Film Properties of Polyazomethines (2). Poly(imide-azomethine)s Derived from Ester-containing Dialdehydes, Tetracarboxylic Dianhydrides, and a Fluorinated Diamine

Poly(imide-azomethine)s (PIAzMs) were prepared from 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA) and terephthalaldehyde (TPAL) with 2,2'-bis(trifluoromethyl) benzidine (TFMB) to improve the dielectric constants and the film transparency while keeping low CTE characteristics. However, this approach using CBDA instead of pyromellitic dianhydride (PMDA) was less effective for improving the film transparency than initially expected. The results were explained by a rather strong contribution around the azomethine units to the electronic conjugation. As an alternative approach, novel dialdehyde monomers, 4-formylphenyl-4'-formylbenzoate (FPFB) and bis(4-formylphenyl) terephthalate (FPTP) were synthesized in which two formyl groups of the monomers were separated via some non-conjugated para-ester spacers. The PIAzM film derived from FPFB, PMDA, and TFMB achieved excellent combined properties, an extremely low CTE (-0.81 ppmK(-1)), a low water absorption (0.33%), a very high tensile modulus (6.3 GPa), sufficient film flexibility (epsilon(b) = 14%), good thermal stability, and improved transparency. A mechanism for the generation of a negative CTE was also proposed. The use of FPTP with a longer ester spacer allowed further improvement of the film transparency (cut-off wavelength = 362 nm) and the optically estimated dielectric constant (epsilon(cal) = 2.97) on the basis of reduced electronic conjugation. The results suggest that the ester-containing PIAzMs can be promising candidates as a new type of heat-resistant dielectric materials.