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Diffusion, Mass Uptake, and Free Volume Behavior of Polymer Thin and Ultra-Thin Films

Richard Lawson, Peter J. Ludovice, and Clifford L. Henderson. School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, GA 30332-0100

Diffusion of penetrants through polymer thin films is important in a variety of applications ranging from microelectronics to gas separations. Previous studies of diffusion behavior in ultra-thin films have shown a significant decrease in diffusion coefficient as the film thickness decreases. [1-3] These studies only investigate films up to a thickness of around 200 nm; at this thickness, the diffusion coefficient is still several orders of magnitude lower than that of the bulk. Our recent experiments have shown that the decrease in diffusion coefficient occurs over a much larger range of thicknesses than would be expected based on an interfacial effect argument that has been previously used to explain the behavior. Quartz crystal microbalance experiments have been carried out in a variety of polymers to determine the diffusion coefficient and mass uptake behavior for a number of different film thicknesses and film aging times. For example, the diffusion coefficient for water uptake into a poly(methyl methacrylate) film increases by over four orders of magnitude as film thickness is increased approximately 20 nm to approximately 2 microns in thickness where the film behavior becomes bulk like. Over this range, the diffusion coefficient scales with thickness squared. A discussion of the effect of molecular weight and aging time on diffusion coefficient and mass uptake in polymer thin films will also be presented along with a discussion of the physical origin of this scaling relationship of diffusion coefficient. Finally, positron annihilation lifetime spectroscopy studies will be presented and analyzed to elucidate the free volume behavior in such thin film materials and the connections between this free volume and the observed diffusion behavior.

[1] C. L. Soles, R. L. Jones, J. L. Lenhart, V. M. Prabhu, W.L. Wu, E. K. Lin, D. L. Goldfarb, M. Angelopoulos. Proc. SPIE 5039 (2003) 366.

[2] B. D. Vogt, C. L. Soles, H. J. Lee, E. K. Lin, W. L. Wu. Polymer 46 (2005) 1635.

[3] B. D. Vogt, C. L. Soles, H. J. Lee, E. K. Lin, W. L. Wu. Langmuir 20, (2004), 1453.