Publikationen

The friction and wear behavior of PEEK and a multi-scale particle filled PEEK was characterized by using a velocity increasing approach during one sliding wear test without changing of polymer specimen and counterbody. The results derived demonstrated that the incorporation of various fillers remarkably improved the tribological performance of PEEK, especially under severer load conditions, e.g. under a pv-condition of 4 MPa and 0.5–4 m/s, the mean specific wear rate reduced from 19 mm3 N−1 m−1 to 0.42 mm3 N−1 m−1 after incorporation of rigid fillers, and similarly, the lowest friction coefficient was obtained from PEEK composite, which is about 0.1. In addition, similar friction coefficients and wear rates of PEEK and PEEK composite are gained from velocity increasing tests compared to those evaluated from constant velocity tests, which means that the successful application of the velocity increasing technique in the investigation of the tribological performance of polymer materials can obviously reduce the time and material expenses. Finally, based on the observations of the worn surface, in addition to the rolling effect of the particles between body and counterbody and the alleviation of stress concentration on the SCFs due to improvement of the matrix stiffness by incorporating rigid particles, an additional protective effect of particles on the SCFs was observed. The particles accumulate in front of the fibers, which reduces the direct contact between the counterbody and SCF, and therefore, the debonding and removals of fibers were alleviated.

AbstractWe demonstrate that the inclusion of subsurface microfluidic features in a soft polymer surface, whose internal pressure can be varied, may be used to modify the effective radius of curvature of the contact and thus switch the magnitude of the adhesion of the surface to a harder counterface from one state to another – in other words to control the 'stickiness' of the surface. In these circumstances, adhesion depends on van der Waal forces and can be described by the classic treatment of Johnson, Kendall and Roberts. The pressurization of the subsurface chambers results in the rapid emergence of surface features with reduced radii of curvature which drastically decreases the area available for contact so providing a reduction of up to 90% in the adhesion force of a surface from its original 'sticky' state. It is demonstrated that this mechanism can provide a quick, reversible and clean detachment.

Localized inclusions of liquids provide solid materials with many functions, such as self-healing, secretion, and tunable mechanical properties, in a spatially controlled mode. However, a strategy to control the distribution of liquid droplets in solid matrices directly obtained from a homogeneous solution has not been reported thus far. Herein, we describe an approach to selectively localize liquid droplets in a supramolecular gel directly obtained from its solution by using evaporative lithography. In this process, the formation of droplet-embedded domains occurs in regions of free evaporation where the non-volatile liquid is concentrated and undergoes a phase separation to create liquid droplets prior to gelation, while a homogeneous gel matrix is formed in the regions of hindered evaporation. The different regions of a coating with droplet embedment patterns display different secretion abilities, enabling the control of the directional movement of water droplets.

Nanocomposites consisting of conductive polymers and functional nanoparticles have recently been employed in photodetectors and imagers. Here, we present a novel hybrid-organic photodetector (HPD) that was optimized for the detection of X-rays meeting the specific needs of medical imaging. Devices were fabricated using inorganic lead sulfide (PbS) nanocrystal (NC) quantum dots (QDs) and a blend of poly (3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Quantum dots convert X-rays directly into charge-carriers that then migrate through the organic blend to the contacting electrodes. The performance of such devices depends on the thickness and probably on the morphology of the active layer. We discuss the synthesis and characterization of the PbS quantum dots, their incorporation into a HPD, and the performance of the HPD in X-ray sensing. Scanning electron microscopy and transmission electron microscopy show the PbS-QD distribution in the organic matrix. We find a strong tendency of the QDs to phase-separate from the organic blend.

We present an elegant route for the fabrication of ordered arrays of vertically-aligned silicon nanowires with tunable geometry at controlled locations on a silicon wafer. A monolayer of transparent microspheres convectively assembled onto a gold-coated silicon wafer acts as a microlens array. Irradiation with a single nanosecond laser pulse removes the gold beneath each focusing microsphere, leaving behind a hexagonal pattern of holes in the gold layer. Owing to the near-field effects, the diameter of the holes can be at least five times smaller than the laser wavelength. The patterned gold layer is used as catalyst in a metal-assisted chemical etching to produce an array of vertically-aligned silicon nanowires. This approach combines the advantages of direct laser writing with the benefits of parallel laser processing, yielding nanowire arrays with controlled geometry at predefined locations on the silicon surface. The fabricated VA-SiNW arrays can effectively transfect human cells with a plasmid encoding for green fluorescent protein.

Novel types of Transparent Conductive Materials (TCMs) based on metal nanostructures are discussed. Dispersed metal nanoparticles can be deposited from liquids with moderate thermal budgets to form conductive films that are suitable for thin-film solar cells, displays, touch screens, and nanoelectronics. We aim at new TCMs that combine high electrical conductivity with optical transparency and mechanical flexibility. Wet-processed films of randomly arranged metallic nanowires networks are commercially established and provide a relatively cost-effective, scalable production. Ultrathin gold nanowires (AuNWs) with diameters below 2nm and high aspect ratios have recently become available. They combine mechanical flexibility, high optical transparency, and chemical inertness. AuNWs carry oleylamine capping ligands from synthesis that cause high contact resistances at their junctions. We investigated different annealing processes based on temperature and plasma treatment, to remove the ligands after deposition and to allow electrical conductivity. Their effect on the resulting nanostructure and on the material properties was studied. Scanning Electron Microscopy (SEM) and optical spectroscopy revealed changes in the microstructure for the different post-treatments. We found that the conductivity and the stability of the TCM depended strongly on its final microstructure. We demonstrate that the best results are obtained using H2-plasma treatment.

Flow field-flow fractionation is a powerful method for the analysis of nanoparticle size distributions, but its widespread use has been hampered by large analyte losses, especially of metal nanoparticles. Here, we report on the colloidal mechanisms underlying the losses. We systematically studied gold nanoparticles (AuNPs) during asymmetrical flow field-flow fractionation (AF4) by systematic variation of the particle properties and the eluent composition. Recoveries of AuNPs (core diameter 12 nm) stabilized by citrate or polyethylene glycol (PEG) at different ionic strengths were determined. We used online UV–vis detection and off-line elementary analysis to follow particle losses during full analysis runs, runs without cross-flow, and runs with parts of the instrument bypassed. The combination allowed us to calculate relative and absolute analyte losses at different stages of the analytic protocol. We found different loss mechanisms depending on the ligand. Citrate-stabilized particles degraded during analysis and suffered large losses (up to 74%). PEG-stabilized particles had smaller relative losses at moderate ionic strengths (1–20%) that depended on PEG length. Long PEGs at higher ionic strengths (≥5 mM) caused particle loss due to bridging adsorption at the membrane. Bulk agglomeration was not a relevant loss mechanism at low ionic strengths ≤5 mM for any of the studied particles. An unexpectedly large fraction of particles was lost at tubing and other internal surfaces. We propose that the colloidal mechanisms observed here are relevant loss mechanisms in many particle analysis protocols and discuss strategies to avoid them.

Binary mixtures of nanoparticles self-assemble in the confinement of evaporating oil droplets and form regular supraparticles. We demonstrate that moderate pressure differences on the order of 100 kPa change the particles' self-assembly behavior. Crystalline superlattices, Janus particles, and core-shell particle arrangements form in the same dispersions when changing the working pressure or the surfactant that sets the Laplace pressure inside the droplets. Molecular dynamics simulations confirm that pressure-dependent interparticle potentials affect the self-assembly route of the confined particles. Optical spectrometry, small-angle X-ray scattering and electron microscopy are used to compare experiments and simulations and confirm that the onset of self-assembly depends on particle size and pressure. The overall formation mechanism reminds of the demixing of binary alloys with different phase diagrams.