Measurement of Diffusivities of Small Molecules in Polymers Using FT-IR-ATR

Abstract

A numerical method is described for obtaining diffusion coefficients ( D) of infrared-absorbing species in polymers using attenuated total reflectance infrared spectroscopy. It involves measurement of changes in the integrated intensity of diffusant infrared bands as a function of time while the diffusant moves into or out of the polymer film. The technique is illustrated with latex paint films containing a calcium carbonate extender which diffuses out of the paint films upon exposure to sulfurous acid. The values of D obtained from two different CaCO₃ infrared bands were in good agreement with D obtained from independent bulk weight loss measurements. The method should be equally applicable to realtime diffusion analysis and to samples containing “frozen” concentration profiles of diffusant (as studied here). The results obtained in this preliminary study indicate that the method is probably best suited for measurement of diffusion coefficients smaller than 10⁻⁹ cm²/s.