Hartwig, Andreas
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Hartwig, Andreas
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Hartwig, Andreas
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Item type:Publication, Peptidbasierte Materialentwicklung(2012-07-26); ; ; Adhesion represents a key joining technology and nature has been developing adhesives for millions of years. These biological adhesives can be found in all natural habitats, where they perform extraordinary bonding, partly under extreme conditions. For example, the blue marine mussel Mytilus edulis is able to adhere to a number of surfaces in sea water: e.g. glass, plastic, metal, wood, and even PTFE. The transfer of these bonding properties to technical and medical adhesives represents a major challenge, but also could lead to enormous benefits for industrial technology or medical treatment. Although the composition of protein based adhesives, as the ones of marine mussel are well known currently, the exact principles of the initial interactions with surfaces remains unclear. For the development of new bioinspired adhesives a potential way is the assessment of the adsorption properties of some mussel inspired peptides towards titanium and silica. For evaluating the influence of the essential amino acid DOPA, derivatives of the mussel peptide Mefp-1 were synthesised by solid phase peptide synthesis. The effect of sequence length and oxidation state was investigated using the quartz crystal microbalance (QCM) technique. This method was supplemented with XPS-analytics. Additionally, the tensile strengths of peptide glued tooth fragments were determined. Furthermore the eligibility of the peptide to perform e.g. with adhesional properties, after it was bioconjugated or immobilised on a surface, were investigated.doctoral thesis349 179 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Gezielte Einstellung der Morphologie kationisch polymerisierter Epoxidnetzwerke zur Implementierung funktioneller Eigenschaften(2016-11-04); ; ; Material improvements in polymer science are often based on morphology adjustment through incorporation of additives, especially telechelics. For this, a view on natural materials, such as bone, spider silk, human enamel, or nacre reveals that crystallinity is a dominating factor for improved mechanical properties. Furthermore, synthetic materials, e.g. elastomers, thermoplastics, and shape-memory polymers have been pointed out to show functional as well as increased mechanical properties by a defined crystallinity. The use of crystalline domains in thermosets, such as epoxy resins, is only rarely discussed in the literature. This work shows how enhancements in both strength and toughness are achievable in cationically polymerized, epoxy based thermosets due to a semi-crystalline character of the resulting network. Furthermore, functional properties are generated by the materials morphology and composition. The investigations of these materials show combinations of versatile thermo-mechanical properties with moldability, shape-memory behavior, and stress relaxation performance which emphasize them as smart materials. Semi-crystalline polyester polyols, such as poly( -caprolactone) or poly(omega-pentadecalactone), are suitable telechelics to introduce crystalline domains in epoxy networks. Additionally, these telechelics are able to react with the epoxy resin due to a chain transfer reaction known as activated monomer (AM) mechanism. Crystallinity adjustment in epoxy resins is possible by the control of reaction mechanisms and reaction conditions during cationic polymerization. For this, an enhanced segregation into crystalline domains as well as nano-domain formation is observed by suppressing the AM mechanism e.g. by esterification of the polyester polyol end groups or by increased reaction rates due to high curing temperatures. In the last case, the polymerization by epoxide propagation proceeds preferably compared to the AM mechanism. Strong phase separation during polymerization leads to both enhanced strength and toughness, but also to superior adhesion properties when the crystal sizes are small; preferably below one micrometer. Moldability of those crosslinked networks has been observed under certain conditions due to the occurrence of a transesterification reaction. It is shown that reaction conditions and the choice of polyester are responsible for the occurrence of crystallinity and; furthermore, for its unique properties as universal smart material.doctoral thesis455 408 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Intrinsische Selbstheilung von Beschichtungssystemen(2018-12-18); ; ; In the present dissertation, two novel polyurethane-polyurea-based polymeric materials equipped with a high number of reversible hydrogen bonds (HBs) were examined in detail and characterized for their intrinsically self-healing properties. The chemical (covalent bonds) and physical (HB) network density of the polymers was varied by the self-healing reagent 1- (2-aminoethyl) imidazolidone (UDETA). Two different stimuli were identified for successful self-healing - increased relative humidity (23 degree C, 22-90 % RH) and elevated temperature (80-130 degree C). For self-healing at room temperature the presence of reversible HBs which provide higher molecular dynamics of the binding partners for self-healing process after bond breakage is of central importance. This circumstance was experimentally confirmed by the (temperature)-modulated differential scanning calorimetry (MDSC) measurement. It was found that the polymer absorbed water which is "strongly" bonded via HB within the polymeric network. This water causes a plastification of the material. Time domain Nuclear Magnetic Resonance (TD-NMR) measurements showed that with increasing UDETA content, a higher mobile fraction is present in the polymer. The mobility of the polymeric chains can be increased by incorporation of water, achieving a positive effect on the self-healing efficiency of the polymers. The importance and availability of different HB species in the polymer were examined in detail with Fourier Transformation Infrared (FT-IR) spectroscopy after equilibrating the samples with D2O. With increasing UDETA content the self-healing polymers show an increasing number of free carbonyl groups which were not bound via HB with varying strength. This work has successfully demonstrated the synthesis of intrinsically self-healing polyurethane-polyurea polymers and elucidated fundamental mechanistic questions of self-healing. The investigations underline the high potential of this investigated coating in a potential application.doctoral thesis491 430 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Gezielt abbaubare Polymersysteme: Synthese und Degradationsmechanismen(2011-08-22); ; ; Polymers containing UV-labile groups were prepared and decomposition by UV irradiation with low dose rates was examined. Two different degradation principles were used: direct cleavage of triazene groups and catalytic reaction of carbonate groups with a strong acid. The reaction of the UV labile group and the change of the mechanic properties due to the reaction were analyzed. In contrast to high dose rates no ablation of the triazene containing polymers took place but cross-linking. The cross-linking reaction is explained by the addition of a radical formed during irradiation to an aromatic ring. In contrast the polycarbonates could be easily degraded at higher temperatures but the degree of degradation was strongly dependent on the nature of the acid.doctoral thesis339 211 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Aufklärung der elektrochemischen Mechanismen bei der Anodisierung in hochviskosen Elektrolytlösungen(2017-05-19); ; ; A novel pressure-sensitive adhesive tape was used for anodization of aluminum. The main difference between this anodization tape and conventional bath anodization is the viscosity of the electrolyte. While the viscosity of bath anodization is in the same range of water, the viscosity of the adhesive matrix is higher by seven orders of magnitude. The impact of the high viscous electrolyte on the anodization process and the morphology of the anodic oxide layers was investigated. The limitation of mass transport in the high viscous electrolyte slows down oxide formation and dissolution during anodic oxide growth. With in-situ electrochemical impedance spectroscopy a reduction of the effective area during tape anodization could be determined. Step-like voltage changes during the anodization process lead to the formation of different sublayers in the porous oxide layer, which allow the calculation of oxide formation and oxide dissolution rates for both anodization types.doctoral thesis387 175 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Einfluss von Wasser auf die Kohlenstofffaser/Epoxid-Polymermatrix Wechselwirkung(2014-01-31); ; ; In this study the carbon fibre/epoxy polymer interaction is investigated in single fibre composites with different polymer matrices in dry, water-saturated and redried state. To provide a basis, these epoxy polymers varying in curing degree and amine-to-epoxy ratio are characterized with respect to their physical, chemical and mechanical properties and water absorption tests are performed. The fibre/polymer interaction is examined with the single fibre fragmentation test. Despite of different properties of the epoxy polymers, no significant effect in the strength of the fibre/polymer interaction is observed. The presence of water instead leads to a strong decrease in interfacial shear strength. However, after redrying the strength of the fibre/polymer interaction is comparable with the origin, dry state. The results of this work show that there is a significant reducing effect of absorbed water in epoxy polymers and its carbon fibre composites but also, that this effect is mostly reversible.doctoral thesis576 367 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Einstellung der Netzwerk- und Domänenstruktur kationisch polymerisierter Epoxide(2013-02-14); ; ; Cationically polymerised and highly crosslinked epoxy thermosets containing partially crystalline polyols are observed. The network- and domain formation is characterised with regards to possible phase separation of the partially crystalline polyole component. After thermoanalytical and mechanical characterization of the polymers, a proposed molecular order of the polyole component was discussed to be responsible for the observed toughening of the epoxy polymers. Due to the phase separated morphology kovalently crosslinked polymers showing a shape-memory effect are prepared and characterised. The results are transfered on a thermoplastic polyurethane containing block copolymers of different crystallinities to observe the influence of crystalline interactions on a stretchable polymer.doctoral thesis298 125 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Steuerung funktioneller Materialeigenschaften eines Polybenzoxazin-Netzwerks über die Integration von Poly(caprolacton)-Oligomeren(2019-07-17); ; ; The present dissertation deals with the specific control of the functional properties of a bisphenol A based polybenzoxazine (PBA-a). Therefore, the thermoset was combined with the thermoplastic oligomer poly(caprolactone) (PCL). The influence of covalently integrated versus unbound PCL on polybenzoxazines properties was the main focus of this cumulative work and for that reason systematically analyzed by varying the PCL content in the polymer mixtures. The polyester end groups were functionally modified with a tosyl leaving group. In contrast to the hydroxy-terminated basic structure, the modified PCL could be covalently integrated into benzoxazines network. The copolymers homogeneous morphology observed by scanning electron micrographs of PCL/PBA-a revealed that the covalent incorporation of the polyester chains could prevent phase separation of the components as occurred for the unmodified PCL/PBA-a blend-like structures. A slightly decreased network density, due to the incorporated PCL chains, reduced the brittleness of samples with low contents of modified PCL (wPCL 30 %). Thus, toughness values of up to 1.66 MPa were achieved compared to 0.48 MPa for neat PBA-a. By increasing the content of the tosylated PCL in the samples (wPCL = 30 - 50 %) network density was lowered accordingly, resulting highly flexibilized polymers with an almost 10 times increased elongation at break compared to PBA-a. Due to the macroscopic phase separation, a further increase of the amount of unmodified PCL led to samples with even higher brittleness. Polymers with high levels of tosyl-modified PCL (wPCL = 60 - 80 %) exhibited thermoresponsive shape memory properties, which could be attributed to the presence of both covalently incorporated and free PCL in these samples with mixed bonding mode. The combination of the widened network and increased crystallinity are key factors to the displayed shape memory effect. In this work, benzoxazine networks with variable and functional properties have been successfully prepared due to the covalent integration of PCL oligomers. The deeper understanding of the structure-property relationship will facilitate a broader application of polybenzoxazine based materials.doctoral thesis401 376 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Untersuchung der molekularen Konformation und der intermolekularen Wechselwirkung chiraler Verbindungen mittels VCD-Spektroskopie(2010-08-27); ; ; Vibrational circular dichroism (VCD) spectroscopy is the differential absorption of left- and right-circular polarized light of a chiral molecule during a vibrational transition. In the present work, VCD spectroscopy has been applied to study the molecular conformations and intermolecular interactions of chiral molecules and polymers. In particular, several compounds featuring the alpha-methylbenzyl motif have been studied in order to derive similarities in their spectra. Furthermore, the diastereomeric interaction between chiral ions on the one hand and metal complexes with chiral amines on the other hand have been studied. The helical structures of chiral polymers were studied in the third part of the thesis.doctoral thesis333 406 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Organisch modifizierte Schichtsilicate als thermische Polymerisationsinitiatoren für die Darstellung von Nanokompositen auf Basis von Epoxiden(2004-07-12); ; ; Montmorillonite containing minerals with exchangeable sodium ions are available in large quantities. By exchanging the sodium ions for cationic tensides the surface of the silicate layers becomes organophilic and the distance between the silicate layers the gallery distance increases from 1 nm to about 1.8 nm. Due to this modification the silicate layers can be distributed into a polymeric matrix yielding nanocomposites. If epoxy resins are used as the matrix, the exfoliation of the layered silicate creates polymer clay nanocomposites with improved properties compared to pure epoxy resins or conventional composites. It has been reported in the literature that the diglycidyl ether of bisphenol A undergoes self-polymerization when heated with acidic onium ion exchanged forms of montmorillonite. However nanocomposite formation is accomplished through a dramatic transition from gel state to a dry powder with a very low bulk density. During the thesis two solutions were developed to prevent the rapid liquid to powder transformation and it is now possible to obtain cured transparent epoxy resin based nanocomposites without adding any further hardener. Both methods control the balance between the extragallery reaction rate of the epoxy system and the diffusion rate of monomers into the galleries, which affects the innergallery reaction rate. One possibility is to use the influence of additives for improving the diffusion rate of low viscosity epoxy resins. The other approach leads to a new structure of the ion exchanged montmorillonite. This new structure improves the diffusion of monomers into the galleries and leads to cured and transparent nanocomposites without any further additive or hardener. The organophilic clays and the formed polymers were studied by DSC, TGA, EDX, SEM, XRD and TEM.doctoral thesis271 100