21 search hits
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Ordering of Nanoparticles by Wrinkle-Assisted Self-Assembly : Controlling Plasmonic Coupling Effects
(2011)
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Alexandra Schweikart
- Structures of spatial scale between 10Å and 1000Å are known as nanomaterials and have attracted immense interest over the last decades (Nobel Prize in physics in 2010 was awarded for the nanomaterial graphene). Materials within this scale show a large surface-to-volume ratio and amplify surface-related properties. Governing and manipulating material on this almost atomic level is one of the most active fields in modern natural science. Nanoscale technology, such as some of the processes involved in steel production and painting, has been empirically utilized in human society for centuries, however, a scientific investigation of phenomena on this spatial scale only began in 1857 when Michael Faraday reported on the synthesis and colors of gold colloids. In 1959 interest in the nanoscale was stimulated by an American physicist, Richard Feynman, in his famous “There’s plenty of room at the bottom” address, and the term nanotechnology first appeared in 1974 from the Japanese Norio Tanigucho. Since these pioneering works, thousands of publications have been focused on the synthesis, modification, properties and assembly of nanoparticles. Great progress has been attained in the preparation of nanoparticles of any desired size, shape and composition. Metal nanoparticles are particularly attractive due to their spectacular size and shape dependent optical and electronic properties. Color variations of nanoparticle suspension for example arise from changes in the composition, size and shape of nanoparticles, as well as from the proximity of other metal nanoparticles. The average distances of nanoparticles in thin films influence the spectral features because of inter-nanoparticle coupling. These effects are often the result of changes in the so-called surface Plasmon resonance, the frequency at which conduction electrons oscillate in response to the alternating electric field. Provided nanoparticles form ordered arrays, they can additionally have unique and fascinating optical properties because of photonic band gap effects with potential applications such as detectors, circuits, light sources, polymeric opals or meta-materials. The present work deals with the controlled placement of nanoparticles by physical con-straints. Exact placement of nanoparticles allows for the control of the inter-nanoparticle distance and thus determines the coupling effects (here: Plasmon coupling) which arise upon interaction with electromagnetic radiation. Different coupling leads to different distance-dependent signals and such substrates can serve as sensors if, for example, Raman spectroscopy is carried out for detection of the signal. Currently, most templates are created using lithographic techniques. Particularly if structures on the sub-micron scale are desired, electron beam lithography has to be used which involves environmentally harmful etching processes. Within this work we show how controlled wrinkling of a thin rigid film on a soft, elastomeric substrate, can be used as an alternative to fabricate nano-templates without using any lithography. As a substrate, a silicon elastomer poly (dimethylsiloxane) (PDMS) was used. Upon stretching such substrates uniaxially, an enlarged surface was exposed to oxygen plasma and converted to silica by oxidation. After releasing the strain, periodic wrinkles appeared perpendicular to the applied strain. Under defined conditions, such wrinkles have a regular sinusoidal topology featuring a single dominant wavelength and amplitude. The formation process could easily be tuned by tuning the plasma exposure to generate periodically structured templates between few hundreds of nanometers and several microns. In this work, wrinkled templates were tailored such that suitably sized nanoparticles could be arbitrarily assembled into a hierarchical structure by drying colloids out of suspension in a channel-like confinement offered by wrinkles in contact with a flat substrate. Using the same template geometry (same wavelength and amplitude of wrinkles) but different particle concentration of spherical polystyrene beads (r = 55nm) we found parallel particle-structures ranging from single parallel lines at low particle concentration to dense prismatic ridges at high particle concentration. The wavelength of the wrinkled template defined the spacing between the particle lines. Moreover, we performed Monte Carlo (MC) computer simulations in collaboration with the theoretical physics department (Prof. Dr. Matthias Schmidt and Dr. Andrea Fortini) at the Uni-versity of Bayreuth to assess the dominant driving forces during the assembly process. Be using MC, colloidal particle assemblies can be characterized in terms of their equilibrium configuration that minimizes the free energy. Simulations were performed on particles in a box delimited by a flat hard wall and a sinusoidal hard wall according to our experimental system. These simulations precisely predicted the exact assembled geometry in thermal equilibrium. Comparing results of simulation and experiment we found perfect agreement between the equilibrium structures. We discovered the confinement itself to be mainly responsible for the assembled morphology of nanoparticle, which makes the process independent of the detailed chemistry of particles. In addition we obtained very similar structures with the same assembly strategy but using gold nanoparticles (r = 33 nm) instead of polymeric particles. We fabricated lines of gold nanoparticles assembled in a single file and lines two particles wide using similar particle concentration but different sizes of the confinement template. The different morphologies of the lines give rise to different optical signals as collective oscillation of conduction electrons result in different interaction with electromagnetic radiation. Surface Plasmon resonance due to Plasmon coupling between adjacent particles arises. Different morphology-dependent signals of nanoparticles in contact within the lines were detected by surface enhanced Raman spectroscopy (SERS). The electromagnetic field was measured to be randomly distributed along the particle lines with strong enhancements at so-called hot spots located at gaps between neighboring nano-particles. To confirm the measured signal we compared theoretical simulations using the finite-difference time-domain (FDTD) method and experimentally measured dark-field spectroscopy signal along differently shaped lines of particles within a collaboration with Weihai Ni and Dr. Ramón Alvarez-Puebla at the University of Vigo in Spain. Good agreement between theory and experiment indicated that indeed plasmonic coupling of the individual nanoparticles is responsible for the observed SERS effects: Using wrinkle-assisted self-assembly it is possible to control the organization of the colloidal particles on the substrate, with a consequent control over the formation of hot spots and the resulting SERS intensity. Such ordered multiplicities of hot spots give rise to quantitative SERS signals with high sensitivity which has applications as diverse as biological detectors, optical filters and sensors. In addition, this work deals with chemical modification of the wrinkled structure to render it accessible to different solvents as PDMS tends to swell in organic solvents and suffers from poor mechanical stability. Additionally, wrinkles fabricated through a buckling instability of a stiff supported layer under compression are not tension free on the microscopic level and suffer from relaxation on a longer time scale. We introduce in this work two different methods to replicate wrinkles by molding. In micro thermoforming, the wrinkled surface was used as a mold (or caliber) to structure different kinds of polymers (polystyrene and poly (methylmethacrylate)) by pressing the originally wrinkled structure onto a ductile material which preserves the nanostructure after curing. The second methodology was carried out in collaboration with PD. Dr. Kerstin Koch and Michael Bennemann at the Nees institute in Bonn and employed a two-step molding process, where wrinkles were molded against wax and in a second step, the structured wax was cast against epoxy resin. Both methods revealed perfect copies of the wrinkled original with high fidelity even at dimensions as small as a few hundred nanometers and hold no residual stresses because there is only one component. Wrinkles made of tough polymers are now accessible to various solvents which make them potential substrates for microfluidics. In the last part of this work, wrinkles are used as stamps in so-called micro contact printing (µCP). In this technique, a structured elastomeric stamp is used to transfer a surface-active molecule out of solution to a flat substrate by mechanical contact. Patterns of different charge density can be created which have applications in the field of biosensors, diagnostic immunoassays and cell culturing. Traditionally, stamps for µCP are prepared by a two step process where a lithographically fabricated structured silicon master serves as mold. An elastomeric polymer is cast against the caliber and preserves the structure after curing and detaching. As already mentioned lithography is expensive and involves environmentally harmful etching processes. Within this work we introduce the one step wrinkling process to fabricate structured stamps. Even though the diversity of stamp geometries created by wrinkling is limited, the simplicity compared to lithographic techniques is evident. The process of wrinkle formation includes plasma oxidation, which renders the topmost surface hydrophilic. Therefore, charged macromolecules out of aqueous solution were adsorbed onto the surface. The coated relief structure was used as a stamp to transfer the molecules selectively from the elevated parts of the wrinkles to another flat, oppositely charged surface by means of µCP. The topography of the resulting pattern was characterized by Atomic Force Microscopy (AFM) imaging as alternating charged pattern of printed (elevated) and non-printed areas. By varying the geometry of the wrinkled stamp (amplitude and wavelength) we studied the limits in which successful µCP with wrinkles can be carried out. We found the limits for wavelength of the wrinkles below 355nm and amplitudes below 40nm at which the printed structure disappeared because material was transferred from the wrinkles’ hills as well as from the bottom parts. The height of the transferred structure increase with increasing wavelength and amplitude of the wrinkles but tended to a limit of 6-7nm, even though the topology of the stamp increases. The smallest structure found in lateral dimensions was as small as 50nm, appearing as areas where no material was transferred.
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Homogen und heterogen katalysierte Dimerisierungsreaktionen von Olefinen
(2011)
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Tanja Englmann
- Ziel der vorliegenden Arbeit war der Einsatz von Übergangsmetallkomplexen als homogene und heterogene Katalysatoren für die Dimerisierung von 1-Penten und 1-Hexen. Weiterhin sollten verschiedene Einflussfaktoren auf die Aktivitäten und Selektivitäten der Dimerisierungsreaktionen ermittelt und optimiert werden. Hierzu wurden Di(imino)pyridin-Übergangsmetallkomplexe synthetisiert und bei homogenen Dimerisierungsreaktionen eingesetzt. Die Synthese der Di(imino)pyridin-Ligandvorstufen verlief über eine Kondensationsreaktion von Diacetylpyridin und unterschiedlich substituierten Anilinen. Mit den hieraus gewonnenen Verbindungen wurden mit Salzen der 3d-Übergangs-metalle die entsprechenden Komplexe synthetisiert. Bei der Variation des Zentralmetalls erzielte der Di(imino)pyridin-Eisen(III)komplex nach Aktivierung mit MAO bei der Dimerisierung von 1-Hexen die besten Ergebnisse. Die in para-Position des Anilinringes halogenierten Di(imino)pyridin-Eisen(III)komplexe lieferten die höchsten Aktivitäten sowohl bei der Dimerisierung von 1-Hexen, als auch bei Kupplungsreaktionen von 1-Hexen mit 1-Penten. Die Selektivitäten für Dimere lagen bei allen Di(imino)pyridin-Eisen(III)komplexen mit substituierten Iminophenylringen um 90%. Neben linearen Produkten wurden auch methyl-verzweigte Alkene gaschromatographisch detektiert. Die Produktverteilungen der Codimerisierungen weisen Ähnlichkeit zur Binomialverteilung auf. Das bedeutet, dass überwiegend Undecene sowie Hexa- und Heptadecene, also „Co-Produkte“, gebildet wurden. Bei allen Produktzusammensetzungen wurde ein geringfügig höherer Anteil an Hexen-Dimeren festgestellt, was bedeutet, dass die Insertion von 1-Hexen etwas schneller erfolgt, als die von 1-Penten. Mit neun verschiedenen Metallocenkomplexen konnte 1-Penten bzw. 1-Hexen mit Selektivitäten über 90% erfolgreich homogen dimerisiert werden. Die Produkt-verteilung bei der Codimerisierung von 1-Penten mit 1-Hexen zeigte ein binomiales Verhalten, wobei in allen Fällen überwiegend „Co-Produkte“ gebildet wurden. So liegen die Dimere im Verhältnis 1:2:1 (C10:C11:C12) vor, während sich für die Trimere (Pentadecene bis Octadecene) ein Verhältnis von 1:3:3:1 ergibt. Im Gegensatz zu den Di(imino)pyridin-Komplex-katalysierten Reaktionen wurden mit Metallocenkomplexen ausschließlich lineare Produkte erhalten. Bei Variation des Verhältnisses der 1-Penten/1-Hexen-Mischung verschob sich die Binomial-Verteilung erwartungsgemäß auf die Seite der Produkte des höher konzentrierten Monomers. Auch unter der Verwendung von methyl-verzweigten Olefinen konnte die binomiale Produktverteilung beobachtet werden. Allerdings konnten mit internen Alkenen keine Umsätze erzielt werden. Ebenso negativ verlief die Aktivierung von Olefinen, die eine Methylgruppe in β-Position besitzen. Sterisch gehinderte Alkene mit tert-Butylgruppen oder neo-Pentylgruppen zeigten ebenfalls keine Tendenz zur Dimerisierung. Durch Zugabe geeigneter Additive, wie Tributylphosphan oder Aluminium-Pulver konnte die Selektivität für Dimere und damit der Anteil der Undecen-Fraktion bei der Codimerisierung von 1-Penten mit 1-Hexen ebenso verbessert werden, wie die Aktivität. Zur Übertragung dieser homogenen Katalyse auf ein heterogenes System wurden 40 heterogene Katalysatoren mit Cp2ZrCl2 hergestellt. Unter Verwendung von SiO2, Al2O3, SiO2/Al2O3, Zeolithen und MCM-41 konnte gezeigt werden, dass das Trägermaterial großen Einfluss auf die Aktivitäten bei der Dimerisierung von 1-Hexen besitzt. Die besten Ergebnisse hinsichtlich der Aktivität (TON = 2934) erzielten die Silicagele, während unter Verwendung von MCM-41 die beste Dimeren-Selektivität erhalten wurde. Die Beladung des Trägers, die Aktivierung durch den Cokatalysator, sowie die WHSV spielten eine große Rolle bei der Selektivität der gebildeten Produkte. Somit konnte nach der Optimierung dieser Parameter der Produktanteil der Hexen-Dimeren wesentlich verbessert werden. Im Vergleich zur homogenen Variante konnten bei der heterogenen Codimerisierung von 1-Penten und 1-Hexen mit aktiviertem Zirkonocendichlorid höhere Selektivitäten für (Co-)Dimere als auch höhere Aktivitäten erzielt werden.
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Structure and dynamics of new intelligent copolymer hydrogels and hydrogel nanoparticle hybrids
(2011)
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Yvonne Hertle
- Polymeric gels consist of a chemically or physically crosslinked polymer network swollen with a certain amount of solvent and most of these gels show an ability of reversible swelling or shrinking due to small changes in their environment (as changes in pH, temperature or electric field). In the group of hydrogels, poly(N isopropylacrylamide) (poly(NIPAM)) crosslinked with N,N' methylene-bisacrylamide (BIS) is the most well-known member of the class of thermoresponsive "smart" polymers. This work covers the characterization of thermoresponsive poly(NIPAM) gels an shows different kinds of possibilities tuning their properties. The first part of this thesis presents different methods for the preparation of crosslinker gradient macroscopic gels with the dimensions of some cubic centimetres. The swelling behaviour from different zones of the macrogel with a varying crosslinker content was studied as a function of temperature. Furthermore, the internal dynamics of a poly(NIPAM-co-butenoic acid) copolymer macrogel was investigated by neutron spin echo and compared to the results for a chemically identical microgel. Due to the different preparation conditions of the macro- and microgel, a difference in the collective diffusion of the network was expected. Beside this, copolymer microgel particles based on NIPAM and N tert butylacrylamide (NtBAM) were synthesized. The particle size and the swelling behaviour of the obtained colloidal microgels was characterized with respect to the content of the comonomer using different scattering techniques and electron microscopy. In addition, an attempt was made to describe theoretically the temperature induced deswelling with the Flory-Rehner theory. The latter part of this thesis focuses on hybrid materials based on magnetic nanoparticles and thermoresponsive microgels. First of all, different approaches for the synthesis of cobalt and nickel nanoparticles and their protection against oxidation were made. Furthermore, these magnetic particles were incorporated as an inorganic core in poly(NIPAM) core-shell systems, as well as randomly distributed particles within the gel network.Polymergele sind aus chemisch oder physikalisch vernetzten Polymerketten aufgebaut und weisen dabei, bedingt durch ihre Netzwerkstruktur, ein Quellverhalten auf. Die meisten Vertreter dieser Klasse zeigen zusätzlich durch kleine Veränderungen in ihrer Umgebung, wie eine Änderung des pH-Wertes, der Temperatur oder eines elektrischen Feldes, eine reversible Volumenänderung. Das wohlbekannteste Mitglied der sogenannten thermoresponsiven, "intelligenten" Gele ist ein Hydrogel auf der Basis von N,N' Methylenbisacrylamid-vernetztem Poly(N Isopropylacrylamid) (Poly(NIPAM)). Die vorliegende Arbeit beschäftigt sich mit den charakteristischen Eigenschaften von thermoresponsiven Gelen auf Basis von Poly(NIPAM) und den vielfältigen Möglichkeiten diese Eigenschaften gezielt zu steuern. Im ersten Teil der Arbeit werden unterschiedliche Methoden zur Herstellung von Makrogelen basierend auf Poly(NIPAM) mit einem Quervernetzergradient entwickelt, wobei die hier synthetisierten Gele eine Größe von einigen Kubikzentimetern aufweisen. Für eine anschließende Charakterisierung des Quellverhaltens wurde das Quellverhältnis alpha in Abhängigkeit von der Temperatur für unterschiedliche Bereiche des Makrogels bestimmt. Die so erhaltenen Quellkurven konnten dann anhand des Quellvermögens den jeweiligen Bereichen mit unterschiedlichem Quervernetzergehalt zugeordnet werden. Zusätzlich wurde die interne Dynamik eines Poly(NIPAM-co-Butensäure) Copolymer-Makrogels mit Hilfe von Neutronen Spin-Echo Experimenten analysiert. Die durch die Messungen erhaltenen Ergebnisse konnten mit denen chemisch-identischer Mikrogele verglichen werden. Dabei wurde aufgrund der unterschiedlichen Synthesebedingungen von Makro- und Mikrogelen ein Unterschied in der kollektiven Netzwerkdiffusion erwartet. Der gefundene Unterschied in der Netzwerkdynamik war allerdings geringer als erwartet und liegt im Bereich von etwa 10%. Ein weiterer Teil der vorliegenden Arbeit beschäftigt sich mit der Synthese von Mikrogel-Copolymeren auf Basis von NIPAM und N-tert-Butylacrylamid (NtBAM). Mit Hilfe unterschiedlicher Streumethoden, sowie durch Elektronenmikroskopie, wurde sowohl Partikelgröße, als auch Quellverhalten der erhaltenen kolloidalen Mikrogele charakterisiert. Dabei galt es zu bestimmen, welchen Einfluss der Comonomergehalt auf die Eigenschaften des Mikrogels hat. Weiterhin wurde das thermoresponsive Quellen des Gelnetzwerks mit den Vorhersagen der Flory-Rehner Theorie verglichen. Hybridmaterialien aus magnetischen Nanopartikeln und thermoresponsiven Poly(NIPAM) Mikrogelen werden im letzten Teil der Arbeit vorgestellt. Zuerst wurden unterschiedliche Syntheseansätze für Cobalt- und Nickel-Nanopartikel, sowie deren Schutz vor Oxidation, verfolgt. Des Weiteren wurde ein Kern-Schale System angestrebt, in welchem die magnetischen Partikel als Kern vorliegen. Zum anderen wurde beabsichtigt, eine statistische Verteilung der Nanopartikel im Gelnetzwerk zu erreichen.
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Surface Modification of Spherical Particles with Bioactive Glycopolymers
(2011)
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André Pfaff
- Glycopolymers containing different kinds of carbohydrates were grafted from various spherical templates, whereby the glycopolymer chains were prepared via controlled radical polymerization techniques, namely ATRP and RAFT. A library of carbohydrate-displaying spheres and their interaction with lectins is presented. Glucose- or acetylglucosamine-displaying nanospheres were prepared via the combination of emulsion polymerization and photo-induced conventional polymerization or ATRP, respectively. The particles were able to stabilize gold nanoparticles to form catalytically active hybrid particles that were capable of reducing p-nitrophenol in the presence of NaBH4. Investigation of the interactions between acetylglucosamine-displaying polymers and a series of lectins revealed a selective binding towards the lectin wheat germ agglutinin (WGA) whereby the binding affinity of the protein to the polymer brushes was found to be magnitudes higher than to acetylglucosamine unimers. Lectin precipitation experiments revealed that 1 mg of glycopolymer brush was able to precipitate 0.5 mg of WGA. Surface modification of poly(divinylbenzene) microspheres was performed using two different glycomonomers and various types of grafting techniques. “Grafting through” of a mannose-displaying glycomonomer yielded core-shell particles with densely grafted glycopolymer arms that were found to show no binding affinity towards a series of lectins. The nexus of the carbohydrate moiety to the polymer backbone seemed to hamper the key-lock interaction of the sugar and protein. Grafting experiments of a galactose-displaying glycomonomer yielded particles with grafting densities ranging from 0.20 to 0.35 chains per nm2 depending on the utilized grafting approach. These particles showed a selective binding towards the lectin Ricinus communis agglutinin (RCA120), whereby each grafted glycopolymer chain was capable of binding to 0.7 molecules of RCA120. Furthermore, the particles were found to have a superior binding affinity towards RCA120 in comparison to microspheres covered with galactose unimers. The preparation of core-shell particles consisting of a poly(divinylbenzene) micro-sphere core and a shell of highly branched glycopolymers was achieved via self-condensing vinyl copolymerization of an initiator-monomer and acetylglucosamine-displaying glycomonomer. It was found that an increase in incorporated inimer, which results in more compact and branched structures, directly led to an increase in particle coverage (1.6 – 2.4 wt.-%). Carbohydrate-lectin binding studies revealed that the incorporation of approximately 50% of the hydrophobic inimer led to an increase in adsorption of 26% compared to a less branched glycopolymer and 16% compared to linear glycopolymer grafted particles. These results indicated that the three-dimensional glycopolymer architecture directly affects the strength of the key-lock interaction of sugar and sugar-binding protein. Studies on the interactions between glycopolymers and lectins were extended towards the cellular uptake of fluorescent, magnetic galactose-displaying core-shell nanospheres. These particles were prepared by grafting a galactose-displaying glycocopolymer onto silica-encapsulated iron oxide particles via thiol-ene chemistry. Due to the carbohydrate-containing shell, these particles could be localized not only in the cytoplasm but also in the nucleus of human lung cancer cells. This cell line expresses a galactose-binding protein which indicates that carbohydrate-lectin interactions are responsible for the uptake of the functionalized particles. In general, these studies show the capability of utilizing carbohydrate-lectin interactions for potential applications like lectin precipitation and cellular imaging.
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Biochemische Charakterisierung und Wirkoptimierung potentieller Chemotherapeutika auf Basis der natürlichen Chinone Doxorubicin und Thymochinon
(2011)
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Katharina Effenberger-Neidnicht
- Eine Reihe von natürlichen Chinonen weist eine Vielzahl an pharmakologisch sehr interessanten Eigenschaften auf, antitumorale Aktivitäten eingeschlossen. Das etablierte Chemotherapeutikum Doxorubicin beispielsweise – ein Sekundärmetabolit von Streptomyces peucetius var. caesius – findet seine klinische Anwendung bei der Behandlung verschiedenster Krebserkrankungen wie etwa Leukämien, Lymphomen, Karzinomen und Sarkomen. Dennoch ist die Verwendung von Doxorubicin als Chemotherapeutikum durch seine Kardiotoxizität und die Ausbildung von Resistenzen limitiert. Um diese Nebenwirkungen möglichst gering zu halten, ist die kumulative Dosis auf 550 mg / m² festgesetzt. Durch geeignete Derivatisierung von Doxorubicin mit verschiedenen gesättigten und ungesättigten bzw. Terpen-terminierten Fettsäuren sollten unerwünschte Nebenwirkungen – wie etwa die Anfälligkeit gegenüber Mehrfachresistenzen – minimiert werden, ohne jedoch einen Verlust der Wirksamkeit zu riskieren. Das Ziel dieser Arbeit war es, mithilfe biochemischer Arbeitsmethoden diejenigen Wirkstoff-Kandidaten zu finden und zu charakterisieren, die diese Kriterien erfüllen. Es konnten Doxorubicinderivate identifiziert werden, die in der Lage waren, limitierende Mehrfachresistenzen zu überwinden: Besonders erwähnenswert ist das Heptadecansäure-Doxorubicinhydrochlorid-Hydrazon, das anders als die übrigen Fettsäurederivate eine deutliche Wachstums-inhibierende Wirkung gegen die verschiedenen Tumor-Zelllinien aufweist. Zudem übertrifft es aufgrund einer geringeren Anfälligkeit gegenüber dem ABC-Transportprotein P-gp die Wirkung von Doxorubicin an mehrfachresistenten KB-V1/VBL Zervixkarzinom-Zellen um das 3-Fache. Die lange Fettsäureseitenkette dient dabei als eine Art Anker. Die Wirkungsweise von Doxorubicin und seinen Derivaten ist vorwiegend apoptotisch, wobei sowohl klassische Wege als auch alternative Wege über das Endoplasmatische Retikulum und die Generierung von reaktiven Sauerstoffradikalen (ROS) und Ceramiden beteiligt sind. Weiterhin konnte auch für die meisten Derivate die zelluläre DNA als Haupt-Target identifiziert werden. Die Interkalation der Derivate in die DNA führt aufgrund der Blockierung von DNA-bindenden Enzymen zu einem Verbleib der Zellen in der DNA-Replikation-Phase und somit zur Auslösung der Apoptose. Auch einfache Verbindungen wie das p-Benzochinon Thymochinon – Bestandteil des Schwarzkümmel-Extraktes (Nigella sativa) – sind aufgrund von antioxidativen und antitumoralen Effekten potentielle Kandidaten zur Weiterentwicklung zum Chemotherapeutikum. Durch die Funktionalisierung von Thymochinon mit verschiedenen gesättigten und ungesättigten bzw. Terpen-terminierten Fettsäuren sollte die geringe antitumorale Wirkung verbessert werden; die Derivate sollten wiederum mithilfe geeigneter biochemischer Arbeitsmethoden charakterisiert werden. Es konnten einige Derivate identifiziert werden, die die Wirkung und Selektivität der Ausgangsverbindung bei Weitem überschreiten: Einerseits zeigt das DHA-Thymochinon mit Langzeit-IC50-Werten im nano-molaren Bereich überdurchschnittliche Effektivität bei gleichzeitig verbesserter Tumorselektivität. Andererseits weist das (–)-Mentholderivat die geringste MDR-Sensitivität, also die geringste Anfälligkeit gegenüber ABC-Transportern auf. Die Wirkungsweise von Thymochinon und seinen Derivaten ist vorwiegend apoptotisch; es werden sowohl klassische als auch alternative Wege der Apoptose-Initiation angesprochen. Als Haupt-Wirkort von konnten DNA- bzw. RNA-reiche Regionen im Zellkern identifiziert werden.
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Amphiphilic Diblock Copolymers: Study of Interpolyelectrolyte Complexation in Organic Media and Nanoencapsulation of Melatonin
(2011)
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Evis Karina Penott-Chang
- Two oppositely charged homopolyelectrolytes poly(2-(methacryloyloxy)ethyl¬dimethyl-ethyl¬ammonium bromide) (PDMAEMAQ) and poly(acrylic acid) (PAA), and amphiphilic diblock copolymers based on polystyrene and the ionizable block poly(acrylic acid) were synthesized via Atom Transfer Radical Polymerization (ATRP). All polymers were characterized using 1H NMR and gel permeation chromatography to confirm their structure, molecular weight distribution and to follow the conversion. Poly(2-(dimethylamino)ethyl methacrylate), PDMAEMA, was quaternized with ethyl bromide to produce PDMAEMAQ with a quaternization degree of 98%. Furthermore, poly(acrylic acid) segments were obtained after hydrolysis of the poly(t-butyl acrylate) block. After characterization of all polymers, interpolyelectrolyte complexation in chloroform was carried out. A novel method was developed to transfer the insoluble polyelectrolytes into the organic solvent and subsequently form polymer/polymer interpolyelectrolyte (IPECs) in organic media. Therein, the polyelectrolyte were first reacted with oppositely charged low molecular weight surfactants (sodium dodecyl sulfate, SDS, and cetyltrimethylammonium bromide, CTAB) to form polyelectrolyte-surfactant complexes (PESCs). In organic solvents, analogously to the formation of IPECs in aqueous media, interpolyelectrolyte complexation takes place upon the direct mixing of organic solutions of two complementary PESCs. This process is accompanied by an entropically favorable release of the surfactant counterions (in the form of ion pairs or their aggregates in low polarity organic solvents), which were previously associated with the ionic groups of the polyelectrolytes in solution. These reactions are fast and lead to frozen and non-equilibrium macromolecular co-assemblies. The size and the morphologies of the IPECs in chloroform were extensively investigated using transmission electronic microscopy (TEM), scanning force microscopy (SFM), dynamic/static light scattering techniques, 1HMR and turbidimetric titrations, for two different systems: (i) homopolyelectrolyte/homopolyelectrolyte and (ii) homopolycation/negatively charged amphiphilic diblock copolymer. For the first system, the possible particle structures consist either of particles with a core formed by IPECs stabilized by fragments of the excess polymeric component or of vesicles (polymersomes). In system (ii), particles of micellar type with a core assembled from electrostatically coupled segments of the polymeric components can be found, surrounded by a corona built up either from a mixture of polystyrene blocks and excess segments of PDMAEMAQ+DS- chains or from a mixture of polystyrene blocks and excess parts of PA-CTA+ blocks, depending on which polymeric component was present in surplus during the interpolyelectrolyte complexation. Finally, nanocapsules loaded with melatonin were fabricated using a simple nanoprecipitation route employing a mixture of a diblock copolymer based on poly(methyl methacrylate) and PDMAEMA (PMMA-b-PDMAEMA) in combination with poly(ε-caprolactone), PCL. The diblock copolymers were synthesized via ATRP using PMMA-macroinitiators for the DMAEMA polymerization. Shape and size of the nanocarriers were visualized by TEM, cryogenic TEM and scanning electron microscopy (SEM). Standard TEM for nanocapsules showed an oily core surrounded by a thin layer composed of PCL/PMMA-b-PDMAEMA. Cryo-TEM also indicated the presence of spherical nanoobjects with a diffuse polymer corona. Encapsulation efficiencies were determined assaying the nanoparticles by HPLC and values of ca. 30-35% are shown by the nanocapsules. DLS measurements further confirmed well-defined unimodal particle size distributions for all formulations. It was also possible to successfully incorporate platinum nanoparticles into the nanocarrier, as evidenced by TEM, which opens up possibilities for promising applications like monitoring the circulation of the drug carrier within the body.
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Polymeric Nanoparticles for the Modification of Polyurethane Coatings
(2011)
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Sandrine Tea
- Rubber-based nanomodifiers were successfully synthesized following two different strategies and were used as impact modifiers in polyurethane (PU) automotive clearcoats to improve chip resistance. Various narrowly distributed polybutadiene-b-poly(methyl methacrylate) (B-M) block copolymers differing in composition and molecular weights were synthesized and studied with respect to their self-assembly in organic selective solvents. Dynamic light scattering and transmission electron microscopy measurements revealed that spherical micelles were obtained in acetonitrile for all block copolymers, independently of the polymer concentration. Their radii varied from 11 to 69 nm depending on the molecular weight of the initial linear block copolymer and their aggregation behavior in acetonitrile followed the model established by Förster and Antonietti for strongly segregated block copolymers. In DMF and acetone, block copolymers with 85 %wt PMMA were dissolved as unimers. For lower methacrylate contents, the sizes of the obtained spherical micelles were decreasing from DMF to acetone independently of the polymer concentration. The calculated interaction parameters confirmed acetonitrile as the best solvent for PMMA followed by DMF and acetone as the poorest one. The size of the spherical aggregates could be tuned by the molecular weight and/or by the nature of the selective solvent. Polybutadiene-b-poly(n-butly acrylate) (B-nBA), polybutadiene-b-poly(n-butyl methacrylate) (B-nBMA) and polybutadiene-b-poly(t-butyl methacrylate) (B-tBMA) did not show such a large choice in selective solvents and spherical micelles were obtained in DMF, DMAc and acetone respectively. Cross-linking of the polybutadiene core of the obtained micelles was performed in solution using two different methods: cold vulcanization and radical reaction upon the decomposition of a photo-initiator under UV radiation. Both methods allow retaining the spherical shape of the micelles leading to narrowly distributed non fusible nanospheres. In the case of B-M nanoparticles, the degree of cross-linking seemed independent of the amount of cross-linker used. Unlikely, B-nBMA and B-nBA nanoparticles exhibited increasing degrees of cross-linking with the amount of photo-initiator introduced. Their degrees of cross-linking were particularly lower than those of B-M nanoparticles. The hydrolysis of the t-BMA corona of the nanoparticles obtained from B-tBMA linear block copolymers self-assembly in selective solvent resulted in water soluble nanoparticles carrying acid functions and thus potentially exhibiting pH-responsive behavior. Various hyperstars consisting of a hyperbranched PB core and (meth)acrylate arms were synthesized by anionic self-condensing vinyl copolymerization (SCVCP) of divinylbenzene and butadiene followed by the anionic polymerization of the linear (meth)acrylate arms. The amount of hyperbranched products resulting from SCVCP could be enhanced by introducing additional DVB to the reaction while polymerizing. The topology of the hyperbranched PB cores was confirmed by viscosity measurements. All Mark-Houwink-Sakurada exponents were significantly below the value for linear PB. The initiation of (meth)acrylate arms was confirmed by NMR spectroscopy. Upon the arm-growth reaction, the branched topology was retained as witnessed by further viscosity measurements. The introduction of cross-linked nanoparticles based on linear block copolymers did not disturb the transparency of PU coatings. Even after curing reaction, the nanoparticles were well-dispersed into the coating. TEM observations confirmed this last result where neither aggregation nor flocculation of the cross-linked nanoparticles was observed. Hyperstar polymers were found to undergo self-assembly upon the curing reaction leading to “onion-like” structured aggregates, in the case of PMMA hyperstars, with sizes as large as 200 nm. Aggregates of the same size order were observed for the other hyperstars but no defined structures were found. For all hyperstar modified coatings, the transparency of the films was altered. In both cases, cross-linked nanoparticles and hyperstar modified coatings, improvements of chip resistance were observed. The improvements were even better with increasing amount of cross-linked nanoparticles but no effect was noticed on the hardness of the coatings. Similar trends were observed for the hyperstar modified coatings.
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Novel Host Materials for Blue Phosphorescent Organic Light-Emitting Diodes
(2011)
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Pamela Schrögel
- Organic light-emitting diodes (OLEDs) have been commercially used in full-colour active matrix (AMOLED) displays for a couple of years. Only recently, a new application of OLEDs in the field of lighting has been opened up. For white emission monochrome systems of the three primary colours red, green and blue need to be combined. The major issue from the materials’ point of view is still the lack of stable host-emitter systems for blue emission. This thesis deals with the development of new host materials for blue phosphorescent emitters. The host material has to meet a complex profile of requirements. As most crucial feature the triplet energy of the host material has to exceed the triplet energy of the emitter. An increase of triplet energy of the host material is achieved by reducing the conjugated π-system in the host molecule. This thesis describes three synthetic approaches to high triplet energies by confining the π-conjugation: by introducing torsion in the molecular structure, by choosing a meta-linkage and by a non-conjugated linkage. The first and second approach was applied to carbazole-based host materials, whereas the third was demonstrated on phosphazene-based host materials. In the first approach, the molecular structure of a well-known carbazole-based host material, 4,4’-bis(carbazol-9-yl)-2,2’-biphenyl (CBP), was optimised by introducing torsion via methyl or trifluoromethyl substituents in the 2- and 2’-positions of the central biphenyl moiety to yield twisted CBP-derivatives. By confining the conjugated system in combination with selective methyl substitution a series of host materials with superior thermal and photophysical properties was obtained. Compared with the triplet energy of 2.58 eV for CBP, high triplet energies of 2.95 eV could be realised for the twisted CBP-derivatives. In addition, appropriate substitution of the crystalline CBP results in amorphous materials with high glass transition temperatures of up to 120°C. In cyclic voltammetry the electrochemical properties were studied. Here, it was found that the systematic variation of the substitution patterns enables fine-tuning of the energetic positions of the HOMO and LUMO. This helps to avoid injection barriers at materials’ interfaces in the OLED device. By blocking the activated sites in the host molecules a stability of the electrochemically oxidised species against dimerisation could be demonstrated. In the second approach, the conjugation in the same parent carbazole-based compound CBP was reduced by choosing a meta-type of linkage instead of the common para-linkage of the carbazole substituents to the central biphenyl unit. As a result of the meta-linkage, triplet energies of more than 2.90 eV were achieved. No further increase in triplet energy was observed by introducing additional torsion in the molecular structure as described in the first approach. Moreover, the thermal properties were optimised by selective methyl substitution to yield host materials with glass forming properties and high glass transition temperatures of up to 120°C. All host materials were tested in a comparative OLED device study in combination with a phosphorescent emitter with saturated blue emission. For the best host material of this series an external quantum efficiency of 9.7 % and a high brightness of 10 800 cd/m2 were achieved. Both series of carbazole based host materials – the twisted and the meta-linked CBP-derivatives – were synthesised by Ullmann reaction of a dihalogenated biphenyl unit with two (substituted) carbazole units under classic conditions. Noteworthy is the intermediate 5,5’-diiodo-2,2’-dimethyl-biphenyl – a simple and versatile building block in the synthesis of materials with confined conjugation. The synthesis by direct iodination of 2,2’-dimethylbiphenyl, to the best of our knowledge, has not been described in literature before. In the third approach, the class of low molecular weight phosphazenes, which is less described in the context of OLED-materials, was chosen as hosts for blue phosphorescent emitters. As a common characteristic all host materials consist of a six-membered ring of alternating phosphorus and nitrogen atoms. Each phosphorus atom bears two aromatic substituents attached via a non-conjugated linkage. Depending on the type of linkage to the central phosphazene core two sets of host materials can be distinguished: phenoxy substituted phosphazenes with phosphorus-oxygen bonds and phenyl substituted phosphazenes with phosphorus-carbon bonds. The phenoxy substituted derivatives were synthesized by nucleophilic substitution of the chlorine atoms in hexachlorocyclotriphosphazene with phenolates as nucleophils whereas the phenyl substituted derivatives were prepared by cyclocondensation of three equivalents of phosphinic amides. Due to their superior thermal properties compared to the phenoxy substituted series the phenyl substituted phosphazenes are better suited for the use in OLED devices. They exhibit particularly high triplet energies of up to 3.4 eV. Thus, they can be combined with deep blue phosphorescent emitters. Another specialty of the phenyl substituted phosphazenes is a balanced charge carrier transport characteristic. To conclude, each of the three presented approaches yields host materials with triplet energies high enough for a combination with blue phosphorescent emitters. Regarding the morphological stability the extensively studied carbazole based host materials exceed the novel phosphazene based host materials.
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The Surface Charge of Soft and Hard Sphere Colloidal Particles - Experimental Investigation and Comparison to Theory
(2011)
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Christian Schneider
- The focus of this thesis was aimed at the investigation of colloidal particle stability. In a first step we established a method to assess the repulsive interaction energy of dispersed colloids based on the measurement of the rate of slow coagulation with light scattering. Due to an energy resolution in the order of magnitude of the thermal energy, the method was termed microsurface potential measurements (MSPM). We then used the MSPM to measure the potential at the outer Helmholtz plane (oHp), the diffuse potential, which determines the electric double layer of surface charged colloidal particles. The MSPM were performed on anionic particles in the presence of di- and trivalent counterions as a function of the bulk electrolyte concentration. We found that the the diffuse potential does only weakly depend on the magnesium but strongly on the lanthanum ion concentration. In both cases the absolute value of the the diffuse potential decreases with increasing electrolyte concentration. The absolute values of the the diffuse potential are always lower for the trivalent counterions as compared to the divalent results. To supplement the results of the MSPM, we measured the zeta-potential of the particles under similar conditions. Here we detected charge reversal in the experiments with the di- and trivalent counterions. In the salt concentration range of the MSPM the zeta-potential and the the diffuse potential were closely related for both ion species but could not be described by Poisson-Boltzmann based models. In the case of the trivalent counterions, we could experimentally verify the strong influence of counterion adsorption in the destabilization of the surface charged colloids. Furthermore, we showed that the zeta-potential is not suited for calculating the particle stability in the experiments involving trivalent counterions and found strong experimental indications for counterion correlations. We also used MSPM to investigate an anionic SPB in the presence of trivalent counterions. For this purpose we measured the interaction force of two planar polyelectrolyte brush layers across an aqueous medium containing trivalent counterions with the surface forces apparatus. We found that steric repulsion does not occur. The repulsion only arises from residual charges inside the brush layers. From the resulting force curves we were able to deduce an interaction profile of SPB particles in aqueous solution containing multivalent counterions. Thus, we were able to measure the effective repulsive energy of SPB particles using MSPM with an accuracy of the thermal energy. Due to the increase of confined lanthanum counterions in the brush layer the electrostatic repulsion decreased with rising lanthanum concentration. Furthermore, the experimental results were well predicted by a mean-field model. For the first time, we described the means to measure and predict the repulsive energies of SPB particles in aqueous solution in the presence of multivalent counterions. In a next step we refined the theoretical basis of the MSPM and expanded the electrolyte concentration range of the stability experiments. We also measured the form factors of the SPB doublets and found pronounced deviations between the data points and the predictions of the Rayleigh-Debye approximation. We showed that the MSPM are now accurate enough to measure the effective charges per SPB particle with a sub millimolar concentration resolution. Furthermore, we used the mean-field model to predict the particle stability and the effective charge per SPB particle. In both cases we found the deviations between the experimental data and the model to be within an error margin of 20%. Therefore we predicted the particle stability of SPBs in aqueous solution for the first time. In conclusion, this thesis provides a deeper insight into the mechanisms of particle stability and coagulation of electrostatically and electrosterically stabilized dispersions. It offers a new method to investigate the repulsive interactions between colloidal particles which is applicable to a wide variety of colloidal systems. Moreover, we made the first steps toward a more complete understanding of the stability of SPB particles, which is important for potential industrial applications of these kind of systems.
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Neuartige Schichtverbindungen zur Herstellung von Nanokompositen mit verbesserten Flammschutzeigenschaften.
(2011)
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Michael R. Schütz
- Die vorliegende Arbeit wurde teilweise in Kooperation mit der Firma Dronco AG, Wunsiedel durchgeführt. Im Focus der Arbeit stand die Synthese von Füllstoffen für Polymer-Nanokomposite, wobei die erhaltenen Materialien höhere thermische Stabilitäten sowie bessere Performance im Bereich Flammschutz zeigen sollten. Es wurde grundsätzlich mit drei verschiedenen Füllstoffen gearbeitet. Zwei silicatische und ein hydroxidischer Füllstoff wurden synthetisiert und in verschiedenen Polymermatrizes dispergiert, wobei die Wahl des Matrixpolymers zunächst eine untergeordnete Rolle spielte. Zur Synthese des einen silicatischen Füllstoffs wurde ein molekularer Silanprecursor benutzt, aus dem ein Silsesquioxane Füllstoff in situ im benutzten PF-Prepolymer gebildet wurde. Der Wirkmechanismus sowie die Bildung des entstehenden Komposits wurden aufgeklärt. Aus dem eingesetzten Triethylaminopropylsilan entsteht nach Hydrolyse und Kondensation in situ ein Silsesquioxannetzwerk, das das PF-Polymer homogen durchzieht. Somit konnten keine Silsesquioxanpartikel oder Agglomerate im Matrixpolymer nachgewiesen werden. Aus der homogenen Verteilung des Silsesquioxannetzwerks im Polymer ergeben sich signifikant verbesserte mechanische und thermooxidative Eigenschaften. Verbesserte Flammschutzeigenschaften konnten für dieses System jedoch nicht nachgewiesen werden. Weiterhin wurden synthetische Schichtsilicate als Füllstoffe benutzt. Die Synthese dieser Schichtsilicate wurde am Lehrstuhl AC I über die letzten Jahre optimiert. Sie stellen sehr interessante, neue Ausgangstoffe für die Entwicklung von nanoskopischen Zuschlagstoffen dar. Im Speziellen wurden ein Na-Hectorit sowie ein Li-Hectorit benutzt, die beide herausragende Eigenschaften besitzen um als Füllstoff im Bereich Flammschutz von Polymeren eingesetzt zu werden. Diese Schichtsilicate zeichnen sich insbesondere durch hohe Aspektverhältnisse im Vergleich zu natürlichen Schichtsilicaten wie MMT aus. Der benutzte Natrium-Hectorit wurde mit Hilfe von Kationenaustauschreaktionen zunächst scherlabil gemacht und anschließend mit Hilfe einer Rührwerkskugelmühle exfoliert um das Aspektverhältnis zu maximieren. Die Reaktionsfähigkeit der erhaltenen Plättchen wurde anschließend durch einen weiteren Austausch der Zwischenschichtkationen auf die äußeren Oberflächen beschränkt. Diese Grenzfläche wurde dann mit einem oligomeren Modifikator belegt und damit der Phasentransfer in ein organisches Medium ermöglicht. Die so erhaltenen Schichtsilicatpakete, die ausschließlich an der Oberfläche modifiziert waren, wurden dann in PS dispergiert. Die erhaltenen Füllstoffe zeigten eine sehr gute Dispergierbarkeit und ein hohes Aspektverhältnis, was sich durch ausgezeichnete mechanische, thermische und Flammschutzeigenschaften äußerte. Der Anteil des organischen Modifikators am reinen Füllstoff war auf Grund dieser optimierten Nanokomposit-Synthese beträchtlich geringer als bei vergleichbaren Füllstoffen. Der Lithium-Hectorit wurde in der vorliegenden Studie mit einem natürlichen MMT verglichen. Mit typischen Aspektverhältnissen im Bereich 1000 des synthetischen Hectorit im Vergleich zu nur ca. 50 für das natürliche Schichtsilicat konnte zum ersten Mal eindeutig der Einfluss des Aspektverhältnisses auf die Flammschutzeigenschaften untersucht werden. Hier wurde eindeutig gezeigt, dass ein maximiertes Aspektverhältnis zu deutlich besseren Flammschutzeigenschaften führt. Ein weiterer Focus lag auf dem Vergleich von verschiedenen blending-Methoden. Hier konnte eine optimale Dispergierung in der Polymermatrix nur mit Lösungsmittelmethoden gewährleistet werden, eine Trocknung und ein anschließendes Schmelz-blending führten zu kaum redispergierbaren Agglomeraten und in der Folge zu „schlechteren“ Kompositen. Durch das hohe Aspektverhältnis und die hohe Homogenität des Hectorit-Nanokomposits (Lösungsmittel-blended) konnte bei einem solchen Material zum ersten Mal ein intumeszens-artiges Verhalten beschrieben werden. Als dritter Füllstoff wurden LDHs synthetisiert. Diese schichtartigen Hydroxide sollten auf Grund ihrer Morphologie, der hohen Konzentration an thermisch abspaltbaren Hydroxyl¬gruppen und der variablen Zusammensetzung, die auch strukturelle Radikalfänger umfasst, optimale Flammschutzadditive darstellen. Hierzu wurden eisenhaltige LDHs mit hohen Aspektverhältnissen synthetisiert. Die direkte Synthese mit etablierten Methoden war hier nicht möglich, da die Löslichkeit des intermediär gefällten FeOOH zu gering ist. Deshalb wurde eine Komplex-unterstützte Synthese entwickelt, die es auf einfachem Weg möglich macht, phasenreine, Fe3+-haltige LDHs mit akzeptablem Aspektverhältnis zu erhalten. Diese Arbeit ist eine kumulative Dissertation. Die detaillierten Ergebnisse werden in den angehängten Publikationen beschrieben.
