Dissertation/Thesis Abstract

Mesostrukturierte Metalloxide und Polyoxometallate mittels Ionogener Diblockcopolymere - Synthese, Charakterisierung und Anwendung
by Lunkenbein, Thomas, Dr.Nat., Universitaet Bayreuth (Germany), 2013, 250; 10693293
Abstract (Summary)

This thesis focuses on the synthesis, characterization, and application of mesostructured metal oxides and polyoxometalates templated by ionogene diblock copolymers. In particular three previously reported problems hampering the formation of mesostructured metal oxides were addressed. Fast hydrolysis and condensation of reactive metal alkoxide-precursors foster the formation of large and unstructurable oxidic precursors. Furthermore, a simple and direct access to the desired morphology is often inhibited by the dynamic nature of the micelles formed by the template which is influenced by changes in pH, concentration, or ionic strength. Finally, the weak connection between the inorganic precursor and the organic template is attributed to weak attractive interactions between both materials and therefore limits the design of a periodic mesostructure. In general, hydrolysis, condensation, and hybrid formation have to proceed on the same time frame to reliably avoid macrophase separation. To overcome this macrophase separation the department of ACI has recently developed a concept that addresses and solves all three problems. In the following these approaches are explained in the order mentioned above. Inorganic oligomeric clusters or pre-synthesized colloids replaced fast hydrolyzing and condensing metal alkolxides as inorganic precursors. Those clusters do not tend to condense since they occupy energetic minima on the condensation pathway toward extended oxidic structures. In addition, 1-dimensional core-crosslinked polymer brushes were employed for overcoming, the dynamic nature of the micelles. Those rigid cylindrical polymer brushes were invariant against outer influences. Furthermore, those 1-dimensional nanoobjects exhibited ionizable coronas which were sufficiently basic to be easily protonated. Therefore, the strong Coulomb interactions formed guarantee a strong connection of the inorganic oxides to the organic templates. Within this thesis the concept of charge-induced mesostructuring using molecular oxidic precursor was extended as presented hereafter. Foremost, pre-synthesized rutile and anatase colloids were incorporated into 1-dimensional polymer structures. The crystal structure of rutile and anatase could be directed by the use of different acids (HCl: rutile; acetic acid: anatase) in the hydrolysis reaction of the titanium alkoxide precursor. Those TiO2 polymorphs possessed positive zeta-potential at low pH-values. Hence, the template required negative charges at a pH around one. Therefore, in cooperation with the department MC II poly(styrenesulfonic acid-block-allylmethacrylate) (PSS-b-PAMA) were synthesized as cylindrical anionic antagonists. The PSS-corona is negatively charged even at low pH-values. The resulting 1-dimensional hybrid materials exhibited the same crystal structure as the precursor, a homogenous distribution of the nanoparticles in the polymer matrix, and a non-woven spaghetti-like microstructure with relatively high specific surface area after drying. In a more detailed study the packing of the resulting 1-dimensional hybrid materials and their corresponding specific surface areas were investigated as a function of the length of the cylinders. In this study, cylindrical hybrid materials were used that consisted of heteropoly acid of the Keggin-Type polyoxometalate (Keggin POM) incorporated into 1-dimensional poly(butadiene-block-2-vinylpyridine) (PB-b-P2VP) polymer brushes. The aspect ratio of the 1-dimensional hybrid structures was found to vary only marginally with short sonication times. Furthermore, this short sonication resulted in a perfect dispersion of the anisotropic hybrid materials, and minimized the interparticular contact points within the microstructure. As a result, those pre-treated hybrid materials exhibited a maximum accessible surface area. An extension of the described synthesis protocol could be successfully applied toward Keggin POMs with varying charges, heteroatoms or metal cations. Catalysis tests of the hybrid cylinders revealed differences in the activity of the nanorods in the acidic catalyzed decomposition of isopropanol. To enlarge the idea of charged-induced mesostructuring a new synthesis concept was developed yielding inverse hexagonally ordered polymer/Keggin POM mesophases. In this context poly(buadien-block-2-dimethylaminoethyl methacrylate) (PB-b-PDMAEMA) with a high degree of polymerization of the PB block was synthesized as organic template. Furthermore, the choice of solvent and the Keggin POM content were key factors within this process. To render this material interesting for diverse applications such as catalysis, the polymeric template has to be removed completely. Therefore, a systematical study was conducted. This study was conducted with thermal as well as aggressive methods. Amongst others the systematic head treatment studies showed that the mesostructure had been collapsed before the carbon was removed completely. Aggressive methods, such as plasma etching, allowed successful access to the mesopores of microtome cut (< 50 nm) nanocomposite films. Addressing the heat treatment of the as-synthesized ordered polymer/ Keggin POM the oxidic walls were carburized resulting in ordered carbide/ carbon nanocomposites. Ordered carbide/ carbon nanocomposites can be considered interesting for catalysis or charge storage. For this purpose the polymeric template was not removed, but instead used as carbon source within the carburization reaction. Hence, the as-synthesized ordered mesophases gained direct and simple access to porous carbide/ carbon nanocomposites and showed catalytic activity in the decomposition of ammonia. This work is a cumulative thesis. The detailed results are described in the attached publications.

Indexing (document details)
School: Universitaet Bayreuth (Germany)
School Location: Germany
Source: DAI-C 81/1(E), Dissertation Abstracts International
Subjects: Polymer chemistry
Keywords: Diblock copolymers
Publication Number: 10693293
ISBN: 9781392821329
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