A Study on Inking PDMS Stamp of Micro-Contact Printing

Author

Yeh-Min Lin

Journal

LOPE-C

Publication Date

06/28/2011

Abstract

Since the invention of its chemisorption version in 1993, micro-contact printing (CP) using a poly(dimethylsiloxane) (PDMS) stamp has received significant attention, and various physisorption-based CP methods were subsequently proposed for even broader applications including fabrication of organic electronics. Up to date, analysis of physisorption CP was all limited to adhesion work analysis for successful transfer of the ink film from PDMS to the substrate; no analysis has been devoted to quality inking on PDMS stamp. Without quality inking, one cannot realize the full benefit of the CP. This paper presents the first report which is primarily focused on inking the PDMS. Its contributions include (1) derivation and application of a necessary condition for adsorption of ink molecules on PDMS, a pre-requisite for any successful coating method, in terms of the surface free energies (with both dispersion and polar components) of the PDMS, ink molecules, and solvent and (2) observation and remedy of a fingering instability in spin-coating of non-polar polymers on PDMS. The necessary condition of adsorption was derived based on the Young’s equation of contact angle and successfully applied to spin-coating of the polar polymer, the conductive poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). However, for non-polar polymers such as the semiconductive poly(3-hexylthiophene) (P3HT), the necessary condition of adsorption alone could not elucidate the whole story of spin-coating on PDMS, in particular, regarding an interesting fingering instability. Attributing the fingering instability to non-uniform contact line due to sporadic absorption of the solvent of the ink solution by PDMS, a solvent pre-wetting technique was introduced. It is believed that by pre-wetting the PDMS with a solvent whose Hildebrand solubility parameter significantly different from that of PDMS, excessive absorption of the solvent of the ink solution could be effectively reduced, and consequently the fingering instability be avoided, rendering smooth inking results on PDMS.

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