Magnetostructural investigations were carried out on pyrene-1-yl (Pyr) bearing nitronylnitroxide (NN) and iminoylnitroxide (IN) radicals. PyrNN gives two allotropes: one has spin-paired dyads with ΔE = J/k ≈ -178 K, and the other is only half spin-paired with ΔE = J/k ≈ -102 K and the other half paramagnetic. PyrIN also gives two allotropes, an anti conformation that is spin paired in the crystal lattice with ΔE = J/k = -410 K, and a syn conformation that is disordered and paramagnetic. PyrNN also was discovered to co-crystallize with C6F6 in 2:1 ratio to give chains of radical networks linked into networks exhibiting low dimensional 1-D or 2-D antiferromagnetic exchange behavior. Furthermore, PyrNN was discovered to form a 2:1:2 co-crystal with octafluoronaphthalene (OFN) and entrapped solvent dichloromethane (DCM), in which the radical is ``shepherded'' into forming chains of radical-radical contacts on the peripheries of (PyrNN-OFN-PyrNN)n pi-stacks, giving weak, low dimensional inter-radical antiferromagnetic (AFM) exchange interactions. Anthraquinone-substituted nitronylnitroxide radical (AntQNN) was synthesized and found to form two crystal polymorphs. Magnetostructural investigations carried on these indicated that both have antiferromagnetic (AFM) exchange behavior attributed to chain-type inter-radical contacts: one with J1D/k ≈ -3 K, and one with J1D/k ≈-17 K. Five different anthracene nitroxide-type biradicals were synthesized: 27AntdNN, 27AntdIN, 26AntdNN, 26AntdIN, and 9Br27AntdNN. Room temperature solution state, and frozen solution state electron spin resonance (ESR) studies were carried on all of these biradicals. Crystallographic packing information was successfully obtained for 27AntdIN, 26AntdNN, 26AntdIN, and 9Br27AntdNN. Magnetic susceptibility measurements were carried on 27AntdNN, 27AntdIN, 26AntdIN and 9Br27AntdNN. The NN derivatives showed both ferromagnetic (presumed intramolecular) and antiferromagnetic exchange interactions in the solid state. Four different anthraquinone nitroxide-type biradicals were synthesized: 27AntQdNN, 27AntQdIN, 26AntQdNN, 26AntdIN. ESR studies were carried on these biradicals, and showed that 27AntQdNN is not a stable organic radical. While ESR spectra confirmed that 27AntQdNN, 27AntQdIN are biradicals, ESR spectra with isolated monoradical behavior were obtained for 26AntQdNN, 26AntQdIN. Iodine substituted meta-phenylene nitroxide biradicals, IPhdNN, IPhNNIN, and IPhdIN were synthesized. Room temperature and frozen solution ESR studies showed triplet states with strong intramolecular spin interaction. Magnetic behavior and crystallography for IPhdIN (which incorporates DCM), showed halogen bonding between molecules that assists formation of chains between radical sites.

Geometrically hindered motions of a single large solute (particle or polymer) can be imaged in real time via optical microscopy. The dynamics of fluorescent colloidal particles near surfaces and in porous membranes were monitored using confocal microscopy. A method of analysis to estimate diffusivity of particles in the axial direction by observing their intensity fluctuations was developed. The intensity fluctuations correspond to the Brownian motion of the particles in the axial direction. The method was successful in capturing the hindered diffusion of particles close to surfaces and in pores. This study provides a novel route to monitor the dynamics of particles, including biomacromolecules, near surfaces, through porous substrates and biological tissues. Ionic liquid (IL) as a medium for room temperature preservation of biomacromolecules has been proposed and, to investigate the possibility, physicochemical and enzymatic properties of proteins in the neat hydrophilic IL, ethylmethyl imidazolium ethyl sulfate [EMIM][EtSO4] were studied. Spectroscopic techniques were employed to probe the secondary and tertiary structure of proteins whereas light scattering and viscometry were used to estimate the hydrodynamic size. The secondary structure of the protein was retained in the ionic liquid but the tertiary structure was found to change. Alterations in protein conformation/activity were investigated after transfer of the dissolved protein from the IL to buffer. Further, suitability of ionic liquid gels as protein encapsulation and preservation media was assessed.

The unique electronic, magnetic, and optical properties of nanoparticles (NPs) make them useful building blocks for nanodevices and biofabrication. Site-selective immobilization/deposition of NPs on surfaces at desired positions is an important fabrication step in realizing the potential of nanomaterials in these applications. In this thesis, my research has focused on developing new strategies for mono- and multilayered-NP deposition on surfaces, increasing the stability of NP-assembles upon various surfaces for practical use of NP-based devices. Chemically directed dithiocarbamate binding of amine groups to NPs in the presence of CS2 was used for enhancing the robustness of NP assembles. Such patterning methodologies have allowed me to use site-directed NP immobilization in applications as diverse as microcontact printing, nanomolding in capillaries, nanoimprint lithography, and photolithography. Also, I have developed a simple and reliable one-step technique to form robust dendrimer-NP nanocomposites using dithiocarbamate-based chemistry. These composites are able to encapsulate and release various therapeutics, providing controllable sustained release and to separate small molecules and biomacromolecules.

Rheology is a particularly useful tool for elucidating the nature of polymer structures. The polymer at the gel point, the critical gel, has a power-law relaxation modulus, G(t) = St$\sp{-n}$. This simple constitutive equation is an expression of structural self-similarity which evolves as a result of crosslinking. We tested the composition dependence of the critical gel properties--the strength S and the relaxation exponent n--by studying an array a precursor materials, with different chain length, stoichiometry, and diluent concentration. All materials studied exhibited power-law rheological behavior, however, the critical gel properties were found to depend on the precursor composition. Several end-linked polydimethylsiloxanes with various composition yielded n values between 0.18 and 0.92, which is almost the entire possible range (0 $<$ n $<$ 1), while the gel strength varied by over five decades. Series of experiments at different stoichiometry and dilution can be reduced with S = G$\sb0\lambda\sb0\sp{\rm n}$, where G$\sb0$ and $\lambda\sb0$ may be material characteristic constants. The rate of change of the dynamic modulus was found to scale as a power law of frequency. The exponent, termed the dynamic critical exponent, describes the evolution of mechanical properties. It appears to take on a universal value $\kappa$ = 0.21 $\pm$ 0.02. A simple constitutive equation in the form of a stretched exponential can model the evolution of mechanical properties very well.

A novel technique for the production of toughened polymers using LCST behavior as a mechanism for the production of rubbery domains is discussed. The polymer blend of isotactic polyvinylmethylether (PVME) with polystyrene (PS) is used. Synthesis, fractionation and characterization of isotactic PVME is reviewed. Thermodynamic effects of tacticity on miscibility are extensively investigated using light and neutron scattering. A simple critical point analysis is presented which indicates an entropic nature to the tacticity effect in this blend. Flory-Huggins-Staverman (F-H-S) theory is next applied to the tacticity effect. This more elaborate analysis also indicates an entropic nature to the tacticity effect. Accounting for polydispersity results in a predicted fractionation in the phase separated blends which is supported by mechanical data. F-H-S theory was used to generate a functional form for the interaction parameter in terms of the temperature and composition dependence of miscibility. From these functions a dramatic shift in the kinetics of phase separation with tacticity is predicted. Experimental data affirms this prediction. A novel, modified Cahn-Hillard analysis is presented for analysis of intermediate stages. Neutron scattering data yields the composition and temperature dependence of the statistical segment length, b, of tactic PVME. A functional form for b is derived which predicts the equilibrium melting point and melting point depression behavior for the blends. A relationship between b and thermodynamic miscibility from a geometric perspective is discussed. The tacticity effect can be described in terms of an interaction parameter whose change with tacticity in terms of entropy is functionally related to the volume of an interacting group and in terms of enthalpy is functionally related to the surface area of an interacting group. Two related studies are presented. The first pertains to a shift in glass transition of thin PS films with thickness as investigated using ellipsometry. The second study used neutron reflection data to disprove the supporting argument for a minus two thirds dependence of surface tension with molecular weight. An alternative theory for the molecular weight dependence of surface tension is presented.

The reduction of 1,2-catecholatochloroboron by lithium in liquid ammonia/dimethoxyethane produces a reactive mixture containing multiple products. Formation of complexes with 12-crown-4 allows the isolation of one of these, (12-C-4)LiB(O$\sb2$C$\sb6$H$\sb4)\sb2.$ This complex was characterized by chemical analyses, multinuclear NMR, and single crystal x-ray diffraction. Chemical and spectroscopic data suggest that the remainder of this mixture contains boron in an asymmetric electronic environment and a low oxidation state. Reaction of this mixture with TiCl$\sb4$ forms an air stable complex with the formula TiB$\sb2{\cdot2}$((12-C-4)$\cdot$Ti(O$\sb2$C$\sb6 $H$\sb4)\sb2$). $\sp{11}$B and $\sp{47,49}$Ti NMR measurements of this complex indicate that boron and titanium inhabit a highly asymmetric electronic environment possibly with a considerable B-Ti interaction. The complex TiB$\sb2{\cdot}$((dme)$\cdot$Ti(SC$\sb2$H$\sb5)\sb2$(NH$\sb2)\sb2\rbrack,$ was prepared in a similar way. Reaction of this complex with bidentate protic species produced a series of polymeric complexes. TGA of theses polymers showed that mass loss was essentially complete by 400-600$\sp\circ$C. Pyrolysis of this series to 1000$\sp\circ$C formed green ceramic products that were characterized by volatiles analysis, chemical analysis, $\sp{11}$B MAS-NMR, XPS and powder diffraction. Annealing the green products at 1650$\sp\circ$C caused a carbothermic reduction that removed oxide impurities and formed crystalline TiB$\sb2$/TiC. However, this reduction only occurred when the green ceramic contained $>$5% amorphous carbon. In all other cases, the annealed ceramics contained TiBO$\sb3$/TiC. Pyrolysis of TiB$\sb2{\cdot}2$((dme)$\cdot$Ti(O$\sb2$C$\sb6$H$\sb4)\sb2\rbrack$ in a fluidized bed reactor produced a green ceramic with a minimum particle size of 100 nm. For this product, formation of TiB$\sb2$/TiC by carbothermic reduction was complete by 800$\sp\circ$C, 450$\sp\circ$C lower that conventional preparations. Reaction of a boryl-titanium complex with solid supports containing surface hydroxide groups led to thin films of titanium boride.

Evaluation of stress transfer mechanisms is critical in the design of composite structures and laminates. Due to anisotropy incurred during production, assessment of stress transfer processes is crucial for polymeric materials. In this study, two cases are evaluated in which parameters governing stress transfer play a critical role. These are: (1) Single fiber composites, and (2) Coatings. The first case examines the consequences of transferring stress from an isotropic matrix to embedded orthotropic fibers. The second case assesses the transferring of stress from anisotropic coatings to isotropic substrates. Fibers used in this investigation are highly oriented high-density polyethylene, poly (paraphenylene terephthalamide), and polydiacetylene (poly(2,4-hexadiyne-1,6-diol bisphenyl urethane)) single crystals. Poly (4,4$\prime$-oxydiphenylene pyrromellitimide), known as PMDA-ODA polyimide, is used as the coating material. The investigation of stress transfer was carried out using finite element analysis and analytical solutions. Elastic constants utilized for the analysis were obtained by molecular simulation methodologies. Crystalline and orientational order of the polymeric coating are characterized. This characterization was accomplished using wide-angle X-ray diffraction, electron diffraction, infrared spectroscopy, and Raman spectroscopy. Also, an external reflection infrared spectroscopic study of ultra-thin polyimide films revealed that molecular anisotropy remained higher and molecular packing lower for films thinner than 150 A. Results from these two case studies indicate that stress transfer observed at the ends of fibers or at the edges of coatings, is influenced by increases in the degree of material anisotropy. Assumptions of isotropy in polymeric materials undergoing analysis may lead to serious error in the characterization of stress transfer. Stress transferred from the isotropic matrix to anisotropic fibers leads to a state of radial compression near the fiber ends. Transverse stresses approach the value of the transverse strength of anisotropic fibers when strained uniaxially. For coatings, a complicated state of stress develops at the edges, at which out-of-plane and shear stresses are significant. Shear stress distributions near the ends, and equal in-plane stress, exhibit a slower decay when the degree of anisotropy is higher.

<p> Solar radiation is a vast, distributed, and renewable energy source which Humanity can utilize via the photovoltaic effect. The goal of photovoltaic technology is to minimize the true costs, while maximizing the power conversion efficiency and lifetime of the cell/module. Interface-related approaches to achieving this goal are explored here, for two technologically-important classes of light absorbers: crystalline-silicon (c-Si) and metal halide perovskite (MHP). The simplest solar cell consists of a light absorber, sandwiched between two metals with dissimilar work functions. Carrier-selective contacts (CSC&rsquo;s), which are ubiquitous in modern solar cells, are added to improve the electrical performance. Solar cells require asymmetric carrier transport within the cell, which can be effected via electrostatic and/or effective fields, and CSC&rsquo;s augment the asymmetry by selectively transporting holes to one contact, and electrons to the other contact. </p><p> The proper design and implementation of a CSC is crucial, as the performance, lifetime, and/or cost reduction of a solar cell can be hampered by a single interface or layer. A framework, consisting of eight core requirements, was developed from first-principles to evaluate the effectiveness of a given CSC. The framework includes some requirements which are well-recognized, such as the need for appropriate band offsets, and some requirements which are not well-recognized at the moment, such as the need for effective valence/conduction band density of states matching between the absorber and CSC.</p><p> The application of the framework to multiple silicon-based and MHP-based CSC&rsquo;s revealed the difficulties of effectively designing and implementing a CSC. A poly(3-hexylthiophene)/c-Si heterojunction was found to be a near ideal hole-selective contact (HSC). Three metal oxide/c-Si heterojunctions initially expected to yield comparable electron-selective contacts (ESC&rsquo;s), titanium dioxide/c-Si (TiO₂/c-Si), zinc oxide/c-Si (ZnO/c-Si), and tin dioxide/c-Si (SnO₂/c-Si), were instead discovered to be widely different. The TiO₂/MHP heterojunction was found to be a moderately ideal ESC, and the nickel oxide/MHP (NiOX/MHP) heterojunction is expected to be a good HSC. If interfacial lead di-iodide (PbI₂) is intentionally or unintentionally deposited at the interfaces of a MHP solar cell, it is expected to be detrimental to the operation of the NiOX/MHP HSC, but not to the TiO₂/MHP ESC.</p><p>

The assembly of nanoparticles at the interface of immiscible fluids holds promise for the preparation of new materials that benefit from both the physical properties of the nanoparticles and the chemistry associated with the ligands. Shaking nanoparticle solutions in organic solvents with water, results in the formation of nanoparticle-coated droplets that range in size from 10 µm to 200 µm. A strategy to control the size of these emulsions is described, by passing the droplets through commercial track-etch membranes with known pore sizes. Extrusion reduces the droplet size by breaking the droplets while passing theough the membrane pores, and reforming in the presence of excess nanoparticles in solution to form droplets as small as 1-5 µm. Crosslinking of nanoparticles at a liquid interface lends greater stability to the interfacial assembly, leading to ultrathin nanoparticle-based capsules, which possess mechanical integrity even after removal of the interface. Two approaches towards crosslinking are used in this thesis. Norbornene-functionalized CdSe/ZnS are used to afford facile capsule visualization by fluorescence confocal microscopy, as well as ease of crosslinking in mild conditions by means of ring-opening metathesis polymerization (ROMP). The crosslinked capsules can be used to encapsulate materials, and display size-selective retention capability, governed by the interstitial spaces between the nanoparticles. In a second approach to making hybrid capsules and sheets, horse spleen ferritin bionanoparticles and aldehyde-functionalized CdSe quantum dots are co-assembled at an oil-water interface. The cross-linked materials formed by reaction of the aldehyde functionality on the quantum dots with the surface-available amines on the ferritin bionanoparticles can be disrupted by addition of acid, thus leading to pH-degradable capsules and sheets. The driving force for assembly of nanoparticles at liquid interfaces is the reduction of the interfacial energy between the two liquids. The factors governing the amount of interfacial stabilization provided by the nanoparticles, namely the size and ligand coverage of the nanoparticles, are examined using the example of mixed assemblies of two different types of nanoparticles. Assemblies of 10 nm cobalt nanoparticles are disrupted upon the addition of 2.5 nm CdSe nanoparticles. The studies in this thesis demonstrate that the lower density of ligand coverage on CdSe quantum dots can overcome the large difference in size between the two nanoparticles, thus displacing the cobalt nanoparticles from the interface. Finally, preliminary results using amphiphilic graft copolymers instead of nanoparticles for interfacial stabilization of liquids are discussed. The resulting capsules are used for encapsulation and release of nanoparticles. In a technique termed repair-and-go, these nanoparticle-filled capsules are used for repairing cracked surfaces by passing the capsules over hydrophilic substrates containing hydrophobic cracks.

Anhydrous proton conducting polymers are highly sought after for applications in high temperature polymer electrolyte membrane fuel cells (PEMFCs). N-heterocycles (eg. imidazole, triazole, and benzimidazole), owing to their amphoteric nature, have been widely studied to develop efficient anhydrous proton transporting polymers. The proton conductivity of N-heterocyclic polymers is influenced by several factors and the design and development of polymers with a delicate balance among various synergistic and competing factors to provide appreciable proton conductivities has been a challenging task. In this thesis, the proton transport (PT) characteristics of polymers functionalized with two diverse classes of functional groups— N-heterocycles and phenols have been investigated and efforts have been made to develop the molecular design criteria for the design and development of efficient proton transporting functional groups and polymers. The proton conduction pathway in 1H-1,2,3-triazole polymers is probed by employing structurally analogous N-heterocyclic (triazole, imidazole, and pyrazole) and benz-N-heterocyclic (benzotriazole, benzimidazole, and benzopyrazole) polymers. Imidazole-like pathway was found to dominate the proton conductivity of triazole and pyrazole-like pathway makes only a negligible contribution, if any. Polymers containing benz-N-heterocycles exhibited higher proton conductivity than those with the corresponding N-heterocycles. Pyrazole-like functional groups, i.e. the molecules with two nitrogen atoms adjacent to each other, were found not to be good candidates for PT applications. A new class of proton transporting functional groups, phenols, has been introduced for anhydrous PT. One of the highlighting features of phenols over N-heterocycles is that the hydrogen bond donor/acceptor reorientation can happen on a single -OH site, allowing for facile reorientational dynamics in Grotthuss PT and enhanced proton conductivities in phenolic polymers. Unlike the case of N-heterocycles, comparable conductivities were achieved between poly (3,4,5-trihydroxy) styrene and the corresponding small molecule, pyrogallol. This observation suggests that reorientation should be considered as a crucial design parameter for PT functional groups. The PT characteristics of phenol-based biaryl polymers are studied and compared with the analogous phenol-based linear styrenic polymers. The two-dimensional disposition of -OH moieties in biaryl polymers, although resulted in lower apparent activation energies (Ea), did not improve the net proton conductivity due to the accompanying increase in glass transition temperature (Tg). Thus, the ease of synthesis and lower Tg values of phenol-based styrene polymers make the styrenic polymer architecture preferable over the biaryl architecture. Finally, the synthesis of a series of poly(3,4-dihydroxy styrene)-b-polystyrene block copolymers has been demonstrated via anionic polymerization. These block copolymers will provide an opportunity to systematically investigate the effect of nanoscale morphology on proton transport.