MPAB-based colorless and transparent copoly(amide imide)s enabled by dianhydride composition control

Colorless and transparent copoly(amide imide) (Co-CPAI) films were synthesized from N, N′-[2,2′-bis(trifluoromethyl)-4,4′-biphenylene]bis(4-aminobenzamide) (MPAB) and two dianhydride systems composed of 4,4′-oxydiphthalic anhydride (ODPA) with either 1,2,3,4-cyclobutane tetracarboxylic dianhydride (CBDA) or 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) to investigate the influence of dianhydride composition on the thermomechanical, optical, and solubility properties. Computer simulated three-dimentional structures suggested that CBDA introduces moderate chain bending and a corresponding increase in free volume relative to ODPA, whereas 6FDA produces a much more twisted and expanded chain conformation with the largest free volume. The intermediate composition (MPAB/ODPA/CBDA or 6FDA = 1.0/0.5/0.5) exhibited the highest thermomechanical performance owing to the synergistic effects of copolymerization, which provide an optimal balance among chain rigidity, conformational irregularity, intermolecular interactions, and free volume. The 6FDA-based copolymers generally exhibited higher glass-transition temperatures, thermal decomposition stability, and mechanical properties, whereas the conformationally constrained cyclobutane unit of CBDA effectively restricted local segmental motion, contributing to lower CTE values and superior dimensional stability. All Co-CPAI films exhibited excellent optical transparency (> 85% transmittance at 500 nm), low yellow indices (≈ 1–2), and good solubility in polar aprotic solvents. These results demonstrate that controlling the dianhydride composition is an effective molecular design strategy for developing high-performance transparent polyimides for flexible electronic and optoelectronic applications.