Optical determination of molecule diffusion coefficients in polymer films

Abstract

We present a method using surface-plasmon leaky mode spectroscopy for the analysis of the vapor–molecule diffusion process in polymer films leading to the diffusion coefficients of the molecules. The solution of the diffusion differential equation in the case of thin films yields a molecule concentration profile. Linearity between refractive index increase and molecule concentration gives the same variation for the index increase. Thus, a theoretical expression for the measured reflectivity can be derived using the transfer-matrix formalism for layered media, and theoretical fits to measured reflectivity data finally yield the corresponding diffusion coefficients. For the polymer Teflon®AF1600 diffusion coefficients for benzene and toluene are determined to 4.9×10−10 and 2.7×10−10 cm2/s, respectively.