Diffusion of associating and solvating penetrants in polymers above and below the glass transition

Polymer applications usually involve the diffusion of one or more strongly-interacting species in the polymer, which makes it important to understand diffusion in these systems on a molecular level. In this work, molecular self-association and intersolute or polymer-solute solvation are the two types of molecular interactions that have been considered.; First, a transport model was developed to study the effect of unequal transport rates and intersolute solvation on the selective batch extraction of a binary solute mixture using a dense polymeric sorbent. The model shows that separation factors higher than the equilibrium value can be obtained at intermediate times and at high levels of extraction, under certain conditions.; Second, transport models were derived for diffusion with self-association or intersolute solvation through a membrane. Numerical solutions for the transient portion of the cumulative flux expression were used to determine the effect of association or solvation on the breakthrough time, and the results were interpreted in terms of local sources and sinks of penetrant.; Third, FTIR-ATR spectroscopic data for the diffusion of ethanol and methanol in rubbery polybutadiene was analyzed. The OH stretching region consists of two major bands; one for free hydroxyl groups and one for hydrogen bound hydroxyl groups. Mathematical models assuming cluster immobility and local equilibrium between the two “species” were developed. The mechanisms of self-association, the size and structure of clusters, and the diffusion coefficients of the unassociated penetrant were determined from fitting both sets of data with the derived models.; Finally, experiments were performed for the diffusion of methanol in glassy polycarbonate using FTIR-ATR spectroscopy. Deconvolution of time-evolved methanol spectra resulted in an overshoot for the unassociated alcohol while the clusters simply increased monotonically to the equilibrium value. A transport model accounting for the kinetics of cluster formation was developed to capture this overshoot behavior. The rate constant for cluster formation can be interpreted in terms of a relaxation time scale within the glassy polymer matrix. Spectroscopic evidence that the unassociated OH groups hydrogen bond with the carbonyl groups in the polymer backbone was also obtained.