Effects of rigid end-capped groups on the structure and properties of colorless polyamide-imide films

In this work, we use 2-aminobenzimidazole and 2-aminobenzoxazole to end-cap colorless polymer, PAITFDB-TPC6FDA, in order to investigate methods for reducing the CTE value of products and their effects on other properties. Results show that, as the ratio of 2-aminobenzimidazole to the raw material, diamine TFDB, increases from 0:100 to 2:99, the CTE value of products gradually decreases from 20.0 to 12.9 ppm/K (a decrease of 36%). Nevertheless, the CTE value of products rebounds as the dosage of the end-capped groups exceeds this ratio. Among all the products obtained, PAI-1M film has the best overall properties: its CTE is 12.9 ppm/K, Td5 is 518 degrees C, Tg is 366 degrees C, TS is 191 MPa, T430 is 85.1%, WA is 85.4% and CA is 0.8%. The two end-capped groups are of highly linear and rigid structures. They also help to enhance the hydrogen bonding between molecules, resulting in a decrease in d-spacing values. The results of molecular configuration simulation and calculation indicate that, using 2-aminobenzimidazole to end-cap the chains of PAI molecules is beneficial for suppressing the formation of the chargetransfer complexes (CTCs), and thereby offsetting impacts of the reduced molecular chain spacing on the transparency of films. Based on these two influencing factors, we confidently conclude that linear and rigid endcapped groups can effectively improve the dimensional stability and mechanical properties of PAI films without damaging their optical properties.