Rheological and morphological evidence of binary liquid crystalline phases in solutions of an organo-soluble cyano-substituted p-aramid

Poly(p-phenylene terephthalamide) is a primary precursor for super p-aramid fibers, but its poor solubility allows fiber spinning only in sulfuric acid. As an alternative, poly(2-cyano-p-phenylene terephthalamide) (CY-PPTA) is of great interest because of its organo-solubility while maintaining high performance of the resulting fibers. Herein, we observed abnormal binary liquid crystalline (LC) phases of nematic and cholesteric orders in organo-soluble CY-PPTA solutions and explored the rheological and morphological evidence of such phase composition. Rheologically, CY-PPTA solutions exhibited two critical concentrations (C*s) at 9 and 15 wt% with concentration-dependent phase transitions: isotropic, nematic-rich, and cholesteric-rich. Compared to the nematic-rich solutions, the cholesteric-rich solutions exhibited higher rheological heterogeneity and stronger shear dependency. Morphologically, nematic- and cholesteric-rich phases were identified by schlieren and fingerprint textures, respectively. Moreover, the circular dichroism spectra demonstrated a helical ordering in the cholesteric-rich phase. The evolution of the unusual cholesteric ordering resulted from the distorted conformation of CY-PPTA chains induced by the electron-withdrawing ability of cyano groups. Cholesteric ordering was ultimately transferred into the film morphology after casting, as shown by the herringbone surface pattern. Thus, understanding the abnormal binary LC phases would provide a sound theoretical background to control complicated hierarchical structure and resulting final properties of the organo-soluble CY-PPTA.