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Massgeschneiderte Schichtsilicate für Materialwissenschaftliche Anwendungen (2010)

Michael Möller

Im Rahmen dieser Arbeit ist es gelungen, flexible und transparente Beschichtungen mit absolut herausragenden Barriereigenschaften auf Basis synthetischer Schichtsilicate herzustellen. Während der Entwicklung eines Polymer-Schichtsilicat Nanokomposites wurden unterschiedliche Schichtsilicate sowie mehrere Konzepte der Partikelmodifikation getestet. Primäres Ziel war es, Plättchen mit hohen Aspektverhältnissen zu erzeugen, welche in Nanokompositen die Durchlässigkeit für Gase etc. verringern sollten. In enger Zusammenarbeit mit M. Sc. Hussein Kalo, der für die Schmelzsynthese der verwendeten Schichtsilicate des Hectorit-Typs verantwortlich war, wurden ein Na-Hectorit (Na0,5[Mg2,5Li0,5]<Si4>O10F2) und ein Li Hectorit (Li0,8[Mg2,2Li0,8]<Si4>O10F2) für die weiteren Experimente optimiert. Diese beiden Schichtsilicate weisen neben einer hohen Reinheit und Homogenität überragende Partikelgrößen im µm-Bereich und vorteilhafte chemische Eigenschaften wie beispielsweise eine genau steuerbare intrakristalline Reaktivität und Kolloidchemie auf. Basierend auf dem genannten Na-Hectorit konnte ein nasschemischer Prozess weiterentwickelt werden, der sowohl eine maßgeschneiderte Exfolierung der Schichtsilicattaktoide als auch das gezielte Einstellen deren Steifigkeit ermöglicht. Durch Kationenaustausch mit Ionen hoher Hydrationsenthalphie lassen sich die Kohäsionskräfte innerhalb eines Taktoides stark verringern und so die Exfolierungseffizienz deutlich steigern. Im Gegenzug erhöht ein Austausch mit Kaliumionen (geringe Hydrationsenthalpie) die Steifigkeit der Taktoide und überführt sie somit in glimmerähnliche Plättchen. Neben den offensichtlichen mechanischen Verbesserungen bringt solch ein Prozess auch ökonomische Vorteile mit sich: Eine nachfolgende Grenzflächenkompatibilisierung kann mit einem Bruchteil der sonst nötigen Menge an amphiphilen Kationen durchgeführt werden. Diese glimmerähnlichen Partikel stellen einen optimalen Kompromiss hinsichtlich Aspektverhältnis und Steifigkeit dar. Die Ionenaustauschreaktionen, die im Kern dieses Prozesses stehen, wurden im Detail untersucht. Unter speziellen Bedingungen des Kationenaustausches eines Na-Hectorites mit Kaliumionen bilden sich spontan geordnete Wechsellagerungen aus. Der jahrelang in der Literatur kontrovers diskutierte Mechanismus der Entstehung streng alternierender Abfolgen von quellfähigen und nicht-quellfähigen Schichten konnte aufgrund der hohen Reinheit und Homogenität des verwendeten Hectorites zum ersten Mal anhand experimenteller Daten herausgearbeitet werden. Die Kombination verschiedener unabhängiger Analytikverfahren lieferte ein überzeugendes Gesamtbild: Die Triebkraft der Ausordnung in eine regelmäßige Wechsellagerung sind Selektivitätsunterschiede der verwendeten Kationen, was letztendlich zu einer alternierenden Kationendichte im Zwischenschichtraum als thermodynamisches Minimum führt. Im Zuge der Optimierung der Kompositmaterialien für Barriereanwendungen wurde als weiteres Schichtsilicat –Li Hectorit– verwendet. Die extrem gute Quellfähigkeit und die enormen Partikelgrößen dieses Hectorites führen mit geringem Aufwand zu Aspektverhältnissen von über 1000, wodurch dieses Material für die Verwendung in Hochbarriere-Kompositen prädestiniert ist. Zunächst wurde die natürliche Barriere von reinen Li-Hectorit Filmen gemessen. Im nächsten Schritt wurde dieser Füllstoff in eine Polymermatrix eingearbeitet, um einen transparenten und flexiblen Hochbarriere-Komposit herzustellen. So reduzierte beispielsweise eine 7 µm-dicke Nanokomposit-Beschichtung auf Polyurethanbasis die Sauerstoffdurchlässigkeit einer Polypropylenfolie auf ein Niveau von 0,05%. Bei dieser Arbeit handelt es sich um eine kumulative Dissertation. Die Ergebnisse werden daher thematisch sortiert und detailliert in den anhängenden Publikationen beschrieben.

Synthesis, Characterization, and Properties of Tailored Functional Block Copolymers (2011)

Robin Pettau

This thesis covers the design, synthesis, characterization, and application of functional block copolymers (BCP) based on a polymer analogous approach and includes three main subjects. The first subject is the implementation of a specially constructed reactor setup for sequential anionic polymerization that allows parallel block copolymer synthesis based on one identical A-block on a lab scale. For this reason, this setup facilitates the preparation of block copolymer series in a combinatorial fashion. It consists of one main reactor and three secondary reactors with individual temperature control. The addition of monomers or additives to each reactor can be handled separately. AB diblock copolymer and ABC triblock copolymer series were prepared with different lengths of the final block as well as different chemical structures of the last block. The second subject covers the synthesis, characterization, processing and application of new liquid crystalline azobenzene-containing block copolymers designed as materials for holographic data storage. Therefore, these polymers contained an amorphous, optical inert poly(methyl methacrylate) (PMMA) or polystyrene (PS) matrix and a functional segment based on polyhydroxystyrene (PHS). Different lengths of flexible spacers and/or mixtures of two spacer lengths were employed to connect the mesogenic chromophores to the polymer backbone. The structure-property relation of functionalized BCPs and the resulting mesophase was investigated. Holographic experiments were conducted on selected examples of the photo-addressable polymers. Smectic annealed samples or amorphous quenched samples were obtained by different sample preparation methods to investigate the influence of the liquid crystalline order. While the initial sensitivity to light induced orientation of the polymer systems remained unaffected, the writing times and level of postdevelopment were improved for quenched samples. Variation in spacer lengths resulted in decreasing smectic order with decreasing spacer length as well as for mixtures of two different spacer lengths promoting lower writing times in the holographic experiments. Additionally, the temperature dependence of the temporal evolution of the refractive index modulation in the smectic polymers was studied. A significant decrease of writing times and an enhancement of the postdevelopment were revealed at elevated temperatures. Stable holographic gratings could be obtained even at 100 °C. 1.1 mm thick samples, that are a prerequisite for volume holographic data storage with a high data storage density, were prepared by injection molding of blends of photoaddressable BCPs with PMMA or PS. Preliminary results confirmed the long-term stability of inscribed holographic gratings and demonstrated angular multiplexing of holographic volume gratings. The third subject covers the synthesis and characterization of new cyanobiphenyl-containing ABA triblock copolymers and their application as BCP gelators for the low molecular weight liquid crystal (LC) 4-cyano-4’-(pentyl)biphenyl (5CB). Based on the selective solubility of the A and B blocks in the nematic solvent, ABA triblock copolymers can be used for the thermoreversible gelation of 5CB. To this end, ABA and ABA’ triblock copolymers comprised of PS A-blocks and a cyanobiphenyl-functionalized PHS B-block with a high degree of polymerization were prepared by the combination of anionic polymerization, using two different synthetic routes, and polymer analogous attachment of the mesogens. Series of linear gelators were prepared with variations in B-block length, A-block lengths and star shaped BCPs by coupling linear ABA’ triblock copolymers. Structure-property relations of the cyanobiphenyl-functionalized polymers regarding the mesophase characterization revealed a dependence of solubility in the nematic 5CB on spacer length. A comprehensive study was conducted to investigate the influence of the BCP backbone and architecture on the gelation of 5CB. Oscillating rheology measurements and thermal characterization were employed to investigate the thermoreversible LC gels. Most of the BCP gelators achieved gelation of 5CB at a mass concentration of 5 wt%. The properties of the different gels where compared at this fixed concentration. The influence of the gelator backbone on the gel properties was investigated by comparing different sets of triblock copolymers. While a short functionalized B-block resulted in high network density and, thus, a high elasticity of the gel the length of the A-blocks influenced the node stability. The LC gel using a star-shaped gealtor exhibited a significantly higher elasticity than with the respective linear block copolymer gelator.

Neuartige Schichtverbindungen zur Herstellung von Nanokompositen mit verbesserten Flammschutzeigenschaften. (2011)

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.

Synthesis of reponsive homo- and block copolymers - application to the generation of inorganic-organic nanohybrids (2010)

Pierre-Eric Millard

Responsive homopolymers and multi-responsive block copolymers were prepared via reversible addition-fragmentation chain transfer (RAFT) and atom transfer radical polymerization (ATRP). Self-assembly in solution depending on environmental stimuli was investigated and exploited to create responsive micelles. New cross-linking strategies were thoroughly performed in aqueous solution to allow a controlled preservation and a high shape-persistence of the colloid particles, even when exposed to non-selective environmental conditions. The synthesis of poly(N-isopropylacrylamide) (PNIPAAm) was investigated by ATRP for subsequent polymer-protein nanohybrid generation. This temperature-responsive polymer was polymerized directly in pure water at a low temperature (4 ºC) by using a functional ATRP initiator which allows post-polymerization conjugation. Without the addition of Cu(II), the kinetics were extremely fast, typically less than one minute for a full conversion. By adjusting the ratio of Cu(I)/(Cu(II) and selecting a very active ligand, all polymerizations proceeded in a controlled fashion to near quantitative conversion without evidence of termination. N-isopropylacrylamide and acrylic acid (AA) were also homopolymerized by RAFT in aqueous media using a novel strategy. Instead of using a diazo-initiator, which generally decomposed at high temperatures, gamma-irradiation was used to initiate polymerization at ambient temperature. This type of radiation has many advantages. A very tiny and constant amount of radicals can be generated, which is perfect for the RAFT process. Moreover, the rate of initiation only has a low level of dependence on temperature and can be used in a wide range of temperatures. Finally, compared to UV-initiation, gamma-irradiation can penetrate the reaction solution deeper and without evidence of irreversible decomposition of the dithioester end group. Therefore, RAFT polymerizations of NIPAAm and AA were achieved with a very good level of control, even at high monomer conversions. This new process was then extended to many other water-soluble monomers for generating homopolymers and block copolymers. Among these, acrylamide, N,N-dimethylacrylamide, 2-hydroxyethyl acrylate and poly(ethylene glycol) methacrylate gave the best results. This technique proved to be very efficient at generating very long and narrowly distributed polymers (up to a degree of polymerization of 10,000) and at designing block copolymers. High molecular weight PNIPAAm-b-PAA copolymers, synthesized by RAFT polymerization under gamma-radiation, were used to generate multi-responsive cross-linked micelles. These block copolymers were self-assembled in water at pH 7 by increasing the temperature over the lower critical solution temperature. The PNIPAAm became hydrophobic and formed the micellar core and the hydrophilic PAA block generated the corona which prevented full aggregation of the system. Then, by amidification at elevated temperatures of the carboxylic moieties via a trifunctional primary amine, the structure was found to remain even after cooling down the system. The shell-cross-linked micelles formed were utilized to generate inorganic-organic nanohybrids by the in situ reduction of gold or silver salts to generate nanoparticles inside the nanocarrier. Another strategy of cross-linking was also investigated by using amino-functional silsesquioxane nanoparticles. In water around neutral pH values and room temperature, these particles interacted with the carboxylic groups of a high molecular weight PNIPAAm-b-PAA by hydrogen bonding and ionic interactions to generate an insoluble complex. Due to the presence of the hydrophilic PNIPAAm block, defined spherical micelles were obtained. The inorganic-organic particles were successfully cross-linked by subsequent amidification to preserve the structure, even at a high pH. Different temperature properties of the hybrids were observed depending on the pH value, due to the residual charge in the micellar core. At a neutral pH, shrinking of the corona was observed, while at a high pH (pH 13) a fully reversible aggregation of the system occurred.

The Surface Charge of Soft and Hard Sphere Colloidal Particles - Experimental Investigation and Comparison to Theory (2011)

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.

Novel Host Materials for Blue Phosphorescent Organic Light-Emitting Diodes (2011)

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.

Polymeric Nanoparticles for the Modification of Polyurethane Coatings (2011)

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.

Amphiphilic Diblock Copolymers: Study of Interpolyelectrolyte Complexation in Organic Media and Nanoencapsulation of Melatonin (2011)

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.

Biochemische Charakterisierung und Wirkoptimierung potentieller Chemotherapeutika auf Basis der natürlichen Chinone Doxorubicin und Thymochinon (2011)

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.

Surface Modification of Spherical Particles with Bioactive Glycopolymers (2011)

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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