Novel Silicone Polymeric Material with High Thermal Stability for Optical Waveguides

The optical loss and refractive index controllability of a novel silicone ladder copolymer (polyvinylphenylsilsesquioxane: PVSQ) were studied with respect to the application of the copolymer to optoelectronic components. PVSQ copolymers were prepared from trichlorophenylsilane and trichloro (vinyl) silane in the various mol ratios. They had a ladder structure consisting of siloxane bonds as the main chain and phenyl groups or vinyl groups as the side chain. PVSQ demonstrated excellent transparency at the wavelengths used in optical communication, 0.6–1.7 µm. The optical losses of PVSQ with 5 mol% vinyl group content in the side chain, were 0.99 and 1.05 dB/cm at the wavelengths of 1.30 and 1.55 µm, respectively. The in-plane refractive index of the polymers at 0.65 µm could be controlled to between 1.558 and 1.466 by changing the contents of phenyl groups and vinyl groups in the side chain. The out-of-plane refractive index could also be controlled to between 1.562 and 1.466. The thermal decomposition temperature of PVSQ, defined as that at which 10% weight loss occurred in N2 atmosphere, decreased with increasing vinyl group content, but all still showed decomposition temperatures above 500°C, ensuring sufficient thermal stability for polymer waveguide applications. Furthermore, the correlation between the vinyl group content and the refractive indices of the PVSQ polymers were discussed based on results of the molecular orbital (MO) calculations. The MO calculations were found to be useful for qualitative estimation of the change in the refractive index of compounds with a systematic derivative structure, such as PVSQ polymers.