| Abstract: | Shear-induced chain orientation and relaxation of chain orientation in a thermotropic liquid-crystalline poly(4-hydroxybenzoic acid (73mol%)-co-2-hydroxy-6-naphthoic acid (27 mol%)) (Vectra A950) were studied by means of transmitted light intensity through crossed polarizers in a shear cell. Shear rates ranged between 4 and 1000 s?1 and the level of chain orientation increased with increasing shear rate and increasing temperature. Overall, the times for relaxation were very short. The longest relaxation times (?21s) were obtained at the lowest shear temperatures and at intermediate shear rates. At high shear rates, the relaxation times were generally short (?2s) and independent of temperature. The rapid relaxation of orientation after high rate shearing (100–1000 s?1) indicates that shear-induced crystallization is inhibited under these conditions. A certain degree of chain orientation was still present although the relaxation as judged by the transmitted light intensity was complete. Signs of recrystallization were observed if the samples were not preheated to 320°C prior to shearing. By simultanecous shearing and cooling, high degrees of orientation were achieved (f ? 0.7) as revealed by X-ray diffraction and infrared dichroism. Infrared spectroscopy showed that annealing of oriented samples at 250°C led to an increase in thermal stability of the orientation with annealing time. |
