(346c) In Situ Pressure-Contact Time-Resolved Fourier Transform Infrared Attenuated Total Reflectance Spectroscopy: A New Method to Measure Liquid Diffusion in Free-Standing Polymer Films

A conventional method (ex situ gravimetry) for measuring liquid diffusion in free-standing polymer films typically consists immersing a film into a liquid, manually removing the film at a given time interval, removing the film and blotting the excess liquid from the surface of the film, weighing the film, re-immersing the film in the liquid, and then repeating this process many times to construct weight gain versus time data. However, this ex situ technique can introduce significant error when the sample size is small (typical of polymers synthesized at the academic laboratory scale) or take significant experimental time when the sample is thick, and this technique does not provide any molecular level detail. In this study, a new technique was developed to accurately measure liquid diffusion in free-standing polymer films with molecular level detail: in situ pressure-contact time-resolved Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy. Previously, many studies have demonstrated the accurate measurement of liquid diffusion in polymer films using FTIR-ATR spectroscopy; however, polymers typically require solution casting on to an ATR element (or crystal) to ensure adequate adhesion or contact between the polymer film or crystal, where the infrared signal is measured inside the polymer close to the polymer/crystal interface. The new method developed in this study allows for an accurate measurement of liquid transport in polymer films for the case where the films cannot be easily solution cast or solution casting can alter the properties of polymer films that are processed using alternative methods (e.g., extrusion). A custom-made cell was designed to introduce the liquid above the polymer film while also applying an adequate pressure between the polymer film and the ATR crystal to ensure adequate adhesion for accurate FTIR-ATR measurements. Liquid transport was investigated in free-standing polymer films with this new technique and compared with liquid transport measurements in free-standing polymer films using conventional gravimetry and liquid transport measurements in solution-cast polymer films using the conventional FTIR-ATR technique. Experiments were performed on both rubbery and glassy polymer films and also as a function of temperature and film thickness. Non-Fickian diffusion was observed in glass polymer films, specifically diffusion-relaxation phenomena was observed, where the relaxation behavior differed for free-standing films compared to solution-cast films. The results from this new technique will be discussed in detail and in comparison to conventional gravimetry and conventional FTIR-ATR results.