Vertically Aligned Polyimide Prepared from a Lyotropic Precursor Containing Clay Nanosheet that Facilitates Vertical Stacking of Smectic Layer

Abstract

This paper reports that adding a small amount of nanosheets (nanoclay, NC) with a hydrophobic surface to a solution of lyotropic liquid-crystalline poly(amic ester) (PAE) facilitates the vertical alignment of the main chain of PAE as a precursor of polyimide. This method allows the preparation of thin films of preferentially vertically aligned (VA) polyimide (PI), even on hydrophilic substrates, using a conventional solution casting process. The vertical alignment mechanism of the PAE main chains was investigated using precise orientation analyses based on grazing-incidence wide-angle X-ray scattering (GI-WAXS), infrared p-polarized multiple-angle incidence resolution spectroscopy (IR pMAIRS), and polarized micro-ATR-FTIR spectroscopic imaging (micro-ATR imaging) methods. GI-WAXS and IR pMAIRS revealed that the NC nanosheets were horizontally oriented in the film, enhancing the orientation order parameter (S) of the PAE. Micro-ATR imaging indicated that S increased homogeneously in the composite film, even though a small portion of the NC was aggregated. These analyses demonstrate that the smectic-layer growth of the lyotropic PAE from the hydrophobic interfaces in the solution was the key to generating the vertical alignment of the PAE main chain. This study provides a versatile strategy for fabricating VA films by adding nanosheets to various precursors that exhibit lyotropic smectic phases.