Fundamental properties of alkoxide-derived titania gel films providing plastics surface with high refractive index

In order to obtain the information of the fundamental properties of alkoxide-derived titania gel films on plastics, we prepared the films on polycarbonate (PC) substrates by spin-coating, dried them at 120 °C for 24 h, and evaluated their refractive index, uncracking critical thickness, flexibility, chemical durability and adhesion to the substrate. First, dried gel films were prepared from solutions of various types of alkoxides, i.e., from Ti(OR)₄-HNO₃-H₂O-ROH solutions (R = C₂H₅, C₃H₇ⁱ, or C₄H₉ⁿ) with or without CH₃COOH as a chelating agent (CA). Among them, those prepared from Ti(OC₃H₇ⁱ)₄ had higher refractive indices than the others. Next, dried gel films were prepared from Ti(OC₃H₇ⁱ)₄-HNO₃-H₂O-(C₂H₅OH or C₃H₇ⁱOH) with or without CA, where HCOOH, CH₃COOH, CH₃COCH₂COCH₃ were employed as CA. The addition of CA in solutions tended to lower the refractive index of the dried gel films. The uncracking critical thickness was ca. 30–80 nm, with larger critical thickness given by the addition of CA in solutions, which was attributed to the lower degrees of the increase in in-plane tensile stress during heating. The critical thickness on PC substrates was found to be smaller than that on Si(100) substrates (ca. 100–150 nm) possibly due to the larger thermal expansion coefficient of PC than Si(100). Cross-cut adhesion test showed that all these Ti(OC₃H₇ⁱ)₄-derived dried gel films have excellent adhesion to PC substrates. Chemical durability was evaluated on these dried films by soaking them in water and ethanol at room temperature for 30 min. No significant changes in thickness or refractive index on soaking were detected under such a limited test condition. These dried films on PC substrates were shown to be bent without cracking until the radius of curvature decreased down to about 16 mm.