Effects of compressed carbon dioxide on the phase equilibrium and molecular order of a lyotropic polyamide solution

The effects of compressed carbon dioxide at slightly subcritical temperatures on solutions of a chloro-substituted, para-linked aromatic polyamide (PPTA-Cl) in dimethylacetamide were investigated by depolarized light spectroscopy. Pressurization of isotropic solutions resulted in polymer precipitation at a sharply defined pressure. Pressurization of nematic solutions resulted in an increase in the intensity of scattered light with no evidence of precipitation. A plausible interpretation of this observation is the occurrence of liquid-liquid phase separation into polymer-rich (anisotropic) and polymer-lean (isotropic) phases induced by the compressed carbon dioxide. The results suggest that compressed, near-critical antisolvents can be used to control the degree of anisotropy in liquid-crystalline polymeric systems.