Patterning dewetting in thin polymer films by spatially directed photocrosslinking Gregory T. Carroll a, * , Nicholas J. Turro a,b , Jeffrey T. Koberstein b a Department of Chemistry, Columbia University, 3000 Broadway, MC 3157, NY 10027, United States b Department of Chemical Engineering, Columbia University, 500 West 120th Street, NY 10027, United States article info Article history: Received 5 May 2010 Accepted 28 July 2010 Available online 2 August 2010 Keywords: Dewetting Patterning Photocrosslinking Thin films abstract In this report we examine the dewetting of spin-cast poly (styrene) films in a confined geometry. We designed a platform for laterally confining PS by photo-patterning crosslinks in spin-coated thin films. Heating the patterned film above the glass transition temperature of PS results in localized dewetting patterns in regions that were not crosslinked, while the crosslinked pattern serves as a rigid barrier that confines the retraction of the uncrosslinked polymer in micron-sized domains. The barriers also provide a favorable surface that the liquid PS wets onto, forming a rim at the boundary of crosslinked and uncross- linked polymer. The resulting patterns are shown to be dependent on the irradiation and annealing time, the dimensions of the uncrosslinked region and the thickness of the film. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction Self-organization processes offer exciting opportunities in gen- erating micro- and nano-scale architectures and materials. The resulting interfacial structures generated by such processes can imbue a material with unique physical properties. For example, the undulating pattern of the inner membrane of the mitochondria results in a very large surface area, increasing the amount of possi- ble locations for the presentation of ATP synthesizing proteins [1]. The surfaces of many organisms contain micro- and nano- structured motifs that provide important functions such as the self-cleaning abilities of the lotus leaf [2,3] and wings of the cicada [4], water-harvesting by the Namib desert beetle [5] and the hydrophobic ‘‘water-walking” legs of the water strider [6,7].A common goal in materials science is the utilization of self-organiz- ing processes to create three-dimensional structures and patterns on surfaces to facilitate the fabrication of devices. Pertinent applications include anti-fog and anti-reflection coatings [8], water-harvesting surfaces [9], decontaminating surfaces [10], surface roughness-enhanced adsorption [11], surface-enhanced spectroscopic signals [12], micro- and nano-containers [13–16] and the study of fluid flow in confined spaces [17,18]. Many reports have described methods that allow for polymer films and other materials to organize into characteristic structures [19–23]. Surface tension can lead to interesting morphological structures in polymer films. An important surface-tension driven process is the dewetting of thin polymer films where a macromo- lecular layer retracts from a surface that does not favor spreading [24–26]. A classic example is the dewetting of poly styrene (PS) on silicon wafers. Smooth thin (typically less than 100 nm) PS films can be prepared on Si substrates by spin-coating, however upon heating the films above T g , the polymer dewets, forming droplets with finite contact angles. Dewetting proceeds by the formation of isolated holes, followed by their growth through the retraction of the hole perimeter into the liquid film. A rim of liquid is created ahead of the retracting front, which eventually contacts other rims forming ribbons. The ribbons eventually break into drops splayed in a cellular structure. Additionally, a bicontinuous pattern has been observed for very thin films [27]. The origin of the holes at the earliest stages of the dewetting process has been attributed to two mechanisms: spinodal decomposition and nucleation onto defect sites [28]. In the spinodal mechanism, thermal undulations (with wavelength k) increase the interfacial area, which raises the free energy (in proportion to the surface tension, c). The undu- lations also change the local film thickness h, and hence locally change the free energy per unit area of the van der Waals interac- tions in the film: F ðhÞ¼A=12ph 2 ; ð1Þ where A is the Hamaker constant of the polystyrene/SiO 2 system. When the spinodal parameter d 2 F/dh 2 < 0 (i.e. A > 0), the van der Waals interactions cause the film to rupture for wavenumbers (2pk) smaller than a critical value as given by: k 2 c ¼1=kd 2 F =dh 2 : ð2Þ 0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.07.070 * Corresponding author. E-mail address: gtcarroll@gmail.com (G.T. Carroll). Journal of Colloid and Interface Science 351 (2010) 556–560 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis