Browsing by Author "Madsen, Louis A."

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    Accurate Calculations of Molecular Properties with Explicitly Correlated Methods
    Zhang, Jinmei (Virginia Tech, 2014-08-13)
    Conventional correlation methods suffer from the slow convergence of electron correlation energies with respect to the size of orbital expansions. This problem is due to the fact that orbital products alone cannot describe the behavior of the exact wave function at short inter-electronic distances. Explicitly correlated methods overcome this basis set problem by including the inter-electronic distances (rij) explicitly in wave function expansions. Here, the origin of the basis set problem of conventional wave function methods is reviewed, and a short history of explicitly correlated methods is presented. The F12 methods are the focus herein, as they are the most practical explicitly correlated methods to date. Moreover, some of the key developments in modern F12 technology, which have significantly improved the efficiency and accuracy of these methods, are also reviewed. In this work, the extension of the perturbative coupled-cluster F12 method, CCSD(T)F12, developed in our group for the treatment of high-spin open-shell molecules (J. Zhang and E. F. Valeev, J. Chem. Theory Comput., 2012, 8, 3175.), is also documented. Its performance is assessed for accurate prediction of chemical reactivity. The reference data include reaction barrier heights, electronic reaction energies, atomization energies, and enthalpies of formation from the following sources: (1) the DBH24/08 database of 22 reaction barriers (Truhlar et al., J. Chem. Theory Comput., 2007, 3, 569.), (2) the HJO12 set of isogyric reaction energies (Helgaker et al., Modern Electronic Structure Theory, Wiley, Chichester, first ed., 2000.), and (3) the HEAT set of atomization energies and heats of formation (Stanton et al., J. Chem. Phys., 2004, 121, 11599.). Two types of analyses were performed, which target the two distinct uses of explicitly correlated CCSD(T) models: as a replacement for the basis-set-extrapolated CCSD(T) in highly accurate composite methods like HEAT and as a distinct model chemistry for standalone applications. Hence, (1) the basis set error of each component of the CCSD(T)F12 contribution to the chemical energy difference in question and (2) the total error of the CCSD(T)F12 model chemistry relative to the benchmark values are analyzed in detail. Two basis set families were utilized in the calculations: the standard aug-cc-p(C)VXZ (X = D, T, Q) basis sets for the conventional correlation methods and the cc-p(C)VXZ-F12 (X = D, T, Q) basis sets of Peterson and co-workers that are specifically designed for explicitly correlated methods. The conclusion is that the performance of the two families for CCSD correlation contributions (which are the only components affected by the explicitly correlated terms in our formulation) are nearly identical with triple- and quadruple-ζ quality basis sets, with some differences at the double-ζ level. Chemical accuracy (~4.18 kJ/mol) for reaction barrier heights, electronic reaction energies, atomization energies, and enthalpies of formation is attained, on average, with the aug-cc-pVDZ, aug-cc-pVTZ, cc- pCVTZ-F12/aug-cc-pCVTZ, and cc-pCVDZ-F12 basis sets, respectively, at the CCSD(T)F12 level of theory. The corresponding mean unsigned errors are 1.72 kJ/ mol, 1.5 kJ/mol, ~ 2 kJ/mol, and 2.17 kJ/mol, and the corresponding maximum unsigned errors are 4.44 kJ/mol, 3.6 kJ/mol, ~ 5 kJ/mol, and 5.75 kJ/mol. In addition to accurate energy calculations, our studies were extended to the computation of molecular properties with the MP2-F12 method, and its performance was assessed for prediction of the electric dipole and quadrupole moments of the BH, CO, H2O, and HF molecules (J. Zhang and E. F. Valeev, in preparation for submission). First, various MP2- F12 contributions to the electric dipole and quadrupole moments were analyzed. It was found that the unrelaxed one-electron density contribution is much larger than the orbital response contribution in the CABS singles correction, while both contributions are important in the MP2 correlation contribution. In contrast, the majority of the F12 correction originates from orbital response effects. In the calculations, the two basis set families, the aug-cc-pVXZ (X = D, T, Q) and cc-pVXZ-F12 (X = D, T, Q) basis sets, were also employed. The two basis set series show noticeably different performances at the double-ζ level, though the difference is smaller at triple- and quadruple-ζ levels. In general, the F12 calculations with the aug-cc- pVXZ series give better results than those with the cc-pVXZ-F12 family. In addition, the contribution of the coupling from the MP2 and F12 corrections was investigated. Although the computational cost of the F12 calculations can be significantly reduced by neglecting the coupling terms, this does increase the errors in most cases. With the MP2-F12C/aug-cc-pVDZ calculations, dipole moments close to the basis set limits can be obtained; the errors are around 0.001 a.u. For quadrupole moments, the MP2-F12C/aug-cc-pVTZ calculations can accurately approximate the MP2 basis set limits (within 0.001 a.u.).
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    Adsorption of Biomacromolecules onto Polysaccharide Surfaces
    Zhang, Xiao (Virginia Tech, 2014-10-02)
    Plant cell wall polysaccharides are abundant natural polymers making them potential sources for sustainable and biodegradable materials. Interfacial behavior, including adsorption and enzymatic degradation, of several plant cell wall polysaccharides and their derivatives were studied with a quartz crystal microbalance with dissipation monitoring (QCM-D), surface plasmon resonance (SPR) and atomic force microscopy (AFM). Xyloglucan adsorption isotherms were obtained to probe how cellulose-hemicellulose interactions were affected by the type of cellulose substrate and molar mass of xyloglucan. Xyloglucan as small as a heptasaccharide still adsorbed irreversibly onto cellulose. Carboxymethyl cellulose (CMC) adsorption onto cellulose and viscoelastic properties and water contents of the adsorbed CMC layers were obtained from a combination of QCM-D and SPR data. The CMC samples formed hydrated and viscoelastic layers compared to the relatively rigid xyloglucan layer. Pectin model surfaces were prepared by pectin adsorption from citric phosphate buffer onto gold substrates. These pectin model surfaces were used for subsequent interaction studies with xyloglucan and enzymatic degradation behavior. There is a strong correlation between the degree of esterification (DE) and film resistance to degradation with the high DE being the most susceptible to degradation. The adsorption of two mixed linkage glucans (MLG), barley and lichen MLG, onto regenerated cellulose (RC) surfaces in the absence and presence of other matrix polysaccharides was studied. Viscoelastic properties of the resulting layer were compared as a function of the proprotion of '-(1''3) linkages with lichen MLG forming softer gel-like layers on RC. The lichen MLG layers were further used for enzymatic degradation studies with respect to enzyme concentration, temperature, pH and ionic strength. These studies show that polymer adsorption is a promising strategy to modify material surfaces and provides fundamental understanding of interactions and biodegradation of cell wall polysaccharides at solid/liquid interfaces.
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    Adsorption Studies of Polysaccharides and Phospholipids Onto Cellulose
    Du, Xiaosong (Virginia Tech, 2011-12-12)
    Interactions between biomolecules and cellulose films at solid/liquid interfaces was studied by surface plasmon resonance spectroscopy (SPR), quartz crystal microbalance with dissipation monitoring (QCM-D) and in situ atomic force microscopy (AFM) measurements. This dissertation shows the porous character of nanocrystalline cellulose films as the key feature for enhanced adsorption of chemically modified polysaccharides and provides quantitative analysis of polymer supported phospholipid structures as a stable platform for studying membrane-related processes. Smooth cellulose I films were prepared by spincoating cellulose nanocrystal suspensions onto positively charged self-assembled monolayers on gold. The adsorption of pullulan cinnamate (PC) onto cellulose surfaces increased with increasing degree of cinnamate substitution. The interactions between PCs with higher degree of substitution (DS) and porous nanocrystalline cellulose (NC) films presumably generated looped multilayer PC structures that adsorbed more than twice as much onto NC films than onto regenerated cellulose (RC) films. PC chains not only covered the NC surface but also penetrated into the porous film. The porous features of NC film are responsible for the greater adsorption of polymer chains relative to tightly packed RC films. Adsorption of phospholipid vesicles onto RC and NC films was also studied. Aggregates of intact vesicle were observed on NC surfaces with high water content ~ 84 % by mass. Phospholipid patches with smooth features were found to assemble onto RC surfaces with a lower degree of hydration ~ 30 % by mass. Vesicle membrane breakage was triggered by a destabilizing agent, LysoPC. The great mass decrease, and changes in dissipation and degree of hydration for phospholipid structures after exposure to LysoPC corresponded to the transformation from vesicles to layered structures. Initial binding of LysoPC micelles to unruptured vesicles was clearly resolved in SPR, whereas the huge mass decrease associated with bound water hides the initial adsorption of LysoPC onto vesicles in QCM-D experiments. The intitial binding of LysoPC micelles onto vesicle membranes lasted for 200 seconds with a maximal increase of 14 % by mass prior to vesicle collapse. The role of cholesterol in phospholipid interactions with model cellulose surfaces was also considered. Supported vesicle layers over RC surfaces were observed for vesicle membranes containing ≥ 6.3 % by mole cholesterol, whereas phospholipid or phospholipid with lower cholesterol content formed disconnected lipid islands on RC surfaces. Meanwhile, intact vesicles were always observed on NC surfaces for phospholipid/cholesterol blends regardless of the cholesterol content. The intact vesicles on cellulose surfaces were attributed to the ability of cholesterol to accommodate vesicle deformation. These studies showed the impact of mesoscale structure of cellulose films on adsorbates. It sheds light on the role of the lignin-carbonhydrate-complex in plant cell wall structure and will inform the next generation of biomimetic nanocomposites. The designed polymer supported biomimetic membranes provide a perfect platform to develop intact and ruptured protoplast systems for the study of plant cell wall self-assembly.
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    Advancing characterization techniques for structure-property determination of in-situ lignocelluloses
    Chowdhury, Sudip (Virginia Tech, 2011-07-21)
    The global progression towards sustainable energy, materials and chemicals requires novel and improved analytical tools to understand and optimize lignocellulosic biomass utilization. In an effort to advance lignocellulose characterization, gain insights into biomass processing, and obtain novel perspectives on cell wall ultrastructure, this study utilizes three principal polymer characterization techniques, namely compressive-torsion dynamic mechanical analysis (DMA), deuterium quadrupolar nuclear magnetic resonance (2H NMR) and rheo-infrared spectroscopy. A novel parallel-plate compressive-torsion DMA protocol is developed to analyze very small solvent-plasticized biomass specimens with or without mechanical integrity. The benefits and limitations of this technique are demonstrated by comparing it to a conventional tensile-torsion DMA while analyzing various solvent-plasticized lignocelluloses. The rheology of wood in various organic solvents is studied through dynamic thermal scans, Time/temperature superposition (TTS) and fragility analysis. Plasticizing solvents and wood grain orientation significantly affected the lignin glass-transition temperature. Dynamic TTS reveals that while all storage modulus data shift smoothly, the thermorheological complexity of solvent-plasticized wood becomes evident in loss component master curves. It is argued that the plasticized lignocellulose TTS is insightful and potentially useful, although it fails to satisfy the classic TTS validity criteria. Subsequently, it is justified that the fragility analysis is a better suited treatment than the WLF model to investigate cooperative segmental motions of plasticized wood. Deuterium quadrupolar NMR reveals a new perspective on the orientation of amorphous wood polymers and two distinct amorphous polymer domains: a highly oriented phase in the S2 layer of the secondary cell wall and an isotropic phase postulated to occur in the compound middle lamella (CML). If the origin of the isotropic phase is confirmed to arise from the CML, then this technique provides a way to independently investigate the morphology and phase dynamics of CML and S2 in an intact tissue, and should bring novel insights into deconstructive strategies specific to the oriented and unoriented domains. Finally the effects of a wood-adhesion promoter (hydroxymethyl resorcinol, HMR) on in-situ wood polymers are studied to elucidate the still unresolved HMR-lignocellulose interactions. DMA, creep-TTS and 2H NMR reveal that HMR increases the crosslink density and restricts the mobility of wood amorphous phase. Rheo-IR spectroscopy shows that the molecular stress-transfer mechanism is altered within the wood cell wall.
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    Arabinoglucuronoxylan and Arabinoxylan Adsorption onto Regenerated Cellulose Films
    Ni, Ying (Virginia Tech, 2013-09-03)
    Cellulose and hemicelluloses have attracted increasing interest as renewable biopolymers because of their abundance. Furthermore, the recognition of biomass as a sustainable and renewable source of biofuels has driven research into the assembly and disassembly of polymers within plant cell walls. Cellulose thin films are useful in the study of interactions between cellulose and hemicelluloses, and quartz crystal microbalances with dissipation monitoring (QCM-D), surface plasmon resonance (SPR) and atomic force microscopy (AFM) are widely used to investigate polymer adsorption/desorption at liquid/solid interfaces. In this study, smooth trimethylsilyl cellulose (TMSC) films were spincoated onto gold QCM-D sensors and hydrolyzed into ultrathin cellulose films upon exposure to aqueous HCl vapor. The adsorption of arabinoglucuronoxylan (AGX) and arabinoxylan (AX) onto these cellulose surfaces was studied. The effects of structure, molar mass and ionic strength of the solution were considered. Increasing ionic strength increased AGX and AX adsorption onto cellulose. While AGX showed greater adsorption onto cellulose than AX by QCM-D, the trend was reversed in SPR experiments. The combination of QCM-D and SPR data showed a greater amount of water was trapped within the AX films. Both adsorbed AGX and AX films were subsequently visualized by AFM. Images from AFM showed AGX and AX adsorbed as aggregates from water, while AGX and AX adsorbed from CaCl2 yielded smaller xylan particles with more numerous globular structures on the cellulose surfaces. Images from AFM of xylan films on bare gold surfaces also showed layers of uniform aggregates that were consistent with AX and AGX aggregation in solution.
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    Block Copolymer Solutions: Transport and Dynamics, Targeted Cargo Delivery, and Molecular Partitioning and Exchange
    Li, Xiuli (Virginia Tech, 2020-01-23)
    Block copolymers have been extensively applied in diverse fields including packaging, electrolytes, delivery devices, and biosensors. Multiple investigations have been carried out on polymeric materials for cargo delivery purpose to understand how they behave over time. Block copolymer micelles (BCMs) have demonstrated superiority to deliver cargo, especially in drug delivery due to their encapsulation of hydrophobic agents. This dissertation will mainly study BCMs for potential applications in cargo delivery. Methods to study BCMs, including NMR spectroscopy, relaxometry and diffusometry, can provide valuable molecular information, such as chemical structure, translational motion, inter- or intramolecular interaction, dynamics, and exchange kinetics. Therefore, this dissertation describes applications of versatile NMR methods to reveal the fundamental behaviors of block copolymer self-assemblies, such as their dynamic stability, cargo partitioning, polymer chain exchange, and chain distribution in solution. We have investigated two BCM systems. Poly(ethylene oxide)-b-(ε-caprolactone) (PEO-PCL) is a model system to study BCM dynamic stability. PEO-PCL can self-assemble into spherical micelles at 1% w/v in D2O-THF-d8 mixed solvents. We used NMR diffusometry to quantify diffusion coefficients and populations of micelles and unimers (i.e. free polymer chains in solution) over a range of temperature (21 – 50 °C) and solvent composition (10 – 100 vol % THF-d8). By mapping the micelle-unimer coexistence phase diagrams, we are able to enhance our ability to understand and design micelle structure and dynamics. Moreover, we can also probe the chain exchange kinetics between micelles using a new technique we developed – time-resolved NMR spin-lattice relaxation (T1) or TR-NMR. This technique is an analog to time-resolved small-angle neutron scattering (TR-SANS), which can monitor specific signal intensity changes caused after mixing of isotope-labeled micelle solutions. A second system, Pluronic® F127 (PEO99PPO69PEO99) is a test system to study BCM structure and dynamic changes upon drug uptake. Pluronic® F127 is a commercial copolymer that can solubilize different hydrophobic drugs in micelles. We successfully encapsulated three model drugs into Pluronic® F127 BCMs and investigated the effects of polymer concentration and drug composition on drug partitioning fractions. Also, we proposed to design and synthesize a series of block copolymers with varied glass transition temperatures in core-forming blocks. Using NMR diffusometry, we have measured the existing unimer concentrations in micellar solutions and correlated these results with chain mobility and internal chemical composition. Lastly, we have extended our expertise in NMR and polymers into the study of ion-containing polymer systems (polyelectrolytes). A critical problem in polymer science is the inability to reliably measure the molecular weight of polyelectrolytes. We are developing methods to solve this problem by using NMR diffusometry, rheology, scattering, and scaling theories to accomplish general molecular weight measurements for polyelectrolytes. In short, this dissertation describes studies to provide more perspectives on structural and dynamic properties at various time and length scales for polymeric materials. NMR measurements, in combination with many other advanced techniques, have given us a better picture of soft matter behaviors and provided guidance for synthesis and processing efforts, especially in block copolymer micelles for delivery purposes.
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    Block Copolymer-derived Porous Polyimides and Carbon for High-Performance Energy Storage
    Guo, Dong (Virginia Tech, 2022-05-12)
    Block copolymer-derived nanoporous materials are featured with microstructures defined by the microphase separation of constituent blocks, enabling various applications in energy storage. Dictated by the molecular weights and volume fractions of constituent blocks, the microphase separation forms nanoscale microstructures of 1-100 nm. Selective removal of a sacrificial phase produces nanopores with tailored pore width, continuity, and tortuosity. The remaining phase customizes the properties of resulting nanoporous materials, including specific surface area, electrical conductivity/insulation, and mechanical performance. Therefore, block copolymer-derived porous materials are felicitous for use in high-performance energy storage. This dissertation presents the utilization of block copolymers to derive nanoporous materials: i) high-modulus polyimide separators for lithium-metal batteries, and ii) high-surface-area carbon electrodes for fast-charging zinc-ion batteries. In lithium-metal batteries, the dendritic growth of lithium leads to deteriorating performance and severe safety concerns. Suppressing lithium dendrites is imperative to guarantee both high performance and safe cycling. Mesoporous polyimide separators are promising for dendrite suppression: i) the mesopores are smaller than the width of lithium dendrites, preventing lithium dendrites from penetrating the separator. ii) The high-modulus polyimide ceases the growth of lithium dendrites. Herein, this dissertation reports a mesoporous polyimide separator produced by thermalizing polylactide-b-polyimide-b-polylactide at 280 °C. The mesoporous polyimide separator exhibits a median pore width of 21 nm and a storage modulus of 1.8 GPa. When serving as a dendrite-suppressing separator in lithium-metal batteries, the mesoporous polyimide separator enables safe cycling for 500 hours at a current density of 4 mA/cm2. In zinc-ion batteries, developing cathodes compatible with fast charging remains a challenge. Conventional MnO2 gravel cathodes suffer from low electrical conductivity and slow ion (de-)insertion, resulting in poor recharging performance. In this dissertation, porous carbon fiber (PCF) supported MnO2 (PCF@MnO2), comprising nanometer-thick MnO2 deposited on block copolymer-derived PCF, serves as a fast-charging cathode. The high electrical conductivity of PCF and fast ion (de-)insertion in nanometer-thick MnO2 both contribute to a high rate capability. The PCF@MnO2 cathode, with a MnO2 loading of 59.1 wt%, achieves a MnO2-based specific capacity of 326 and 184 mAh/g at a current density of 0.1 and 1.0 A/g, respectively. This dissertation investigates approaches to utilizing block copolymers-derived nanoporous materials for high-performance energy storage. Those approaches are envisaged to inspire the design of block copolymer-derived nanoporous materials, and advance the development of "beyond Li-ion" energy storage.
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    Cation and Anion Transport in a Dicationic Imidazolium-Based Plastic Crystal Ion Conductor
    Kidd, Bryce Edwin (Virginia Tech, 2013-07-10)
    Here we investigate the organic ionic plastic crystal (OIPC) 1,2-bis[N-(N\'-hexylimidazolium-d2(4,5))]C2H4 2PF6- in one of its solid plastic crystal phases by means of multi-nuclear solid-state (SS) NMR and pulsed-field-gradient (PFG) NMR. We quantify distinct cation and anion diffusion coefficients as well as the diffusion activation energies (Ea) in this dicationic imidazolium-based OIPC. Our studies suggest a change in transport mechanism for the cation upon varying thermal and magnetic treatment (9.4 T), evidenced by changes in cation and anion Ea. Moreover, variable temperature 2H SSNMR lineshapes further support a change in local molecular environment upon slow cooling in B0. Additionally, we quantify the percentage of mobile anions as a function of temperature from variable temperature 19F SSNMR, where two distinct spectral features are present. We also comment on the pre-exponential factor (D0), giving insight into the number of degrees of freedom for both cation and anion as a function of thermal treatment. In conjunction with previously reported conductivity values for this class of OIPCs and the Stokes-Einstein relation, we propose that ion conduction is dominated by anion diffusion between crystallites (i.e., grain boundaries). Using our experimentally determine diffusion coefficient and previously reported PF6- hydrodynamic radius (rH), viscous (" = 4.1 Pa " s) ionic liquid (IL) is present with a cation rH of 0.34 nm. NMR measurements are very powerful in elucidating fundamental OIPC properties and allow a deeper understanding of ion transport within such materials.
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    Characterization of Cellulose and Chitin Thin Films and Their Interactions with Bio-based Polymers
    Kittle, Joshua Daniel (Virginia Tech, 2012-04-02)
    As the two most abundant natural polymers on earth, cellulose and chitin have attracted increasing attention as a source of renewable energy and functional materials. Thin films of cellulose and chitin are useful for studying interactions of these materials with other natural and synthetic molecules via techniques such as quartz crystal microbalance with dissipation monitoring (QCM-D) and surface plasmon resonance (SPR). Because of the difficulty of extracting native cellulose, regenerated cellulose (RC), sulfated nanocrystalline cellulose (SNC), and desulfated nanocrystalline cellulose (DNC) thin films are often studied in its place. In this work, QCM-D solvent exchange studies showed that water contents of RC, SNC and DNC films were proportional to the film thickness (d). Accessibility and degradation of the films was further analyzed via substrate exposure to cellulase. Cellulase adsorption onto RC films was independent of d, whereas cellulase adsorption onto SNC and DNC films increased with d. Enhanced access to guest molecules for SNC and DNC films relative to RC films revealed they are more porous. The porosity of these cellulose films aided in understanding the observed differences of xyloglucan (XG) adsorption onto their surfaces. Xyloglucan adsorption onto RC, SNC, and DNC was studied by QCM-D and SPR. The amount of adsorbed XG increased in the order RC < SNC < DNC. XG adsorption onto RC films was independent of d, whereas XG adsorption was weakly dependent upon d for SNC films and strongly dependent upon d for DNC films. However, XG adsorbed onto "monolayer" thin films of RC, SNC, and DNC in approximately the same amount. These results suggested that the morphology and surface charge of the cellulose substrate had a limited effect upon XG adsorption and that accessible surface area of the cellulose film may be the factor leading to apparent differences in XG adsorption for different surfaces. The porosity and surface charge of SNC films presented a unique opportunity to examine polyelectrolyte adsorption and subsequent dewatering of the SNC substrate. The adsorption of a series of cationically derivatized dextran (cDex) polyelectrolytes with various degrees of substitution (DS) onto SNC was studied using QCM-D and SPR. As the hydrophobic character of the cDex samples increased, the water content of the adsorbed cDex layer decreased. For cDex with the greatest hydrophobic content, nearly 50% by mass of the initial water present in the porous SNC film was removed upon cDex adsorption. This study indicated that the water content of the film could be tailored by controlling the DS and hydrophobic character of the polyelectrolyte. This work also presents the first report of smooth, homogeneous, ultrathin chitin films, opening the door to surface studies of binding interactions, adsorption kinetics, and enzymatic degradation. The chitin films were formed by spincoating trimethylsilyl chitin onto gold or silica substrates, followed by regeneration to a chitin film. The utility of these chitin films as biosensors was evident from QCM-D and SPR studies that revealed bovine serum albumin adsorbed as a monolayer.
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    Computational Study of the Properties and Stabilities of Endohedral Metallofullerenes
    Fuhrer, Timothy J. (Virginia Tech, 2013-04-23)
    The chemistry of fullerenes, which are a class of carbon allotropes that can be prepared by vaporization of graphite in an electric arc in a low pressure atmosphere,1 has become a topic of much experimental and theoretical study over the past 25 years.  Herein we present a series of theoretical studies related to recently discovered or studied endohedral metallofullerenes (EMF) and a theory as to the selective stability of certain isomers of EMFs. Computational treatments of the anions of C80 and C94 are presented and compared in an effort to gain an understanding and predictive model for which isomers of each cage size EMF will be most stable.  A model is proposed in which the pentagons of fullerene anions are seen as charge localization centers that repel one another, making the pyracyclene bonding motif much more unstable for fullerene anions than for fullerene neutral cages. Computational treatments are also presented for two newly discovered EMFs, Y2C2@C92 and Gd2@C79N.  Y2C2@C92 is reported to exhibit a previously undiscovered mode of internal cluster rotation, while Gd2@C79N is shown to have unusual stability for an azofullerene with a large spin quantum number (15/2). Finally, computational techniques are employed to predict the thermodynamic feasibility of a chemical reaction replacing one metal atom in a trimetallic-nitride template (TNT) endohedral metallofullerene with different metal atom.  At least two of these are predicted to be thermodynamically practical.
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    Crystallization and Melting Studies of Poly(ε-caprolactone) and Poly(ethylene oxide) using Flash™ Differential Scanning Calorimetry and Preparation and Characterization of Poly(δ-valerolactone) Fractions
    Vincent, Matthew Ryan (Virginia Tech, 2019-07-03)
    The isothermal crystallization and melting temperatures of poly(ε-caprolactone) were correlated using fast differential scanning calorimetry. The melting kinetics was found to be independent of isothermal crystallization temperature and time. The conventional Hoffman-Weeks method could not be used to determine the equilibrium melting temperature because the observed melting temperatures were greater than the crystallization temperatures by a constant, so the Gibbs-Thomson method was used instead, yielding an equilibrium melting temperature of 103.4 ± 2.3°C. A modification was proposed to the non-linear Hoffman-Weeks equation that included a non-linear undercooling dependence for the kinetic fold surface free energy upon crystallization and permitted accurate modeling of the observed melting behavior. The isothermal crystallization rates of four narrow molecular weight poly(ethylene oxide) fractions were characterized using fast differential scanning calorimetry for crystallization temperatures spanning 100°C range with the lower limit approaching the glass transition. A transition from homogeneous to heterogeneous primary nucleation was observed at −5°C. The kinetic analysis suggested that the crystal growth geometry depends strongly on temperature, where rod-like structures begin to appear near the glass transition temperature, highly branched solid sheaves grow throughout the homogeneous primary nucleation temperature range, and spherulites grow in the heterogenous primary nucleation range. Poly(δ-valerolactone) was synthesized using microwave-assisted techniques. Narrow molecular weight fractions were obtained using successive precipitation fractionation. Preliminary isothermal crystallization studies suggest that conventional thermal analysis methods are not adequate to measure the melting temperatures accurately due to reorganization during heating.
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    Cyclopentadiene-Maleimide Platform for Thermally Reversible Polymers
    Stegall, Jeremy Brent (Virginia Tech, 2014-12-04)
    This dissertation describes a new platform for the synthesis of thermally reversible polymers, based on Diels-Alder reactions of bis-cyclopentadienes (bis-CPDs) and bis-maleimides (bis-MIs), that meets two main objectives. First, the bis-CPD must resist characteristic self-coupling. Second, the CPD-MI adducts should undergo the retro-Diels-Alder (rDA) reaction (i.e., thermal depolymerization) in a temperature regime that is comparable or slightly higher than that of the freely reversible bis-furan/bis-MI polymers (rDA between 80 °C and 130 °C) but much lower than that of bis-CPD homopolymers (rDA above 160 °C). Structure-reactivity relationships gleaned from the literature and from related but as yet unpublished work in our own laboratories led to our main hypothesis that a CPD moiety bearing one sterically encumbering substituent such as isopropyl (𝑖Pr) or tert-butyl (𝑡Bu) and one electronwithdrawing substituent such as perfluoroaryl would have the desired reactivity and adduct stability in combination with an 𝑁-substituted maleimide. Synthetic considerations led to a bisCPD monomer design in which two alkylcyclopentadiene groups (alkyl = 𝑖Pr or 𝑡Bu) are connected by an octafluorobiphenylene linker. As an initial fundamental step (Chapter 3), 1-(nonafluorobiphenyl-4’-yl)-4-tertbutylcyclopentadiene (1) was synthesized to provide a monofunctional model for the proposed difunctional CPD monomer. Reactions of 1 and 𝑁-(4-fluorophenyl)maleimide (FMI) afforded up to five regio- and stereo-isomeric adducts (of fourteen possible). Variable-temperature reactivity studies combined with NMR spectroscopic analysis, X-ray crystallography, and computational modeling enabled product distributions to be understood according to a conventional kinetic-vs- iii thermodynamic framework. These studies also predicted the microstructure of polymers derived from the proposed bis-CPD monomer, which is structurally analogous to 1, and bis-MIs. Moreover, 1 does not undergo DA self-coupling under ordinary conditions (T < 180 °C). Thermolysis studies of the major adducts revealed that the rDA becomes observable on a laboratory timescale (hours) at about 140 °C, which is at the upper end of the temperature range reported for furan+MI adducts but well below that of CPD+CPD adducts. In contrast, adducts formed from either of the analogous monosubstituted cyclopentadienes (𝑡BuC₅H₅ and C₆F₅C₅H₅) do not undergo rDA below 180 °C. These results strongly support the proposed bis-CPD monomer design. In a second fundamental step (Chapter 4), the hypothesis that an electron-withdrawing CPD substituent would destabilize a CPD-MI adduct was further tested by reacting 𝑁-(4- fluorophenyl)maleimide with a series of triarylated cyclopentadienes (1,2,3-Ar₃C₅H₃ and 1,2,4- Ar₃C₅H₃, Ar = C₆F₅, C₆F₄CF₃, and Ar = C₅F₄N). The perfluorophenyl- and perfluorotolylsubstituted compounds were previously reported, but the perfluoropyridyl-substituted cyclopentadienes were prepared for this study using SNAr reactions of pentafluoropyridine and sodium cyclopentadienide. The least electron deficient cyclopentadiene in each series (Ar = C₆F₅) reacted the most quickly and with the highest ultimate equilibrium binding constant, confirming the electron-effects hypothesis as well as the underlying presumption that DA reactions of even relatively electron-poor CPDs with MI would behave according to normal-electron-demand principles. In the main section of this dissertation (Chapter 5) the proposed bis(cyclopentadiene)s reacted with a series of previously reported bis(maleimides) to form linear polymers having molecular weights (Mn) up to 40 kDa. Relationships among the length and flexibility of the bis-MI linker (C₆H₁₂, C₁₂H₂₄, C₆H₄OC₆H₄, and (C₂H₄O)₂), the identity of the CPD alkyl substitutent (CHMe₂, CMe₃ and CMe₂Ph) and the glass transition temperature (Tg) as measured by differential scanning calorimetry (DSC) were understood in terms of a general model of local segmental mobility and free volume. Solution thermolysis of a model polymer system (bis-MI linker = C₆H₁₂ (7), CPD alkyl substituent = 𝑡Bu) showed a rapid decrease in molecular weight at 160 °C as determined by size exclusion chromatography (SEC). Solution thermolysis in the presence of excess FMI (as a trap for free CPD moieties) revealed that the onset temperature for rDA on a laboratory time scale (hours) was as low as 120 °C. In the bulk, thermolysis above 250 °C under vacuum led to recovery of a small portion of the bis-CPD monomer, but bulk thermolysis at 200 °C did not reveal a change in molecular weight as determined by SEC. The current interpretation of these observations is that limited mobility in these glassy polymers prohibits retro-DA decoupling. These findings largely validate the main hypothesis of this dissertation.
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    The Design and Study of Lanthanide-Chelating Macromolecular Diagnostic and Delivery Agents
    Bryson, Joshua Matthew (Virginia Tech, 2009-08-07)
    Macromolecular magnetic resonance imaging (MRI) contrast agents have unique localization and contrast enhancement properties. We have designed and studied a monodisperse paramagnetic β-cyclodextrin click cluster (Gd10) decorated with Gd-containing arms and unique contrast enhancing polymers. To synthesize Gd10, a novel alkyne-functionalized diethylenetriaminetetraacetic acid chelate was created and coupled to a per-azido-β-cyclodextrin core and chelated with Gd(III) to yield the precursor macromolecule. Luminescence measurements were carried out using an analogous structure Eu(III)-containing structure and indicated that each lanthanide has an average of 1.8 water exchange sites. Gd10 yields a high relaxivity profile (6.2 mM⁻¹ s⁻¹ per Gd(III) at 9.4 T). Gd10 shows toxicity higher than clinically used contrast agents such as Magnevist&trade in vitro in cardiomyoblast cells. No acute toxicity was observed in the rats (n = 9) and contrast enhanced image analysis indicates renal processes may be involved in clearance. The contrast enhancing polymers we developed are new macromolecular beacons that allow the delivery of nucleic acids to be visualized at different biological scales. They contain repeated oligoethyleneamines, for binding and compacting nucleic acids into nanoparticles, and Gd(III)/Eu(III) chelates. The chelated lanthanides allow the visualization of the delivery vehicle via microscopy and via magnetic resonance imaging (MRI). We demonstrate that these new delivery beacons effectively bind plasmid DNA(pDNA) and protect their cargo nucleic acids from nuclease damage. The lanthanide-chelate materials have been found to efficiently deliver pDNA into cultured cells and do not exhibit toxicity. Micrographs of cultured cells exposed to the nanoparticle complexes formed with fluorescein-labeled pDNA and the europium-chelated polymers reveal effective intracellular imaging of the delivery process. MRI of bulk cells exposed to the complexes formulated with pDNA and the gadolinium-chelated structures show bright image contrast, allowing visualization of effective intracellular delivery on the tissue-scale. Because of their versatility as imaging probes, these delivery beacons posses remarkable potential for tracking and understanding nucleic acid transfer in vitro and have promise for in vivo imaging applications. In later studies the Ln-chelating polymers were co-polymerized with dimethylgalacterate which definitively increases luciferase gene expression (up 50x enhancement) and cellular uptake (up to 2x enhancement).
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    Design and Synthesis of Supramolecular Structures for the Controlled Release of Sulfur Signaling Species
    Carrazzone, Ryan Joseph (Virginia Tech, 2022-02-08)
    In the early 2000s, hydrogen sulfide (H₂S) was added to the family of molecules known as gasotransmitters, a class of endogenously produced and freely diffusing biological signaling molecules. Since this discovery, biologists and chemists have sought to understand the physiological roles of H₂S and to elucidate the potential benefits of exogenous H₂S delivery. As a result, many synthetic small molecule donor compounds have been created to deliver H₂S in response to various biologically relevant stimuli. Furthermore, macromolecular and supramolecular H₂S donor systems have been created to protect donors in the biological milieu, extend release kinetics, or control H₂S release conditions. Thus, H₂S-donating nanostructures with precisely tuned release rates provide invaluable tools for further investigating the biological roles and therapeutic potential of H₂S. This work describes two polymer micelle systems for the controlled delivery of H₂S. The first system is based on H2S-releasing polymer amphiphiles with varying degrees of a plasticizing comonomer incorporated into the core-forming block. The glass transition temperature of the core-forming block varied predictably with incorporation of the plasticizing comonomer. Accordingly, the half-life of H₂S release decreased from 4.2 h to 0.18 h with increasing core-forming block mobility. The second system is based on H₂S releasing polymer amphiphiles with varying degrees of crosslinking in the core-forming block. The crosslinked system was designed to achieve control over H₂S release rate with minimal dilution of donor in the core-forming block. The half-life of H₂S release increased from 117 min to 210 min with increasing crosslink density in the core-forming block, further demonstrating that H₂S release rates can be precisely controlled by tuning micelle core mobility. Beyond control over H₂S release rate, further study of the biological roles of H₂S requires donor systems with precisely triggered release. To this end, this dissertation also discusses efforts to investigate fundamental micelle–unimer relationships. This section includes an evaluation of the impact of core-forming block mobility on micelle–unimer coexistence utilizing a model polymer amphiphile system. Unimer populations correlated with glass transition temperatures of the core-forming block, suggesting the need to consider micelle core mobility when discussing polymer chain phase behavior of amphiphilic block copolymers. Finally, this work discloses new methods for the radical polymerization of poly(olefin sulfones) with control over molecular weight. POSs are a unique class of polymers with great potential for stimuli-responsive depolymerization to generate sulfur dioxide (SO₂), a signaling gas related to H₂S.
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    The Design, Synthesis and Study of Mixed-Metal Ru,Rh and Os, Rh Complexes with Biologically Relevant Reactivity
    Wang, Jing (Virginia Tech, 2013-01-23)
    A series of mixed-metal bimetallic complexes [(TL)2M(dpp)RhCl2(TL)]3 (M = Ru and Os, terminal ligands (TL) = phen, Ph2phen, Me2phen and bpy, terminal ligands (TL) = phen, bpy and Me2bpy ), which couple one Ru or Os polyazine light absorber (LA) to a cis-RhIIICl2 center through a dpp bridging ligand (BL), were synthesized using a building block method. These are related to previously studied trimetallic systems [{(TL)2M(dpp)2RhCl2]5+, but the bimetallics are synthetically more complex to prepare due to the tendency of RhIII halide starting materials to react with diimine ligands to form cis-[Rh(NN)2Cl2]+ motifs. The bimetallic complexes, [(phen)2Ru(dpp)RhCl2(bpy)]3+, [(phen)2Ru(dpp)RhCl2(phen)]3+, [(Ph2phen)2Ru(dpp)RhCl2(phen)]3+, [(Me2phen)2Ru(dpp)RhCl2(phen)]3+, [(bpy)2Ru(dpp)RhCl2(bpy)]3+, [(bpy)2Ru(dpp)RhCl2(Me2bpy)]3+ and [(bpy)2Os(dpp)RhCl2(phen)]3+, were characterized and studied by electrochemistry, electronic absorption spectroscopy, ESI-mass spectrometry, steady-state and time-resolved emission spectroscopy. Ï¿" ï¿" The electrochemical properties of bimetallic complexes with polyazine ligands exhibit a reversible one-electron metal-based oxidation, a quasi-reversible RhIII/IICl2 overlapped with a small amount of RhII/ICl and an irreversible RhII/ICl2 �reductions prior to the reversible bridging ligand dpp0/- �reduction. ï¿" ï¿" The title bimetallic complexes are efficient light absorbers due to the [(TL)2MII(dpp)] light absorber subunit. The bimetallics display ligand-based ï¿"'ï¿"* transitions in the UV region and metal-to-ligand charge transfer (MLCT) transitions in the visible region of the spectrum with approximately half the absorption extinction coefficient values relative to the trimetallics in the spectrum. The Os,Rh bimetallic complex, [(bpy)2Os(dpp)RhCl2(phen)]3+, displays Os(dï¿")'dpp(ï¿"*) CT transition at 521 nm and a low energy absorption band at 750 nm in the near-infrared region representing direct 1GS'3MLCT excitation due to the high degree of spin orbital coupling in Os complexes. The bimetallic complexes [(phen)2Ru(dpp)RhCl2(bpy)]3+, [(phen)2Ru(dpp)RhCl2(phen)]3+, [(Ph2phen)2Ru(dpp)RhCl2(phen)]3+, [(Me2phen)2Ru(dpp)RhCl2(phen)]3+, [(bpy)2Ru(dpp)RhCl2(bpy)]3+ and [(bpy)2Ru(dpp)RhCl2(Me2bpy)]3+ display Ru(dï¿")'dpp(ï¿"*) MLCT transitions centered at 505, 508, 515, 516, 510 and 506 nm, respectively. The bimetallic complex [(Ph2phen)2Ru(dpp)RhCl2(phen)]3+ displays enhanced absorption. Ï¿" ï¿" The photophysical properties of Ru,Rh bimetallic complexes are close to those of trimetallic analogues. In room temperature acetonitrile, both bimetallic and trimetallic complexes display a weak and short-lived emission from the Ru(dï¿")'dpp(ï¿"*) 3MLCT excited state. For example, the bimetallic complex [(phen)2Ru(dpp)RhCl2(bpy)]3+ emits at 766 nm and the trimetallic complex [{(phen)2Ru(dpp)}2RhCl2]5+ emits at 760 nm. At 77 K in 4:1 ethanol/methanol glass, the bimetallics, as well as trimetallics, exhibit a more intense blue-shifted emission with a longer lifetime, which is from the same 3MLCT excited state. At 77 K, the low temperature emission from the same 3MLCT state of [{(phen)2Ru(dpp)}2RhCl2]5+ blue-shifts to 706 nm with the emission lifetime of 1.8 ms and the bimetallic [(phen)2Ru(dpp)RhCl2(bpy)]3+ emits at 706 nm (t = 1.8 ms). The Ru,Rh complexes 3MLCT excited states can populate Ru(dï¿")'Rh(ds*) triplet metal-to-metal charge transfer (3MMCT) excited states through intramolecular electron transfer at room temperature, which is impeded in the rigid matrice at 77 K due to the large reorganizational energy and restricted molecular motion. The emission of Os,Rh bimetallic complex [(bpy)2Os(dpp)RhCl2(phen)]3+ could not be detected by our instruments likely due to its expected red-shifted emission which lies outside our detector window. ï¿" ï¿" �The Ru,Rh bimetallics display interesting and efficient photo-reactivity with DNA activated by visible light. The DNA gel shift assay, selective precipitation, ESI-mass spectrometry and polymerase chain reaction (PCR) studies suggest that Ru,Rh bimetallic complexes photobind to DNA following visible light excitation. This reactivity is not observed for analogous Ru,Rh,Ru trimetallics due to the steric protection of the Rh site in that motif. The bimetallic [(TL)2Ru(dpp)RhCl2(TL)]3+ systems can photobind and photocleave DNA through low-lying 3MMCT excited states when excited by the low energy visible light, with or without molecular oxygen. This is unusual but desirable reactivity for photodynamic therapy (PDT) drug development. The Os,Rh bimetallic complex [(bpy)2Os(dpp)RhCl2(phen)]3+ photobinds and photocleaves DNA under red therapeutic light excitation without molecular oxygen, an unprecedented result. Polymerase chain reaction experiments were used to evaluate the impact on DNA amplification of the DNA photo-modification and photo-damage induced by [(bpy)2Os(dpp)RhCl2(phen)]3+ under red light irradiation. Either photobinding or photocleavage induced by red light excitation of [(bpy)2Os(dpp)RhCl2(phen)]3+ on DNA inhibits amplification via PCR methods, a model for in vivo replication. Moreover, significant thermal stability of DNA photo-modification over 90 "C is required for PCR. A red light-activated drug that acts in an oxygen-independent mechanism to impede DNA amplification is unique in this field and desirable for study as a new class of PDT drugs.
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    Development and Characterization of Advanced Polymer Electrolyte for Energy Storage and Conversion Devices
    Wang, Ying (Virginia Tech, 2017-01-09)
    Among the myraid energy storage technologies, polymer electrolytes have been widely employed in diverse applications such as fuel cell membranes, battery separators, mechanical actuators, reverse-osmosis membranes and solar cells. The polymer electrolytes used for these applications usually require a combination of properties, including anisotropic orientation, tunable modulus, high ionic conductivity, light weight, high thermal stability and low cost. These critical properties have motivated researchers to find next-generation polymer electrolytes, for example ion gels. This dissertation aims to develop and characterize a new class of ion gel electrolytes based on ionic liquids and a rigid-rod polyelectrolyte. The rigid-rod polyelectrolyte poly (2,2'-disulfonyl-4,4'-benzidine terephthalamide) (PBDT) is a water-miscible system and forms a liquid crystal phase above a critical concentration. The diverse properties and broad applications of this rigid-rod polyelectrolyte may originate from the double helical conformation of PBDT molecular chains. We primarily develop an ionic liquid-based polymer gel electrolyte that possesses the following exceptional combination of properties: transport anisotropy up to 3.5×, high ionic conductivity (up to 8 mS cm⁻¹), widely tunable modulus (0.03 – 3 GPa) and high thermal stability (up to 300°C). This unique platform that combines ionic liquid and polyelectrolyte is essential to develop more advanced materials for broader applications. After we obtain the ion gels, we then mainly focus on modifying and then applying them in Li-metal batteries. As a next generation of Li batteries, the Li-metal battery offers higher energy capacity compared to the current Li-ion battery, thus satisfying our requirements in developing longer-lasting batteries for portable devices and even electric vehicles. However, Li dendrite growth on the Li metal anode has limited the pratical application of Li-metal batteries. This unexpected Li dendrite growth can be suppressed by developing polymer separators with high modulus (~ Gpa), while maintaining enough ionic conductivity (~ 1 mS/cm). Here, we describe an advanced solid-state electrolyte based on a sulfonated aramid rigid-rod polymer, an ionic liquid (IL), and a lithium salt, showing promise to make a breakthrough. This unique fabrication platform can be a milestone in discovering next-generation electrolyte materials.
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    Development of Carbohydrate-based Diblock Polymers for Nucleic Acid Delivery
    Sizovs, Antons (Virginia Tech, 2012-05-04)
    The delivery of nucleic acids remains the major obstacle for nucleic acid-based therapies such as gene therapy and gene silencing therapies based on RNA interference. In this dissertation we have developed and studied nucleic acid delivery vehicles based on cationic diblock glycopolymers that contain glucosamine and trehalosamine. Practical procedures were developed to synthesize 2-methacrylamido-2-deoxy glucose and 6-methacrylamido-6-deoxy trehalose starting with commercially available carbohydrates and utilizing trimethylsilyl protecting group chemistry. These monomers were polymerized via reversible addition-fragmentation chain transfer (RAFT) polymerization to yield glycopolymers with the desired lengths and low polydispersity indexes. Glycopolymers were chain-extended with aminoethylmethacrylamide to produce cationic diblock copolymers. The ability of cationic diblock copolymers to bind nucleic acids was demonstrated with gel electrophoresis and heparin exclusion assays. Complexes of the synthesized polymers with nucleic acids were studied with dynamic light scattering to reveal nanoparticles of 100-250 nm that were stable in the presence of serum proteins. Quartz crystal microbalance experiments showed that serum proteins adsorb on polytrehalose coated gold surfaces and it was suggested that these interactions may help mask the polytrehalose coated nanoparticles from potential actions of the immune system. Polytrehalose was also shown to suppress water crystallization similarly to trehalose by lowering the energies associated with the water/ice phase transition. The property was utilized to freeze-dry siRNA containing polyplexes which could be re-dissolved in water after lyophilization to yield nanoparticles. The polyplexes formulated with cationic diblock copolymers were shown to efficiently enter cervical cancer cells (HeLa cell line) and glioblastoma cells (U-87 cell line) and to deliver their nucleic acid cargo. Polyglucose-containing polymers were efficient mediators of exogenous gene expression in HeLa cells, and polytrehalose- containing polymers were effective in promoting the target gene down-regulation via RNA interference by delivered siRNA.
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    Development, Characterization, and Fundamental Studies on Molecular Ionic Composites and PBDT Hydrogels
    Zanelotti, Curt Joseph (Virginia Tech, 2022-01-28)
    This dissertation aims to develop, characterize, and fundamentally understand a new class of materials termed "molecular ionic composites" (MICs). MICs show promise as next-generation solid electrolytes for batteries. MICs form when mixing a rigid polyanion with purely ionic fluids, and they behave mechanically as a solid but contain a high density of ions that move nearly as in a neat liquid. Specifically, prototypical MICs are based on solutions of the rigid-rod polyelectrolyte poly(2,2'-disulfonyl-4,4'-benzideneterephthalamide) (PBDT), which forms a double helix, combined with imidazolium-based ionic liquids (ILs). The IL comprises 75-97 wt% of the final solid, even though the Young's modulus can reach ~ 2 GPa at 80 wt% IL. We propose that these properties are driven by a biphasic internal structure in MICs corresponding to IL-rich "puddles" (an interconnected liquid phase) and PBDT-IL associated "bundles" where IL ions form the collective electrostatic associations that cause the MICs to be a solid. Through this dissertation I will discuss a wide variety of MICs that have been created through the use of two different formation processes, the "ingot" method and the "solvent casting" method, which allow for the use of many different ionic fluid sources to further tune MIC properties. The following chapters build to the fundamental knowledge and our current understanding of the wide variety of materials that can be created from PBDT and IL.
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    Double helical conformation and extreme rigidity in a rodlike polyelectrolyte
    Wang, Ying; He, Yadong; Yu, Zhou; Gao, Jianwei; ten Brinck, Stephanie; Slebodnick, Carla; Fahs, Gregory B.; Zanelotti, Curt J.; Hegde, Maruti; Moore, Robert Bowen; Ensing, Bernd; Dingemans, Theo J.; Qiao, Rui; Madsen, Louis A. (Nature Publishing Group, 2019-02-18)
    The ubiquitous biomacromolecule DNA has an axial rigidity persistence length of ~50 nm, driven by its elegant double helical structure. While double and multiple helix structures appear widely in nature, only rarely are these found in synthetic non-chiral macromolecules. Here we report a double helical conformation in the densely charged aromatic polyamide poly(2,2′-disulfonyl-4,4′-benzidine terephthalamide) or PBDT. This double helix macromolecule represents one of the most rigid simple molecular structures known, exhibiting an extremely high axial persistence length (~1 micrometer). We present X-ray diffraction, NMR spectroscopy, and molecular dynamics (MD) simulations that reveal and confirm the double helical conformation. The discovery of this extreme rigidity in combination with high charge density gives insight into the self-assembly of molecular ionic composites with high mechanical modulus (~ 1 GPa) yet with liquid-like ion motions inside, and provides fodder for formation of other 1D-reinforced composites. © 2019, The Author(s).
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    Effects of Thermal Treatments on Perfluorosulfonate Ionomer Membranes
    Yan, Bing (Virginia Tech, 2010-07-27)
    Perfluorosulfonate ionomer (PFSI) membranes were annealed at elevated temperature for various periods of time in order to investigate the morphological effects of thermal treatments. For Nafion® 117, the DSC thermograms of Na+-, Cs+- and tetramethylammonium(TMA+)-form membranes show an endothermic peak develops upon annealing at 200ºC, indicating the development of crystallinity in the membrane. For these three samples annealed under same conditions, the heat of fusion (ΠH) values of the endothermic event increases with increasing counterion size. Larger tetraalkylammonium ions, tetraethylammonium(TEA+) and tetrapropylammonium(TPA+), result in no significant peak upon annealing at 200ºC. DSC thermograms of annealed Na+-form 3M Ionomer show no peak upon annealing and DSC thermograms of annealed TMA+-form 3M Ionomer show a very small peak that develops with annealing time at high equivalent weights. Annealed TMA+-form Dow Ionomer, which has a side chain shorter than both Nafion® and 3M Ionomer and a smaller mole% of side chains at the same equivalent weight, shows a relatively high ΠH value, which might also be related to its blocky nature. These results show that the isothermal crystallization kinetics of PFSI is affected by the counterion attached to the sulfonate group, the length of side chain, the mole% of side chains and the nature of the membrane. Water uptake analysis has been performed on annealed membranes, and the result shows that water uptake decreases with increasing degree of crystallinity.