Solution-processable transparent polyimides with low coefficients of thermal expansion and self-orientation behavior induced by solution casting

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• This work proposes novel fluorinated polyimides (PIs).
• PI-1 (#1) gave transparent films with high Tg (328 °C), low CTE (9.9 ppm K−1), and low YI.
• PI-2 (#2) showed further improved transparency with low CTE and excellent solubility.
• These are useful as novel high-temperature plastic substrates in display devices.

A fluorinated tetracarboxylic dianhydride (amide-type TA-TFMB) was prepared from trimellitic anhydride chloride and 2,2′-bis(trifluoromethyl)benzidine (TFMB). A chemically imidized polyimide (PI) derived from TA-TFMB and TFMB was rather soluble in various solvents. Solution casting of this PI (TA-TFMB/TFMB) led to a flexible, non-turbid, and seemingly almost colorless PI film with a high Tg of 328 °C and a considerably low coefficient of thermal expansion (CTE) of 9.9 ppm K−1 which results from significant in-plane chain orientation induced during solution casting. The self-orientation mechanism is discussed. The properties of TA-TFMB/TFMB were compared with those of some relevant systems. The results suggest that an electron-withdrawing effect of the 2,2′-CF3 substituents of TA-TFMB and a twisted conformation of the central biphenyl moiety greatly contribute to the suppressed coloration of the TA-TFMB/TFMB film. The use of a TA-TFMB counterpart (ester-type TA-TFBP) was effective for further enhancing the transparency owing to reduced charge-transfer interaction. However, the thermal properties of TA-TFBP/TFMB were not always satisfactory. Copolymerization using 2,3,6,7-naphthalenetetracarboxylic dianhydride led to a PI film with an increased Tg of 277 °C and a very low CTE of 12.6 ppm K−1 without significant decreases in the transparency and the solubility. Thus, this work proposes promising candidates as novel heat-resistant plastic substrate materials in display devices.