Investigation of the band texture occurring in acetoxypropylcellulose thermotropic liquid crystalline polymer using rheo-optical, rheological and light scattering techniques

The optical evolution of the band texture occurring in acetoxypropylcellulo se thermotropic polymer has been investigated as a function of temperature and primary shear rate. Two distinct kinds of band texture were observed wh ich are referred to here as the 'fast' and 'slow' band textures with regard to their rate of evolution. The fast band texture appears very quickly fol lowing the cessation of shear and then disappears. The slow band texture is much finer than the fast band texture and appears to exist both during and after the appearance of the fast band texture. The evolution behaviour of the fast band texture is interpreted in terms of the shifting of a three-re gion evolution curve. Particular attention has been paid to investigating t he influence of temperature on the formation of the fast band texture. Rheo -optical experiments show that the minimum shear rate required to form the fast band texture increases as a power-law function of the temperature. By subsequently performing steady flow measurements over a range of temperatur es, the minimum shear stress required to form the fast band texture has bee n found to be independent of temperature and to increase linearly with the molecular weight of the sample. Results obtained from dynamic tests are com pared with similar tests conducted previously on a lyotropic hydroxypropylc ellulose water solution (Harrison and Navard 1999). The results of the comp arison provide evidence in support of a connection between the behaviour of the dynamic functions and the optical evolution of the slow band texture. These results suggest that nematic and cholesteric fluids call relax throug h several different possible mechanisms, each, of which results in a period ic band texture following the cessation of shear.