Effects of UV Crosslinking under High Temperature on the Refractive Indices and Aggregation Structures of Benzophenone-containing Polyimides

Abstract

For controlling the optical and thermal properties of photo-sensitive benzophenone (BP)containing polyimides (PIs), an attempt was made to fix the dense polymer chain packing induced by high pressure using intermolecular crosslinking reactions. The effects of temperature and pressure during UV irradiation on the reaction rates of photo-crosslinking and refractive indices were evaluated. After UV irradiation at ambient temperature and pressure, refractive indices of the PIs increased due to the densification of chain packing. The degree of crosslinking was further increased with increasing the irradiation temperature. The PI films irradiated at 400 MPa exhibited higher refractive indices than those irradiated at 0.1 MPa, despite the nearly identical reaction rates. The fluorescence intensities of the PIs irradiated at 400 MPa showed smaller dependence on the pressure than those irradiated at 0.1 MPa, which indicates that the dense chain packing formed under high pressures was partially and effectively fixed by UV irradiation. The increase in refractive indices of the PIs irradiated at elevated pressures is attributable to the densification of aggregation structures, which was controllable by pressure under UV irradiation.