(694f) Micromolding of UV Curable Coatings

Du, Y. - Presenter, University of Minnesota
McCormick, A., University of Minnesota, Twin Cities
Francis, L. F., University of Minnesota
In micromolding, a microstructured coating is created by pressing a stamp into a coating, UV curing with the stamp in place, and then removing the stamp. The process can be implemented in roll-to-roll systems and nanometer to micrometer features can be created; however, one limitation of UV-micromolding is low throughput mainly due to slow curing speed. Additionally, defects may be introduced into the microstructured coating because of strong adhesion between the mold and the coating.

In this research, we explored new formulation approaches based on thiol-ene-acrylate ternary systems, which are insensitive to oxygen during the reaction, cure fast and form highly homogeneous polymer networks. The curing rates and the limiting extents of cure of various thiol-ene-acrylate coating formulations, characterized by real-time Fourier transform infrared spectroscopy (FTIR), increase with the amount of acrylate oligomers in coating formulations. By changing the acrylate concentration in the coating formulation, cured resins with a wide range of mechanical properties have been achieved, from stiff highly cross-linked polymers to soft elastomers. At the same time, the surface energy of a cured coating is independent of the acrylate concentration.

Microchannel arrays composed of 50 µm wide and 25 µm deep channels were successfully replicated with our thiol-ene-acrylate coating systems using polydimethylsiloxane stamps fabricated from SU-8 photoresist masters. The pattern quality strongly depends on the duration of UV exposure and the mechanical properties of cured coatings. For a specific coating formulation, there exists a lower limit of the curing dosage for successful pattern replication. Well-defined surface microstructures are completely transferred to coatings with various elastic moduli. However, the mechanical properties of coatings still need to be optimized to avoid damage of surface features during the demolding step.

To understand the mechanism of debonding between the stamp and the structured coating surface, the effects of the demolding direction and the coating modulus on the demolding force were investigated by T-peel testing. Results reveal different demolding characteristics between the coatings demolded parallel and perpendicular to surface micro-channels. Furthermore, a smaller demolding force is observed during the demolding of low-modulus coatings.