Surface modification of polymeric nanocomposite thin films using supercritical carbon dioxide Tadanori Koga a, *, C. Li a , Y. Sun a , A. Brazin b , M. H. Rafailovich a , J. C. Sokolov a , J. F. Douglas c and D. Mahajan a,d a Department of Materials Science & Engineering, State University of New York at Stony Brook, Stony Brook, New York 11794-2275, USA b HAFTR High School, Cedarhurst, New York 11516, USA c Polymers Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA d Energy Science & Department, Brookhaven National Laboratory, Upton, New York 11973-5000, USA We report on an efficient and environmentally friendly means to modify surface properties of polymer films supported for nanoparticles. Ultrathin polystyrene (PS) films (<300 A ˚ ), in which inorganic nanoparticles were embedded, were exposed to supercritical carbon dioxide (scCO 2 ). The swollen structure was then preserved by quickly evaporating CO 2 . X-ray reflectivity (XR) results showed that this procedure produced polymeric nanocomposite films with a low-density region of about 150 A ˚ at the polymer/air interface. The formation of the low-density layer was independent of the nature of the particles, indicating that the surface modification through exposure to scCO 2 may be a universal phenomenon regardless of a choice of nanoparticles. KEY WORDS: supercritical carbon dioxide; inorganic nanoparticles; low-density polymer thin films; glass transition; X-ray reflectivity. 1. Introduction The use of nanoparticles supported on polymers as catalysts has revealed that the catalytic activity of such materials is greatly influenced by the nature of polymer matrix and reaction solvent [1]. Recently, supercritical carbon dioxide (scCO 2 ), which is readily accessible with a critical temperature (T c ) of 31.3 °C and critical pressure (P c ) of 7.38 PMa, has been extensively used as a ‘‘green’’ medium for homogenous and heteroge- neous catalytic reactions due to its special characteristics as a solvent [2]. In particular, the physical and transport properties of scCO 2 exhibit a novel hybrid of gas-like and liquid-like properties, and its solvation power can be easily and widely tuned by changing the temperature and pressure [3,4]. These properties result in a great improvement of the mass transport to the catalyst active site, especially in reactions controlled by external diffusion. In this paper we report the effect of scCO 2 on the structure of polymer thin films with embedded nano- particles. Recently, we have found that density inho- mogeneities (density fluctuations) present in the supercritical state enhanced the solvent quality of scCO 2 for polymer thin films at the ‘‘density fluctuation ridge’’. This ridge is a locus of points that define the maximum long-range density fluctuation amplitude in CO 2 .A large enhancement in the solvent quality was observed to occur for all polymer films, even when the bulk miscibility with CO 2 was very poor [5–10]. We also showed that when the polymer films were glassy at room temperature, it was possible to rapidly evaporate the CO 2 and preserve the swollen structures as it was [11]. In order to effectively use nanoparticles as catalysts, it is important to develop a proper matrix which provides support while at the same time allows for flow or reaction gases across the active surfaces. The increase in porosity introduced by scCO 2 exposure provides an attractive method for producing these scaffolds. The major advantage is that the porosity can be created without further processing of the polymer with corrosive solvents or heat treatments which may oxidize or alter the surface properties of the particles. We studied functionalized gold nanoparticles embedded in a polystyrene (PS) matrix as a model system. Gold nanoparticles can be highly efficient catalysts because the catalytic functions can be con- trolled by changing the size and shape of the nanoparticles [12]. In addition, gold nanoparticles added to polymers are known to significantly enhance various physical properties such as UV absorption, electrical conductivity and optical dispersion [13–15]. In order to probe the effect of aspect, we also studied thin films with functionalized clay particles, which are often added to polymers in order to increase thermal and UV stability, as well as enhancement of mechan- ical properties [16,17]. We used X-ray reflectivity (XR) to study film structures with the nanoparticles on the molecular scale. The XR data showed that the polymeric nanocomposite films were swollen by scCO 2 . The linear dilation was slightly smaller than that previously reported for the homopolymer films, but no void formation occurred despite the presence of the nanoparticles. These results were surprising because there have been previous reports of *To whom correspondence should be addressed. E-mail: tkoga@notes.cc.sunysb.edu Topics in Catalysis Vol. 32, Nos. 3–4, March 2005 (Ó 2005) 257 DOI: 10.1007/s11244-005-2907-4 1022-5528/05/0300–0257/0 Ó 2005 Springer Science+Business Media, Inc.