolysis, the surfaces were silicon rich and the complexes exhibited a substantial increase in Ms. Early aging data suggests that these complexes are oxidatively stable in air after mechanical grinding.

Aqueous dispersions of macromolecule-magnetite complexes are desirable for biomedical applications. A series of vinylsilylpropanol initiators, where the vinyl groups vary from one to three, were prepared and utilized for the synthesis of heterobifunctional poly(ethylene oxide) oligomers with a free hydroxy group on one end and one to three vinylsilyl groups on the other end. The oligomers were further modified with carboxylic acids via ene-thiol addition reactions while preserving the hydroxyl functionality at the opposite terminus. The resulting carboxylic acid heterobifunctional PEO are currently being investigated as possible dispersion stabilizers for magnetite in aqueous media.


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Synthesis and Characterization of Surface-Functionalized Magnetic Polylactide Nanospheres

Type of Document Dissertation
Author Ragheb, Ragy Tadros
URN etd-04092008-121127
Title Synthesis and Characterization of Surface-Functionalized Magnetic Polylactide Nanospheres
Degree PhD
Department Macromolecular and Science Engineering
Advisory Committee
Advisor Name Title
Dr. Judy S. Riffle Committee Chair
Dr. James E. McGrath Committee Member
Dr. Richey M. Davis Committee Member
Dr. S. Richard Turner Committee Member
Keywords

* confined impingement jet mixing
* nanoprecipitation
* surface-functionalized
* nanospheres
* magnetic
* magnetite
* poly(ethylene oxide)
* L-lactide)
* poly(D
* polylactide

Date of Defense 2008-03-28
Availability unrestricted
Abstract

Polylactide homopolymers with pendent carboxylic acid functional groups have been designed and synthesized to be studied as magnetite nanoparticle dispersion stabilizers. Magnetic nanoparticles are of interest for a variety of biomedical applications including magnetic field-directed drug delivery and magnetic cell separations. Small magnetite nanoparticles are desirable due to their established biocompatibility and superparamagnetic (lack of magnetic hysteresis) behavior. For in-vivo applications, it is important that the magnetic material be coated with biocompatible organic materials to afford dispersion characteristics or to further modify the surfaces of the complexes with biospecific moieties. The acid-functionalized silane endgroup was utilized as the dispersant anchor to adsorb onto magnetite nanoparticle surfaces and allowed the polylactide to extend into various solvents to impart dispersion stability. The homopolymers were complexed with magnetite nanoparticles by electrostatic adsorption of the carboxylates onto the iron oxide surfaces, and these complexes were dispersible in dichloromethane. The polylactide tailblocks extended into the dichloromethane and provided steric repulsion between the magnetite-polymer complexes. The resultant magnetite-polymer complexes were further incorporated into controlled-size nanospheres. The complexes were blended with poly(ethylene oxide-b-D,L-lactide) diblock copolymers to introduce hydrophilicity on the surface of the nanospheres with tailored functionality. Self-assembly of the PEO block to the surface of the nanosphere was established by utilizing an amine terminus on the PEO to react with FITC and noting fluorescence.

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Physical Properties of Macromolecule-metal oxide nanoparticle complexes: Magnetophoretic Mobility, Size, and Interparticle Potentials

Type of Document Dissertation
Author Mefford, Olin Thompson
URN etd-06262007-152333
Title Physical Properties of Macromolecule-metal oxide nanoparticle complexes: Magnetophoretic Mobility, Size, and Interparticle Potentials
Degree PhD
Department Macromolecular and Science Engineering
Advisory Committee
Advisor Name Title
Judy S. Riffle Committee Chair
Brian Love Committee Member
James McGrath Committee Member
Rick Davis Committee Member
Tim St. Pierre Committee Member
Keywords

* SQuID
* TEM
* polydimethylsiloxane
* magnetophoretic mobility
* DLS
* rheology
* DLVO theory
* magnetite
* nanoparticle

Date of Defense 2007-06-14
Availability unrestricted
Abstract

Magnetic nanoparticles coated with polymers hold great promise as materials for applications in biotechnology. In this body of work, magnetic fluids for the treatment of retinal detachment are examined closely in three regimes; motion of ferrofluid droplets in aqueous media, size analysis of the polymer-iron oxide nanoparticles, and calculation of interparticle potentials as a means for predicting fluid stability. The macromolecular ferrofluids investigated herein are comprised of magnetite nanoparticles coated with tricarboxylate-functional polydimethylsiloxane (PDMS) oligomers. The nanoparticles were formed by reacting stoichiometric concentrations of iron chloride salts with base. After the magnetite particles were prepared, the functional PDMS oligomers were adsorbed onto the nanoparticle surfaces.

The motion of ferrofluid droplets in aqueous media was studied using both theoretical modeling and experimental verification. Droplets (~1-2 mm in diameter) of ferrofluid were moved through a viscous aqueous medium by an external magnet of measured field and field gradient. Theoretical calculations were made to approximate the forces on the droplet. Using the force calculations, the times required for the droplet to travel across particular distances were estimated. These estimated times were within close approximation of experimental values.

Characterization of the sizes of the nanoparticles was particularly important, since the size of the magnetite core affects the magnetic properties of the system, as well as the long-term stability of the nanoparticles against flocculation. Transmission electron microscopy (TEM) was used to measure the sizes and size distributions of the magnetite cores. Image analyses were conducted on the TEM micrographs to measure the sizes of approximately 6000 particles per sample. Distributions of the diameters of the magnetite cores were determined from this data. A method for calculating the total particle size, including the magnetite core and the adsorbed polymer, in organic dispersions was established. These estimated values were compared to measurements of the entire complex utilizing dynamic light scattering (DLS). Better agreement was found for narrow particle size distributions as opposed to broader distributions.

The stability against flocculation of the complexes over time in organic media were examined via modified Derjaguin-Landau-Verwey-Overbeek (DLVO) calculations. DLVO theory allows for predicting the total particle-particle interaction potentials, which include steric and electrostatic repulsions as well as van der Waals and magnetic attractions. The interparticle potentials can be determined as a function of separation of the particle surfaces. At a constant molecular weight of the polymer dispersion stabilizer, these calculations indicated that dispersions of smaller PDMS-magnetite particles should be more stable than those containing larger particles. The rheological characteristics of neat magnetite-PDMS complexes (i.e, no solvent or carrier fluid were present) were measured over time in the absence of an applied magnetic field to probe the expected properties upon storage. The viscosity of a neat ferrofluid increased over the course of a month, indicating that some aggregation occurred. However, this effect could be removed by shearing the fluids at a high rate. This suggests that the particles do not irreversibly flocculate under these conditions.


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Large area electro-optical tactile sensor:Characterization and design of a polymer, nanoparticle based tunneling device

Type of Document Dissertation
Author Maheshwari, Vivek Chandra
Author's Email Address vmaheshw@vt.edu
URN etd-12242006-233123
Title Large area electro-optical tactile sensor:Characterization and design of a polymer, nanoparticle based tunneling device
Degree PhD
Department Macromolecular and Science Engineering
Advisory Committee
Advisor Name Title
David F Cox Committee Chair
Ravi F Saraf Committee Co-Chair
Alan R Esker Committee Member
Garth L Wilkes Committee Member
Richey M Davis Committee Member
Keywords

* Thin Film
* Large Area Device
* Nanoparticles
* Polyelectrolyte
* Tunneling
* Self-assembly
* Electronic Skin
* Tactile Sensor
* Electroluminescence

Date of Defense 2006-12-04
Availability unrestricted
Abstract

Touch (or tactile) sensors are gaining renewed interest as the level of sophistication in the application of minimally invasive surgery and humanoid robots increases. The spatial resolution of current large-area tactile sensors (greater than 1 cm2) lag human fingers by over an order of magnitude. Using metal and semiconducting nanoparticles, a ~100 nm thick, large area thin-film device working on the principles of electron tunneling is self-assembled, such that the change in current density through the film and the electroluminescence light intensity are linearly proportional to the local stress. By pressing a United States 1 cent coin (and also a copper grid) on the device a well resolved stress image by focusing the electroluminescence light directly on CCD is obtained. Both the lateral and height resolution of texture are comparable to human finger at similar stress levels of ~10 KPa.

The fabrication of the film is based on self-assembly of polyelectrolytes, and metal and semiconducting nanoparticles in a layered architecture. The polyelectrolyte layer functions as the dielectric tunneling barrier and the nanoparticles function as the base for tunneling electrons. The assembly of the device can be simplified by incorporating the functionality of the polyelectrolyte and the nanoparticles in a single composite medium. A non-micellar mineralization process for the synthesis of multifunctional nanocomposite materials is also reported as a possible building block for the assembly of tactile sensor. The non-micellar method results in the synthesis of monodisperse semi-conducting nanoparticles templated on polymer chains dissolved in solution at high yield. The monodispersity is achieved due to the beaded necklace morphology of the polyelectrolyte chains in solution where the beads are nanometer-scale nodules in the polymer chain and the nanoparticles are confined to the beads. The resultant structure is a nanoparticle studded necklace where the particles are imbedded in the beads. Multiple cycles of the synthesis on the polymer template yield nanoparticles of identical size, resulting in a nanocomposite with high particle fraction. The resultant nanocomposite has beaded-fibrilar morphology with imbedded nanoparticles, and can be solution cast to make electroluminescent thin film devices. The concept is further modified for synthesis of metal nanoparticles on polyelectrolyte templates with isolated beaded morphology.
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Durability study of proton exchange membrane fuel cells via experimental investigations and mathematical modeling

Type of Document Dissertation
Author Liu, Dan
URN etd-07182006-090606
Title Durability study of proton exchange membrane fuel cells via experimental investigations and mathematical modeling
Degree PhD
Department Macromolecular and Science Engineering
Advisory Committee
Advisor Name Title
Scott Case Committee Chair
Garth Wilkes Committee Member
James McGrath Committee Member
John J. Lesko Committee Member
Mike Ellis Committee Member
Keywords

* Cyclic Profile
* Aging
* Proton Conductivity
* Mechanical Properties
* Durability
* Proton Exchange Membrane Fuel Cell

Date of Defense 2006-07-11
Availability unrestricted
Abstract

In this dissertation, novel approaches to PEMFC durability research are summarized. These efforts are significantly different from most other studies on durability in that rather than focusing on chemical degradation, more attention is given to the mechanical aspects of the PEMFC system. The tensile stress-strain behavior of Nafion� 117 (N117) and sulfonated poly(arylene ether sulfone) random copolymer (BPSH35) membranes is explored under ambient conditions, with respect to the effects of initial strain rate, counterion type, molecular weight and the presence of inorganic fillers. A three-dimensional �bundle-cluster� model is proposed to interpret the tensile observations, combining the concepts of elongated polymer aggregates, proton conduction channels as well as states of water. The rationale focuses on the polymer bundle rotation/interphase chain readjustment before yielding and polymer aggregates disentanglement/ reorientation after yielding.

In addition, the influence of uniaxial loading on proton conductivity of N117 and BPSH35 membranes is investigated. When the membranes are stretched, their proton conductivities in the straining direction increase compared to the unstretched films, and then relax exponentially with time. The behavior is explained on the basis of the morphological variations of hydrophilic channels, accompanied by the rotation, orientation and disentanglement of the copolymer chains in the hydrophobic domains, as illustrated with the help of our bundle-cluster model.

Finally, the long-term aging of hydrogen-air PEMFCs is examined with a cyclic current profile and under constant current conditions. The end-of-period diagnosis is performed for both MEAs at 100h aging intervals, including a series of cell polarization, impedance and electrochemical experiments. The results demonstrate that hydrogen crossover is the most significant result of degradation for the MEA under cyclic aging mode due to the formation of pinholes at approximately 500-600h, and mass transport limitations are the major degradation sources for constant current mode. A phenomenological mathematical model is set up to describe the PEMFC aging process under both cyclic and constant conditions.


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Synthesis and Characterization of Multiphase, Highly Branched Polymers

Type of Document Dissertation
Author Fornof, Ann R.
URN etd-04212006-184840
Title Synthesis and Characterization of Multiphase, Highly Branched Polymers
Degree PhD
Department Macromolecular and Science Engineering
Advisory Committee
Advisor Name Title
Timothy E. Long Committee Chair
Don Leo Committee Member
Garth L. Wilkes Committee Member
James McGrath Committee Member
Judy Riffle Committee Member
Thomas Ward Committee Member
Keywords

* hyperbranched
* polyurethanes
* rheology
* ionenes

Date of Defense 2006-04-20
Availability unrestricted
Abstract

Rheological modification is frequently cited as a key application for hyperbranched polymers. However, the high degree of branching in these polymers restricts entanglement and the resultant mechanical properties suffer. Longer distances between branch points may allow entanglements. Highly branched polymers, where linear units are incorporated between branch points, are synthesized with an oligomeric A2 plus a monomeric B3. Higly branched polymers differ from traditional hyperbranched polymers in that every monomeric repeating unit of a hyperbranched polymer is a potential branch point, which is not true for highly branched polymers.

The oligomeric A2 plus B3 synthetic methodology was used for the synthesis of highly branched ionenes and polyurethanes. Highly branched ionenes, which have a quaternary ammonium salt in the main chain, were synthesized with a modified Menshutkin reaction. The oligomeric A2 was comprised of well-defined telechelic tertiary amine endcapped poly(tetramethylene oxide). Reduced mechanical properties were observed for highly branched polymers compared to linear counterparts.

Highly branched polyurethanes were synthesized with polyether soft segments including poly(ethylene glycol), poly(tetramethylene glycol), and poly(propylene glycol). Degree of branching was determined via a novel 13C NMR spectroscopy approach, which is described herein. The classical degree of branching was supplemented with an alternative degree of branching equation, which was tailored for highly branched architectures. The melt and solution viscosities of highly branched poly(ether urethane)s were orders of magnitude lower than the linear analogs. For the first time, the presence of entanglements was confirmed for highly branched polymers. Doping the highly branched polyurethane with lithium perchlorate, a metal salt, resulted in a significantly higher melt viscosity. The ionic conductivity of the highly branched polyurethane when doped with a metal salt was orders of magnitude higher than the linear analog.

Soybean oil was oxidized for synthesis of soy-based polyol monomers. Three regimes were determined, and for the first time, a correlation between hydroxyl number and a resonance from the double bonds of soybean oil in 1H NMR spectroscopy was described. The relationship was used to accurately describe oxidation of soybean oil with time, temperature, and air flow rate. Soybean oil oxidation was catalyzed, and tack-free films were formed.


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High Temperature Polymers for Proton Exchange Membrane Fuel Cells

Type of Document Dissertation
Author Einsla, Brian Russel
Author's Email Address beinsla@vt.edu
URN etd-04262005-140621
Title High Temperature Polymers for Proton Exchange Membrane Fuel Cells
Degree PhD
Department Macromolecular and Science Engineering
Advisory Committee
Advisor Name Title
James E. McGrath Committee Chair
Alan R. Esker Committee Member
Donald J. Leo Committee Member
John G. Dillard Committee Member
Judy S. Riffle Committee Member
Keywords

* poly(arylene ether)
* polybenzimidazole
* polybenzoxazole
* polyimide
* sulfonated copolymers
* proton exchange membrane
* fuel cell

Date of Defense 2005-04-01
Availability unrestricted
Abstract

Novel proton exchange membranes (PEMs) were investigated that show potential for operating at higher temperatures in both direct methanol (DMFC) and H2/air PEM fuel cells. The need for thermally stable polymers immediately suggests the possibility of heterocyclic polymers bearing appropriate ion conducting sites. Accordingly, monomers and random disulfonated poly(arylene ether) copolymers containing either naphthalimide, benzoxazole or benzimidazole moieties were synthesized via direct copolymerization. The ion exchange capacity (IEC) was varied by simply changing the ratio of disulfonated monomer to nonsulfonated monomer in the copolymerization step. Water uptake and proton conductivity of cast membranes increased with IEC. The water uptake of these heterocyclic copolymers was lower than that of comparable disulfonated poly(arylene ether) systems, which is a desirable improvement for PEMs. Membrane electrode assemblies were prepared and the initial fuel cell performance of the disulfonated polyimide and polybenzoxazole (PBO) copolymers was very promising at 80 C compared to the state-of-the-art PEM (Nafion®); nevertheless these membranes became brittle under operating conditions. Several series of poly(arylene ether)s based on disodium-3,3’-disulfonate-4,4’-dichlorodiphenylsulfone (S-DCDPS) and a benzimidazole-containing bisphenol were synthesized and afforded copolymers with enhanced stability. Selected properties of these membranes were compared to separately prepared miscible blends of disulfonated poly(arylene ether sulfone) copolymers and polybenzimidazole (PBI). Complexation of the sulfonic acid groups with the PBI structure reduced water swelling and proton conductivity.

The enhanced proton conductivity of Nafion® membranes has been proposed to be due to the aggregation of the highly acidic side-chain sulfonic acid sites to form ion channels. A series of side-chain sulfonated poly(arylene ether sulfone) copolymers based on methoxyhydroquinone was synthesized in order to investigate this possible advantage and to couple this with the excellent hydrolytic stability of poly(arylene ether)s. The methoxy groups were deprotected to afford reactive phenolic sites and nucleophilic substitution reactions with functional aryl sulfonates were used to prepare simple aryl or highly acidic fluorinated sulfonated copolymers. The proton conductivity and water sorption of the resulting copolymers increased with the ion exchange capacity, but changing the acidity of the sulfonic acid had no apparent effect.



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Wood/Polymeric Isocyanate Resin Interactions: Species dependence

das, sudipto
Keywords

* solid-state nuclear magnetic resosnance
* differential scanning calorimetry
* dynamic mechanical analysis
* thermogravimetric analysis
* creep
* wood species
* polymeric isocyanate resin

Abstract

The performance of polymeric diphenylmethane diisocyanate (PMDI) resin is known to be highly dependent on the wood species. This species dependence may be due to differences in: cure chemistry, interphase morphology, or both of these factors. This study addresses aspects of the cure chemistry and interphase morphology of wood/PMDI bondlines; specifically these effects are compared using two woods: yellow-poplar and southern pine.

In this study, the cure chemistry of wood-PMDI system was analyzed with solid state NMR (SSNMR) using wood samples cured with doubly labeled (15N,13C) PMDI resin. The kinetics of PMDI cure in the presence of wood was analyzed with differential scanning calorimetry. Thermogravimetric analysis was used to analyze the effect of resin impregnation on the degradation patterns of wood. The wood-PMDI bond morphology was probed with dynamic and static (creep) mechanical analyses in both dry and plasticized conditions. The effect of resin on wood polymer relaxations was quantitatively analyzed by both the time-temperature superposition principle and the Kohlrausch-Williams-Watts equation.

The presence of a small but statistically significant species effect was observed on both the cure chemistry and bond morphology of wood-PMDI system at low cure temperatures. The cure of PMDI resin was found to be significantly faster in pine relative to corresponding poplar samples. Resin impregnation showed a significant species dependent effect on the wood mechanical properties; the resinated pine samples showed increase in compliance while the corresponding poplar samples became stiffer.

The in situ lignin relaxation was studied with both dynamic and static modes, using plasticized wood samples. Results showed that the lignin relaxation was slightly affected by resin impregnation in both woods, but the effect was relatively larger in pine. Static experiments of dry wood samples showed a significant reduction in the interchain interactions of wood polymers in pine samples, exclusively. Investigation of plasticized pine samples, which focuses on the in situ lignin relaxations, showed only minor changes with resin impregnation. This led us to hypothesize that the large changes observed in dry samples, were due to the in situ amorphous polysaccharides. The wood-PMDI interactions were significantly reduced upon acetylation of wood. This study also discusses three new and highly sensitive methods for the analysis of wood-resin interactions.
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The Development of Compression Moldable Polymer Composite Bipolar Plates for Fuel Cells

Cunningham, Brent David

* laminate
* wet-lay
* PEM
* polymer composite
* bipolar plate
* fuel cell

Abstract

The development, design, and modeling of a rapid continuous processing scheme is developed to economically manufacture conductive polymer composite bipolar plates for fuel cells. Bipolar plates are required to possess several important properties for fuel cell operation, with the most sought after being electrical conductivity and mechanical strength. The polymer composite material generated at Virginia Tech is based on material generated by a wet-lay process and uses polyethylene terepthalate (PET) or polyphenylene sulfide (PPS) as the binder, although PPS is mainly used. In order to reach sufficient conductivity for use in generating bipolar plates, the polymer is doped with high levels of conductive graphite particles in the range of 70-80 wt%. The polymer system is reinforced with 6-9 wt% glass or carbon fibers. When compression molded into a solid, flat preform, the wet-lay material exhibits excellent bulk (in-plane) conductivity (> 250 S/cm). The material also exhibits tensile and flexural strengths of 57.5 and 95.8 MPa, respectively, higher than other polymer composite material being considered for bipolar plate production. However, formability and through-plane conductivity needs improvement.

The laminate bipolar plates developed at Virginia Tech are made using wet-lay material in the core and a thermoplastic/graphite mixture on the surfaces. The wet-lay material provides mechanical integrity, while a powder form of PVDF or PPS and graphite mixture added to the surfaces to improve through-plane conductivity and formability.

The manufacturing scheme for the production of laminate bipolar plates is based on the pre-consolidation of the wet-lay material, which establishes a solid, flat surface for the continuous addition of laminate powder. Because the laminate powder only requires heating, radiation heating is used in the process design to pre-heat the preform prior to compression molding. The heated preform passes underneath a press, where forming of channels takes place along with cooling of the bipolar plate. It is estimated that the entire process can take one minute to produce a bipolar plate. The cost of manufacturing a bipolar plate is estimated to be $8/kW, below the goal of $10/kW. The annual production is determined to be 250,000, with over 500,000 possible depending on certain design factors.
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Bioresorbable Electrospun Tissue Scaffolds of Poly(ethylene glycol-b-lactide) Copolymers for Bone Tissue Engineering

Badami, Anand Shreyans

* tissue engineering
* differentiation
* cell proliferation
* electrospinning
* poly(ethylene oxide)
* polylactide copolymers

Abstract

Poly(�-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(�-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(�-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 �m. 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.

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The Influence of a Client Preference on Auditor Judgment: An Investigation of Temporal Effects and Client Trustworthiness

Jenkins, James Gregory Jr.
Abstract

The purpose of this dissertation is to investigate auditors' judgments and decisions in the presence of an explicitly stated client preference. This investigation considers two factors. First, the temporal placement (i.e., timing) of the client preference is varied to allow for an examination of differential effects associated with the receipt of an early client preference and a late client preference. Second, client trustworthiness is varied so that participants may have a basis upon which to evaluate the client's representations (i.e., preferences). Practicing auditors, who were either managers or senior managers at a national accounting firm, participated in the study by completing two audit tasks in which the two factors were manipulated.

Findings indicate that explicitly stated client preferences resulted in significantly different decision processes, but did not significantly influence auditors' judgment processes. However, further analysis indicated that there was no significant client preference (CP) effect observed for auditors' final decisions. Therefore, it appears that the influence of the client's preference was transitory. Taken together, these findings suggest that the CP did not result in a loss of auditors' objectivity.

Auditors' judgments and decisions were sensitive to the client's relative trustworthiness. This finding suggests that auditors are responsive to a client's credibility when evaluating the client's representations. This result is expected given since generally accepted auditing standards require auditors to consider a source's credibility. However, it is surprising that auditors' evidence evaluation efforts were not differentially sensitive to the client's trustworthiness. Such a finding may indicate that the participating auditors' evidence evaluation efforts are more influenced by firm policy than individual judgment.
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Activity-Based Costing & Warm Fuzzies - - Costing, Presentation & Framing Influences on Decision-Making ~ A Business Optimization Simulation

Abstract

Activity-Based Costing is presented in accounting text books as a costing system that can be used to make valuable managerial decisions. Accounting journals regularly report the successful implementations and benefits of activity-based costing systems for particular businesses. Little experimental or empirical evidence exists, however, that has demonstrated the benefits of activity-based costing under controlled conditions. Similarly, although case studies report conditions that may or may not favor activity-based costing decision making, controlled studies that measure the actual influence of those conditions on the usefulness of activity-based costing information are few.

This study looked at the decision usefulness of activity-based costing information under controlled, laboratory settings. An interactive computer simulation tested the ability of 48 accounting majors to optimize profits with and without activity-based costing information and tested to see if presentation format or decision framing would influence their outcomes.

The research showed that the activity-based costing information resulted in significantly better profitability decisions and required no additional time. Presentation in graphic (bar charts) or numeric (tabular reports) format did not influence profitability decisions but the graphs took longer for analysis and decision making. Decision framing influences were shown to beneficially affect profitability decisions but did not require additional time. Decision framing was especially helpful with the non-activity based costing information; it had no significant effect on activity-based costing performance.


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