Conformation of Polyimide Backbone Structures for Determination of the Pretilt Angle of Liquid Crystals

The aromatic polyimides (Pis) containing pendent tert-butyl side groups were synthesized via polycondensation of 1-(4-aminophenoxy)-4-(4-aminophenyl)-2,6-di-tert-butyl-benzene (4), 4-(4-aminophenyl)1-(1-(4-amino-2-trifluoromethylphenoxy)-2,6-di-tert-butyl-benzene with various aromatic tetracarboxylic dianhydrides. The conformation of 2,6-di-tert-butyl-1-(4-phthalimidophenoxy)-4-(4-phthalimidophenyl)benzene created the two orthogonally molecular planes. Uniform alignment layers, the three Pis possessing high pretilt angles ranging from 87.9 to 88.7 degrees. have been achieved after mechanical rubbing of the PI thin film Surfaces at room temperature and subsequent annealing. This unusual high pretilt angle of Pis containing only short side groups is quite different for most of the typical Pis. The unusual high pretilt angles were predominately governed by their flat-straight conformations. (1)H NMR spectrum of the diamine 4 revealed that the protons of 4-aminophenoxy moiety are not chemical shift equivalent. This is because the steric hindrance of the bulky tert-butyl groups prevents the benzene ring, of the 4-aminophenoxy moiety from rotating freely. These protons produced individual broad singlet signals. These novel Pis exhibited a low dielectric constant (2.75 to 3.11), low moisture absorption (0.65 to 1.74), excellent solubility. and high glass-transition temperatures (337-456 degrees C). The nonfluorinated Pis derived from the new diamines and the rigid pyromellitic diamhydride (PMDA) were Soluble in N-methyl-2-pyrrolidinone, N,N-dimethylacetamide. and in-cresol.