Vertically aligned polyimide films prepared from lyotropic liquid-crystalline smectic precursors

Vertically aligned (VA), fully aromatic polyimide (PI) thin films were successfully prepared from a lyotropic smectic liquid-crystalline precursor of poly(amic ester) (PAE) containing perfluorooctyl (Rf) terminal groups using a “push-coating” method. Because the repeating units of PAE form the smectic layer, the main chains of PAE are oriented perpendicular to the layer. A homeotropically aligned smectic structure is induced by the step-by-step stacking growth of the layer from the monolayer of the Rf groups self-organized on the hydrophobic interfaces (interfaces between the PAE solution and air or a surface-modified substrate), leading to a vertically aligned PAE. By optimizing the preparation conditions, the values of the orientational order parameter (S) of the push-coated PAE film exceeded 0.7. The homeotropically aligned PAE films were imidized by heating to 300 °C, and the high S values were maintained in the subsequent imidization to give VA PI films. Despite the large standard deviation in the temperature variation of the optical path length of the VA PI films, the small and reversible variations in S confirmed that the vertical orientation of the PI films was thermodynamically stable.