Enthalpy-Driven Transition of Liquid Crystalline Textures of Poly(2-cyano-p-phenylene terephthalamide) in N,N-Dimethyl Acetamide/Lithium Chloride

The solution of poly(2-cyano- p -phenylene terephthalamide) (CY-PPTA) in N , N -dimethyl acetamide/lithium chloride exhibited a concentration-dependent transition of liquid crystalline textures from nematic-rich to cholesteric-rich with increasing concentration above the critical concentration. The 2D correlation analysis of the FTIR spectra revealed that the dipolar interactions between cyano groups of the polymer increased as concentration increased. The polarized cyano group withdrew the delocalized π -electrons in the C-N bond of the amide group of the polymer, increasing its single bond character. The molecular simulation based on this spectroscopic evidence proposed a change of molecular shapes of CY-PPTA from linear rigid rod to semi-rigid helix. The semi-rigid helical conformation where the single bond character was prevalent over the delocalized one in the C-N bond would develop a cholesteric liquid crystalline texture.