Partial Dewetting of Polyethylene Thin Films on Rough Silicon Dioxide Surfaces Fajun Zhang,* Gabriel Baralia, Adrian Boborodea, Christian Bailly, Bernard Nysten, and Alain M. Jonas* Unite ´ de Physique et de Chimie des Hauts Polyme ` res, Universite ´ catholique de Louvain, Place Croix du Sud 1, B-1348 Louvain-la-Neuve, Belgium Received April 18, 2005. In Final Form: June 9, 2005 The effect of roughness on the dewetting behavior of polyethylene thin films on silicon dioxide substrates is presented. Smooth and rough silicon dioxide substrates of 0.3 and 3.2-3.9 nm root-mean-square roughness were prepared by thermal oxidation of silicon wafers and plasma-enhanced chemical vapor deposition on silicon wafers, respectively. Polymer thin films of ∼80 nm thickness were deposited by spin-coating on these substrates. Subsequent dewetting and crystallization of the polyethylene were observed by hot-stage optical microscopy in reflection mode. During heating, the polymer films melt and dewet on both substrates. Further observations after cooling indicate that, whereas complete dewetting occurs on the smooth substrate surface, partial dewetting occurs for the polymer film on the rough surface. The average thickness of the residual film on the rough surface was determined by ellipsometry to be a few nanometers, and the spatial distribution of the polymer in the cavities of the rough surface could be obtained by X-ray reflectometry. The residual film originates from the impregnation of the porous surface by the polymer fluid, leading to the observed partial dewetting behavior. This new type of partial dewetting should have important practical consequences, as most real surfaces exhibit significant roughness. Introduction The stability of polymer thin films on the surface of solid substrates is an important topic in fields, such as coatings, corrosion protection, membranes, biological interfaces. So far, significant progress has been achieved regarding the mechanisms of dewetting 1-6 and stabiliza- tion of polymer thin films. 7-11 An especially intriguing dewetting behavior of polymer thin films on solid sub- strates is partial dewetting, 12-19 where the polymer dewets while leaving an ultrathin continuous film on the solid substrates. There are currently three reported cases of partial dewetting. First, studies on the wetting properties of homopolymer films on polymer brushes of identical chemical composition indicate that a polymer may dewet its own brush for purely entropic reasons associated with conformational restrictions, depending on the molar mass and grafting density. 12-14 This phenomenon, usually referred to as autophobic dewetting, can be considered as an example of partial dewetting, since the final morphology consists of droplets of polymer over a remaining thin film of identical composition. In a related way, partial dew- etting may be observed for polymers interacting strongly with a given substrate, when the adhesive forces of the polymer to the substrate are stronger than the cohesive forces between molecules themselves. 15,16 In this case, partial dewetting is most likely due to the difference in conformation of adsorbed and nonadsorbed molecules. This type of partial dewetting was reported for liquid-crystalline polymer films 15 and poly(ethylene oxide) on UV-treated oxidized silicon substrates. 16 The third case of partial dewetting, nano-dewetting, was identified by Mu ¨ ller- Buschbaum et al., 17,18 who studied the dewetting of a polystyrene thin film atop different silicon substrates. These authors found a nano-dewetting structure consisting of small nanoscale dimples coexisting with the usual microscopic dewetting structure. 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