3 search hits
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Crystalline Morphologies of Poly(butadiene)-b-Poly(ethylene oxide) Block Copolymers in n-Heptane
(2009)
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Adriana Mirela Mihut
- This thesis reports the development of micellar crystalline morphologies in a selective solvent. The phase diagram of solution morphologies as a function of the molecular composition of the semicrystalline poly(butadiene)-b-poly(ethylene oxide)(PB-b-PEO) block copolymers was investigated. The crystalline morphologies discussed here have been generated from selective solvent condition (70°C in n-heptane) via two thermal pathways: (A) by direct immersion into liquid nitrogen, where n-heptane becomes a poor solvent for both blocks at very low temperatures, and (B): by quenching to the crystallization temperature of the PEO, i.e., determined by the length of PEO block. In pathway B, n-heptane is a poor solvent only for the PEO block. At 70°C, the block copolymers self-assembled into micellar structures consisting of a PEO molten core and a soluble PB corona. As crystallization takes place in the PEO core, a fast quenching into liquid nitrogen results in the formation of crystalline micelles retaining the shape present in the molten state at 70°C (pathway A). In the case of pathway B, the competition between the PEO core crystallization and the self-assembly of the micellar units, is the driving force that dictates the morphological development, therefore crystallization breaks out the melt morphology. These studies, demonstrated that the PB-b-PEO block copolymers are a promising system models for developing a general route towards tunable crystalline morphologies. In a symmetric PB-b-PEO block copolymer, crystalline morphologies like spheres and meanders formed upon quenching into liquid nitrogen and at 30°C, respectively. The meander morphology consisting of branched lamellae with a crystalline PEO ribbon-like core and ellipsoidal endings was observed for the first time in solution. Investigations of the crystal development revealed that this structure formed via crystallization-induced aggregation of spherical micelles upon cooling. A systematic study of the effect of crystallization kinetics on the formed morphology upon crystallization-induced aggregation of spherical micelles of a symmetric PB-b-PEO block copolymer was discussed. We demonstrated that the resulting morphology is controlled by two competitive effects, namely, by the nucleation and growth of the PEO micellar core: at lower crystallization temperatures (Tc ≤ 30°C), a high nucleation rate leads to a meander-like morphology formation, whereas at higher crystallization temperatures (Tc > 30°C), a low nucleation rate favors the formation of twisted lamellae. For a highly asymmetric PB-b-PEO block copolymer, crystallization at -30°C induced the formation of crystalline micelles retaining the spherical shape present in the molten state at 70°C. However, a quenching into liquid nitrogen facilitated a transition to rod-like micelles caused by changes of solvent quality for the PB coronar chains. This triggers the onset of an interfacial instability, therefore the spherical micelles preferred to reorganize into a morphology with a smaller interfacial curvature. The low crystallinity of the PEO core imposed a stronger tendency of the rods to aggregate and to thicken into more stable morphologies as needle-like structures, with a preferred growth direction along the long axis. Finally, the micellar morphology diagram of the PB-b-PEO block copolymers has been studied as a function of the crystallization temperature and molecular composition of the blocks via two thermal pathways. Pathway A allowed morphological transitions from spheres to rods, worms or twisted cylinders with the increase of the crystalline content of the PEO core. In Pathway B, the sequence of spheres, cylinders, lamellae, platelets and dendrites structures is observed with the increases of the PEO block length. The aggregation number of the spherical micelles is affected by the weight fraction and crystallinity of the PEO block. Moreover, an increased chain folding was observed at a high PEO composition which reduced the lamellar thickness of the crystals. The competition between the PEO core crystallization and the aggregation of the micellar units leads to coexistence regions of lamellae with platelets and cylinders with platelets. The novelty of this thesis relies on the development of novel crystalline morphologies in a selective solvent, as well as, in the detailed analysis of the major parameters that govern morphological formation in a controlled manner.
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Novel Semiconductor Block Copolymers for Organic Electronic Devices: Synthesis, Properties and Applications
(2009)
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Michael Sommer
- Diese Arbeit beschreibt die Synthese und Charakterisierung von neuartigen, maßgeschneiderten Donor-Akzeptor (D-A) Blockcopolymeren mit elektronisch funktionellen Blöcken, sowie deren Anwendung in organischen Feldeffekttransistoren und organischen Solarzellen. Die hergestellten D-A Blockcopolymere können in zwei Klassen unterteilt werden: Blockcopolymere mit einem amorphen und einem kristallinen Block und Blockcopolymere mit zwei kristallinen Blöcken. Die Synthese dieser neuen Materialien verlangt die geschickte Kombination von klassischer organischer Chemie mit einer oder zwei Polymerisationsmethoden. Die Besonderheit solcher aufwendigen Blockcopolymere liegt in ihrer Fähigkeit zur Mikrophasenseparation. Die dadurch entstehenden Domänengrößen liegen im Bereich der Exzitonendiffusionslänge, wodurch D-A Blockcopolymere als äußerst vielversprechend für Ladungstrennung und Ladungstransport gelten. Die Selbstaggregation der D-A Blockcopolymere wird vom Zusammenspiel verschiedener Kräfte geleitet: Kristallisation eines oder zweier Blöcke und Mikrophasenseparation. Solche Materialien mit definierten Moleküleigenschaften sind bisher sehr wenig erforscht und ermöglichen es, die D-A Grenzfläche in dünnen Filmen präzise einzustellen. Um definierte Blockcopolymerarchitekturen herzustellen, wurden zwei verschiedene Polymerisationsmethoden mit lebendem Charakter verwendet, kombiniert und angepasst: Nitroxid-vermittelte radikalische Polymerisation (NMRP) und Grignard Metathese Polymerisation (GRIM). Die Grignard Metathese Polymerisation wurde erfolgreich optimiert und verwendet, um mehrere Poly(3-hexylthiophen)P3HT-Blöcke mit kontrolliertem Molekulargewicht und niedriger Polydispersität herzustellen. Weiterhin wurde eine einfache und zielgerichtete Eintopfreaktion entwickelt, um P3HT-Makroinitiatoren für die NMRP herzustellen. Ausgehend von diesen Makroinitiatoren wurden mehrere definierte Blockcopolymere mit P3HT und Perylen Bisimidacrylat PerAcr mit unterschiedlicher Komposition und unterschiedlichem Molekulargewicht synthetisiert. Die Besonderheit von P3HT-b-PPerAcr liegt in der kristallin-kristallinen Blockcopolymerarchitektur, wobei das erste Segment hauptkettenkristallin und das zweite Segment seitenkettenkristallin ist. Der kristallin-kristalline Charakter wurde mittels differentieller Wärmeflußkalorimetrie und Röntgenstreuung bestätigt, wobei eine Koexistenz von lamellaren P3HT- und eindimensionalen PPerAcr Stapeln festgestellt wurde. Die Koexistenz dieser Aggregate ist maßgeblich von der Komposition, dem Molekulargewicht, und der Vorbehandlung von P3HT-b-PPerAcr abhängig. Während thermisch vorbehandelte Proben eine verstärkte Ausbildung von kristallinen PPerAcr Domänen zeigen, fördert die Lösungsmitteldampfbehandlung die Aggregation von P3HT. Dieser Effekt wird übereinstimmend bei der Untersuchung der optischen, thermischen, morphologischen und elektrischen Eigenschaften gefunden. Die Herstellung von organischen Solarzellen mit P3HT-b-PPerAcr als aktiver Schicht ergab einen Rekordwert der externen Quantenausbeute von 31 %, was für die beiden Komponenten P3HT und Perylenbisimid den höchsten gemessenen Wert darstellt.
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New Approaches to the Synthesis of Porous and/or High Surface Area Transition Metal Oxides
(2009)
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Ram Sai Yelamanchili
- We have explored the applicability of hypothesized approaches to the synthesis of porous and/or high surface area transition metal oxides. In addition, applicability and advantage of charged templates where strong Coulomb interactions favour the supramolecular arrangements/assembly were studied. The problems related with the dynamics of polymeric nanostructures for the synthesis of predesigned mesostructures could be avoided by crosslinking micelles, strictly speaking non-continuous phase in the bulk structure. Thereby, we presented a new approach for the grafting of Keggin POMs around the core-crosslinked PB-P2VP worm-like polymer templates (A 1 and 2). The produced POM-1 exhibits high dispersion, improved surface area and is thus expected to be useful in catalytic, electrochemical and biotechnology related applications. The general applicability of the method to other Keggin POMs and spherical polymer nanostructures were studied. Developed Keggin POMs-1 to 6 showed high dispersion of Keggin POM and surface areas. To the best of our knowledge, our approaches lead to Keggin POM nanocomposites with the highest surface areas reported todate. As-synthesized Keggin POM nanocomposites are amorphous. We have studied the removal of polymer template and crystallization of hybrid to corresponding metal oxides through step-wise calcinations under argon followed by air. We have presented another approach to the synthesis of high surface area and mesoporous keggin POM framework materials using amphiphilic PI-PDMAEMA block copolymers (A 3). The calcined mesoporous materials exhibit Keggin POM hexagonal pore structure with high keggin POM dispersion, improved surface area. These developed materials are expected to be useful in catalytic applications. A fundamental principle involved in this method is that an attractive interaction between the organic block copolymer and the keggin POM precursors is obtained via Coulombic interactions through in situ quaternization (protonation) of PDMAEMA part, which also ensure the formation of a homogeneous hybrid material without any macrophase separation. Further, step-wise calcinations under argon and air lead to evolution of mesoporous keggin POM material. To the best of our knowledge, this is the first hexagonally ordered mesoporous Keggin POM framework material. We have presented a low-temperature, non-hydrothermal synthesis route to rutile nanocrystals. Both rutile and anatase nanocrystals exhibit positive surface charges. In contrary to the above approaches where polymer templates are cationic and inorganic precursors are anionic, in this case, inorganic nanocrystals are cationic and polymer templates are anionic. In this approach, we have demonstrated that crystalline TiO2 nanocomposites with well-defined crystalline form could be directly synthesized at temperatures as low as 40 oC by mesostructuring the positively charged crystalline titania colloids over anionic spherical polyelectrolyte brush particles under aqueous conditions. Stepwise calcinations first under argon followed with a second calcination in air lead to the complete removal of polymer template without collapse and hollow porous spheres with crystalline framework are obtained. Porosity and surface areas increased dramatically after stepwise calcinations. Moreover, the porous rutile nanomaterials are photocatalytically active. We proved that our hypothesis to the synthesis of crystalline TiO2 nanocomposites with well-defined crystalline form and morphologie is feasible.
