Effects of prestrain applied to poly(ethylene terephthalate) substrate on size, porosity, and geometry of TiO_2 particles and optical properties of TiO_2/PET specimens

摘要

A mold is designed to create a prestrain in a poly(ethylene terephthalate) (PET) substrate before the deposition of TiO_2 film to imitate the deposition process on a cylindrical partial-arc drum. Four prestrain levels, namely 0%, 2%, 4%, and 6%, are adopted in the preparation of TiO_2/PET specimens to investigate their effects on the stress/strain formed in the PET substrate and the film morphology. The contact pressure distribution between the PET substrate and the partial-arc drum is first theoretically derived as the boundary condition. Finite element analyses for stress/strain in the PET substrate are then carried out. A significant increase in the average stress/ strain is produced by increasing the prestrain of the PET substrate. The increase in the prestrain leads to a decrease of the contact angle and an increase of the mean size of the TiO_2 particles deposited at the interface of the TiO_2 film and the PET substrate. The mean length (L~*) and width (W~*) of the voids in the TiO_2/PET specimens strongly depend on the specimen position. An increase in the prestrain, and thus the mean stress/strain, results in a nonlinear increase in the mean width and length of voids. The porosity pattern shows that the average slenderness ratio (average L~*/W~* value) of voids increases with decreasing average tilt angle (θ) of the lateral surfaces in the porous TiO_2 film. Slimmer voids generally form with a larger void depth. The light absorption (Ab) significantly increases when the prestrain is increased from 0% to 2%;it then becomes asymptotic to a constant value as the prestrain is further increased. The reflectance decreases slightly with increasing substrate prestrain. If θ is fixed, the absorption decreases with increasing average slenderness ratio of voids. The reflectance behavior is exactly opposite to that of the absorption.