Low-CTE polyimides derived from 2,3,6,7-naphthalenetetracarboxylic dianhydride

2,3,6,7-naphthalenetetracarboxylic dianhydride (NTDA) showed high reactivity in the polymerization of polyimide precursors [poly(amic acid)s (PAAs)] with various aromatic and cycloaliphatic diamines with an exception of trans-1,4-cyclohexanediamine (CHDA). On the other hand, another isomer 1,4,5,8-NTDA did not allow the formation of high molecular weight PAAs. The poor reactivity of 1,4,5,8-NTDA is probably attributed to the more stable six-membered anhydride structure. The polyimide (PI) films derived from 2,3,6,7-NTDA with some diamines possessing stiff/linear structures, i.e., p-phenylenediamine (PDA), 4-aminophenyl-4'-aminobenzoate (APAB), and 2,2'-bis(trifluoromethyl)benzidine (TFMB) exhibited no distinct glass transitions on the dynamic mechanical thermal analysis or a considerably high T-g exceeding 400 degrees C, extremely low CTE values close to that of silicon wafer or lower, and relatively low degrees of water absorption simultaneously in addition to excellent thermal stability. A polyimide system derived from 2,3,6,7-NTDA and 4,4'-oxydianiline (4,4'-ODA) achieved a low CTE approximate to that of copper foil (20.0 ppm K-1) in spite of the presence of flexible ether linkages in the structure while retaining excellent film toughness (elongation at break > 80%). The low CTE characteristics observed probably results from the longer naphthaldiimide mesogenic unit which acts more effectively for the imidization-induced in-plane orientation. The properties of 2,3,6,7-NTDA-based PIs were compared with those of PIs derived from a fixed diamine with different dianhydrides, i.e., pyromellitic dianhydride (PMDA) and 3,3',4,4'-biphenyltetracarboxylic dianhydride (s-BPDA) to elucidate the merits of the use of 2,3,6,7-NTDA. The results revealed that 2,3,6,7-NTDA is a useful monomer for lowering both CTE and water absorption and enhancing Tg.