Effects of prestrain applied to poly(ethylene terephthalate) substrate on size, porosity, and geometry of TiO2 particles and optical properties of TiO2/PET specimens

A mold is designed to create a prestrain in a poly(ethylene terephthalate) (PET) substrate before the deposition of TiO2 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 TiO2/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 TiO2 particles deposited at the interface of the TiO2 film and the PET substrate. The mean length (L ⁎) and width (W ⁎) of the voids in the TiO2/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 (View the MathML sourceθ¯⁎) of the lateral surfaces in the porous TiO2 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 View the MathML sourceθ¯⁎ is fixed, the absorption decreases with increasing average slenderness ratio of voids. The reflectance behavior is exactly opposite to that of the absorption.