Sub-micrometer Sized, 3D-Surface-attached Polymer Networks by Microcontact Printing: Using UV-Crosslinking Efficiency to Tune Structure Height.

The lateral dimensions of micro- and nanostructures obtained by microcontact printing (μCP) can be easily varied by selecting stamps with the desired spacing and pattern. However, the height of these structures cannot be tuned as easily, and in most cases only 2D structures are obtained. Here, we show how the chemical cross-linking properties of polymer inks designed for μCP can be used to obtain 3D structures with heights ranging from 3 to 750 nm using the same μCP stamps. This is technologically relevant because the ink concentration affects the quality and resolution of the printed image, and therefore can only be varied in a certain range. By exploiting the cross-linking efficiency to tune the height, an additional parameter is available to reach the desired structure height without compromising the image quality. The inks were made from copolymers containing a low percentage of different UV cross-linkable repeat units: nitrobenzoxadiazole (NBD), coumarin (COU), and/or benzophenone (BP). The base polymer of the here presented model system was an antimicrobially active poly(oxanorbornene) (SMAMP), however the concept should be transferable to many other polymer backbones. We describe the fabrication and characterization of the printed micro- and nanostructures made from pure SMAMP, NBD-SMAMP, coumarin-SMAMP, BP-SMAMP, BP-NBD-SMAMP and BP-coumarin-SMAMP polymer inks. The photo-dimerization of COU during UV irradiation at λ = 254 nm was confirmed by UV-Vis spectroscopy. Since NBD and COU are fluorescent, the polymer could be visualized by fluorescence microscopy. Additionally, their height profiles were measured by atomic force microscopy (AFM). The heights of the 3D surface-attached polymer networks obtained from the here presented polymer inks correlated with the gel-content of the corresponding unstructured polymer layers, and thus with the cross-linking efficiency of the NBD, COU and BP cross-linkers. Due to being covalently cross-linked, these 3D-surface attached polymer structures were solvent-stable and stable in aqueous surroundings.