Small Molecule Diffusion in Polymer Ultra-Thin Films
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0899-N05-05.1
Small Molecule Diffusion in Polymer Ultra-Thin Films Ivan Ordaz, Lovejeet Singh, Peter J. Ludovice, Clifford L. Henderson* School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, GA 30332-0100 * Corresponding Author: E-mail: [email protected], Phone: (404)-385-0525 ABSTRACT The influence of film thickness and polymer molecular weight on the diffusion coefficient of water in poly(methyl methacrylate) thin films supported on gold coated surfaces has been studied using vapor sorption experiments via quartz crystal microbalance (QCM) methods. Diffusion coefficients for films ranging in thickness from approximately 1 µm to 50 nm were determined. It is observed that the diffusion coefficient of water in PMMA on weakly interacting substrates such as gold is a strong function of film thickness, and that the diffusion coefficient decreases drastically as film thickness is reduced below a critical thickness value. Furthermore, it is found that polymer molecular weight also appears to play an important role in determining the diffusion behavior of such polymer thin film systems. 1. INTRODUCTION Polymer thin and ultra-thin films are a critical element today in a variety of applications including fuel cell membranes, semiconductor manufacturing, organic electronics, biomedical and tissue engineering, and industrial gas separations. [1,2,3,4] In such applications, the current lack of a fundamental understanding of the physiochemical properties of polymer thin films and their dependence on sample dimensions poses a significant roadblock to the rational design of improved materials and processes for these applications. Recent studies by the authors and others have shown that a wide variety of polymer film properties deviate from bulk behavior as the film thickness decreases below some critical value. The scaling of this critical film thickness has been shown to vary over a wide range of length scales and to depend on the particular property of interest. In addition, no single theory proposed thus far can adequately explain all of the observed thin film behavior, and in many cases the observed changes in film properties appear to be inconsistent with one another. For example, it has been observed that the apparent glass transition temperature (Tg) of supported polymer thin films can decrease when the film is coated on a weakly interacting substrate. Current explanations for this behavior are based on increases in polymer chain mobility near the film free surface. [5] On the other hand, recent experiments by the authors have shown that the diffusion coefficient of small molecules (e.g. water, solvents, and gases) in such supported thin films decreases dramatically as the film thickness decreases. If the phenomena are due to variations in polymer chain mobility and dynamics as others have suggested, the Tg results suggest higher chain mobility in thinner films while the diffusion results would seem to indicate decreased chain mobility and slower segmenta
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