The Role of Enthalpic Effects on the Nematic to Cholesteric Transition of Liquid Crystalline Textures of a 2-CN-Substituted Para-Aramid

Abstract

The lyotropic liquid crystalline (LC) system of poly(2-cyano-p-phenylene terephthalamide) (CY-PPTA) solutions in the polar aprotic solvent of N,N-dimethyl acetamide containing a small amount of lithium chloride (DMAc/LiCl) exhibited a transition of LC textures with increasing concentration above the critical concentration (c*) of 9 wt%. The schlieren texture of the nematic phase was predominantly observed at the low concentrations but increasing concentration made the fingerprint texture of the cholesteric phase prevail notably above 15 wt%, indicating a change of molecular conformation from rigid rod to semi-rigid helix. This cholesteric-rich texture was not observed in H2SO4 over the entire range of concentrations examined.

The 2D correlation analysis of the concentration-dependent FTIR spectra verified that enthalpic effects played the leading role of driving the textural transition in DMAc/LiCl. As the concentration of CY-PPTA increased, the dipolar interactions between cyano groups of CY-PPTA increased particularly in the presence of highly polar DMAc molecules. The higher dipolar interactions at the higher concentration brought about a higher polarization of cyano groups. The further increase of concentration gave rise to the withdrawal of electrons by the polarized cyano group from the conjugated amide group through both inductive and resonance effects. The resultant redistribution of electrons in the amide and cyano groups changed the bond character of the amide C‒N bonds on the molecular axis of CY-PPTA from rigid partial double to flexible single bonds. This transformation of the bond characters led to the change of molecular conformation from rigid rod to semi-rigid helix, responsible for the macroscopic transition of LC textures from nematic-rich to cholesteric-rich with increasing concentration.

In between the two limiting concentration regimes giving the nematic- and cholesteric-rich textures, the CY-PPTA solutions in DMAc/LiCl showed a strong coexistence of nematic and cholesteric textures over the concentration range from 11 to 13 wt%. Ageing more than 2 weeks separated the solutions into two different layers, in which the nematic- and cholesteric-rich textures were observed in the upper and lower layers, respectively. The cholesteric-rich layer of higher concentration (14.4 wt%) had a more compact structure of CY-PPTA molecules of higher molecular weight when compared with the nematic-rich layer of lower concentration (11.1 wt%).

The cholesteric texture, generated by para-aramids, was reconfirmed by casting the CY-PPTA solutions in DMAc/LiCl into films. The reconstruction of the cholesteric molecular order during solidification of the sheared solutions yielded a characteristic band pattern on the film surface. That is, in addition to the primary set of bands perpendicular to the shearing direction due to the molecular relaxation of the sheared polymer molecules, a secondary set of bands was additionally observed, which was superimposed on the primary bands resulting from the rewinding of the cholesteric molecular order. This overall herringbone-like surface pattern was clearly obtained above 15 wt%, which coincided well with the boundary concentration giving the cholesteric-rich texture in the precursor solutions. The cross-section of the CY-PPTA films presented a layered structure on account of the helical molecular arrangement throughout the films.