High-Refractive-Index Polysiloxanes Containing Naphthyl and Phenanthrenyl Groups and Their Thermally Cross-Linked Resins
Max Briesenick, Markus Gallei, Guido Kickelbick
Abstract
High-refractive-index polysiloxanes containing naphthyl, phenanthrenyl, phenyl, and methyl groups have been synthesized using a polycondensation reaction starting from substituted di- and trialkoxysilanes. The obtained polymers comprised linear siloxane and partially cross-linked silsesquioxane units and showed optical transparencies of up to 99% at a thickness around 120 μm and high refractive indices of up to 1.622. The polymeric structures contained stabilized silanol groups that were further cross-linked at increased temperatures of 200 °C resulting in the formation of hybrid inorganic–organic resins. These typical thermal treatments at 200 °C for 72 h kept the transparencies as high as 98% and slightly lowered the RIs to values up to 1.610. A detailed structure evaluation of the resulting systems showed, depending on the size of the polycyclic aromatic substituent, excimer formation, which is based on weak interactions of the aromatic groups in the polymeric material. After thermal consolidation, glass-transition temperatures of the cross-linked systems were in the range of 18–74 °C depending on the composition. Thermal stabilities of the final resins reached higher values than commonly used siloxane resins up to 470 °C. The final materials are potential resins for high-temperature optical applications.