50 search hits
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Footprint synthesis for the FLUXNET site Waldstein/Weidenbrunnen (DE-Bay) during the EGER experiment
(2008)
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Lukas Siebicke
- no abstract
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ExchanGE processes in mountainous Regions (EGER) Documentation of the Intensive Observation Period (IOP2) June, 1st to July, 15th 2008
(2008)
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Andrei Serafimovich
Lukas Siebicke
Katharina Staudt
Johannes Lüers
Martina Hunner
Tobias Gerken
Stephanie Schier
Tobias Biermann
Friederike Rütz
Jannis von Buttlar
- no abstract
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ExchanGE processes in mountainous Regions (EGER)- Documentation of the Intensive Observation Period (IOP1) September, 6th to October, 7th 2007
(2008)
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Andrei Serafimovich
Lukas Siebicke
Katharina Staudt
Johannes Lüers
Tobias Biermann
Stephanie Schier
Thomas Foken
- no abstract
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Mesoscale Modeling of Phase Behavior in Thin Films of Cylinder-Forming ABA Block Copolymers
(2008)
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Andriana Horvat
- In this thesis modeling results on structure formation in thin films of cylinder-forming block copolymers are presented and discussed. The computational study of the equilibrium phase behavior in thin films is complemented by detailed comparison with a real experimental system. Additionally, the dynamics in such films at various length and time scales (the dynamics of individual defects and the dynamics of surface relief structures) is studied. The strength of the presented thesis is the comparison of thin block copolymer film equilibrium and dynamic behavior in experiments and in computer simulations. This comparison supplies an in-depth understanding of the processes in thin films and near the surfaces in thick films and allows to identify the important control parameters of nanopattern formation. Chapters 4 and 5 report on the phase behavior of thin films of asymmetric block copolymers. In addition to the surface induced alignment of hexagonally ordered cylinders, an adjustment to the planar symmetry of the surface by formation of surface reconstructions is found to dominate the phase behavior in thin films. The large parameter space covered by the simulations allows to distinguish the effects of the two constraints characteristic for thin films: the surface field and the film thickness. The deviations from the bulk cylinder structure, both in the vicinity of surfaces and in thin films are identified as surface reconstructions. The stability regions of different phases are modulated by the film thickness via interference and confinement effects. The results give evidence of a general mechanism that govern the phase behavior in thin films of modulated phases: The interplay between the strength of the surface field and the deformability of the bulk structure determines how the system rearranges in the vicinity of the surface. Chapters 6 and 7 present a systematic study of defects in thin films of cylinder-forming block copolymers. In particular, the peculiarities of both classical and specific topological are considered in detail, and a strong relationship between the defect structures and the chain mobility in block copolymers is observed. In the systems studied, representative defect configurations provide connectivity of the minority phase in the form of dislocations with a closed cylinder end or classical disclinations with incorporated alternative, nonbulk structures with planar symmetry. In solvent-annealed films with enhanced chain mobility, the neck defects (bridges between parallel cylinders) were observed. This type of nonsingular defect has not been identified in block copolymer systems before. It is shown, that topological arguments and 2D defect representation, sufficient for lamellar systems, are not sufficient to determine the stability and mobility of defects in the cylindrical phase. In-situ scanning force microscopy measurements are compared with the simulations based on DDFT. The close match between experimental measurements and simulation results suggests that the lateral defect motion is diffusion-driven. Finally, the morphological evolution is considered with the focus on the motion and interaction of the representative defect configurations. Chapter 7 reveals dynamic simulations and in-situ SFM measurements of defect annihilation. Along with the lateral movement of defects, the annihilation frequently proceeds through local structural transitions. The role of the observed structural evolution is discussed in the context of the equilibrium phase behavior of cylinder-forming thin films, studied in chapters 4-5. Chapter 8 presents a study of terrace formation in thin films of a cylinder-forming block copolymers by a computational DDFT method. The results are compared with in situ SFM measurements of SBS block copolymer thin films. This chapter focuses on the early stage of terrace formation, where 80% of height changes occur. Experimental and simulation results agree that the change of the local height is strongly connected to the changes in the local microstructure. The detailed pathways of the structural transitions, as revealed by simulations, suggest a diffusion of block copolymer chains along the microstructure interfaces and indicate an important role of cylinders with necks as a material-transport-channel between neighboring terraces in thin cylinder-forming films. Both systems (in experiment and in simulations) show excellent quantitative agreement in detail of structural phase transitions and in the dynamics of the step development, suggesting that the underlying transport mechanisms are governed by diffusion.
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Synthesis and Characterization of immobilized Gold Nanoparticles and Binary Gold Nanoalloys on Cationic Spherical Polyelectrolyte Brushes and their Application as a Catalyst
(2008)
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Marc Claudius Schrinner
- First of all the up scaled synthesis for cationic and anionic spherical polyelectrolyte brushes (SPB) was introduced and a reproducible method for the synthesis was established (Chapter 3.1). For a better understanding of anionic SPB the complexation of the anionic polyelectrolyte chains with the cationic surfactant cetyl trimethyl ammonium bromide was studied in detail. The models were proved by cryogenic transmission electron microscopy (cryo-TEM) and dynamic light scattering (DLS), if it’s in good agreement with the resulting systems (Chapter 3.2). It was possible to show, that cationic SPB could be used for the generation of gold nanoparticles (Chapter 3.3). The synthesized carrier systems were characterized in detail by transmission electron microscopy (TEM), cryo-TEM and disc centrifuge (DCP). The more detail examination of the Au/SPB system by DLS, TEM/cryo-TEM, showed that a reversible immobilization system for gold nanoparticles was synthesized. The immobilized gold nanoparticles@SPB could be complexed by cyanid ions and oxygen. After the complexation of the gold nanoparticles we get the previous carrier system back. Detailed studies by wide angle X-ray scattering (WAXS) and high resolution transmission electron microscopy (HR-TEM) show an amorphous morphology of the gold nanoparticles in the range of 1 nm. This kind of generation allow us to generate gold nanoparticles in the range of 1.0 and 2.5 nm on the surface of the carrier particle. In the following part of the thesis the concepts described above were used for the synthesis of binary gold nanoalloys (Chapter 3.4 and 3.6). The binary systems Au-Pt, Au-Ru, Au-Rh and Au-Ir were sucessfully generated. In the next step the nanoparticular structure of these binary systems were completely clarified by different HR-TEM methods and WAXS. It was shown, that there is a different behaviour between macroscopic and nanoscopic world. Bulk Au-Pt alloys show miscibility gaps, whereas Au-Pt nanoalloys have no such miscibility gap. This alloy obeys the Vegard’s law. For the generation of facetted Pt nanocrystals (Chapter 3.5), the complexation of gold atoms by cyanid ions and oxygen is used. By this way it’s possible to synthesize well-ordered PtNP structures starting from the Au-Pt nanoalloy. In the second part of the thesis the immobilized gold nanoparticles and the binary gold nanoalloys applied as catalysts for industrial interesting oxidation reactions of alcohols and epoxidation reactions (Chapter 3.4 and 3.6) were studied. All reactions could be conducted at room temperature and in water as reaction media. The catalytic activities have a strong dependency on the composition in the nanoalloy. Cryo-TEM characterization showed us no change of the morphology of the catalyst before and after a catalyst cyclus. Concluding this thesis showed successfully a new route for the synthesis of monodispers and well defined gold nanoparticles, gold nanoalloys and facetted platinum nanocrystals. The particle sizes ranges between 1.0 and 7.0 nm. All systems can be used as green catalysts. This is an important point in the discussion of sustainability. All dispersions are not light and air sensitive, so they can be handled without any problems.
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Computational Analysis of the Proton Transfer to the Secondary Quinone of Type II Photosynthetic Reaction Centers.
(2008)
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Eva-Maria Krammer
- Für das Leben einer Vielzahl unterschiedlicher Arten wird molekularer Sauerstoff benötigt. Auf der Erde ist der wichtigste Prozess zur Herstellung von molekularem Sauerstoff die Photosynthese. Ein entscheidender Schritt der Photosynthese wird durch den Typ II des photosynthetischen Reaktionszentrums (RC) katalysiert: Die Umwandlung von chemischer Energie in einen elektrochemischen Gradienten durch die Reduzierung und Protonierung eines Coenzym Q Moleküls, dass in der QB Bindungstasche des Proteins gebunden ist. Die Pigmente des Typ II RC, nämlich des pflanzlichen Photosystem II RC (PSII RC) und dessen evolutionären Vorfahren, dem bakteriellen RC (bRC), sind in zwei (pseudo)-symmetrischen Zweigen angeordnet, dem A- und dem B-Zweig. In Typ II RC Proteinen werden die Elektronen entlang des A-Zweiges auf QB übertragen, während der B-Zweig keine Elektronen übertragen kann. In dieser Arbeit wurde der Konservierungsgrad von Resten untersucht, für die eine Beeinflussung der Redoxeigenschaften der Pigmente und der Lenkung des Elektronentransfers entlang des A-Zweiges bekannt ist. Die Qualität einer Konservierungsanalyse hängt massgeblich von einem korrekten multiplen Sequenzalignment ab. Da bRC und PSII RC nur eine sehr kleine Sequenzidentität haben, wurden profile Hidden Markov Modelle verwendet, welche die strukturellen Informationen der Proteine berücksichten, um ein korrektes Alignment zu erhalten. Die Konservierungsanalyse zeigte, dass die Abstimmung von Redoxeigenschaften der Pigmente und die Lenkung des Elektronentransfers im bRC konserviert sind, aber in PSII RC abweichen. Zwischen bRC und PSII RC Proteinen gibt es dementsprechend auch Unterschiede in den Eigenschaften der beiden Coenzym Q (QA und QB) Bindungstaschen, die es ermöglichen, dass im PSII RC QA unter Stressbedingungen (wie hoher Lichtintensität) protonieren kann, während eine solche Protonierung im bRC nicht möglich ist. Interessanterweise wurden im bRC von Rhodobacter (Rb.) sphaeroides zwei alternative Bindungspositionen für QB (proximal und distal) festgestellt. Experimente deuteten an, dass QB seine Orientierung um 180 Grad ändert, während es sich von der distalen in die proximale Position bewegt. Zusammen mit kristallographischen Experimenten zeigten meine quantenchemischen und elektrostatischen Berechnungen, dass im bRC von Rb. sphaeroides QB wahrscheinlich die gleiche Orientierung in beiden Positionen einnimmt. Eine Kopplung des Protonierungszustands der terminalen Protonendonoren (GluL212 und AspL213 der L Untereinheit) und der Population der beiden QB Bindungspositionen erklärt die beobachtete pH- und Zustandsabhängigkeit der QB Population. Darüber hinaus müssen diese Reste protoniert sein, um das erste Reaktionszwischenprodukt, das zellschädigende Semichinon gebunden zu halten. Im Unterschied zum Elektronentransfer entlang des A-Zweiges unterscheidet sich der Mechanismus des Protonentransfers zu QB massgeblich zwischen PSII RC und bRC. Im bRC von Rb. sphaeroides wurden die wesentlichen Reste des Protontransfers zu QB experimentell bestimmt und Protoneneintrittspunkte wurden vorgeschlagen. Die genaue Organisation des Protonentransfers zu QB ist allerdings nicht bekannt. Zwei sich ausschliessende Ideen werden diskutiert: Die Protonen werden entweder über unterschiedliche Pfade oder über ein grosses Netzwerk ohne klar definierte Pfade, einem Protonenschwamm, transportiert. Die Auswertung eines multiplen Sequenzalignments der bRC Untereinheiten zeigte, dass die wesentlichen, nicht auf der Proteinoberfläche liegenden Reste des Protonentransfers konserviert sind. Die vorgeschlagenen Protoneneintrittspunkte sind aber nicht im gleichen Ausmass konserviert. Zusätzlich zeigte die Auswertung des Wasserstoffbrückennetzwerks der bRC Proteine von Rb. sphaeroides und Blastochloris viridis jeweils ein grosses Netzwerk, dass vom Cytoplasma bis zur QB Bindungstasche reicht. Interessanterweise haben diese Netzwerke einen ähnlichen Aufbau und beinhalten alle wesentlichen nicht auf der Oberfläche liegenden Reste des Protonentransfers, unterscheiden sich aber in den ermittelten Protoneneintrittspunkten. Sowohl die Konservierungsstudie und als auch die Analyse des Netzwerkes widersprechen der Idee von unabhängigen Protonentransferpfaden und unterstützen die Idee des Protonenschwamms. Durch die Kombination unterschiedlicher Ansätze wie der Konservierungsanalyse basierend auf multiplen Sequenzalignments, der Kontinuumselektrostatik, der Quantenchemie und der Analyse der Wasserstoffbrückennetzwerke, gelang es in dieser Arbeit ein breiteres Wissen "uber die molekularen Details der QB Bindingungstasche und des Elektronen- und Protonentransfer zu QB zu gewinnen.
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Liquid Crystalline Copper(II) Complexes of Poly(propylene imine) Dendrimers
(2008)
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Laura Torre Lorente
- In the last years hybrid systems consisting of organic materials and metal centres have attracted considerable attention since they combine characteristics of metals with those of organic compounds. Particularly interesting are liquid crystalline systems because they cause to supramolecular ordered systems with taylored physical properties. Dendrimers with donor sites can be used as chelates for metal complexation and their well defined structure enables control over the number and topology of the metal centres. With this motivation new liquid crystalline, polynuclear complexes of mesogenic 3,4-decyloxybenzoyl-functionalized poly(propylene imine) PPI dendrimers from 1st to 5th generation as ligands and copper(II) as paramagnetic centres have been synthesised. The dendrimeric complexes were prepared by coordination of copper(II) to the corresponding ligands. The maximum loadings (average number of copper centres per dendrimer) reached for generations 1st to 5th are 1.9, 6.3, 12.0, 24.2, and 45.9, respectively. In order to investigate the influence of the copper loading on the properties of the complexes, a series of 2nd generation complexes, covering the whole range of copper loadings (0.3 to 6.3), has been prepared by varying either the stoichiometry of the reaction or the working-up procedure. TEM of the dendrimeric copper(II) complexes revealed relatively monodisperse particles with circular structures have been observed with mean particle diameter of 2 nm for the 2nd up to 3 nm for the 5th generation. According to estimations from X-ray experiments, these sizes correspond roughly to the dimensions of the dendrimeric polar core (tertiary amine scaffold and amidobenzoyl groups in the outer part), where the coordination of copper takes place. EELS spectroscopy has been also performed on the complexes and confirms the presence of copper. The complexes up to 3rd generation were identified by MALDI-ToF-MS as single molecules corresponding to a dendrimeric ligand with different intramolecular copper loadings. For the complexation the copper was used as its nitrate trihydrate Cu(NO3)2.3H2O. Under the reaction conditions nitric acid developes and takes part in the composition of almost every complex as indicated by the N and O contents, which were determined by elemental analysis. In the FTIR spectra of the complexes characteristic bands of quaternary ammonium salts are observed which gives evidence for the protonation of the tertiary amines and excludes them as complexation sites for copper in the dendrimers. FTIR analysis revealed the amide groups as the coordinating groups of copper(II) and gives evidence of the coexistence of N- and O-amido copper coordination in the complexes. Indications for copper-coordinated bidentate nitrate have been observed. The 1st generation complexes can be separated in two types of complexes (´blue´, ´green´), which can be selectively prepared and are related to each other. The individual structures have been studied by EPR spectroscopy (by Dr. N. E. Domracheva), which revealed the coexistence of both types of complexes within the dendrimers of the higher generations and showed a N2O3 (NO4) coordination for copper(II) in the ´blue´ (´green´) complexes. At low temperatures an antiferromagnetic exchange has been observed. This behaviour is caused by a mu-1,3 nitrate group bridging two copper centres, which gives rise to highly ordered structures with an estimated copper to copper distance of 6.9 Å. All investigated PPI copper (II)-complexes are liquid crystalline. For both the complexes and the ligands the segregation of the polar core and the apolar alkoxy terminal chains in different domains is responsible for the appearance of mesophases. The complexation of copper(II) together with the protonation of the tertiary amine groups contribute to the formation of broader mesophases for the complexes as compared to that of the ligands. The complexes exhibit a hexagonal columnar (p6mm) mesophase. Moreover, for complexes up to the 4th generation a columnar rectangular mesophase (probably p2mg) is induced below the hexagonal mesophase, if a certain copper content is exceeded. By wide-angle X-ray diffraction of the complexes highly ordered, columnar mesophases with stacking distances of ca. 6.9 Å were identified, which is in agreement with the EPR data. Hence, it is assumed that copper centres, belonging to different dendrimeric ligand molecules, can coordinate by a nitrate in a mu-1,3 bridging mode, which causes the ordered stacking of the dendrimer units along the columns in the mesophases.
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Smart organic-inorganic nanohybrids of functionalized silsesquioxane nanoparticles
(2008)
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Manuela Schumacher
- The formation and characterization of smart organic-inorganic nanohybrids was studied. The inorganic part was formed by N,N-di(2,3-di¬hydroxy¬propyl)¬3-amino¬propyl¬functional silsesquioxane nanoparticles being highly functionalized with ca. 14 tertiary amino groups per particles, each amino group bearing four hydroxyl groups. Two different polymer systems were used for the organic side: amphiphilic block copolymer micelles of poly(n-butyl acrylate)-block-poly(acrylic acid) and star-shaped poly(acrylic acid)s, the latter serving as a model system for frozen micelles. In all cases the mixing of aqueous solutions of anionic block copolymer micelles or the anionic stars with solutions of the silsesquioxane nanoparticles led to the easy and straightforward formation of organic-inorganic nanohybrids. The structure of the complex nanohybrids depends on pH and salinity. The amount of incorporated silsesquioxane nanoparticles within the micelles or the stars under varying external stimuli was determined using a large number of methods. Complexation preserved the original size of the micelles - consisting of a PnBA core and a PAA corona - according to dynamic light scattering and static light scattering as well as light scattering titration measurements and asymmetric flow field-flow fractionation experiments. Fourier-transform infrared spectroscopy and dialysis measurements with fluorescently labelled silsesquioxane nanoparticles confirmed the nanohybrid formation over a relatively wide range in pH. Cryogenic transmission electron microscopy micrographs indicated a core-shell structure of the nanohybrids with gradual decreasing density of silsesquioxane nanoparticles. LS titrations gave an insight in the postulated interaction mechanism. Complexation in acidic media is driven by hydrogen-bonding and ionic interaction; in alkaline media nanohybrids are mainly formed due to ionic interaction. Depending on ionic strength, attractive Coulomb interactions may be either sufficient to promote complexation even at high pH, where hydrogen-bonding is absent (low ionic strength), or are screened (high ionic strength), resulting in less favourable interactions between micelles and silsesquioxane nanoparticles. The reason for the size conservation is most probably due to the kinetically frozen micellar core and the compensation of increased steric repulsion due to complexation and attractive interactions between the silsesquioxane nanoparticle and the charged PAA. The maximum of the interaction at 0.1 M NaCl could be deduced to be in the range 3.5 < pH < 7.5 NaCl. At low salinity (0.01 M NaCl) more nanoparticles were incorporated within the micelles. Nanohybrids exist even up to very basic conditions (pH < 9.5). The responsiveness of the system on pH and salinity as external stimuli was demonstrated by LS titration, dialysis and FT-IR measurements, thermogravimetric analysis (TGA) and AFFFF measurements. Quantifying the amount of nanoparticles incorporated in the micelles turns out to be a arduous task. SLS of dialysed and undialysed samples and AFFFF of undialysed samples clearly showed increased molecular weights of the formed nanohybrids. TGA - requiring an exhaustive dialysis procedure prior to the measurements - provided information about the amount of incorporated silsesquioxane nanoparticles within the micelles. Isothermal titration calorimetry (ITC) provided the possibility to investigate the complexation mechanism in greater detail. Small angle neutron scattering (SANS) experiments, conducted at basic conditions, provided information on the inner structure of the nanohybrids. A newly developed fitting model enabled the quantification of the amount of interacting nanoparticles under these conditions. All methods to determine the amount of nanoparticles incorporated within the micelles sustained the formation of the organic-inorganic nanohybrids. The absolute number of nanoparticles per micelle is quite high (in the range from 160 to 4300, depending on the used method and conditions), however, the calculated numbers of nanoparticles per acrylic acid unit are quite low (in the range from 0.002 to 0.053). The stars showed behaviour comparable to that of the micelles. According to DLS and SANS experiments their size was preserved during complexation. SANS and LS titration measurements demonstrated the increased mass of the nanohybrid stars compared to the net stars. Cryo-TEM micrographs confirmed the formation of organic-inorganic nanohybrid stars, indicating a morphology with gradually decreasing density of nanoparticles. An appropriate fitting model for the SANS data for this challenging system was developed that proved the interaction between the silsesquioxane nanoparticles and the PAA and enabled the calculation of the amount of entrapped silsesquioxane nanoparticles within one star. The determined values were comparable to the ones calculated for the micellar nanohybrids.
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CO2 and Isotope Flux Measurements above a Spruce Forest
(2008)
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Johannes Ruppert
- The measurement of the turbulent carbon dioxide (CO2) exchange by the eddy covariance (EC) method has become a fundamental tool for the quantitative determination of the atmospheric CO2 net ecosystem exchange (NEE) and the investigation of the carbon mass balances of ecosystems. Such measurements require a high degree of quality control in order to prevent systematic errors. The determination of the annual sum of NEE and filling of data gaps is complicated by characteristic diurnal and seasonal variation in the governing gross flux components of assimilation, i.e. photosynthetic uptake of CO2, and respiration. In this dissertation, a set of criteria is suggested for the identification of high quality NEE data. They are applied to data obtained above a spruce forest in the Fichtelgebirge Mountains in Germany. The application of the quality criteria resulted in less systematic distribution of data gaps compared to a commonly applied criterion based on the friction velocity u-star measured above the canopy. The suggested method is therefore able to reduce the risk of double accounting of nighttime respiration fluxes and systematic error in the annual sum of NEE. The isotopic flux partitioning method can be applied to quantify the assimilation and respiration flux components. Especially above forest ecosystems, it requires isotope flux measurements with high analytical precision in order to resolve small gradients in the isotopic signature of the turbulent exchange. A conditional sampling instrument was developed and tested in laboratory and field experiments. By combining the hyperbolic relaxed eddy accumulation method (HREA), whole-air sampling and high precision isotope ratio mass spectrometry (IRMS), 13CO2 and CO18O isotopic flux densities (isofluxes) could be measured with an estimated uncertainty of 10-20% during a three day intensive measuring campaign of the field experiment WALDATEM-2003 (Wavelet Detection and Atmospheric Turbulent Exchange Measurements 2003). Thorough quality control was applied at all stages of the experiment, including the data evaluation. The sampling process and the assumption of similarity in the turbulent exchange characteristics of different scalars (scalar similarity) were assessed by simulation of HREA sampling based on high temporal resolution data of the turbulent energy and gas exchange. Above three different vegetation types, distinct diurnal changes of scalar similarity were observed and attributed to events on time scales longer than 60 s, which most likely represent changes in the source/sink strength or convective or advective processes. Poor scalar-scalar correlations indicate the risk of systematic underestimation of fluxes measured by HREA. There is some evidence for good scalar similarity and a generally linear relation between bulk CO2 mixing ratios and its isotopic signatures in the turbulent exchange. However, the slope of that relation was observed to change temporarily so that especially for the EC/flask method temporal and spatial scales represented in flask samples must carefully be considered. HREA isoflux measurements have a footprint similar to the footprint of EC measurements and are therefore able to integrate small-scale heterogeneity in ecosystems. CO2 mixing ratios and delta-13C and delta-18O isotopic signatures measured in updraft and downdraft whole-air samples allowed determining ecosystem integrated and truly flux weighted isotopic signatures of the atmospheric ecosystem gas exchange and ecosystem isotope discrimination Delta-e and Delta-E on half-hourly timescales. The observed diurnal variability demonstrates the need for their repeated high precision measurement at ecosystem scale for the evaluation of isotopic mass balances. For the isotopic flux partitioning method, additional data on the integrated canopy isotope discrimination Delta´-canopy from independent measurements or validated models is indispensable. An observed fast equilibration of isotopic disequilibria D13C and D18O between the assimilation and respiration fluxes may indicate that the successful application of the isotopic flux partitioning method is limited to short periods after significant environmental changes on the scale of few days.
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Nonlinear macroscopic description of liquid crystalline elastomers in external fields
(2008)
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Andreas Menzel
- We concentrate on a continuum characterization of the macroscopic behavior of side-chain liquid single crystal elastomers (SCLSCEs). These materials consist of chemically crosslinked polymer backbones, to which mesogenic units are attached as side-groups. Due to specific routes of synthesis SCLSCEs feature a monodomain of the liquid crystalline order in the ground state. Their macroscopic coupling of liquid crystalline order and elastic mechanical deformations makes them unique. In particular, we investigate the macroscopic behavior of cholesteric and nematic SCLSCEs in external electric and mechanical fields. We characterize the respective liquid crystalline state using the director field and describe the current state of mechanical distortion by a displacement field. The specific coupling between these two components is taken into account explicitly by additional macroscopic variables. These are the relative rotations between the director orientation and the polymer network. Using this kind of description, we first study the influence of an external electric field on the state of a cholesteric SCLSCE. For this purpose, the field direction is chosen to be parallel to the helical axis of the cholesteric mesogen alignment. Director reorientation and mechanical distortions are analyzed to linear order. In the case of low electric field amplitudes, we find an effect that has been termed rotatoelectric. Here, with increasing electric field amplitude, the director arrangement rotates around the helical axis, relative to the polymer network. This effect is specific for cholesteric SCLSCEs. We discuss several aspects important for an experimental observation of this effect. Next, we investigate the dielectric instabilities of a cholesteric SCLSCE in a Frederiks splay geometry at higher electric field amplitudes. On the one hand, we find a scenario that corresponds to the Frederiks transition in conventional low molecular weight liquid crystals. Here, the director reorientation arises in a way that is spatially homogeneous in the directions perpendicular to the cholesteric helical axis. On the other hand, however, we also find a qualitatively different instability. The latter is characterized by spatial undulations of the director reorientation, which occur in at least one direction perpendicular to the cholesteric helical axis. We recover the same results in the case of an external magnetic field. Besides, we discuss elastic mechanical compressions or dilations of a cholesteric SCLSCE in the directions parallel and perpendicular to the cholesteric helical axis. Here, small amplitudes of deformation lead to a distortion of the cholesteric helical structure. In the simplest case, we obtain an elongation or compression of the cholesteric helix along its axis. Furthermore, we propose ways to experimentally access so-far unknown values of the material parameters involved. We proceed by developing a model to characterize the nonlinear macroscopic behavior of the materials. For this purpose, we identify two coupled preferred directions in nematic and cholesteric SCLSCEs. One of them is imprinted into the polymer network during the process of synthesis to align the mesogens in a liquid crystalline monodomain. On the other hand, the actual average mesogen orientation may deviate from this imprinted direction and is described by the director field. We derive expressions characterizing nonlinear relative rotations between these two coupled preferred directions and we include them as macroscopic variables into our description. Using our model, we first investigate the shear deformation of a nematic SCLSCE. If the shear plane contains the director, the latter will be reoriented due to the mechanical deformation. In addition, however, we find as a nonlinear effect that the director reorientation acts back onto the elastic mechanical distortion of the material. This leads to compressive and dilative strain deformations. Finally, we study the specific stress-strain behavior of nematic SCLSCEs. It has been found for nematic SCLSCEs stretched perpendicularly to the initial director orientation that their director reorients towards the stretching direction. This reorientation of the director sets in above a critical threshold strain. In the strain regime where the director reorientation occurs, the slope of the corresponding stress-strain curve is significantly decreased. We demonstrate that our model describes this nonlinear behavior. Furthermore, we compare the predictions of our model with experimental data. As a result, we find that nonlinear relative rotations play the central role in the macroscopic characterization of the behavior of the materials. However, we also conclude that the macroscopic stress-strain behavior can be qualitatively influenced by those contributions to the elastic response that are not connected to the director reorientation and relative rotations.
