Buch, Englisch, 120 Seiten, GB, Format (B × H): 148 mm x 210 mm
Buch, Englisch, 120 Seiten, GB, Format (B × H): 148 mm x 210 mm
ISBN: 978-3-86387-769-9
Verlag: Mensch & Buch
Diese Thesis basiert auf der Anwendung der Kernmagnetikresonanzspektroskopie (NMR) in Kombination mit dynamischer Kernspinpolarisation (DNP) zur Untersuchung von Molekülstrukturen auf der Oberfläche von Zellulosematerialien. Da DNP-verstärkte NMR die Sensitivität um mehrere Größenordnungen erhöht und dadurch die Messung von Kernen wie 13C und 15N in ihrer natürlichen Häufigkeit erlaubt, ist es ein bedeutendes Instrument zur Analyse der chemischen Strukturen von heterogenen Katalysatoren und funktionalisierten Cellulose Materialien. Es bedeutet auch, dass diese Methode ein gutes Mittel darstellt, um die chemische Struktur der Celluloseoberfläche von Nano-Cellulose bis hin zu Papiermaterialien zu erforschen. An drei Beispielen wurde gezeigt, wie effektiv die Anwendung von DNP bei der Strukturbestimmung von funktionalisierten Zellulosematerialien und Papierhybridmaterialien ist.
Als erstes Beispiel wurde ein Material von auf kristalliner Nanozellulose gedoptem Imidazol (Cell-Im) mit 13C and 15N CPMAS NMR und DNP untersucht, welches ein hervorragender fester Protonenleiter für Brennstoffzellen ist. Interaktionen und Dynamiken der Imidazol-Wasserstoffatome an der Zelluloseoberfläche wurden detailliert charakterisiert. Zuerst wurde die lokale Struktur des Imidazols an der Oberfläche der Zellulose von Cell-Ims mittels 13C CPMAS Spektroskopie bei 105 K (DNP) und bei Raumtemperatur (NMR) untersucht. Danach wurden durch 15N CPMAS NMR Experimente an Cell-Im-15N, Cell-Im und Im, unter DNP Bedingungen, zwei verschiedene Arten von Imidazol entdeckt (kristallin und amorph). Weiterhin deuten die 15N CPMAS NMR und 1H-15N HETCOR Experimente von Cell-Im-15N bei unterschiedlichen Temperaturen darauf hin, dass bei den Protonen in Cell-Im Dynamiken vorliegen.
Diese Versuche wurden durchgeführt, um schnelle Austauschprozesse der Protonen zu beobachten und um die Austauschraten, sowie die Verteilung der Aktivierungsenergien für die Prozesse in diesem Material zu bestimmen. Um darüber hinaus die Interaktionen zwischen dem Imidazol, dem Wasser und der Zellulose zu untersuchen, wurden 1H-15N HETCOR Experimente bei verschiedenen Temperaturen durchgeführt. Hier wurde festgestellt, dass das Imidazol an der Zelluloseoberfläche durch zwei Arten von Wasserstoffbrückenbindungen stabilisiert wird, nämlich durch Bindungen zu Zellulose-OH-Gruppen und zu Wassermolekülen. Gemäß der erhaltenen NMR Daten, wurde ein möglicher Protonentransfermechanismus im Cell-Im Material vorgeschlagen, welcher die hoch effiziente Protonenleitung in diesem Material bewirkt.
Im zweiten Beispiel wurde durch DNP-verstärkte NMR die Struktur von Rhodaminderivaten (RhB) aufgeklärt die an kristalline Nanozellulose (Cell-RhB) immobilisiert sind. Solche Verbindungen sind sensitiv gegenüber äußeren Einflüssen, wie dem pH-Wert, Hitze und UV-Licht. Die Anwesenheit von RhB in Cell-RhB wurde durch AFM und FT-IR bestätigt. Die Carboxyl-Bindungsstelle wurde durch DNPverstärkte 13C CPMAS NMR nachgewiesen. Farbwechselexperimente mit Cell-RhB wurden durchgeführt, wobei der optische Farbwechsel einem Ringöffnungs- und Schließungsprozess der Rhodaminspiroamidgruppe zuzuschreiben ist, welcher genauer mit 15N DNP NMR untersucht wurde. Die Verlagerung der 15N NMR Signale in den DNP Experimenten deuten an, dass während der Hitzebehandlung durch elektrostatische Interaktion innerhalb der begrenzten Umgebung an der Zelluloseoberfläche eine temporäre Bindung aufgebaut wird.
Im dritten Beispiel wurden Festkörper-NMR-Techniken angewendet, um die Struktur von, mit Benzophenon enthaltenden Copolymer (P(DMAA-co-MABP)) modifizierten, Papiermaterialien zu erforschen. Um die kovalenten Bindungen zwischen Copolymer und Zellulose zu identifizieren, wurde die Struktur von Zellulose, dem Copolymer und dem modifizierten Papier mittels konventioneller 13C CPMAS NMR untersucht. Zudem wurde die kovalente Bindung mit dem Copolymer mit DNP-Verstärkter 15N CPMAS bestätigt.
Autoren/Hrsg.
Weitere Infos & Material
Cellulose is an environmental friendly and widely used polymer, which shows many interesting properties, making it a versatile material for a wide range of applications, such as superhydrophobic cellulose, pH/temperature-responsive cellulose and low-cost paper-based diagnostic devices. To design custom-made cellulose or hybrid paper materials, their chemical structures need to be explored in detail on the molecular level.
Cellulose is a polysaccharide composed basically of linear chains of anhydroglucose repeat units linked together through ß (C1?C4) glycosidic bonds. The polymeric order, chain length and chains association confer to cellulose-based material good mechanical properties and insolubility in water or diluted acids and alkalis at ordinary temperatures.
Because of the insolubility of the modified cellulose materials, solid state analysis techniques are required. Because of the disordered structure of the modified cellulose samples in this thesis, traditional techniques for the chemical structure characterization such as powder/single-crystal X-ray, atomic force, and electron microscopy are only of limited use, since they often require samples with a long-range order. In such case, solid-state NMR technique can be used in the analysis of amorphous samples, which probes the local structure through a NMR-active nucleus. However, the current solid-state NMR spectroscopy suffers greatly from its low sensitivity. In particular, the low concentration of the functional groups on the cellulose surface makes the solid state NMR characterization hard to perform. E.g. in these systems NMR experiments on natural abundance nuclei (e.g. 15N, 17O) may need weeks to years to get an acceptable result. In this thesis, DNP-enhanced 13C, 15N CPMAS combined with conventional NMR techniques are applied to the surface structural changes in cellulose-based derivatives.
In the past decade, the low sensitivity of conventional NMR was significantly improved by the availability of commercially high-field Dynamic Nuclear Polarization (DNP) spectrometers. DNP is a method that permits NMR signal intensities of samples to be greatly enhanced, and is therefore potentially a tool for structural and mechanistic studies of cellulose coated molecules. In a DNP experiment, the large polarization of an unpaired electron (e.g. radical) is transferred to the nuclei of interest by microwave irradiation. For example, the maximum theoretical enhancement achievable is ca. 660 for protons. DNPenhanced NMR (DNP-NMR) has been applied to study material structures which could not be measured before. Furthermore, the implementation of DNP-enhanced multidimensional NMR experiments (e.g. DQ–SQ, HETCOR) in a time equivalent to that needed for isotopically labeled systems opens a door for the detection of interactions between nuclei in the material.
In the present thesis, DNP-enhanced solid-state NMR spectroscopy is employed to study cellulose fibers functionalized with various organic molecules. The efficiency of DNP-NMR technique allows the structure determination of the surface modified cellulose/paper materials and furtherly reveal the mechanism of their structure change.
In the first part of the thesis, we study the hydrogen bonding interactions between imidazole (Im), cellulose (Cell) and water in Imidazole doped cellulose materials (Cell-Im) by a combination of conventional NMR and DNP-enhanced solid state NMR. Imidazole doped cellulose materials (Cell-Im) with a content of imidazole of 1.0 mmol g-1, which shows excellent intermediate temperature (ca. 433 K) proton conductivity with high application potential as proton exchange membrane for fuel cells. NMR technique is applied to explore the structure of this complex system and the mechanism of the proton transport of Cell-Im, detailed characterization is performed employing a combination of DNP and variable temperature solid-state NMR techniques. DNP-enhanced 13C and 15N CP NMR measurements are applied to reveal the interactions between cellulose and imidazole via hydrogen bonds. Variable temperature 15N CP NMR experiments in the range between 243 K to 353 K illustrate proton exchange dynamics on the imidazole with their corresponding activation barriers which are employed for deeper understanding of the proton transport mechanism in Cell-Im. Combining with the results of 1H-15N HETCOR NMR spectra which illustrate the connectivity between protons and nitrogen atoms of imidazole a more detailed picture on the proton transport is derived. The NMR experiments provide a set of valuable information to understand the process in the imidazole-based high-temperature proton conductivity materials.
To compare with physically adsorbed cellulose materials, the covalent bond modified cellulose show better stability and recyclability performance, especially when the materials is applied in the solution. In this thesis, rhodamine spiroamide groups are covalently bonded with the cellulose fiber(Cell-Im) leading to a hybrid compound being responsive to heating, pH-value, and UV light, which shows great potential in the paper-based sensor. The samples Cell-Im were prepared by our cooperator in a parallel project. It was hitherto unknown how the chemical structures of active functional groups are affected during their dynamic switching process in a confined environment on the cellulose surface. As referred to earlier reports, the fluorescent and correlated visible color change can be provoked, which assigns to a ring opening and closing process. Amine and amide groups in rhodamine spiroamide play the crucial role during this switching process of the fluorescence and correlated optical color.
In our case, rhodamine spiroamide groups covalently bonded with cellulose show a low doping concentration, ca. 0.2 ± 0.01 mmol g-1. As a consequence, the structure determination of rhodamine spiroamide groups on cellulose surface constitutes a significant challenge. Herein, solid-state NMR spectroscopy combined with signal-enhanced DNP was used to measure 13C and 15N in natural isotope abundance, allowing the determination of structural changes during the switching process.
Cellulose-based fibers show rarely thermal expansion, which opens the door for further the multifunctions of paper. The applications of paper as a novel device platform have attracted an increasing interest during the last decade, with a focus on paper-based microfluidic devices. Numerous studies aimed to covalent graft functional polymers with paper substrates. To design chemically and thermally stable paper materials, a covalent linkage between the copolymer material and the cellulose fibers inside the paper is critical. Recently, the benzophenone-containing copolymer modified paper attracts lots of attentions. Since benzophenone group is activated by UV-light and consequently covalently bond with aliphatic groups in the cellulose fibers. The copolymer is often used to enhance cross-linking of the cellulose fibers and to increase the wet tensile strength of paper. A hydrophilic copolymer (P(DMAA-co-MABP)) was immobilized on the surface of cellulose fibers and was prepared in a parallel project. However, the reactions between the benzophenone and aliphatic groups are not selective. Lots of efforts have been taken to prove the existence of the linkage copolymer-copolymer or copolymer-cellulose. Because of the signal overlapping and the low concentration of the copolymer on the cellulose fibers, it is hard to provide the convincible evidences for the covalent linkage.
In this thesis, the photoactive copolymer polydimethylacrylamide-co-methacrylate-benzophenone (P(DMAA-co-MABP)) was immobilized in the paper with the content of MABP is 6 * 10-4 mmol g-1. First, 13C CPMAS NMR was applied for the unambiguous identification of the copolymer and cellulosespecific bands and thus the linkage between copolymer and cellulose. The signal intensity of the C6 in cellulose decreases after the reaction with P(DMAA-co-MABP), which indicates the occurrence of copolymer-cellulose linkage. Furthermore, 15N DNP-NMR were employed to prove the linkage between copolymer-copolymer. Moreover, the various DNP enhancements with different radicals illustrate the distribution of the radical in the modified paper. All the information obtained from NMR spectra provide a deeper view of the benzophenone containing copolymer modified paper structure, which contributes to a deeper understanding of how the copolymer influence local and overall properties.
This thesis is organized as follow: An introduction of the whole thesis is presented in the Chapter 1. Chapter 2 summarizes the theory of solid-state NMR, as well as a short description of DNP techniques and radicals. Chapter 3 shows the general introduction of cellulose material and the application of DNP NMR study cellulose materials. In chapter 4, we present the application of conventional solid-state NMR technique with DNP techniques to explore the structure of imidazole doped cellulose material. The 15N chemical shifts revealed the chemical environment of the imidazole in Cell-Im. According to the NMR data, the proton transportation mechanism for the Cell-Im is depicted. Chapter 5 describes the application of DNP-NMR for the structure determination of the multi-responsive cellulose nanocrystalrhodamine materials (Cell-RhB). The Cell-RhB samples were prepared with different pH solutions and studied through DNP-enhanced 13C and 15N solid-state NMR. Chapter 6 describes the application of DNPNMR to investigate benzophenone-containing copolymer (P(DMAA-co-MABP)) modified paper. Chapter 7 summarizes the whole results obtained in this thesis.
