Publikationen

The molecular alumosiloxanes (O-SiPh2-O-SiPh2-O)3Al2 and (O-SiPh2-O-SiPh2-O)4[Al(OH)]4 react either with water in diethyl ether, or with 1,4-butane-diol to form the new macro-cyclic compounds Al6(OH)8(O-SiPh2-O-SiPh2-O)4(O-SiPh2-O-SiPh2-OH)2·6H2O (2) or Al6(OH)8(O-SiPh2-O-SiPh2-O)4(O-SiPh2-O-SiPh2-O-CH2–CH2–CH2–CH2–OH)2 (3). As shown by single crystal structure analyses, both compounds 2 and 3 have a center of symmetry, resemble each other structurally and display in their center an [Al2(OH)8]2– unit, which is coordinated in a similar fashion to a 24-membered [Al4(O-SiPh2-O-SiPh2-O)4]2+ cycle branched at two aluminum atoms displaying either two (O-SiPh2-O-SiPh2-OH) (2) or two (O-SiPh2-O-SiPh2-O-CH2–CH2–CH2–CH2–OH) (3) arms. The [Al2(OH)8]2– groups are connected to the cycles through oxygen atoms of their hydroxide groups which link to the aluminum atoms of the ring (of the four aluminum atoms in the ring two have a double contact and two a single). Parallel to this bonding, the aluminum atoms of [Al2(OH)8]2– have either a water molecule in their coordination sphere, which is incorporated in a complex hydrogen bridged network including the silanol function (2), or are bonded to the –CH2-OH group of the siloxane-alcohol arm of the cycle (3). The aluminum atoms of the central part are in both compounds in the centers of two distorted edge sharing oxygen octahedra (mean Al–O = 1.881(7) Å (2), 1.893(7) Å (3)), while the other four aluminum atoms display a tetrahedral oxygen environment (Al–O = 1.752(8) Å (2), 1.754(8) Å (3)). Whereas all hydroxy groups of the [Al2(OH)8]2– unit in 2 are engaged in hydrogen bonding including further water molecules which make part of the network, the [Al2(OH)8]2– unit in 3 shows fewer hydrogen bridges. We have here a rare example of the same chemical species in a more hydrophilic and a less hydrophilic surrounding. This allows a detailed study of the impact of secondary hydrogen bonding on the structure.

The atomic structures of SrTiO3 (STO)/LaNiO3 (LNO)/STO heterostructure interfaces were investigated by spherical aberration-corrected (CS) (scanning) transmission electron microscopy. Atomic displacement and lattice distortion measurements and electron energy loss spectroscopy (EELS) were used to quantitatively analyze the distortion of the interfacial octahedra and the bond length at the interfaces. Combined with high-resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy analyses, two distinct interfacial atomic terminating layers are unambiguously determined. Ensuing quantitative HRTEM measurements revealed that the Ni-O bond length in the interfacial octahedral is elongated at the bottom interface (-NiO2-SrO-). Atomic displacement shows structural relaxation effects when crossing the interfaces and lattice distortions across the interface is more pronounced in LNO than in STO. The Ti/O atomic ratio, La and Ti relative atomic ratio as derived by EELS quantification indicate non-stoichiometric composition at the interfaces. Distinct fine structures of Ti-L2,3 edge and O-K edge at the bottom and top interfaces are observed. By comparison, we are able to estimate Ti valency at both interfaces. Combining the structural distortions and Ti valency, the polar discontinuity and charge transfer at the interfaces are discussed.

We here present the nucleation and growth of calcium carbonate under the influence of synthetic peptides on topographically patterned poly(dimethylsiloxane)
(PDMS) substrates, which have a controlled density of defects between the wrinkles. Experiments with two lysine-rich peptides derived from the extracellular conserved domain E22 of the mollusc chitin synthase Ar-CS1, AKKKKKAS (AS8) and EEKKKKKES (ES9) on these substrates showed their influence on the calcium carbonate morphology. A transition from polycrystalline composites to single crystalline phases was achieved with the peptide AS8 by changing the pH of the buffer solution. We analyzed three different pH values as previous experiments showed that E22 interacts with aragonite biominerals more strongly at pH 7.75 than at pH 9.0. At any given pH, crystals appeared in characteristic morphologies only on wrinkled substrates, and did not occur on the flat, wrinkle-free PDMS substrate. These results suggest that these wrinkled substrates could be useful for controlling the morphologies of other mineral/peptide and mineral/protein composites. In nature, these templates are formed enzymatically by glycosyltransferases containing pH-sensitive epitopes, similar to the peptides investigated here. Our in vitro test systems may be useful to gain understanding of the formation of distinct 3D morphologies in mollusc shells in response to local pH shifts during the mineralization of organic templates.

This research is the first of its kind to study the comparison between spherical and flat probe adhesion behavior as a function of viscoelasticity. Viscoelastic properties were tailored through the use of acrylate networks synthesized from tert-butyl acrylate and poly(ethylene glycol) dimethacrylate (PEGDMA) solutions. The molecular weight and the weight fraction of PEGDMA crosslinker was altered to maintain a constant glass transition temperature of approximately 57 °C, but systematically vary the viscoelastic properties and the rubbery moduli (1-62 MPa). Dynamic mechanical analysis was performed to characterize the low-strain thermo-mechanical behavior of the materials. Viscoelastic behavior of the materials was characterized by creep testing and was observed to inversely correlate with crosslinking density. The samples tested with the spherical probe exhibited low pull-off forces at temperatures well above and well below the glass transition temperature of the material. A maximum in pull-off force was observed in the vicinity of the glass transition temperature owing to the viscoelastic energy dissipative processes. The peak in pull-off force was observed to decrease with an increase in crosslinking density and modulus. Adhesion measurements using the flat probe demonstrated a strong dependence of pull-off force on the modulus of the material above the glass transition temperature. It is concluded that viscoelasticity is a dominating factor in increasing the pull-off force values in the vicinity of the glass transition, while it plays a little or no role for temperatures +/-20 °C away from transition region, opening the possibility of thermally switchable adhesives.

Four acrylate-based networks were developed such that they possessed similar glass transition temperature (~-37 °C) but varied in material stiffness at room temperature by an order of magnitude (2-12 MPa). Thermo-mechanical and adhesion testing were performed to investigate the effect of elastic modulus on adhesion profiles of the developed samples. Adhesion experiments with a spherical probe revealed no dependency of the pull-off force on material modulus as predicted by the Johnson, Kendall, and Roberts theory. Results obtained using a flat probe showed that the pull-off force increases linearly with an increase in the material modulus, which matches very well with Kendall's theory.

Adhesion and buckling of single PDMS micropillars were investigated as a function of compressive preload. The micropillars had diameters of 10, 12, 14, and 20 µm and aspect ratios of 1 to 3.3. Adhesion generally increased with a decrease in the aspect ratio. A dependence of pull-off strength on the compressive preload stress was found for micropillars that underwent buckling. When buckling was reversible, tip contact recovered upon unbuckling, which resulted in only a slight reduction of adhesion. In situ observation studies identified irreversible buckling, i.e., lack of tip-contact re-formation, resulting in adhesion loss. It is concluded that the edge radius of the tip, which acts as a circumferential crack, controls adhesion. Fibril buckling is found to be broadly consistent with the predictions of Euler buckling theory.

Crystalline monolayers of polymer particles are useful templates for surface microstructuring. Here, the authors discuss the use of oxygen plasma to tune interparticle distances in such films. A systematic evaluation of the etch process depending on particle size, plasma power, etching time and particle density was performed. The size evolution of individual particles was analyzed using scanning electron microscopy and compared with different models of the etching process. The authors conclude that none of the existing etch models fit the data very well. Analysis of the particle shape throughout the etching process indicates that changes in particle geometry occur depending on their original size and density. In dense films, bridges form between the particles’ original contact points. Particles increasingly deviate from a spherical geometry. Such shape changes are not captured by current models of the etching process. The authors propose a mechanism to explain the formation of bridges between the particles and their role in the preservation of long-range order. This paper is supplemented by supporting information. The associated files are available online for published issues or can be obtained directly from the managing editor (sohini.banerjee@icepublishing.com) for Ahead of Print articles.

The effect of the addition of trioxadecanoic acid (TODS) on the performance of Dye sensitized solar cells (DSSCs) was studied. An increase of the overall light to current conversion efficiency was observed, due to a significant increase of the photocurrent density. The increase of the photocurrent density referred to the more homogeneous porous structure and the relevant increase of the surface area, which allowed higher amount of dye loading. This resulted in a positive impact on the enhancement of the photo current density. The impedance spectroscopy showed that the addition of TODS resulted in a decrease of the charge transfer resistance of the cell, which explained by increasing the charge regeneration.

In this study, polyetheretherketone composites were compounded using a two-screw extruder followed by injection moulding. The effects of multi-fillers on the mechanical properties and crystallization performances were investigated. Differential scanning calorimetry results indicate that the addition of fillers slightly increases the crystallization temperature and crystallinity. Compared to neat polyetheretherketone, the incorporation of inorganic filler leads to a significant improvement in matrix hardness, matrix stiffness and a slight increase in tensile strength. However, the material ductility, the impact strength and the fracture toughness of polyetheretherketone composites decrease. Fractography analyses show that the addition of fillers restraints the ductile deformation of polymers, which is responsible for the reduction of material ductility, impact strength as well as fracture toughness of polyetheretherketone composites.

The tribological properties of Poly-ether-ether-ketone (PEEK) were studied systematically by multiple scratch tests in both unidirection and bidirection mode on the micro- and nano-length scale. The tip geometry has a strong influence on the scratch friction behavior, in particular on the scratch initiation and on the resulting damage patterns. Plowing contributions to friction are significant for nano-scale tips during the initial scratch cycles. Shear contributions dominate for both nano- and micro-scale once a groove has been established by multiple scratches. While the damaging mechanisms are the same for both micro- and nano-scratch tests, the resulting damaging patterns differ depending on the scratching mode (unidirection or bidirection) and the normal load. Patchy layers of material formed by scratching are torn into fracture by nano-scale tips, while they are stretched to flakes by the micro-scratch indenter.