Understanding the diffusion of small molecules in glassy polymer films is very important to applications where selectivity is important, including membrane separations, barrier materials, the controlled-release of pharmaceuticals, and chemical sensors. The non-equilibrium nature of glassy polymers results in sorption and diffusion behavior that can be considerably more complicated than that observed in rubbery polymers. Time-resolved, Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy has been used increasingly to study diffusion in polymers and has proven to be very accurate and reliable. FTIR-ATR spectroscopy is capable of identifying changes in the local environment for both the penetrant and the polymer, resulting in information at the molecular level during the transport process.

In this study, FTIR-ATR spectroscopy was used to study the diffusion of a small molecule, acetonitrile, in a glassy polymer, cellulose acetate (CA) from the vapor phase. By monitoring the IR absorbances of the nitrile group in acetonitrile and the carbonyl group in cellulose acetate, the kinetics of sorption/desorption and the rates of penetrant-induced swelling/deswelling, respectively, were studied. An additional physical mechanism, resulting in a time delay prior to the appearance of a Fickian-like concentration profile, was uncovered with this technique. A dual mode transport model with local equilibrium relaxation was proposed and successfully used to capture this phenomenon, revealing that a finite hole-filling rate in the dual mode framework is necessary to fully describe transport in glassy polymers. A modified dual mode transport model, taking into account penetrant-induced plasticization in addition to local equilibrium relaxation, was also used and compared to the original version. Differences in the two models were most apparent when describing desorption and were ascribed to differences in the redistribution of molecules between the two modes at the start of the desorption process. Swelling and deswelling rates in the acetonitrile/CA system were predicted using the dual mode model, with and without the modification. Predictions were excellent for swelling, but the inability to predict deswelling was attributed to limitations inherent in the two models.

This work revealed that local equilibrium must be relaxed to fully describe diffusion in glassy polymers. The model developed here should find use in sensor applications of FTIR-ATR spectroscopy, where transient behavior is the key to performance.