Effects of prestrain applied to poly(ethylene terephthalate) substrate on microstructures and morphologies of porous TiO₂ film and optical scattering behaviors of light

A mold was designed to simulate a thin ceramic film coating on a soft, flexible substrate using a rotating deposition system. With this mold, three prestrains (2%, 4%, and 6%) were applied to a polyethylene terephthalate (PET) substrate before the deposition of a thin TiO₂ film. The contact angle of the substrate and, thus, the mean TiO₂ particle size were affected by the prestrain. The effects of the mean particle size of TiO₂ on the surface roughness and cavity area ratio of the porous film and on the scattering behavior of light were investigated. A goniophotometer and Advanced System Analysis Program were employed for the light analyses of bidirectional scatter distribution functions and their calibration. A spectrometer with an integrating sphere was applied to determine the total scatters (TSs) of transmittance and reflection. An increase in the prestrain increased the mean particle size of TiO₂ deposited on the substrate and, thus, the mean surface roughness, cavity/void depth, and cavity area ratio. PET/TiO₂ specimens with various prestrains were prepared that satisfy the Harvey-like model but without the isotropic, diffusive property in scatter. The bidirectional transmittance distribution function area and TS-transmittance results are governed by the mean particle size and, thus, the cavity/void geometries and surface roughness. These values decrease with increasing PET prestrain. The bidirectional reflection distribution function area and TS-reflection, however, are governed by the adsorbed carbon and its absorption thickness. These values increase with increasing C1s peak value in x-ray photoelectron spectroscopy spectra.