A method for measuring moisture swelling of amorphous PET films by using Rayleigh interferometer is reported

Plots of the optical path of light transmission in the polymer vs. soaking time were obtained

and from the same curves

the dependence of time on the moisture swelling

saturated water absorption and diffusion coefficient of PET films were estimated. These results are in agreement with those obtained by weighing method. It is shown that optical interference method is a simple but sensitive method for the study of water absorption and swelling in transparent polymer films.