24 search hits
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New Double-Responsive Micelles of Block Copolymers Based on N,N-Diethylacrylamide: Synthesis, Kinetics, Micellization, and Application as Emulsion Stabilizers
(2005)
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Xavier André
- Thermo- and pH-responsive block copolymers based on (meth)acrylic acid and N,N-diethylacrylamide were synthesized and their aqueous solution behavior was studied. Such bishydrophilic block copolymers represent an interesting class of stimuli-responsive water-soluble materials whose macroscopic properties can be triggered at the molecular level by tuning the temperature, the pH and the ionic strength of the solution. A new method was introduced for the synthesis of well-defined poly(N,N-diethylacrylamide) (PDEAAm) via living anionic polymerization using ethyl alpha-lithioisobutyrate (EiBLi) in the presence of triethylaluminium (Et3Al) as Lewis acid in tetrahydrofuran (THF) at -78 °C. Kinetic investigations were performed using in-situ Fourier-transform Near-Infrared (FT-NIR) fiber-optic spectroscopy. This is the first mechanistic study of the anionic polymerization of a dialkylacrylamide. The polymerization follows first order kinetics with respect to the effective concentration of active chains, [P*]0, but shows complex kinetics with respect to the actual monomer and initial aluminum concentrations. Upon addition of Et3Al, the polymerization rate constant, kp decreases, which is explained by the formation of an amidoenolate chain end/Et3Al complex of lower reactivity. It involves two equilibria: between noncoordinated and Et3Al-coordinated chain ends (deactivation of chain ends) as well as between free and Et3Al-activated monomer (activated monomer mechanism). These two effects are in a delicate balance that depends on the ratio of the concentrations of Et3Al, monomer, and chain ends. Thus, the polymerization rate of this system is governed simultaneously by the complex interplay between the activation of monomer (dependent on monomer and Et3Al concentrations) and the deactivation of chain ends (dependent on the ratio of concentrations of Et3Al to initiator). Polymers with narrow molecular weight distribution are obtained, indicating that the rate of interconversion between the different chain end species is greater than the polymerization rate. In contrast, such well-defined polymers are not found in the absence of Et3Al. PDEAAm polymers, synthesized using organolithium initiator in the presence of Et3Al, are rich in heterotactic (mr) triads and exhibit Lower Critical Solution Temperatures (LCST) in water with a cloud point at Tc = ca. 31 °C. By extending this synthetic concept and using poly(tert-butyl acrylate)-Li, and poly(tert-butyl methacrylate)-Li as macroinitiators, well-defined poly(tert-butyl acrylate)-block-PDEAAm, and poly(tert-butyl methacrylate)-block-PDEAAm block copolymers were obtained. Although the blocking efficiencies remained below 70 % a separation of block and homopolymers was easily possible. The narrowly distributed (AA)45-b-(DEAAm)360 block copolymer obtained after hydrolysis of the protecting tert-butyl groups exhibits interesting ‘schizophrenic’ micellization behavior in response to temperature, to pH, and to ionic strength of the aqueous media. Due to its asymmetric composition, two opposite micellar structures are expected. Indeed, the existence of different micellar aggregates, i.e., ‘crew-cut’ micelles with a PDEAAm core and inverse star-like micelles with PAA core, was proven by several analytical techniques, like Small-Angle Neutron Scattering (SANS), Dynamic and Static Light Scattering (DLS, SLS) and Cryo Transmission Electron Microscopy (cryo-TEM). Furthermore, all the transitions were found to be reversible. Finally, the synthesized bishydrophilic block copolymers were used for batch emulsion polymerizations of styrene, methyl methacrylate and n-butyl acrylate. In all cases, latexes with remarkable long-term stabilities were obtained, which is a very interesting feature from the colloidal point of view. The stabilization efficiency was found to be essentially adjustable by the pH due to the loss of the PDEAAm segment inside the latex particle. A detailed analysis of the particle size and particle size distribution was carried out using a variety of methods, including DLS, TEM and Asymmetric Flow Field-Flow Fractionation (AF-FFF).
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Instabilities in layered liquids induced by external fields
(2003)
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Günter Auernhammer
- In this thesis, we have shown that the inclusion of a nematic degree of freedom in the macroscopic hydrodynamic description of smectic-A-like liquids leads to a number of interesting results. While the director and the layer normal are coupled such that they are parallel in equilibrium, in non-equilibrium situations, the director needs not be parallel to the smectic layer normal. This is in contrast to standard smectic-A hydrodynamics. Using irreversible thermodynamics and symmetry arguments, we derived a complete set of macroscopic hydrodynamic equations for the director variables, the layer displacement, the velocity field, and the moduli of the nematic and smectic order parameters. Recent experiments find that the parallel orientation of smectic-A- like liquids is destabilized by an applied shear. After destabilization, two typical scenarios are observed in a steady state situation: i) The layers are oriented perpendicular to the vorticity direction of the flow, i.e., they lie in the plane spanned by the velocity and the gradient direction (`perpendicular' orientation). ii) Closed multi-lamellar vesicles (`onions') form. A number of experiments indicate that the onset of this reorientation is controlled by the applied shear rate. In contrast to standard smectic-A hydrodynamics where shear in the parallel orientation has no effect on the layers, this destabilizing effect comes out naturally from our extended smectic-A hydrodynamics. The argumentation goes along the following lines. The shear field exerts a torque on the director that must be balanced by the coupling to the layer normal. In the limit of small angles, balancing these torques leads, in the steady state, to a shear-induced director tilt proportional to the shear rate. The preferred thickness of a smectic layer is directly connected to the projection of the averaged molecular axes on the layer normal, or, in terms of our model, the thickness is proportional to the projection of the director on the layer normal. If the director is tilted, this projection is shorter. This decrease of the projection is equivalent to an effective dilation, because the actual layer thickness is larger than the preferred layer thickness. Similar to the case of low molecular weight smectic-A liquid crystals under a dilative strain, this effective dilation leads to an undulation instability. To investigate the stability of the parallel alignment, we performed a linear and weakly non-linear analysis of the governing equations. The initial state is the above described spatially homogeneous director tilt with the smectic layers in the parallel orientation. The linear stability analysis showed an undulation instability which sets in above a critical tilt angle (or equivalently, a critical shear rate). This critical tilt angle turned out to depend strongly on the material parameters. For a typical low molecular weight thermotropic liquid crystal, we estimated the critical tilt angle to be on the order of a few degrees. The linear stability analysis also revealed that the nematic and smectic order is modulated close to the boundaries. Since the probability for the formation of defects is larger in regions with a decreased modulus of the order parameter, these variations in the modulus of the order parameter open the way for a destabilization of the layered structure. We note that a detailed investigation of this point is beyond the scope of the present work. Finally, we could exclude an oscillatory instability for all physically reasonable regions in parameter space. The weakly non-linear analysis shows that the bifurcation is supercritical for most physically relevant regions in the parameter space. A detailed comparison to an independent approach was undertaken in a collaboration with simulation physicists from the Max-Planck- Institute for Polymer Research in Mainz. In a molecular dynamics simulation, a model layered liquid consisting of chains of four particles (AABB) was considered. The interaction potential of particles not connected by springs is attractive for like particles and repulsive for particles of a different nature. The simulation demonstrated the two main predictions of our analytic theory: The director tilts in the flow direction and, above a critical shear rate, the layers show stationary undulations with a wave vector in the vorticity direction. Besides this good qualitative agreement, a reasonable quantitative agreement for the critical shear rate was found.
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Self-Assembly of Block Copolymers in External Fields
(2002)
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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.
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Shear-induced alignment in block copolymer solutions
(2005)
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Gabi Cantea
- The alignment of the ordered microdomains of block copolymers in solution has been accomplished by using mechanical shear fields. Two pathways to monitore the achievement of aligned structures have been used: rheo-optical and in situ rheo-SAXS methods. The AC diblock copolymer and ABC triblock terpolymer with high molecular weights and different morphologies were synthesized via anionic polymerization and the alignment has been monitored from the solutions of neat AC diblock, ABC triblock and their blend in a non-volatile solvent. Before proceeding to the alignment protocols the thermodynamics of the diblock copolymer solutions has been studied by rheological means. The slightly asymmetric diblock copolymer polystyrene-b-poly(t-butyl methacrylate) (ST) as solutions at different concentrations was investigated by scanning of moduli over a range of temperatures and the morphological transitions were detected. This led us to conclude that dioctyl phthalate is a selective solvent for this diblock copolymer (ST) and a transition from the initially lamellar toward a cylindrical or spherical morphology takes place before order-disorder transition. A route to check the induced asymmetry due to the solvent selectivity was accomplished by applying the Leibler dilution approximation theory. While for the less asymmetric diblock copolymer ST(72K) the dilution approximation theory could be approached, for the higher asymmetric ST(117K) a failure of the theory has been encountered. For the system ST(72K) where the theory was still valid, as a result an expression for the interaction parameter has been developed. The alignment kinetics were performed first by making use of the rheooptical method and the monitored retardation was further used for calculation of birefringence. A perfectly symmetric diblock ST with high molecular weight (100K) as solution in dioctyl phthalate was used for the alignment protocols. Previous investigation of such system did not show any order-order transition, thus for this particular symmetric diblock the same lamellar morphology was preserved in the swollen state. Over a wide range of frequencies and strain amplitudes the monitored birefringence was always positive meaning a perpendicular alignment with the normals of lamellae along the vorticity axis and perpendicular on the plane formed between flow direction and gradient velocity axes. This prefered alignment has been explained due to the low viscoelastic contrast between the polystyrene and poly(t-butyl methacrylate) blocks which did not allow the sliding toward a parallel alignment found for systems with a large viscoelastic contrast. Thus, choosing an appropriate chemical sequence of blocks a selective type of macroscopic alignment by LAOS can be achieved. The introduction of the third elastomeric block between the thermoplastic outer blocks (S and T), namely SBT triblock terpolymer, lead to significant changes in terms of alignment in the sense of tunable intermediate (perpendicular and transverse) aligned states which finally led to the same final parallel orientation in the diluted state. The in situ rheo-SAXS method applied to the same system elucidated the intermediate mechanism leading to a final parallel aligned state such as a coexistence of parallel and perpendicular states which gave rise to a transverse alignment in rheooptical method. As a finally aligned state resulting from in situ rheo-SAXS in oscillatory mode the perpendicular one was found within a short time scale (1h), while the rheooptical method revealed a perpendicular state at short time scale (1h) and parallel state at long time scale (10h). Finally, the investigation of the lamellar non-centrosymmetric blend SBT:ST=60:40 allowed preferential parallel alignment above a critical strain amplitude instead below the critical strain amplitude only perpendicular alignment is achieved. While the rheooptical method applied to a bcc morphology of a solution of SBM triblock terpolymer in DOP did not give evidence for an induced morphological transition, in situ rheo-SAXS was a powerful tool to illustrate such a transition. Moreover, we have shown that more complicated morphologies like knitting pattern of SEBM (as cast film from chloroform) which reveals a bcc morphology as a solution in DOP could be investigated and macroscopically aligned via in situ rheo-SAXS while the rheooptical method could not be used due to the non transparent system. The induced columnar structure could be monitored in 2D SAXS patterns which have never been reported before. In summary, it was demonstrated that mechanical field alignment of block copolymer domains in solution allowed to generate highly anisotropic structures even for block copolymers with high molecular weights under ambient conditions and for a large variety of morphologies.
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Phasenverhalten von Blockcopolymeren und deren Wechselwirkung mit normalen Kohlenwasserstoff-Tensiden
(2005)
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Elham Eghbali
- Die Mizellbildung dreier Blockcopolymer-Systemen in Wasser und deren Wechselwirkungen mit normalen Kohlenwasserstoff-Tensiden wurden durch Messungen der Oberflächenspannung, Elektronenmikroskopie, Lichtstreuung und Kleinwinkelneutronenstreuung untersucht. Für das System PEO18-PEB64-PEO18 sind die wässrigen Lösungen der Blockcopolymere trüb und metastabil und trennen sich innerhalb von ein paar Wochen in zwei Phasen. Das Blockcopolymer ist oberflächenaktiv und erniedrigt die Oberflächenspannung von Wasser. Die elektronenmikroskopischen Aufnahmen einer 1% igen Lösung des Blockcopolymers in Wasser weisen auf eine Polymorphie der Aggregate und einen fehlenden Gleichgewichtszustand hin. Unilamellare und multilamellare Vesikel und fadenförmige Mizellen sind nebeneinander auf den Aufnahmen erkennbar. Mit der Zugabe eines anionischen Tensids, SDS, ändern sich die optischen Eigenschaften sowie die Nullviskosität der Lösung und die Morphologie der Aggregate. Unterhalb der cmc des Blockcopolymers wurde keine Wechselwirkung mit Tensiden erkannt. Oberhalb der cmc lagern sich die Tensidmoleküle an die Aggregate der Blockcopolymere an. Mit zunehmender Tensidkonzentration verschwinden die größeren Aggregate und Vesikel und die Trübung der Lösung nimmt ab. Die dominante Morphologie ändert sich mit der zunehmenden Tensidkonzentration von Vesikeln zu fadenförmigen Mizellen, zu großen Mischaggregaten und zu kleinen SDS-Mizellen in welche einzelne Polymerketten gelöst sind. Diese Mischmizellen befinden sich im Gleichgewicht mit reinen SDS-Mizellen. Bei Sättigung der Blockcopolymere mit Tensid bleibt die Oberflächenspannung konstant und bei einem Wert wie dem von reinem Tensid. Die Lösungen sind dann klar und haben eine niedrige Viskosität. Die Sättigungs-Tensidkonzentration nimmt linear mit der Polymerkonzentration zu. Für das System PnBA100-PAA150 sind die wässrigen Lösungen der Blockcopolymere trüb und trennen sich in zwei Phasen. Mit der Neutralisation der Polysäure-Gruppe nimmt die Trübung ab. Für dieses System wurden zwei interessante Eigenschaften beobachtet: 1- Keine Oberflächenaktivität 2-Abhängigkeit der Viskosität von dem alpha-Wert Auf den elektronenmikroskopischen Aufnahmen sind die Änderungen der Morphologie und der Teilchenanordnung mit dem alpha-Wert erkennbar. Bei alpha=0 koexistieren unterschiedliche Morphologien wie kugelförmige Mizellen und Stäbchen, deren Größe und Form darauf hinweisen, daß das System sich nicht in einem thermodynamischen Zustand befindet. Bei alpha>0.1 sind die Teilchen ausschließlich kugelförmig. Die Polymer-Moleküle aggregieren in kugelförmige Aggregate mit einer Kern-Schale Struktur. Mit zunehmendem alpha-Wert erkennt man eine hohe Ordnung der Aggregate und einen ziemlich gleichmässigen Abstand zwischen den Teilchen. Die Polyelektrolyt-Kette streckt sich mit zunehmendem alpha-Wert, bei alpha=0,5 hat sie die höchste Länge erreicht und die Mizellen besitzen die höchste Ordnung. Aus der SANS-Messung läßt sich zwischen den Teilchen ein Abstand von ca. 104 nm und ein Durchmesser von ca. 99 nm für die Teilchen berechnen. Dies entspricht einer dichten Packung der Aggregate, welche sich in der hohen Viskosität der Lösung wiederspiegelt. Sowohl kationische, anionische und zwitterionische Tenside lagern sich an das Blockcopolymer an. Das kationische Tensid bindet an die negativ geladenen Säuregruppen und bildet einen nicht löslichen Komplex, welcher sich bei niedrigeren Tensidkonzentrationen um den Kern der Blockcopolymere legt. Die Mizelle hat dann einen schalenförmigen Aufbau mit dem Butylacrylat-Block im Zentrum, der mit einer Schale von dem unlöslichen Komplex umgeben ist. Der Rest der Polyelektrolyt-Kette bildet die Corona.) Bei höheren Tensidkonzentrationen fällt der Komplex aus der Lösung. Das Tensid bindet sich kooperativ an die Polysäure und ist nicht gleichmäßig verteilt. Mit dem anionischen Tensid SDS dominieren hydrophobe Wechselwirkungen. Das Tensid dringt in den Kern der Mizelle hinein. Nach Absättigung der vorhandenen Grenzfläche und Auflösung der Blockcopolymermizellen aggregieren die Tensid-Moleküle in normale Mizellen. Mit dem zwitterionischen Tensid C14DMAO wird die Wechselwirkung bei niedrigen alpha-Werten durch Protonenübertragung und elektrostatische Kräfte und bei höheren alpha-Werten durch hydrophobe Kräfte beherrscht. Das Blockcopolymer PMMA60-PAA90 aggregiert in Wasser ohne die Oberflächenspannung zu erniedrigen. Die wässrigen Lösungen von PMMA60-PAA90 schäumen obwohl das Blockcopolymer die Oberflächenspannung des Wassers kaum ändert. Die Viskosität von 1 %igen Blockcopolymer-Lösungen ändern sich nur wenig mit dem alpha-Wert und bleiben für den ganzen Neutralisationsbereich niedrig. Auf der Kryo-TEM Aufnahme der Mizellen ist eine starke Ordnung erkennbar, die aber keine hohe Viskosität verursacht. Der Unterschied kann ebenfalls an der Länge der Polyelektrolyt-Ketten liegen.
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Mesoscale Modeling of Phase Behavior in Thin Films of Cylinder-Forming ABA Block Copolymers
(2008)
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Andriana Horvat
- In this thesis modeling results on structure formation in thin films of cylinder-forming block copolymers are presented and discussed. The computational study of the equilibrium phase behavior in thin films is complemented by detailed comparison with a real experimental system. Additionally, the dynamics in such films at various length and time scales (the dynamics of individual defects and the dynamics of surface relief structures) is studied. The strength of the presented thesis is the comparison of thin block copolymer film equilibrium and dynamic behavior in experiments and in computer simulations. This comparison supplies an in-depth understanding of the processes in thin films and near the surfaces in thick films and allows to identify the important control parameters of nanopattern formation. Chapters 4 and 5 report on the phase behavior of thin films of asymmetric block copolymers. In addition to the surface induced alignment of hexagonally ordered cylinders, an adjustment to the planar symmetry of the surface by formation of surface reconstructions is found to dominate the phase behavior in thin films. The large parameter space covered by the simulations allows to distinguish the effects of the two constraints characteristic for thin films: the surface field and the film thickness. The deviations from the bulk cylinder structure, both in the vicinity of surfaces and in thin films are identified as surface reconstructions. The stability regions of different phases are modulated by the film thickness via interference and confinement effects. The results give evidence of a general mechanism that govern the phase behavior in thin films of modulated phases: The interplay between the strength of the surface field and the deformability of the bulk structure determines how the system rearranges in the vicinity of the surface. Chapters 6 and 7 present a systematic study of defects in thin films of cylinder-forming block copolymers. In particular, the peculiarities of both classical and specific topological are considered in detail, and a strong relationship between the defect structures and the chain mobility in block copolymers is observed. In the systems studied, representative defect configurations provide connectivity of the minority phase in the form of dislocations with a closed cylinder end or classical disclinations with incorporated alternative, nonbulk structures with planar symmetry. In solvent-annealed films with enhanced chain mobility, the neck defects (bridges between parallel cylinders) were observed. This type of nonsingular defect has not been identified in block copolymer systems before. It is shown, that topological arguments and 2D defect representation, sufficient for lamellar systems, are not sufficient to determine the stability and mobility of defects in the cylindrical phase. In-situ scanning force microscopy measurements are compared with the simulations based on DDFT. The close match between experimental measurements and simulation results suggests that the lateral defect motion is diffusion-driven. Finally, the morphological evolution is considered with the focus on the motion and interaction of the representative defect configurations. Chapter 7 reveals dynamic simulations and in-situ SFM measurements of defect annihilation. Along with the lateral movement of defects, the annihilation frequently proceeds through local structural transitions. The role of the observed structural evolution is discussed in the context of the equilibrium phase behavior of cylinder-forming thin films, studied in chapters 4-5. Chapter 8 presents a study of terrace formation in thin films of a cylinder-forming block copolymers by a computational DDFT method. The results are compared with in situ SFM measurements of SBS block copolymer thin films. This chapter focuses on the early stage of terrace formation, where 80% of height changes occur. Experimental and simulation results agree that the change of the local height is strongly connected to the changes in the local microstructure. The detailed pathways of the structural transitions, as revealed by simulations, suggest a diffusion of block copolymer chains along the microstructure interfaces and indicate an important role of cylinders with necks as a material-transport-channel between neighboring terraces in thin cylinder-forming films. Both systems (in experiment and in simulations) show excellent quantitative agreement in detail of structural phase transitions and in the dynamics of the step development, suggesting that the underlying transport mechanisms are governed by diffusion.
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Holographische Datenspeicherung in nanostrukturierten azobenzolhaltigen Polymeren
(2006)
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Michael Häckel
- In der vorliegenden Arbeit werden verschiedene Diblock-Copolymere mit azobenzolhaltigen Seitenketten auf ihr Potential für Anwendungen als wiederbeschreibbares Medium für die holographische Datenspeicherung untersucht. Die Seitenketten dieser Polymere können durch Bestrahlung mit Licht angeregt und umorientiert werden. Infolge der Anisotropie der Seitenketten führt die Umorientierung in den beleuchteten Bereichen des Materials zu Doppelbrechung und damit zu einer räumlichen Brechungsindexmodulation für polarisiertes Licht. Bei Blockcopolymeren tritt Mikrophasenseparation auf. Diese ermöglicht im räumlichen Mittel eine Verdünnung der Azobenzolfarbstoffe bei gleichbleibend hoher lokaler Konzentration in den eingeschlossenen Minoritätsphasen. Die makroskopische Verdünnung ist zur Reduzierung des Absorptionskoeffizienten notwendig, damit das zum Schreiben verwendete Licht Proben mit einer Dicke im Bereich von 1 mm durchdringen kann. Eine hohe lokale Konzentration ist andererseits für die Stabilisierung der eingeschriebenen Information notwendig. Es ist bekannt, dass in azobenzolhaltigen Homopolymeren und statistischen Copolymeren eine stabile Orientierung bei gleichzeitiger schneller lichtinduzierter Umlagerung durch flüssigkristalline Phasen erreicht werden kann. Um festzustellen, ob die kooperative Umorientierung der Seitenketten und ihre gegenseitige Stabilisierung auch in den Minoritätsphasen von Blockcopolymeren auftreten, wurden solche Blockcopolymere untersucht, die in den Seitenketten des photoadressierbaren Blocks methoxysubstituierte Azobenzolgruppen und nichtabsorbierende Dreikernmesogene in unterschiedlichem Verhältnis enthielten. Mit zunehmendem Anteil der mesogenen Seitenketten stieg trotz des gleichzeitig abnehmenden Anteils der Farbstoffgruppen die erreichbare Brechungsindexmodulation eingeschriebener holographischer Gitter an. Damit konnte gezeigt werden, dass sich auch in Blockcopolymeren die mesogenen Seitenketten gemeinsam mit den Farbstoffgruppen umorientieren lassen. Mit steigendem Mesogenanteil stieg die Stabilität der eingeschriebenen Gitter ebenfalls an. Ab einem Anteil von 35 mol-% mesogenen Seitenketten im photoadressierbaren Block war diese so hoch, dass innerhalb eines Zeitraumes von zwei Jahren keine Relaxation der eingeschriebenen Orientierung beobachtet wurde. Nachdem an Gittern, deren Dicke wesentlich größer als die Gitterperiode ist, nur dann Beugung auftritt, wenn die Bragg-Bedingung erfüllt ist, können mehrere Hologramme an der selben Stelle des Mediums eingeschrieben und unabhängig voneinander wieder gelesen werden. Dadurch erhält man zu den üblichen zwei Dimensionen eines flächigen optischen Datenspeichers den Winkel als dritten Freiheitsgrad. An 1,1 mm dicken Spritzgussproben von Mischungen aus einem Blockcopolymer und Polystyrol wurden Experimente zum Winkelmultiplexing einfacher holographischer Gitter durchgeführt. Es gelang, sowohl überlagerte Intensitätsgitter als auch überlagerte Polarisationsgitter einzuschreiben. In azobenzolhaltigen Polymeren wächst die Brechungsindexmodulation holographischer Gitter extrem nichtlinear mit der Belichtungszeit an. Dennoch konnten in der Praxis Intensitätsgitter mit gleicher Belichtungszeit eingeschrieben werden, die am Ende der Einschreibvorgänge nahezu identische Beugungseffizienzen aufwiesen. Bis zu 200 holographische Intensitätsgitter konnten an der selben Stelle des Materials erzeugt werden. In Mischungen von Polystyrol mit Blockcopolymeren, die nichtabsorbierende mesogene Seitenketten oder mesogene Farbstoffgruppen enthielten, wurde sowohl bei einzelnen Hologrammen als auch im Fall mehrerer überlagerter Gitter eine sehr gute Stabilität beobachtet. Als nächsten Schritt hin zur Speicherung realer Daten wurden ausgedehnte Hologramme von zweidimensionalen Testbildern gespeichert. Mehrere dieser Hologramme konnten ebenfalls erfolgreich an der selben Stelle unter unterschiedlichen Winkeln geschrieben und rekonstruiert werden. Die Umorientierung von Azobenzolseitenketten ist reversibel. Die Materialien sind daher wiederbeschreibbar. Es konnte ein geeignetes Verfahren entwickelt werden, das es ermöglicht, Hologramme auf rein optischem Wege nahezu vollständig zu löschen und das Medium mehrere tausend Male wiederzubeschreiben. Zum Löschen wurde jeweils ein zweites holographisches Gitter verwendet, das zu dem ursprünglich eingeschriebenen um 180° phasenverschoben war. Dadurch wurde die Information bereits nahezu vollständig gelöscht. Anschließend wurde mit einem einzelnen Laserstrahl, dessen Polarisation um 90° gedreht war, nachbelichtet, um die Farbstoffgruppen wieder in die Polarisationsrichtung der Schreibstrahlen zu orientieren. Durch diese beiden Schritte konnte die Beugungseffizienz der Hologramme um mehr als zwei Größenordnungen abgeschwächt werden und es wurde nach mehr als 1000 Schreib-Lösch-Zyklen wurde weder eine Abnahme der Beugungseffizienz eingeschriebener Gitter noch eine Verschlechterung des Löschverhaltens beobachtet.
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Donor-Acceptor Block Copolymers in Organic Electronics - Spectroscopy, Charge Transport, Morphology and Device Application
(2010)
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Sven Hüttner
- Organic electronic devices have attracted increasing attention over the last decade. The use of organic materials allows the creation of large area, flexible and low-cost lightemitting devices, transistors and photovoltaics. The development of new organic materials contributes to a successful commercialisation. The present work deals with the characterisation of novel donor-acceptor block copolymers and their constituent polymer blocks that are well-suited for organic photovoltaics. Block copolymers phase-separate and self-assemble into nanostructured morphologies due to the covalent linkage of the two blocks. The interplay between intermolecular interactions, mesoscopic crystalline structures and the block copolymer microphase separation determine the material properties and therefore the device characteristics. Thus, these block copolymers offer a unique platform to study the electronic and photophysical properties of confined donor-acceptor systems. This work is concerned with the fundamental characterisation of these properties as well as the application in organic field effect transistors and organic solar cells. The acceptor polymer block poly(perylene bisimide acrylate) (PPerAcr) consists of perylene bisimide (PBI) units that are linked to a polyacrylate backbone. We have investigated the homopolymer PPerAcr, a model block copolymer in conjunction with polystyrene (PS), as well as fully functionalised block copolymers with a donor block either made of poly(vinyl triphenylamine) (PvTPA) or poly(3-hexylthiophene) (P3HT). These polymers offer a set of electronically active materials with several hierarchical structures: The PBI moieties feature intermolecular pi-pi interactions that lead to crystalline side chains of PPerAcr that form a lattice of one-dimensional stacks of PBI. Further nanoscopic structures are induced by the combination of PPerAcr with another amorphous block or another semi-crystalline block such as P3HT due to phase separation. Since PPerAcr is used as an electron transporting material in all subsequent block copolymers, its structural, optical and electronic properties are investigated in detail. The intermolecular interactions of the PBI moieties favour not only charge transport, but also affect the optical properties, due to the electronic coupling of the transition dipole moments. Thus, optical spectroscopy such as absorption and fluorescence spectroscopy give access to information about the intermolecular packing, which is correlated with temperature dependent X-ray diffraction studies. The strong intermolecular packing of the PBI units can be overcome by solvent-vapour exposure. This is especially helpful to induce polymer chain mobility, enabling the completion of block copolymer phase separation for example. This method was studied in detail by means of in-situ spectroscopy and ellipsometry during controlled solvent-vapour exposure. Spincoated films of PvTPA-b-PPerAcr exhibit an incomplete phase separation and can be transformed into an ordered lamellar morphology by solvent-vapour annealing. In addition to PvTPA, we have characterised further poly(triarylamines) with different electron-rich substituents at the TPA units in OFETs. All these polymers are amorphous side-chain polymers. We found the charge carrier mobility to be independent of the molecular weight, though allowing an adjustment of their thermal properties for device fabrication. This is in contrast to P3HT, which is a semi-crystalline, conjugated main chain polymer. X-ray diffraction, steady state and time-resolved spectroscopy, as well as the transistor device characterisation were employed to establish a charge transport - morphology relation for the donor-acceptor block copolymers P3HT-b-PPerAcr containing two crystalline blocks. Controlling the crystallisation preferences of the two blocks leads to a new processing route for OFETs with tunable p-type, ambipolar, or n-type transport through a one-time thermal annealing step. The application of P3HT-b-PPerAcr in organic photovoltaic devices showed also very promising results with high external quantum efficiencies. Subsequently, the photophysics of P3HT-b-PPerAcr by means of absorption and fluorescence spectroscopy as well as time-resolved transient absorption spectroscopy were investigated. All block copolymers exhibited an ultra-fast charge-pair formation and a strongly reduced photoluminescence, suggesting domain sizes of only some nanometres. Although efficient charge separation could be accomplished, a good charge percolation was lacking due to small domain sizes. Furthermore the herein presented results emphasis the fundamental importance of morphology and interfacial properties such as crystallinity. These findings motivate the further use of block copolymers as compatibilising agents for polymer blends to improve their interface and morphology.
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Structural Analysis of Cylindrical Particles by Small Angle X-ray Scattering
(2005)
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Li Li
- The objective of this work is to analyze nano-scaled cylindrical particles by small angle X-ray scattering (SAXS). Three systems with cylinder-shaped particles: (1) Laponite particles in aqueous solutions, (2) Poly(carbon suboxide) particles in binary water/DMF solutions, and (3) Suprastructural aggregates of coil-ring-coil block copolymers in cyclohexane, have been studied by SAXS performing either a Kratky-Compact-Camera in our laboratory or ID2 beamline of the European Synchrotron Radiation Facility (ESRF) in Grenoble. The synthetic clay particles, Laponite RD, have been chosen as ideal disc-shaped model particles. In combination of SAXS with static light scattering, the scattering intensities of a concentration serial (volume fraction from 0.0002 to 0.0016) were measured in almost three orders of magnitude of the scattering vector q. Through extrapolation of concentration the scattering intensity at vanishing concentration, i.e. the form factor P(q) of particles was achieved. It shows q-2 decay at intermediate q range, which indicates that the shape of single Laponite particle in aqueous solution is platelet. The plateau of the form factor at low q range implies that there is no aggregate or cluster structure, and the Laponite particles are dispersed completely under the investigated conditions. More detailed structural information was then obtained by fitting of P(q) with disc model. The radii of the discs exhibit a large polydispersity. A radius of 10.5 nm with Schulz-Zimm distribution of Rw/Rn = 1.5 (where Rw and Rn denote weight and number average radius, respectively) was found to fit the form factor perfectly. The thickness of one single platelet was determined to be 0.9 nm. The weight averaged molecular weight and radius of gyration were determined to be 930 kg/mol and 13.4 nm, respectively. The inter-particle interactions of Laponite particles were investigated by the structure factor S(q), from which the effective diameter of interparticle interactions deff was determined for the first time. The strong electrostatic Coulomb repulsion between charged Laponite particles was attributed to the much higher value of deff (= 46 nm), in comparison to 2Rg (= 27 nm). The recently developed multicomponent interaction site model was performed by Harnau to predict these experimental structure factors. An effective potential of interaction, which pays attention to a screened Coulomb interaction as well as an attractive interaction, leads to the best description of the model to the experimental data. By means of SAXS, the size of synthetic polymer carbon suboxide ((C3O2)n) dissolved in binary water/DMF solutions was determined for the first time with radius of gyration Rg = 1.7 nm and molecular weight Mw = 2760 g/mol, which corresponds to a polymerization’s degree of about 40. This value is much larger than literature one (5-10). The form factor of polymer carbon suboxide can be described by a semiflexible chain model. The radius of gyration in cross-section RC and molecular weight per unit length ML were obtained to be 0.3 nm and 350 g/(mol.nm), respectively, which can confirm the fact that the chemical structure of poly(carbon suboxide) is repeated pyronic ring, as suggested in most literatures. Thus the structure and size of polymer carbon suboxide were characterized completely by SAXS. Finally, SAXS was employed to analyze a suprastructural aggregation system derived by self-assembly of coil-ring-coil block copolymers. This is a newly synthesized subclass of rod-coil block copolymers composed of a nanometer-sized shape-persistent macrocycle and two covalently attached polystyrene (PS) coils. The solubility of the rigid ring is largely enhanced due to the attachment of the flexible side groups. With suitable length of the flexible side groups (Mw (PS) = 2500 g/mol) the block copolymers can form colloidal-sized aggregates in selected solvent cyclohexane, which were concluded to be of cylindrical shape with the rigid rings packing densely in a tubular way and the flexible side groups arranging outside of the ring. Such aggregated cylinder brushes can be further confirmed to exist as a mixture of cylinder bundles by analyzing the local structural parameter ML (= 25730 g/(mol.nm), molecular weight per nm length of cylindrical objects). In comparison of this value with M0 (= 6500 g/mol, molecular weight of single coil-ring-coil block copolymer) and d (= 0.6 nm, distance of adjacent densely packed rings), the number fraction of coexisted single cylinder, bi- and tri-cylinder bundles was resulted to be 1:1:2. Through fitting by using approximated circular cylinder model the radius of single cylinders was determined to be 2.6 nm (polydispersity 20 percent) with a hollow inside of radius of 1.2 nm.
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Donor-Acceptor Block Copolymers for Charge Separation at Nanostructured Interfaces
(2006)
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Stefan Lindner
- The motivation for this thesis was the synthesis and characterization of novel materials exhibiting nanostructured interfaces for electro-optical studies. Therefore a series of functionalized block copolymers, acceptor labeled polymers and low molecular weight model compounds were synthesized in which hole transport (donor), electron transport (acceptor) and light absorbing functionalities were incorporated. My approach was to use functionalized block copolymers. Block copolymers exhibit microphase separation with domain sizes on a nanometer scale by the interplay between immiscibility and molecular connectivity. I used a controlled radical polymerization technique, the nitroxide mediated radical polymerization (NMRP), to get block copolymers with one block consisting of an electron transport material and the other one of a hole transport material. Triphenylamine was used as hole conductor in combination with perylene bisimide as dye and electron conductor. First, a soluble perylene bisimide monomer had to be synthesized. This was achieved by an unsymmetrical synthesis starting from the perylene-3,4:9,10-tetracarboxylic bisanhydride. For the solubility a swallow-tail substituent was introduced and the other imide group was functionalized with an acrylate to get the monomer. Starting the polymerization with 4-vinyltriphenylamine, different PvTPA 23 macroinitiators were synthesized. A series of block copolymers 24C-24F were prepared using the same PvTPA macroinitiator 23C, thus only varying the perylene bisimide block. Furthermore, a series of block copolymers 24A-24C were synthesized using different PvTPA macroinitiators 23A-23C. Thus block copolymers with different molecular weights, but similar ratios of the blocks could be prepared. The controlled nature of NMRP allowed the architecture of these block copolymers with low polydispersities and controlled molecular weight. The block copolymers exhibited microphase separation, revealing elongated nanowire like structures for those with high perylene bisimide content. Most of these block copolymers exhibit a constant width of 13 nm for the nanowire like structure of the perylene bisimides. This was the first examples of microphase separation of block copolymers carrying electron transport and hole transport blocks. The electrochemical properties of the block copolymers were studied using cyclic voltammetry. The LUMO of the perylene bisimide block is -3.65 eV and the HOMO of the triphenylamine block is -5.23 eV. Therefore the maximum built-in potential and theoretically achievable photovoltage Voc is 1.58V. The efficiency of the block copolymer solar cells is one order of magnitude higher than that of the comparable blend device. It could also be shown that the block copolymer in the solar cell is microphase separated, revealing domain sizes from 10 to 50 nm, whereas the blend on the other hand is macrophase separated. This is the first report of charge separation at a nanostructured bulk interface in a block copolymer consisting of an electron transport and a hole transport material exhibiting microphase separation. These results are thus proof-of-principle for the nanostructured bulk heterojunction solar cells using block copolymers. Furthermore, fluorescent acceptor labeled polymers were synthesized using a series of monomers in order to obtain a single dye unit attached to various polymer chains. These polymers were prepared by nitroxide mediated radical polymerization with an alkoxyamine initiator that is covalently bound to a perylene bisimide moiety. It could be shown with MALDI-TOF mass spectrometry that a single perylene bisimide unit is incorporated in each polymer chain. By using 4-vinyltriphenylamine monomers bifunctional polymers (8) containing electron donating moieties and a single electron acceptor unit were obtained. The polymerization of standard monomers such as styrene and acrylates, gave polymers (9-12) with only a single electron acceptor unit. Also novel electron acceptors consisting of perylene bisimide and fullerene moieties 15 and 17 were prepared and characterized. Although these dyads do not exhibit any ground state electronic coupling between the individual moieties, the emissive properties of the perylene bisimide units are strongly influenced by the covalently bound fullerene. The fluorescence of the perylene bisimide moiety is quenched by 99 % due to energy and electron transfer between the fullerene and the perylene bisimide. Beside the use as a model system these dyads are also capable of being used in organic solar cells. PCBM, the fullerene derivative which is usually used in polymer solar cells, is barely absorbing light and therefore perylene bisimide functionalized fullerenes may be an alternative as they strongly absorb light in the visible region.
