Water-sorption behaviors of poly(3,4′-oxydiphenylene pyromellitimide) films depending on the thickness variation

The effect of film thickness on the water-sorption behaviors of poly(3,4′-oxydiphenylene pyromellitimide) (PMDA-3,4′ODA) films was gravimetrically investigated and interpreted with a Fickian diffusion model in films. The diffusion coefficient increased with increasing film thickness, whereas the water uptake and the activation energy decreased. Overall, the water-sorption behaviors of PMDA-3,4′ODA films are strongly dependent on the changes in morphological structure, which originated from the variation in the film thickness. As the film thickness increased, the molecular in-plane orientation decreased, consequently leading to the increased diffusion coefficient and decreased activation energy. In contrast, the water uptake decreased with increasing film thickness because of the increase in the out-of-plane packing order. The diffusion coefficient and activation energy were strongly dependent on the in-plane orientation in the films.