Bayreuther Graduiertenschule für Mathematik und Naturwissenschaften (BayNAT)
Design, Synthesis and Application of Cylindrical Polymer Brushes: From Nanostructures to Advanced Materials
- This thesis focuses on the design, synthesis and application of cylindrical polymer brushes (CPBs). Herein, we investigated the scission behavior of polyelectrolyte CPBs on different surfaces, developed novel synthetic pathways for well-defined CPBs via reversible addition-fragmentation chain transfer (RAFT) polymerization, designed and prepared complex functional CPBs for light-harvesting and energy transfer, and utilized CPBs as templates for the synthesis of novel one-dimensional (1D) organic/inorganic hybrid nanostructures.
The ‘grafting-from’ approach was chosen as the general method to synthesize well-defined CPBs with various chemical and structural compositions. The linear polymer backbones (polyinitiators) were obtained by anionic polymerization or RAFT polymerization, whereas the side chains were grafted by atom transfer radical polymerization (ATRP) or RAFT polymerization. The obtained CPBs possess a narrow molecular weight distribution in both the backbone and the side chains.
The polymer backbone of core-shell CPBs consisting of a poly(oligo(ethylene glycol) methyl ether methacrylate) (POEGMA) core block and a poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) shell block was ruptured upon drying on solid surfaces, when suf-ficient Coulombic interactions between the shell block and the surface were formed. We controlled this scission behavior by tuning the surface interactions through switching the surface nature, shell quaternization, varying pH, or adding multivalent counterions. This study demonstrates that core-shell CPBs serve as a tool to directly compare the weak intermolecular forces with the strong carbon-carbon covalent bonds.
A novel ‘grafting-from’ approach was developed to overcome the challenges of synthesizing well-defined CPBs from a linear polymer backbone with a high density of RAFT functionalities. In this so-called “CTA-shuttled” R-group approach, a certain amount of low-molecular-weight chain transfer agent (CTA) was added to the polymerization system, serving as shuttles to transfer active radicals among the individual growing CPBs. Well-defined CPBs with polystyrene or poly(tert-butyl acrylate) branches and core-shell CPBs with polystyrene-block-poly(N-isopropylacrylamide) branches were synthesized, with the molecular weight distribution much narrower than that from the conventional R-group approach. Monte Carlo simulations confirmed that the advantage of the “CTA-shuttled” R-group approach consists in the release of the active radicals from the trapping CPB systems.
Imitating the natural “energy cascade” architecture, we developed single-molecular, rod-like nano-light harvesters (NLHs) on the basis of CPBs. Herein, a number of block copolymer side chains carrying light absorbing antennae groups (9,9-diethylfluorene, energy donors) were tethered to a linear polymer backbone containing emitting groups (anthracene, energy acceptors). These NLHs provide very efficient energy absorption and energy transfer from antennae to energy acceptors. Furthermore, we were able to manipulate the efficiency of energy transfer by tuning the distance between energy donors and energy acceptors in physical and/or chemical ways. This CPB-based NLH architecture presents a novel concept to design light harvesting materials and can readily be transplanted to any other applications in photoelectronic devices.
Core-shell CPBs with a poly(acrylic acid) (PAA) core block and a PDMAEMA shell block were employed as templates for the preparation of various rare-earth metal cations (Ln3+) incorporated silica hybrid nanoparticles (NPs). A tight chelation of Ln3+ ions in the PAA core and a crosslinked silica layer deposited on the shell provide a very stable encapsulation of Ln3+ ions within the hybrid NPs and thus a high biocompatibility. The silica hybrid NPs obtain unique and diverse properties from the incorporated Ln3+ ions, such as visible photoluminescence, paramagnetic behavior, and a longitudinal relaxation time (T1) shortening effect. This novel template-directed approach succeeds in combining different functional centers via loading in-situ mixed Ln3+ ions into individual CPBs resulting in multicomponent hybrid NPs, which possess both visible photoluminescence and T1 contrast enhancement and can thus be applied as multimodal bioimaging probes.
Functional Triblock Terpolymers for Multicompartment Micelle and Janus Particle Synthesis
- This thesis describes the synthesis of ABC triblock terpolymers with functional moieties via living anionic polymerization, followed by Janus particle (JP) and multicompartment micelle (MCM) synthesis from the as prepared triblock terpolymers.
A synthesis method that can accomplish the tasks of the preparation of spherical as well as non-spherical JPs that are well-defined and in the nanometer size range is based on converting self-assembled triblock terpolymer bulk structures via selective cross-linking of the middle block. Until now such soft JPs were prepared mainly from polystyrene-block-polybutadiene-block-poly(methyl methacrylate) and polystyrene-block-polybutadiene-block-poly(tert-butyl methacrylate). However, these polymers do not offer many possibilities of chemical alterations and stimuli-responsive elements.
Therefore, potential new functional monomers for the use in JP synthesis from triblock terpolymer bulk structures were identified and their anionic polymerization examined, p-tert-butoxystyrene (tS) and 4-(dimethylaminomethyl)styrene (DMAMS). Polymers with low polydispersity indices could be prepared from both monomers in tetrahydrofuran (THF) with sec-butyllithium (sec-BuLi) as initiator.
Poly(p-tert-butoxystyrene) (PtS) was hydrolyzed to poly(p-hydroxystyrene) (PHS) which is water-soluble at high pH values, opening the possibility to prepare water-soluble JPs. The pH-responsive behavior of poly(4-(dimethylaminomethyl)styrene) could be confirmed and for the first time an LCST behavior was documented with cloudpoints of 59.3 °C at pH 7 and 28.5 °C at pH 8.
PtS was then used in the preparation of two triblock terpolymers, poly(tert-butoxystyrene)-block-polybutadiene-block-poly(tert-butyl methacrylate) (tSBT) and poly(tert-butoxystyrene)-block-polybutadiene-block-poly(2-(dimethylamino)ethyl methacrylate) (tSBD). tSBT exhibited a lamella-cylinder (lc) bulk morphology with polybutadiene (PB) spheres surrounded by alternating lamellae of PtS and poly(tert-butyl methacrylate) (PtBMA). However, the bulk structure of tSBD consisted of a symmetrical lamella-lamella pattern that is not suitable for JP synthesis.
From tSBT bulk material, three different types of non-spherical JPs could be obtained. Photo-cross-linking of the lamella-cylinder (lc)-morphology by co-casting a radical photo-initiator and UV exposure resulted in the expected Janus cylinders. When the bulk material was first swollen in acetonitrile and cross-linked by cold vulcanization, Janus sheets were obtained. Swelling in acetonitrile/decane emulsion lead to a new type of JPs, Janus ribbons. In both cases a phase transition had occurred; in case of the Janus sheets a thin PB layer had formed between the original PB cylinders, resulting in an undulated-lamella morphology. For the formation of Janus ribbons a connecting PB phase had formed in every second interspace along the major axis of the cylinders. Casting a tSBT film from tert-butanol, a non-solvent for PB, also enabled the synthesis of spherical JPs. This way, the importance and versatile application of swelling agents and cross-linking methods for the preparation of JPs from bulk structures was demonstrated. The obtained Janus cylinders were hydrolyzed to have one PHS and one poly(methacrylic acid) (PMAA) hemicylinder, resulting in water-soluble particles.
Further, solution structures of tSBD and tSBT triblock terpolymers were investigated. In water, tSBD formed core-corona micelles that exhibited pH-responsive and LCST behavior due to the responsive poly(2-(dimethylamino)ethyl methacrylate) corona. Employing the novel method of directed hierarchical self-assembly, “football” MCMs were obtained from tSBD whereas tSBT formed “clover” structures. Cross-linking of the B block in such MCMs and their subsequent dissolution in a solvent for all three blocks can be used to prepare spherical JPs. However, as tSBD MCMs existed in water, no sufficient cross-linking method could be found to cross-link the organic PB phase within the aqueous solution. For tSBT MCMs in ethanol the standard approach of adding a photo-initiator to the MCM solution followed by UV exposure was successfully employed and spherical JPs were obtained. These were again hydrolyzed to acquire water-soluble JPs. Apart from some isolated single Janus spheres, cryogenic transmission electron microscopy mainly revealed the formation of “clover” and “hamburger” oligomers possibly due to the slightly better solubility of PMAA. Here, the applicability of the concept of directed hierarchical self-assembly to create MCMs was demonstrated for two different triblock terpolymers and the corresponding method of solution-based JP synthesis was successfully conducted for tSBT.
Investigations towards a better understanding of arsenic-sulfur speciation in aquatic environments: Formation, stability, structural characterization, and conflicting analyses
- Arsenic is a widespread contaminant of global concerns due to its neurotoxicity and carcinogenicity. Particularly critical is the speciation of arsenic, influencing its mobility, toxicity and retention capability. Recently, it was analytically proven that arsenic-sulfur (As-S) species play a dominant role for arsenic cycling in sulfidic systems. The geochemistry of As-S species is not well investigated, yet, and especially the nature of these species, thioarsenites vs. thioarsenates, has been under intense debate. The major objective of the present PhD work was to improve the current knowledge about As-S species by structural characterization, investigations of their occurrence, formation and transformation, and evaluation of the analytical techniques, X-ray absorption spectroscopy (XAS) and ion chromatography coupled to ICP-MS (IC-ICP-MS). By XAS it was shown that under strictly anoxic conditions thioarsenites form in arsenite-sulfide mixes with sulfide (SH-) excess and as co-occurring intermediates during acidic transformation of thioarsenates. Thioarsenites can be specified as highly labile, converting rapidly to thioarsenates in the presence of traces of oxygen, e.g. during standard IC-ICP-MS analyses. Excess hydroxide (OH-) either due to high pH or sample dilution in ultrapure water inhibits the formation of thioarsenites by SH--OH- competition. These facts make the current IC-ICP-MS method even under anoxic conditions unsuitable for thioarsenite analyses. However, thioarsenites were shown to be necessary intermediates for formation of thioarsenates. Thioarsenates determined in natural oxic systems are thus most likely the product of rapid in-situ thioarsenite oxidation. Direct thioarsenite determination is currently only possible by XAS with a limitation on > 5 mM-solutions for structural evaluations. The characteristic coordination and bond length (RAs-S 2.23-2.28 Å) makes thioarsenites distinguishable from thioarsenates (RAs-O 1.70 Å, RAs-S 2.13-2.18 Å). The individual thioarsenates are distinct in their coordination and absorption edge energies, successively decreasing about 1 eV per sulfur atom. Generally, the absorption edge energies decrease in the order arsenate > thioarsenates > arsenite > thioarsenites. This primary XAS-dataset enables the evaluation of (thio)arsenites and (thio)arsenates in mixed solutions. Despite the greater stability of thioarsenates vs. thioarsenites, they also have been shown to transform under certain conditions. Upon acidification they convert to thioarsenites (anoxic) or arsenite (oxic) with subsequent As-S precipitation. The presence of FeII in anoxic solutions or heating (80 °C) results in their decay to substantial amounts of arsenite. Thioarsenates are also easily oxidized by synthetic oxidants, air purging or naturally along hot spring drainage channels. For trithioarsenate, the major species of alkaline hot springs in Yellowstone National Park, two transformation processes have been identified: successive ligand exchange to arsenate, observed naturally and by using a strong oxidant, and the decay to arsenite (and trithioarsenate) in natural systems and under moderately reducing conditions. However, transformation under natural conditions was up to 500 times faster and is likely catalyzed by Thermocrinis spp.. Naturally important are also processes promoting mobilization or immobilization of arsenic from and at mineral surfaces. Arsenopyrite and orpiment belong to the most abundant (Fe-)As-S minerals with particular importance as host rocks for gold refractory. Oxidative leaching of both minerals yielded up to 50% thioarsenates. The release of thioarsenates from orpiment, at pH 7 and 12, is possibly caused by thioarsenite oxidation. Contrary, physisorption of OH- is the proposed mechanism for arsenopyrite with thioarsenate formation only at highly alkaline pH. The immobilization of monothioarsenate by sorption on ironhydroxide was less effective and kinetically slower compared to arsenate and arsenite. The presence of iron in As-S systems was hitherto considered to counteract thioarsenate occurrence. This was refuted by finding up to 17% thioarsenates in Czech spring waters. However, those Fe As S systems are a challenge for sample preservation. While acidification results in As-S precipitation and thioarsenate transformation, flash-freezing as preferred for thioarsenates induces ironhydroxide precipitation. An anoxic gas headspace, a strong matrix and an organic solvent supported the stability of pure thioarsenate solutions, whereas in the presence of iron a combination of EDTA-addition and cryo-preservation is required. Overall, the present PhD thesis reveals the importance of thioarsenites and thioarsenates for arsenic cycling. The results significantly increase the present knowledge on As-S geochemistry and help to define potential for future studies.
Struktur-/Funktionsbeziehung rekombinant hergestellter Proteine aus dem Byssusfaden der Miesmuschel Mytilus galloprovincialis
- Marine Muscheln der Gezeitenzone, wie die Miesmuschel Mytilus galloprovincialis, haften mittels eines Byssus am Untergrund. Der Muschelbyssus besteht aus einzelnen Fäden, welche aufgrund von Strömungen und Wellengang extremen mechanischen Belastungen ausgeliefert sind und aus diesem Grund eine hohe Energieaufnahmefähigkeit besitzen. Unter anderem dieser Umstand macht die Byssusfäden zu einem interessanten biologischen Material. Byssusfäden gliedern sich in drei morphologisch unterscheidbare Abschnitte mit unterschiedlichen mechanischen Eigenschaften, einen elastischen proximalen Teil, einen distalen Teil mit hoher Steifigkeit und einen adhäsiven Plaque, welcher die Substratbindestelle darstellt. Der Byssus besteht fast ausschließlich aus Proteinen, wovon wiederum ein großer Teil des Fadens durch fibrilläre Kollagene repräsentiert ist, welche in eine Proteinmatrix eingebettet sind.
Nahezu alle Proteine des Byssusfadens weisen z. T. einen hohen Grad an posttranslationalen Modifikationen auf, allen voran die Hydroxylierung von Tyrosinen zu 3,4-Dihydroxy-phenylalaninen (DOPA). Diese Reste nehmen eine Schlüsselrolle für die kohäsiven und adhäsiven Eigenschaften der Byssusproteine ein. Ferner spielt die Oxidation von DOPA zum entsprechenden o-Chinon eine wichtige Rolle bei der Quervernetzung der Proteine im Zuge der Reifung bzw. chinonbasierter Gerbung (quinone tanning) der Fäden. Diese Reaktion wird von einer im Byssusfaden detektierten Catecholoxidase katalysiert, von der bis dato jedoch keine Aminosäuresequenz bekannt war. Im Rahmen dieser Arbeit wurde erstmalig die Sequenz einer solchen mutmaßlichen Byssuscatecholoxidase in einer cDNA-Bank des Muschelfußes identifiziert und eingehend bioinformatisch hinsichtlich Lokalisierung und potenzieller Tertiärstruktur charakterisiert. Das entsprechende Protein weist einige Charakteristika auf, die die Identität mit einer von J. H. Waite aus dem Byssusfaden isolierten Catecholoxidase wahrscheinlich machen. Neben einer katalytischen Domäne, welche Homologie zu bekannten Catecholoxidasen bzw. Tyrosinasen aufweist, zeigt das Protein ferner weitere spezielle Bereiche, die eine strukturelle Beteiligung des Proteins am Byssusfaden nahelegen.
Den Hauptteil der Arbeit stellt die Analyse eines der bekannten Matrixproteine dar, des Proximalen Fadenmatrixproteins 1 (PTMP1), welches zwei Von-Willebrand-Faktor Typ A-ähnliche (VWA) Domänen besitzt. Hierzu wurde zunächst die Sequenz von PTMP1 aus einer cDNA-Bank des Muschelfußes isoliert und kloniert. Anschließend erfolgte die heterologe Expression und rekombinante Produktion des Proteins und einiger davon abgeleiteten Varianten in E. coli. Da alle Proteine in Form unlöslicher bakterieller Inclusion Bodies gebildet wurden, erfolgten die Etablierung einer Reinigungs- und Rückfaltungsstrategie sowie die chromatographische Analyse und spektroskopische Charakterisierung. Dabei wies PTMP1 zwei monomere Isoformen mit unterschiedlicher Disulfidverbrückung auf. Die Kristallstruktur von PTMP1, welche die erste bekannte Kristallstruktur eines Byssusproteins darstellt, zeigte eine neuartige Anordnung der beiden VWA-Domänen, welche dabei durch einen hochgradig stabilisierten Linker verbunden sind. Ferner konnte im Kristall die Bindung zweier Zinkionen an PTMP1 nachgewiesen werden, wobei jedoch nur das MIDAS-Motiv der A1-Domäne von PTMP1, jedoch nicht das der A2-Domäne diese Bindung aufwies. Die zweite Bindestelle wurde stattdessen im Bereich des Domänenlinkers detektiert. PTMP1 wies eine außergewöhnliche strukturelle Stabilität auf, insbesondere gegenüber thermischer Denaturierung. Die zuvor postulierte Fähigkeit von PTMP1 zur Bindung von Kollagenen konnte bestätigt und vertiefend analysiert werden. Dabei zeigte sich ein signifikanter Einfluss der Bindung von der
Ionenstärke, sodass von elektrostatischen Interaktionen zwischen PTMP1 und Kollagen ausgegangen werden kann. Darüber hinaus besitzt PTMP1 die Fähigkeit zur Bindung tripelhelikaler kollagenartiger Strukturen. Die geordnete Assemblierung präformierter Kollagenfibrillen erwies sich in Anwesenheit von PTMP1 als signifikant gestört. Darüber hinaus beeinflusst PTMP1 die Bildung von Fibrillen aus löslichem Kollagen, was eine Beteiligung dieses Matrixproteins an der Assemblierung der Byssuskollagene und somit einen Einfluss auf die mechanischen Eigenschaften des proximalen Byssusfadenteils nahelegt.
The origins of olivine fabric transitions and their effects on seismic anisotropy in the upper mantle
- Convecting mantle plays a central role in the thermal and geochemical evolution of the Earth. It provides the principal force responsible for major geological features such as mountains and ocean basins. Plate tectonics and its violent consequences such as earthquakes and volcanoes are all manifestations of the dynamics of the convective mantle. Shearing forces generated by mantle convection leads to lattice preferred orientation (LPO) of the major upper mantle mineral phases. LPO that develops in this way is thought to be the principal cause behind seismic anisotropy in the upper mantle, which can consequently be used to chart convective flow of the mantle.
Strong changes in seismic anisotropy occur in the top 300 km of the upper mantle where olivine is the principal mineral. In this study a solid media high pressure deformation apparatus, called the deformation-DIA or D-DIA, has been used to deform aggregates of San Carlos olivine in simple shear geometry at pressures between 3 and 8.5 GPa and temperatures from 1300-1500°C. As part of this project a high pressure and temperature solid-media cubic assembly was developed to facilitate these experiment that employed alumina pistons cut at 45° to shear the sample but minimized cold deformation of the sample by employing initially porous alumina in the sample column. Once stable high pressures and temperature were reached the cubic assembly was deformed by compressing two vertically oriented anvils of the D-DIA, while the four horizontally oriented anvils were maintained at a constant loading force. This assembly shortening led to shearing of the olivine sample. Recovered samples were analyzed for fabric development employing electron backscattered diffraction (EBSD) and microstructure was observed using transmission electron microscopy (TEM).
Experiments were performed at each pressure and temperature as a function of strain rate and H2O content. In dry olivine deformation experiments performed at slower strain rates an A-type fabric dominated at all pressures and temperatures, implying deformation by dislocation glide through the (010) slip system. At higher strain rates evidence for the B-type fabric was observed, suggesting increased activity of the (010) slip system at higher stresses. Recrystallization grains size and dislocation densities were used to estimate stresses in the samples and a good correlation was observed between strain rate and estimated flow stresses. Dry experiments from 8.5 GPa and 1500°C exhibited no LPO, which may be an indication for deformation through diffusion accommodated grain boundary sliding at these conditions. No indication was found that pressure influences the dominant slip system in olivine, in contrast to previous studies. It is considered that previously reported incidences of pressure effects can in fact be attributed to the development of higher stresses in experiments performed at higher pressures.
Fabrics in H2O bearing olivine deformed at similar conditions revealed the overriding dominance of the C-type fabric, developed through action of the (100) slip system. Variations in pressure, temperature and strain rate had little influence on this fabric development. TEM observations confirmed the presence of dislocations with slip systems consistent with the development of the macroscopic fabrics. Viscoplastic self consistent modeling was employed to understand the development of fabric in the samples and to estimate the relative contributions of variations slip systems to the developed fabrics.
These results are used to construct an olivine fabric map which is found to be consistent with some previous studies at lower pressures. It is argued that the decrease in seismic anisotropy observed in the top 300 km of the upper mantle cannot originate from a pressure induced change in the dominant olivine deformation fabric. Instead it is argued that changes in the H2O content of olivine with depth cause a shift in the dominant fabric from A-type to C-type, with a possible excursion through the E-type fabric, dominant slip system (001), which was, however, not observed in this study. Modeling is used to show that this variation in fabric with depth can cause the observed weakening the seismic anisotropy in the upper mantle if the olivine H2O content increases from below 100 ppm at 50 km to 250 ppm at 300 km. Rather than implying an increased in the H2O content of the mantle with depth, however, it is argued that this change in olivine H2O content can be caused by changes in the H2O olivine-pyroxene partition coefficients with depth, for a fixed bulk mantle H2O content of 200 ppm.
Similar deformation experiments performed on a peridotite assemblage at 8.5 GPa and 1300°C indicate identical olivine fabrics to those observed in monomineralic experiments at the same conditions. Fabrics for diopside and enstatite were found to be similar to those found in previously performed lower pressure experiments.
Experiments on a piezoelectric single crystal of GaPO4 were performed in the D-DIA and 6-ram MAVO press at high pressures in order to measure charge on the crystal developed through the application of deviatoric stresses. Electrical charges were measured through the use of an operational amplifier. Experiments performed at room temperature using a developed cubic assembly were successful in measuring quantifiable electrical charges resulting from the advancement of the deformation anvils by as little as 0.5 µm. Although the piezoelectric constant for this material is not yet calibrated at high pressures, stresses were estimated from the measured charges and measureable values were in the range 4-350 MPa.
Flow and transport processes as affected by tillage management under monsoonal conditions in South Korea
- A sustainable agriculture, which provides enough yields to satisfy the food demand and minimizes the impacts on ecosystem services such as provision of high water quality, is challenging especially in monsoon regions. In this thesis, plastic mulched ridge cultivation (RTpm) under monsoonal conditions and its impact on flow processes and nitrate transport was investigated.
On hillslopes, we monitored surface and subsurface flow processes in four plastic mulched potato fields using a network of tensiometers, FDR sensors, runoff collectors and flow dividers as well as Brilliant Blue FCF tracer experiments. The obtained datasets were used to calibrate the process-based models HYDRUS 2/3D and EROSION 3D in order to quantify drainage water fluxes, surface runoff and erosion rates of RTpm compared to ridge tillage without coverage (RT) and conventional flat tillage (CT). In a flat terrain, N fate under fertilizer rates at 50, 150, 250 and 350 kg NO3− ha−1 was investigated in a plastic mulched radish field using suction lysimeters, tensiometers and a 15N tracer experiment. We used datasets of nitrate concentrations and matric potentials to calibrate a water flow and solute transport model using the numerical code HydroGeoSphere.
RTpm affects soil water dynamics dominantly during dry periods, when ridge soil was drier compared to furrow soil caused by the protective plastic coverage and root water uptake in ridges. Hence, pressure head gradients induced lateral flow from furrows to ridges. Under monsoonal conditions, soil was fully saturated and down slope lateral flow occurred in the coarse textured topsoil. The dye tracer experiments showed that matrix flow dominated in the sandy topsoil. Lateral funnel flow above the tillage pan was the prominent preferential flow path. Unexpectedly, macropore flow in deeper soil horizons was not detected. The field and modeling studies revealed that surface runoff was substantially increased under RTpm compared to RT and CT. However, the field topography primarily controlled surface runoff and erosion rates. The concavity of the field led to flow accumulation and high erosion losses in the center of the field, while a convex shape resulted in less soil erosion.
NO3− leaching was found to be the prominent pathway especially during the early season. Furthermore, the biomass production did not significantly differ between NO3− fertilizer rates of 150 to 350 kg ha−1. Hence, we recommend NO3− fertilizer application of 150 kg ha−1, a better fertilizer placement and split applications. We simulated whether the given recommendations on fertilizer best management practices (FBMPs) decreased NO3− leaching amounts. Compared to RT under conventional fertilization in ridges and furrows, the simulations showed that NO3− leaching can be considerably reduced up to 82% by combining RTpm, fertilizer placement only in ridges and split applications with a total fertilizer NO3− amount of 150 kg ha−1.
Based on these findings, the impact of RTpm on flow and transport processes has to be evaluated differently depending on terrain complexity. In a flat terrain, where surface runoff processes are absent or minimal, RTpm has several advantages. Beside functions such as weed control, and earlier plant emergence due to higher temperatures, plastic mulching decreases drainage water and NO3− leaching. Thus, RTpm enhances nutrient retention below the plastic mulch and reduces NO3− groundwater contamination risk. On slopes, where precipitation contributes substantially to surface runoff, RTpm even increases runoff, soil erosion and surface leaching of agrochemicals into aquatic systems.
This thesis provides several recommendations, aiming to minimize environmental impacts and to decrease costs of fertilizer and herbicide inputs. To reduce surface runoff and soil erosion at sloped fields, we suggest applying perforated plastic mulch and a ridge configuration following contours of the field. Furthermore, we recommend omitting application of herbicides in furrows to allow weed growth, which slows down runoff processes. These suggestions would increase infiltration and subsurface flow processes automatically become more important. However, absent preferential flow to deeper soil layers indicated a low groundwater contamination risk. Since funnel flow above the tillage pan was found to be the most important preferential flow path, we propose the establishment of riparian buffer zones. This would also help to reduce the discharge of sediments and fertilizers via surface runoff into the streams. Finally, FBMPs such as fertilizer placement only in ridges and split applications were found to decrease nitrate leaching considerably. Hence, we suggest applying FBMPs with impermeable plastic mulch in flat terrain, while on hill slopes FBMPs should be applied with perforated plastic mulch. To reduce the leaching and erosion risk after harvest when the plastic mulched ridges are removed, we recommend cultivating cover crops.
Janus Particles at Interfaces
- This thesis describes the synthesis and the characterization of both polymeric and hybrid Janus particles of well-defined size, shape and functionality and their high potential for applications in colloidal and material science.
Soft Janus particles, based on polystyrene-block-polybutadiene-block-poly(methyl methacrylate) (SBM) triblock terpolymers, represent a fascinating group of polymeric materials because their size, shape and functionality directly influences their adsorption behavior at liquid-liquid interfaces.
The adsorption behavior of Janus cylinders at liquid-liquid interfaces was studied using the pendant drop technique. The interfacial tension decreases with increasing Janus cylinder length and concentration. From the time evolution of the interfacial tension the characteristics of early and late stages of the Janus cylinder adsorption were specified. A series of TEM images of the liquid-liquid interface taken during the cylinder adsorption confirm these observations. Janus cylinders behave differently at the interfaces as compared to the block terpolymer precursor SBM and to cylinders of comparable sizes with a polybutadiene core and a homogeneous polystyrene shell.
Understanding the influence of particle size and architecture on the adsorption process is a very important criterion for an efficient industrial use of the Janus particles. To establish the effect of the Janus character together with the effect of particle shape on the interfacial activity and orientation of the Janus particles at an liquid-liquid interface, we present a combination of experimental and simulation data together with detailed studies elucidating the mechanisms governing the adsorption process of Janus spheres, Janus cylinders and Janus discs. These studies demonstrate that changes in the geometry of the particles strongly influence the stabilization of the liquid-liquid interface. As the shape changes from spheres to discs and cylinders, different adsorption kinetics, different packing behavior, different energy barriers and finally different equilibrium values for the interfacial tension can be found.
Another main point of this thesis was the synthesis of functional and/or stimuli-responsive hybrid core-shell-corona Janus particles based on inorganic colloids and the characterization of their unique properties and fascinating self-assembly behavior. The first step towards these Janus particles was to understand in detail the formation of core-shell-corona particles with a homogeneous corona, and then in a second step, to use our new knowledge to create hybrid core-shell-corona Janus particles.
We developed an easy and completely reproducible preparation and characterization of the solution behavior and functional properties of superparamagnetic and/or fluorescent, thermo-responsive inorganic/organic hybrid nanogels with an intermediate protective silica shell and an interactive polymer layer. These well-defined multifunctional nanogels were prepared via two consecutive encapsulation processes of superparamagnetic and/or fluorescent semiconductor nanocrystals with a silica layer and a crosslinked and responsive polymer poly(N-isopropylacrylamide) (PNIPAAm) corona. The precise adjustment of the conditions allows to achieve a reliable encapsulation and to either entrap several particles or single ones and to precisely tailor the thickness of the silica shell. Full functionality of the encapsulated nanocrystals is retained, but excellent wettability, biocompatibility, flexible surface chemistry, increased chemical stability are implemented together with a thermo-responsive polymer corona.
On the basis of our well-characterized core-shell particles we took advantage of the variable surface chemistry of the silica shell to combine the properties of the superparamagnetic core-shell nanoparticles with the catalytic character of nickel complexes in hybrid core-shell-corona nanoparticles forming heterogeneous nanocatalysts. In that way a heterogeneous catalyst was created for facile product separation in the catalytic conversion of olefins.
In the next level, an efficient strategy for the large-scale synthesis of well-defined hybrid Janus particles with a silica core (˂˂ 100 nm) and a stimuli-responsive PDMAEMA hemicorona was developed. The synthesis is based on a modified version of the Pickering emulsion technique in combination with surface-initiated atom transfer radical polymerization (ATRP) in a “grafting from” approach. First, 30 nm silica nanoparticles are immobilized at the interface of sub-micrometer sized droplets of poly(vinyl acetate). Since the nanoparticles are partially embedded, one hemisphere is protected. After the modification with an ATRP-initiator and the detachment of the modified silica particles, PDMAEMA was grafted from one hemisphere via ATRP. The obtained Janus nanoparticles are well-defined in size and shape and show stimuli-responsive structural changes depending on pH and temperature.
Iron spin crossovers at high pressures and temperatures and their effects on materials relevant tot he Earth’s lower mantle and core
- Iron is the most abundant element by mass in the Earth. The iron content and its spin or oxidation state have a major influence on the physical properties of the main phases in the Earth’s interior. Therefore it is of vast importance to understand the behavior of iron in mineral phases at the temperature and pressure conditions of the Earth’s interior. This cumulative thesis investigates Fe spin crossovers in iron-containing magnesium aluminum silicates, iron-bearing silicate glasses, the iron carbide Fe3C and the effect of Fe spin crossovers on the Fe/Mg partitioning between perovskite and ferropericlase in pyrolitic model system of the Earth’s lower mantle. The goal is first to understand the nature of the Fe spin crossover in respect to its oxidation state and second to estimate the consequences of their occurrence to processes and the structure in the Earth. Central tools in these studies are laser heated diamond anvil cells, to reach the pressure and temperature conditions of the Earth’s interior, Mössbauer spectroscopy, which is a sensitive probe for detecting structural and spin changes in Fe-bearing materials, and analytical transmission electron microscopy, as a probe of chemistry and oxidation state on the nm-scale. In this cumulative thesis I present the results of five research articles. For the analysis of conventional and recently developed synchrotron energy domain Mössbauer spectra the computer program MossA is introduced, which builds the basis for the analysis and interpretation of the results for the other studies. Based on synchrotron Mössbauer spectroscopy and electrical conductivity measurements of Fe-bearing silicate aluminum perovskite it is shown that Fe3+ occupies the dodecahedral A-site of the perovskite structure and remains in the high-spin state throughout the pressure and temperature conditions of the Earth’s lower mantle. Furthermore, a study on the electronic behavior of Fe in a Fe2+-rich aluminous silicate glass and a Fe3+-rich sodium silicate glass infers that no sharp high spin to low spin crossover occurs in silicate melts in the Earth’s lower mantle. This result excludes the possibility of negatively buoyant melts in the lower mantle in an early magma ocean solely due to strong preferential partitioning of iron into the melt phase, which would be induced by a Fe low-spin bearing melt. New insights into to decoupled partitioning behavior of Fe2+ and Fe3+ between the two dominant phases of the Earth’s lower mantle, perovskite and ferropericlase, are presented. The intermediate spin to low spin crossover of Fe2+ in perovskite at about 110 GPa seems to have a strong effect on partitioning and oxidation state of Fe. It leads to a change of the partitioning behavior of Fe between perovskite and ferropericlase and induces a reduction of Fe3+ to Fe2+ in perovskite. Finally, a Mössbauer spectroscopic and single-crystal x-ray diffraction study of Fe3C reveals a two-stage loss of magnetism in Fe3C at high pressures at room temperature: a ferro- to paramagnetic transition around 8-10 GPa and a para- to nonmagnetic transition at about 22 GPa.
"The New Chemistry" - Sustainable Catalysis with Alcohols
- Subject of the thesis are new iridium complexes stabilized by anionic P,N- or P,N,P-ligands. These complexes were used in homogeneous catalysis. Furthermore, mechanistic studies were performed to provide an insight into the catalytic cycles. Synthesis protocols for a multitude of different product classes have been developed.
The iridium complex 1, stabilized by a neutral P,N-ligand, reacts under basic conditions with 2-aminopyridines. By elimination of dipyridylamine the new catalyst species 2a was formed, which is more stable than catalyst 1.
Based on this finding eight new anionic P,N-ligands and the resulting iridium complexes were synthesized.
After optimization of the reaction conditions (solvent, base, temperature and catalyst loading) these catalysts were used in BH (borrowing hydrogen)/HA (hydrogen autotransfer) reactions. The selective monoalkylation of anilines with primary alcohols was investigated. In comparative experiments the superiority of the new class of catalysts versus the original catalyst 1 was clearly shown. Under mild reaction conditions (70 °C) the selectivity profile with respect to the monoalkylation has been preserved.
The catalytic protocol was subsequently extended to the alkylation of aromatic diamines. Therefore various diaminobenzenes were used as substrates. Also Dapsone®, an important drug in treatment of leprosy could be used as starting material. We succeeded in both symmetric and unsymmetric monoalkylations of diamines. Due to the selectivity profile of the catalyst regarding aromatic amines, also unprotected amino alcohols could be used as alkylating reagents.
By the use of tridentate P,N,P-ligands, a novel class of more stable catalysts compared to complexes 2a-9a, could be developed. Due to sealing the synthesis reactor with a semipermeable membrane, the retransfer of the “borrowed” hydrogen could be prevented and H2 is released. Dehydrogenation and condensation steps are now possible instead of BH/HA. By reacting secondary alcohols with β-amino alcohols, pyrroles were accessible.
After adapting the synthesis protocol to this new class of products the tolerance of functional groups was tested. Diversely functionalized alcohols were used. Under mild reaction conditions (90 °C) and very low catalyst loadings (down to 0.03 mol% iridium), a large number of novel pyrroles was accessible. Using this protocol 21 differently substituted α,α-pyrroles, 12 bicyclic pyrroles, symmetrically as well as non-symmetrically substituted oligopyrroles and three β-aminopyrroles were synthesized. The catalyst resting state was identified by NMR experiments and X-ray structure analysis to be an iridium trihydride. This trihydride is formed under catalytic conditions, by treatment of the pre-catalyst with alcohols or in hydrogen atmosphere.
In the final part of the work, a catalytic pyridine synthesis was developed. In this so far unknown heterocycle synthesis up to four different substituents could be introduced within a single reaction step. 2,6-, 2,5-, 2,4- and 2,3-substituted pyridines were synthesized selectively by using variously substituted primary or secondary alcohols and γ-amino alcohols. Furthermore, both the synthesis of bicyclic pyridines as well as the synthesis of pyridines that bear chiral substituents is possible.
Dual-Responsive Polymer and Hybrid Systems: Applications for Gene Delivery and Hydrogels
- This thesis focused on the synthesis of functional materials based on water-soluble and responsive polymers, in particular poly((2-dimethylamino)ethyl methacrylate) (PDMAEMA). The dual-responsive behavior and polycationic character at physiological pH of PDMAEMA lead to outstanding properties and thus, to a versatile component for water-based applications. The main concept of the thesis was to combine the ability for gene delivery of PDMAEMA with the magnetic properties of iron oxide nanoparticles to enable an activity of the gene vector in an applied external magnetic field. Another point was to apply the dual-responsive behavior of PDMAEMA (temperature and pH) for physically cross-linked hydrogels.
Initial studies on magnetic dual-responsive gene vectors revealed a facile synthesis of PDMAEMA-grafted iron oxide nanoparticles utilizing dopamine as physically binding anchor group for the polymer chains. Here, a dopamine-based ATRP initiator was applied for the surface modification of the nanoparticles, which enabled a controlled polymerization technique via the “grafting-from” approach. Gene transfection experiments with CHO-K1 cells show that the transfection efficiency is significantly higher than for poly(ethyleneimine) (PEI), which is regarded as the “gold standard” among the polycationic gene vectors. Although the hybrid particles show a considerably high molecular weight (4.3 MDa), which should lead to a significant increase of the cytotoxicity as observed for linear PDMAEMA their cytotoxicity is remarkably low, lower than that of PEI. Thus, the excellent performance in gene delivery experiments can be attributed to the star-like architecture of the PDMAEMA. Moreover, the uptake of our superparamagnetic gene vector into the cells enables a magnetic cell separation by applying an external magnetic field.
However, due to the non-covalent bonds of dopamine to the iron oxide nanoparticles, the PDMAEMA chains undergo a detachment with time from the nanoparticle surface. This led to the synthesis of PDMAEMA-based magnetic core-shell-corona nanoparticles. Here, the iron oxide nanoparticles were covered with a thin silica shell in order to link the PDMAEMA chains covalently to the inorganic core via silane chemistry. This approach revealed stable dual-responsive hybrid nanoparticles with irreversible binding of the polymer chains and a high long-term stability in aqueous media. These hybrid star-like particles also show excellent gene delivery. The inter-polyelectrolyte complex formation between the PDMAEMA corona of the core-shell-corona particles and pDNA showed that the pDNA molecules are individually complexed with single nanoparticles at N/P ratios (polymer nitrogen / pDNA phosphorous) where the best transfection results are obtained. The magnetic cell separation was further improved by using a Magnetic Activated Cell Sorting system (MACSTM). The magnetically separated cells maintain a high transfection efficiency as well as viability and could even be further cultivated.
Another aspect of this thesis was to include PDMAEMA as stimuli-responsive block in a double switchable block copolymer-based hydrogel. For this purpose, we chose a physically cross-linked ABCBA pentablock terpolymer system, which was polymerized via sequential ATRP and consist of a water-soluble PEO middle block, two dual-responsive (temperature/pH) PDMAEMA B-blocks as well as two thermo-responsive poly(di(ethylene glycol) methyl ether methacrylate) (PDEGMA) A-blocks (PDEGMA-b-PDMAEMA-b-PEO-b-PDMAEMA-b-PDEGMA). The aggregation behavior in dilute solution was investigated via temperature-dependent Dynamic Light Scattering (DLS) revealing that both stimuli-responsive blocks can be triggered separately and the coil-to-globule transition temperatures of the stimuli-responsive blocks were found to be strongly dependent on the block lengths for low molecular weights. In concentrated solutions, however, rheology studies did not show a further change in the mechanical properties after gelation for the investigated ABCBA pentablock terpolymer compositions. As a result, the principle of our complex system points towards a successful synthesis of a dual-responsive ABCBA pentablock terpolymer hydrogel system, which may show two distinct phase transition even for the gel state, if longer block lengths of the outer A- and B-blocks would be applied.