Preparation of heat-resistant poly(amide-imide) films with ultralow coefficients of thermal expansion for optoelectronic application

The novel poly(amide imide) (PAI) films derived from three kinds of amide-containing diamines and different aromatic dianhydrides were synthesized and characterized. These PAI films exhibited excellent mechanical and thermal properties due to the rigidity of main chains and existence of strong hydrogen bonding interactions. For PAI films based on the diamine N,N′-(1,4-phenylene)bis(4-aminobenzamide) (PABA), their tensile strength and modulus even exceeded 280 MPa and 10 GPa, accompanied with Tg and T5 values higher than 408 °C and 520 °C, respectively. The results indicated that CTE values of these PAI films were greatly affected by rigidity, orientation and packing of chains as well as hydrogen bonding interactions. PABA-based PAI films with the highest amide content and linearly rigid backbones had ultralow in-plane CTE values ranging from −4.6 to −0.8 ppm/°C in the temperature range of 30–300 °C combined with the largest ∆n values of 0.22–0.23, suggesting a remarkable negative correlation between CTE and ∆n. The hydrogen bonding interactions were proved to be maintained at high temperature, and it was crucial for the regulation and control of thermal expansion. These heat-resistant PAI films with high tensile strength and ultralow CTE can be used as flexible polymeric substrates for optoelectronic application.