Synthesis and Properties of Lyotropic para-Linked Aromatic Poly(amic ethyl ester)s

The paper describes the synthesis and structure‐property relations of para‐linked aromatic poly(amic ethyl ester)s with the intention to realize lyotropic precursor solutions for rigid‐rod polyimides. As compared to the commonly used flexible precursors, these preorganized precusors should be superior to obtain perfectly oriented polyimide films, orientation layers, and fibers. The poly(amic ethyl ester)s were synthesized by polycondensation of 2,5‐diethylbis(ethoxycarbonyl)terephthaloyl acid dichloride and several para‐functionalized diamines bearing fluoro substituents, bulky side groups, or non‐coplanar structures. Certain introduced structural modifications improve the solubility in NMP to concentrations up to 50 wt.‐% and lyotropic nematic solutions could be obtained. Polymers without fluoro atoms in the ortho position to the polyamide bond have a strong tendency to form a birefringent gel at room temperature which turns into a shearable lyotropic phase above 50°C. This transformation was studied by temperature dependent FTIR spectroscopy and WAXS. However when the ortho position is fluorinated the precursors form a lyotropic phase at room temperature under shear. The macroscopic surface morphology of the dried gels of concentrated solutions is dramatically affected by the chain stiffness. para‐Poly(amic ester)s show a distinct wormlike morphology, indicating molecular order within the gel, whereas the corresponding flexible meta‐poly(amic ester)s show a structureless, smooth surface.