ABSTRACT
The contact geometry of microstructured adhesive surfaces is of high relevance for adhesion enhancement. Theoretical considerations indicate that the stress distribution in the contact zone is crucial for the detachment mechanism, but direct experimental evidence is missing so far. In this work, we propose a method that allows, for the first time, the detection of local stresses at the contact area of biomimetic adhesive microstructures during contact formation, compression and detachment. We use a mechano-sensitive polymeric layer, which turns mechanical stresses into changes of fluorescence intensity. The biomimetic surface is brought into contact with this layer in a well-defined fashion using a microcontact printer, while the contact area is monitored with fluorescence microscopy in situ. Thus, changes in stress distribution across the contact area during compression and pull-off can be visualized with a lateral resolution of 1 µm. We apply this method to study the enhanced adhesive performance of T-shaped micropillars, compared to flat punch microstructures. We find significant differences in the stress distribution of the both differing contact geometries during pull-off. In particular, we find direct evidence for the suppression of crack nucleation at the edge of T-shaped pillars, which confirms theoretical models for the superior adhesive properties of these structures.
ABSTRACT
Photoresponsive polymer brushes constitute an attractive platform for tuning surface properties and functionality. Since the degree of photoconversion can be controlled by the light dose, functional states with intermediate properties between those of the nonexposed and fully exposed brushes are accessible. Here we investigate the light-modulated interfacial, adhesion, and frictional properties of photosensitive polymer brushes with a methacrylate backbone and ionizable -COOH side groups modified with the photoremovable group 6-nitroveratryloxycarbonyl (NVOC). The original brush (PNVOCMA) gradually changes into a charged poly(methacrylic acid) (PMAA) brush upon exposure to ultraviolet light due to the photoremoval of the chromophore and generation of free COOH groups. We show how the physical properties of the brush can be gradually tuned with the exposure dose using condensation microscopy, atomic force microscopy (AFM), force mapping, and friction force spectroscopy.
ABSTRACT
Knowledge of the mechanical properties of singular clay lamellae is of crucial importance for the optimization of clay-polymer nanocomposites. On the basis of controlled stress release, singular 2:1 clay lamellae show regular wrinkles on a deformable substrate. A subsequent two-dimensional Fourier transformation gives an in-plane modulus of the clay lamella of approximately 150 GPa. Only readily-available topographical atomic force microscopy is required for analysis rendering that fast and facile procedure generally applicable for nanoplatelet characterization.
ABSTRACT
Transparency, flexibility, and especially ultralow oxygen (OTR) and water vapor (WVTR) transmission rates are the key issues to be addressed for packaging of flexible organic photovoltaics and organic light-emitting diodes. Concomitant optimization of all essential features is still a big challenge. Here we present a thin (1.5 µm), highly transparent, and at the same time flexible nanocomposite coating with an exceptionally low OTR and WVTR (1.0 × 10(-2) cm(3) m(-2) day(-1) bar(-1) and <0.05 g m(-2) day(-1) at 50% RH, respectively). A commercially available polyurethane (Desmodur N 3600 and Desmophen 670 BA, Bayer MaterialScience AG) was filled with a delaminated synthetic layered silicate exhibiting huge aspect ratios of about 25,000. Functional films were prepared by simple doctor-blading a suspension of the matrix and the organophilized clay. This preparation procedure is technically benign, is easy to scale up, and may readily be applied for encapsulation of sensitive flexible electronics.
