(17c) Probing and Understanding the Diffusion Behavior in Polymer Thin Films

A variety of physical properties have recently been reported to depend on the sample dimensions for polymers in cases where the dimensions approach the 100 nm length scale. In this work, the diffusion behavior of spin cast polymer thin films was studied in detail as a function of film thickness. Diffusion coefficients of water molecules in poly(methyl methacrylate) (PMMA) films were studied via sorption-desorption experiments using a quartz crystal microbalance (QCM) system. The diffusion coefficients were found to decrease from 10-8 cm2/s in thick films to 10-13 cm2/s in ultra-thin films. In order to probe if there is a characteristic length scale set by the polymer chain size, the effect of PMMA molecular weights on this behavior was tested and no significant differences were observed. Diffusion coefficients in these films were also investigated as a function of aging time and were not found to significantly change. In contrast to the diffusion coefficients, the total mass uptake of water molecules in these films scaled linearly with film thickness which implies that the fractional free volume in these films is constant regardless of film thickness. The films were subjected to rigorous annealing to probe if potentially different residual casting solvent levels in the thick and thin films contribute to the observed diffusion coefficient changes. No substantial change was observed in the diffusion behavior with respect to different annealing conditions. Positron Annihilation Lifetime Spectroscopy (PALs) was also used to probe the free volume (FV) content distribution as a function of film thickness in PMMA. The results from PALs show that thicker films have a broader distribution of FV than thinner films. This change in FV distribution may be a key factor that leads to the significant drop in diffusion coefficient in thinner PMMA films. The origin of this FV change is being studied.