We have developed a method to fabricate permanent surface topologies as ripples, wells, and channels in polymers [1]. The method is a combination of plasma, UV or ion treatment of the polymer surface (step I) and swelling (step II; see image), for which we use the term Structured Surface Swelling (SSS).

	Structured Surface Swelling (SSS) to the left: Procedure of SSS; above: AFM image of a microwell array in an extruded polystyrene (PS) surface after SSS through a mask and swelling in toluene

A mask with holes is set on top of a polymer substrate (here: extruded polystyrene / PS). Plasma, UV light or a beam of argon ions is applied through the holes of the mask, leading to a locally cross-linked polymer surface (step I, B). Alternatively, a material like gold is deposited locally through the holes of the mask on top of the polymer surface (A). Then the mask is removed and the pre-structured polymer is exposed to a good solvent (here: toluene). The polymer between the cross-linked or gold-coated areas starts to swell to give surface protrusions (here: as an array of microwells). After removal of the solvent the protrusions persist. Usually, we observed depths up to some micrometer. Depending on the size of the holes in the mask we could fabricate repeated surface protrusions below 1 µm. We use our surface structuring technique to investigate physical processes in restricted volumes. The next image shows an example for a crystallization of K₂SO₄, a salt, performed simultaneously in an array of microwells.

Microscopic image of the crystallization of K2SO4 in an array of microwells in PS (width of each well: 45 µm) The array has been fabricated with gold as depicted in the scheme above (A). The gold plates have been thiolised afterwards with -Hydroxythiol to render the bottom of the microwells hydrophilic. After a simple upstroke of the modified polymer microwell array from an aqueous solution of K2SO4, water droplets form exclusively in the interior of the hydrophilic microwells, forming crystals during evaporation.

Beside these implications for application [2,3], we study the physical origin for the formation of the persistent protrusions. They only occur if the polymer has been extruded or stretched prior to SSS. Therefore, it is likely that the protrusions occur due to relaxation of stress and/or orientation of polymer molecules within the surface upon exposure to a solvent. Indeed, if the polymer is exposed to moderate plasma (low power, short time) without use of a mask, persistent surface ripples are observed after contact with solvent [4]. The ripples appear to be more parallel at higher stretching ratios (see image).

AFM images of surface protrusions in PS substrates after exposure to air plasma (30 W, 4 min) and consecutive swelling in toluene left image: hot-pressed PS substrate; right image: stretched PS substrate by a factor of 2

These results show that the morphology of such protrusions within the polymer surface can be tuned by the preparation of the substrates prior to SSS. Currently, we are working on experiments to better understand the origin of the protrusions. This project is part of the Center for Microchemistry, Nanochemistry and Engineering at the University of Siegen and funded by the Deutsche Forschungsgemeinschaft (DFG) as part of the Research Unit 516 (GR2003/2-1 FOR516).

References:

1. E. Bonaccurso, H.-J. Butt, K. Graf, Microarrays by Structured Substrate Swelling, European Polymer Journal, 2004, 40, 975-980. [Full text]
   
2. K. Graf, H.-J. Butt, E. Bonaccurso, "Herstellung von Microarrays durch Lösungsmitteldämpfe", Patentveröffentlichung Universität Siegen, DE 10253077 A1 vom 27.05.2004.
   
3. E. Bonaccurso, K. Graf, The nanostructuring effect of plasma and solvent treatment on polystyrene, Langmuir, 2004, 20, 11183-11190. [Full text]