Anomalous swelling of polymer films has been reported by a number of research groups near the critical pressure of carbon dioxide.^1-3 However, the origins of this behavior have been elusive to identify: hypotheses have included density fluctuations of the CO₂ with enhanced swelling through the full film,⁴ adsorption at the polymer surface,³ and accumulation of CO₂ at the substrate/polymer interface. Simulations have suggested that the anomalous swelling is due to adsorption at the polymer surface,3 but experiments that alter the polymer chemistry are inconsistent with this result.¹ Neutron reflectivity has been used to directly measure moisture accumulation at buried polymer-inorganic oxide interfaces.^5,6 However, the limited contrast afforded by CO₂ does not allow for facile determination of the CO₂ distribution within the film. However at high q for films swollen near the critical point of CO₂, there is attenuation of the Kiessig fringes;⁷ similar features at high q has been attributed to non-uniform distribution of molecules within a polymer film (notable at the buried interface).⁶ The contrast difficulties however have limited the interpretation of the distribution for polymer-CO₂ systems.

In this work, a reactive modification route to indirectly observe the distribution of CO₂ at polymer-silica interfaces is proposed. Selective condensation of TEOS within a block copolymer template results in the in-situ formation of a silica-polymer interface. Using a BCC sphere morphology allows for accumulation of CO₂ at this interface during the synthesis to be observed post synthesis through the pore size. A significant increase in the pore size is observed near the critical pressure; this occurs at a similar CO₂ activity as the enhanced swelling of polymer films supported on silicon wafers.

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1. Li, Y.; Park, E. J.; Lim, K. T. L.; Johnston, K. P.; Green, P. F. Role of interfacial interactions on the anomalous swelling of polymer thin films in supercritical carbon dioxide. Journal of Polymer Science Part B-Polymer Physics 2007, 45 (11), 1313-1324.

2. Koga, T.; Akashige, E.; Reinstein, A.; Bronner, A.; Seo, Y. S.; Shin, K.; Rafailovich, M. H.; Sokolov, J. C.; Chu, B.; Satija, S. K. The effect of density fluctuations in supercritical fluids: new science and technology for polymer thin films. Physica B-Condensed Matter 2005, 357 (1-2), 73-79.

3. Wang, X. C.; Sanchez, I. C. Anomalous sorption of supercritical fluids on polymer thin films. Langmuir 2006, 22 (22), 9251-9253.

4. Koga, T.; Seo, Y. S.; Zhang, Y. M.; Shin, K.; Kusano, K.; Nishikawa, K.; Rafailovich, M. H.; Sokolov, J. C.; Chu, B.; Peiffer, D.; Occhiogrosso, R.; Satija, S. K. Density-fluctuation-induced swelling of polymer thin films in carbon dioxide. Physical Review Letters 2002, 89 (12).

5. Vogt, B. D.; Soles, C. L.; Jones, R. L.; Wang, C.-Y.; Lin, E. K.; Wu, W.; Satija, S. Interfacial effects on moisture absorption in thin polymer films. Langmuir 2004, 20, 5285-5290.

6. Vogt, B. D.; Soles, C. L.; Lee, H.-J.; Lin, E. K.; Wu, W. Moisture absorption into ultrathin hydrophilic polymer films on different substrate surfaces. Polymer 2005, 46, 1635-1642.

7. Koga, T.; Ji, Y.; Seo, Y. S.; Gordon, C.; Qu, F.; Rafailovich, M. H.; Sokolov, J. C.; Satija, S. K. Neutron reflectivity study of glassy polymer brushes in density fluctuating supercrifical carbon dioxide. Journal of Polymer Science Part B-Polymer Physics 2004, 42 (17), 3282-3289.