Polyimides Containing Trans-1,4-cyclohexane Unit (II). Low-K and Low-CTE Semi- and Wholly Cycloaliphatic Polyimides

Polyimide precursors were successfully polymerized from trans-1,4-cyclohexanediamine (CHDA) with pyromellitic dianhydride (PMDA) and 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA) under very limited conditions: at controlled extents of silylation for CHDA in a mixed solvent containing hexamethylphosphoramide (HMPA). Flexible and transparent polyimide films were obtained upon thermal imidization of the precursor films on a substrate after residual solvent extraction from the precursor films by using adequate alcohols. Semi-aromatic PMDA/CHDA polyimide film displayed a considerably lower linear coefficient of thermal expansion (CTE = 9.6 ppm K^—1) and a relatively low dielectric constant (2.92). On the other hand, wholly cycloaliphatic CBDA/CHDA film attained a low CTE (26 ppm K^—1) and a considerably lower dielectric constant (2.66), simultaneously. Factors influencing the imidization-induced in-plane orientation, namely, effect of the steric structures of cycloaliphatic units are discussed in comparison with polyimides from hydrogenated PMDA (1,2,4,5-cyclohexanetetracarboxylic dianhydride).