Diffusion measurements using ATR-FTIR spectroscopy: acetone diffusion in polypropylene - use of penetrant fluid pressure to improve sample/IRE contact

Abstract

In this article, we present a simple method for improving the contact between a film sample and the internal reflection element (or crystal) when diffusion into thin polymer films is measured with attenuated total reflectance (ATR)-Fourier transform infrared spectroscopy. Intimate film/crystal contact is particularly important for making measurements on premade films and materials that cannot be solution-coated onto the crystal. This method is based on controlling the penetrant fluid pressure above a threshold value (>230 kPa) in the ATR flow cell. Measurements of acetone diffusion into a commercial polypropylene film at 300 K and varied pressures indicated that the diffusion time constant was constant at pressures above this threshold. We also monitored the absorbance of a polymer band that had no overlap with the acetone spectrum to examine whether adequate sample-film/crystal contact was reached and maintained. From these observations, we concluded that an apparently good match between the experimental data and a model calculation does not alone justify confidence in the accuracy of the calculated diffusion time constant. Additionally, the practice of using a reference band to correct the uncertainty in absorbance for bands of interest (due to imperfect sample/crystal contact) yielded inconsistent results. We also report further measurements of acetone diffusivity in a polypropylene film at temperatures ranging from 278 to 308 K that yielded an estimated activation energy for diffusion of 98 kJ/mol.