abstract: The focus of this thesis is to study dissolved organic carbon composition and reactivity along the Colorado and Green Rivers. Dissolved organic carbon (DOC) in large-scale, managed rivers is relatively poorly studied as most literature has focused on pristine unmanaged rivers. The Colorado River System is the 7th largest in the North America; there are seventeen large dams along the Colorado and Green River. DOC in rivers and in the lakes formed by dams (reservoirs) undergo photo-chemical and bio-degradation. DOC concentration and composition in these systems were investigated using bulk concentration, optical properties, and fluorescence spectroscopy. The riverine DOC concentration decreased from upstream to downstream but there was no change in the specific ultraviolet absorbance at 254 nm (SUVA254). Total fluorescence also decreased along the river. In general, the fluorescence index (FI) increased slightly, the humification index (HIX) decreased, and the freshness index (β/α) increased from upstream to downstream. Photo-oxidation and biodegradation experiments were used to determine if the observed changes in DOC composition along the river could be driven by these biogeochemical alteration processes. In two-week natural sunlight photo-oxidation experiments the DOC concentration did not change, while the SUVA254 and TF decreased. In addition, the FI and ‘freshness’ increased and HIX decreased during photo-oxidation. Photo-oxidation can explain the upstream to downstream trends for TF, FI, HIX, and freshness observed in river water. Serial photo-oxidation and biodegradation experiments were performed on water collected from three sites along the Colorado River. Bulk DOC concentration in all samples decreased during the biodegradation portion of the study, but DOC bioavailability was lower in samples that were photo-oxidized prior to the bioavailability study. The upstream to downstream trends in DOC concentration and composition along the river can be explained by a combination of photo-chemical and microbial degradation. The bulk DOC concentration change is primarily driven by microbial degradation, while the changes in the composition of the fluorescent DOC are driven by photo-oxidation. / Dissertation/Thesis / Masters Thesis Chemistry 2015

This research has focused on understanding the mechanicPhysics and chemistry based constitutive framework for thermo-chemo-mechanical responses of polymeric materialsPhysics and Chemistry Based Constitutive Framework for Thermo-Chemo-Mechanical Responses of Polymeric Materialss and multi-physics of soft materials with rate-and temperature-dependent matrices. Such materials are oftentimes exposed to extreme environmental conditions such as Ultra-Violet (UV) light, elevated temperatures, and oxygen which degenerate their mechanical properties and contribute to their permanent failure. The irreversible changes in the mechanical response of polymers induced by such deleterious processes is commonly referred to as polymer aging. The ultimate goal of this work has been to identify the relevant damage processes affecting the lifetime of polymeric materials, and to develop predictive physics- and chemistry-based, thermodynamically consistent constitutive frameworks to evaluate their response under extreme environmental condition. A series of interconnected experimental, theoretical, and numerical studies were developed regarding the chemical, morphological, and mechanical changes that polymers and elastomers exhibit under thermo-photo-chemo-mechanical conditions. Emphasis was placed on linking the aggravation of macrostructural changes (such as cross-link breakage/formation and transformation of linkages) to the macromechanical response of aged polymers, and the development of a mathematically verifiable procedure for incorporating stored and dissipated energies – obtained through chemical experiments – into the thermodynamic formalism. Fracture was considered using the phase-field approach to brittle failure through development of robust and efficient numerical algorithms intended to solve the highly coupled and nonlinear displacement-damage problems. Results demonstrate that several chemical characterization tests such as equilibrium swelling, differential scanning calorimetry (DSC), quartz crystal microbalance with dissipation (QCMD-D), and dynamic mechanical analysis (DMA) can indeed reveal crucial information regarding the physio-chemical changes manifested within polymer networks. Information obtained from these tests can then be employed to propose accurate predictive evolution functions for the mechanical as well as the fracture properties towards a complete physics- and chemistry-based constitutive framework. Numerical analyses were performed within finite element software Abaqus via several user-element and user-material subroutines (UEL, VUMAT) to investigate the predictive capabilities of the developed frameworks in describing complex loading configurations including fracture. The developed constitutive frameworks are all thermodynamics-based and rely solely on the outputs obtained through appropriate chemical characterization techniques. Not only are the predicted results highly accurate, but also and most importantly, the developed constitutive equations are completely self-contained and bypass the need for extra fitting variables. / Doctor of Philosophy / Material science is a fundamental field of research. Understanding how materials behave under various operating conditions can help scientists and engineers propose efficient and economical designs with the aim of potentially establishing a robust foundation for our infrastructure. This work focused on the study and prediction of the deleterious effects of several environmental factors such as elevated temperature, Ultra-Violet (UV) light, and oxygen on the mechanical and failure responses of polymer systems. Several interconnected experimental, theoretical, and numerical studies were developed with the aim of characterizing the chemical, morphological, and mechanical changes that such material systems exhibit under coupled dissipation phenomena. In particular, this research aimed to investigate the aggravation of macrostructural changes that manifested themselves within polymer systems upon exposure to thermo-oxidation and photo-oxidation. Predictive constitutive frameworks were developed based on principles of thermodynamics and continuum damage theories to understand the effects that heterogeneous aging has on the mechanical and fracture responses of these materials. Results achieved in this work helped fill several gaps on both the theoretical as well as numerical sides towards a complete physics and chemistry-based constitutive framework for the analysis of multi-physics phenomena in soft materials. Overall, results shed light on our understanding of the aging process and the predictive capabilities of our proposed equations in describing such degenerative processes as thermo-chemo-mechanical aging. Findings from this work will contribute to the design of high-performance polymers in other applications such as implanted bio-medical devices. Ultimately, describing aging under extreme environmental conditions will contribute to the understanding and prediction of plastic fragmentation processes and therefore, microplastic pollution.