Synthesis and characterisation of novel soluble polyimides for the application as liquid crystal vertical alignment layers

Abstract

A new triphenylamine-based functional diamine monomer named N, N-bis (4-aminophenyl)-4-(nonyloxy-biphenyl)-4′-aminophenyl ether (N9) was successfully synthesised from cesium fluoride-mediated N,N-diarylation of 4-(nonyloxy-biphenyl)-4′-aminophenyl ether with p-fluoronitrobenzene and subsequent reduction of the resultant dinitro compound. A series of polyimides (PIs) with triphenylamine (TPA) moieties were prepared from the newly synthesised diamine monomer, 3,3′-dimethyl-4,4′-methylene-dianiline and 4,4′-oxydiphthalic anhydride through a conventional two-step procedure that included a ring-opening polyaddition to give polyamic acids, followed by chemical cyclodehydration. The PIs were completely amorphous and exhibited good solubility in organic solvents. PI films attained from a casting method showed high transmittance above 90% in the wave length range of 400–700 nm and could align liquid crystals vertically before and after rubbing treatment. The PIs derived from N9 exhibited much higher thermal stability compared with the corresponding PIs from 4-dodecyloxy-biphenyl-3′,5′-diaminobenzoate (D12). The results demonstrated that the introduction of TPA units into the polymer backbone could largely enhance the thermal stability of PIs while sustaining its solubility.

Keywords:

• soluble polyimides
• thermal properties
• triphenylamine
• liquid crystals
• alignment layers