Combinatorial Mapping of Polymer Film Wettability On Gradient Energy Surfaces
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Combinatorial Mapping of Polymer Film Wettability On Gradient Energy Surfaces Karen Ashley1, A. Sehgal2, Eric J. Amis2, D. Raghavan1*, and A. Karim2* Polymer Division, Department of Chemistry, Howard University, Washington DC 20059. [email protected], [email protected] 2 Polymers Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, [email protected], [email protected], [email protected] 1
ABSTRACT Combinatorial methods were used for studying dewetting of thin hydrophobic polystyrene (PS) and hydrophilic poly(DL-lactic acid) (PDLA) films on chemically modified gradient energy surfaces. Substrate libraries were prepared by immersing passivated Si (Si-H surface functionality) in Piranha solution (H2SO4/H2O2/H2O) at a controlled rate, yielding a systematic variation of solvent contact angles across the surface. Additionally, chlorosilane-treated Si surfaces were exposed to UV radiation in a gradient fashion under ozone atmosphere such that a range from hydrophobic to hydrophilic conditions was obtained across the surface (≈ 3 cm). Solvent droplet contact angles of water and diiodo methane were used to quantify the spatial variation of surface energy along one axis across the surface. Libraries of thin films of PS or PDLA coatings on gradient energy surfaces orthogonal to gradients in film thickness were screened for dewetting behavior using automated optical microscopy. Contrasting trends in the wettability of PS and PDLA were visibly apparent as a function of surface energy of the substrate. The number density of polygons of the dewet PS films was found to obey a power law relationship with both film thickness and substrate surface hydrophilicity. INTRODUCTION Film stability and dewetting are important for applications in coatings ranging from photoresists, paints, adhesives, to lubricants, and biomaterials. Recently, much effort has been directed towards understanding the parameters that control the stability of thin polymeric films on solid substrates [1-6]. The parameters that influence film stability include substrate chemistry and roughness, time of equilibration, temperature of equilibration, polymer chemistry and molecular weight. Owing to the multivariable parameter space and the complexity of interactions between these variables, considerable theoretical and experimental effort has been directed towards understanding the physics of film stability on a substrate. Current experimental and theoretical literature of thin film stability on substrates only addresses chemically homogeneous Si or Silicon oxide substrates where the oxide film thickness is used to control the surface interaction [4, 7]. With the exception of recent theoretical work, the perturbing influence of surface energy gradients or quantification of the effect of surface energy on film stability has not been systematically studied [8]. We employ an approach to study the role of surface energy on film dewetting by etching silicon substrates with acid for different lengths of time. This approach provides a method to
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