Drying-induced structure formation in polymer films

Abstract

The thesis focusses on mechanisms of structure formation in drying polymer films. Most thoroughly investigated were structures, where irradiation with light created the pattern. Light was always converted to a source of heat, for instance by adding a dye to the material. The dye’s absorption amounts to a localized heat source. Sine the rate of evaporation is high at the irradiated spot, there is a flow of liquid towards the irradiated spot. In consequence, a “Mexican hat” develops. The phenomenon is related to what is called “evaporative lithography” in the literature. Novel to this work is the combined action of solvent flow and buckling. Buckling leads to a characteristic pattern, which is a small peak on top of a wider pedestal. Importantly, the buckling-induced features are narrower than the features induced by flow. Buckling was only found when employing dextran. Presumably, buckling is the consequence of an elastic skin. For reasons which are not understood to date, the minimum feature size obtainable with the flow mechanism only is limited to about 300 μm. The size of the laser spot is a few tens of microns. Starting from this experience other means of localized heating were explored, as well. Films were dried films on a polyester sheet with printed black lines onto the back. When drying a polystyrene film deposited on this kind of substrate under an IR-lamp, the black line absorbed heat. The films after drying showed elevations at the position of the black lines. We showed that the amplitude of the surface features depends on several parameters such as the initial thickness of the film, the molecular weight of polystyrene and the pitch size, where increasing the pitch size leads to increasing of amplitude of surface features. Motivated by the need to create patterned structures during drying, we investigated the properties of cellulose nanocrystals (CNC). CNC does have an impact on structure formation. When cellulose nanocrystals are dispersed in water, they form a stable suspension. Depending on concentration, there is a phase separation, where the phase with lower density is isotropic, while the other phase forms a chiral nematic liquid crystal. Upon drying, CNC suspensions form fingerprint-like textures. We reported the use of a CNC/latex blend as an ink for security printing. The inks is recognized as being special in diffuse reflection, if polarizers are inserted between the light source and the paper and, also, between the paper and the observer. Such devices are easily realized; their application is trivial. Ink made of CNC appears peculiar because it scrambles the polarization from the background without being a strong scatterer, itself. Application requires a dark background. Applied to black paper, the contrast strongly depends on whether the printed area is viewed with polarizers being parallel or with crossed polarizers.%%%%Die Dissertation befasst sich mit verschiedenen Mechanismen der Strukturbildung bei trocknenden Polymerfilmen. In der Regel wurde Licht als der…