Formation of a Banded Texture in Solutions of Liquid-Crystalline Polymers 11. Poly(n-Hexylisocyanate) in Toluene

The kinetics of band formation has been followed during the relaxation of anisotropic solutions of poly(n-hexylisocyanate) in toluene previously subjected to a shear rate, for a time t_s. There is a threshold at low, or t_s, below which bands do not appear (formation time t_b→ ∞). Past the threshold, t_s decreases with either or t_s, and eventually bands are seen immediately after stopping the flow (t_b = 0). However, for still larger particularly for the more viscous solutions and when the flowing solution is well oriented, t_b may increase again. The results are similar to those already reported for hydroxypropylcellulose in water and support a previous suggestion for the origin of the banded texture.