Structure and properties of thin films of binary rodlike/flexible polyimide mixtures

Abstract

AbstractBinary mixtures of a rodlike poly(p‐phenylene pyromellitimide) (PMDA‐PDA) and a flexible 6F‐BDAF polyimide synthesized from hexafluoroisopropylidene diphthalic anhydride and 2,2‐bis(4‐aminophenoxy‐p‐phenylene) hexafluoropropane were prepared by solution‐blending of the meta‐PMDA‐PDA poly(amic ethyl ester) and 6F‐BDAF poly(amic acid) precursors, followed by solvent evaporation and thermal imidization. Mixtures containing different molecular weights of 6F‐BDAF poly(amic acid) were studied. The size scale of the phase separation, as measured by light scattering, is ca. 1 μm or smaller in most cases. The domain size is primarily set by the demixing of the precursor polymers during solvent evaporation, with no significant coarsening observed during the thermal imidization. The observed variation of the domain size with molecular and process parameters such as composition, molecular weight, and film thickness is discussed in terms of the miscibility of the precursor polymers, rate of solvent evaporation, and solidification. Dynamic mechanical thermal analysis and dielectric relaxation measurements indicate that the glass transition temperature of 6F‐BDAF is unaffected in all of the mixtures studied, indicating complete demixing of rodlike and flexible polyimides in agreement with theory. X‐ray photoelectron spectroscopy results show a strong surface segregation of 6F‐BDAF in mixtures containing as low as 10% by weight of the 6F‐BDAF component in the bulk. The mixtures with PMDA‐PDA as the major matrix component therefore exhibit excellent mechanical toughness, dimensional stability up to 500°C, low coefficients of thermal expansion (< ca. 10 ppm/°C), and low dielectric constants (<3.0). On the other hand, the surface properties of the mixtures are dominated by the flexible 6F‐BDAF, resulting in excellent polymer/polymer self‐adhesion (lamination) properties between fully imidized films.