(651d) Diffusion and Mass Uptake Behavior of Polymer Thin and Ultra-Thin Films

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. 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 interface effect argument that has been previously used to explain the behavior. Quartz crystal microbalance experiments have been carried out to determine the diffusion coefficient and mass uptake for a number of different film thicknesses. The diffusion coefficient for water uptake into a poly(methyl methacrylate) film increases from sub-20 nm thicknesses over four orders of magnitude until if approaches bulk values at a thickness of around 2 microns. Over this range, the diffusion coefficient scales with thickness squared. We have also carried out studies on the molecular weight effect on diffusion coefficient to determine the universal scaling for these materials. Aging effects have also been studied, and the QCM allows us to determine the aging behavior of both diffusion coefficient and solubility. In addition to these experiments, positron annihilation lifetime spectroscopy (PALS) studies have been carried out to determine the thickness dependence on the size and distribution of free volume in these thin films. By combining information from all these sources, we have gained insight into the physical origin of this scaling of diffusion coefficient in thin and ultra-thin films.