OPUS 4 | Chemie

Self-Assembly of Block Copolymers in External Fields (2002)

Alexander Böker

The influence of external fields on the microdomain structure of block copolymers has been studied. Both surface fields and electric fields have been considered. External electric fields are used to create macroscopically oriented bulk samples. In order to circumvent limitations associated with the application of external fields to melts of high molecular weight block copolymers and multiblock copolymers of complex architecture, a new solvent-based procedure is introduced, i.e. the block copolymer microdomains are aligned by application of an electric field (E ~ 1 - 2 kV/mm) during solvent casting of bulk samples. In order to elucidate the dominating parameters and governing mechanisms, the microdomain orientation kinetics of concentrated block copolymer solutions exposed to a DC electric field is investigated by time-resolved synchrotron small-angle X-ray scattering (SAXS) at the ID02 beamline at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. As a first model system, a lamellar polystyrene-b-polyisoprene block copolymer dissolved in toluene is used. The orientation kinetics follows a single exponential behavior with characteristic time constants varying from a few seconds to some minutes depending on polymer concentration, temperature, electric field strength, and system size. Furthermore, two mechanisms governing the electric field alignment of a lamellar block copolymer from concentrated solutions are identified. It is shown that depending on the segregation power (c µ fP, c µ 1/T) a single mechanism dominates the orientation process, i.e. in a weakly segregated system (low concentration or high temperature) the migration of boundaries prevails, whereas a stronger phase separated system (high concentration or low temperature) predominantly exhibits rotation of grains. In addition, the orientation kinetics slows down with increasing polymer concentration, which can be correlated to the respective solution viscosity and the mechanism of orientation. Moreover, the influence of the electric field strength on the orientation kinetics is determined, including a threshold value below which no electric field induced orientation could be achieved on the time scale of the experiment. The time constants of the fastest processes were in the range of 0.5 sec, reaching a final orientation described by order parameters of up to P2 = -0.35. Finally, the variation of temperature yields control of the governing mechanisms at a fixed polymer concentration. In additional studies, the dielectric contrast of the block copolymer components was varied systematically (PS-b-PI, PS-b-PMMA, PS-b-PtBMA, PS-b-PHEMA-b-PMMA, PS-b-P2VP). It is found that a high dielectric contrast leads to faster alignment kinetics (e.g. the time constants of the fastest processes for a PS-b-P2VP diblock system in THF are in the range of 0.3 sec) and reduces the threshold field strength (around 200 V/mm for PS-b-P2VP). Furthermore, it could be shown that the interplay between degree of phase-separation, solution viscosity and dielectric contrast is crucial to decide if a given polymer/solvent system can be used for electric field-induced microdomain alignment. For example, it was found that PS-b-PtBMA shows electric field-induced orientation of the microdomains while PS-b-PMMA does not. This can be explained by the larger interaction parameter cST compared to cSM leading to a phase-separated solution at lower viscosities. In a similar way, the introduction of a high dielectric constant middle block (PHEMA) into a PS-b-PMMA, which additionally enhances phase separation, is shown to be the key to creating a well-performing methacrylate-based block copolymer system for electric field induced alignment from solution. Finally, we could show that the even more complex lamellar and core-shell cylindrical PS-b-P2VP-b-PtBMA high molecular weight triblock copolymer systems could be oriented by virtue of an electric field from solution. In summary, it was demonstrated that electric field alignment of block copolymer domains from solution is a powerful tool to generate highly anisotropic bulk block copolymer samples. The large variety of parameters which we can control allows us to further improve the preparation of macroscopically aligned melt samples via solvent casting in the presence of an electric field.

Development of Thermoplastic Elastomers with Improved Elastic Properties Based on Semicrystalline Block Copolymers (2002)

Holger Schmalz

In this work the synthesis and characterization of two novel types of thermoplastic elastomers (TPEs), exhibiting improved elastic properties compared to commercially available systems, is described. The first type comprises multiblock copoly(ether ester)s with semicrystalline hard segments and triblock copolymer soft segments. The second class of TPEs are systems based on ABA and ABC triblock copolymers with two glassy and one or two semicrystalline end blocks, respectively. The used strategy for increasing the elasticity of conventional copoly(ether ester)s based on poly(butylene terephthalate) (PBT) hard segments and polyether soft segments is the replacement of the continuous PBT hard phase in these systems by a disperse PBT hard phase. The incorporation of nonpolar segments is possible by using poly(ethylene oxide)-block-poly(ethylene-stat-butylene)-block-poly(ethylene oxide) (PEO-b-PEB-b-PEO) triblock copolymers, where the polar PEO blocks act as compatibilizer between the nonpolar PEB segments and the polar PBT segments during the melt polycondensation. The incorporated nonpolar PEB segments induce an enhanced microphase separation in the melt, which in turn results in the formation of a disperse PBT hard phase. Mechanical testing reveals a significantly improved elastic recovery compared to that of conventional copoly(ether ester)s exhibiting a continuous PBT hard phase. Morphological investigations reveal that this novel copoly(ether ester)s consist of a semicrystalline PBT hard phase and an amorphous soft segment phase, which is divided into a pure PEB phase, a PEO-rich phase besides a mixed PBT/PEO phase, and a pure amorphous PBT phase. The second part of this thesis deals with ABC triblock copolymers with one or two semicrystalline end blocks. Two main aspects were addressed: i) the interplay between morphology and crystallization, and ii) the comparison of ABA and ABC triblock copolymers with glassy (A), elastomeric (B) and crystalline (C) blocks. Several polyethylene-block-poly(ethylene-alt-propylene)-block-poly(ethylene oxide) (PE-b-PEP-b-PEO) triblock copolymers have been synthesized by sequential anionic polymerization of butadiene, isoprene, and ethylene oxide (PB-b-PI-b-PEO) followed by homogeneous catalytic hydrogenation. Anionic polymerization in a one-pot procedure was accomplished by using the phosphazene base t-BuP4, which enables the polymerization of ethylene oxide (EO) in the presence of Li+ counterions. Kinetic investigations on the EO polymerization reveal an unexpected induction period. It is concluded, that different factors contribute to the induction period, like break up of lithium alkoxide aggregates by t-BuP4, and chain length effects. Characterization of the PB-b-PI-b-PEO and PE-b-PEP-b-PEO triblock copolymers utilizing differential scanning calorimetry and special self-nucleation (SN) techniques reveals a strong influence of the confinement active during crystallization on the crystallization and SN behavior of the semicrystalline PEO and PE blocks. For low PEO contents large supercoolings are necessary to induce crystallization of PEO. Furthermore, the SN behavior of the PEO blocks is different compared to that of semicrystalline homopolymers, i. e. domain II (SN domain) is absent. This is a direct result of the confined crystallization of PEO within small isolated microdomains. In contrast, for the PE blocks a heterogeneous nucleation mechanism and the presence of all three SN domains, usually present in crystallizable homopolymers, is observed. In this case, PE crystallizes without any confinement from a homogeneous mixture of PE and PEP segments, which can be ascribed to their small segmental interaction parameter. In TPEs based on ABA triblock copolymers with two glassy end blocks, e. g. polystyrene-block-poly(ethylene-alt-propylene)-block-polystyrene (PS-b-PEP-b-PS), the middle block chains can either loop back into the same PS domain or form bridges between two different PS domains. However, only the bridges contribute to the elastic properties, which limits the elastic recovery of theses systems. The influence of a semicrystalline end block on the elastic properties has been investigated by comparison of PS-b-PEP-b-PE and the corresponding PS-b-PEP-b-PS triblock copolymers. For small elongations (< 300%) the PE containing triblock copolymers exhibit a significantly improved elastic recovery. This can be attributed to the increased bridge fraction induced by the immiscibility of the two different end blocks. In contrast, for high elongations (> 300%) the situation is reversed and the PS-b-PEP-b-PS triblock copolymers reveal better elastic properties. Obviously, glassy PS domains show a higher resistance against distortion compared to that of semicrystalline PE domains, especially at high strains.

Novel Precursors for Polymer-Protein-Conjugate Synthesis via Reversible Addition-Fragmentation Chain Transfer Polymerization (2003)

Christine Maria Schilli

The RAFT polymerization of N-isopropylacrylamide with two different chain transfer agents, namely benzyl 1-pyrrolecarbodithioate and cumyl 1-pyrrolecarbodithioate, yielded polymers with narrow molecular weight distributions as well as Mn values that were in good agreement with the calculated ones. A comparison between the Mn values determined from gel permeation chromatography, GPC, and the values from MALDI-TOF mass spectrometry showed that the molecular weights obtained from GPC using polystyrene standards were considerably higher. A relation between log Mn,MALDI and log Mn,GPC was established, which permitted construction of a calibration curve for PNIPAAm polymers. In-situ Fourier-transform near-infrared spectroscopy was applied for the reliable determination of monomer conversions and it indicated living characteristics. Both polymerization processes showed an induction period that seems to be correlated with a retardation in rate, where the induction time is higher for the cumyl chain transfer agent as compared to the benzyl chain transfer agent of the same concentration. The induction periods decrease with decreasing transfer agent concentration and were explained in terms of the different stabilities of the respective radicals that add to monomer in the reinitiation step. The more stable cumyl radical adds slower than the benzyl radical. Both UV spectroscopy and MALDI-TOF mass spectrometry confirm the presence of the expected dithiocarbamate endgroups. MALDI-TOF characterization of the polymer samples showed the transfer agent endgroups together with some initiator-derived polymers. Endgroups that seemed to originate from disproportionation or transfer were the result of fragmentation under MALDI conditions as was shown by a post source decay analysis and MALDI-TOF characterization of the hydrolyzed polymer. With amine-reactive diacetone acrylamide, 2-vinyl-4,4-dimethyl-5-oxazolone and N-hydroxysuccinimide methacrylate, new monomers were polymerized via RAFT in a controlled manner. Poly(diacetone acrylamide) and poly(2-vinyl-4,4-dimethyl-5-oxazolone) showed low polydispersities and good control over molecular weight, where poly(N-hydroxysuccinimide methacrylate) displayed relatively high polydispersities despite the controlled polymerization evident from the monomodal GPC traces. These amine-reactive polymers were subsequently used for successful conjugation to the primary amino group of the model peptide glycine-leucine. For poly(N-isopropylacrylamide)-block-poly(acrylic acid), PNIPAAm-b-PAA, it was demonstrated that hydrogen bonding between N-isopropylacrylamide and acrylic acid units influences strongly its behavior in both the solid state and in solution. The block copolymers form micelles in aqueous solutions in dependence of pH and temperature. Cloud point measurements indicated the formation of larger aggregates at pH 4.5 and temperatures above LCST, whereas micelles formed at pH 5-7 and temperatures above LCST. At pH 5.6 and 50 °C, only micelles were found, whereas, at lower temperatures, larger aggregates and micelles coexist. Formation of larger aggregates by hydrogen bonding interactions was revealed by IR and Raman spectroscopy as well as by cryogenic transmission electron microscopy and dynamic light scattering. Differential scanning calorimetry yielded glass transition temperatures of PNIPAAm-b-PAA that were well above the transition temperatures of the homopolymers, demonstrating molecular interactions between the acrylic acid and the N-isopropylacrylamide blocks. Conjugation of sulfhydryl-terminated PNIPAAm to thiol disulfide exchange reagents and maleimides was probed for later conjugation to proteins. Evaluation of the different cross-linking systems resulted in the choice of maleimides as cross-linkers for subsequent conjugation to the protein streptavidin. Sulfhydryl-terminated PNIPAAm-b-PAA was conjugated to the streptavidin mutant S139C using a bismaleimide cross-linker and also direct conjugation via disulfide linkage. Both conjugations were successful and proceeded with more than 50 % conversion. Conjugation of PNIPAAm and PNIPAAm-b-PAA was also achieved by non-covalent attachment of the biotinylated polymers to wild-type streptavidin. Conjugates of wild-type streptavidin with biotinylated PNIPAAm-b-PAA were found to remain dissolved at temperatures above LCST even at very low pH values, which was in contrast to the observed precipitation of the unconjugated block copolymer at pH <= 4.5. Conjugates of wild-type streptavidin with biotinylated PNIPAAm of different molecular weights formed aggregates in aqueous solutions above LCST and a dependence of aggregate size on the size of the polymer was found

Phase Behavior and Structural Transitions in The Mixtures of Cationic Surfactants and Hydrophobic Counterions (2003)

Rami Abdel-Rahem

Anionic hydrophobic counterions with certain geometry adsorb onto the surface of cationic surfactant micelles and they minimize the repulsion between the headgroups, so the charge density on the surface is reduced. As a result of this, the micelle spontaneously changes its morphology due to a new packing for the head groups. The adsorption of 2-hydroxy-1-naphthoic acid 2,1 HNC and 6-hydroxy-2-naphthoic acid 6,2 HNC onto the surface of the cationic surfactant cetyltrimethylammonium hydroxide was studied. The results were compared to the published system 3-hydroxy-2-naphthoic acid 3,2 HNC/CTAOH. When an increasing amount of 2,1 HNC is introduced into a micellar solution of 100 mM CTAOH, one finds low viscous micellar solution, viscoelastic gel (consisting of rod like micelles), turbid region (two phase region), and viscoelastic liquid crystalline gel (consisting of multilamellar vesicles MLV with yield value). The complex viscosity (0.01 Hz) of 100 mM CTAOH rises by six orders of magnitude as the rodlike micelles form.It decreases then to the turbid region, and then rises again approximately six orders of magnitude. The second rising of the complex viscosity is accompained by the formation of a liquid crystalline phase which consists of multilamellar vesicles. This has been proven by DICM, FF-TEM and Cryo-TEM. The vesicles were polydisperse and ranged from 100 to 1000 nm in diameter. SANS detected the transition in the microstructure which was caused by changing the concentration of 2,1 HNC in the system. SANS calculations show results similar to that obtained by microscopic methods. Surprising rheological behavior was measured in the rodlike micelle region, at which storage modulus was about one order of magnitude higher than loss modulus and both were parallel in the frequency range 0.001-10 Hz. Such behavior usually indicates the presence of vesicles in the liquid crystal phases. It was proved that other rheological measurements can be used to distinguish the tow types, namely, amplitude sweep measurements, first normal stress difference N1 (Weissenbeg effect), the effect of adding electrolyte, and stress relaxation curves. When 6,2 HNC (new substitution of HNC) is added with an increasing amount to 100 mM CTAOH, a new phase behavior is observed. Here the structure changes from small micelle aggregates into rodlike micelles, and then a two phase region consisting of L1-phase and un-reacted 6,2 HNC is formed. No transition into MLV has been detected. In the case of 3,2 HNC and 2,1 HNC, the hydroxyl and the carboxyl group are neighboring, so they can effectively share in reducing the repulsion between the headgroups while the rings are in interaction with hydrocarbon tails. For 6,2 HNC the hydroxyl group is in position number 6 on the aromatic rings, which means that hydroxyl group is distant from the carboxyl group, thereby, less screening for the cationic charge in the micelle surface is obtained. Substitution of HNC plays a main role in controlling the microstructure and other physical properties such viscosity, Krafft point, ..etc. In the second part of this work, the hydrophobic counterion is fixed (2,1 HNC), and the length of the cationic surfactant‘s chain is changed from C16 into C14, C12, C10 and C8. For the system 2,1 HNC/ tetradecyltrimethylammonium hydroxide TTAOH similar phase behavior as 2,1 HNC/CTAOH is observed. At 2,1 HNC/TTAOH ratio r aproximately 1, formation of MLV is observed. After the neutralization addition of excess amount of 2,1 HNC is possible since the insoluble molecular form of 2,1 HNC becomes solublized in the formed MLV. Conductivity measurements prove that 2,1 HNC stays in the molecular form after the neutralization. A difference in the rheological behavior of the system 2,1 HNC/TTAOH compared to 2,1 HNC/CTAOH is seen. In the rodlike micelles region of 2,1 HNC/TTAOH, the solutions exhibit a short relaxation time compared to 2,1 HNC/CTAOH system. FF-TEM and SANS proved the formation of polydisperse MLV in this system with a maximum diameter of about 2000 nm and wall thickens of about 28 nm. As a result of this work, it is concluded that the role of the hydrophobic counterions with certain geometry could be looked upon as a co-surfactant with a shorter chain length which changes the bending rigidity, of the bilayer. They are surface active species that bind strongly on the micelle surface and change the packing parameter of the headgroups. It is suggested that the hydrophobicity of the counterion plays an important role in deciding the structure of the supramolecular assemblies such as vesicles, or micelles. As a consequence one can change the morphology of micelle species by changing the ratio of counterion /surfactant ion. These studies also suggest that by mixing cationic surfactant and hydrophobic counterion with varying cationic surfactants chain lengths, one can have a control over the supramolecular structures formed.

The Aggregation Behavior of Mixture of Alkylmethylaminoxides with Their Protonated Analogues in Aqueous Solution (2005)

Yuji Yamashita

The C12C8MAO aqueous solution at 100 mM consisted of two phases which were optically isotropic and low viscous. Addition of chloric acid induced a phase transition, and the following lamellar (L-alpha) phase was formed in the range of low protonation degree, X = 0.007 – 0.35. When the surfactant was protonated further (X > 0.35), the single L-alpha phase separated again into two isotropic phases. The abnormal phase sequence could be interpreted by the result that mixtures of protonated and non-protonated C12C8MAO were more surface active than each component. The surface and interfacial measurements showed synergism of mixing two components. This synergistic effect arised from the peculiar interaction of hydrogen bonding between protonated and non-protonated head groups. This short-range interaction would cause the C12C8MAO molecules to be more lipophilic with protonation, resulting in the phase separation at high protonation degree. The SAXS measurements in the L-alpha phase also showed the synergistic effect between the head groups. The rheological measurements and microscope observations demonstrated that the morphologies of L-alpha phase could be controlled by preparation routes. It was found that the vesicles were transformed into the classical lamellar phase by the simple process of heating and cooling through the phase transition (L-alpha <-> L-alpha;/L1) temperature. Furthermore, the classical, planar lamellar morphology could be prepared by means of kinetic protonation of the C12C8MAO molecules using hydrolysis reaction. Any classical L phase was modified to the vesicle form under shearing, and its transformation was irreversible in terms of shear force. Various acids were treated as protonation agent in 100mM C14DMAO aqueous solution, and their contributions to the viscosity of solution were examined ranging X = 0 to 1. It was elucidated that the aggregate structure remarkably depended on the ion-pair (counte-ion) thermodynamics. For the Cl-, HCOO-, and H2PO4- ions, no remarkable structural change took place with increasing protonation. For the Br-, NO3-, Oxalate, Tartarate, Tartronate, and SO42- ions, the small micelles of C14DMAO grew up with protonation. For the ClO4-, SCN-, and Salicylate ions, the L-alpha phase was formed with protonation. The interfacial curvature thus followed mostly the sequence of Hofmeister series, while the sulfuric ion was completely excluded from the series because of its divalency. The viscosity change was interpreted quantitatively by the hydration free energy (Delta-Ghyd) of counter-ion. However the divalent counter-ions did not arrange in the same order as the monovalent ones, that could be considered to arise from (1) depressing pKa of C14DMAO with increasing amount of acid and (2) electrostatic force within the diffuse double layer. For different counter-ions, the characteristic scaling laws of viscosity evolution against X were observed. The scaling laws also obeyed Delta-Ghyd. Sulfate, however, could not be manipulated by Delta-Ghyd: regardless of its strong hydrophilicity (high Delta-Ghyd), the excess amount of SO42- caused the micelles to grow up. The micelle growth therefore would be attributed by counter-ion condensation onto the micelles rather than hydrophilicity itself. Comprehensively it was proved that Delta-Ghyd and the ionic valency (electrostatic force) of counter-ion played strikingly significant roles in the structural properties. Trifluoro acetic acid CF3COOH behaved as hydrophobic acid, however, the viscosity trend could stand in neither the sequence of Delta-Ghyd nor the Hofmeister series. The interfacial tension measurments suggested that the CF3COO- ion was incorporated into the micelle, behaving like co-surfactant. Synergism on mixing was also observed in the CF3COOH system, by which the minimal CMC was obtained. The interaction parameter beta indicated that CF3COOH caused the stronger synergistic effect than HNO3. However, some experiments showed the successively hydrophobic C14DMAO with protonation. This would be due to the orientation of hydrophobic trifluoro group in the palisade layer of micelle. The increasing volume fraction of hydrophobic moiety in the surfactant molecule (OleylDMAO) built up the bilayer structure even using the strongly hydrated acids. The L-alpha formation was referred to synergism because the isotropic micellar phase was present on both the sides of high and low protonation degrees. The L-alpha phase melted on elevating temperature, and the subsequent L1 phase was highly viscoelastic. The L-alpha - L1 transition temperatures for different acids were almost correlated by the enthalpy of hydration (DeltaHhyd). And the viscoelastic properties of the L-alpha phases were dependent on the Hofmeister series.

Keteneylidenetriphenylphosphorane as a Versatile C-2 Building Block Leading to Tetronic Acids with Potential Herbicidal and anti-HIV Activity (2004)

Gary John Gordon

The cumulated ylide keteneylidenetriphenylphosphorane (Ph3PCCO) has shown great potential in the construction of heterocyclic compounds. The formation of heterocycles arises from the unique dipolar electronic structure of the cumulated ylide, which combines ylide and ketene properties. Upon reaction of alcohols, amines and thiols with keteneylidenetriphenylphosphorane the intermediate ketene cation gets intercepted by the more strongly nucleophilic counter anion (alkoxide, amide, thiolate) yielding monomer “acyl” ylides. Since acyl ylides enter into Wittig alkenation reactions far more quickly than the starting ylide, multi-component or domino reactions between the latter and a carbonyl compound becomes possible leading to beta,gamma-unsaturated carbonyl derivatives. A further strength of keteneylidenetriphenylphosphorane is its low toxicity, easy accessibility, simple handling and its ability to enter into extended domino reactions, thus this molecule is extremely useful in modern synthetic applications. Our group has recently extended this procedure to the formation of tetronates from keteneylidenetriphenylphosphorane and alpha-hydroxycarboxylic esters. In cases were the ester contains an beta,gamma-unsaturated alkene the corresponding tetronates can be easily converted to either tetronic acids or 3-dispirodihydrofurandiones by careful control of the reaction conditions. Tetronic acids are an important class of heterocycles which exhibit a large array of biological properties. In recent years tetronic acids derivatives have been found to be important HIV-1 protease inhibitors. This work is concerned with using keteneylidenetriphenylphosphorane as a new route to highly functionalised HIV-protease inhibitors. New synthetic techniques such as microwave irradiation is investigated as a means to accelerate Claisen rearrangement reactions. The mechanistic pathway of Claisen/abnormal Claisen rearrangements is investigated in detail. Further examples of 3-dispirodihydrofurandiones are provided with a more in-depth study of the reaction mechanism. The nucleophilic ring opening of these 3-dispirodihydrofurandiones to give 3-substituted tetronic acids is also investigated. These tetronic acids have been found to have potential as lead compounds in the herbicidal industry.

Template-Controlled Synthesis of Magnetic/Semiconducting Nanoparticles within Amphiphilic Core-Shell Cylindrical Polymer Brushes (2004)

Mingfu Zhang

Core-shell cylindrical polymer brushes with poly(t-butyl acrylate)-b-poly(n-butyl acrylate) (PtBA-b-PnBA) diblock copolymer side chains were synthesized via the “grafting from” technique using a combination of anionic polymerization (for the synthesis of the backbone) and atom transfer radical polymerization (ATRP, for the synthesis of the side chains). The formation of well-defined brushes was confirmed by 1H-NMR and GPC. The selective hydrolysis of the PtBA block of the side chains resulted in novel amphiphilic core-shell cylindrical polymer brushes with poly(acrylic acid)-b-poly(n-butyl acrylate) (PAA-b-PnBA) side chains. The characteristic core-shell cylindrical structure of the brushes was directly visualized on mica by scanning force microscopy (SFM). Amphiphilic brushes with 1500 block copolymer side chains and a length distribution of lw/ln = 1.04 at a total length ln = 179 nm were obtained. These amphiphilic polymer brushes can be regarded as unimolecular cylindrical micelles, because of the core-shell structure and the amphiphilicity of side chains. The amphiphilic brushes can be used as single molecular templates for the synthesis of inorganic nanoparticles, because the carboxylic acid groups (or carboxylate groups, after neutralization) in the polymer core can coordinate with various metal ions. The hydrophilic core of polymer brushes, poly(acrylic acid), was neutralized by NaOH and afterward iron cations (Fe3+ and Fe2+) were loaded into the polymer core via ion exchange. The formation of the polychelates of polymer brushes and iron cations was confirmed and characterized by various techniques such as Fourier transform infrared spectroscopy (FTIR), UV/vis spectroscopy, transmission electron microscopy (TEM) and SFM. A peculiar “pearl necklace” morphology was observed for the polychelates, which is caused by the physical cross-linking of the side chains via multivalent iron cations. Formation of crystalline alpha-Fe2O3 (hematite) was observed during the He-Ne laser irradiation in the confocal Raman microscopy measurement of the polychelate containing Fe3+ ions. Magnetic nanoparticles were successfully produced from the coordinated iron cations within polymer brushes via single molecule templating technique, as confirmed by various techniques such as SFM, TEM, and UV/visible spectroscopy. Superconducting quantum interference device (SQUID) magnetization measurements show that the hybrid nanocylinders are superparamagnetic at room temperature. The polymer shell provides not only the stability of the nanoparticles but also the solubility of the hybrid nanocylinders. After the formation of the magnetic nanoparticles, the carboxylate coordination sites within the polymer brushes are liberated and ready for further coordination with more iron ions, thus it is possible to increase the amount and/or particle size of the nanoparticles by multi-cycles of iron ion loading and particle formation. The as-prepared hybrid nanocylinders combine the promising properties of polymers and superparamagnetic nanoparticles, and may find potential applications such as in ferrofluids. Similarly, using the amphiphilic core-shell cylindrical polymer brush with PAA core and PnBA shell as template, wire-like assemblies of CdS nanoparticles were successfully synthesized under mild solution conditions, as confirmed by various characterization techniques. Quantum confinement of the CdS nanoparticles was observed, indicated by the blue shift of the absorbance edge in UV/visible spectrum. The technique using a single cylindrical molecule as template for inorganic nanoparticle fabrication presented in this thesis is not restricted to magnetic/semiconductor nanoparticles, but can also be used for the preparation of a number of metal, metal oxide, and metal chalcogenide nanoparticles.

Synthesis, Molecular Structure and Reactivity of Di(1-cyclohepta-2,4,6-trienyl) thioether, S(C7H7)2 and Tri(1-cyclohepta-2,4,6-trienyl) amine, N(C7H7)3 (2004)

Jinnan Liu

This thesis describes the synthesis, characterization and coordination chemistry of di(1-cyclohepta-2,4,6-trienyl) thioether, S(C7H7)2 (1), tri(1-cyclohepta-2,4,6-trienyl) amine, N(C7H7)3 (24), and di(1-cyclohepta-2,4,6-trienyl) amine, NH(C7H7)2 (25). The reaction of tropylium bromide, C7H7Br, with the gas hydrogen sulfide (H2S) leads to the di(1-cyclohepta-2,4,6-trienyl) thioether, S(C7H7)2 (1), which can undergo Diels-Alder reactions with maleic anhydride, maleimide and N-phenyl maleimide to give the adducts S(C7H7)(C7H7C4H2O3) (4a), S(C7H7)(C7H7C4H3NO2) (4b) and S(C7H7)(C7H7C10H7NO2) (4c), respectively. In these three compounds, one cyclohepta-2,4,6-trienyl ring has remained intact, whereas the other seven-membered ring was involved in the Diels-Alder reaction. The advantage of the potential ligand S(C7H7)2 (1) in comparison to other simple sulfanes is that it can act as a chelate ligand, using one of the non-planar seven-membered rings. In general, the central double bond of one of the seven-membered rings becomes coordinated to the metal. The disadvantage of the sulfane 1 is its sensitivity towards oxidation and its low thermal stability. The reactions of either Cr(CO)5(thf) or Cr(CO)4(eta4-C7H8) with one equivalent of 1 lead to the monosubstituted derivative Cr(CO)5[PhCH2S(C7H7)] (7), in which one seven-membered ring has remained unchanged and the other seven-membered ring has been transformed into a benzyl substituent. From the reaction of 1 with a mixture of chromium(carbonyl)(acetonitrile) complexes, Cr(CO)6-x(CH3CN)x (x = 1, 2, 3), the pentacarbonyl Cr(CO)5[S(C7H7)2] (6) could be obtained, in which both cycloheptatrienyl ring substituents are freely pending. Using the same method as for the chromium complexes, tetracarbonyl molybdenum Mo(CO)4[(eta2-C7H7)S(C7H7)] (10) was obtained from the reactions of either Mo(CO)5(thf) or Mo(CO)4(eta4-C7H8) with the sulfane 1. In complex 10 the ligand S(C7H7)2 is coordinated to the metal both through a lone pair of electrons at the sulfur atom and the central C=C double bond of a cyclohepta-2,4,6-trienyl substituent. Mo(CO)5[S(C7H7)2] (9) could be obtained from Mo(CO)6-x(CH3CN)x (x = 1, 2, 3); in analogy to 6, the sulfane ligand in 9 is coordinated to molybdenum exclusively through a lone pair of electrons at the sulfur atom. From the intermediate W(CO)5(thf), the pentacarbonyltungsten complex W(CO)5[S(C7H7)2] (13) was obtained, which could be converted thermally to the benzyl complex W(CO)5[PhCH2S(C7H7)] (14). In the reaction with the mixture of tungsten(carbonyl)(acetonitrile) complexes, W(CO)6-x(CH3CN)x (x = 1, 2, 3), the compounds W(CO)4[(eta2-C7H7)S(C7H7)] (15) and W(CO)5[S(C7H7)2] (13) were obtained; complex 15 was the main product. The reactions of either Mn2(CO)10 (under irradiation) or Mn(CO)5X (X = Cl, Br) with di(1-cyclohepta-2,4,6-trienyl) thioether (1) led to the organothiolato-bridged dimer Mn2(CO)8[S(C7H7)]2 (17) in addition to ditropyl, (C7H7)2. Subsequent substitution of two carbonyl ligands in 17 by stronger sigma-donor-pi-acceptor ligands such as tert-butyl isocyanide and trimethyl phosphite (tBuNC and P(OMe)3) gave the complexes Mn2(CO)6[tBuNC]2[S(C7H7)]2 (18) and Mn2(CO)6[P(OMe)3]2[S(C7H7)]2 (19). Both 17 and 18 contain a planar Mn2S2 core with the 1-cyclohepta-2,4,6-trienyl substituents in anti-position. In addition, some investigations with tri(1-cyclohepta-2,4,6-trienyl) amine, N(C7H7)3 (24), and di(1-cyclohepta-2,4,6-trienyl) amine, NH(C7H7)2 (25) were carried out. In analogy to the sulfane S(C7H7)2 (1), 24 and 25 can undergo Diels-Alder reactions with maleimide and N-phenyl maleimide to give the educts (C7H7)2N(C7H7C4H3O2N) (26b), (C7H7)N(C7H7C10H7NO2)2 (26c), (C7H7)NH(C7H7C4H3O2N) (27b) and (C7H7)NH(C7H7C10H7NO2) (27c), respectively. Similar to the synthesis of 24 and 25, the mixed tertiary amines di(1-cyclohepta-2,4,6-trienyl)phenylamine, N(Ph)(C7H7)2 (28), and (1-cyclohepta-2,4,6-trienyl)diphenylamine, N(Ph)2(C7H7) (29) were prepared using aniline or diphenylamine. The reaction of N(C7H7)3 (24) with [C7H7]BF4 (1:1) leads to the N-tropylidene-N-(1-cyclohepta-2,4,6-trienyl)immonium tetrafluoroborate salt, [(C7H6)N(H)(C7H7)]BF4 (30). Compound 30 has also been obtained using NH(C7H7)2 (25) as the educt. As expected, the tertiary amine N(Ph)(C7H7)2 (28) reacted with [C7H7]BF4 to give the dark-red salt [(C7H6)N(Ph)(C7H7)]BF4 (31). Attempts to use the amine N(C7H7)3 (24) as a ligand in transition metal complexes were unsuccessful. However, NH(C7H7)2 (25) reacts with the mixture of tungsten(carbonyl)(acetonitrile) derivatives to give the chelate tetracarbonyl complex W(CO)4[(C7H7)NH(eta2-C7H7)] (32), which is similar to the sulfane complexes 10 and 15. Compared with the versatile phosphane ligand tri(1-cyclohepta-2,4,6-trienyl) phosphane, P(C7H7)3, the analogous amine N(C7H7)3 (24) is unable to act as a coordination ligand, probably as a result of the steric shielding of the lone pair of electrons at the nitrogen atom.

Complex nanostructures in triblock terpolymer thin films (2004)

Sabine Ludwigs

The thin film phase behavior of poly(styrene)-block-poly(2-vinylpyridine)-block-poly(tert-butyl methacrylate) (PS-b-P2VP-b-PtBMA) triblock terpolymers with volume fractions f(PS) : f(P2VP) : f(PtBMA) = 1 : 1.2 : x, with x ranging from 3.05 to 4, is studied with a combinatorial gradient approach. Gradients in film thickness are prepared via thin film flow coating of dilute solutions in chloroform. Upon controlled annealing in nearly saturated solvent vapor the films form terraces of well-defined step height. The dependence between morphology and film thickness is studied with optical microscopy, tapping mode SFM, and SEM. Though showing different morphologies in the bulk, the same sequence of surface structures is found with increasing film thickness for the whole range of compositions: a disordered phase in the thinnest regions, a liquid-like distribution of upright standing cylinders, cylinders oriented parallel to the film, and finally a hexagonally ordered perforated lamella structure (PL) on the first terrace with a thickness of d = (37+3) nm. Higher terraces also exhibit PL as surface structures. Due to the chemical nature of the block components and the particular stoichiometry of the polymer a wetting layer with a PtBMA-rich top layer is formed next to the substrate. By imposing an additional gradient in substrate surface energy, orthogonal to the gradually increasing film thickness, the perforated lamella is shown to be a stable phase, regardless of the chemical nature of the substrate, which makes this structure and methodology robust for application in nanotechnology. The complex phase behavior observed in thin films is supported by mesoscale computer simulations based on dynamic density functional theory. Thin films of the above mentioned triblock terpolymers are modeled as a melt of A3B4C12 Gaussian chains which is confined in a slit with film thickness H. By adjusting the interaction parameters between the polymer components and the surfaces, the experimentally observed sequence of surface structures as function of the film thickness can be successfully modeled. At well-defined film thickness the perforated lamella structure is formed. In analogy with earlier work on a two-component system these structures are identified as surface reconstructions of the bulk structure. In particular, the core-shell PL can be seen as analogue to the PL surface reconstruction of cylinder-forming AB and ABA systems. The influence of film thickness, surface field, and the interaction parameters between the different polymer components on the phase behavior is also explored. A large spectrum of surface structures is observed in analogy to the experiments. Further attention has been given to the perforated lamella structure. This structure can be visualized as P2VP/PS/P2VP lamellae which are perforated by channels of PtBMA interconnecting between two outer layers of PtBMA. A highly ordered PL structure could be prepared with a very small number of defects over an area of about 12 x 4 µm2. Because of the special functionalities of the triblock terpolymer a rather versatile nanostructure was produced. By selective UV-depolymerization of the PtBMA matrix phase, the PL phase might potentially be used for lithographic applications similar to the case of perpendicularly oriented poly(methyl methacrylate) (PMMA) cylinders in PS-b-PMMA block copolymer thin films. Furthermore, a responsive membrane can be created by selective removal of the matrix phase. The remaining PL has a P2VP shell which might be either switched via the pH-value or loaded with metal components. A polymer-analogous reaction of the matrix phase of the PL to poly(methacrylic acid) via acid-catalyzed hydrolysis leads to a pH-responsive nanostructure without altering the overall structure. With SFM in aqueous environment structural changes of the PL phase are studied as function of the pH-value. Upon changing the pH of the surrounding medium a strong swelling of the original film thickness is observed at pH-values > 6 to a maximum degree of 7.5-fold swelling. This swelling is explained with a conformational change of the matrix phase poly(methacrylic acid). The hexagonal arrangement of the pattern is not affected. The first two blocks PS and P2VP act as skeleton of the PL phase which withstands the mechanical forces exerted on the strongly swollen PMAA. In contrast to the PL phase core-shell cylinders oriented parallel to the interfaces cannot withstand these forces and are solubilised at high pH-values.

Synthesis, characterisation and application of low molecular weight and polymeric 1,3-di-2-thienylbenzo[c]thiophenes (2004)

Roman Kisselev

The synthesis and characterisation of the new class of compounds, dithienylisothianaphthene phenyldiamines (DTITNPDs) is described. These bifunctional hole transport dyes combine well-known hole-transport property of triaryl amines and thiophenes as well as low band gap nature of isothianaphthene (ITN) moiety. The synthetic strategy is chosen in such a way to obtain low molecular weight and polymeric DTITNPDs. Low molecular weight DTITNPDs are synthesised by Pd-catalysed amination of dibromo dithienylisothianaphthene with secondary amines. On the other hand, poly(DTITNPD)s are obtained via polycondensation of diiodo dithienylisothianaphthene and bis(secondary amine)s using a modified Ullmann reaction. The multi-step syntheses of dibromo dithienylisothianaphthene and novel diiodo dithienylisothianaphthene are optimised. The preparation of new bis(secondary amine)s are also described. Moreover, the influence of substituents on optical, electro-chemical and thermal properties of DTITNPDs and poly(DTITNPD)s is also investigated. The low molar mass (monomers) and polymeric DTITNs are also obtained. These compounds also possess hole transport property of thiophene and low band gap nature of ITN. Poly(DTITN)s are synthesised from corresponding monomers using FeCl3 oxidative polymerisation. Multi-step syntheses of DTITN monomers are also presented here. The main highlight of this work is the realisation of solution processable and film-forming and air-stable poly(DTITN) and poly(DTITNPD)s in addition to the model compounds DTITNs and DTITNPDs. The model compounds, polymeric DTITNPDs as well as poly(DTITN)s are characterised by means of 1H-NMR-, FT-IR-, MS- and UV-Vis- spectroscopy. Their thermal and electro-chemical behaviour is studied using TGA, DSC and CV measurements. All intermediates, synthesised in this work are also fully characterised by spectroscopic methods discussed above, except UV-Vis spectroscopy. Novel DTITNs and DTITNPDs show good thermal and electro-chemical stability as well as ability to form smooth thin films. DTITNPDs show lower band gap, solubility in common organic solvents and better thermal stability compared to DTITNs. For the application in organic electro-optical devices materials with improved optical and charge transport properties are required. Moreover, these hole transport dyes should match the energy levels (HOMO/LUMO) of the electron transport partner for efficient charge transfer/injection. In this respect, the main attention is placed on variation of energy levels in synthesised molecules by structure modification. The structure modification in DTITN usually changes the LUMO level in the molecule. In contrast to the DTITN, the introduction of different substituents into diphenylamine allows manipulation of HOMO level in DTITNPDs. Thus, the combination of DTITN and triarylamines leads to DTITNPD, where the values of both energy levels can be varied. Novel DTITNPDs exhibit smaller band gap compared to DTITNs. The better delocalised HOMO level in the DTITNPDs compared to DTITNs leads to novel hole transport dyes with Eg less than 1.8 eV. The low molar mass DTITNPDs are tested in plastic solar cells and multi-layer solar cells in combination with electron transport perylene bisimide derivatives and fullerene (C60). The poly(DTITN) is used in plastic solar cell in combination with a soluble fullerene derivative, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). All of these compounds show good performance in solar cells. External quantum efficiency (IPCE spectrum) for the last solar cell shows a maximum of 40 % at 350 nm and a maximum of 15 % at 540 nm, at maximum wavelength of absorption. A promising result is obtained using low molar mass DTITNPD as red-emitter in OLEDs. When the red emitter doped in Alq3 at concentration of 1 % is used as emitting layer in OLED, the pure red electroluminescence with maximum brightness of 13830 cd/m2 at operating voltage of 12 V is observed. This device exhibits a high efficiency of 3.8 cd/A at 6 V bias, emitting bright red electroluminescence with CIE coordinates of x = 0.66 and y = 0.34, closely resembling the desired standard red colour (NTSC standard: x = 0.67 and y = 0.34) set for RGB displays.

Chemie

Refine

Author

Year of publication

Language

Keywords

107 search hits