Effects of pre-strain applied to a poly(ethylene terephthalate) substrate before TiO2 film deposition on the contact angle of the substrate and the morphology of the specimen

Abstract

A mold is designed to create various pre-strain levels in poly(ethylene) terephthalate (PET) substrates before the deposition of TiO2 film to imitate the deposition process on a cylindrical drum. Four pre-strain levels, namely 0%, 2%, 4%, and 6%, are used in the preparations of TiO2/PET specimens to investigate the effects of pre-strain on the film morphology and the stress/strain formed in the PET substrate on the contact angle. Finite element analyses for stress/strain in the PET substrate are carried out, including that for the contact pressure distribution over the contact surface between the PET substrate and the partial-arc cylindrical drum. A significant increase in the stress/strain is produced with increasing pre-strain of the PET substrate. The mean length and width of the voids formed in the TiO2/PET specimens strongly depend on the specimen position. An increase in the pre-strain, and thus the stress/strain, results in an increase of the mean width and length of voids. The contact angle increases with decreasing stress/strain of the PET substrate. An increase in the pre-strain of the PET substrate leads to a reduction in the contact angle. Bent PET specimens have wavier surface profiles compared to that of the as-received PET. The mean amplitude value decreases with increasing pre-strain. The effect of stress on the contact angle formed on the wavy surface can be explained by the Wenzel equation. The decrease in the contact angle can enhance the absorption of the TiO2 film, and thus increase the distance between two adjacent valleys in the porous TiO2 film. The extrusions/intrusions in the porous PET after bending and the different stress/strain distributions over the PET substrate cause drastic changes in the local morphology of the TiO2/PET specimen.