Colorless Polyimides Derived from Novel Role-Dividing Spiro-Type Monomers: Strategies to Overcome the Trade-Off Between Low Linear Coefficients of Thermal Expansion and Low Thickness-Direction Birefringence Without Fillers

This study presents unique polymeric materials applicable to plastic substrates for use in flexible-display devices that overcome the trade-off between low linear coefficients of thermal expansion (CTE) and low thickness-direction birefringence (Δnth) while combining a very high Tg, sufficiently high thermal stability, excellent optical transparency, good solubility, and minimum-required ductility. Polyimide (PI) films obtained from 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA) with 2,2′-bis(trifluoromethyl)benzidine (TFMB) under different conditions resulted in widely varying CTE values and provided a clear CTE–Δnth correlation, which can be regarded as a virtual lower boundary in the CTE–Δnth relationship for various PI systems. The pristine CBDA/TFMB and CpODA/TFMB (CpODA = norbornane-2-spiro-α-cyclopentanone-α′-spiro-2″-norbornane-5,5″,6,6″-tetracarboxylic dianhydride) systems were modified using numerous specifically designed monomers, i.e., a vertical-alignment-type liquid-crystalline diamine and cardo-type and spiro-type monomers. However, it was very challenging to overcome the trade-off between low CTE and low Δnth, that is, to significantly exceed this lower boundary by modifying the pristine systems, while ensuring other target properties. One of the keys to achieving the present goal was compatibility with chemical imidization or one-pot polymerization processes (i.e., high solubility of the PIs), because these processes were more advantageous in reducing CTE and enhancing film transparency than the conventional two-step process. The modifications using phenyl-substituted xanthene-pendant 2,7-diaminofluorene and fluorene-pendant 2,3,6,7-xanthenetetracarboxylic dianhydride exhibited a prominent effect on overcoming the trade-off without the help of any fillers, while combining other excellent target properties. Polarized FT-IR difference spectra measured at varying incidence angles suggested that these side groups, which are connected perpendicularly to the PI main chains, align in the Z-direction, rationalizing the observed prominent effect. Thus, unique high-temperature transparent materials applicable to plastic substrates were successfully obtained in this study.