Abstract
We report a range of surface characteristics from strong and partial dewetting to complete wetting for thin films of cylinder and lamellae forming block copolymers (c-BCP and l-BCP, respectively) of polystyrene-b-polymethylmethacrylate (PS-b-PMMA) on flexible substrates. BCP of various molecular weights (Mw) was applied on elastomeric polydimethylsiloxane (PDMS) substrates of distinct cross-link densities and various surface energy (SE) obtained by exposure to UV-ozone. We demonstrate that lower Mw c-BCP films dewet faster than l-BCP films of higher Mw applied on PDMS of lowest cross-link density and SE < 25 mJ/m2. Partial dewetting was observed for c-BCP films applied on the lowest cross-link density substrates with 25 mJ/m2 < SE < 32 mJ/m2, with a sharper transition from dewetting to wetting for l-BCP films. Finally, complete wetting (stable films) occurred for both c-BCP and l-BCP films on PDMS substrates with lowest cross-link density and SE > 32 mJ/m2, as well as on PDMS of the highest cross-link density with SE > 25 mJ/m2. In this extended SE wetting regime, c-BCP show vertical orientation when applied on both types of cross-linked PDMS in a narrow neutral SE range (39–40) mJ/m2, while being oriented parallel to the substrate when SE > 40 mJ/m2. The ratio of vertically oriented l-BCP however decreases gradually when applied on substrates with a higher SE range (40–65 mJ/m2) in comparison to the sharply transitioning c-BCP orientation, reflecting the intrinsic stability of vertical lamellae over a wider range of substrate SE, consistent with theoretical estimates. These results have important ramifications for design of next-generation flexible electronics utilizing BCP thin films.
Highlights
Block copolymer films have a unique capability of selfassembly which makes them facile soft materials and a topic of current interest
We examine how to control the orientation of c-BCP and l-BCP by changing the PDMS substrate surface energy and its mechanical modulus through degree of cross-linking
Motivated by needs for flexible electronics of the future, particularity related to directed self-assembly (DSA) of BCPs on flexible substrates, we have mapped a large range of two BCP morphologies and studied their surface phenomena from dewetting to partial wetting to complete wetting on modified surface energy (SE) and varied flexibility/modulus PDMS substrates
Summary
BCP ordering and orientation in thin films have been intensively studied on supporting hard substrates such as glass, silicon, quartz, etc. [14,15,16]. Mansky et al and other researchers have performed extensive theoretical and practical studies on random copolymer brush layer grafted onto the substrate and film interfaces to control the interactions between them [39,40,41] These systems have the potential to affect a gradual substrate modification by creating a range of surface energy values. This idea was to further develop such double neutral surfaces comprising of random copolymer brushes to heighten the perpendicular orientation all along the BCP film [42] The power of these interfacial interactions is usually altered by exchanging each block composition from the random brushes. We examine how to control the orientation of c-BCP and l-BCP by changing the PDMS substrate surface energy and its mechanical modulus through degree of cross-linking
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