Physical Aging of Thin Glassy Polymer Films Monitored by Optical Properties

The change in refractive index of thin films formed from three glassy polymers, polysulfone, a polyimide, and poly(2,6-dimethyl-1,4-phenylene oxide), measured by ellipsometry was used to track their physical aging. These thin films with thicknesses less than 1 μm were aged for up to 6000 h at 35 °C. A pronounced aging response via refractive index change, attributed to the densification of the glassy polymers, was observed for each film. The Lorentz−Lorenz equation was used to relate changes in refractive index to densification, or volume relaxation, with aging time. The volumetric aging rate was shown to be dependent on the polymer structure and the film thickness. These thin films age at rates orders of magnitude more rapid than expected for bulk or thick films.