Effect of Terephthaloyl Chloride Comonomer on Thermal and Optical Properties of Colorless and Transparent Polyimide Films Based on 4, 4′-(Hexafluoroisopropylidene) Diphthalic Anhydride and Bis(3-aminophenyl) Sulfone

In this study, we prepared a series of polyimide (PI) films based on 4,4ʹ-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) and bis(3-aminophenyl) sulfone (3DDS), incorporating various amounts of terephthaloyl chloride (TPC) as a comonomer, and reported their thermal and optical properties. Fourier transform infrared spectroscopy confirmed the chemical structure of the synthesized PI films and the successful imidization of all PI films through thermal treatment. Differential scanning calorimetry analysis showed that the glass transition temperature ( T g ) of pure PI films based on 6FDA and 3DDS was approximately 275 °C, increasing sequentially with an increase in TPC content. For instance, the T g of PI with 20 mol% TPC increased by about 5 °C to 280.24 °C compared to the T g of the pure PI film, indicating improved thermal stability. Thermogravimetric analysis revealed a slight decrease in the thermal decomposition temperature with increasing TPC content, which can be attributed to the decomposition of the amide group induced by TPC. The coefficient of thermal expansion (CTE) of pure PI films was evaluated to be 60 ppm/℃, significantly decreasing with increasing TPC content. More specifically, the CTE decreased to 41 ppm/℃ when TPC was introduced at a level of 20 mol%. Based on these results, it is reasonable to conclude that the introduction of TPC improved the thermal dimensional stability of pure PI films based on 6FDA and 3DDS by increasing intermolecular forces induced by hydrogen bonding from the amide group. Overall, these results suggest that the 6FDA and 3DDS-based PI films containing TPC as a comonomer have improved thermal properties and are suitable for high-dimensional stability polymer substrates for display applications.