24 search hits
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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.
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Soft Compartmentalized Polymer Colloids: Janus Particles, Multicompartment Structures, Inorganic-Organic Hybrids and Applications
(2008)
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Andreas Walther
- Compartmentalized polymer-based colloids with nanoscopic dimensions and different topologies were prepared based on various block copolymer architectures. The polymers were prepared via anionic polymerization or a controlled radical polymerization technique (RAFT). Self-assembly both in solution and in bulk were rigorously exploited to create the multicompartment architectures. Several new crosslinking strategies, in bulk and in solution, were thoroughly investigated to allow a controlled preservation and a high shape-persistence of the colloidal particles even when exposed to non-selective solvents. Cylindrical and disc-like Janus particles were investigated according to their self-assembly behavior into superstructures. The Janus discs undergo back-to-back stacking in organic solvent. In aqueous solution, a size-dependent aggregation was found. While the smaller Janus discs are unimolecularly dissolved with a significant polystyrene surface exposed to the water, the larger Janus sheets can shield the insoluble side by a large bending in an intramolecular fashion. Janus cylinders self-assemble on two hierarchical levels. Upon exposure to a selective solvent, they self-organize into fibers. The length of these fibers depends on the concentration and a critical aggregation concentration exists below which self-assembly is absent. Secondly, the Janus cylinders form fibrillar networks with tunable pore sizes when deposited from more concentrated solution. The surface-active properties of spherical Janus particle were exploited for the investigation of two possible applications of both academic and industrial relevance. In Pickering emulsion polymerization, extremely well-defined latexes with long-term stability could be prepared in a very facile fashion. A control of the particle size by changing the concentration of Janus particles could easily be achieved. Secondly, the nanostructuring of polymer blends was shown for a PS/PMMA model system. The system exhibits a control on two length scales. The first is the controlled decrease of the domains of the dispersed phase and the second is the controlled spacing between the particles at the interface. The particles are exclusively located at the interface and the nanostructuring can be obtained while matching macroscopic processing constraints, i.e. high-shear blending in a mini mixer. The self-assembly of bis-hydrophilic triblock terpolymers with two outer hydrophobic blocks was investigated for a variety of different hydrophilic end blocks. The overall architecture of the solution structures could be tailored by changing the hydrophobic-to-hydrophilic balance. Additionally, the interaction between the corona-forming blocks has an influence on the particle shapes as well. The micelles possess coronas with appealing and tunable properties, due to the presence of a hydrophobic core and hydrophilic biocompatible and stimuli-responsive segments. The self-assembly of miktoarm star terpolymers, bearing arms of polystyrene (PS), polybutadiene (PB) and poly(2-vinylpyridine) (P2VP), was analyzed both in solution as well as in the bulk state. In solution, the miktoarm star terpolymers form multicompartment micelles with a glassy (PS) and a soft compartment (PB), all rendered water-soluble by the P2VP corona. Strikingly, the soft PB compartments show hydrophobic bridges in aqueous medium which is of high interest as they can be used as a second motif for sensing, adhesion control or interaction with cellular membranes. The transfer of a hexagonally ordered cylindrical bulk phase via crosslinking of the PB domain of a bulk structure of a similar miktoarm star terpolymer allowed the preparation of novel multicompartment cylinders. The structures possess perfectly parallel aligned compartments. Two symmetric and opposing PS and P2VP compartments surround a central ribbon-like PB compartment. The P2VP compartments could be used to generate perfectly aligned bi-axial nanowires inside spatially separated compartments within close proximity. Due to the presence of an amphiphilic corona, the extent of the compartmentalization can be tuned from separated nanowires into one homogenous nanowire simply by exchanging the solvent. The complexity and high control of the structure of this multicompartment cylinder is unmatched and can most likely not be obtained by solution based self-assembly.
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Polymeric Nanoparticles for the Modification of Polyurethane Coatings
(2011)
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Sandrine Tea
- Rubber-based nanomodifiers were successfully synthesized following two different strategies and were used as impact modifiers in polyurethane (PU) automotive clearcoats to improve chip resistance. Various narrowly distributed polybutadiene-b-poly(methyl methacrylate) (B-M) block copolymers differing in composition and molecular weights were synthesized and studied with respect to their self-assembly in organic selective solvents. Dynamic light scattering and transmission electron microscopy measurements revealed that spherical micelles were obtained in acetonitrile for all block copolymers, independently of the polymer concentration. Their radii varied from 11 to 69 nm depending on the molecular weight of the initial linear block copolymer and their aggregation behavior in acetonitrile followed the model established by Förster and Antonietti for strongly segregated block copolymers. In DMF and acetone, block copolymers with 85 %wt PMMA were dissolved as unimers. For lower methacrylate contents, the sizes of the obtained spherical micelles were decreasing from DMF to acetone independently of the polymer concentration. The calculated interaction parameters confirmed acetonitrile as the best solvent for PMMA followed by DMF and acetone as the poorest one. The size of the spherical aggregates could be tuned by the molecular weight and/or by the nature of the selective solvent. Polybutadiene-b-poly(n-butly acrylate) (B-nBA), polybutadiene-b-poly(n-butyl methacrylate) (B-nBMA) and polybutadiene-b-poly(t-butyl methacrylate) (B-tBMA) did not show such a large choice in selective solvents and spherical micelles were obtained in DMF, DMAc and acetone respectively. Cross-linking of the polybutadiene core of the obtained micelles was performed in solution using two different methods: cold vulcanization and radical reaction upon the decomposition of a photo-initiator under UV radiation. Both methods allow retaining the spherical shape of the micelles leading to narrowly distributed non fusible nanospheres. In the case of B-M nanoparticles, the degree of cross-linking seemed independent of the amount of cross-linker used. Unlikely, B-nBMA and B-nBA nanoparticles exhibited increasing degrees of cross-linking with the amount of photo-initiator introduced. Their degrees of cross-linking were particularly lower than those of B-M nanoparticles. The hydrolysis of the t-BMA corona of the nanoparticles obtained from B-tBMA linear block copolymers self-assembly in selective solvent resulted in water soluble nanoparticles carrying acid functions and thus potentially exhibiting pH-responsive behavior. Various hyperstars consisting of a hyperbranched PB core and (meth)acrylate arms were synthesized by anionic self-condensing vinyl copolymerization (SCVCP) of divinylbenzene and butadiene followed by the anionic polymerization of the linear (meth)acrylate arms. The amount of hyperbranched products resulting from SCVCP could be enhanced by introducing additional DVB to the reaction while polymerizing. The topology of the hyperbranched PB cores was confirmed by viscosity measurements. All Mark-Houwink-Sakurada exponents were significantly below the value for linear PB. The initiation of (meth)acrylate arms was confirmed by NMR spectroscopy. Upon the arm-growth reaction, the branched topology was retained as witnessed by further viscosity measurements. The introduction of cross-linked nanoparticles based on linear block copolymers did not disturb the transparency of PU coatings. Even after curing reaction, the nanoparticles were well-dispersed into the coating. TEM observations confirmed this last result where neither aggregation nor flocculation of the cross-linked nanoparticles was observed. Hyperstar polymers were found to undergo self-assembly upon the curing reaction leading to “onion-like” structured aggregates, in the case of PMMA hyperstars, with sizes as large as 200 nm. Aggregates of the same size order were observed for the other hyperstars but no defined structures were found. For all hyperstar modified coatings, the transparency of the films was altered. In both cases, cross-linked nanoparticles and hyperstar modified coatings, improvements of chip resistance were observed. The improvements were even better with increasing amount of cross-linked nanoparticles but no effect was noticed on the hardness of the coatings. Similar trends were observed for the hyperstar modified coatings.
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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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Orientation and Phase Behavior of Block Copolymers in External Electric Fields
(2007)
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Kristin Schmidt
- The influence of external electric fields on the microdomain structure of block copolymers has been studied. The results range from an analysis of the mechanism and kinetics of the reorientation process, via the discussion of the driving force, to first investigations on the influence of an electric field on the phase behavior. The electric field induced effects on concentrated block copolymer solutions were investigated by in-situ synchrotron small-angle x-ray scattering. The first part concerns the analysis of the mechanism and kinetics of the alignment of lamellar forming diblock copolymer solutions as well as a quantitative study of the reorientation kinetics of various block copolymers in order to clarify the driving force of reorientation in a DC electric field. The second part of this thesis describes the influence of an electric field on the phase behavior of block copolymers. It is shown that a gyroid phase (G) as well as a hexagonally perforated lamella phase (HPL) exposed to an electric field undergo a phase transition to cylinders (C) and lamellae (L), respectively. Furthermore, an anisotropic deformation of the chain conformation in various lamellar and cylindrical block copolymer solutions via electric fields is demonstrated.
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Crystalline-core micelles based on triblock terpolymers with polyethylene middle blocks
(2012)
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Joachim Schmelz
- This thesis is focused on the crystallization-induced structure formation of polyethylene containing triblock terpolymers in organic solvents to surface-compartmentalized worm-like crystalline-core micelles (wCCMs). Obtaining profound knowledge of the parameters controlling the self-assembly process allowed the production of a variety of complex one-dimensional micellar architectures with many potential applications, such as adaptive surfactants.
At first, the basic parameters that control the crystallization-induced self-assembly were explored using symmetric polystyrene-block-polyethylene-block-poly(methyl methacrylate) (PS-b-PE-b-PMMA) triblock terpolymers and a PS-b-PE-b-PS triblock copolymer. In good solvents for the PE block, e.g. THF and toluene, the selective formation of wCCMs was observed over a wide range of concentration, applied crystallization temperature and polymer composition. Whereas wCCMs produced by PS-b-PE-b-PS showed a homogeneous PS corona, a patch-like compartmentalization of the corona was observed if the micelles were formed by PS-b-PE-b-PMMA. As THF shows equal solvent quality for both corona blocks, wCCMs with almost alternating PS and PMMA compartments of about 15 nm were observed in this solvent. However, if structure formation was conducted in bad solvents for PE, such as dioxane or dimethylacetamide, spherical micelles with amorphous PE cores were formed already before crystallization. Hence, the subsequent crystallization of PE resulted in spherical CCMs with a patchy or a homogeneous corona depending on the used triblock. These findings allow the highly selective production of stable spherical or worm-like CCMs from the same polymer.
As the corona structure of the patchy micelles self-assembled from triblock terpolymers was mainly deduced from transmission electron microscopy (TEM) performed on dried samples, a small-angle neutron scattering (SANS) study was performed in order to elucidate the morphology in solution. Therefore a partly deuterated triblock terpolymer was synthesized and measured at different contrasts to allow the selective detection of the different corona compartments. The resulting SANS curves could be interpreted using a form factor model for core-shell cylinders with alternating PS and PMMA hemishells including interparticle interactions, thus validating the TEM observations. Notably, Janus-type and patchy cylinders can be clearly distinguished using the applied form factor model.
Moreover, the controlled formation of wCCMs with tunable corona composition and structure was achieved using the cocrystallization of different triblock copolymers. Via random cocrystallization of PS-b-PE-b-PMMA and PS-b-PE-b-PS the corona morphology could be tuned continuously from a mixed corona at low PMMA content over spherical PMMA patches of increasing number and size to alternating PS and PMMA patches. This approach allows to manufacture wCCMs with predefined corona structure omitting the need to synthesize a new tailor-made triblock terpolymer for every desired morphology.
By establishing the controlled crystallization-driven self-assembly of triblock terpolymers with PE middle blocks, it was further possible to prepare wCCMs with predefined average lengths up to 500 nm and length polydispersities as low as Lw/Ln = 1.1. Here, self-assembled spherical CCMs of PS-b-PE-b-PS were used as seeds for the controlled growth of PS-b-PE-b-PS unimers. Upon further addition of PS-b-PE-b-PMMA unimers these grew epitaxially onto the preexisting wCCMs, resulting in triblock co-micelles that consisted of middle blocks with a homogeneous PS corona and outer blocks with alternating PS/PMMA compartments. These structures represent not only the first block co-micelles including blocks with a patchy corona, but also the first ones produced from purely organic block copolymers.
In view of application, the ability of patchy wCCMs formed by PS-b-PE-b-PMMA to stabilize interfaces was investigated using pendant-drop tensiometry. The observed reduction of the interfacial tension at the toluene/water interface was significantly higher than that of comparable triblock terpolymer single chains and that of wCCMs with a homogeneous PS corona. Interestingly, the obtained equilibrium interfacial tension equaled that of Janus cylinders with similar dimensions. To explain this unexpected finding the corona chains were proposed to adapt to the interface via selective collapse and shielding of the incompatible part of the corona chains. Studying wCCMs formed by several triblock terpolymers with different compositions, the interfacial activity was found to increase with increasing overall length of the corona chains, and to a certain extent with the molar fraction of PS units in the corona.
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Novel Precursors for Polymer-Protein-Conjugate Synthesis via Reversible Addition-Fragmentation Chain Transfer Polymerization
(2003)
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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
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"Smart" Hydrogels based on Trishydrophilic Triblock Terpolymers
(2010)
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Stefan Reinicke
- The work presented in this thesis focuses on the synthesis of double stimuli-responsive, trishydrophilic triblock terpolymers and their utilization for the construction of “smart” hydrogel systems, responding to a variety of external stimuli. The central focus was put on ABC triblock terpolymers composed of a pH-sensitive A block, a water soluble B block and a thermo-sensitive or multi-responsive C block. This concept was used for the construction of hydrogels responding independently to pH, temperature, and UV light. It was further applied to the formation of polymer/nanoparticle hybrid micelles suitable for the formation of magneto-responsive hydrogels (ferrogels). At first, a new route for the synthesis of block copolymers, containing ethylene oxide and glycidol derivatives, was developed. The crucial aspect of this procedure, based on sequential anionic polymerization, was the utilization of the phosphazene base t-BuP4, enabling the anionic polymerization of epoxide monomers in the presence of lithium counterions. It was shown, that ethoxyethyl glycidyl ether polymerizes easily under the established polymerization conditions without unwanted termination. Hence, we were able to synthesize well-defined block copolymers containing vinyl and epoxide monomers in a one-pot reaction, without performing additional intermediate steps. This new synthetic route was then utilized to synthesize a series of poly(2-vinylpyridine)-block-poly(ethylene oxide)-block-poly(glycidyl methyl ether-co-ethyl glycidyl ether) (P2VP-b-PEO-b-P(GME-co-EGE)) triblock terpolymers suitable for pH and temperature dependent hydrogel formation. The reversible gelation for this particular system relies on two distinct mechanisms. Under conditions, where only one outer block is insoluble, core-shell-corona (CSC) micelles are formed, resulting in gelation via close cubic packing of the micelles. On the other hand, the micelles are also able to crosslink through their corona when both outer blocks are insoluble. As a direct consequence, a temperature triggered gel-sol-gel transition occurred at pH = 7, accompanied by a unique gel strengthening. Solubility and gelation studies were performed by DLS, rheology and SANS. The influence of polymer concentrations and block lengths on the gelation behavior and gel properties was studied. In order to derive information about the exact structure of the cubic lattice formed in the low temperature gel phase (simple cubic or body centered cubic), a 19 wt% aqueous solution of a particular P2VP-b-PEO-b-P(GME-co-EGE) triblock terpolymer at pH = 7 was further investigated using SANS under steady shear. By application of shear stress, the irregularly arranged polydomains of the sample oriented macroscopically along a preferred direction, which led to highly defined, strongly anisotropic 2D scattering patterns. The interpretation of these patterns confirmed the presence of a body centered cubic packing. The gel-sol transition upon temperature increase can be explained by a shrinkage of the shell of the CSC micelles. To increase the versatility of the established hydrogel concept, we further synthesized ABC triblock terpolymers with different responsive polymers as C blocks. This required an alternative synthetic route, combining anionic polymerization and ATRP via “click” chemistry. After optimization of each synthetic step, exemplary poly(2-vinylpyridine)-block-poly(ethylene oxide)-block-poly(oligo(ethylene glycol) methacrylate) (P2VP-b-PEO-b-POEGMA) and poly(2-vinylpyridine)-block-poly(ethylene oxide)-block-poly(dimethyl- aminoethyl methacrylate) (P2VP-b-PEO-b-PDMAEMA) triblock terpolymers were synthesized, respectively, and characterized regarding their solubility and gelation behavior. At pH > 5, P2VP-b-PEO-b-PDMAEMA forms CSC micelles with a P2VP core, and a pH- as well as thermo-sensitive PDMAEMA corona. This particular structure represents a hydrogel, whose temperature dependent response can be easily changed from a gel-sol to a sol-gel transition by increasing the pH from 8 to 9. At pH = 7.5 on the other hand, gel formation is induced by the addition of hexacyanocobaltate(III) ions due to electrostatic interactions between the multivalent cobaltate ions and the charged DMAEMA units, causing a physical crosslinking of the CSC micelles. The gel can subsequently be disintegrated by an exposure to UV-light, based on a UV-catalyzed aquation of the trivalent hexacyanocobaltate(III) ions ions to divalent aquapentacyanocobaltate(III)-ions. In the last part, a new approach was developed to create a novel type of magnetic field-responsive hydrogels (ferrogels), in which the nanoparticles are tightly bound to the polymer matrix. The P2VP block of the previously synthesized P2VP-b-PEO-b-P(GME-co-EGE) triblock terpolymers was quaternized to a low extent and complexed with negatively charged, citrate stabilized maghemite (γ-Fe(III)-oxide) nanoparticles. Using different analytical methods it was shown that well-defined CSC hybrid micelles were obtained with cores formed by a complex of P2VP and 3-4 nanoparticles per core. Concentrated solutions of these micelles are able to form gels depending on temperature, as revealed by rheology measurements. Due to the presence of the maghemite particles, it is possible to induce gelation via remote heating using AC magnetic fields, which was demonstrated by high frequency magnetocalorimetry.
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Nanostructure formation in thin polymer films
(2004)
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Nicolaus Rehse
- In the first part of this thesis an improved process is presented to prepare laterally structured substrates via hierarchical self organization. A miscut silicon surface annealed at 1400 K under ultra high vacuum conditions is used. The resulting facets are stable against oxidation and form a topographic pattern which can be further modified to a chemical pattern via evaporation of gold on every other facet. By controlling the time of annealing, we create structures with a reproducible mean width ranging from 40 to 400 nm. Despite the rather complex ultra high vacuum treatment and an additional evaporation step, we are able to produce substrates in a relatively short time (36 h). These substrates show a nanometer sized structure over an area of 0.5 cm². The automation of the cleaning process and a controlled heating during the annealing increases the yield of high-quality, stepped substrates. These structures allowed us to study the behavior of ultra-thin polystyrene films on topographically structured substrates. The film thickness of some nanometers is comparable to the radius of gyration of the polymers. The substrate corrugation causes a regular variation of the film thickness. We start with a homogeneous film, which is annealed above the glass transition temperature. During annealing the films are stable or form long polymer nanochannels, which lie in the grooves of the substrate structure. The balance of the radius of gyration and the film thickness controls the stability of the polymer film, while the corrugation only triggers the dewetting. The same behavior is found for films on flat substrates. Here small contaminations nucleate the formation of holes. Evaporation of gold stripes and their modification with self assembled monolayers leads to chemical patterned substrates. This expands the possibilities to manipulate the substrate wettability on the nanometer scale. The second part of the thesis describes the formation of ordered structures in block copolymer films. ABC triblock copolymers show a large variety of morphologies in thin films. We have shown that surface reconstructions play an important role in the structure formation process of these structures. In very thin films, where the film thickness is smaller than the long period of the polymer's micro domains, confinement effects overlap with the surface effects. The component with the lowest surface energy is accumulated at the free surface. It needs a subtle balance between the different surface energies (external fields) and the interaction of the three polymer blocks (internal fields) to create a surface reconstruction. This was shown by variation of the chemistry of the end block and by changing the sequence of blocks in the experiment. To analyze the surface reconstruction we used selective staining along with scanning electron microscopy, selective etching in oxygen plasma in combination with scanning probe microscopy, as well as quantitative TappingMode atomic force microscopy. Surface reconstructions of block copolymers show remarkable similarities with reconstructions of single crystal surfaces. In both cases the driving force for a rearrangement is the decrease in surface free energy of the ideal surface. A second analogy between the lamella forming SBM triblock copolymer and Si(100) is the fact that two non-equivalent layers of matter aligned parallel to the free surface lead to two different terminations at the surface. This shows that the phenomenon of surface reconstructions is not limited to classic crystals. The results of this thesis give new insights in the behavior of polymers at surfaces and in thin films. This gives the opportunity to create or manipulate nanometer sized structures accurately via self assembly, external stimuli, or a combination of both.
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Synthesis, Characterization, and Properties of Tailored Functional Block Copolymers
(2011)
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Robin Pettau
- This thesis covers the design, synthesis, characterization, and application of functional block copolymers (BCP) based on a polymer analogous approach and includes three main subjects. The first subject is the implementation of a specially constructed reactor setup for sequential anionic polymerization that allows parallel block copolymer synthesis based on one identical A-block on a lab scale. For this reason, this setup facilitates the preparation of block copolymer series in a combinatorial fashion. It consists of one main reactor and three secondary reactors with individual temperature control. The addition of monomers or additives to each reactor can be handled separately. AB diblock copolymer and ABC triblock copolymer series were prepared with different lengths of the final block as well as different chemical structures of the last block. The second subject covers the synthesis, characterization, processing and application of new liquid crystalline azobenzene-containing block copolymers designed as materials for holographic data storage. Therefore, these polymers contained an amorphous, optical inert poly(methyl methacrylate) (PMMA) or polystyrene (PS) matrix and a functional segment based on polyhydroxystyrene (PHS). Different lengths of flexible spacers and/or mixtures of two spacer lengths were employed to connect the mesogenic chromophores to the polymer backbone. The structure-property relation of functionalized BCPs and the resulting mesophase was investigated. Holographic experiments were conducted on selected examples of the photo-addressable polymers. Smectic annealed samples or amorphous quenched samples were obtained by different sample preparation methods to investigate the influence of the liquid crystalline order. While the initial sensitivity to light induced orientation of the polymer systems remained unaffected, the writing times and level of postdevelopment were improved for quenched samples. Variation in spacer lengths resulted in decreasing smectic order with decreasing spacer length as well as for mixtures of two different spacer lengths promoting lower writing times in the holographic experiments. Additionally, the temperature dependence of the temporal evolution of the refractive index modulation in the smectic polymers was studied. A significant decrease of writing times and an enhancement of the postdevelopment were revealed at elevated temperatures. Stable holographic gratings could be obtained even at 100 °C. 1.1 mm thick samples, that are a prerequisite for volume holographic data storage with a high data storage density, were prepared by injection molding of blends of photoaddressable BCPs with PMMA or PS. Preliminary results confirmed the long-term stability of inscribed holographic gratings and demonstrated angular multiplexing of holographic volume gratings. The third subject covers the synthesis and characterization of new cyanobiphenyl-containing ABA triblock copolymers and their application as BCP gelators for the low molecular weight liquid crystal (LC) 4-cyano-4’-(pentyl)biphenyl (5CB). Based on the selective solubility of the A and B blocks in the nematic solvent, ABA triblock copolymers can be used for the thermoreversible gelation of 5CB. To this end, ABA and ABA’ triblock copolymers comprised of PS A-blocks and a cyanobiphenyl-functionalized PHS B-block with a high degree of polymerization were prepared by the combination of anionic polymerization, using two different synthetic routes, and polymer analogous attachment of the mesogens. Series of linear gelators were prepared with variations in B-block length, A-block lengths and star shaped BCPs by coupling linear ABA’ triblock copolymers. Structure-property relations of the cyanobiphenyl-functionalized polymers regarding the mesophase characterization revealed a dependence of solubility in the nematic 5CB on spacer length. A comprehensive study was conducted to investigate the influence of the BCP backbone and architecture on the gelation of 5CB. Oscillating rheology measurements and thermal characterization were employed to investigate the thermoreversible LC gels. Most of the BCP gelators achieved gelation of 5CB at a mass concentration of 5 wt%. The properties of the different gels where compared at this fixed concentration. The influence of the gelator backbone on the gel properties was investigated by comparing different sets of triblock copolymers. While a short functionalized B-block resulted in high network density and, thus, a high elasticity of the gel the length of the A-blocks influenced the node stability. The LC gel using a star-shaped gealtor exhibited a significantly higher elasticity than with the respective linear block copolymer gelator.
