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Tuning topography and patterns of thin polymer films on structured substrates

Konstantinos Roumpos. Inauguraldissertation der Fakultät für Mathematik und Physik 2021. DOI: 10.6094/UNIFR/194127

Plant surfaces are characterized by a large variety of topographical patterns that confer them unique properties. Such patterns can be mimicked in thin polymer films utilizing dewetting. In this project, we employed dewetting and mechanical deformation of thin polymer films on substrates which were either covered with partially dewetted polymer films or elastically deformable micro-pillar arrays for forming and tuning hierarchical pattern formation, with the aim of generating plant-inspired surface topographies. The material of choice was atactic polystyrene. Substrates containing networks of filaments of polystyrene were used in order to study adhesion forces between the filaments, the substrate and a polystyrene film which was deposited on top of them. The polystyrene network of filaments wes achieved by dewetting a polystyrene film until the formation of the Voronoi tessellation, a polygon pattern also found in nature and in biological systems such as plant leaves. The polystyrene film was then transferred on top of the patterned substrate. The position of the film with respect to the underlying substrate is determined by the balance of the forces of adhesion between the film and the filaments and the elastic properties of the film. A model description that includes the contact mechanics of thin films adhering to curved surfaces was established in an attempt to describe the experimental results. When the sample was heated above the glass transition temperature (Tg) of atactic polystyrene, dewetting was initiated within the transferred film at locations of amplified stresses, such as the edges of the underlying filaments, leading to a second stage of pattern formation. Moreover, the pattern formation of dewetting films as well as the impact of periodic disturbances with concurrent determination of the appearing force fields were studied as well. Therefore, thin polystyrene films were placed on top of a micro-pillar array, which was made of poly(n-butyl acrylate). When dewetting was initiated, the pillars deflected by the flow of material around a dewetting hole. The forces acting on the pillars were proportional to their deformation. The deformation of the pillars was measured by optical microscopy. This enabled us to visualize the force field around a dewetting hole. Concurrently, the dewetting hole was being distorted and diverged from its initial circular shape. Analyzing the force field, the sum of forces acting on the pillars increased proportionally to the perimeter of the hole; the ratio of the sum of forces and the perimeter was equal to the surface tension of atactic polystyrene. During this patternforming process the elastic deformation of the pillars was balancing the driving capillary forces. Moreover, at later stages of dewetting, an imprint of the underlying square pattern of the pillar array on the envelope of the dewetting holes could be observed, showing the squaring of a circle during a pattern forming process by periodic perturbations.
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