Molekulare und supramolekulare Eigenschaften von neuen Donor-Akzeptor-Polymeren

Die organische Halbleitertechnologie befindet sich seit Jahrzehnten im Focus des Interesses, da sie eine kostengünstige und umweltverträgliche Alternative zu anorganischen Silizium-basierten Halbleitern darstellt. Die Möglichkeit der gezielten Funktionalisierung von definierten Strukturen durch synthetische Methoden, welche eine große Vielfalt an Materialien ermöglicht, steht dabei besonders im Vordergrund. Die Modifikation von physikalischen Eigenschaften ermöglicht dabei eine stark erleichterte Anpassung für den geplanten Anwendungsbereich. Im Rahmen der vorliegenden Dissertation wurden organische Halbleitermaterialien basierend auf Cyclopenta[2,1-b:3,4-b']dithiophen (CDT) dargestellt und hinsichtlich ihrer strukturellen und elektronischen Eigenschaften untersucht. In Kombination mit Benzo[c][1,2,5]thiadiazol (BTZ) und weiteren Akzeptoren wurden zunächst Donor(D)-Akzeptor(A)-Polymere synthetisiert und Struktur-Eigenschaft-Beziehungen aufgestellt. So konnte ein sehr hochmolekulares Polymer CDT-BTZ-Polymer (Mn = 36 kg mol-1, PDI = 2.6) erhalten werden, welches sich durch eine hohe lamellare Ordnung und eine gemessene Ladungsträgermobilität in FETs von über 5.0 cm2V-1s-1 bei Raumtemperatur auszeichnete; bei niedrigen Temperaturen (240 K) war letztgenannte 6.5 cm2V-1s-1. Aufgrund dieses hohen Ladungstransports und der Abwesenheit niedermolekularer Polymerketten innerhalb des Polymers konnte erstmals eine Messung eines HALL-Effektes bewerkstelligt werden. Dies war der erste Beweis eines Band-artigen Ladungstransportes an einem Polymerhalbleiter. Des Weiteren wurde durch synthetische Veränderung der Grundstruktur des Polymers zu längeren Alkylketten eine anisotrope Anordnung der Polymerketten erreicht und die Ladungsträgermobilität (6.5 cm2V-1s-1 bei Raumtemperatur) weiter gesteigert. Darauf aufbauend wurde der Einfluss von stereoisomeren Seitenketten an CDT-BTZ-Polymeren auf Packungsverhalten, Parametern (Sperrstrom, Einschaltstrom) in FETs und Löslichkeit in organischen Lösungsmitteln untersucht. Durch cis-trans-Isomerisierung der Seitenketten wurde hier eine neue Methode zur Optimierung des Packungsverhaltens von Polymeren in dünnen Filmen und Lösung gefunden. Zuletzt wurden D-π-A-Farbstoffen, welche CDT als Verbrückungseinheit (π) beinhalten, dargestellt. Durch Variation von D und A konnten Struktur-Eigenschaft-Beziehungen in der Anwendung in Solarzellen (Feststoffsolarzellen, Flüssigsolarzellen) gefunden werden. Die Untersuchungen der photoinduzierten Absorption und der Photolumisenzenzquantenausbeute lieferten dabei Erklärungen für physikalische Prozesse wie Ladungsinjektion- und rekombination.

Nanoscopic studies of conjugated polymer blends by (electric) scanning probe microscopy

Conjugated polymers and conjugated polymer blends have attracted great interest due to their potential applications in biosensors and organic electronics. The sub-100 nm morphology of these materials is known to heavily influence their electromechanical properties and the performance of devices they are part of. Electromechanical properties include charge injection, transport, recombination, and trapping, the phase behavior and the mechanical robustness of polymers and blends. Electrical scanning probe microscopy techniques are ideal tools to measure simultaneously electric (conductivity and surface potential) and dielectric (dielectric constant) properties, surface morphology, and mechanical properties of thin films of conjugated polymers and their blends.rnIn this thesis, I first present a combined topography, Kelvin probe force microscopy (KPFM), and scanning conductive torsion mode microscopy (SCTMM) study on a gold/polystyrene model system. This system is a mimic for conjugated polymer blends where conductive domains (gold nanoparticles) are embedded in a non-conductive matrix (polystyrene film), like for polypyrrole:polystyrene sulfonate (PPy:PSS), and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). I controlled the nanoscale morphology of the model by varying the distribution of gold nanoparticles in the polystyrene films. I studied the influence of different morphologies on the surface potential measured by KPFM and on the conductivity measured by SCTMM. By the knowledge I gained from analyzing the data of the model system I was able to predict the nanostructure of a homemade PPy:PSS blend.rnThe morphologic, electric, and dielectric properties of water based conjugated polymer blends, e.g. PPy:PSS or PEDOT:PSS, are known to be influenced by their water content. These properties also influence the macroscopic performance when the polymer blends are employed in a device. In the second part I therefore present an in situ humidity-dependence study on PPy:PSS films spin-coated and drop-coated on hydrophobic highly ordered pyrolytic graphite substrates by KPFM. I additionally used a particular KPFM mode that detects the second harmonic electrostatic force. With this, I obtained images of dielectric constants of samples. Upon increasing relative humidity, the surface morphology and composition of the films changed. I also observed that relative humidity affected thermally unannealed and annealed PPy:PSS films differently. rnThe conductivity of a conjugated polymer may change once it is embedded in a non-conductive matrix, like for PPy embedded in PSS. To measure the conductivity of single conjugated polymer particles, in the third part, I present a direct method based on microscopic four-point probes. I started with metal core-shell and metal bulk particles as models, and measured their conductivities. The study could be extended to measure conductivity of single PPy particles (core-shell and bulk) with a diameter of a few micrometers.

A molecular dynamics study of the influence of chain branching on the properties of polymer systems

Die vorliegende Arbeit beschäftigt sich mit dem Einfluß von Kettenverzweigungen unterschiedlicher Topologien auf die statischen Eigenschaften von Polymeren. Diese Untersuchungen werden mit Hilfe von Monte-Carlo- und Molekular-Dynamik-Simulationen durchgeführt.Zunächst werden einige theoretische Konzepte und Modelle eingeführt, welche die Beschreibung von Polymerketten auf mesoskopischen Längenskalen gestatten. Es werden wichtige Bestimmungsgrößen eingeführt und erläutert, welche zur quantitativen Charakterisierung von Verzweigungsstrukturen bei Polymeren geeignet sind. Es wird ebenso auf die verwendeten Optimierungstechniken eingegangen, die bei der Implementierung des Computerprogrammes Verwendung fanden. Untersucht werden neben linearen Polymerketten unterschiedliche Topolgien -Sternpolymere mit variabler Armzahl, Übergang von Sternpolymeren zu linearen Polymeren, Ketten mit variabler Zahl von Seitenketten, reguläre Dendrimere und hyperverzweigte Strukturen - in Abhängigkeit von der Lösungsmittelqualität. Es wird zunächst eine gründliche Analyse des verwendeten Simulationsmodells an sehr langen linearen Einzelketten vorgenommen. Die Skalierungseigenschaften der linearen Ketten werden untersucht in dem gesamten Lösungsmittelbereich vom guten Lösungsmittel bis hin zu weitgehend kollabierten Ketten im schlechten Lösungsmittel. Ein wichtiges Ergebnis dieser Arbeit ist die Bestätigung der Korrekturen zum Skalenverhalten des hydrodynamischen Radius Rh. Dieses Ergebnis war möglich aufgrund der großen gewählten Kettenlängen und der hohen Qualität der erhaltenen Daten in dieser Arbeit, insbesondere bei den linearen ketten, und es steht im Widerspruch zu vielen bisherigen Simulations-Studien und experimentellen Arbeiten. Diese Korrekturen zum Skalenverhalten wurden nicht nur für die linearen Ketten, sondern auch für Sternpolymere mit unterchiedlicher Armzahl gezeigt. Für lineare Ketten wird der Einfluß von Polydispersität untersucht.Es wird gezeigt, daß eine eindeutige Abbildung von Längenskalen zwischen Simulationsmodell und Experiment nicht möglich ist, da die zu diesem Zweck verwendete dimensionslose Größe eine zu schwache Abhängigkeit von der Polymerisation der Ketten besitzt. Ein Vergleich von Simulationsdaten mit industriellem Low-Density-Polyäthylen(LDPE) zeigt, daß LDPE in Form von stark verzweigten Ketten vorliegt.Für reguläre Dendrimere konnte ein hochgradiges Zurückfalten der Arme in die innere Kernregion nachgewiesen werden.

Synthesis and characterization of polymers containing oligothiophenes

In the present work a series of thiophene oligomers of three and six thiophene units were synthesized, starting from thiophene, and characterized. Polymers containing these electroative side groups were then prepared by two strategies. The oligomers were attached to existing polymer systems and were connected to a polymerizable unit leading to monomer containing the oligothiophenes as side groups. Subsequently the properties of the monomers and the polymers were investigated. A butylcellulose derivative carrying terthienyl side chains (BCTTE, 26) was synthesized starting from cellulose acetate and 5-(2-chloroethyl)-2,2':5',2'-terthiophene (4). The polymer had a degree of substitution (DS) of the butyl and terthienyl side chains of DSbutyl = 1.9 and DSterth = 0.35, respectively. It was successfully spread on a Langmuir-Blodgett (LB) trough and then transferred to several solid substrates. X-rays reflectometry showed an ordered architecture of the cellulose backbones. However, the terthiophene side groups were found as isotropically aligned by polarized UV-Vis spectroscopy. When used as anode material in the electropolymerization of 3-pentylthiophene (28), polythiophene was grafted onto the cellulose backbone through the terthienyl side groups. The polythiophene chains showed an average anisotropic alignment of 20 % along the LB dipping direction, calculated by means of polarized UV-Vis spectroscopy. A second butylcellulose derivative carrying sexithienyl side chains (BCST) was synthesized and investigated, starting from butylcellulose and 2-[b ', b ''-dipentyl-5'''-(2-hydroxyethyl)-2,2': 5',2':5',2'':5'',2'':5'',2'''-sexithiophen-5-yl]-ethyl p-toluensulfonate (7). The polymer showed formation of stable LB monolayers at the air-water interface, but its transfer onto solid substrates was not successful. A poly(p-phenylene-ethynylene) bearing sexithienyl side chains (BzAcST, 31) was prepared by reaction of the two monomers 2-[b ', b ''-dipentyl-5'''-(2-hydroxyethyl)-2,2': 5',2':5',2'':5'',2'':5'',2'''-sexithiophen-5-yl]-ethyl 2,5-diiodobenzoate (15) and 2-[b', b ''-dipentyl-5'''-(2-hydroxyethyl)-2,2':5',2':5',2'':5'',2'':5'',2'''-sexithiophen-5-yl]-ethyl 2,5-diethynylbenzoate (18). The polymer was obtained as insoluble product. Upon oxidation with FeCl3 (doping) of the polymer suspension, BzAcST showed an electrical conductivity of ó = 2.5 . 10 -6 S/cm, a typical value for semiconductors. The IR spectrum of the doped polymer presented the diagnostic bands of oxidized sexithiophene in good agreement with literature results. Along with the monomer and polymer synthesis, an a,a '-disubstituted sexithiophene, b ', b ''-dipentyl-5,5'''-bis-(2-hydroxyethyl)-2,2':5',2':5',2'':5'',2'':5'',2'''-sexithiophene (6a),was synthesized and characterized. The UV-Vis absorption of the chromophore wasinvestigated as a function of temperature and different solvents, showing a blue-shift of the absorption maximum with increasing temperature and a red-shift changing the solvent from hexane to ethanol to toluene. Monitoring the change of the UV-Vis spectrum upon electrochemical oxidation, the oxidized chromophore showed a new broad absorption band, red shifted with respect to the p -p* transition of the neutral state. Upon reduction, the new band disappeared and the UV-Vis spectrum of the chromophore was restored. Such oxidation-reduction cycles were totally reversible. This feature, together with the absorption maximum falling in the visible region, makes this chromophore a suitable compound for the development of an electrochemical sensor.Attempts to polymerize acrylic monomers carrying sexythienyl side chains both via radical polymerization, as in the case of 2-[b ', b ''-dipentyl-5'''-(2-hydroxyethyl)-2,2': 5',2':5',2'':5'',2'':5'',2'''-sexithiophen-5-yl]-ethyl acrylate (8), and anionic polymerization, as in the case of 2-{b ', b ''-dipentyl-5'''-[2-(tertbutyldimethylsiloxy)ethyl]-2,2':5',2':5',2'':5'',2'': 5'',2''' -sexithiophen-5-yl}-ethylacrylate (29), were not successful, probably due to the steric hindrance of the oligothiophene side group. However, due to the time consuming and therefore restricted availability of the monomers, a screening of the polymerization conditions towards the formation of polymeric material was not possible.

Decay of imprinted surface waves in polymers

CONCLUSIONS The focus of this work was the investigation ofanomalies in Tg and dynamics at polymer surfaces. Thethermally induced decay of hot-embossed polymer gratings isstudied using laser-diffraction and atomic force microscopy(AFM). Monodisperse PMMA and PS are selected in the Mwranges of 4.2 to 65.0 kg/mol and 3.47 to 65.0 kg/mol,respectively. Two different modes of measurement were used:the one mode uses temperature ramps to obtain an estimate ofthe near-surface glass temperature, Tdec,0; the other modeinvestigates the dynamics at a constant temperature aboveTg. The temperature-ramp experiments reveal Tdec,0 valuesvery close to the Tg,bulk values, as determined bydifferential scanning calorimetry (DSC). The PMMA of65.0 kg/mol shows a decreased value of Tg, while the PS samples of 3.47 and 10.3 kg/mol (Mw-surface Tg. The transition widthbetween the onset of an amplitude reduction and a finalstate is smaller in the case of PS than in the case of PMMA.This suggests a higher degree of cooperation between thepolymer chains for PMMA than for PS chains even near thesurface. A reduction of the investigated near-surface region byusing smaller grating constants and AFM did not show achange in the near-surface Tdec,0. Decay experiments were performed at a variety ofconstant temperatures. Master plots are produced by shifting the decay curves, on the logarithmic time scale,with respect to a reference curve at Tref. From thisprocedure shift factors were extracted. An Arrhenius analysis of the shift factors reveals adecreasing non-Arrhenius (fragile) behavior with molecular weight for PMMA. PS is fragile for all Mw asexpected for linear polymers. Non-Arrhenius behavior allowsone to fit the shift factors to the William-Landel-Ferry(WLF) equation. The WLF parameters for the varying molecular weights ofPMMA and PS were extracted and compared to the values frombulk rheology measurements. Assuming cg1=16+/-2 at Tg, assuggested by Angell, the glass temperature was determinedfrom the dynamic decay experiments. Within the experimentalerrors, the values for Tg,surf(c1=16) and T_g,bulk(c1=16)tend to be smaller than Tdec,0 and Tg,bulk fromtemperature-ramp and DSC measurements, but confirm thecourse of the values with increasing Mw. The comparison of the fragilities (temperaturedependence of the polymer properties at Tg) near the surfaceand in the bulk shows a higher fragility for PS near thesurface, a lower one for PMMA with molecular weights of 4.2and 65.0 kg/mol. The different surface behavior of PS istraced back to a lower degree of cooperation and a largerfree volume fraction.

Electronic energy transfer processes in pi-conjugated polymers

Currently pi-conjugated polymers are considered as technologically interesting materials to be used as functional building elements for the development of the new generation of optoelectronic devices. More specifically during the last few years, poly-p-phenylene materials have attracted considerable attention for their blue photoluminescence properties. This Thesis deals with the optical properties of the most representative blue light poly-p-phenylene emitters such as poly(fluorene), oligo(fluorene), poly(indenofluorene) and ladder-type penta(phenylene) derivatives. In the present work, laser induced photoluminescence spectroscopy is used as a major tool for the study of the interdependence between the dynamics of the probed photoluminescence, the molecular structures of the prepared polymeric films and the presence of chemical defects. Complementary results obtained by two-dimensional wide-angle X-ray diffraction are reported. These findings show that the different optical properties observed are influenced by the intermolecular solid-state interactions that in turn are controlled by the pendant groups of the polymer backbone. A significant feedback is delivered regarding the positive impact of a new synthetic route for the preparation of a poly(indenofluorene) derivative on the spectral purity of the compound. The energy transfer mechanisms that operate in the studied systems are addressed by doping experiments. After the evaluation of the structure/property interdependence, a new optical excitation pathway is presented. An efficient photon low-energy up-conversion that sensitises the blue emission of poly(fluorene) is demonstrated. The observed phenomenon takes place in poly(fluorene) derivatives hosts doped with metallated octaethyl porphyrins, after quasi-CW photoexcitation of intensities in the order of kW/cm2. The up-conversion process is parameterised in terms of temperature, wavelength excitation and central metal cation in the porphyrin ring. Additionally the observation of the up-conversion is extended in a broad range of poly-p-phenylene blue light emitting hosts. The dependence of the detected up-conversion intensity on the excitation intensity and doping concentration is reported. Furthermore the dynamics of the up-conversion intensity are monitored as a function of the doping concentration. These experimental results strongly suggest the existence of triplet-triplet annihilation events into the porphyrin molecules that are subsequently followed by energy transfer to the host. After confirming the occurrence of the up-conversion in solutions, cyclic voltammetry is used in order to show that the up-conversion efficiency is partially determined from the energetic alignment between the HOMO levels of the host and the dopant.

Electronic energy transfer processes and charge carrier transport in pi-conjugated polymers

Conjugated polymers have attracted tremendous academical and industrial research interest over the past decades due to the appealing advantages that organic / polymeric materials offer for electronic applications and devices such as organic light emitting diodes (OLED), organic field effect transistors (OFET), organic solar cells (OSC), photodiodes and plastic lasers. The optimization of organic materials for applications in optoelectronic devices requires detailed knowledge of their photophysical properties, for instance energy levels of excited singlet and triplet states, excited state decay mechanisms and charge carrier mobilities. In the present work a variety of different conjugated (co)polymers, mainly polyspirobifluorene- and polyfluorene-type materials, was investigated using time-resolved photoluminescence spectroscopy in the picosecond to second time domain to study their elementary photophysical properties and to get a deeper insight into structure-property relationships. The experiments cover fluorescence spectroscopy using Streak Camera techniques as well as time-delayed gated detection techniques for the investigation of delayed fluorescence and phosphorescence. All measurements were performed on the solid state, i.e. thin polymer films and on diluted solutions. Starting from the elementary photophysical properties of conjugated polymers the experiments were extended to studies of singlet and triplet energy transfer processes in polymer blends, polymer-triplet emitter blends and copolymers. The phenomenon of photonenergy upconversion was investigated in blue light-emitting polymer matrices doped with metallated porphyrin derivatives supposing an bimolecular annihilation upconversion mechanism which could be experimentally verified on a series of copolymers. This mechanism allows for more efficient photonenergy upconversion than previously reported for polyfluorene derivatives. In addition to the above described spectroscopical experiments, amplified spontaneous emission (ASE) in thin film polymer waveguides was studied employing a fully-arylated poly(indenofluorene) as the gain medium. It was found that the material exhibits a very low threshold value for amplification of blue light combined with an excellent oxidative stability, which makes it interesting as active material for organic solid state lasers. Apart from spectroscopical experiments, transient photocurrent measurements on conjugated polymers were performed as well to elucidate the charge carrier mobility in the solid state, which is an important material parameter for device applications. A modified time-of-flight (TOF) technique using a charge carrier generation layer allowed to study hole transport in a series of spirobifluorene copolymers to unravel the structure-mobility relationship by comparison with the homopolymer. Not only the charge carrier mobility could be determined for the series of polymers but also field- and temperature-dependent measurements analyzed in the framework of the Gaussian disorder model showed that results coincide very well with the predictions of the model. Thus, the validity of the disorder concept for charge carrier transport in amorphous glassy materials could be verified for the investigated series of copolymers.

High quality colloidal crystals with different architectures and their optical properties

In this present work high quality PMMA opals with different sphere sizes, silica opals from large size spheres, multilayer opals, and inverse opals were fabricated. Highly monodisperse PMMA spheres were synthesized by surfactant-free emulsion polymerization (polydispersity ~2%). Large-area and well-ordered PMMA crystalline films with a homogenous thickness were produced by the vertical deposition method using a drawing device. Optical experiments have confirmed the high quality of these PMMA photonic crystals, e.g., well resolved high-energy bands of the transmission and reflectance spectra of the opaline films were observed. For fabrication of high quality opaline photonic crystals from large silica spheres (diameter of 890 nm), self-assembled in patterned Si-substrates a novel technique has been developed, in which the crystallization was performed by using a drawing apparatus in combination with stirring. The achievements comprise a spatial selectivity of opal crystallization without special treatment of the wafer surface, the opal lattice was found to match the pattern precisely in width as well as depth, particularly an absence of cracks within the size of the trenches, and finally a good three-dimensional order of the opal lattice even in trenches with a complex confined geometry. Multilayer opals from opaline films with different sphere sizes or different materials were produced by sequential crystallization procedure. Studies of the transmission in triple-layer hetero-opal revealed that its optical properties cannot only be considered as the linear superposition of two independent photonic bandgaps. The remarkable interface effect is the narrowing of the transmission minima. Large-area, high-quality, and robust photonic opal replicas from silicate-based inorganic-organic hybrid polymers (ORMOCER® s) were prepared by using the template-directed method, in which a high quality PMMA opal template was infiltrated with a neat inorganic-organic ORMOCER® oligomer, which can be photopolymerized within the opaline voids leading to a fully-developed replica structure with a filling factor of nearly 100%. This opal replica is structurally homogeneous, thermally and mechanically stable and the large scale (cm2 size) replica films can be handled easily as free films with a pair of tweezers.

Spectroscopic signatures from first principles calculations: from surface adsorbates to liquids and polymers

In dieser Arbeit werden drei wasserstoffverbrückte Systeme in der kondensierten Phase mit Hilfe von first-principles-Elektronenstruktur-Rechnungen untersucht, die auf der Dichtefunktionaltheorie (DFT) unter periodischen Randbedingungen basieren. Ihre lokalen Konformationen und Wasserstoffbrückenbindungen werden mittels ab-initio Molekulardynamiksimulationen berechnet und weiterhin durch die Bestimmung ihrer spektroskopischen Parameter charakterisiert. Der Schwerpunkt liegt dabei auf lokalen Strukturen und auf schnellen Fluktuationen der Wasserstoffbrückenbindungen, welche von zentraler Bedeutung für die physikalischen und chemischen Eigenschaften der betrachteten Systeme sind. Die für die lokalen, instantanen Konformationen berechneten Spektren werden verwendet, um die physikalischen Prozesse, die hinter den untersuchten Phänomenen stehen, zu erklären: die Wasseradsorption auf metallischen Oberflächen, die Ionensolvatisierung in wässrigen Lösungen und der Protonentransport in protonleitenden Polymeren, welche Prototypen von Membranen für Brennstoffzellen sind. Die Möglichkeit der Vorhersage spektroskopischer Parameter eröffnet vielfältige Möglichkeiten des Dialogs zwischen Experimenten und numerischen Simulationen. Die in dieser Arbeit vorgestellten Ergebnisse zeigen, dass die Zuverlässigkeit dieser theoretischen Berechnungen inzwischen für viele experimentell relevante Systeme ein quantitatives Niveau erreicht hat.

Dynamics, rheology and critical properties of colloidal fluid mixtures

Liquids under the influence of external fields exhibit a wide range of intriguing phenomena that can be markedly different from the behaviour of a quiescent system. This work considers two different systems — a glassforming Yukawa system and a colloid-polymer mixture — by Molecular Dynamics (MD) computer simulations coupled to dissipative particle dynamics. The former consists of a 50-50 binary mixture of differently-sized, like-charged colloids interacting via a screened Coulomb (Yukawa) potential. Near the glass transition the influence of an external shear field is studied. In particular, the transition from elastic response to plastic flow is of interest. At first, this model is characterised in equilibrium. Upon decreasing temperature it exhibits the typical dynamics of glassforming liquids, i.e. the structural relaxation time τα grows strongly in a rather small temperature range. This is discussed with respect to the mode-coupling theory of the glass transition (MCT). For the simulation of bulk systems under shear, Lees-Edwards boundary conditions are applied. At constant shear rates γ˙ ≫ 1/τα the relevant time scale is given by 1/γ˙ and the system shows shear thinning behaviour. In order to understand the pronounced differences between a quiescent system and a system under shear, the response to a suddenly commencing or terminating shear flow is studied. After the switch-on of the shear field the shear stress shows an overshoot, marking the transition from elastic to plastic deformation, which is connected to a super-diffusive increase of the mean squared displacement. Since the average static structure only depends on the value of the shear stress, it does not discriminate between those two regimes. The distribution of local stresses, in contrast, becomes broader as soon as the system starts flowing. After a switch-off of the shear field, these additional fluctuations are responsible for the fast decay of stresses, which occurs on a time scale 1/γ˙ . The stress decay after a switch-off in the elastic regime, on the other hand, happens on the much larger time scale of structural relaxation τα. While stresses decrease to zero after a switch-off for temperatures above the glass transition, they decay to a finite value for lower temperatures. The obtained results are important for advancing new theoretical approaches in the framework of mode-coupling theory. Furthermore, they suggest new experimental investigations on colloidal systems. The colloid-polymer mixture is studied in the context of the behaviour near the critical point of phase separation. For the MD simulations a new effective model with soft interaction potentials is introduced and its phase diagram is presented. Here, mainly the equilibrium properties of this model are characterised. While the self-diffusion constants of colloids and polymers do not change strongly when the critical point is approached, critical slowing down of interdiffusion is observed. The order parameter fluctuations can be determined through the long-wavelength limit of static structure factors. For this strongly asymmetric mixture it is shown how the relevant structure factor can be extracted by a diagonalisation of a matrix that contains the partial static structure factors. By presenting first results of this model under shear it is demonstrated that it is suitable for non-equilibrium simulations as well.

Single polymers at surfaces: adsorption and detachment

This thesis is concerned with the adsorption and detachment of polymers at planar, rigid surfaces. We have carried out a systematic investigation of adsorption of polymers using analytical techniques as well as Monte Carlo simulations with a coarse grained off-lattice bead spring model. The investigation was carried out in three stages. In the first stage the adsorption of a single multiblock AB copolymer on a solid surface was investigated by means of simulations and scaling analysis. It was shown that the problem could be mapped onto an effective homopolymer problem. Our main result was the phase diagram of regular multiblock copolymers which shows an increase in the critical adsorption potential of the substrate with decreasing size of blocks. We also considered the adsorption of random copolymers which was found to be well described within the annealed disorder approximation. In the next phase, we studied the adsorption kinetics of a single polymer on a flat, structureless surface in the regime of strong physisorption. The idea of a ’stem-flower’ polymer conformation and the mechanism of ’zipping’ during the adsorption process were used to derive a Fokker-Planck equation with reflecting boundary conditions for the time dependent probability distribution function (PDF) of the number of adsorbed monomers. The numerical solution of the time-dependent PDF obtained from a discrete set of coupled differential equations were shown to be in perfect agreement with Monte Carlo simulation results. Finally we studied force induced desorption of a polymer chain adsorbed on an attractive surface. We approached the problem within the framework of two different statistical ensembles; (i) by keeping the pulling force fixed while measuring the position of the polymer chain end, and (ii) by measuring the force necessary to keep the chain end at fixed distance above the adsorbing plane. In the first case we treated the problem within the framework of the Grand Canonical Ensemble approach and derived analytic expressions for the various conformational building blocks, characterizing the structure of an adsorbed linear polymer chain, subject to pulling force of fixed strength. The main result was the phase diagram of a polymer chain under pulling. We demonstrated a novel first order phase transformation which is dichotomic i.e. phase coexistence is not possible. In the second case, we carried out our study in the “fixed height” statistical ensemble where one measures the fluctuating force, exerted by the chain on the last monomer when a chain end is kept fixed at height h over the solid plane at different adsorption strength ε. The phase diagram in the h − ε plane was calculated both analytically and by Monte Carlo simulations. We demonstrated that in the vicinity of the polymer desorption transition a number of properties like fluctuations and probability distribution of various quantities behave differently, if h rather than the force, f, is used as an independent control parameter.

Electrostatic supramolecular assembly of charged dendritic polymers and their biological application

The aim of this study was the development of functional multilayer films through electrostatic layer by layer (LbL) assembly of dendritic macromolecules, the investigation of the fundamental properties of these multilalyered films and the study of their biological applications. rnThe synthesis of the anionic hyperbranched polyglycerols (hbPG) and the preparation of multilayers made of hbPG/phosphorus dendrimer as well as the influences of deposition conditions on multilayers were reported. The thicknesses of multilayer films increase with a decrease of molecular weight of anionic hbPGs. The multilayer films fabricated by low molecular weight hbPGs grow less regularly due to the less charged carboxylic acid groups providing the relative weaker electrostatic forces for the deposition. The thicknesses of multilayer films are reduced with increasing pH values and decreasing the concentration of NaCl. The observed changes of multilayer thickness and surface morphology could be interpreted with the aid of theories regarding the charge density and conformation of the anionic hbPG chains in solution. rnBesides the study of fundamental properties of hbPG/phosphorus multilayer films, antifouling thin films derived from hbPG layers were developed. The antifouling properties of hbPG layers were found to correlate with factors of the molecular weight of anionic hbPG and the film thickness. It was demonstrated that anionic hbPG single layer with highest molecular weight can reduce non specific protein adsorption more efficiently than single layer with lower molecular weight and all the hbPG bilayers possessed excellent property of antifouling. rnPhosphorus dendrimer multilayers were successfully prepared as the platforms to detect DNA immobilization and hybridization. The effect of NaCl concentration on the multilayer film thickness was evaluated to obtain the optimized film thickness. Making use of the multilayer deposited at the optimized condition as a substrate, a high loading of DNA probes was achieved through covalent coupling of probe DNA with the as-formed multilayer films. The hybridization of target DNA with immobilized probe DNA was then carried out and studied by SPFS. The limit of detection upon hybridization was estimated on various dendrimer multilayer platforms. The minimum detection concentration for DNA hybridization is in the same order of magnitude compared with other neutral phosphorus dendrimer systems. Furthermore, the LbL deposition of phosphorus dendrimer multilayers provided a mild and simple way to prepare platforms as DNA microarrays. rnBased on the phosphorus dendrimer multilayer systems, dendritic star polymers were employed which have more reactive groups than that phosphorus dendrimers. The as-assembled dendritic star polymer multilayer films exhibited such distinct morphology characteristics that they underwent extensive structural reorganization upon post-treatment under different pH conditions. Kinetic binding of probe DNA molecules on the outermost negatively charged dendritic surface was studied by SPR as well. The binding capacities of probe DNA on the multilayer surfaces fabricated from the first-generation and the second-generation of dendritic star polymers were compared. The improved binding capacity was achieved from the second-generation of dendritic star polymer multilayer films due to their more reactive groups. DNA hybridization reaction on dendritic multilayer films was investigated by SPFS. The similar hybridization behaviors were found on both multilayer surfaces. Meanwhile, the hybridization kinetic affinities were compared with that of phosphorus dendrimer multilayer surfaces and showed improved detection sensitivity than phosphorus dendrimer multilayer films.rn

Triphenylene-based polymers for organic electronics

An efficient synthesis has been developed toward a novel series of conjugated blue emitting polymers containing triphenylene as repeating unit for polymer light emitting diodes (PLEDs). Soluble triphenylene-based co- and homo-polymers have been synthesized by the palladium-catalyzed Suzuki-Miyaura and the nickel-catalysed Yamamoto polycondensation reactions, respectively. The photophysical properties as well as the application of the polymers in PLED devices are presented here.rnIn addition a simple GNR fabrication method that allows for the production of atomically precise GNRs of different topologies and widths is introduced. This bottom-up approach consists in the surface-assisted coupling of suitably designed molecular triphenylene precursors into linear polyphenylenes and their subsequent cyclodehydrogenation and results in GNRs whose topology, width and edge periphery are defined by the precursor monomers. Various types of atomically precise GNRs thus eventually become available for experimental investigation and exploitation of their many predicted and technologically highly interesting properties. Furthermore, it is anticipated that this bottom-up approach of GNR fabrication will allow the engineering of chemical and electronic properties and the yet elusive realization of theoretically predicted structures such as intraribbon quantum dots, superlattice structures, or magnetic devices based on specific GNR edge states.rn

Structure and properties of nanostructures from polyelectrolytes and porphyrins: Self-Assembly in aqueous solution

In this work self-assembling model systems in aqueous solution were studied. The systems contained charged polymers, polyelectrolytes, that were combined with oppositely charged counterions to build up supramolecular structures. With imaging, scattering and spectroscopic techniques it was investigated how the structure of building units influences the structure of their assemblies. Polyelectrolytes with different chemical structure, molecular weight and morphology were investigated. In addition to linear polyelectrolytes, semi-flexible cylindrical bottle-brush polymers that possess a defined cross-section and a relatively high persistence along the backbone were studied. The polyelectrolytes were combined with structural organic counterions having charge numbers one to four. Especially the self-assembly of polyelectrolytes with different tetravalent water-soluble porphyrins was studied. Porphyrins have a rigid aromatic structure that has a structural effect on their self-assembly behavior and through which porphyrins are capable of self-aggregation via π-π interaction. The main focus of the thesis is the self-assembly of cylindrical bottle-brush polyelectrolytes with tetravalent porphyrins. It was shown that the addition of porphyrins to oppositely charged brush molecules induces a hierarchical formation of stable nanoscale brush-porphyrin networks. The networks can be disconnected by addition of salt and single porphyrin-decoratedrncylindrical brush polymers are obtained. These two new morphologies, brush-porphyrin networks and porphyrin-decorated brush polymers, may have potential as functional materials with interesting mechanical and optical properties.

From amphiphilic block copolymers to ferrocenyl-functionalized polymers for biosensoric applications

The present thesis can be divided in three main parts. In all parts new polymer architecturesrnwere synthesized and characterized concerning their special features.rnThe first part will emphasize the advantage of a polystyrene-block-(hyperbranchedrnpolyglycerol) copolymer in comparison to an analogue polystyrene-block-(linear polyglycerol)rncopolymer. Therefore a synthethic route to prepare linear block copolymersrnhas been developed. Two strategies were examined. One strategy was based on thernclassic, sequential anionic polymerization; the second strategy was based on arn“Click-Chemistry” coupling reaction. In a following step glycidol was hypergraftedrnfrom these block copolymers by applying a hypergrafting reaction with glycidol. Thernbehavior of the amphiphilic block copolymers synthesized was studied in differentrnsolvents. Furthermore the polarity of the solvent was changed to form the correspondingrninverse micelles. DLS, SLS, SEC-MALLS-VISCO, AFM and Cyro TEMrnmeasurements were performed to obtain a visual image from the appearance of thernaggregates. It was found that a linear-hyperbranched architecture is necessary, ifrnwell defined, monodisperse aggregates are required, e.g. for the preparation of orderedrnnanoarrays. Linear-linear block copolymers formed only polydisperse aggregates.rnAdditionally it was found that size distribution could be improved dramaticallyrnby passing the aggregates through a SEC column with large pores. The SEC columnsrnacted like a template in which the aggregates adopt a more stable conformation.rnIn the second part anionic polymerization was employed to synthesize silaneendfunctionalizedrnmacromonomers with different molecular weights based on polybutadienernand polyisoprene. These were polymerized by a hydrosilylation reaction inrnbulk to obtain branched polymers, using Karstedt’s catalyst. Surprisingly the additionrnof monofunctional silanes during the polymerization had only a minimal effect concerningrnthe degree of polymerization. It was possible to introduce silanes without increasingrnthe overall number of reaction steps by a very convenient “pseudo-copolymerization”rnmethod. All branched polymers were analyzed by SEC, SEC-MALLS,rnSEC-viscometry, 1H-NMR-spectroscopy and DSC concerning their branching ratio.rnThe branching parameters for the branched polymers exhibited similar characteristicsrnas hyperbranched polymers based on AB2 monomers. Detailed kinetic study showedrnthat the polymerization occurred very rapidly in comparison to the hydrosilylation polymerizationrnof classical AB2 type carbosilanes monomers.rnThe last part will deal with ferrocenyl-functionalized polymers. On the one hand,rnferrocenyl-functionalized polyglycerols (PG) were studied. Esterification of PGs withrndifferent molecular weight using ferrocenemonocarboxylic acid gave the ferrocenylrnfuntionalized polymers in high yields. On the other hand three different block copolymersrnwere prepared with different ratios of styrene to butadiene units (10:1, 4:1, 2:1).rnThe double bonds of the 1,2-PB block were hydrosilylated using silanes bearing onern(HSiMe2Fc) or two (HSiMeFc2) ferrocene units. High degrees of functionalizationrnwere obtained (up to 83 %). In this manner, six different ferrocenyl-rich block copolymersrnwith different fractions of ferrocene were prepared and analyzed, employingrnNMR-spectroscopy, SEC, SEC/MALLS/viscometry, DLS and cyclic voltammetry. Thernredox properties of the studied polymers varied primarily with the nature of the silanernunit attached. Additionally, the redox properties in solution of the studied polymersrnwere influenced by the block length ratio of the block copolymers. Unexpectedly, withrnincreasing block length of the ferrocenyl block the fraction of active ferrocenes decreased.rnNevertheless, in case of thin monolayer films this behaviour was not observed.rnAll polymers (PG and PS-b-PB based) exhibited good electrochemical propertiesrnin a wide range of solvents, which rendered them very interesting for biosensoricrnapplications.