44 search hits
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Turnover and fluxes of carbon and nitrogen in a spruce forest under natural and extreme meteorological conditions
(2010)
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Kerstin Schulze
- Climate models predict an increase in the intensity and frequency of extreme meteorological climate events like extended summer droughts, heavy rainfall or intensive frost periods with largely unknown effects on microbial activity and pysico-chemical soil properties and their impact on availability of soil organic matter. The influence of drying/rewetting (A/W) and freezing/thawing (G/A) events on solution chemistry and leaching losses of soils is barely known. This thesis aimed to study the effects of A/W and G/A events on soil solution chemistry and solute fluxes, in particular, of dissolved organic carbon (DOC) and inorganic nitrogen (NH4+, NO3-) in a podzol soil under a Norway spruce forest. A field experiment was designed to study the effects of (i) summer drought by exclusion of natural throughfall and subsequent rewetting and of (ii) soil frost by removal of natural snow cover. In complementary laboratory experiments with undisturbed soil columns, (i) drying/rewetting cycles were simulated with different rewetting intensities and (ii) freezing/thawing cycles were induced using different freezing temperatures. In the second part of this work, total C and N stocks as well as radiocarbon signatures of soil organic carbon (SOC) from different soil horizons and density fractions were investigated. A/W increased the DOC concentrations in the organic layer and upper mineral soil. More DOC was released from the organic layer to the mineral soil. However, the effects on total DOC leaching were smaller due to reduced water fluxes. Specific UV absorbance and emission fluorescence detected a switch in the release of easily decomposable DOC to hardly decomposable DOC during the wetting phase. Prolonged summer drought and incomplete rewetting due to hydrophobicity of SOM in the organic layer and upper mineral horizon reduced net N mineralisation as well as concentrations and fluxes of the NH4+ and NO3-. The net nitrification rate in the organic layer was more negatively influenced than net ammonification, indicating that nitrifiers are more sensitive to drought stress than ammonifiers. The effect of soil frost strongly depended on soil freezing temperature. Only soil frost at temperature below -8°C led to short periods of additional DOC production in the organic layer. Spectroscopic properties and ∆14C signatures of DOC implied a disruption of soil aggregates and desorption of older DOC from the mineral associated organic matter fraction of the Oa horizons by G/A events. Severe soil frost below -8°C inhibited the activity of nitrifiers and ammonifiers with decreased NH4+ and NO3- concentrations and fluxes in the mesocosm experiment. A delayed (by 4 months) increase in NO3- concentration in the upper soil horizon by moderate soil frost (-5°C) was attributed to reduced Immobilisation by heterotrophic microorganisms. Summarised, drying and the effect of hydrophobicity led to long-term, severe soil frost to short-term reduction in N mineralisation and N leaching. The effect of increased NO3- concentrations as delayed response to G/A needs further research in case of potentially changes in the N balance. Drying as well as freezing induced changes in the soil structure and properties and led to increased DOC concentrations. Moderate soil temperature had much less effects on C and N in this temperate forest soil. The results of this thesis demonstrated the potential of extreme meteorological events on the quality and availability of dissolved C and N. Both, A/W and G/A cycles decreased C and N mineralisation, increased the sink strength of the soil by the accumulation of SOC and N, considering constant C and N litter input. However, optimal temperature and moisture conditions in other seasons could compensate the sink strength of soils. This work underpins the need for holistic and long-term investigations to understand and model the impact of extreme meteorological conditions on the dynamics of dissolved C and N.
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Synthesis of reponsive homo- and block copolymers - application to the generation of inorganic-organic nanohybrids
(2010)
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Pierre-Eric Millard
- Responsive homopolymers and multi-responsive block copolymers were prepared via reversible addition-fragmentation chain transfer (RAFT) and atom transfer radical polymerization (ATRP). Self-assembly in solution depending on environmental stimuli was investigated and exploited to create responsive micelles. New cross-linking strategies were thoroughly performed in aqueous solution to allow a controlled preservation and a high shape-persistence of the colloid particles, even when exposed to non-selective environmental conditions. The synthesis of poly(N-isopropylacrylamide) (PNIPAAm) was investigated by ATRP for subsequent polymer-protein nanohybrid generation. This temperature-responsive polymer was polymerized directly in pure water at a low temperature (4 ºC) by using a functional ATRP initiator which allows post-polymerization conjugation. Without the addition of Cu(II), the kinetics were extremely fast, typically less than one minute for a full conversion. By adjusting the ratio of Cu(I)/(Cu(II) and selecting a very active ligand, all polymerizations proceeded in a controlled fashion to near quantitative conversion without evidence of termination. N-isopropylacrylamide and acrylic acid (AA) were also homopolymerized by RAFT in aqueous media using a novel strategy. Instead of using a diazo-initiator, which generally decomposed at high temperatures, gamma-irradiation was used to initiate polymerization at ambient temperature. This type of radiation has many advantages. A very tiny and constant amount of radicals can be generated, which is perfect for the RAFT process. Moreover, the rate of initiation only has a low level of dependence on temperature and can be used in a wide range of temperatures. Finally, compared to UV-initiation, gamma-irradiation can penetrate the reaction solution deeper and without evidence of irreversible decomposition of the dithioester end group. Therefore, RAFT polymerizations of NIPAAm and AA were achieved with a very good level of control, even at high monomer conversions. This new process was then extended to many other water-soluble monomers for generating homopolymers and block copolymers. Among these, acrylamide, N,N-dimethylacrylamide, 2-hydroxyethyl acrylate and poly(ethylene glycol) methacrylate gave the best results. This technique proved to be very efficient at generating very long and narrowly distributed polymers (up to a degree of polymerization of 10,000) and at designing block copolymers. High molecular weight PNIPAAm-b-PAA copolymers, synthesized by RAFT polymerization under gamma-radiation, were used to generate multi-responsive cross-linked micelles. These block copolymers were self-assembled in water at pH 7 by increasing the temperature over the lower critical solution temperature. The PNIPAAm became hydrophobic and formed the micellar core and the hydrophilic PAA block generated the corona which prevented full aggregation of the system. Then, by amidification at elevated temperatures of the carboxylic moieties via a trifunctional primary amine, the structure was found to remain even after cooling down the system. The shell-cross-linked micelles formed were utilized to generate inorganic-organic nanohybrids by the in situ reduction of gold or silver salts to generate nanoparticles inside the nanocarrier. Another strategy of cross-linking was also investigated by using amino-functional silsesquioxane nanoparticles. In water around neutral pH values and room temperature, these particles interacted with the carboxylic groups of a high molecular weight PNIPAAm-b-PAA by hydrogen bonding and ionic interactions to generate an insoluble complex. Due to the presence of the hydrophilic PNIPAAm block, defined spherical micelles were obtained. The inorganic-organic particles were successfully cross-linked by subsequent amidification to preserve the structure, even at a high pH. Different temperature properties of the hybrids were observed depending on the pH value, due to the residual charge in the micellar core. At a neutral pH, shrinking of the corona was observed, while at a high pH (pH 13) a fully reversible aggregation of the system occurred.
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Soil-Landscape Modelling in an Andean Mountain Forest Region in Southern Ecuador
(2010)
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Mareike Ließ
- Soil-landscapes are diverse and complex due to the interaction of pedogenetic, geo-morphological and hydrological processes. The resulting soil profile reflects the balance of these processes in its properties. Early conceptual models have by now resulted into quantitative soil-landscape models including soil variation and its unpredictability as a key soil attribute. Soils in the Andean mountain rainforest area of southern Ecuador are influenced by hillslope processes and landslides in particular. The lack of knowledge on the distribution of soils and especially physical soil properties to understand slope failure, resulted in the study of this particular soil-landscape by means of statistical models relating soil to terrain attributes, i.e. predictive soil mapping. A 24 terrain classes comprising sampling design for soil investigation in mountainous areas was developed to obtain a representative dataset for statistical modelling. The soils were investigated by 56 profiles and 315 auger points. The Reference Soil Groups (RSGs) Histosol, Stagnosol, Umbrisol, Cambisol, Leptosol and Regosol were identified according to the World Reference Base for Soil Resources (WRB). While soil profiles and auger points were described in their horizon composition, thickness, soil cohesion, bulk density and texture were analysed in soil profiles only. The prediction of soil parameters was carried out with Classification and regression tree (CART) and Random Forest (RF) method. At this, prediction uncertainty was addressed with hundredfold model runs based on different random Jackknife partitions. Problems with the prediction of the RSGs, likely caused by inconsequence within the WRB (absolute and relative values as decision criteria), resulted in the proposal of “incomplete soil classification”, which relates the thickness of the diagnostic WRB horizons to the upper 100 soil centimetres. Histosol and Stagnosol have been distinguished as dominant RSGs within the inves-tigation area. While Histosol probability depended on hydrological parameters; highest Stagnosol probability was predicted on slopes < 40° and above 2146 m a.s.l. Whether the first mineral soil horizon displays stagnic properties or not, likely depends on physical soil properties in addition to terrain parameters. Incomplete soil classification resulted in histic and stagnic soil parts dominating the first 100 cm of the soil volume for most of the research area. Comparing CART and Random Forest (RF) in their model performance to predict topsoil texture and bulk density as well as mineral soil thickness by hundredfold model runs with random Jackknife partitions, RF predictions resulted more powerful. Altitude a.s.l. was the most important predictor for all three soil parameters. Increasing sand/ clay ratios with increasing altitude, on steep slopes and with overland flow distance to the channel network are caused by shallow subsurface flow removing clay particles downslope. Deeper soil layers are not influenced by the same process and therefore showed different texture properties. Terrain parameters could only explain the spatial distribution of topsoil properties to a limited extent, subsoil properties could not be predicted at all. Other parameters that likely influence soil properties within the investigation area are parent material and landslides. Strong evidence was found that topsoil horizons did not form from the bedrock underlying the soil profile. Parent material changes within short distance and often within one soil profile. Landslides have a strong influence on soil-landscape formation in shifting soil and rock material. Soil mechanical and hydrological properties in addition to terrain steepness were hypothesized to be the major factors in causing soil slides. Thus, the factor of safety (FS) was calculated as the soil shear ratio that is necessary to maintain the critical state equilibrium on a potential sliding surface. The depth of the failure plane was assumed at the lower boundary of the stagnic soil layer or complete soil depth, depending on soils being stagnic or non-stagnic. The FS was determined in dependence of soil wetness referring to 0.001, 0.01, 0.1 and 3 mm/h net rainfall rate. Sites with a FS ≥ 1 at 3 mm/h (complete saturation) were classified as unconditionally stable, sites with a FS < 1 at 0.001 mm/h as unconditionally unstable. The latter coincided quite well with landslide scars from a recent aerial photograph.
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The reactivity of ferric (hydr)oxides towards dissolved sulphide
(2010)
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Katrin Hellige
- Ferric (hydr)oxides are ubiquitous with different characteristics such as stability, reactivity and surface properties and play an important role in redox reactions in many environments such as soils, marine sediments, lakes, and ground water. Under anoxic conditions, ferric (hydr)oxides are reduced by dissolved sulphide or by microorganisms. This reaction generates Fe(II) which may precipitate as iron hydroxide, adsorb to the ferric (hydr)oxide surfaces and transform the ferric (hydr)oxides into more stable minerals, or precipitate as iron sulphide. During the reductive dissolution adsorbed species like arsenic may be released from the oxide surfaces to solution. Furthermore, the generation of ferrous iron in ground water systems, their transport through the groundwater-surface water interface, and subsequent iron oxidation and precipitation contribute to the acidification of lakes or sediments as a result of both mining activities and natural processes. Hence, the redox reactions between dissolved sulphide and ferric (hydr)oxides are of fundamental importance for the elemental cycles of sulphur and iron and in particular for the carbon and electron flow in groundwater, soil, and lake systems. The overall chemical pathway of the reactions and their kinetics are reasonably understood. There is less knowledge on the transient stages and the electron transfer processes during the reactions which involve the formation of amorphous or disordered, as well as, nucleation of (metastable) crystalline phases at the reacting interface as a function of time. Furthermore, the interaction between dissolved sulphide and ferric (hydr)oxides can be regard as a key reaction ultimately leading to pyrite formation in both marine and freshwater sediments. However, the knowledge on the pathways and on the controlling factors of pyrite formation is still limited. Therefore this work focused on anoxic abiotic kinetic batch and flow-through experiments with various ferric (hydr)oxides and dissolved sulphide at pH 4 and pH 7. TEM, X-ray diffraction, Mössbauer spectroscopy, and wet chemistry were used to explore the nanocrystalline products which formed over time during the reaction. Furthermore, these experiments should be contribute to the elucidation of the role of Fe2+ regarding the iron sulphide formation and the transformation of Fe(III) oxides. The electron transfer reaction between dissolved sulphide and ferric (hydr)oxides and the deeper insight into the processes occurring at the ferric (hydr)oxides surfaces were investigated in chapter 2 and 3. Batch experiments with dissolved sulphide and ferrihydrite, lepidocrocite, and goethite were performed under well-defined conditions at pH 7 and at room temperature in a glove box with a special emphasis on the characterization of nanocrystalline products forming at different time steps over a reaction time of 14 days. The temporal evolution of the chemical species and the solid phases indicate that the reaction progress was highly dynamic. After two weeks we observed the formation of secondary minerals and pyrite in all experiments as a result of excess-Fe(II) formation. Ferrihydrite was transformed completely via dissolution-precipitation processes into more stable minerals such as magnetite, hematite, pyrite, and into minor amounts of goethite. In the experimental solution with lepidocrocite and goethite the host mineral remained and we detected only pyrite as new mineral. Small amounts of goethite were transformed to hematite while the pyrite formation in the experimental solution with lepidocrocite was accompanied by traces of magnetite. In chapter 4, the reaction kinetic of dissolved sulphide and ferric (hydr)oxides were studied under abiotic, anoxic, and flow-through conditions at pH 4 and 7 and at room temperature over a time period of 6 hours. Various synthetic Fe(III) (hydr)oxides with a broad range of crystallinity and different surface and bulk properties were used in order to assess how variations in these properties influence the kinetics of chemical Fe(III) (hydr)oxide reduction. These experiments showed, as well as, the batch experiments, that the formation of Fe(II) and S(0) was decoupled. In the presence of ferrihydrite and lepidocrocite the generated Fe(II) due to the reaction with dissolved sulphide adsorbed to their surfaces and was accompanied by an electron transfer which led to the formation of excess-Fe(II). These processes seem to be accelerating the reductive dissolution of ferrihydrite and lepidocrocite by dissolved sulphide. Goethite behaved differ: the adsorption of Fe(II) onto the goethite surface occurred without an electron transfer. Thus, the generated Fe(II) controls the reductive dissolution of various ferric (hydr)oxides by dissolved sulphide.
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Azobenzene-functionalized molecular glasses for holographic applications
(2010)
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Roland Walker
- Holography is an optical imaging technique, with which an authentic copy of the original object can be created, even in the absence of the object itself. This means, that in contrast to conventional photography, the information of depth is not lost. Holography is based on writing an interference grating in a photosensitive volume element. Hereby two light sources are generating an interference pattern, which causes chemical or physical changes in the photosensitive material. By illumination of the stored diffraction grating, the original information can be reconstructed. One of the most important classes of photoaddressable chromophores utilized in holography are azobenzene compounds. Owing to the rich photochemistry of these chromophores, materials incorporating azobenzenes can be used as photoswitches, allowing fast and reversible control over the chemical, physical or optical properties of the entire system. Therefore, azobenzene-containing compounds are envisioned as smart light-responsive materials for various holographic applications. This thesis describes the synthesis and characterization of azobenzene-containing molecular glasses as well as their application as functional materials in specific holographic experiments. By utilizing a modular design principle, we were able to fine-tune their physical and photo-physical properties and optimize the molecular structure in view of the formation of surface relief nanostructures as well as inscription of holographic volume gratings. Understanding the formation of surface relief nanostructures and discovering ways of controlling the process is of importance, as uniform surface relief gratings (SRGs) with adjustable spacing and amplitude are of interest. Therefore a new series of azobenzene-containing molecular glasses based on a triphenylamine core has been synthesized and photo-physically characterized. A clear relationship between the chemical structure of these molecules and SRG build-up was established: the rate of formation and the maximal achievable amplitude of SRGs strongly depend on the optical susceptibility at the wavelength of the writing laser. Furthermore, we found that different polarizations of the laser beams also have a major influence. With this knowledge we were able to efficiently form SRGs with amplitude heights of up to 600 nm by tailoring the molecular structure of the material and selecting specific experimental conditions. Furthermore, it has been demonstrated that these surface patterns are stable enough to be transferred to a polymer surface with replica molding techniques. This concept has the potential to be practically applied for holographic optical elements. Holography is a most promising solution for optical data storage, as in contrast to conventional optical storage media, the entire volume of the medium is used instead of only a few thin layers. Unfortunately, current rewritable materials still exhibit certain challenges, most important, sufficiently fast writing times. Therefore, material concepts especially for improving the recording time as well as the long-term stability of holographic volume gratings are presented. By employing azobenzene-containing molecular glasses in blends with photoaddressable polymers, we were able to merge the excellent long-term stability of the polymer systems with the higher photo-physical sensitivity of the molecular glasses, thus creating a superior holographic material which combines the advantages of both material classes. In order to find a suitable blending material, we synthesized series of photochromic azobenzene-containing molecular glasses and screened them with respect to their photo-physical properties. The best combination of structural variations was chosen for the blending experiments. Already a blend comprising as less as ten wt% of molecular glass allowed us to decrease the holographic writing time of a photoaddressable block copolymer system by a factor of three while increasing the recording sensitivity by the factor five. In addition to molecular glasses with ordinary azobenzene chromophores we also examined low molecular weight materials functionalized with bisazobenzene moieties. This enabled us to achieve higher maximum refractive index modulations. Liquid-crystalline behaviour could be introduced with the incorporation of substituents at the bisazobenzene moiety. Subsequent investigations of the photo-physical properties revealed a long-term stable photo-orientation solely based on small molecular compounds, making such materials an interesting alternative to established systems. In summary, this thesis demonstrates that azobenzene-containing molecular glasses are a worthwile focus for research, as they are an amazingly versatile and adaptable class of materials suitable for a large number of different applications.
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Role of Dissolved Organic Nitrogen in the Soil Nitrogen Cycle of Forest Ecosystems
(2010)
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Bettina Schmidt
- In the last years, dissolved organic N (DON) has been shown to be a crucial part of the soil N cycle in forest ecosystems. Despite this, information on its dynamics, sources and fate is still lacking. Especially data from (sub)tropical forest ecosystems are scarce. Therefore, this study investigated (i) the magnitude and drivers of DON fluxes in a subtropical montane forest, (ii) the biodegradability of DON from forest floors, (iii) the abiotic formation mechanism for DON in forest floors as postulated by the Ferrous Wheel Hypothesis and (iv) the link between DON and dissolved organic C (DOC) dynamics. In a field study (2005-2008), average DON fluxes in forest floor percolates and seepage (60 cm) of a subtropical montainous cypress forest (16 and 8 kg N ha-1 yr-1, respectively) were similar to fluxes in other (sub)tropical ecosystems, and dominated total N fluxes. Dissolved organic N concentrations in the soil were independent of the water flux (meaning that no dilution effect was visible). This implies that first, the pool size of potentially soluble DON is variable and second, that this pool is hard to deplete. In contrast, the linear relationship between soil organic solute and water fluxes was positive, showing that precipitation is an important driver for DON losses in this ecosystem. Although this has also been reported from temperate ecosystems, this relationship did not hold when analyzing the combined data from various (sub)tropical and temperate forest ecosystems. The biodegradability of DON was highest in inoculated spruce-Oi water extracts in a 21-day incubation experiment, while in extracts from beech-Oi and Oa horizons, DON concentrations only slightly decreased. Dissolved organic N was recalcitrant in spruce-Oa and cypress-Oa extracts, indicating that this DON could add to the formation of stable soil N pools. As various additions of NO3- never influenced DON biodegradation, it is concluded that microbes do not necessarily prefer mineral N over DON as substrate. Mineralization was always more important than microbial uptake in samples without NO3- additions, and denitrification only played a minor role in spruce-Oi samples (as indicated by a negative balance of all N species after 21 days). Fluorescence excitation-emission spectroscopy and subsequent parallel factor analysis identified four groups of fluorophores in the extracts. The initial concentration of two of these so-called factors was correlated with DON biodegradation, but protein-like fluorescence (which has been suggested as a proxy for dissolved organic matter biodegradation) was shown to be independent of DON biodegradation due to similar excitation-emission-maxima of recalcitrant compounds. Therefore, these factors might not always be suitable to predict DON biodegradation. The abiotic reaction of NO2- with DOC (as postulated by the last step of the Ferrous Wheel Hypothesis) was tested in a second incubation experiment in extracts with varying DOC concentrations and qualities and NO2- additions under oxic conditions. Concentrations of added NO2- never decreased within 60 min, indicating, that no DON formation from added NO2- took place. The results show, that the last step of the Ferrous Wheel Hypothesis (which has been suggested to be fast) is unlikely to occur in forest floors. Dissolved organic N and C fluxes were both highly dependent on precipitation at the cypress site, suggesting a strong link between these two classes of compounds. This assumption was supported by the first incubation experiment, where both DON and DOC biodegradation were not influenced by NO3- additions. Moreover, DOC dynamics closely resembled DON dynamics, which suggests that DON biodegradation could be driven by microbial C demand. Therefore, the often used separation of DON and DOC into functionally different compound classes is not always warranted. In conclusion, this study emphasized the need to include DON in biogeochemical N studies of both temperate and (sub)tropical ecosystems, and provided new and important insights regarding DON biodegradation, possible DON sources in forest floors and the link between DON and DOC dynamics in forest ecosystems.
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"Smart" Hydrogels based on Trishydrophilic Triblock Terpolymers
(2010)
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Stefan Reinicke
- The work presented in this thesis focuses on the synthesis of double stimuli-responsive, trishydrophilic triblock terpolymers and their utilization for the construction of “smart” hydrogel systems, responding to a variety of external stimuli. The central focus was put on ABC triblock terpolymers composed of a pH-sensitive A block, a water soluble B block and a thermo-sensitive or multi-responsive C block. This concept was used for the construction of hydrogels responding independently to pH, temperature, and UV light. It was further applied to the formation of polymer/nanoparticle hybrid micelles suitable for the formation of magneto-responsive hydrogels (ferrogels). At first, a new route for the synthesis of block copolymers, containing ethylene oxide and glycidol derivatives, was developed. The crucial aspect of this procedure, based on sequential anionic polymerization, was the utilization of the phosphazene base t-BuP4, enabling the anionic polymerization of epoxide monomers in the presence of lithium counterions. It was shown, that ethoxyethyl glycidyl ether polymerizes easily under the established polymerization conditions without unwanted termination. Hence, we were able to synthesize well-defined block copolymers containing vinyl and epoxide monomers in a one-pot reaction, without performing additional intermediate steps. This new synthetic route was then utilized to synthesize a series of poly(2-vinylpyridine)-block-poly(ethylene oxide)-block-poly(glycidyl methyl ether-co-ethyl glycidyl ether) (P2VP-b-PEO-b-P(GME-co-EGE)) triblock terpolymers suitable for pH and temperature dependent hydrogel formation. The reversible gelation for this particular system relies on two distinct mechanisms. Under conditions, where only one outer block is insoluble, core-shell-corona (CSC) micelles are formed, resulting in gelation via close cubic packing of the micelles. On the other hand, the micelles are also able to crosslink through their corona when both outer blocks are insoluble. As a direct consequence, a temperature triggered gel-sol-gel transition occurred at pH = 7, accompanied by a unique gel strengthening. Solubility and gelation studies were performed by DLS, rheology and SANS. The influence of polymer concentrations and block lengths on the gelation behavior and gel properties was studied. In order to derive information about the exact structure of the cubic lattice formed in the low temperature gel phase (simple cubic or body centered cubic), a 19 wt% aqueous solution of a particular P2VP-b-PEO-b-P(GME-co-EGE) triblock terpolymer at pH = 7 was further investigated using SANS under steady shear. By application of shear stress, the irregularly arranged polydomains of the sample oriented macroscopically along a preferred direction, which led to highly defined, strongly anisotropic 2D scattering patterns. The interpretation of these patterns confirmed the presence of a body centered cubic packing. The gel-sol transition upon temperature increase can be explained by a shrinkage of the shell of the CSC micelles. To increase the versatility of the established hydrogel concept, we further synthesized ABC triblock terpolymers with different responsive polymers as C blocks. This required an alternative synthetic route, combining anionic polymerization and ATRP via “click” chemistry. After optimization of each synthetic step, exemplary poly(2-vinylpyridine)-block-poly(ethylene oxide)-block-poly(oligo(ethylene glycol) methacrylate) (P2VP-b-PEO-b-POEGMA) and poly(2-vinylpyridine)-block-poly(ethylene oxide)-block-poly(dimethyl- aminoethyl methacrylate) (P2VP-b-PEO-b-PDMAEMA) triblock terpolymers were synthesized, respectively, and characterized regarding their solubility and gelation behavior. At pH > 5, P2VP-b-PEO-b-PDMAEMA forms CSC micelles with a P2VP core, and a pH- as well as thermo-sensitive PDMAEMA corona. This particular structure represents a hydrogel, whose temperature dependent response can be easily changed from a gel-sol to a sol-gel transition by increasing the pH from 8 to 9. At pH = 7.5 on the other hand, gel formation is induced by the addition of hexacyanocobaltate(III) ions due to electrostatic interactions between the multivalent cobaltate ions and the charged DMAEMA units, causing a physical crosslinking of the CSC micelles. The gel can subsequently be disintegrated by an exposure to UV-light, based on a UV-catalyzed aquation of the trivalent hexacyanocobaltate(III) ions ions to divalent aquapentacyanocobaltate(III)-ions. In the last part, a new approach was developed to create a novel type of magnetic field-responsive hydrogels (ferrogels), in which the nanoparticles are tightly bound to the polymer matrix. The P2VP block of the previously synthesized P2VP-b-PEO-b-P(GME-co-EGE) triblock terpolymers was quaternized to a low extent and complexed with negatively charged, citrate stabilized maghemite (γ-Fe(III)-oxide) nanoparticles. Using different analytical methods it was shown that well-defined CSC hybrid micelles were obtained with cores formed by a complex of P2VP and 3-4 nanoparticles per core. Concentrated solutions of these micelles are able to form gels depending on temperature, as revealed by rheology measurements. Due to the presence of the maghemite particles, it is possible to induce gelation via remote heating using AC magnetic fields, which was demonstrated by high frequency magnetocalorimetry.
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Chromosome dynamics during cell divisions in Drosophila melanogaster: The role of Rad21 in meiotic cohesion and dynamic analysis of the condensin subunit CapG in early embryonic mitotic divisions
(2010)
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Sonal Nagarkar
- Faithful segregation of genetic material is an essential hallmark of cell division. In eukaryotic cells, the DNA is replicated during S phase into two identical copies, which reside intimately paired (cohesed) in the nucleus as dispersed and entangled interphase chromatin fibers. At the onset of mitosis, the chromatin fibers start to resolve and by the end of metaphase they are compacted and individualized into a pair of cylindrical structures called sister chromatids, which remain connected until anaphase onset by residual sister chromatid cohesion in their centromeric regions. The compaction process is known as chromosome condensation, which is a prerequisite for accurate segregation of sister chromatids in anaphase. Chromosome condensation and sister chromatid cohesion require multisubunit protein complexes, the condensin and the cohesin complexes, respectively. Both complexes are composed of two core SMC subunits and a set of non-SMC subunits, which are conserved among most eukaryotes. In the first part of my thesis, I have analyzed the localization and dynamic behavior of a functional, EGFP-fused variant of CapG, one of the non-SMC subunits of the condensin I complex in Drosophila melanogaster. In vivo fluorescence microscopy of early embryonic mitotic divisions revealed that CapG-EGFP is mainly nuclear during interphase and that it starts to enrich at centromeric proximal regions in late interphase. Thereafter, CapG-EGFP spreads onto the chromosome arms concomitantly with the initiation of chromosome condensation (ICC) and loading is complete already in prophase at the time of nuclear envelope breakdown. Furthermore, FRAP analyses revealed that a major proportion of CapG-EGFP is stably bound to chromatin during metaphase and only a minor fraction shows a dynamic association with chromatin. These results are similar, but not identical, to findings previously obtained for another non-SMC subunit, CapH/Barren, suggesting interactions of the individual non-SMC subunits with chromatin outside a bona fide condensin complex. Since a non-SMC cohesin subunit homologous to the typical meiotic Rec8 protein found in other eukaryotes appears to be missing in Drosophila, I have assessed in the second part of my thesis a possible cohesive role for the mitotic subunit Rad21 during female meiosis. Furthermore, a potential redundancy during oogenesis between Rad21 and another candidate cohesin subunit, C(2)M, was analyzed. Forced proteolysis of Rad21 during oogenesis resulted in delocalization of the canonical cohesin core subunit Smc1 from oocyte chromatin. Furthermore, immunofluorescence and fluorescence in situ hybridization analyses revealed a high proportion of premature homolog disjunction and premature sister chromatid separation in the developing mutant oocytes and also during the meiotic divisions. Moreover, it was established that Rad21 has a role in the maintenance of the synaptonemal complex (SC), as shown by delocalization of the transversal SC component C(3)G. Taken together, these results suggest that Rad21 is indeed involved in sister chromatid cohesion during female meiosis in D. melanogaster. Since in the absence of Rad21 and the concomitant presence of C(2)M meiotic sister chromatid cohesion is compromised, Rad21 appears to play the major role in meiotic sister chromatid cohesion in D. melanogaster and a functional redundancy between C(2)M and Rad21 is unlikely.
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Donor-Acceptor Block Copolymers in Organic Electronics - Spectroscopy, Charge Transport, Morphology and Device Application
(2010)
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Sven Hüttner
- Organic electronic devices have attracted increasing attention over the last decade. The use of organic materials allows the creation of large area, flexible and low-cost lightemitting devices, transistors and photovoltaics. The development of new organic materials contributes to a successful commercialisation. The present work deals with the characterisation of novel donor-acceptor block copolymers and their constituent polymer blocks that are well-suited for organic photovoltaics. Block copolymers phase-separate and self-assemble into nanostructured morphologies due to the covalent linkage of the two blocks. The interplay between intermolecular interactions, mesoscopic crystalline structures and the block copolymer microphase separation determine the material properties and therefore the device characteristics. Thus, these block copolymers offer a unique platform to study the electronic and photophysical properties of confined donor-acceptor systems. This work is concerned with the fundamental characterisation of these properties as well as the application in organic field effect transistors and organic solar cells. The acceptor polymer block poly(perylene bisimide acrylate) (PPerAcr) consists of perylene bisimide (PBI) units that are linked to a polyacrylate backbone. We have investigated the homopolymer PPerAcr, a model block copolymer in conjunction with polystyrene (PS), as well as fully functionalised block copolymers with a donor block either made of poly(vinyl triphenylamine) (PvTPA) or poly(3-hexylthiophene) (P3HT). These polymers offer a set of electronically active materials with several hierarchical structures: The PBI moieties feature intermolecular pi-pi interactions that lead to crystalline side chains of PPerAcr that form a lattice of one-dimensional stacks of PBI. Further nanoscopic structures are induced by the combination of PPerAcr with another amorphous block or another semi-crystalline block such as P3HT due to phase separation. Since PPerAcr is used as an electron transporting material in all subsequent block copolymers, its structural, optical and electronic properties are investigated in detail. The intermolecular interactions of the PBI moieties favour not only charge transport, but also affect the optical properties, due to the electronic coupling of the transition dipole moments. Thus, optical spectroscopy such as absorption and fluorescence spectroscopy give access to information about the intermolecular packing, which is correlated with temperature dependent X-ray diffraction studies. The strong intermolecular packing of the PBI units can be overcome by solvent-vapour exposure. This is especially helpful to induce polymer chain mobility, enabling the completion of block copolymer phase separation for example. This method was studied in detail by means of in-situ spectroscopy and ellipsometry during controlled solvent-vapour exposure. Spincoated films of PvTPA-b-PPerAcr exhibit an incomplete phase separation and can be transformed into an ordered lamellar morphology by solvent-vapour annealing. In addition to PvTPA, we have characterised further poly(triarylamines) with different electron-rich substituents at the TPA units in OFETs. All these polymers are amorphous side-chain polymers. We found the charge carrier mobility to be independent of the molecular weight, though allowing an adjustment of their thermal properties for device fabrication. This is in contrast to P3HT, which is a semi-crystalline, conjugated main chain polymer. X-ray diffraction, steady state and time-resolved spectroscopy, as well as the transistor device characterisation were employed to establish a charge transport - morphology relation for the donor-acceptor block copolymers P3HT-b-PPerAcr containing two crystalline blocks. Controlling the crystallisation preferences of the two blocks leads to a new processing route for OFETs with tunable p-type, ambipolar, or n-type transport through a one-time thermal annealing step. The application of P3HT-b-PPerAcr in organic photovoltaic devices showed also very promising results with high external quantum efficiencies. Subsequently, the photophysics of P3HT-b-PPerAcr by means of absorption and fluorescence spectroscopy as well as time-resolved transient absorption spectroscopy were investigated. All block copolymers exhibited an ultra-fast charge-pair formation and a strongly reduced photoluminescence, suggesting domain sizes of only some nanometres. Although efficient charge separation could be accomplished, a good charge percolation was lacking due to small domain sizes. Furthermore the herein presented results emphasis the fundamental importance of morphology and interfacial properties such as crystallinity. These findings motivate the further use of block copolymers as compatibilising agents for polymer blends to improve their interface and morphology.
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Documentation and Instruction Manual for the Krypton Hygrometer Calibration Instrument
(2010)
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Thomas Foken
Hubert Falke
- no abstract
