Evaporation of Solvents from Polymer Surfaces

Evaporation of solvents from polymer surfaces

Contact person: PD Dr. rer. nat. Elmar Bonaccurso

An increasingly used technique for polymer microstructuring is inkjet etching. Solvent drops are deposited on polymers via an ink-jet nozzle. After evaporation of the drop a crater-shaped surface topology is left. This surface structure is formed owing to a flow of dissolved polymer to the rim of the pinned evaporating droplet. The shape of the microcraters is also governed by the surface tension of the drop, acting at the three-phase contact line (TPCL), by the capillary pressure at the bottom of the drop, by swelling, by dissolution of the polymer into the solvent, etc. Craters with diameters down to 20 µm and aspect ratios up to 0.07 have been made.

We can simplify the structuring procedure and use slow evaporating drops of non-solvents. This suppresses the evaporation-induced flow of polymer to the rim, while crater-like structures still occur. Part of the structuring results from the permanent uptake of solvent and from the combined effect of surface tension and capillary pressure of the drop acting on a thin softened polymer layer. Structures with aspect ratios of 0.5 can be obtained.

(a) Scheme of crater formation: (i) a solvent drop is deposited onto a flat polymer surface; (ii) the solvent starts to diffuse into the polymer, dissolving and swelling it; (iii) the drop deforms the softened polymer due to surface tension and evaporates with pinned TPCL. Solvent flows to the rim transporting polymer with it and depositing it there; (iv) after solvent evaporation a crater is left behind. Crater profiles produced by depositing five series of toluene drops on a polystyrene plate. Each series consists of a different number of 35 µm sized drops, deposited at an interval of 1 s from each other.

(b) A characteristic microstructure is left in a polymer surface after evaporation of a sessile ethylene glycol/water (EG/H2O) droplet in a solvent-rich environment. Regular arrays of size-controlled microvessels can be fabricated. The resulting height profiles can be modeled and approximated with elastic theory, based on the interplay between the interfacial tension, Ƴlv, and the capillary pressure, ΔP, of the liquid.

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