Soluble Polyimides Based on Aromatic Diamines Bearing Long-Chain Alkyl Groups

Although polyimides have been widely used in microelectronics, one of the disadvantages of these polymers is that these are insoluble in common solvents. We have systematically investigated the synthesis and characterization of soluble polyimides based on aromatic diamines bearing long-chain alkyl groups, and also those containing dendritic moieties. In this paper, the effects for the enhancement of solubility based on aromatic diamines such as alkyloxydiaminobenzene (AODB-X, X = carbon number of alkyl chain, 10-14), alkyldiaminobenzophenone (ADBP-9-14), diaminobenzoic acid alkylester (DBAE-8-14) and alkyldiaminobenzamide (ADBA-9-14) are discussed in detail and the conclusions regarding enhancement of solubility are as follows: (1) the effect of diamines decreases in the order AODB > ADBP > ADBA > ADBE, (2) long-chain alkyl groups bearing twelve carbon atoms are the most effective, (3) long-chain alkyl groups bearing even numbers of carbon atoms are more effective, and (4) branched alkyl chains are more effective. The polyimides having dendritic side chains were also investigated. For example, the terphenylene diamine monomer having a first-generation monodendron, 3,4,5-tris(n-dodecyloxy) benzoate and the monomer having a second-generation monodendron, 3,4,5-tris[-3',4',5' -tri(n-dodecyloxy) benzyloxy] benzoate were successfully synthesized and the corresponding soluble polyimides were obtained by polycondensation with tetracarboxylic dianhydride monomers such as benzophenone tetracarboxylic dianhydride (BTDA). These polyimides were soluble in various polar solvents and also exhibited good thermal stability. Their application as alignment layers for LCDs was also investigated, and it was found that these polyimides having dendritic side chains were applicable for the vertically aligned nematic liquid crystal displays (VAN-LCDs).