Different polymer foils, namely polyimide, FEP, PFA and PVDF were tested on a setup designed to measure the static coefficient of friction between them. The setup was designed according to the requirements of a damping device based on electrostatically tunable friction. The foils were tested under different mechanically applied forces and showed reproducible results for the static coefficient of friction. With the same setup the measurements were performed under an electric field as the source of the normal force. Up to a certain electric field the values were in good agreement. Beyond this field discrepancies were found.

No abstract

Controlled, bottom-up self-assembly of ordered and hierarchical structures remains a major challenge and increasingly attracts attention in basic and technology-driven research. A simple process is described for the generation of such structures, which is based on slow solvent evaporation of a polymer solution blended with a crystal-forming species (Krogmann's salt). Upon drying, the viscosity of the polymer-blend solution increases in a progressing solidification zone, which precisely controls crystal growth by limiting the transport of the crystallizing units through this gel-like solidification zone and gives rise to a position- and time-dependent diffusion rate. The progressing solidification zone also leads to a preferential crystallographic orientation on a centimeter scale and introduces an instability that drives spatial pattern formation and hierarchical ordering on five distinct levels, ranging from the atomic positions in crystals to the assembly on a microscale and up to a centimeter length scale. Together with a quantitative description, the presented findings are envisaged to improve the understanding and application of periodic precipitation processes.

The damping capacity of a novel composite film, designed to exhibit high dielectric strength and a high friction coefficient for an electrostatic tuneable friction damper, is tested on a cantilever beam. Such a system consists of a carbon fibre reinforced polymer stiffening element which is reversibly laminated onto a host structure with a dielectric material by means of electrostatic fields. Damping is achieved when the maximum shear at the interface between the stiffening element and structure exceeds the shear strength of the electrostatically laminated interface. The thin films tested consist of barium titanate particles and alumina platelets in an epoxy matrix. Their high dielectric constant and high coefficient of friction compared to a commercial available polymer film, polyvinylidene fluoride, lead to a reduction of the required electric field to stiffen and damp the cantilever beam. Reducing the operating voltage affects different aspects of the studied damper. The cost of possible applications of the frictional damper can be reduced, as the special components necessary at high voltages become redundant. Furthermore, the enhanced security positively affects the damping system’s appeal as an alternative damping method.