Shinji Ando

Ternary composites of linear and hyperbranched polyimides with nanoscale silica for low dielectric constant, high transparency, and high thermal stability

A new series of ternary polyimide–silica composites was developed to obtain polymer films with low dielectric constant, high optical transparency, and good thermal stability. By using a linear polyamic acid with triethoxysilane termini and a hyperbranched polyimide with peripheral hydroxyl groups (HBPIBPADA-TAP(OH)), the ternary composites were fabricated through in situ imidization and sol–gel reaction with tetraethoxysilane. The results show that the composite films exhibit significantly improved properties due to the strong silica cross-linkages between organic–inorganic phases. The triethoxysilane termini can effectively enhance transparency because of the homogeneous dispersion of the inorganic phase in the PI matrices and the improved dispersibility through their strong covalent and partial hydrogen bonding with inorganic silica networks. With an appropriate content of 30% HBPIBPADA-TAP(OH) and 20% SiO2 in the linear polyimide (PI6FDA-APB(Si)), the dielectric constant (Dk) can reach the lowest value of 2.19 at 100 kHz. The highest transmittance of 96% at 450 nm is obtained for a ternary hybrid containing 20% SiO2 and 10% HBPIBPADA-TAP(OH). The incorporation of HBPIBPADA-TAP(OH) does not cause negative effects on the thermal stability. The ternary hybrid containing 20% SiO2 and 10% HBPIBPADA-TAP(OH) also exhibits the lowest coefficient of thermal expansion (CTE) of 27.8 ppm °C−1, when compared with 29.9 ppm °C−1 for the binary composite as PI6FDA-APB(Si) with 20% SiO2. These properties can well match the requirements for potential applications in the microelectronics insulator fields as interlayer dielectrics of advanced electronic devices.

Highly dispersible ternary composites with high transparency and ultra low dielectric constants based on hyperbranched polyimide with organosilane termini and cross-linked polyimide with silica

Flexible insulating materials that are both thermally and mechanically stable, highly transparent, and have low dielectric constants are highly desirable for electronic applications. With these technical needs, a highly disperse inorganic matrix is the most important factor in polyimide–inorganic composites. We report an optimised method for the preparation of a hyperbranched polyimide using HBPIBPADA-TAP(Si). This method involves modifying the polymer termini by coupling (3-isocyanatopropyl)triethoxysilane to HBPIBPADA-TAP(OH) via the hydroxyl (–OH) groups at peripheral positions of the polymer chain. Then, based on the HBPIBPADA-TAP(Si) with silane-modified termini, linear PI6FDA-APB(Si) and tetraethoxysilane cross-linking agent were used to prepare ternary composites, PI6FDA-APB(Si)_HBPIBPADA-TAP(Si)_SiO2, by sol–gel cross-linking reaction. The dielectric constant (Dk) of PI6FDA-APB(Si)_HBPIBPADA-TAP(Si)-30%_SiO2-20% was very low, 2.04, and the optical transparencies of the ternary hybrid composite films also improved over those of similar composites due to the synergistic interactions between HBPIBPADA-TAP(Si) and PI6FDA-APB(Si) that improves phase dispersion. The highest transparency, 95% at 450 nm, was obtained for PI6FDA-APB(Si)_HBPIBPADA-TAP(Si)-30%_SiO2-20%, a significant improvement from that (87%) of the binary composite of PI6FDA-APB(Si)_SiO2-20%. The glass transition temperature (Tg) of PI6FDA-APB(Si)_HBPIBPADA-TAP(Si)-30%_SiO2-20% is 212.6 °C, which is the highest in the ternary composite series. PI6FDA-APB(Si)_HBPIBPADA-TAP(Si)-40%_SiO2-20% has the largest storage modulus, 2952.0 MPa at 180 °C. The tan δ values of the composite films decreased from 0.96 to 0.73 with increasing HBPIBPADA-TAP(Si) content. The ternary hybrid composites with densely cross-linked SiO2 covalent networks developed in this study have improved dielectric, optical, thermal, and mechanical properties. Our fabrication method paves the way to the facile production of high-performance flexible and transparent electronic circuits that could be used in a broad range of applications in future electronics.

Polarization dependence of thermo-optic coefficients in polyimide films originating from chain orientation and residual thermal stress

Control of polarization dependences of thermo-optic (TO) coefficients, i.e., the temperature dependence of birefringence (d(Δn)/dT), is critical issue for optical materials. Especially in active optical components like TO switches, polarization dependent behaviors are caused by d(Δn)/dT of materials. Origins of polarization dependence of TO coefficient were investigated for seven kinds of polyimide (PI) films in terms of main chain orientation and residual thermal stress. The values of d(Δn)/dT of PI films originated from chain orientation vary from nil to negative as the degrees of in-plane orientation increase. Magnitudes of d(Δn)/dT depend on intrinsic birefringences, second order orientation coefficients, and their temperature dependence. On the other hand, those of d(Δn)/dT originated from residual stress are negative for PI films exhibiting large in-plane coefficients of thermal expansion (CTE) and vary to zero as their CTE decreases due to reduced residual stress. d(Δn)/dT originating from chain or...