Polyelectrolyte Complexes and their Therapeutic Potential
- This thesis describes the preparation of polyelectrolyte nanostructures, the characterization of interpolyelectrolyte complexes (IPECs) made from these structures and their use in a therapeutic context. The therapeutic use of such IPECs connects the two major topics of this work: First, the delivery of genes into eukaryotic cells in vitro by means of new star-shaped polycations was explored. Second, the structure of ionic multicompartment micelles (MCMs) when complexed with polyions was studied and the performance of these nanostructures as delivery agents of anti-cancer drugs both in vitro and in vivo was tested.
To better understand structure-property relationships of polycations relevant for gene delivery, a library of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) homopolymers was synthesized. Star-shaped polymers with a different number of arms and molecular weights were created by using sugar-based or inorganic nanoparticles as core molecules with varying number of initiation sites. The cytotoxicity as well as transfection performance of polyplexes from these polymers and plasmid DNA was determined for different PDMAEMA-nitrogen/DNA phosphate ratios in CHO-K1 cells. A decrease of the cytotoxicity of polymers with a given molecular weight was observed with increasing degree of branching, i.e., with increased arm-number. Star-shaped PDMAEMA with roughly 20 arms (Si-PDMAEMA) showed exceptionally high transfection efficiency. The superior transfection behavior of this specific polymer was demonstrated in non-dividing or differentiated (C2C12 and human T lymphocytes) cell lines. Additionally, polymeric micelles were produced from a polybutadiene-block-PDMAEMA diblock copolymer in aqueous solution and subsequently used for gene transfection. Their transfection efficiency was in the same range as that of Si-PDMAEMA, hinting towards a general design principle for highly effective gene vectors. This consists of a star-shaped architecture of PDMAEMA chains emanating from a common center.
The second major part of this thesis deals with the structure of ionic MCMs complexed with diverse polycations as well as the drug delivery capabilities of some of these complex micellar structures. MCMs from polybutadiene-block-poly(1-methyl-2-vinyl pyridinium)-block-poly(methacrylic acid) (BVqMAA) triblock terpolymers were used as the basis for further structural modifications. These micelles exhibit a core-shell-corona morphology, where PB forms the core of the micelles, a discontinuous (patchy) shell consisting of an IPEC between P2VPq and PMAA is present and finally a corona of excess PMAA stabilizes the micelles in aqueous solution. At sufficiently high pH a portion of the corona carries negative charges, which were then used to form further IPECs with either cationic homopolymers or double-hydrophilic block copolymers featuring one positively charged block. If polycations other than poly(2-vinyl pyridine) such as quaternized PDMAEMA were used for the complexation, a new and distinguishable IPEC compartment was formed on top of the already existing P2VPq/PMAA IPEC. In the case of MCMs with a short to moderate block length of the corona, i.e., the degree of polymerization (DP) of PMAA was between 345 to 550 units, a layered arrangement of the newly formed IPEC compartment was found.
For BVqMAA micelles with a long PMAA corona (DP of PMAA = 1350) complexed with different quaternized homopolymers, a patchy arrangement of the newly formed IPEC compartment instead of a layered one was found. The formation of this new structure is due to an interfacial energy minimization between the new IPEC and the compartmentalized core of the micelles that became possible due to the exceptionally long PMAA corona of the precursor micelles.
Finally, MCMs from BVqMAA were tested for their capacity to deliver a hydrophobic anti-cancer drug for photodynamic therapy to lung cancer cells in vitro and in vivo. The influence of the corona composition was studied by forming complexes with a double-hydrophilic diblock copolymer, poly(L-lysine)-block-poly(ethylene glycol) (PLL-b-PEG). A new cylinder-on-sphere morphology was observed in electron microscopy for BVqMAA/PLL-b-PEG complex micelles at high complexation ratios between lysine and MAA units. Highest cytotoxicity and uptake were found for pure BVqMAA micelles, both decreasing with increasing amount of PLL-b-PEG attached to the micelles. In mice, a prolonged blood circulation time in the range of several hours was exclusively observed when fully PEGylated micelles were injected. Those micelles were the only ones showing an enhanced accumulation in a subcutaneous tumor model 24 h after intravenous injection. The amount of drug delivered to the tumor tissue by the micelles suppressed tumor growth for up to 21 days after a single dose injection and photoirradiation step. The potential of complex micelles on the basis of BVqMAA MCMs could thus be proven.
Effect of Dopants on the Local Atomic Structure and Sintering Behavior of Bismuth Sodium Titanate
- The most commonly used piezoceramic is lead zirconate titanate Pb(ZrxTi(1-x))O (PZT). It possesses outstanding piezoelectric properties which can be modified for numerous applications by the addition of dopants. However, because of environmental and health concerns regarding lead, lead-free alternatives are demanded by politics.
One of the two most promising lead-free replacement materials is the ferroelectric bismuth sodium titanate (Bi0.5Na0.5)TiO3 (BNT). Like PZT, it crystallizes in the perovskite structure.
Since the dielectric and piezoelectric properties of pure BNT ceramics are insufficient for application, BNT is often modified by the addition of dopants. These influence a great variety of material properties to different degrees, e.g. the sintering behavior, the dielectric and piezoelectric properties and their respective temperature stabilities. Doping of BNT aims to decrease the sintering temperature in order to avoid Bi vaporization, to increase the depolarization temperature and to enhance the piezoelectric coefficient.
The effects of numerous dopants on the resulting performance of BNT were studied extensively in the literature. However, so far little attention has been payed to the way in which dopants interact with the piezomaterial. Nevertheless, it is the understanding of these relationships that would make targeted modifications and improvements of BNT possible.
The primary goal of this study was to investigate and explain the effects of a model dopant -cobalt- on the phase formation, sintering behavior and microstructure of BNT as well as on the resulting dielectric and piezoelectric properties. In this regard, a core issue was to determine the preferred lattice site of Co in BNT.
BNT was synthesized from oxide powders using the classic solid-state route and sintered at temperatures ranging from 1000 °C to 1150 °C. Cobalt was added in concentrations between 0.1 mol% and 2.6 mol% Co prior to the calcination as Co3O4.
About one third of the total cobalt amount was incorporated into the BNT lattice on the perovskite B-site, that is, it substituted for Ti. The cobalt in BNT appeared to be in equilibrium with the secondary phase Co2TiO4, which invariably formed at cobalt concentrations greater than 0.1 mol% Co. For charge balancing reasons, oxygen vacancies were created in the lattice of cobalt-doped BNT. These markedly enhanced the diffusivity. As a result, the sintering temperature of doped BNT decreased with increasing cobalt concentrations, and high final densities were achieved. However, in highly doped BNT sample swelling occurred at elevated temperatures of the sintering cycle. This phenomenon was attributed to evaporating oxygen caused by the valence transition of Co3+ to Co2+.
Up to 950 °C, BNT was found to densify via solid state sintering mechanisms. Above this temperature, a small amount of liquid phase was present, which probably formed from decomposing BNT because of a slight Ti-deficiency due to doping. Increased Bi vaporization from the melt above 1000 °C appeared to have stabilized sodium cobalt titanate, an additional secondary phase.
The rotation of the iso-lines in the kinetic field diagram of doped BNT was interpreted such that the activation energy for grain growth was higher than the activation energy for densification. Possible reasons are the solute-drag effect and the pinning of domain walls by secondary phase particles.
Both the depolarization temperature and the piezoelectric coefficient d33 decreased with increasing cobalt concentrations. The dielectric properties deteriorated as well. This was attributed to the high electrical conductivity of the doped samples, which prevented full poling.
Polyelectrolyte Coatings with Internal Hierarchy
- The results presented in this thesis are focused on the surface modification by polyelectrolytes and polyelectrolyte copolymers. The internal structural hierarchy originate thereby from the self-assembly processes at different length scales. To generate different levels of hierarchy, the coatings were constructed by using either the layer-by-layer (LbL) deposition method (lateral chemical structure), the adsorption of supramolecular aggregates (lateral topographycal structure), or the combination of both. Using these techniques, one can control the properties of the coatings by varying the chemical structure of the polyelectrolytes, for instance, their charge density, thus providing a convenient way for their functionalization and the ability to tune properties of the surface. Therefore, we were working with systems which have variable charge densities. With this approach, we were able to produce thin and ultrathin nanostructured films with tunable properties and functionality.
Structural analysis of nanoparticles by small angles X-ray scattering
Christophe N. Rochette
- The objective of this work was to analyze nano-scaled particles by the combination of small angle x-ray scattering (SAXS), electron microscopy (TEM and cryo-TEM) and dynamic light scattering (DLS). Two systems with totally di_erent morphologies and compositions have been investigated: spherical particles of calcium-phosphate-protein complexes and hamburgers of semi-crystalline polybutadiene/polyethylene nanoparticles.
The study of the calcium-phosphate-protein complexes consisted in looking for the influence of a protein, Fetuin-A, also called ahsg, onto the calci_cation at early stage. For the purpose, calcium and phosphate ions were mixed with or without the presence of Fetuin-A in a buffr solution of pH=7.4. In a first step, DLS measurements were realized to better appreciate the effect of the total weight percentage of Ca2+ and PO43- ions. These experiments, withtout addition of ahsg, demonstrated that for a lower weight fraction, the particles formed are smaller. Studies with addition of Fetuin-A affected the calcification during the first minute of this process. The early formation of calcium phosphate complexes has been successfully followed by TR-SAXS. A very fast nucleation of nanoparticles within 1 second has been detected. For the first time, the role of the glycoprotein Fetuin-A at the very early stage of calcification has been qualitatively highlighted: ahsg inhibits the aggregation of calcium phosphate particles. Thus, Fetuin-A plays an important role in the fetal serum in the pre-formation of the skeleton of Vertebrates. This study demonstrated that a physiological concentration of this glycoprotein (15 µM) is suffcient to completely inhibit the aggregation of calcium phosphate particles.
Freely suspended nanoparticles of syndiotactic polybutadiene have been studied. By using the combination of cryo-TEM and SAXS, it has been shown that they consist of remarkably thin polymer crystalline lamella. Different models have been compared in order to theoretically fit the experimental SAXS data: homogeneous and heterogeneous (two and three different electron densities within one particle) nanoparticles. The presence of amorphous and crystalline polybutadiene has been demonstrated by the X-ray diffraction. The necessity of using an additional layer of SDS for the modeling is explained by the abundancy of SDS added after the synthesis of the polybutadiene particles (weight ratio ca. 1:1) and by the modeling of the SAXS theoretical intensities which were not sufficient without taking into account the presence of SDS. After the formation of semi-crystalline nanoparticles of PE, these new nanoparticles open a new route to the synthesis of nanopolymers of interesting physico-chemical properties such as semi-conductors or photovoltaic compounds.
Finally, in contrast to recent literature on bulk polyethylene (PE), this thesis investigated freely suspended nanoparticles of PE. As suggested by Weber and coworkers, the combination of SAXS and cryo-TEM has been used for this study. The structure of individual PE nanocrystals has been determined in detail and an improved model of the form factor (SAXS) has been developped in close collaboration with Priv.-Doz. Dr. Ludger Harnau. The second part of this thesis mainly deals with the annealing of these PE particles. For the first time, it is shown that the effect of the annealing process results in a doubling of the crystalline layer of the PE nanoparticles. This behaviour could be traced back to the unlooping of the PE chains. In addition, a linear relationship between the reciprocal of the crystalline layer and the annealing temperature has been experimentally drawn. This line was predicted by the Gibbs-Thomson equation according to the literature. This result is important because it allows controlling the crystalline thickness and physical properties of the system, by the temperature.
Effects of copper on calcium metabolism and detoxification mechanisms in freshwater bivalve species of Anodonta
- Copper (Cu) is one of the metals contaminating European fresh water ecosystems. Filter feeding bivalves have high bioaccumulation potential for transition metals as Cu. While copper is an essential micronutrient for living organisms, it causes serious metabolic and physiological impairments when in excess.
The objectives of this thesis are to get knowledge on toxic effects and detoxification mechanisms of copper in Anodonta cygnea and Anodonta anatina, two mussel species widely distributed in continental waters. Because Ca plays a fundamental role in shell formation and in numerous biological processes, Cu2+ effects on cellular plasma membrane calcium transport were studied first. In the second step, the investigations focused on Cu2+ detoxification mechanism involving cysteine (Cys) rich compounds known to play a major role in homeostasis of essential trace metals and in cellular metal detoxification.
Under our experimental conditions, copper inhibition of Ca2+-ATPase activity was observed in the gills and the kidneys, and inhibition of Na+/K+-ATPase in the gills and the digestive gland (DG) upon 4 d of exposure to 0.35 micro mol/L Cu2+. At day 7 of exposure to environmental Cu2+ concentrations total recoveries was observed in the kidneys and the gills for Ca2+-ATPase activity, and in the DG for Na+/K+-ATPase, but not at high doses. Ca and Na transport inhibition may entail disturbance of osmo-regulation and lead to continuous under-supply of Ca. Recoveries of Na+/K+-ATPase and Ca2+-ATPase enzymes function suggest that metal-detoxification is induced.
Phytochelatins (PC) are Cys-rich oligopeptides synthesised by phytochelatin synthase from glutathione in plants and fungi. Phytochelatin synthase genes have recently been identified in invertebrates; this allows us to hypothesize a role of PC in metal detoxification in animals.
In the second part of this work, PC and their precursors as well as metallothionein were analyzed in the gills and in the DG of Anodonta cygnea exposed to Cu2+. Our results showed for the first time the presence of PC2-4 in invertebrates. PC were detected in control mussels not exposed to metal, suggesting a role in essential metal homeostasis. Compared to control, PC2 induction was observed during the first 12 h of Cu2+ exposure. Those results confirm the role of PC as a first line detoxification mechanism in A. cygnea.
Organic Electronics by Self-Assembly
- This thesis deals with the synthesis of new materials and their applications in n type field-effect transistors in order to improve device performance by using simple fabrication procedures at low temperatures.
The first approach deals with field effect transistors based on zinc oxide nanoparticles. Stable dispersions of zinc oxide nanoparticles allow the fabrication of zinc oxide transistors by solution processes. Despite the high intrinsic electron mobilities of zinc oxide, a large surface to volume ratio of nanoparticles often prevents their use as active material in field-effect transistors. Dangling zinc bonds are present at the surface, which act as electron donor, leading to increased conductivity. Switching the transistor off becomes challenging. The use of a tailored perylene bisimide, chemically linked to pyrrolidone groups, allowed the passivation of the dangling zinc bonds at the particle surface. Mixing of this perylene bisimide into nanoparticulate zinc oxide dispersions allowed transistor fabrication by spin-coating at low temperatures. On/off-ratios of the resulting transistors could be enhanced by 3 orders of magnitude to 10^3, so that zinc oxide nanoparticles turn into a semiconductor.
The second approach represents the main part of this thesis focusing on n type self assembled monolayer field-effect transistors (SAMFETs). It is known that approximately 90% of charge transport in field-effect transistors is managed just by an ultra-thin layer close to the dielectric. Due to the absence of bulk current, on/off ratios in SAMFETs are enhanced without disadvantages to charge mobility or threshold voltage. The molecules for SAMFET applications typically consist of a semiconducting core, an endcapper on one side, and a reactive group on the other side, which is fixed to the core via a spacer. In this thesis, the well known perylene bisimides were chosen as semiconducting core. A branched alkyl tail acted as endcapper, whereas a linear C11 alkyl tail takes over the spacer part. As reactive group a phosphonic acid was chosen, which enables covalent fixation to aluminium oxide, a common dielectric for transistor applications. SAMFETs were fabricated by submerging transistor substrates into a dilute solution of the active molecule. During the immersion, the perylene bisimides reacts spontaneously to the aluminium oxide dielectric, forming a monomolecular layer. X-ray photoelectron spectroscopy (XPS) revealed a dense, homogeneous, and smooth monolayer on top of aluminium oxide. X-ray reflectivity (XRR) measurements confirmed the expected three regions, endcapper, semiconducting core, and spacer, perpendicularly ordered to the surface. In plane order was investigated by grazing incidence measurements (GIXD), which resulted in a nano-crystalline layer. SAMFETs showed bulk like electron mobilities of 10^-3 cm2/Vs. High on/off-ratios up to 10^5 and low threshold voltages were achieved. SAMFETs with channel length up to 100 micrometer were measured for the first time. The fact that all measured transistors, short channel as well as long channel, were working, indicated a high degree of reproducibility. Furthermore, by combining n-type and p type SAMFETs, the first CMOS bias inverter, solely based on SAMFETs, with large gain values up to 15, was realized.
To further improve SAMFET performance, the branched alkyl tail was replaced by a short linear fluorinated alkyl tail, with the intention to increase the surface coverage of the chromophores by a more slender design of the molecule. SAMFETs were fabricated with the same simple method as described above. XPS measurements showed a complete coverage of an organic layer with a thickness matching perfectly the simulated length of the molecule. In contrast to the previous SAMFETs, the phosphorous was located mainly at the aluminium oxide surface and not, as before, throughout the organic layer. The fluorine atoms were detected at the top of the layer. Both observations are indicative for higher order of the monolayer. XRR measurements gave a consistent structure of the layer. A tree layer structure was found in which the electron density of the outer layer was enhanced, due to the presence of electron rich fluorine atoms. The thicknesses of all three layers were in good agreement to the calculated distances. Furthermore, GIXD studies revealed an amorphous SAM on top of aluminium oxide, optimal for charge transport without disturbing grain boundaries.
The new SAMFETs were also highly reproducible and showed electron mobilities on the order of 10^-3 cm2/Vs, low onset voltages, and high on/off-ratios in the order of 10^6. Besides SAMFETs on common non-flexible silicon substrates, comparable n-type SAMFETs were also fabricated on polymer based substrates for the first time. Furthermore, a unipolar bias inverter was built, paving the way towards flexible organic electronics by self-assembly.
In summary, all three publications of this thesis deal with the synthesis of semiconducting perylene bisimides and their implementation in n-type field-effect transistors. Reliable transistors and the first integrated CMOS-like circuits based solely on SAMFETs with high performances were achieved, made by simplest solution processes at low temperatures.
Supramolecular polymer additives to improve the crystallization behavior and optical properties of polybutylene terephthalate and polyamides
- The use of additives to enhance the processability and end properties is one of the main measures to enlarge the property profile of commercial thermoplastic polymers. The physico chemical properties of semi crystalline polymers depend to a large extend on the crystal structure and spherulite size, which can be controlled by nucleating agents. These additives can increase the crystallization temperature and thus reduce cycle times during melt processing, affect the physical properties and can in some cases improve the optical properties (clarity and haze). This thesis describes the use of supramolecular nucleating agents to increase the polymer crystallization behavior and to improve the optical properties of polybutylene terephthalate and polyamides.
In the first chapter the influence of the molecular structure of 1,3,5-benzenetrisamides on the nucleation behavior of polybutylene terephthalate was investigated. In order to explore structure property relations, 43 derivatives were screened with regard to their nucleation ability of PBT. For promising compounds the additive dissolution and crystallization behavior in the polymer melt and crystallization temperature of the polymer were investigated. It was revealed that certain 1,3,5- benzenetrisamides are capable to nucleate PBT. They can act as supramolecular polymer additives, are soluble in the PBT melt under the processing conditions and upon cooling self assemble into fine fibrillar supramolecular nano objects inducing the nucleation of PBT. It was found that particularly the trisamide derivatives based on the 1,3,5 triaminobenzene core are much better soluble than the derivatives based on the 1,3,5-benzenetricarboxylic acid core and 2,4,6-trimethyl-1,3,5-trisaminobenzene core. As a result additives featuring too good solubility do not self-assemble prior to the polymer crystallization and hence do not induce nucleation.
This demonstrates that the nucleation efficiency strongly depends on the individual chemical structure of the additives and slight structural changes have large impact. In addition for the first time it was possible to visualize the supramolecular structures within the PBT matrix by selectively hydrolyzing PBT. The morphology of the formed supramolecular nano objects is determined by cooling rate and additive concentration. Faster cooling and lower additive concentrations leads to the formation of needle like structures with smaller lateral dimensions and a narrower size distribution.
In the second chapter a new class of supramolecular nucleating agents on the structural motif of 1,4- cyclohexane bisureas was synthesized specifically for semi crystalline polyamides. The bisureas are temperature stable enough to be applied in high melting polyamides. In total, 29 derivatives were synthesized and investigated with respect to their nucleation and clarification behavior in PA6. The chemical structure of the additives was systematically varied by changing the conformation of the central unit and the length and degree of branching within the peripheral substituents. Bisureas based on the trans conformation of 1,4- cyclohexane are capable to efficiently nucleate the alpha form of PA6 even at concentrations below 200 ppm, with extremely high nucleation efficiencies of around 90 %. In addition, selected derivatives were found to distinctly improve the optical properties haze and clarity, as well as the laser transparency, in injection molded samples. This is the first time semi crystalline polyamides were clarified. The most efficient clarifiers for PA6 were found to decrease the values for haze from nearly 100 % (1.1 mm thick samples) for neat PA6 to 19 % and 11 % at a concentration of 1.5 wt%. By varying the peripheral substituents of the trans-bisureas interesting structure property relations could be revealed. The investigated derivatives with longer flexible substituents were efficient nucleating agents for PA6, but displayed only modest improvements in optical properties. Short, highly branched substituents, preferably in alpha position to the urea groups, were found to be a key segment for the clarification of PA6. Additionally we demonstrated that the process conditions, namely cooling rate and mold thickness, strongly determine the final optical properties. The use of trans-1,4-cyclohexane bisureas could also be expanded to other semi crystalline polyamide homo- and copolymers.
For the first time the concept of supramolecular nucleating and clarifiying agents was sucessfully transferred to semi crystalline polyamides. With the new class of compounds it could be demonstrated that trans-1,4-cyclohexane bisurea derivatives are capable to increase the crystallization behavior and improve the optical properties, particularly the haze values, remarkably.
Coarse-grained Modeling of Protein Dynamics using Elastic Network Models
- Dynamics is crucial for the functioning of biological macromolecules. Because of severe limitations in studying protein dynamics experimentally or with all-atom simulations, coarse-grained methods, especially elastic network models (ENMs), are frequently employed. In the last years, studies on various proteins showed that ENMs reliably reproduce experimental data, despite the simplified protein representation and the purely harmonic potential function. This work on two proteins with very different dynamical properties highlights the remarkable success of ENMs and shows which kind of questions can be answered using coarse-grained methods.
The allosteric enzyme aminoglycoside phosphotransferase(3')-IIIa (APH), which confers resistance against a broad range of aminoglycoside antibiotics to pathogenic bacteria, drastically changes its flexibility upon binding of substrates, but without changing its average conformation. In contrast, the homotrimeric vesicular stomatitis virus glycoprotein G (VSV-G), which triggers the pH-dependent fusion of viral and host membrane, undergoes a large structural rearrangement. A striking difference between the two proteins is their shape. VSV-G contains weakly constrained protein segments, the fusion loops, which can undergo large-scale motions at low energetic cost, whereas APH is not obviously arranged into different protein segments. Nevertheless, ENM calculations show that also APH consists of independently moving segments with correlated internal motion, so-called dynamic domains. The concept of dynamic domains can explain the differential effects of ligand binding on the dynamics of APH.
The first chapter of this thesis describes how experimental evidence for the importance of dynamics successively replaced the former idea of static proteins, and explains the dynamic basis of ligand binding, allostery and conformational changes. In the second chapter, theoretical methods for the analysis of protein dynamics are introduced, with emphasis on the ENM-based methods used in my studies. The studies are summarized in the third chapter. In the study on APH, I employ the Gaussian network model to analyze the ligand-dependent dynamics, the broad substrate specificity and the perturbation-sensitivity of the ligand binding sites. In a second study, ENM-based as well as all-atom molecular dynamics simulations are used to analyze the conformational change of VSV-G. Both approaches detect the fusion loops of VSV-G as most flexible parts of the protein, and thus as most likely starting point for the structural rearrangement, but only the all-atom model can generate deviations from the average structure at low pH. The last study describes the implementation and application of a dynamic domain assignment method, called CovarDom, which is based on covariances of residue fluctuations. Calculation of dynamic domains for a large protein set demonstrates the general applicability of CovarDom.
Hybrids Based on Layered Silicates
- Novel hybrid nanoparticles were synthesized based on combinations of various layered silicates as inorganic core and well-defined polymer chains as a shell. In all cases positively charged 2-(dimethylamino)ethyl methacrylate (DMAEMA) was incorporated into the polymeric structure to serve as a firm anchor onto the negatively charged clay surface via electrostatic adsorption.
First, hybrid nanofillers were synthesized to improve the mechanical properties of a homopolymer matrix by combining a shear-stiff synthetic K-hectorite with a tailored surface activity. For this, the synthetic fluorohectorite with very high aspect ratios was organophilized with a specifically designed macroinitiator created by statistical Reversible Addition Fragmentation Chain Transfer (RAFT) copolymerization of DMAEMA and the initator-monomer 2-(2-bromoisobutyryloxy)ethyl methacrylate (BIEM). The copolymer was firmly anchored through multiple cationic charges distributed over the chain while the multiple initiating functions were used to polymerize the monomer of choice via Atom Transfer Radical Polymerization (ATRP). The final hybrid was equipped with a hydrophobic polymeric shell of poly(methyl methacrylate) (PMMA), which enables dispersion in organic solvents. The hybrid particles were compounded into a polymeric matrix of commercial PMMA and tested with regard to its reinforcing properties. The similarity of the polymeric shell to the homopolymer matrix of the chosen sample composite combined with the inherent stiffness of the inorganic core lead to an increase in tensile modulus of up to 84 % at 5% filler content. Further, patchy hybrid nanodiscs based on natural montmorillonite as core and a shell made from compartments of two different polymers were evaluated as cheap and versatile compatibilizers in an immiscible polymer blend. In a simple one-step modification process a shell comprising patches of either of two polymer species (PMMA and polystyrene, PS), each chosen to be similar in polarity to one of the matrix polymers, was attached to the inorganic core via Coulomb interaction. The behaviour of these particles in a solvent-cast blend of 2:1 PS/PMMA was investigated via transmission electron microscopy (TEM) and dynamic-mechanical analysis (DMA). Particles were found distributed in both of the blend’s domains and at the interface and an improvement of the storage module of 17% was found.
Finally, kaolinite was used as a core to create true hybrid Janus nanodiscs, which were applied for compatibilizing an immiscible polymer blend of 2:1 PS/PMMA. It was possible to create two chemically distinct surfaces on the clay particle by addressing each of its two basal surfaces individually via simple, but selective, surface modification. Two diblock copolymers were used to create the Janus structure, each one with a first block consisting of monomer units bearing the anchoring group for the respective surface and a second block, PS or PMMA, tailored to the polarity of the respective matrix polymer. Thus it was possible to direct the Janus particles straight into the interface between the polymeric domains, visualized by TEM images taken from solvent-cast nanocomposite films.
Structure-Property Correlation of Electron Transport Materials in Organic Devices
- This dissertation deals with organic semiconductors as electron acceptor
(n-type) materials in bulk heterojunction (BHJ) solar cells. Important features of an electron acceptor are strong visible light absorption, sufficient high electron mobility and appropriate energy levels with respect to the donor. Furthermore, the blend morphology of donor and acceptor is crucial for the device performance. Within this thesis, the synthesis and characterization of novel n-type polymers is reported and various techniques to evaluate the above mentioned parameters for n-type small molecules and polymers are presented. The aim was to investigate the impact of chemical structure on the optical and electronic properties and morphology of these semiconductors. Successful strategies how to control and improve light harvesting, electron mobility, blend morphology and solar cell performance were identified. The fundamental question of charge transport properties of the materials was addressed by fabricating single carrier devices using the SCLC (space-charge limited currents) method. The morphology was primarily investigated by atomic force microscopy (AFM) and X-ray diffraction (XRD).
The first part of this thesis focuses on perylene imide based small molecules and polymers. The side groups of a series of N-substituted perylene bisimides (PBI) were found to play a crucial role on crystallinity and charge transport. The nature of the side groups had great impact on the crystalline structure and electron mobility. When hydrophilic oligoethylenglycol (OEG) side groups were present, the perylene molecules aligned in highly ordered hexagonal or lamellar columns and realized high electron mobilities of up to 7∙10E-3 cm2V-1s-1, while the perylene derivative with only hydrophobic alkyl chains only showed 3∙10E-5 cm2V-1s-1. The substituents at the perylene core also had a major impact on the blend morphology of OPV devices when these materials were used in combination with a donor polymer. Here, we were able to tune the extent of phase separation between donor and acceptor via hydrophilic-hydrophobic interactions of donor polymer and acceptor side groups. To improve light harvesting of perylene compounds, the pi-electron system of PBIs was altered and highly soluble, novel perylene side chain polymers (PPDB and PPDI) were synthesized by nitroxide mediated radical polymerization (NMRP). The pendant perylene moieties were perylene diester benzimidazole (PDB) and perylene diester imide (PDI). Compared to polymers bearing PBI side groups, the visible light absorption of PPDB was broadened and red shifted, whereas a narrower and blue shifted absorption was observed for PPDI. Remarkably, also the electronic nature of the two materials was affected by the modification at the perylene core, as PPDB is an n-type semiconductor and PPDI has a more pronounced p-type character. A comparative study of perylene side-chain polymers synthesized by a combination of NMRP and “click” chemistry revealed that the compound with improved optical properties (PPDEB) exhibited worse charge carrier mobility compared to PPBI. Another striking result was found as an amorphous polymer bearing OEG side chains showed a better electron mobility than the corresponding material with alkyl chains, which was liquid crystalline. A very high electron mobility of 1∙10E-2 cm2V-1s-1 was measured.
The second part of this dissertation addresses fullerene based acceptor materials, among which Phenyl-C61-butyric acid methyl ester (PCBM) is the state-of-the-art n-type semiconductor used in OPV. For two fullerene derivatives, Bis-Phenyl-C61-butyric acid methyl ester (bis-PCBM) and Bis-o-quino-dimethane C60 (bis-oQDMC), the LUMO energy levels were higher compared to PCBM. As a result, improved open circuit voltages (Voc) in BHJ solar cells were obtained. The efficiency however did not improve, because of reduced short circuit current densities (Jsc). We found that for the bis-PCBM system, Jsc was limited by low electron transport, while for the bis-oQDMC system an unfavorable blend morphology hampered the performance. The problem of low electron mobility could be overcome by reducing the thickness of the active layer and higher Jsc and overall device performance could be achieved. A drawback of fullerene small molecules is that diffusion, aggregation and crystallization of these molecules within BHJ blends can often negatively affect the stability of the blend morphology and reduce the device performance. We discovered that aggregate and crystallite formation in novel fullerene side chain polymers could be successfully suppressed, whilst high electron mobility and better film properties were achieved.
Altogether, new insights into structure-property relation of organic electron transport materials are presented in this work. Moreover, the detailed analysis of charge transport helped to understand the performance of solar cells.