Nanomechanical characterisation of multilayered organic-inorganic composite films produced by bioinspired processing routes
Zusammenfassung der Projektergebnisse
This project was devoted to exploring the mechanical performance characteristics of nanocomposites whose structure is analogous to that of nacre. To this end, nanoindentation and nano-tensile tests were performed on different types of laminar nanocomposites comprised of an inorganic (either TiO2, or V2O5, or a zirconiumbased ceramic) and an organic (polymer or protein) component. A major objective was to establish a correlation between the films' microstructure and mechanical properties, as expressed in their hardness, Young's modulus and fracture toughness, as a basis for determining the enhancement mechanisms. The second funding period endeavoured to optimize the films' mechanical performance via three different strategies, specifically increasing the crystalline degree of the oxide component, enhancing the bonding strength between the inorganic nanoparticles, as well as incorporating organic polymers that more closely resemble the biopolymers in nacre. Electron microscopy analysis of the (TiO 2/organic polyelectrolyte)n films identified two relevant features that can explain their good mechanical performance. The first one involves protrusions on the oxide particle surface which are expected to increase the stress required to drive the inelastic deformation through the composite, akin to the mineral asperities inside natural nacre. The second feature consists of approximately 10 nm wide TiO2 filaments that connect neighbouring oxide layers through the intermediate PE layers, in close analogy to mineral bridges within nacre. On this basis, the observed optimum thickness ratio of 1:10 between the inorganic and organic layers could be rationalized, since it ensures that the organic layers are sufficiently thick to promote stress distribution via viscoelastic deformation, and simultaneously thin enough to allow a communication between the inorganic layers via the formation of TiO 2 bridges. Further to this, a very promising first step was made toward the biotemplate-induced deposition of (monolithic) TiO 2 films. In fact, TiO2 films grown on substrate-supported hydrophobin monolayers exhibited significantly improved mechanical properties in comparison to TiO 2 films prepared on unaltered SiO2 surfaces. This difference can be attributed to an increased crystalline order in the hydrophobin-nucleated films. Besides the TiO2-based nanocomposites, nacre-like laminar composites incorporating either ZrO 2 or zirconium hydrogen phosphate (ZrP) were investigated. The ZrP was combined with the biopolymer chitosan due to the biocompatibility of these two materials. Both types of laminar films displayed enhanced mechanical properties as compared to the monolithic films, albeit their performance lacked behind that of the TiO 2-based composites. Furthermore, a self-assembly method was developed to obtain high quality paper-like films comprising single crystalline, several micrometer long V2O5 nanofibers. These films display extraordinary mechanical flexibility, which could be linked to the contained water molecules presumably acting like a kind of flexible glue between the densely packed nanofibers. It was shown that the dense packing of the fibers combined with their in-plane alignment imparts a mechanical performance that is superior to the main other types of paperlike materials reported in the literature. The mechanical properties could be further improved by the synthesis of nanocomposites composed of alternative layers of V2O5 nanofibers and an organic polymer like the biocompatible poly-dopamine. Complementary to the nacre-like nanocomposites, five different types of biomaterials were investigated in close association with the DFG project “Biologische Erzeugung von Oxidkeramiken”. These studies aimed at further elucidating the design principles developed by nature, and focused on the mechanical properties of nacre, sponge spicules, as well as the teeth, skeleton and spines of sea urchins. Our findings highlight the sophisticated structural design of nacres that imparts a gradual modulation of mechanical performance. It could furthermore be demonstrated for the first time that the inner layer, which is closest to the animal body, displays highest hardness and elastic modulus. Finally, evidence was gained that magnesium deficiency during the development of sea urchin larvae leads to significantly diminished mechanical stability as a consequence of a changed morphology of the spicules. The achievement of making the artificial nacre–like materials has stimulated two newspapers articles, the first of which appeared on 1st of January 2010 in the Frankfurter Allgemeine Zeitung, and the second one on 5th of January 2010 in the Stuttgarter Zeitung. Moreover, a corresponding press release of the Max-Planck Society had appeared on 14th of December 2009.
Projektbezogene Publikationen (Auswahl)
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“Bio-sintering processes in hexactinellid sponges: Fusion of bio-silica in giant basal spicules from Monorhaphis chuni”. J. Struct. Biol. 168 (2009), 548
W. Müller, X. Wang, Z. Burghard, J. Bill, A. Krasko, A. Boreiko, U. Schlossmacher, H.C. Schröder, M. Wiens
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“Toughening through natureadapted nanoscale design”. Nano Letters 9 (2009), 4103
Z. Burghard, L. Zini, V. Srot, P. Bellina, P. A. van Aken, and J. Bill
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“Bioinspired Deposition of TiO2 Thin Films Induced by Hydrophobins”. Langmuir 26 (2010), 6494
D. Santhiya, Z. Burghard, C. Greiner, L.P.H. Jeurgens, T. Subkowski, J. Bill
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European patent Invention No AE 20090292 “Process for deposition of thin layers of metal oxide”. (2011)
D. Santhiya, Z. Burghard, T. Subkowski, J. Bill
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“Fabrication and characterization of biocompatible nacre-like structures from alpha-zirconium hydrogen phosphate hydrate and chitosan“. J. Coll. Interf. Sci. 367 (2012), 74
S.M. Waraich, B. Hering, Z. Burghard, J. Bill, P. Behrens, H. Menzel
