| Molecular diffusion in polymers plays a significant role in many applications including the membrane separation of biological and chemical mixtures, the controlled-release of pharmaceutical compounds, and the optimal design of fuel-cell membranes, biomaterials, and selective-barrier materials. In many of these applications, the polymer is exposed to a mixture of penetrants that can either interact with the polymer or with each other. An important aspect of optimizing or characterizing these applications is understanding the interactions that occur and how they will affect the overall transport rates across a polymer membrane. To analyze these complex systems, experimental techniques that can identify penetrants, polymer groups, and their interactions on a molecular level is vital.; In the past, the more common techniques for studying diffusion in polymers have been permeation (gases), gravimetric sorption (condensable gases and vapors), liquid sorption (liquids), and the side-by-side diffusion cell (liquids). Unfortunately, these methods only measure the total mass (or total concentration) of penetrant and either cannot distinguish between separate penetrants or cannot probe molecular interactions during the diffusion process. Over the past ten years, time-resolved Fourier-transform infrared, attenuated total reflectance (FTIR-ATR) spectroscopy has been used more frequently to measure single-penetrant diffusion coefficients and has proven to be very accurate and reliable. This technique, however, can also overcome many limitations of other techniques with its ability to sensitively distinguish between different chemical species and detect chemical interactions through shifts in the infrared spectrum.; In this study, diffusion models were combined with time-resolved FTIR-ATR spectroscopic experiments to elucidate complex phenomena, such as penetrant-polymer binding in homogeneous and heterogeneous (phase-segregated) polymers and penetrantpenetrant interactions in self-associating and multicomponent systems. Equilibrium constants, penetrant aggregate sizes, individual or “true” diffusion coefficients, and binding effects, which all can have a significant impact on the transport process, were quantitatively measured using this technique. This work establishes new relationships between experimental results and diffusion parameters, which can aid in the future optimization of polymer processes for a variety of applications. |
