Non-equilibrium molecular dynamics study of an amphiphilic model system

Diese Doktorarbeit untersucht das Verhalten von komplexenFluidenunter Scherung, insbesondere den Einfluss von Scherflüssenauf dieStrukturbildung.Dazu wird ein Modell dieser entworfen, welches imRahmen von Molekulardynamiksimulationen verwendet wird.Zunächst werden Gleichgewichtseigenschaften dieses Modellsuntersucht.Hierbei wird unter anderem die Lage desOrdnungs--Unordnungsübergangs von derisotropen zur lamellaren Phase der Dimere bestimmt.Der Einfluss von Scherflüssen auf diese lamellare Phase wirdnununtersucht und mit analytischen Theorien verglichen. Die Scherung einer parallelen lamellaren Phase ruft eineNeuausrichtung des Direktors in Flussrichtung hervor.Das verursacht eine Verminderung der Schichtdicke mitsteigender Scherrateund führt oberhalb eines Schwellwertes zu Ondulationen.Ein vergleichbares Verhalten wird auch in lamellarenSystemengefunden, an denen in Richtung des Direktors gezogen wird.Allerdings wird festgestellt, dass die Art der Bifurkationenin beidenFällen unterschiedlich ist.Unter Scherung wird ein Übergang von Lamellen parallelerAusrichtung zu senkrechter gefunden.Dabei wird beoachtet, dass die Scherspannung in senkrechterOrientierungniedriger als in der parallelen ist.Dies führt unter bestimmten Bedingungen zum Auftreten vonScherbändern, was auch in Simulationen beobachtet wird. Es ist gelungen mit einem einfachen Modell viele Apsekte desVerhalten vonkomplexen Fluiden wiederzugeben. Die Strukturbildung hängt offensichtlich nurbedingt von lokalen Eigenschaften der Moleküle ab.

Untersuchung zum Entstehungsmechanismus von Smith-Purcell-Strahlung im Nahbereich mit einem 855 MeV Elektronenstrahl geringer Emittanz

Experimental investigations of visible Smith-Purcell-radiation with a micro-focused high relativistic electron beam (E=855 MeV) are presented in the near region, in which the electron beam grazes the surface of the grating. The radiation intensity was measured as a function of the angle of observation and of the distance between electron beam axis and the surface of the grating simultaneously for two different wavelengths (360 nm, 546 nm).In the experiments Smith-Purcell-radiation was identified by the measured angular distribution fulfilling the characteristic coherence condition. By the observed distance dependence of the intensity two components of Smith-Purcell-radiation could be separated: one component with the theoretical predicted interaction length hint, which is produced by electrons passing over the surface of the grating, and an additional component in the near region leading to a strong enhancement of the intensity, which is produced by electrons hitting the surface. To describe the intensity of the observed additional radiation component a simple model for optical grating transition radiation, caused by the electrons passing through the grating structure, is presented. Taking into account the simple scalar model, the results of a Monte-Carlo calculation show that the additional radiation component could be explained by optical grating transition radiation.

Relaxationsuntersuchungen in unterkühlten und glasig erstarrten Polyalkoholen mittels dielektrischer und NMR Spektroskopie

Es wurde eine homologe Reihe von Polyalkoholen mit der allgemeinen Summenformel CNHN+2(OH)N (N=3-6) hinsichtlich ihrer Glaseigenschaften ober- und unterhalb der Glasübergangstemperatur TG untersucht. Dabei kamen die dielektrische und magnetische Resonanzspektroskopie (NMR) zum Einsatz. Es ergab sich oberhalb TG eine systematische Zunahme aller untersuchten dynamischen Parameter wie Fragilität, Breite der angenommenen Korrelationszeitenverteilungen und der Sprungwinkel der primären Glasrelaxation mit zunehmendem N. Dies kann insgesamt als eine Abnahme des Netzwerkcharakters, der durch Wasserstoffbrückenbindungen bedingt ist, bei zunehmender Kettenlänge interpretiert werden. Unterhalb TG entwickelt sich mit zunehmendem N die Sekundärrelaxation von einem 'Wing Szenario' zu einem ausgeprägten Johari - Goldstein (JG) - Prozess. Ein Sprungmodell, welches eine eingeschränkte Reorientierung auf einem Konusrand beschreibt, erzeugt mit Hilfe der parametrisierten dielektrischen Verlustspektren Sprungwinkel, die mit denen aus aktuellen ²H - NMR spektroskopischen Untersuchungen vergleichbar sind. Durch den Vergleich unterschiedlich deuterierter Derivate von Glyzerin (N=3) und Sorbitol (N=6) wurde gefolgert, dass auch unterhalb TG der Netzwerkcharakter mit zunehmender Kettenlänge abnimmt.Aufgrund der hier durchgeführten Untersuchungen konnte eine Zeitskala für einen Johari - Goldstein - Prozess im Modellglasbildner Glyzerin extrapoliert werden. Eine Deutung des Wings als Hochfrequenzausläufer des JG - Prozesses ist dadurch möglich.Der JG - Prozess kann somit als universeller Glasprozess interpretiert werden, der in verschiedenen Glasbildnern in unterschiedlicher Ausprägung auftritt.

Grazing incidence pumped Zr x-ray laser for spectroscopy on Li-like ions

X-ray laser fluorescence spectroscopy of the 2s-2p transition in Li-like ions is promising to become a widely applicable tool to provide information on the nuclear charge radii of stable and radioactive isotopes. For performing such experiments at the Experimental Storage Ring ESR, and the future NESR within the FAIR Project, a grazing incidence pumped (GRIP) x-ray laser (XRL) was set up at GSI Darmstadt using PHELIX (Petawatt High Energy Laser for heavy Ions eXperiments). The experiments demonstrated that lasing using the GRIP geometry could be achieved with relatively low pump energy, a prerequisite for higher repetition rate. In the first chapter the need of a plasma XRL is motivated and a short history of the plasma XRL is presented. The distinctive characteristic of the GRIP method is the controlled deposition of the pump laser energy into the desired plasma density region. While up to now the analysis performed were mostly concerned with the plasma density at the turning point of the main pump pulse, in this thesis it is demonstrated that also the energy deposition is significantly modified for the GRIP method, being sensitive in different ways to a large number of parameters. In the second chapter, the theoretical description of the plasma evolution, active medium and XRL emission properties are reviewed. In addition an innovative analysis of the laser absorption in plasma which includes an inverse Bremsstrahlung (IB) correction factor is presented. The third chapter gives an overview of the experimental set-up and diagnostics, providing an analytical formula for the average and instantaneous traveling wave speed generated with a tilted, on-axis spherical mirror, the only focusing system used up to now in GRIP XRL. The fourth chapter describes the experimental optimization and results. The emphasis is on the effect of the incidence angle of the main pump pulse on the absorption in plasma and on output and gain in different lasing lines. This is compared to the theoretical results for two different incidence angles. Significant corrections for the temperature evolution during the main pump pulse due to the incidence angle are demonstrated in comparison to a simple analytical model which does not take into account the pumping geometry. A much better agreement is reached by the model developed in this thesis. An interesting result is also the appearance of a central dip in the spatially resolved keV emission which was observed in the XRL experiments for the first time and correlates well with previous near field imaging and plasma density profile measurements. In the conclusion also an outlook to the generation of shorter wavelength XRL’s is given.

Noble-metal nanoparticles produced with colloidal lithography: fabrication, optical properties and applications

In this work, metal nanoparticles produced by nanosphere lithography were studied in terms of their optical properties (in connection to their plasmon resonances), their potential application in sensing platforms - for thin layer sensing and bio-recognition events -, and for a particular case (the nanocrescents), for enhanced spectroscopy studies. The general preparation procedures introduced early in 2005 by Shumaker-Parry et al. to produce metallic nanocrescents were extended to give rise to more complex (isolated) structures, and also, by combining colloidal monolayer fabrication and plasma etching techniques, to arrays of them. The fabrication methods presented in this work were extended not only to new shapes or arrangements of particles, but included also a targeted surface tailoring of the substrates and the structures, using different thiol and silane compounds as linkers for further attachment of, i.e. polyelectrolyte layers, which allow for a controlled tailoring of their nanoenvironment. The optical properties of the nanocrescents were studied with conventional transmission spectroscopy; a simple multipole model was adapted to explain their behaviour qualitatively. In terms of applications, the results on thin film sensing using these particles show that the crescents present an interesting mode-dependent sensitivity and spatial extension. Parallel to this, the penetrations depths were modeled with two simplified schemes, obtaining good agreement with theory. The multiple modes of the particles with their characteristic decay lengths and sensitivities represent a major improvement for particle-sensing platforms compared to previous single resonance systems. The nanocrescents were also used to alter the emission properties of fluorophores placed close to them. In this work, green emitting dyes were placed at controlled distances from the structures and excited using a pulsed laser emitting in the near infrared. The fluorescence signal obtained in this manner should be connected to a two-photon processes triggered by these structures; obtaining first insight into plasmon-mediated enhancement phenomena. An even simpler and faster approach to produce plasmonic structures than that for the crescents was tested. Metallic nanodiscs and nanoellipses were produced by means of nanosphere lithography, extending a procedure reported in the literature to new shapes and optical properties. The optical properties of these particles were characterized by extinction spectroscopy and compared to results from the literature. Their major advantage is that they present a polarization-dependent response, like the nanocrescents, but are much simpler to fabricate, and the resonances can be tailored in the visible with relative ease. The sensing capabilities of the metallic nanodiscs were explored in the same manner as for the nanocrescents, meaning their response to thin layers and to bio-recognition events on their surface. The sensitivity of these nanostructures to thin films proved to be lower than that of the crescents, though in the same order of magnitude. Experimental information about the near field extension for the Au nanodiscs of different sizes was also extracted from these measurements. Further resonance-tailoring approaches based on electrochemical deposition of metals on the nanodiscs were explored, as a means of modifying plasmon resonances by changing surface properties of the nanoparticles. First results on these experiments would indicate that the deposition of Ag on Au on a submonolayer coverage level can lead to important blue-shifts in the resonances, which would open a simple way to tailor resonances by changing material properties in a local manner.

Optical resonances of sphere-on-plane geometries

The optical resonances of metallic nanoparticles placed at nanometer distances from a metal plane were investigated. At certain wavelengths, these “sphere-on-plane” systems become resonant with the incident electromagnetic field and huge enhancements of the field are predicted localized in the small gaps created between the nanoparticle and the plane. An experimental architecture to fabricate sphere-on-plane systems was successfully achieved in which in addition to the commonly used alkanethiols, polyphenylene dendrimers were used as molecular spacers to separate the metallic nanoparticles from the metal planes. They allow for a defined nanoparticle-plane separation and some often are functionalized with a chromophore core which is therefore positioned exactly in the gap. The metal planes used in the system architecture consisted of evaporated thin films of gold or silver. Evaporated gold or silver films have a smooth interface with their substrate and a rougher top surface. To investigate the influence of surface roughness on the optical response of such a film, two gold films were prepared with a smooth and a rough side which were as similar as possible. Surface plasmons were excited in Kretschmann configuration both on the rough and on the smooth side. Their reflectivity could be well modeled by a single gold film for each individual measurement. The film has to be modeled as two layers with significantly different optical constants. The smooth side, although polycrystalline, had an optical response that was very similar to a monocrystalline surface while for the rough side the standard response of evaporated gold is retrieved. For investigations on thin non-absorbing dielectric films though, this heterogeneity introduces only a negligible error. To determine the resonant wavelength of the sphere-on-plane systems a strategy was developed which is based on multi-wavelength surface plasmon spectroscopy experiments in Kretschmann-configuration. The resonant behavior of the system lead to characteristic changes in the surface plasmon dispersion. A quantitative analysis was performed by calculating the polarisability per unit area /A treating the sphere-on-plane systems as an effective layer. This approach completely avoids the ambiguity in the determination of thickness and optical response of thin films in surface plasmon spectroscopy. Equal area densities of polarisable units yielded identical response irrespective of the thickness of the layer they are distributed in. The parameter range where the evaluation of surface plasmon data in terms of /A is applicable was determined for a typical experimental situation. It was shown that this analysis yields reasonable quantitative agreement with a simple theoretical model of the sphere-on-plane resonators and reproduces the results from standard extinction experiments having a higher information content and significantly increased signal-to-noise ratio. With the objective to acquire a better quantitative understanding of the dependence of the resonance wavelength on the geometry of the sphere-on-plane systems, different systems were fabricated in which the gold nanoparticle size, type of spacer and ambient medium were varied and the resonance wavelength of the system was determined. The gold nanoparticle radius was varied in the range from 10 nm to 80 nm. It could be shown that the polyphenylene dendrimers can be used as molecular spacers to fabricate systems which support gap resonances. The resonance wavelength of the systems could be tuned in the optical region between 550 nm and 800 nm. Based on a simple analytical model, a quantitative analysis was developed to relate the systems’ geometry with the resonant wavelength and surprisingly good agreement of this simple model with the experiment without any adjustable parameters was found. The key feature ascribed to sphere-on-plane systems is a very large electromagnetic field localized in volumes in the nanometer range. Experiments towards a quantitative understanding of the field enhancements taking place in the gap of the sphere-on-plane systems were done by monitoring the increase in fluorescence of a metal-supported monolayer of a dye-loaded dendrimer upon decoration of the surface with nanoparticles. The metal used (gold and silver), the colloid mean size and the surface roughness were varied. Large silver crystallites on evaporated silver surfaces lead to the most pronounced fluorescence enhancements in the order of 104. They constitute a very promising sample architecture for the study of field enhancements.

Charged colloidal suspensions in confined geometries

Im Zentrum dieser Arbeit steht das Verhalten von geladenen kolloidalen Suspensionen in eingeschränkten Geometrien. Es wurden verschiedene keilförmige Zellen verwendet, die eine kontinuierliche Variation der Abstände zwischen den Platten ermöglichen. In Zellen mit fluid geordneten Suspensionen bei niedrigen Salzkonzentrationen akkumulieren die kolloidalen Partikel in der Keilspitze und bilden kristallin geordnete Strukturen. Systematische Experimente zu diesem Akkumulationseffekt führen zu dem Schluss, dass es um eine elektrostatischer Fallensituation handeln muss, was durch ein einfaches theoretisches, von Löwen et al vorgeschlagenes Modell bestätigt wird. In Abhängig von der Zellhöhe lässt sich in den auftretenden kristallinen Strukturen eine charakteristische Abfolge erkennen. Diese Struktursequenz wurde schon zuvor in eingeschränkten Keilgeometrien beobachtet, jedoch ermöglichen die in unseren Experimenten realisierbaren kleinen Keilwinkel die Beobachtung neuer Strukturen. Einige dieser neuen Strukturen zeigen eine exotische Anordnung die keine atomare Entsprechung besitzen. Basierend auf experimentellen Beobachtungen schlagen wir Modelle für unterschiedliche Übergangsmechanismen zwischen den verschiedenen Strukturen vor, unter der physikalisch motivierten Vorraussetzung, dass sich die Partikel wie einem hohen Druck unterworfene harte Kugeln verhalten. Des Weiteren wurde eine Zelle mit variabler Höhe konstruiert, die zur Untersuchung des vollständigen Phasenverhaltens geladener, zwischen parallelen Platten eingeschlossener Kugeln dient. Die vorläufigen Ergebnisse werden mit theoretischen Prognosen verglichen.

Tunnelkontakte mit Heusler-Elektrode: Spinpolarisation von Co 2Cr 0,6Fe 0,4Al und Einfluss der Barriere

Ziel dieser Arbeit ist die Bestimmung der Spinpolarisation von der Heusler-Verbindung Co2Cr0,6Fe0,4Al. Dieses Ziel wurde durch die sorgfältige Präparation von Co2Cr0,6Fe0,4Al basierten Tunnelkontakten realisiert. Tunnelwiderstandsmessungen an Co2Cr0,6Fe0,4Al-basiertenrnTunnelkontakten ergaben einen Tunnelmagnetowiderstand von 101% bei 4 K. DieserrnTunnelmagnetowiderstand legt eine untere Grenze von 67% für die Spinpolarisation von Co2Cr0,6Fe0,4Al fest.rnrnCo2Cr0,6Fe0,4Al ist eine Heusler-Verbindung, der die Eigenschaften eines halbmetallischen Ferromagneten zugeschrieben werden. Ein halbmetallischer Ferromagnet hat an der Fermikante nur Elektronenspinzustände mit einer Polarisation. Als Folge davon können bei einem spinerhaltenden Tunnelprozess nur Elektronen einer Spinrichtung in den halbmetallischen Ferromagneten tunneln. Mit einem magnetischen Feld und einer durch einen Antiferromagneten fixierten Gegenelektrode, können an einem Tunnelkontakt mit einem spinpolarisierten Ferromagneten deshalb zwei Zustände, eine hohe und eine niedrige Tunnelleitfähigkeit, erzeugt werden. Daher finden spinpolarisierte Tunnelkontakte in Form von MRAM in der Datenspeicherung Verwendung. Bislang wurde jedoch keine Verbindung gefunden, der eine Spinpolarisation von 100% experimentell eindeutig nachgewiesen werden konnte. Für Co2Cr0,6Fe0,4Al lagen die höchsten gemessenen Spinpolarisationen um 50%.rnrnTunnelspektroskopie ist eine zuverlässige und anwendungsnahe Methode zur Untersuchung der Spinpolarisation. Inelastische Tunnelprozesse und eine reduzierte Ordnung an Grenzflächen bewirken einen reduzierten Tunnelmagnetowiderstand. Eine symmetriebrechende Barriere, wie amorphes AlOx, ist Voraussetzung für die Anwendung des Jullière-Modells zur Bestimmung der Spinpolarisation. Das Jullière-Modell verknüpft die Spin-aufgespaltenenrnZustandsdichten der Elektroden mit dem Tunnelmagnetowiderstand. Ohne einernsymmetriebrechende Barriere, zum Beispiel mit MgO als Isolatorschicht, können höhere Tunnelmagnetowiderstände erzwungen werden. Ein eindeutiger Rückschluss auf die Spinpolarisation ist dann jedoch nicht mehr möglich. Mit Aluminiumoxid-basierten Barrieren liefert die Anwendung des einfachen Jullière-Modells eine Untergrenze der Spinpolarisation.rnrnUm die Spinpolarisation von Co2Cr0,6Fe0,4Al durch Tunnelspektroskopie zu bestimmen, musste die Präparation der Tunnelkontakte verbessert werden. Dies wurde ermöglicht durch den Anbau einer neuen Sputterkammer mit besseren UHV-Bedingungen an ein bestehendes Präparationscluster. Co2Cr0,6Fe0,4Al wird mit Hilfe von Radiofrequenz-Kathodenzerstäuben deponiert. Die resultierenden Schichten verfügen nach ihrer Deposition über einen höheren Ordnungsgrad und über eine geordnete Oberfläche. Durch eine Magnesium-Pufferschicht war es möglich, auf diese Oberfläche eine homogene amorphe AlOx-Barriere zu deponieren. Als Gegenelektrode wurde CoFe als Ferromagnet mit MnFe als Antiferromagnet gewählt. Diese Gegenelektrode ermöglicht Tunnelmessungen bis hin zu Raumtemperatur.rnrnMit den in dieser Arbeit vorgestellten optimierten Analyse- und Präparationsmethoden ist es möglich, die Untergrenze der Spinpolarisation von Co2Cr0,6Fe0,4Al auf 67% anzuheben. Dies ist der bisher höchste veröffentlichte Wert der Spinpolarisation von Co2Cr0,6Fe0,4Al.rn

Measurement and modeling of cloud condensation nuclei in continental air

Atmospheric aerosol particles serving as cloud condensation nuclei (CCN) are key elements of the hydrological cycle and climate. Knowledge of the spatial and temporal distribution of CCN in the atmosphere is essential to understand and describe the effects of aerosols in meteorological models. In this study, CCN properties were measured in polluted and pristine air of different continental regions, and the results were parameterized for efficient prediction of CCN concentrations.The continuous-flow CCN counter used for size-resolved measurements of CCN efficiency spectra (activation curves) was calibrated with ammonium sulfate and sodium chloride aerosols for a wide range of water vapor supersaturations (S=0.068% to 1.27%). A comprehensive uncertainty analysis showed that the instrument calibration depends strongly on the applied particle generation techniques, Köhler model calculations, and water activity parameterizations (relative deviations in S up to 25%). Laboratory experiments and a comparison with other CCN instruments confirmed the high accuracy and precision of the calibration and measurement procedures developed and applied in this study.The mean CCN number concentrations (NCCN,S) observed in polluted mega-city air and biomass burning smoke (Beijing and Pearl River Delta, China) ranged from 1000 cm−3 at S=0.068% to 16 000 cm−3 at S=1.27%, which is about two orders of magnitude higher than in pristine air at remote continental sites (Swiss Alps, Amazonian rainforest). Effective average hygroscopicity parameters, κ, describing the influence of chemical composition on the CCN activity of aerosol particles were derived from the measurement data. They varied in the range of 0.3±0.2, were size-dependent, and could be parameterized as a function of organic and inorganic aerosol mass fraction. At low S (≤0.27%), substantial portions of externally mixed CCN-inactive particles with much lower hygroscopicity were observed in polluted air (fresh soot particles with κ≈0.01). Thus, the aerosol particle mixing state needs to be known for highly accurate predictions of NCCN,S. Nevertheless, the observed CCN number concentrations could be efficiently approximated using measured aerosol particle number size distributions and a simple κ-Köhler model with a single proxy for the effective average particle hygroscopicity. The relative deviations between observations and model predictions were on average less than 20% when a constant average value of κ=0.3 was used in conjunction with variable size distribution data. With a constant average size distribution, however, the deviations increased up to 100% and more. The measurement and model results demonstrate that the aerosol particle number and size are the major predictors for the variability of the CCN concentration in continental boundary layer air, followed by particle composition and hygroscopicity as relatively minor modulators. Depending on the required and applicable level of detail, the measurement results and parameterizations presented in this study can be directly implemented in detailed process models as well as in large-scale atmospheric and climate models for efficient description of the CCN activity of atmospheric aerosols.

Simulations of physics and chemistry of polar stratospheric clouds with a general circulation model

A polar stratospheric cloud submodel has been developed and incorporated in a general circulation model including atmospheric chemistry (ECHAM5/MESSy). The formation and sedimentation of polar stratospheric cloud (PSC) particles can thus be simulated as well as heterogeneous chemical reactions that take place on the PSC particles. For solid PSC particle sedimentation, the need for a tailor-made algorithm has been elucidated. A sedimentation scheme based on first order approximations of vertical mixing ratio profiles has been developed. It produces relatively little numerical diffusion and can deal well with divergent or convergent sedimentation velocity fields. For the determination of solid PSC particle sizes, an efficient algorithm has been adapted. It assumes a monodisperse radii distribution and thermodynamic equilibrium between the gas phase and the solid particle phase. This scheme, though relatively simple, is shown to produce particle number densities and radii within the observed range. The combined effects of the representations of sedimentation and solid PSC particles on vertical H2O and HNO3 redistribution are investigated in a series of tests. The formation of solid PSC particles, especially of those consisting of nitric acid trihydrate, has been discussed extensively in recent years. Three particle formation schemes in accordance with the most widely used approaches have been identified and implemented. For the evaluation of PSC occurrence a new data set with unprecedented spatial and temporal coverage was available. A quantitative method for the comparison of simulation results and observations is developed and applied. It reveals that the relative PSC sighting frequency can be reproduced well with the PSC submodel whereas the detailed modelling of PSC events is beyond the scope of coarse global scale models. In addition to the development and evaluation of new PSC submodel components, parts of existing simulation programs have been improved, e.g. a method for the assimilation of meteorological analysis data in the general circulation model, the liquid PSC particle composition scheme, and the calculation of heterogeneous reaction rate coefficients. The interplay of these model components is demonstrated in a simulation of stratospheric chemistry with the coupled general circulation model. Tests against recent satellite data show that the model successfully reproduces the Antarctic ozone hole.

Structure formation as studied by EPR spectroscopy: From simple solutions to nanoparticles

In this thesis, three nitroxide based ionic systems were used to investigate structure and dynamics of their respective solutions in mixed solvents by means of electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopy at X- and W-band (9.5 and 94.5 GHz, respectively). rnFirst, the solvation of the inorganic radical Fremy’s salt (K2ON(SO3)2) in isotope substituted binary solvent mixtures (methanol/water) was investigated by means of high-field (W-band) pulse ENDOR spectroscopy and molecular dynamics (MD) simulations. From the analysis of orientation-selective 1H and 2H ENDOR spectra the principal components of the hyperfine coupling (hfc) tensor for chemically different protons (alcoholic methyl vs. exchangeable protons) were obtained. The methyl protons of the organic solvent approach with a mean distance of 3.5 Å perpendicular to the approximate plane spanned by ON(S)2 of the probe molecule. Exchangeable protons were found to be distributed isotropically, approaching closest to Fremy’s salt from the hydrogen-bonded network around the sulfonate groups. The distribution of exchangeable and methyl protons as found in MD simulations is in full agreement with the ENDOR results. The solvation was found to be similar for the studied solvent ratios between 1:2.3 and 2.3:1 and dominated by an interplay of H-bond (electrostatic) interactions and steric considerations with the NO group merely involved into H-bonds.rnFurther, the conformation of spin labeled poly(diallyldimethylammonium chloride) (PDADMAC) solutions in aqueous alcohol (methanol, ethanol, n-propanol, ethylene glycol, glycerol) mixtures in dependence of divalent sodium sulfate was investigated with double electron-electron resonance (DEER) spectroscopy. The DEER data was analyzed using the worm-like chain model which suggests that in organic-water solvent mixtures the polymer backbones are preferentially solvated by the organic solvent. We found a less serve impact on conformational changes due to salt than usually predicted in polyelectrolyte theory which stresses the importance of a delicate balance of hydrophobic and electrostatic interactions, in particular in the presence of organic solvents.rnFinally, the structure and dynamics of miniemulsions and polymerdispersions prepared with anionic surfactants, that were partially replaced by a spin labeled fatty acid in presence and absence of a lanthanide beta-diketonate complex was characterized by CW EPR spectroscopy. Such miniemulsions form multilayers with the surfactant head group bound to the lanthanide ion. Beta-diketonates were formerly used as NMR shift reagents and nowadays find application as luminescent materials in OLEDs and LCDs and as contrast agent in MRT. The embedding of the complex into a polymer matrix results in an easy processable material. It was found that the structure formation takes place in miniemulsion and is preserved during polymerization. For surfactants with carboxyl-head group a higher order of the alkyl chains and less lateral diffusion is found than for sulfat-head groups, suggesting a more uniform and stronger coordination to the metal ion. The stability of these bilayers depends on the temperature and the used surfactant which should be considered for the used polymerization temperature if a maximum output of the structured regions is wished.

A soft-tetramer model for diblock copolymer melts: structure formation and geometric characterization

The ability of block copolymers to spontaneously self-assemble into a variety of ordered nano-structures not only makes them a scientifically interesting system for the investigation of order-disorder phase transitions, but also offers a wide range of nano-technological applications. The architecture of a diblock is the most simple among the block copolymer systems, hence it is often used as a model system in both experiment and theory. We introduce a new soft-tetramer model for efficient computer simulations of diblock copolymer melts. The instantaneous non-spherical shape of polymer chains in molten state is incorporated by modeling each of the two blocks as two soft spheres. The interactions between the spheres are modeled in a way that the diblock melt tends to microphase separate with decreasing temperature. Using Monte Carlo simulations, we determine the equilibrium structures at variable values of the two relevant control parameters, the diblock composition and the incompatibility of unlike components. The simplicity of the model allows us to scan the control parameter space in a completeness that has not been reached in previous molecular simulations.The resulting phase diagram shows clear similarities with the phase diagram found in experiments. Moreover, we show that structural details of block copolymer chains can be reproduced by our simple model.We develop a novel method for the identification of the observed diblock copolymer mesophases that formalizes the usual approach of direct visual observation,using the characteristic geometry of the structures. A cluster analysis algorithm is used to determine clusters of each component of the diblock, and the number and shape of the clusters can be used to determine the mesophase.We also employ methods from integral geometry for the identification of mesophases and compare their usefulness to the cluster analysis approach.To probe the properties of our model in confinement, we perform molecular dynamics simulations of atomistic polyethylene melts confined between graphite surfaces. The results from these simulations are used as an input for an iterative coarse-graining procedure that yields a surface interaction potential for the soft-tetramer model. Using the interaction potential derived in that way, we perform an initial study on the behavior of the soft-tetramer model in confinement. Comparing with experimental studies, we find that our model can reflect basic features of confined diblock copolymer melts.

Computer simulation methods to study interfacial tensions : from the Ising model to colloidal crystals

In condensed matter systems, the interfacial tension plays a central role for a multitude of phenomena. It is the driving force for nucleation processes, determines the shape and structure of crystalline structures and is important for industrial applications. Despite its importance, the interfacial tension is hard to determine in experiments and also in computer simulations. While for liquid-vapor interfacial tensions there exist sophisticated simulation methods to compute the interfacial tension, current methods for solid-liquid interfaces produce unsatisfactory results.rnrnAs a first approach to this topic, the influence of the interfacial tension on nuclei is studied within the three-dimensional Ising model. This model is well suited because despite its simplicity, one can learn much about nucleation of crystalline nuclei. Below the so-called roughening temperature, nuclei in the Ising model are not spherical anymore but become cubic because of the anisotropy of the interfacial tension. This is similar to crystalline nuclei, which are in general not spherical but more like a convex polyhedron with flat facets on the surface. In this context, the problem of distinguishing between the two bulk phases in the vicinity of the diffuse droplet surface is addressed. A new definition is found which correctly determines the volume of a droplet in a given configuration if compared to the volume predicted by simple macroscopic assumptions.rnrnTo compute the interfacial tension of solid-liquid interfaces, a new Monte Carlo method called ensemble switch method'' is presented which allows to compute the interfacial tension of liquid-vapor interfaces as well as solid-liquid interfaces with great accuracy. In the past, the dependence of the interfacial tension on the finite size and shape of the simulation box has often been neglected although there is a nontrivial dependence on the box dimensions. As a consequence, one needs to systematically increase the box size and extrapolate to infinite volume in order to accurately predict the interfacial tension. Therefore, a thorough finite-size scaling analysis is established in this thesis. Logarithmic corrections to the finite-size scaling are motivated and identified, which are of leading order and therefore must not be neglected. The astounding feature of these logarithmic corrections is that they do not depend at all on the model under consideration. Using the ensemble switch method, the validity of a finite-size scaling ansatz containing the aforementioned logarithmic corrections is carefully tested and confirmed. Combining the finite-size scaling theory with the ensemble switch method, the interfacial tension of several model systems, ranging from the Ising model to colloidal systems, is computed with great accuracy.