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AbstractLake Ogallala, a 260-hectare tailwater reservoir, is subject to wide fluctuations of inflow water quality and quantity. These fluctuations impact lake temperatures, water stage, dissolved oxygen (DO), and nutrients. A two-dimensional, continuous simulation hydrodynamic and water quality model, CE-QUAL-W2, was used to simulate dissolved oxygen levels and to quantify DO sources and sinks in the lake. The elements modeled include surges of low DO and temperature; high chemical oxygen demands and dissolved nutrients; response of in-lake algae, macrophytes, and epiphytes; temperature and bathymetric induced circulation patterns; and weather impacts. The model's epiphyte routine was used to simulate macrophytes. The model was able to replicate the diurnal DO fluctuations, which ranged from 2 to 12 mg/L, as well as other water quality parameters. The study found that the chemical oxygen demand depressed daily DO minima by approximately 1.5 mg/L. This study also found that macrophyte respiration was an important factor in the low daily DO minima, depressing daily DO minima approximately 2 mg/L overnight.

Monovinyl methacrylic and acrylic monomers that exhibit enhanced photopolymerization reactivities were studied to discern a possible secondary polymerization mechanism that involves an anionic contribution to the polymerization. To test for this possible anionic contribution, a strong acid, methanesulfonic acid, was added in small quantities to each of the (meth)acrylic monomers, and the steady-state photopolymerization rate was monitored. Initial control studies with conventional free-radical polymerizing monomers showed that the polymerization rate of these conventional (meth)acrylic monomers are not affected by small amounts of acid. Acid studies of secondary functionalized (meth)acrylate monomers, which typically exhibit rapid polymerization kinetics, demonstrated that the polymerization rate is decreased in these monomers by the addition of the strong acid. More specifically, the acid addition had a greater inhibitory effect on the monomers with faster bulk polymerization rates. The acid addition had a much more profound effect on the polymerization rate of the acrylate monomers compared to analogous methacrylate monomers. More specifically, addition of 250 ppm of the strong acid to the phenyl carbamate acrylate monomer showed that both the steady-state and unsteady-state polymerization profiles were similar to that of a traditional free-radical acrylate monomer. All experiments indicate that an anionic contribution to the polymerization is probable and has a profound impact on the polymerization rate.

Novel monoacrylate monomers with a carbamate secondary functionality and a terminal aryl group were synthesized to study the mechanisms that allow for increased reactivity of these monomers compared to typical monoacrylate monomers. To test for the possibility of hydrogen abstraction from the two-carbon aliphatic spacer group, varying methyl substituents were added to one or both of the spacer carbons. Single methylation at the α and β carbons showed no drastic effect on the polymerization rate. However, dual methylation of the α carbon reduced the overall polymerization rate by approximately 5-fold. Conversely, dual methylation at the β carbon reduced the polymerization rate by only 2-fold. Unsteady state analysis of the α,α-methylated monomer showed that the apparent kinetic constants approach that of traditional monoacrylate monomers. Thus, hydrogen abstraction appears to be a likely explanation for these unique substitution effects on monoacrylate reactivity where the substitution number and location have a profound impact on the polymerization rate of these novel monomers. Disubstitution at the α-position has the most profound influence on the rate.

Novel monoacrylate monomers with a carbamate secondary functionality and a terminal aryl group were synthesized to study the mechanisms that allow for increased reactivity of these monomers compared to that of typical monoacrylate monomers. Systematic studies were conducted by varying electron-withdrawing and electron-donating substituents on the aryl moiety. Two primary hypotheses were studied, which were that resonance and electronic effects or hydrogen-bonding effects lead to enhanced reactivity. Kinetic evaluations of these substituted monomers revealed that electronic and resonance effects are not the primary source of the increased reactivity. Moreover, correlations of dipole moment calculations with steady-state polymerization rate behavior revealed that dipole moment is also not the primary source of the increased reactivity. However, comonomer studies showed that, by adding the novel monoacrylate monomers to more slowly polymerizing traditional acrylate monomers, the overall reactivity of the mixture could be increased dramatically, revealing a possible use as reactive diluents. Additionally, hydrogen-bonding strength was analyzed and correlated with steady-state polymerization rate behavior. The hydrogen-bonding and N−H polarization strength of the monomers is not the primary source of the increased reactivity.

Experimental investigations were made into the effects of monomer structure and functionality on free-radical polymerization kinetics. A more comprehensive understanding of how structural characteristics, monomer traits, and polymerization conditions influence the polymerization mechanisms and network evolution was desired. Variations in the nature of the monomers' secondary functionality and the terminal substitution were the primary variables examined. The three factors hypothesized as important to the advantageous polymerization characteristics observed are hydrogen bonding, hydrogen abstraction, and the electronic characteristics of the monomer. The experimental evaluations presented clearly demonstrate that each of these mechanisms contributes to the reactivity of these monomers and the networks that they form. The combination of these factors leads to cross-linked network formation and enhanced polymerization kinetics, i.e., monovinyl monomers with reactivities that rival those of commonly used divinyl monomers.

The photopolymerization kinetics of typical dental dimethacrylate monomers were studied by differential photocalorimetry. Increasing proportions of the low-viscosity diluent monomer triethylene glycol dimethacrylate (TEGDMA) were added to either Bis-GMA (2,2-bis[p-(2‘-hydroxy-3‘-methacryloxypropoxy)phenylene]propane), EBADMA (ethoxylated bisphenol A dimethacrylate), or UDMA (1,6-bis(methacryloxy-2-ethoxycarbonylamino)-2,4,4-trimethylhexane) to provide three base resins that differed in their hydrogen-bonding potential and, therefore, resulted in compositions covering a broad range of viscosities. When compared at similar diluent concentrations, UDMA resins were significantly more reactive than Bis-GMA and EBADMA resins. At higher diluent concentrations, EBADMA resins provided the lowest photopolymerization reactivities. Optimum reactivities in the UDMA and EBADMA resin systems were obtained with the addition of relatively small amounts of TEGDMA, whereas the Bis-GMA/TEGDMA resin system required near equivalent mole ratios for highest reactivity. The hydrogen-bonding interactions, which substantially influence the Bis-GMA and UDMA resin series, were examined by Fourier transform infrared spectroscopy and resin viscosity. Synergistic effects of base and diluent monomer on the polymerization rate and the final conversion were found for the two base resins having hydrogen-bonding interactions. The structures of the individual monomers and, consequently, the resin viscosities of the comonomer mixtures strongly influence both the rate and the extent of conversion of the photopolymerization process.

The bis-GMA (2,2-bis[4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl]propane) and TEGDMA (triethylene glycol dimethacrylate) copolymerization has been often used as the organic phase of dental composites and is a good example of a dimethacrylate/dimethacrylate polymerization involving a viscous monomer and a reactive diluent. This work focuses on the kinetic behavior of this monomer system and the effect of monomer structure, viscosity, and comonomer concentration on the mobility and reactivity of the evolving network. Unsteady-state polymerizations were performed using various compositions of bis-GMA/TEGDMA to obtain the kinetic parameters as a function of double-bond conversion. Furthermore, TEGDMA and bis-GMA were hydrogenated, i.e., the double bonds were saturated, and utilized to study the effect of each reactive component on the copolymerization. The results suggest that reaction diffusion-controlled termination dominates as the reaction progresses and that bis-GMA plays a more controlling role in the kinetics than TEGDMA. In addition, the hydrogenated monomers were used to determine the effect of viscosity on the homopolymerization of dimethacrylates. It was shown that the addition of a solvent, i.e., hydrogenated TEGDMA, to the bis-GMA system had a more pronounced effect on the polymerization kinetics than TEGDMA polymerized in the presence of viscous hydrogenated bis-GMA. The difference in the kinetic behavior was attributed to the relative mobilities of each monomer.

With an appropriate amine catalyst, formaldehyde addition to acrylates results in α-hydroxymethyl acrylate intermediates that undergo a further condensation reaction to provide ether-linked 1,6-diene monomers capable of efficient cyclopolymerization. While 1,4-diazabicyclo[2.2.2]octane (DABCO) is an effective catalyst in the synthesis of these cyclopolymerizable monomers, a series of polymer-supported amine catalysts were prepared and evaluated in an effort to further simplify this process. 3-Quinuclidinol was reacted with methacrylic anhydride to give the functionalized methacrylate monomer, which was copolymerized with methyl methacrylate and triethylene glycol dimethacrylate. Various polymerization techniques (bulk, solution, and suspension) were used to produce the cross-linked polymeric catalysts. Supports with mole fractions of catalyst ranging from 20% to 98% were examined. In reactions with ethyl acrylate and paraformaldehyde, certain polymer-supported catalysts gave near quantitative conversion to products. The rate of reaction with the polymeric catalysts was less than that of reactions with DABCO or other nonpolymeric model catalysts; however, after reaction times of 20 h, product yields and distributions were quite similar. With the polymeric catalysts, product isolation required only simple filtration or a solvent wash rather than an aqueous extraction step. This technique was also applied to the preparation of cyclopolymerizable resins containing multifunctional oligomers.