Dewetting Polymer Bilayers by Solvent Vapour Annealing: A Pathway to Manufacturing Biomimetic Water Harvesting Surface Coatings

Abstract

Producing patterned surfaces by a simple and controlled process has important implications for an increasing number of technological and biomedical applications. The dewetting of thin polymer films is a robust and efficient route towards the fabrication of surfaces with topographical and chemical patterns from the nano to the macro scale. In this Thesis, a solvent vapour annealing technique is presented which provides control over the dynamics of the dewetting process, as well as the dewetted morphology. Polystyrene (PS) films, which are only slightly metastable on a silicon substrate with a native oxide layer (SiO/Si), were annealed in the saturated vapour of a toluene-ethanol mixture, resulting in the fast dewetting of the PS film from the substrate. The dewetting rate was shown to depend on the ratio of the two solvents: the poor solvent ethanol acted to increase the dewetting driving force, while the good solvent toluene, acted to plasticise the PS. The morphology of the rims surrounding the holes and the PS droplets were also affected by the concentration of ethanol in the solvent mixture. Utilising the principle of good-poor solvent mixture annealing to promote the dewetting of polymer films, a bilayer of poly(4-vinylpyridine) (P4VP) / PS films was dewetted to produce topographically and chemically patterned polymeric surfaces. Acetone and ethanol, poor solvents for P4VP, and water, a non-solvent for P4VP, were combined in binary mixtures and the individual effect of each solvent on dewetting rate, rim and droplet morphology was investigated. Using this solvent vapour annealing technique, nano to macro scale P4VP droplets on PS coated copper substrates were prepared and the water harvesting capability of these patterned surfaces was analysed in a custom-built condensation chamber. The effect of pattern size and density was compared with the condensation performance of an unpatterned hydrophobic coating under different humidity and temperature conditions.