Ordering of liquid crystalline solutions of rigid‐rod aramids using mechanical shearing and electric‐field poling

Abstract

Abstract2′,5′‐Diamino‐4‐(dimethylamino)‐4′‐nitrostilbene was polymerized with terephthaloyl dichloride and 2,6‐difluoroterephthaloyl dichloride in a low‐temperature solution polycondensation to give two novel, fully aromatic rigid‐rod polyamides. Nematic solutions of these polymers were processed into fibers and films that were characterized by wide‐angle X‐ray diffraction measurements. A post‐spin annealing process was employed to enhance the chain orientation in the fibers. The dominating crystal structure was found to be similar to “modification II” of the fibers from poly(p‐phenyleneterephthalamide), but the fibers do not suffer a comparable structural transformation upon heat treatment. A corona‐discharge poling process gave rise to a remarkable gain in average chain orientation in the films. Again the crystal structure was found to be similar to “modification II”. The aramids investigated in this work represent a new approach to the design of liquid‐crystalline rigid‐rod polymers, where different mechanisms of orientation can be combined. In the nematic phase, the rigid‐rod molecules form highly oriented domains that can be oriented using mechanical processes such as shearing. In addition the stilbene units that are fixed in the polymer backbone with their dipole moments oriented transverse to the main chain can be oriented in an electric field. The combination of both orientation mechanisms seems to cause a synergistic effect, probably since each affects different levels of the polymer microstructure in the solid.