<p> The primary aim of this study was to develop mesoporous nanocatalysts for (i) hydrogen production via steam reforming of methanol (SRM) in a tubular reactor, and (ii) syngas conversion to hydrocarbons via Fisher-Tropsch synthesis using silicon microchannel microreactors. The mesoporous catalysts for SRM were prepared by an optimized one-pot hydrothermal synthesis procedure. The catalysts were investigated for SRM activity in a packed bed tubular reactor using metals, namely, Cu, Co, Ni, Pd, Zn, and Sn. The metals were incorporated in different supports -MCM-41, SBA-15, CeO₂, TiO₂, and ZrO₂ to investigate the influence of support on catalyst properties. A sharp contrast in catalyst performance was noticed depending on the type of support employed. For example, in SRM at 250 &deg;C, Cu supported on amorphous silica SBA-15 and MCM-41 produced significantly less CO (&lt; 7%) compared to other crystalline supports Cu-TiO₂ and Cu/ZrO₂ that showed high CO selectivity of &sim;56% and &sim;37%, respectively. Amongst all the metals studied for SRM activity using 1:3 methanol:water mole ratio at 250 &deg;C, 10%Cu-MCM-41 showed the best performance with 68% methanol conversion, 100% H₂ , &sim;6 % CO, 94% CO₂ selectivities, and no methane formation. Furthermore, 10%Cu-CeO₂ yielded the lowest CO selectivity of 1.84% and the highest CO2 selectivity of &sim;98% at 250 &deg;C. Stability studies of the catalysts conducted for time-on-stream of 40 h at 300 &deg;C revealed that Cu-MCM41 was the most stable and displayed consistent steady state conversion of &sim;74%. Our results indicate that, although coking played an influential role in deactivation of most catalysts, thermal sintering and changes in MCM-41 structure can be responsible for the catalyst deactivation. For monomtetallic systems, the MCM-41 supported catalysts especially Pd and Sn showed appreciable hydrothermal stability under the synthesis and reaction conditions. While bimetallic Pd-Co-MCM-41 and Cu-Ni-MCM-41 catalysts produced more CO, Cu-Zn-MCM-41 and Cu-Sn-MCM-41exhibited better SRM activity, and produced much less CO and CH4. In spite of the improved the stability and dispersion of the monometallic active sites in the support, no noticeable synergistic activity was observed in terms of H₂ and CO selectivities in the multimetallic catalysts. For the Fisher-Tropsch (F-T) studies, Co-TiO₂, Fe-TiO₂ and Ru-TiO₂ catalysts were prepared by the sol-gel method and coated on 116 microchannels (50&mu;m wide x 100&mu;m deep) of a Si-microreactor. The F-T process parameters such as temperature, pressure and flow rates were controlled by an in-house setup programmed by LabVIEW^&reg;. The effect of temperature on F-T activity in the range of 150 to 300&deg;C was investigated at 1 atm, a flow rate of 6 ml/min and a constant H₂:CO molar ratio of 2:1. In our initial studies at 220 &deg;C, 12%Ru-TiO₂ showed higher CO conversion of 74% and produced the highest C₂-C₄ hydrocarbon selectivity-of &sim;11% ethane, 22% propane and &sim;17% butane. The overall catalyst stability and performance was in the order of 12%Ru-TiO₂>> 12%Fe-TiO₂ > 12%Co-TiO₂.</p>

<p> Nanomaterials are the focus of intense research efforts in a variety of fields because of dramatic differences in properties when compared to the corresponding bulk materials. Catalysis is one material property that can become more pronounced at the nanoscale. By lowering energy requirements for chemical reactions, catalysts reduce production costs in diverse sectors of the economy, including medicine, transportation, environmental protection, oil and gas, food, and synthetic materials. Transition metals are an important class of catalysts capable of facilitating reduction and oxidation of molecular species. Since the discovery of transition metal catalysts nearly 200 years ago, certain metals were considered more active as catalysts (i.e., Pt, Pd, and Ru), while others (Au) appeared to have negligible catalytic activity as bulk materials. In recent years, gold nanoparticles (AuNPs) have become a fast-growing field of research owing to their unexpected catalytic properties not present in the bulk material. However, unsupported AuNPs are highly prone to flocculation and subsequent reduced catalytic activity. The choice of an appropriate aggregation-resistant stabilizing ligand for these nanoparticles is an important part of maintaining nanoscale catalytic properties. Additional stability is provided by anchoring AuNPs to support materials, allowing for dramatic improvements in catalyst lifetimes. This work discusses the development of novel diamond support materials for improving the stability of catalytically active AuNPs. Synthetic nanodiamond is a widely available, inexpensive, and robust material that has found applications in a wide range of commercial abrasives, lubricants, and composite materials. By exploiting the rich surface chemistry of nanodiamond, we have developed versatile catalyst support materials that offer unrivaled chemical and mechanical stability. Various nanodiamond surface modifications are readily prepared using a combination of chemical vapor deposition, photo-active polymer chemistry, and synthetic organic chemistry techniques. Control over the surface chemistry and properties of the resulting nanodiamond allow for increased stability of AuNPs via surface anchored thiol and amine moieties. The use of diamond as a support material should allow a wide variety of noble and nonprecious metal composite materials to be used as catalysts in harsh chemical environments not suitable for existing support materials.</p><p>

<p> Poly(ethylene terephthalate) (PET) is a semi-crystalline polymer that has mechanical and thermal properties suitable for many applications. The rate of crystallization in manufacturing environments influences the final physical, mechanical, and optical properties of PET. Many industrial PET processes occur under dynamic or non-isothermal conditions and in the melt phase. The final material properties are influenced by the size, dimension, and distribution of crystallites and morphology that develop upon cooling from the melt. PET films of varying thickness for optical applications require clarity and transparency. One way achieving clarity and transparency in PET films is to limit or inhibit the quiescent crystallization, while not completely eliminating useful strain-induced crystals. The crystallization behavior of PET is influenced by many things including molecular weight, catalyst remnants, nucleating additives, and the addition of linear and multifunctional comonomers (i.e. branching agents). Branching agents have been reported to inhibit the crystallization of PET. It is of interest to study the effects of branching agents on branched PET (BPET). </p><p> In this investigation the influence of branching agent concentration and geometry on the non-isothermal crystallization behavior and kinetics of BPET was studied. To study the influence of branching agent concentration and geometry, two structural isomers of benzenetricarboxylic acid (<i> n</i>=3) were used at concentrations of 0.10, 0.25, 0.50, and 1.00 mol% (with respect to purified terephthalic acid). The branching agents used were 1,3,5-benzenetricarboxylic acid (trimesic acid, TMA) and 1,2,4-benzenetricarboxylic acid (trimellitic acid, TMLA). TMA and TMLA were used to study the influence of branching agent geometry because TMA is planar and TMLA is non-planar. Two different series of BPET were made to evaluate the influence of catalyst remnants and process on the non-isothermal crystallization behavior of BPET. The Jeziorny-modified Avrami model, the Ozawa model, and the Mo model were applied to study the effects of the branching agent concentration and geometry on the non-isothermal crystallization kinetics of BPET at various cooling rates (5, 10, 20, 50 &deg;C/min). </p><p> The results from the study showed that equivalent amounts of TMA and TMLA produced different non-isothermal crystallization results even though the molecular weight and catalyst concentration remained approximately constant. Increasing branching agent content did not produce a systematic decrease in the crystallization peak temperatures <i>T</i>c. The Mo model was successful in characterizing the non-isothermal crystallization behavior and kinetics of BPET. The crystallization rate was inhibited at concentration of 0.25 and 0.50 mol% TMA and 0.50 and 1.00 mol% TMLA. However, the crystallization rate was enhanced at 0.10 and 1.00 mol% TMA and 0.10 and 0.25 mol% TMLA. It is thought that at small concentrations of the branching agents, regardless of geometry, the branching agents act as nucleating agents. At other branching agent concentrations it is thought that the branching agent geometry influenced the non-isothermal crystallization behavior.</p>

<p> The aqueous chemistry, precipitation, and crystallization of metal-carbonates comprises a vast field of research that underlies the urgency of CO₂ sequestration, ocean-acidification, and biomineralization. The results of recent experimental and computational studies suggest that amorphous calcium and magnesium carbonates are precipitated from supersaturated aqueous conditions by non-classical aggregation of ion pairs, dimers, dynamically-ordered-liquid-likeoxypolymers (DOLLOPS), and prenucleation clusters (PNCs). We present the first high field (20 T) ⁴³Ca and ²⁵Mg NMR studies of amorphous calcium-magnesium carbonates (ACC, ACMC, AMC) materials. Direct integration of computational techniques with experimental NMR provides a novel step forward toward multi-scale integration of computational and experimental techniques. Supporting information is derived from X-ray diffraction (XRD), thermogravimetric/differential thermal analysis (TGA-DTA), and scanning electron microscopy&mdash;energy dispersive spectroscopy (SEM-EDS) and provides important comparison to the bulk structures and composition. </p><p> High field NMR of amorphous carbonates demonstrates that amorphous carbonates contain various types of local disorder, but does not corroborate the theory of polyamorphism nor nano scale phase separations postulated by other workers. Carbon (¹³C) NMR of ¹³Cenriched materials indicates a degree of Ca-Mg solid solution in ACMCs, as ACMC ¹³C resonances cannot be adequately reconstructed from the pure ACC and AMC ¹³C resonances. However, with increasing Mg-content (and therefore H₂O content) ¹³C NMR resonances are strongly influenced by water-carbonate hydrogen bonding, shifting to lower resonance frequency and broadening. The ¹³C-NMR are well-fit with single Gaussian distributions, suggesting that two-phase models of ACMCs are not required to explain our ¹³C NMR observations. Protoncarbon cross polarization indicates that there is a H population proximal to carbonate groups for all amorphous phases. ⁴³Ca NMR yields line shapes that span the resonance frequency range of all known crystalline calcium carbonate polymorphs and is well fit with a single Gaussian distributions. ⁴³Ca NMR does not support a theory of polyamorphisms, but rather suggests an unstructured, continuous distribution of local environments that is unlike any specific crystalline phase. The mean ⁴³Ca chemical shifts vary 0.77 ppm from compositions x = 0 to 0.5 [x = Mg/(Mg + Ca)], demonstrating that Mg²⁺ has very little influence on the molecular-scale ⁴³Ca environment in ACMCs. Through integration of quantum mechanical calculations, classical MD, and NMR we ascertain a maximum mean Ca-O bond distance in our ACCs/ACMCs of 2.45 &plusmn; 1 &Aring; that is independent of composition. Unlike the indistinguishable local calcium environments, ²⁵Mg NMR of amorphous material gives evidence for several distinct overlapping quadrupolar line shapes. These sites do not generate NMR resonances that are perfect matches for known crystalline polymorphs of magnesian carbonates and extend toward lower resonance frequencies far beyond the range of known equilibrium analogs. By comparison to the range of reference phases, the low frequency singularities of ACMC-AMC resonances are consistent with some population of Mg-O bond distances greater than 2.10 &Aring; and/or some fraction of sites with high coordination numbers (up to 8). The local Mg environment of a protodolomite crystallization [x = Mg/(Mg + Ca) = 0.6] exhibits ²⁵Mg NMR parameters most similar to the asymmetric Mg²⁺ coordination environment of lansfordite [Mg(CO₃)2(H₂O)₄]^2&ndash; or huntite. Although H-C cross polarization indicates no H-bonding with carbonate the XRD gives not longrange indications of huntite. The large effective radius of strongly hydrated Mg in the protodolomite likely provides a driving force for cation ordering in dolomite.</p><p>

The primary objective of this research is to generate reinforcing domains in situ during the processing of polymers by using phase separation techniques. Low molecular weight compounds were mixed with polymers where the process viscosity is reduced at process temperatures and mechanical properties are improved once the material system is cooled or reacted. Thermally induced phase separation and thermotropic phase transformation of low molar mass compounds were used in isotactic polypropylene (iPP) and poly(ether ether ketone) (PEEK) resins. Reaction induced phase separation was utilized in thermosets to generate anisotropic reinforcements. A new strategy to increase fracture toughness of materials was introduced. Simultaneously, enhancement in stiffness and reduction in process viscosity were also attained. Materials with improved rheological and mechanical properties were prepared by using thermotropic phase transformations of metal soaps in polymers (calcium stearate/iPP). Morphology and thermal properties were studied using WAXS, DSC and SEM. Mechanical and rheological investigation showed significant reduction in process viscosity and substantial improvement in fracture toughness were attained. Effects of molecular architecture of metal soaps were investigated in PEEK (calcium stearate/PEEK and sodium stearate/PEEK). The selected compounds reduced the process viscosity due to the high temperature co-continuous morphology of metal soaps. Unlike the iPP system that incorporates spherical particles, interaction between PEEK and metal soaps resulted in two discrete and co-continuous phases of PEEK and the metal stearates. DMA and melt rheology exhibited that sodium stearate/PEEK composites are stiffer. Effective moduli of secondary metal stearate phase were calculated using different composite theories, which suggested bicontinuous morphology to the metal soaps in PEEK. Use of low molecular weight crystallizable solvents was investigated in reactive systems. Formation of anisotropic reinforcements was evaluated using dimethyl sulfone (DMS) as the crystallizable diluent and diglycidyl ether of bisphenol-A (DGEBA)/m-phenylene diamine (mPDA) material system as the epoxy thermoset. Miscible blends of DMS and DGEBA/mPDA form homogenous mixtures that undergo polymerization induced phase separation, once the DGEBA oligomers react with mPDA. The effect of the competition between the crystallization and phase separation of DMS resulted in nano-wires to micro-scale fiber-like crystals that were generated by adjusting the reaction temperature and DMS concentration.

The focus of this prospectus is to study a new and simple process method to prepare and characterize elastomers and hydrogels. A prestressed double network thermoplastic elastomer and hydrogel is prepared by a two step curing process where first network is introduced in the unstrained state, while the second is introduced in the strained state, hence varying prestress after first curing step. The focus of this thesis is towards the understanding of the basic network mechanism governing the final physical, mechanical and thermo-mechanical properties of these prestressed double networks and relating them to their microstructure and morphology. Moreover, the major factors governing the final properties of these networks are being identified including the type of crosslinks, the extent of crosslinking in the two states of stresses/strains, mode of deformation and the behavior is compared with simple theoretical models. The network structure of swollen hydrogel networks has been studied and the effect of various topological constraints ranging from the crosslinks to entangled linear chains to stiff nanofillers have been studied. The study has been utilized to propose a filler reinforcing mechanism for elastomeric networks and also identify the competition between the effect of various constraints in the final steady state and relaxation properties of the swollen hydrogel networks. The final part of this thesis focuses towards the network evolution in ultra high molecular weight poly (tetrafluoroethylene) (PTFE) in its melt state. Initial studies on the viscoelastic properties of PTFE in its melt state has been discussed and later a method to alter the network evolution utilizing supercritical carbon dioxide has been discussed. The effect of supercritical carbon dioxide on the melt of PTFE has been observed by utilizing a new setup to understand the behavior of PTFE in-situ in presence of supercritical carbon dioxide.

This dissertation focuses on the solution-state polymer assembly of conjugated polymers with specific attention to nano- and molecular-scale morphology. Understanding how to control these structures holds potential for applications in polymer-based electronics. Optimization of conjugated polymer morphology was performed with three objectives: 1) segregation of donor and acceptor materials on the nanometer length-scale, 2) achieving molecular-scale ordering in terms of crystallinity within distinct domains, and 3) maximizing the number and quality of well-defined donor/acceptor interfaces. Chapter 1 introduces the development of a mixed solvent method to create crystalline poly(3-hexyl thiophene) (P3HT) fibrils in solution. Chapter 2 describes fibril purification and approaches to robust and functional fibrils, while chapters 3 and 4 demonstrate the formation of hybrid nanocomposite wires of P3HT and cadmium selenide (CdSe) nanoparticles by two methods: 1) co-crystallization of free and P3HT-grafted CdSe for composite nanowires and 2) direct attachment of CdSe nanoparticles at fibril edges to give superhighway structures. These composite structures show great potential in the application of optoelectronic devices, such as the active layer of solar cells. Finally, ultrafast photophysical characterization of these polymers, using time-resolved photoluminescence and transient absorption, was performed to determine the aggregation types present in suspended fibrils and monitor the formation and decay of charged species in fibrils and donor-acceptor systems.

Geckos and other insects have fascinated scientists and casual observers with their ability to effortlessly climb up walls and across ceilings. This capability has inspired high capacity, easy release synthetic adhesives, which have focused on mimicking the fibrillar features found on the foot pads of these climbing organisms. However, without a fundamental framework that connects biological and synthetic adhesives from nanoscopic to macroscopic features, synthetic mimics have failed to perform favorably at large contact areas. In this thesis, we present a scaling approach which leads to an understanding of reversible adhesion in both synthetic and biological systems over multiple length scales. We identify, under various loading scenarios, how geometry and material properties control adhesion, and we apply this understanding to the development of high capacity, easy release synthetic adhesive materials at macroscopic size scales. Starting from basic fracture mechanics, our generalized scaling theory reveals that the ratio of contact area to compliance in the loading direction, A/C, is the governing scaling parameter for the force capacity of reversible adhesive interfaces. This scaling theory is verified experimentally in both synthetic and biological adhesive systems, over many orders of magnitude in size and adhesive force capacity (Chapter 2). This understanding is applied to the development of gecko-like adhesive pads, consisting of stiff, draping fabrics incorporated with thin elastomeric layers, which at macroscopic sizes (contact areas of 100 cm2) exhibit force capacities on the order of 3000 N. Significantly, this adhesive pad is non-patterned and completely smooth, demonstrating that fibrillar features are not necessary to achieve high capacity, easy release adhesion at macroscopic sizes and emphasizing the importance of subsurface anatomy in biological adhesive systems (Chapter 2, Chapter 3). We further extend the utility of the scaling theory under shear (Chapter 4) and normal (Chapter 5) loading conditions and develop simple expressions for patterned and non-patterned interfaces which describe experimental force capacity data as a function of geometric parameters such as contact area, aspect ratio, and contact radius. These studies provide guidance for the precise control of adhesion with enables the development of a simple transfer printing technique controlled by geometric confinement (Chapter 6). Force capacity data from each chapter, along with various literature data are collapsed onto a master plot described by the A/C scaling parameter, with agreement over 15 orders of magnitude in adhesive force capacity for synthetic and biological adhesives, demonstrating the generality and robustness of the scaling theory (Chapter 7).

For the first time, a physical method based on polymer crystallization is employed, to overcome the inherent brittleness of poly (L-lactic acid) (PLLA) by kinetically trapping a low Tg continuous amorphous phase. The decrease in mobility as a result of polymer crystallization is used to arrest the remaining polymer in the amorphous phase. This is achieved by melt blending and co-crystallizing a triblock copolymer with a configuration of the form PDLA-soft block-PDLA with PLLA. When crystallized from a temperature Tb, such that TmPLLA < Tb < Tm stereocomplex, the slow quiescent crystallization of PLLA and the preferential crystallization of the PLA stereocomplex results in formation of a morphology that can be described as stereocomplex crystals dispersed in a soft continuous amorphous phase containing the amorphous PLLA and the non-crystallized triblock copolymer. A systematic investigation of the effect of various parameters on the stereocomplex crystallization, morphology and properties of the PDLA-softblock-PDLA triblock copolymer/ PLLA blends is performed. These parameters include the chemical nature of the midblock, the triblock composition and the triblock architecture. The effect of addition of a molecular plasticizer as a third component to the blend to modify the properties of the amorphous phase is also investigated. Finally, varying functionality epoxy oligomers are investigated as additives to improve the hydrolytic stability and durability of the poly(lactic acid) based blends developed. Based on these studies, poly(lactic acid) based flexible, rubbery, semicrystalline materials containing a physically cross-linked network with the stereocomplex crystals acting as physical cross-links and the amorphous PLLA, triblock and plasticizer acting as mobile amorphous chains between the cross-links has been developed.

The aim of this research was to explore new directions for carbon dioxide. The first project emphasized silyl monolayer synthesis. Silylation reactions were performed in both liquid and supercritical carbon dioxide. Different monofunctional organosilanes reacted with silica surfaces, forming covalently attached monolayers. These monolayers were characterized using contact angle measurements, X-ray photoelectron spectroscopy, and ellipsometry. Reaction kinetics were established, and compared with silylations in organic solvents. The reaction rate in CO2 is higher than that in conventional solvents while the final coverage is slightly lower than the optimized conditions for conventional solvents. Other multi-functional silanes were also studied. The silylation of nanoporous silica surfaces showed bonding densities almost as high as the maximum value reported in literature for small-pore substrates. Overall, CO2 is a good solvent for silylations on silica surfaces. The second project was to synthesize polymer/polymer nanocomposites using a CO2-assisted templating method. Semicrystalline polymers are composed of tens-of-nanometer thick crystalline lamellae and an amorphous matrix. CO2 normally swells only the amorphous and interlamellar regions. The goal of this research was to selectively bring monomers to the amorphous and interlamellar regions with the help of CO2. In situ polymerization and precipitation fixes the structure, replicating the nano-structure of the semicrystalline polymer substrate. Ring-opening metathesis polymerization was performed inside of CO2-swollen poly(4-methyl-1-pentene) (PMP) of high crystallinity. Several polymer/polymer nanocomposites were successfully produced using this method. They were characterized by a variety of techniques, such as transmission electron microscopy (TEM), differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA) and wide angle X-ray diffraction (WAXD). Infrared studies and TEM indicated that one type of composite, polynorbomene/PMP, had a gradient distribution of polynorbornene inside of the PMP matrix. Another composite, polyoctenamer/PMP prepared by cis-cyclooctene polymerization, exhibited very interesting mechanical properties. The poly(dicyclopentadiene)/PMP composites are unique nanometer-scale blends of a highly crosslinked thermoset with a thermoplastic polymer.