Polyimide Orientation Layers Prepared from Lyotropic Aromatic Poly(Amic Ethyl Ester)s

Authors


  • This work was supported by the German Science Foundation (SFB 481), projects A6 and Z2. We are indebted to the group of Prof. G. Krausch for support during the AFM measurements. We thank Carsten Frenz for providing the bis-azo dye used in the LC guest/host mixture.

Abstract

The synthesis and characterization of liquid-crystalline precursor polymer solutions[1] for polyimides permit for the first time the preparation of bulk- and surface-oriented polyimide thin films from the nematic lyotropic state by shear. A special shearing technique was developed and optimized to orient viscous solutions into thin films with thicknesses below 100 nm. The films produced were thermally imidized and characterized by polarized light microscopy, as well as polarized FTIR and UV-vis spectroscopy before and after imidization. The dichroic ratios (DRs) before imidization were determined as 5 by FTIR, and 4.5 by UV-vis spectroscopies. After imidization the DRs increased to 14 and 7, respectively. The shear-oriented layers possess a surface profile in the form of striations, which was characterized by mechanical surface scanning and atomic force microscopy (AFM). The profile height was determined in the nanometer range in contrast to the profile distance in the micrometer range, thus the latter is a magnitude larger than the film thickness. To quantify and compare the orientation potential of the obtained orientation layers, cells with a liquid-crystalline host and a dichroic azo dye as guest were prepared. Interesting for this class of rod-like polyimides is that layers, which were cast from low concentration isotropic solutions and rubbed, exhibited an almost doubled DR of 15 compared to analogously prepared alignment layers based on commercial flexible polyimide systems (DR = 8).

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