Photoactive Additives for Cross-Linking Polymer Films: Inhibition of Dewetting in Thin Polymer Films Gregory T. Carroll, ² Melissa E. Sojka, § Xuegong Lei, ² Nicholas J. Turro,* ,²,‡ and Jeffrey T. Koberstein* ,‡ Department of Chemistry, Columbia UniVersity, 3000 Broadway, MC 3157, New York, New York 10027, Department of Chemical Engineering, Columbia UniVersity, 500 West 120th Street, 10027, New York, New York 10027, and cLS fur Umweltmesstechnik, UniVersitaet Karlsruhe, 76128 Karlsruhe, Germany ReceiVed April 24, 2006. In Final Form: June 19, 2006 In this report, we describe a versatile photochemical method for cross-linking polymer films and demonstrate that this method can be used to inhibit thin polymer films from dewetting. A bifunctional photoactive molecule featuring two benzophenone chromophores capable of abstracting hydrogen atoms from various donors, including C-H groups, is mixed into PS films. Upon exposure to UV light, the bis-benzophenone molecule cross-links the chains presumably by hydrogen abstraction followed by radical recombination. Photoinduced cross-linking is characterized by infrared spectroscopy and gel permeation chromatography. Optical and atomic force microscopy images show that photocrosslinked polystyrene (PS) thin films resist dewetting when heated above the glass transition temperature or exposed to solvent vapor. PS films are inhibited from dewetting on both solid and liquid substrates. The effectiveness of the method to inhibit dewetting is studied as a function of the ratio of cross-linker to macromolecule, duration of exposure to UV light, film thickness, the driving force for dewetting, and the thermodynamic nature of the substrate. Introduction Polymer thin films have important applications in materials science and engineering including the fabrication of sensors and microelectronic, optoelectronic, and biomedical devices and the control of wettability, adhesion, barrier properties, and friction and wear. 1,2 A current problem in thin film technology involves stabilizing a polymer film on a given surface such that the polymer remains wetted on the substrate. A smooth surface is said to be wetted by an adsorbed species when van der Waals interactions at the substrate-liquid, liquid-air, and substrate-air interfaces allow the liquid to spread on the surface such that the contact angle is zero or very close to zero. 3 The effect of the interfacial tensions on the contact angle, θ, is described by the Young equation where γ sv is the solid-vapor interfacial tension, γ sl is the solid- liquid interfacial tension, and γ lv is the liquid-vapor interfacial tension. When a thin film is cast on a nonwettable surface for which the interfacial tensions do not favor wetting, dewetting can occur, a process in which the film retracts from the substrate, typically by forming holes, and organizes into structures that ultimately decay into a stable state. 4 In many cases, this is undesirable because it will compromise the function of any device for which the film is a component. Studies on PS films have shown that the dewetting process generally occurs in three stages. 5 In the first stage, the film breaks up into holes with the mass of the film either redistributed evenly across the film or collected in a rim at the perimeter of the holes. In the second stage, the holes grow and coalesce to form a morphology consisting of a polygonal pattern of unstable ribbons. In the third stage, the ribbons break down into stable droplets. Thin films can be cast on nonwettable substrates by spin- coating. The polymer chains become frozen in a vitrified state that stabilizes the film because the chains lose their mobility. Such glassy films are metastable 6 and will spontaneously dewet the substrate when the polymer chains gain enough mobility by heating above the glass transition temperature, T g , or exposure to solvent vapor. The likelihood of wetting for a thin film is determined by the sign of the spreading coefficient, S 2 If S is positive, the liquid will wet the surface. If S is negative, the liquid dewets the substrate. Numerous examples of systems that dewet have been reported. Polystyrene (PS) has been shown to dewet on various solid and liquid substrates including Si, 5 poly(dimethylsiloxane) (PDMS), 7 and poly(methyl methacrylate) (PMMA). 8 PDMS films with a thickness below 500 nm on silanized Si have been reported to dewet spontaneously. 9 Bilayers composed of polycarbonate deposited on poly (styrene-co-acrylonitrile) dewet when annealed above T g . 10 Films composed of mixtures of deuterated oligomeric styrene and oligomeric ethylene-propylene were found to dewet * To whom correspondence should be addressed. ² Department of Chemistry, Columbia University. ‡ Department of Chemical Engineering, Columbia University. § Universitaet Karlsruhe. (1) Granick, S.; Kumar, S. K.; Amis, E. J.; Antonietti, M.; Balazs, A. C.; Chakraborty, A. K.; Grest, G. S.; Hawker, C.; Janmey, P.; Kramer, E. J.; Nuzzo, R.; Russell, T. P.; Safinya, C. R. J. Polym. Sci., Part B: Polym. Phys. 2003, 41 (22), 2755-2793. (2) Garbassi, F.; Morra, M.; Occhiello, E. Polymer Surfaces: From Physics to Technology Updated Edition; John Wiley & Sons: New York, 1998. (3) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; John Wiley & Sons: New York, 1997. (4) Mueller-Buschbaum, P. J. Phys.: Condens. Matter 2003, 15 (36), R1549- R1582. (5) Reiter, G. Phys. ReV. Lett. 1992, 68 (1), 75-8. (6) Reiter, G.; De Gennes, P. G. Eur. Phys. J. E 2001, 6 (1), 25-28. (7) Reiter, G. Phys. ReV. Lett. 2001, 87 (18), 186101/1-186101/4. (8) Lambooy, P.; Phelan, K. C.; Haugg, O.; Krausch, G. Phys. ReV. Lett. 1996, 76 (7), 1110-13. (9) Redon, C.; Brzoska, J. B.; Brochard-Wyart, F. Macromolecules 1994, 27 (2), 468-71. (10) Faldi, A.; Composto, R. J.; Winey, K. I. Langmuir 1995, 11 (12), 4855- 61. cos θ ) (γ sv - γ sl )/γ lv (1) S ) γ sv - γ sl - γ lv (2) 7748 Langmuir 2006, 22, 7748-7754 10.1021/la0611099 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/05/2006