Synthesis of novel soluble polyimides containing triphenylamine groups for liquid crystal vertical alignment layers

A novel functional diamine containing triphenylamine and biphenyl moieties as well as a long alkyl chain, N,N-bis (4-aminophenyl)-4-(dodecyloxy-biphenyl)-4′-aminophenyl ether (B12), was successfully synthesized. A series of novel polyimides (PIs) with triphenylamine units in the polymer backbone were prepared from B12, 3,3′-dimethyl-4,4′-methylene-dianiline (DMMDA), and 4,4′-oxydiphthalic anhydride (ODPA) via a conventional two-step procedure that included a ring-opening polyaddition to give polyamic acids (PAAs) followed by chemical cyclodehydration. The chemical structures of the intermediates, diamine, and PIs were characterized by Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectroscopy (¹H NMR). All PIs were amorphous and readily soluble in many organic solvents, including tetrahydrofuran. PI films attained from a casting method showed high transmittance (above 90 %) in the wavelength range 400–700 nm and induced highly uniform vertical alignment of liquid crystals (LCs). PIs-B12 derived from B12, DMMDA, and ODPA exhibited much higher thermal stabilities than with the corresponding PIs-D12 from 4-dodecyloxy-biphenyl-3′,5′-diaminobenzoate (D12). The results demonstrated that the introduction of triphenylamine units into the polymer backbone improves the thermal stability of the PIs without deteriorating their solubility.