Synthesis and Self-Assembly of Novel ABC Miktoarm Star Terpolymers
- A novel synthesis for ABC miktoarm star terpolymers and their self-assembly into complex superstructures in aqueous solution are described within this thesis. To this aim a modular route for such materials was developed, combining anionic polymerization and copper-catalyzed azide-alkyne cycloaddition. At the example of ABC miktoarm star terpolymers and an ABA’ miktoarm star copolymer containing a poly(N-methyl-2-vinypyridinium iodide) (P2VPq) segment, the counterion-mediated superstructure-formation of complex shaped aggregates was thoroughly investigated.
The key compound of the combinatorial synthesis is the newly synthesized 4-alkyne-substitued diphenylethylene derivative 1-[(4-(tert-butyldimethylsilyl)ethynyl)phenyl]-1-phenylethylene (“click-DPE”). This was applied in sequential anionic polymerization to prepare well-defined alkyne mid-functional diblock copolymers composed of polybutadiene (PB) as first and poly(tert-butyl methacrylate) (PtBMA), poly(2-vinylpyridine) (P2VP), or poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) as second block. The alkyne-midfunctional diblock copolymers were afterwards conjugated with azido-functional polystyrenes (PS), poly(ethylene oxide) (PEO), PtBMA and PDMAEMA to successfully obtain different novel ABC miktoarm star terpolymers with narrow molecular weight distributions.
For an ABC miktoarm star terpolymer consisting of arms of PB, PtBMA and P2VP it was demonstrated that after quaternization with methyl iodide (yielding BVqT) and dialysis to water the nature of the counterion allows for manipulation of the obtained structures. The miktoarm star architecture together with iodide as counterion is essential for this directed self-assembly. Transformation of iodide to triiodide, via the addition of iodine before dialysis to water, decreases the hydrophilicity of the P2VPq corona and therefore induces the directed self-assembly of spherical micelles with a PB/PtBMA core, into cylinders, superstructures thereof and finally barrel-shaped aggregates of up to 1 µm with an internal lamellar fine structure. Based on their appearance in transmission electron micrographs these were termed “woodlouse” aggregates. The compact particles consist of alternating lamellae of a partially demixed PB/PtBMA phase and a swollen P2VPq phase.
The general applicability of this counterion-mediated hierarchical self-assembly was furthermore demonstrated by using two other miktoarm star systems. For three ABC miktoarm star terpolymers of different composition, consisting of PB, PS and P2VPq segments (BVqS), a dependence of the morphology on the fraction of the hydrophilic block was determined, in analogy to diblock copolymers. For long P2VPq blocks stacked lamellar/disk-like structures evolve from micellar building units. In contrast, a short P2VPq segment yields multilamellar vesicles via fusion of vesicular primary building blocks. The vesicle walls are supposed to consist of a lamellar structure with the PB phase in the centre, shielded from the P2VPq corona by thin PS layers. At the example of one BVqS miktoarm star terpolymer the successful formation of nanohybrids containing gold nanoparticles within the P2VPq phase is demonstrated.
In the second system the low-Tg PB segment was replaced by a second PS block of different length (SVqS’). Even though vesicles serve as initial building units, the triiodide-induced superstructure formation leads to anisotropic aggregation of deformed vesicles, rather than to the fusion into multilamellar vesicles. This is attributed to the two glassy PS core blocks which minimize the dynamics during self-assembly and allow only minor rearrangement of the aggregated structures. Similar to the “woodlouse” aggregates from BVqT, lamellar structured particles of elongated shape were obtained from SVqS’, despite vesicles serving as primary building units. Consequently, the presented triiodide-directed self-assembly into complex superstructures is not restricted to miktoarm star polymers containing a low-Tg segment, as the rearrangement processes take place during the dialysis process, where the organic co-solvent enables sufficient mobility of the core-forming blocks.
Besides the introduction of a novel synthetic approach for the construction of miktoarm star terpolymers and the synthetic advance of the alkyne-functionalized DPE, the presented triiodide-mediated superstructure formation represents an interesting concept for directed self-assembly processes.
The Effect of Spatial and Environmental Drivers on Patterns in Species Richness and Composition
- This thesis includes eight manuscripts with methodological, empirical and theoretical contributions that aim to enhance the understanding of species richness and composition patterns and their underlying drivers. Islands and isolated systems are in the focus of this work.
Islands provide optimal conditions to study biogeographic patterns. Theoretical advances in ecology have been initiated by island biogeography. Theory on island biogeography has particularly been improved by a better representation of time related components including speciation and environmental change. Oceanic islands are not stable systems but follow a characteristic ontogeny. After the volcanic emergence over the sea surface, erosion processes, shaping the island first more heterogeneous and then flatter, transform islands. This thesis shows how particular characteristics of the classic theory of island biogeography can be included in the currently most advanced theoretical framework. While MacArthur & Wilson (1963) particulary focussed on processes (colonisation and extinction) for generating species richness patterns, current theory assumes a defined upper limit for species richness (“carrying capacity”). By reinforcing the importance of processes in the current theory, as suggested in this thesis, it is much simpler to formulate hypothesis that can be tested by empirical data. Carrying capacity is linked to “habitat heterogeneity”, both, in the meaning of topographic variability as well as the number of vegetation units that are present in a given area. This thesis demonstrates that a clear terminology is a prerequisite for a profound understanding of the effects of “heterogeneity” on species diversity patterns in general and the underlying biogeographic processes in particular. The heterogeneity of surfaces influences species diversity not only on scales larger than kilometres but also is important on very fine scales of meters and smaller. Novel methods to measure different aspects of surface variability are introduced and discussed and their effect on species richness and composition of plant species groups in different ecological systems is presented.
Furthermore, this thesis highlights the isolating effect of elevation (elevation-driven ecological isolation hypothesis). Environmental filtering along an elevational gradient differentiates ecosystems. Isolation increases with elevation, as comparable ecosystems are much farther apart at high elevations than is the case for lowland ecosystems. In addition, ecosystems on neighbouring islands or on the continent that serve as source regions for colonising species are smaller in area in high elevations in comparison to low elevation ecosystems. Consequently, an above average speciation rate reflected in a high percentage of endemic species can be expected for higher elevations on islands and high mountains. The elevation driven ecological isolation hypothesis is tested for a number of islands and a new hypothesis indicating a complex interaction with isolation is developed. The difference in isolation between low and high elevation ecosystem diminishes as the overall isolation of the island increases. Thus the relation between the percentage of endemic species and elevation should reverse with an increase in isolation. On very isolated islands, low and high elevation ecosystems are alike isolated but low elevation ecosystems should have an above average speciation rate as they provide more area and higher temperatures relative to the ecosystems above (e.g. metabolic theory of ecology).
The scale dependence of diversity patterns are attributed to ecological processes that operate differently over varying extents and grain sizes. This thesis demonstrates that scale dependencies in distance-decay analyses cannot be traced back to processes that are specific for the ecological scale, but can largely be attributed to sampling design and are highly sensitive to grain size and study extent. Distance-decay analyses are an adequate method to assess spatial turnover in species composition. However, this thesis shows that frequent practise of making comparisons among studies is not possible within the current methodological framework.
Finally, this thesis provides an overview on patterns in species richness and composition and elaborates interconnections between associated theories and underling drivers. Promising novel research questions and directions are identified in the field of island biogeography and in an adequate formalisation of a “heterogeneity” concept.
Die chemische Ökologie von Kurzflügelkäfern der Gattungen Dianous und Stenus (Coleoptera, Staphylinidae)
- Vertreter der Kurzflügelkäfer-Gattungen Stenus und Dianous weisen, wie die meisten Staphylinidae, einen schlanken Körperbau mit reduzierten Elytren auf. Dies führt zu einer erhöhten Beweglichkeit des Abdomens, wodurch Lückensysteme als Habitate erschlossen werden können. Da die reduzierten Elytren das Abdomen jedoch nur unzureichend schützen können, neigen die Käfer zu einer deutlich größeren Anfälligkeit gegen Prädation und Parasitierung. Um diesen Nachteil zu kompensieren, produzieren und speichern Steninae in abdominalen Wehrdrüsen hochgradig biologisch aktive Abwehrverbindungen, die sie auf vielfältige Art und Weise einsetzen.
Um neue Einblicke in die komplexe chemische Ökologie der Steninae zu ermöglichen, wurden in der vorliegenden Dissertation die Morphologie, Ultrastruktur und Funktion der Pygidialdrüsen sowie die biologische Aktivität der Abwehrkomponenten und -sekrete untersucht. Durch Analyse der Biosynthese der stickstoffhaltigen Sekretkomponenten und deren Verteilung über verschiedene Arten wurden ferner Aussagen über potentielle evolutive Trends und phylogenetische Zusammenhänge innerhalb der Steninae möglich.
Die paarig angelegten Pygidialwehrdrüsen der Steninae erstrecken sich lateral des Darmes und dorsal der Geschlechtsorgane über die letzten drei bis vier Abdominalsegmente. Sie bestehen aus zwei großen Reservoiren mit bandförmig aufgelagertem sekretorischen Gewebe sowie zwei weiteren kleinen Reservoiren mit zugehörigem Drüsengewebe an der Basis der Großen. In beiden sekretorisch aktiven Geweben erfolgt die Sekretproduktion in Drüsenzellen, die einen länglichen inneren Extrazellulärraum aufweisen. Die synthetisierten Produkte werden über zahlreiche Mikrovilli der Drüsenzellenmembran in den Extrazellulärraum abgegeben, wo sie von einem cuticulären Tubulus aufgenommen und zum entsprechenden Reservoir abgeleitet werden. Außerhalb der Drüsenzelle ist der sekretableitende Tubulus von einer Kanalzelle umgeben, die zusammen mit der Drüsenzelle eine Drüseneinheit bildet. Während die großen Reservoire mit den entsprechenden Drüsengeweben in Struktur und Funktion bei allen Steninen konserviert vorliegen, unterliegen die kleinen Reservoire und sekretorischen Gewebe meist starken Reduktionen, die bis hin zur Funktionslosigkeit einzelner Strukturen führen können.
Die Sekrete in den großen Reservoiren bestehen aus den Alkaloiden Stenusin, 3-(2-Methyl-1-butenyl)-pyridin und Cicindeloin sowie zum Teil aus deren Dehydro- oder Nor-Verbindungen, wobei sowohl die qualitative, als auch die quantitative Zusammensetzung je nach Art unterschiedlich ausfallen kann. Die kleinen Reservoire enthalten Monoterpene, wie unter anderem 1,8-Cineol, α-Pinen und 6-Methyl-5-hepten-2-on. In Biotests zeigten alle Sekretkomponenten eine signifikante Aktivität als Feeding Deterrent gegen die Ameise Lasius flavus und den Fisch Xiphophorus helleri. Weiterhin war bei den meisten Verbindungen eine deutliche wachstumshemmende Wirkung gegen Bakterien und Pilze feststellbar. Sowohl bezüglich der fraßhemmenden als auch der antimikrobiellen Wirkung zeigten sich hierbei die Alkaloid-Komponenten den Terpenen meist überlegen. In Tests mit zwei reinen Diastereomeren des in vier Stereoisomeren vorkommenden Stenusins zeigte sich über die biologische Aktivität eines Isomerengemisches hinaus, dass Ameisen zwischen verschiedenen Stereoisomeren diskriminieren, während bei Bakterien stets die gleiche Hemmwirkung zu beobachten ist.
Die in den großen Reservoiren gespeicherten Alkaloide weisen alle ein identisches Grundgerüst mit N-heterocyclischem Sechsring und verzweigter Alkyl-Seitenkette auf. Über Verfütterung deuterierter Aminosäuren konnte die biogenetische Verwandtschaft der drei Alkaloid-Hauptkomponenten Stenusin, 3-(2-Methyl-1-butenyl)-pyridin und Cicindeloin nachgewiesen werden. In allen drei Fällen wird der N-Heterocyclus aus der Aminosäure L-Lysin, die Seitenkette aus L-Isoleucin gebildet. Die Biosynthese der Alkaloide erfolgt hierbei zunächst identisch, bis hin zu zwei Vorläuferverbindungen, die letztendlich durch wenige chemische Modifikationen in die Sekret-Alkaloide überführt werden können.
Bezüglich der Morphologie der Pygidialdrüsen sowie der qualitativen und quantitativen Sekretkomposition sind evolutive Trends innerhalb der Steninae zu erkennen. In den phylogenetisch basalen Arten S. comma und S. biguttatus liegt ein stark ausgeprägtes, voll funktionsfähiges kleines Reservoir und Drüsengewebe an der Basis der großen Reservoire vor. Das Drüsensekret enthält Stenusin, Norstenusin und die im kleinen Reservoir gespeicherten Terpene. Im Laufe der Evolution der Steninae wurde sowohl das kleine Reservoir, als auch das entsprechende Drüsengewebe reduziert und der Terpen-Gehalt des Sekrets minimiert. Desweiteren wurde das Pygidialdrüsensekret zunächst um 3-(2-Methyl-1-butenyl)-pyridin (z.B. S. similis) und später um Cicindeloin (z.B. S. solutus) erweitert, wobei Stenusin und Norstenusin mehr und mehr ersetzt wurden.
Impact of time and spatial averages on the energy balance closure
- Secondary circulations are large and relatively stationary eddies, which are caused by the surface heterogeneity and normally reside away from the ground. They are believed to be the cause of the energy balance closure problem at the earth's surface, because their contribution to the turbulent fluxes is missed by a fixed eddy-covariance tower measurement that has a typical averaging time of 30 minutes. In this thesis, data from the LITFASS-2003 experiment was used to investigate the impact of time and spatial averages on the energy balance closure. This data consisted of many observations over a large heterogeneous landscape that could generate secondary circulations; some of which might be still near the earth's surface.
For the time average analysis, the averaging time was extended to increase the possibility that secondary circulations were picked up by the sensor. Two approaches, which were the modified ogive analysis and the block ensemble average, were applied to analyze the data from ground-based measurements. The modified ogive analysis requiring a steady state condition, could extend the averaging time up to a few hours and suggested that an averaging time of 30 minutes was still overall sufficient for the eddy-covariance measurement over low vegetation. The block ensemble average, on the contrary, did not require a steady state condition, but could extend the averaging time to several days. However, this approach could only improve the energy balance closure for some sites during specific periods, when secondary circulations existed in the vicinity of the sensor. Based on this approach, it was found that the near-surface secondary circulations mainly transported sensible heat, which led to an alternative energy balance correction by the buoyancy flux ratio approach, in which the attribution of the residual depended on the relative contribution of the sensible heat flux to the buoyancy flux. The fraction of the residual attributed to the sensible heat flux by this energy balance correction was larger than in the energy balance correction that preserved the Bowen ratio.
In the spatial average analysis, two energy balance correction approaches, the buoyancy flux ratio and the Bowen ratio approaches, were applied to the area-averaged fluxes (composite fluxes) in order to include contribution from secondary circulations. These composite fluxes were aggregated from multiple ground-based measurements. The energy balance corrected fluxes were validated against the spatial average fluxes, which were measured by an aircraft and a large aperture scintillometer (LAS). In this validation, the backward Lagrangian footprint model was used to estimate the source area of the measurement. It was found that both energy balance correction approaches did improve the agreement between time and spatial averages fluxes. This suggested that the contribution from secondary circulations could be properly accounted by the energy balance correction.
All findings in this thesis, therefore, depict that secondary circulations significantly transport energy in the atmospheric surface layer. The energy balance correction, accomplished by using either the Bowen ratio approach or the buoyancy flux ratio approach, is necessary to estimate the actual vertical transport of energy at the earth's surface.
Direct Force Measurements on the Colloidal Scale: From Modified Electrodes to Particle Manipulation
- In this thesis the interfacial surface forces and mechanical properties of thin films have been studied by the colloidal probe technique. One central point is the combination of direct force measurements with an electrochemical setup in order to tune interfacial properties of an electrode modified with an organic layer. In particular the adhesion and ion adsorption have been studied, which are ubiquitous phenomena in the colloid science, electrochemistry, and biology. Moreover, a novel technique has been developed to fabricate chemically and mechanically stable colloidal probes for atomic force microscopy (AFM). Additionally, the elastic properties of polyelectrolyte multilayer films were locally resolved under controlled humidity.
The adhesive behaviour of colloidal particles on modified electrodes has been studied by direct force measurements with a micrometre-sized silica probe attached to an AFM-cantilever. By controlling the external potential applied to the modified electrode by means of a potentiostat, separate adhesion contributions at the modification layers in electrolyte solution were quantified. In particular, to determine the influence of the terminating functional groups, gold electrodes modified with self-assembled monolayers (SAMs) terminated in non-ionizable groups were used. It has been demonstrated that electrostatic double-layer forces dominate the adhesion of colloidal particles on hydrophobic and hydrophilic interfaces. In contrast to hydrophilic interface, for hydrophobic one forces due to the solvent exclusion play a significant role and leads to an offset in the adhesive force, which otherwise can be compensated by the external potential. However, the electrocapillarity is of minor importance and can be neglected.
To quantify the ion adsorption at organic interfaces a novel approach was followed, which is based on direct force measurements with silica colloidal probes on SAM-modified electrodes in electrolyte solutions. By variation of applied potential and concentration of specifically adsorbed ions, given by the solution’s pH, the charging behaviour of hydrophilic SAM-OH and hydrophobic SAM-CH3 has been determined. In difference to electrokinetic techniques, direct force measurements allow to probe the full range of the diffuse layer. The analysis of the diffuse layer potential as a function of externally applied potential provides important information. In particular, the shift of the potential of zero charge (pzc) indicates on the specific ion adsorption in the Stern layer as it alters the charging behaviour of the electrode’s interface. It has been demonstrated that hydronium and hydroxide ions adsorb on both the hydrophobic and hydrophilic interfaces. However, the presence of the background electrolyte (KCl) does not shift pzc and thus its ions have no specific affinity towards the interfacial adsorption. The adsorption of hydronium and hydroxide ions is stronger on hydrophobic, than on hydrophilic interface. This is in agreement with theoretical studies. The simple three-capacitor model based on a Langmuir-type adsorption isotherm provides semi-quantitative description of observed dependence of the diffuse double layer potential on applied potential.
A new technique for colloidal probe preparation was developed. A great challenge for the force measurements with the AFM is to ensure the cleanliness, chemical and mechanical stability of the used probes. The approach is based on high-temperature sintering of micrometer-sized silica particles to AFM cantilever with enhanced contact area. Due to a “neck” formed by nanometer-sized particles the increased mechanical stability of colloidal probes was achieved, which has been quantitatively determined by lateral force spectroscopy. The implementation of sintering procedure for silica colloids allowed the development of the highly stable colloidal probes, whose surface properties could be renewed by heating.
Finally, the mechanical properties of polyelectrolyte multilayer films have been determined by nanoindentation as a function of relative humidity. For these series of measurements again a colloidal probe has been used. It has been demonstrated that films containing polyglutamic acid have Young’s modulus, which depends on humidity. The change of stiffness with ambient humidity has reversible character.
Foamy Virus RNase H - Aktivität, Struktur und Funktion
Das für die Replikation des RNA-Genoms von Foamy Viren (FV) notwendige Enzym, die Protease-Reverse Transkriptase (PR-RT), beinhaltet die Protease-, die Polymerase- und RNase H-Domäne. Letztere ist für den Abbau der RNA im entstehenden RNA/DNA Hybrid verantwortlich. Während die FV PR-RT als Monomer vorliegt, besteht die HIV-1 RT aus einem p66/p51-Heterodimer. Erstaunlicherweise ist die isolierte HIV-1 RNase H im Vergleich zur z.B. E. coli oder separaten MoMLV RNase H nicht aktiv. Aus den Sequenzvergleichen verschiedener RNase H-Domänen ergibt sich, dass die Prototyp FV (PFV) RNase H im Gegensatz zur HIV-1 RNase H einen Sekundärstrukturbereich aufweist, bei dem es sich um die sogenannte C-Helix mit einer sich anschließenden basischen Schleife (basic protrusion) handelt. Da zu Beginn der Arbeit keine 3D-Struktur einer retroviralen RNase H mit basic protrusion bekannt war, sollte in dieser Arbeit die Struktur der PFV RNase H und die Funktion der basic protrusion bei der Substratbindung geklärt werden.
Die Tertiärstruktur der PFV RNase H-Domäne konnte mit NMR-Spektroskopie gelöst werden. Somit war es möglich, die basic protrusion einschließlich der C-Helix zu identifizieren. Die isolierte RNase H-Domäne zeigte in fluoreszenzbasierten Tests sowie in qualitativen RNase H-Versuchen mit radioaktiv markiertem Substrat Aktivität. Um die Funktion der C-Helix und der sich anschließenden basischen Schleife bei der Substratbindung zu analysieren, wurden NMR-Titrationsexperimente durchgeführt. Dafür wurde die PFV RNase H-Domäne zunächst durch den Austausch der zwei katalytisch wichtigen Reste Aspartat 599 und Histidin 724 zu Asparagin inaktiviert (RNase H-(D599N-H724N)), um den Abbau des Substrates während der Messungen zu vermeiden. Die Auswertung von [15N, 1H]-HSQC- und [1H, 15N, 1H]-NOESY-HSQC-Spektren erbrachte eine Übereinstimmung der Tertiärstrukturen der RNase H-(D599N-H724N) mit der wt RNase H.
Die NMR-Titrationsexperimente zeigten, dass die C-Helix in der PFV RNase H wie ein Lineal agiert, das die sich anschließende basische Schleife zum Substrat orientiert. Darüber hinaus besitzt die basic protrusion zusätzlich eine Reihe an positiv geladenen Resten, die gut lösungsmittelzugänglich sind und dadurch erste Kontakte mit dem Substrat ermöglichen. Damit bietet die basic protrusion eine Art Plattform für die Substratbindung. Der HIV-1 RNase H fehlt nicht nur die C-Helix, zusätzlich ist die sich anschließende Schleife vermutlich zu kurz, um das Substrat zu binden. Da diese Schleife außerdem nur über einen basischen Rest verfügt, ist wahrscheinlich auch die Gesamtaffinität dieses Bereichs für die Substratbindung zu gering. Strukturvergleiche der PFV RNase H mit der HIV-1 RT zeigen, dass die fehlende basic protrusion der HIV-1 RNase H durch eine Schleife aus der Verbindungs-Subdomäne der p66-Untereinheit kompensiert wird. Dieser Bereich könnte somit ein neuer Angriffspunkt für Inhibitoren in der antiretroviralen Therapie bei HIV 1 sein.
Um zukünftig weitere Strukturanalysen mit der PR-RT bzw. mit einzelnen Domänen durchführen zu können, wurden in einem weiteren Projekt verschiedene N- und C-terminale Deletionsvarianten der PR-RT des Simian Foamy Virus hergestellt. Durch Aktivitätstests mit diesen Deletionsvarianten konnten die Abgrenzungen für die PR-, die Polymerase-, die RNase H-Domäne und die Verbindungs-Subdomäne in der PR-RT identifiziert werden. Dabei zeigte sich, dass die Region H107-N143 C-terminal von der PR wichtig für die Funktion der Polymerase ist. Die Deletion der RNase H-Domäne und Verbindungs-Subdomäne führte zu einer drastischen Abnahme der Substrataffinität, Integrität und Polymerisationsfähigkeit des Enzyms. Trotzdem konnte eine minimale Polymerase-Domäne bestimmt werden (RT(107 454)), die ohne PR- und RNase H-Domäne sowie ohne die Verbindungs-Subdomäne in der Lage ist, zu polymerisieren. Für die Dimerisierung und damit Aktivierung der PR, die durch Bindung von zwei PR-RTs an das sog. PARM-Element (engl. protease activating RNA motif) auf der genomischen RNA geschieht, sind jedoch die RNase H-Domäne und die Verbindungs-Subdomäne unverzichtbar. Dadurch ist die RNase H nicht nur für die reverse Transkription essenziell; in FV stellt sie auch ein Regulationselement für die PR-Aktivierung dar und ist somit indirekt an der Prozessierung von Gag (Strukturproteine) und Pol (virale Enzyme) beteiligt.
Nostalgia, Home and Be-longing in Contemporary Postapartheid Fiction by Zakes Mda and Ivan Vladislavić
- Nostalgia, Home and Be-Longing in Contemporary South African Fiction: Zakes Mda and Ivan Vladislavić
The dissertation examines the representation of nostalgic longing and the question of home and belonging in selected South African novels by the contemporary writers Zakes Mda and Ivan Vladislavić. I argue that nostalgia is a crucial aspect to be explored in the South African context because it poses uncomfortable questions about one’s sense of belonging in a transforming society and one’s position in the present with regard to a history of colonialism and apartheid. The sentiment of nostalgia and the feeling of displacement bind the selected novels and characters and reveal a deep dissatisfaction with the present. In a comparative study, I show that nostalgia and the question of home have different impacts on the two authors. While the longing for a past home and time supports a reaffirmation of cultural identity in Zakes Mda’s novels, Ivan Vladislavić portrays nostalgia with a critical distance showing that the past is difficult to identify with. Both writers are critical of restorative forms of nostalgia (Boym 2001) and show that home and be-longing are a matter of inclusion and exclusion resulting from a legacy of segregation and Othering. I demonstrate how nostalgia can either contribute to constructively engage with the past and present and envision a future for the South African nation, or how it, on the contrary, can enforce the erection of borders and the (colonial) method of Othering within society.
Energy-domain synchrotron radiation Mössbauer source for physics under extreme conditions
- Iron is one of the most abundant elements on Earth, and it is an important component in minerals. Electronic and magnetic properties of iron-bearing materials significantly affect processes occurring in the deep interior of the Earth. In the materials that make up the Earth’s lower mantle iron may exist in different valence, spin states and crystallographic environments. Most of the existing experimental techniques either do not allow to separately follow evolution of different iron sites or are not suitable for measurements under high-pressure/high-temperature conditions. This makes studies of iron electronic structure under such conditions extremely challenging.
The current Ph.D. thesis is divided into two major parts. The first part is dedicated to the development of a Synchrotron Mössbauer Source (SMS). This device allows energy domain Mossbauer spectroscopy to be performed on a sample under pressures above 100 GPa in laser heated diamond anvil cells. The second part is dedicated to studying the behavior of iron in iron/alumina-bearing silicate perovskite under conditions of the Earth’s lower mantle.
1. Synchrotron Mössbauer Source
There are several techniques that allow magnetic and electronic properties of materials under extreme conditions to be probed: X-ray Emission Spectroscopy (XES), X-ray absorption near edge structure (XANES), Nuclear Resonance Spectroscopes, etc. For elements in which observation of Mössbauer effect is possible the most mature, sensitive, and suitable technique for studies of magnetic and electronic properties is energy-domain Mössbauer spectroscopy.
However, due to low brilliance of utilized radioactive sources and low natural abundance of iron in lower mantle minerals measurements using conventional energy- resolved Mössbauer spectroscopy require very long time and usually are limited to moderate pressures. The problem can be solved by combining the outstanding properties of synchrotron radiation (high brilliance, possibility for extreme focusing) with the energy-resolved approach. In brief, what is needed is a synchrotron source of Mössbauer radiation. Construction of such source was the primary task of my PhD work. The possibility to develop such a source was demonstrated at the Nuclear Resonance beamline ID18 at the European Synchrotron Radiation Facility (ESRF) by Smirnov et al. (1997). The source is based on pure nuclear reflections existing in antiferromagnetic 57FeBO3 crystals.
￼The major technical goals of my PhD work were to (a) construct a device that would be permanently ready for operation, and (b) optimize it to have the best possible resolution and highest possible intensity. In order to achieve these goals theoretical calculations were first conducted to understand how to best improve the performance. Second, several experiments were performed to confirm the theoretically predicted results. Third, several optical schemes of the SMS were tested in order to determine the optimal setup.
As a result of the research and development program a powerful Synchrotron Mössbauer Source (SMS) for high-pressure applications was constructed at the Nuclear Resonance beamline (ID18) of the ESRF. Using results obtained in the combined theoretical/experimental study of angular dependence of energy and temporal distributions of the pure nuclear reflections of iron borate crystal, the SMS was optimized for the highest possible intensity and best possible resolution. The bandwidth of radiation provided by the SMS is between 10-15 neV (2-3 Γ0, where Γ0 is a natural linewidth of Mössbauer resonance for Iron), the intensity is ~2.5×104 photons/s and the typical scanning velocity range is about ±12 mm/s (±0.6 μeV). In contrast to conventional radioactive sources, the SMS gives the possibility to focus the beam to tens of microns. SMS is the in-line monochromator, permanently located in the optics hutch and operational immediately after moving it into the incident beam position. The source can be used with all existing sample environments in the experimental hutches downstream of the beamline.
The implementation of this device opens the possibility for studying systems with complex hyperfine structure utilizing energy-resolved approach under various extreme conditions, for example at high-pressure. Furthermore, the SMS allows for very short collection times of only a few minutes, which enables data to be collected during laser heating. Several high-pressure and high-pressure/high-temperature studies that have already been performed are described in the second part of this Ph.D. thesis. The almost 100% recoilless resonant radiation delivered by the source and its high brightness allow a broad field of SMS applications. The SMS can be utilized in any mode of synchrotron storage ring operation.
2. Study of the spin state of Fe3+ ions in perovskite
Silicate perovskite (Mg,Fe)(Si,Al)O3 is the most abundant phase in the Earth’s lower mantle. Knowledge of its properties is indispensable for understanding lower mantle behavior. Dynamic, thermodynamic, and transport properties of silicate perovskite can be significantly affected by the valence and spin state of iron. Silicate perovskite with 5-10 mol% of Fe (where Fe3+/ΣFe ~50–75% (McCammon et al., ￼1997)) and Al, is dominant phase in Earth lower mantle (~75 vol%) (Zhang et al., 2006; Stackhouse et al., 2007). The behavior of Fe electronic properties under conditions close to those of the Earths lower mantle remains strongly controversial.
The second part of my Ph.D. work is dedicated to an investigation of the spin state of iron in Fe3+- rich silicate perovskite at high pressure. Four different silicate perovskite samples with different stoichiometry were studied using the Synchrotron Mössbauer Source. SMS spectra were collected at room temperature and pressures up to 122 GPa using diamond anvil cells, with or without laser annealing of the samples.
The hyperfine parameters, i.e., centre shift and quadrupole splitting, for the same phases, which were extracted from measured spectra for all perovskite samples studied in this work, are the same at each pressure within experimental error. Moreover, there is no change in Fe3+/ΣFe for individual samples over the entire pressure range of the experiment. The hyperfine parameters of the Fe3+ doublet are consistent with the high-spin state (Gütlich et al., 2011), and their smooth variation with pressure indicates that Fe3+ does not undergo spin crossover within the entire pressure range. All observed changes in the spectra are associated with abrupt changes in the electronic state of Fe2+. The hyperfine parameters of the low QS Fe2+ doublet correspond to the high-spin state (McCammon et al., 2008), while the doublet with high quadrupole splitting, whose intensity grows with pressure at the expense of the Fe2+ high-spin state, corresponds either to intermediate-spin (IS) Fe2+ (McCammon et al., 2008) or a distortion of the site occupied by high-spin Fe2+ (Hsu et al., 2010). Based on results presented in a work of Narygina (2010), we indentify changes in Fe2+ electronic structure as high-spin to intermediate spin transition. Irrespective of the interpretation of the Fe2+ spin state, conclusions regarding the absence of spin crossover in Fe3+ remain valid.
These results show that the previously reported spin crossover of Fe3+ ions does not occur when Fe3+ occupies the A-site. In both alumina-containing and alumina-free silicate perovskites Fe3+ ions remain in the high-spin state up to at least 122 GPa, i.e., almost up to the pressure corresponding to the lower mantle - outer core boundary. The results also indicate that Fe3+ ions do not diffuse from the A-site to the B-site in perovskite after high-temperature annealing at high pressure, Mössbauer spectra of before and after annealing are identical. There is also no evidence for high-spin to low-spin crossover of Fe3+ ions due to site change. In contrast, the results confirm that Fe2+ ions undergo a transition from a high-spin to an intermediate spin state, without reaching a low-spin state within the studied pressure range at room temperature. These results suggest that the seismic velocity anomalies in the lower mantle cannot be attributed to spin crossover in Fe3+.
Structure-Property Correlation of Electron Transport Materials in Organic Devices
- This dissertation deals with organic semiconductors as electron acceptor
(n-type) materials in bulk heterojunction (BHJ) solar cells. Important features of an electron acceptor are strong visible light absorption, sufficient high electron mobility and appropriate energy levels with respect to the donor. Furthermore, the blend morphology of donor and acceptor is crucial for the device performance. Within this thesis, the synthesis and characterization of novel n-type polymers is reported and various techniques to evaluate the above mentioned parameters for n-type small molecules and polymers are presented. The aim was to investigate the impact of chemical structure on the optical and electronic properties and morphology of these semiconductors. Successful strategies how to control and improve light harvesting, electron mobility, blend morphology and solar cell performance were identified. The fundamental question of charge transport properties of the materials was addressed by fabricating single carrier devices using the SCLC (space-charge limited currents) method. The morphology was primarily investigated by atomic force microscopy (AFM) and X-ray diffraction (XRD).
The first part of this thesis focuses on perylene imide based small molecules and polymers. The side groups of a series of N-substituted perylene bisimides (PBI) were found to play a crucial role on crystallinity and charge transport. The nature of the side groups had great impact on the crystalline structure and electron mobility. When hydrophilic oligoethylenglycol (OEG) side groups were present, the perylene molecules aligned in highly ordered hexagonal or lamellar columns and realized high electron mobilities of up to 7∙10E-3 cm2V-1s-1, while the perylene derivative with only hydrophobic alkyl chains only showed 3∙10E-5 cm2V-1s-1. The substituents at the perylene core also had a major impact on the blend morphology of OPV devices when these materials were used in combination with a donor polymer. Here, we were able to tune the extent of phase separation between donor and acceptor via hydrophilic-hydrophobic interactions of donor polymer and acceptor side groups. To improve light harvesting of perylene compounds, the pi-electron system of PBIs was altered and highly soluble, novel perylene side chain polymers (PPDB and PPDI) were synthesized by nitroxide mediated radical polymerization (NMRP). The pendant perylene moieties were perylene diester benzimidazole (PDB) and perylene diester imide (PDI). Compared to polymers bearing PBI side groups, the visible light absorption of PPDB was broadened and red shifted, whereas a narrower and blue shifted absorption was observed for PPDI. Remarkably, also the electronic nature of the two materials was affected by the modification at the perylene core, as PPDB is an n-type semiconductor and PPDI has a more pronounced p-type character. A comparative study of perylene side-chain polymers synthesized by a combination of NMRP and “click” chemistry revealed that the compound with improved optical properties (PPDEB) exhibited worse charge carrier mobility compared to PPBI. Another striking result was found as an amorphous polymer bearing OEG side chains showed a better electron mobility than the corresponding material with alkyl chains, which was liquid crystalline. A very high electron mobility of 1∙10E-2 cm2V-1s-1 was measured.
The second part of this dissertation addresses fullerene based acceptor materials, among which Phenyl-C61-butyric acid methyl ester (PCBM) is the state-of-the-art n-type semiconductor used in OPV. For two fullerene derivatives, Bis-Phenyl-C61-butyric acid methyl ester (bis-PCBM) and Bis-o-quino-dimethane C60 (bis-oQDMC), the LUMO energy levels were higher compared to PCBM. As a result, improved open circuit voltages (Voc) in BHJ solar cells were obtained. The efficiency however did not improve, because of reduced short circuit current densities (Jsc). We found that for the bis-PCBM system, Jsc was limited by low electron transport, while for the bis-oQDMC system an unfavorable blend morphology hampered the performance. The problem of low electron mobility could be overcome by reducing the thickness of the active layer and higher Jsc and overall device performance could be achieved. A drawback of fullerene small molecules is that diffusion, aggregation and crystallization of these molecules within BHJ blends can often negatively affect the stability of the blend morphology and reduce the device performance. We discovered that aggregate and crystallite formation in novel fullerene side chain polymers could be successfully suppressed, whilst high electron mobility and better film properties were achieved.
Altogether, new insights into structure-property relation of organic electron transport materials are presented in this work. Moreover, the detailed analysis of charge transport helped to understand the performance of solar cells.
Hybrids Based on Layered Silicates
- Novel hybrid nanoparticles were synthesized based on combinations of various layered silicates as inorganic core and well-defined polymer chains as a shell. In all cases positively charged 2-(dimethylamino)ethyl methacrylate (DMAEMA) was incorporated into the polymeric structure to serve as a firm anchor onto the negatively charged clay surface via electrostatic adsorption.
First, hybrid nanofillers were synthesized to improve the mechanical properties of a homopolymer matrix by combining a shear-stiff synthetic K-hectorite with a tailored surface activity. For this, the synthetic fluorohectorite with very high aspect ratios was organophilized with a specifically designed macroinitiator created by statistical Reversible Addition Fragmentation Chain Transfer (RAFT) copolymerization of DMAEMA and the initator-monomer 2-(2-bromoisobutyryloxy)ethyl methacrylate (BIEM). The copolymer was firmly anchored through multiple cationic charges distributed over the chain while the multiple initiating functions were used to polymerize the monomer of choice via Atom Transfer Radical Polymerization (ATRP). The final hybrid was equipped with a hydrophobic polymeric shell of poly(methyl methacrylate) (PMMA), which enables dispersion in organic solvents. The hybrid particles were compounded into a polymeric matrix of commercial PMMA and tested with regard to its reinforcing properties. The similarity of the polymeric shell to the homopolymer matrix of the chosen sample composite combined with the inherent stiffness of the inorganic core lead to an increase in tensile modulus of up to 84 % at 5% filler content. Further, patchy hybrid nanodiscs based on natural montmorillonite as core and a shell made from compartments of two different polymers were evaluated as cheap and versatile compatibilizers in an immiscible polymer blend. In a simple one-step modification process a shell comprising patches of either of two polymer species (PMMA and polystyrene, PS), each chosen to be similar in polarity to one of the matrix polymers, was attached to the inorganic core via Coulomb interaction. The behaviour of these particles in a solvent-cast blend of 2:1 PS/PMMA was investigated via transmission electron microscopy (TEM) and dynamic-mechanical analysis (DMA). Particles were found distributed in both of the blend’s domains and at the interface and an improvement of the storage module of 17% was found.
Finally, kaolinite was used as a core to create true hybrid Janus nanodiscs, which were applied for compatibilizing an immiscible polymer blend of 2:1 PS/PMMA. It was possible to create two chemically distinct surfaces on the clay particle by addressing each of its two basal surfaces individually via simple, but selective, surface modification. Two diblock copolymers were used to create the Janus structure, each one with a first block consisting of monomer units bearing the anchoring group for the respective surface and a second block, PS or PMMA, tailored to the polarity of the respective matrix polymer. Thus it was possible to direct the Janus particles straight into the interface between the polymeric domains, visualized by TEM images taken from solvent-cast nanocomposite films.