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Force-distance curves have been acquired with an atomic force microscope on perfluorpolyether films. It is shown that curves acquired on films of different thickness, at different rates and with different dwell times can be overlapped by rescaling the time or distance axis.When the time or distance axis is rescaled,the force depends only on the surface tension of perfluorpolyether and on geometrical properties of thetip (aperture and perimeter of the tip at a given distance from the apex). Hence, curves acquired with thesame tip overlap.
By comparing curves acquired at different rates the dynamics of tip-wetting can be investigated. Fur-thermore, rescaled force-distance curves have been matched with the perimeter of cross sections ofthe tip, i.e. with the perimeter of the three-phase contact line. Such measurements pave the way for anon-destructive investigation of the tip shape.
Blends of poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN) were processed into biaxially drawn films, and samples taken from the bi-oriented films were then investigated by dynamic rheology experiments in the melt state. Storage modulus G and loss modulus G were determined in the frequency range of 10-2-102 rad/s at temperatures between 260 and 300°C. Although the time-temperature superposition (TTS) principle was found to hold in the high frequency regime, a breakdown of TTS was observed at low frequencies, and the terminal behavior of the storage modulus G of the blends departs drastically from the terminal behavior observed for the blend components. This is caused by interfacial surface tension effects. The results indicate that despite the effect of transesterification reactions, the PET/PEN blend systems investigated consist of a microseparate phase of PEN platelets in a matrix of PET. This morphology is produced when the blends are processed into biaxially oriented PET/PEN films, and droplets of PEN are deformed into a lamellar structure consisting of parallel and extended, separate layers. The large interfacial surface area of the bi-oriented PET/PEN blends leads to remarkably strong interfacial tension effects in dynamic rheology measurements.
Surface tension-driven Marangoni convection causes the formation of regular surface structures in drying polymer layers. The shape of the surface structures formed during solvent evaporation depends on layer and interfacial dynamic parameters as well as external factors. The influence of a horizontal radial temperature gradient produced by a point heat source below the polymer layer on the diffusion-controlled Marangoni instability has been studied. In the region of the lateral temperature gradient, radial surface flow coupled with the interfacial instability leads to stripe, ladder, chevron and/or labyrinthine surface structures.
Solvent evaporation causes concentration and temperature gradients at the free surface of a coating polymer solution. Thereby surface-tension-driven flows can result in a surface instability with regular structures within the layer. Different types of surface structures can occur at spots with thickness differences. One can find fractal-like structures especially in the edge zone of a layer. The evolution of such fractal-like structures at the surfaces of coating solutions of polyacrylonitrile (PAN) in dimethylformamide (DMF) during layer hardening is examined. Condensation of water on the surface of the fluid layer exerts a strong influence on the formation of these structures. Solutions of PAN in DMF are important for the production of ultrafiltration membranes. Such membranes are produced by precipitation in a water bath after a short open time.
The movable contact line between two liquids and a gas phase sensitively reacts to small disturbances in the force equilibrium. The shape of the contact line and the adjoining interfaces is determined by the interface and surface tensions, the contact angles, the density differences (hydrostatic pressure), and the Laplace capillary pressure. When these change, the three-phase contact line can deform and even become unstable. Interface and surface tension depend on the concentration and temperature. During mass transport processes (concentration changes) various forms of the instability of the contact line can be observed:
Oscillations of a circular contact line (regular expansion and reduction);
Single deformations (bulges) which quickly disappear again;
Deformations (bulges) which run along the boundary line;
Periodically generated and damped deformations with different modes.
The behavior of the three-phase contact line is of practical importance for coalescence processes and for spontaneous emulsification on liquid surfaces.
When thin layers of polymer solutions dry by solvent evaporation and/or heating from below, the layers can become hydrodynamically instable. As a result various surface structures occur. A great pattern manifold of such structures has been found. By self-organization small surface motions caused by surface tension differences can be amplified, when the ratio of the driving forces to the damping ones characterized by the Marangoni-number exceeds a critical value. Besides greater and hierarchic subdivided cell patterns, periodic line structures with peak-valley-differences of down to 1 nm have been found in the solidified layers. The surface profiles (3D- and line profiles) have been measured by means of a microscope interferometer.