Modeling of complex chemical reactions and macromolecular orientation phenomena in confined geometries

The cooperative motion algorithm was applied on the molecular simulation of complex chemical reactions and macromolecular orientation phenomena in confined geometries. First, we investigated the case of equilibrium step-growth polymerization in lamellae, pores and droplets. In such systems, confinement was quantified as the area/volume ratio. Results showed that, as confinement increases, polymerization becomes slower and the average molecular weight (MW) at equilibrium decreases. This is caused by the sterical hindrance imposed by the walls since chain growth reactions in their close vicinity have less realization possibilities. For reactions inside droplets at surfaces, contact angles usually increased after polymerization to compensate conformation restrictions imposed by confinement upon growing chains. In a second investigation, we considered monodisperse and chemically inert chains and focused on the effect of confinement on chain orientation. Simulations of thin polymer films showed that chains are preferably oriented parallel to the surface. Orientation increases as MW increases or as film thickness d decreases, in qualitative agreement with experiments with low MW polystyrene. It is demonstrated that the orientation of simulated chains results from a size effect, being a function of the ratio between chain end-to-end distance and d. This study was complemented by experiments with thin films of pi-conjugated polymers like MEH-PPV. Anisotropic refractive index measurements were used to analyze chain orientation. With increasing MW, orientation is enhanced. However, for MEH-PPV, orientation does not depend on d even at thicknesses much larger than the chain contour length. This contradiction with simulations was discussed by considering additional causes for orientation, for instance the appearance of nematic-like ordering in polymer films. In another investigation, we simulated droplet evaporation at soluble surfaces and reproduced the formation of wells surrounded by ringlike deposits at the surface, as observed experimentally. In our simulations, swollen substrate particles migrate to the border of the droplet to minimize the contact between solvent and vacuum, which costs the most energy. Deposit formation in the beginning of evaporation results in pinning of the droplet. When polymer chains at the substrate surface have strong uniaxial orientation, the resulting pattern is no longer similar to a ring but to a pair of half-moons. In a final stage, as an extension for the model developed for polymerization in nanoreactors, we studied the effect of geometrical confinement on a hypothetical oscillating reaction following the mechanism of the so called periodically forced Brusselator. It was shown that a reaction which is chaotic in the bulk may be driven to periodicity by confinement and vice-versa, opening new perspectives for chaos control.

Chemical and physical properties of Amazonian aerosol particles

- ZUSAMMENFASSUNG:Die vorliegende Dissertation befasst sich mit der Bestimmung der chemischen und physikalischen Eigenschaften von Aerosolpartikeln im Amazonasbecken, die während Zeiten mit Biomasseverbrennung und bei Hintergrundbedingungen bestimmt wurden. Die Messungen wurden während zwei Kampagnen im Rahmen des europäischen Beitrags zum LBA-EUSTACH Experiment in Amazonien. Die Daten umfassen Messungen der Anzahlkonzentrationen, Größenverteilungen, optischen Eigenschaften sowie Elementzusammensetzungen und Kohlenstoffgehalte der gesammelten Aerosole. Die Zusammensetzung des Aerosols wies auf folgende drei Quellen hin: natürlichen biogenen, Mineralstaub, und pyrogenes Aerosol. Aller drei Komponenten trugen signifikant zur Extinktion des Sonnenlichts bei. Insgesamt ergab sich eine Steigerung der Meßwerte um ca. das Zehnfache während der Trockenzeit im Vergleich zur Regenzeit, was auf eine massive Einbringung von Rauchpartikeln im Submikrometerbereich in die Atmosphäre während der Trockenzeit zurückzuführen ist. Dementsprechend sank die Einzelstreualbedo von ca. 0,97 auf 0,91. Der Brechungsindex der Aerosolpartikel wurde mit einer neuen iterative Methoden, basierend auf der Mie-Theorie berechnet. Es ergaben sich durchschnittliche Werte von 1,42 – 0,006i für die Regenzeit und 1,41 – 0,013i für die Trockenperiode. Weitere klimatisch relevante Parameterergaben für Hintergrundaerosole und für Aerosole aus Biomasseverbrennung folgende Werte: Asymmetrieparameter von 0,63 ± 0,02 bzw. 0,70 ± 0,03 und Rückstreuungsverhältnisse von 0,12 ± 0,01 bzw. 0,08 ± 0,01. Diese Veränderungen haben das Potential, das regionale und globale Klima über die Variierung der Extinktion der Sonneneinstrahlung als auch der Wolkeneigenschaften zu beeinflussen.

Chemical exfoliation of graphene and its application in organic electronics and energy storage devices

Graphene, the thinnest two-dimensional material possible, is considered as a realistic candidate for the numerous applications in electronic, energy storage and conversion devices due to its unique properties, such as high optical transmittance, high conductivity, excellent chemical and thermal stability. However, the electronic and chemical properties of graphene are highly dependent on their preparation methods. Therefore, the development of novel chemical exfoliation process which aims at high yield synthesis of high quality graphene while maintaining good solution processability is of great concern. This thesis focuses on the solution production of high-quality graphene by wet-chemical exfoliation methods and addresses the applications of the chemically exfoliated graphene in organic electronics and energy storage devices.rnPlatinum is the most commonly used catalysts for fuel cells but they suffered from sluggish electron transfer kinetics. On the other hand, heteroatom doped graphene is known to enhance not only electrical conductivity but also long term operation stability. In this regard, a simple synthetic method is developed for the nitrogen doped graphene (NG) preparation. Moreover, iron (Fe) can be incorporated into the synthetic process. As-prepared NG with and without Fe shows excellent catalytic activity and stability compared to that of Pt based catalysts.rnHigh electrical conductivity is one of the most important requirements for the application of graphene in electronic devices. Therefore, for the fabrication of electrically conductive graphene films, a novel methane plasma assisted reduction of GO is developed. The high electrical conductivity of plasma reduced GO films revealed an excellent electrochemical performance in terms of high power and energy densities when used as an electrode in the micro-supercapacitors.rnAlthough, GO can be prepared in bulk scale, large amount of defect density and low electrical conductivity are major drawbacks. To overcome the intrinsic limitation of poor quality of GO and/or reduced GO, a novel protocol is extablished for mass production of high-quality graphene by means of electrochemical exfoliation of graphite. The prepared graphene shows high electrical conductivity, low defect density and good solution processability. Furthermore, when used as electrodes in organic field-effect transistors and/or in supercapacitors, the electrochemically exfoliated graphene shows excellent device performances. The low cost and environment friendly production of such high-quality graphene is of great importance for future generation electronics and energy storage devices. rn