Luminescence measurements of oxygen permeation and oxygen diffusion in thin polymer films

A new method is developed for analyzing the diffusion of oxygen in thin polymer films via fluorescence quenching measurements. We begin by reviewing previous methods, all of which involve approximations, for the measurement of permeability and diffusion coefficient by luminescence quenching; their shortcomings are clarified. An exact analytic theory is developed that successfully couples Fick's laws of diffusion to the Stern-Volmer equation of intensity quenching. Various modes of experimentation with polymeric films are considered. The equations we derive make the unexpected prediction that the rate of emission intensity decay when O-2 diffuses into a polymer film is much faster than the rate of emission intensity enhancement when O-2 diffuses out of the same film, even when the molecular diffusivity remains unchanged. Experiments show that this is indeed the observed behaviour.