Control of Glass Transition, Solubility, and Thermo-Optic Coef-cients of a Siloxane-Containing Polyimide by Silica Hybridization

In order to utilize significant enhancement of thermo-optic coefficients (i.e. temperature dependence of refractive index, dn-dT) of polymers above glass transition temperatures (T g values) for dynamic control of refractive index, the T g of siloxane-containing polyimide (PI), derived from bis(3,4- dicarboxyphenyl)ether dianhydride and bis(3-aminopropyldimethylsilyl)ether, was controlled by hybridization with silica (amorphous SiO2) particles generated by the in situ sol-gel reaction. The T g values for the hybrid films were increased from 55 to 105°C with increasing the silica content. In addition, resistance to the organic solvents was significantly improved by the hybridization. These results indicate that the PI chains were physically or chemically bonded to the silica particles in the hybrids. The characteristic high transparency of the matrix PI in the visible light region was retained in the hybrid films without causing light scattering, which demonstrates that the precipitated silica particles were nano-sized and uniformly dispersed in the hybrid films. The refractive indices of the hybrids decreased from that of the matrix (1.552) to 1.543 as the silica content increased due to low refractive index of silica. The dn-dT values below T g were decreased from -158 to -127 ppm K-1 as the silica contents increased due to the deceases in thermal volume expansion. On the other hand, the hybrid films exhibited an abrupt increase in dn-dT at T g and quite large dn-dT values (in a range -330 to -350 ppm K-1)above T g. The refractive indices and the dn-dT values were stable after annealing at 300°C for 1 h under N2 flow.