A preliminary morphological evidence for the existence of back-flow effect associated with the formation of band texture after cessation of shear for a polymeric liquid crystal

Abstract

The band texture of the hydroxypropylcellulose (HPC) film is observed after cessation of shear by optical microscopy and scanning electron microscopy (SEM). The sinusoidal supramolecular structure associated with the band texture of HPC is verified by polarized optical microscopy, so the band texture corresponds to the spatial periodicity of local director or molecular orientation. On the other hand, however, the pleated morphology, a depth periodicity, is confirmed by SEM and the pleat period is consistent with the band period. What is more, even without polarizer and analyser, the bands can still be found in an optical microscope with the same periodicity as those under cross polars, and these bands might be attributed to a density periodicity. Both the depth and density periodicities result from the mass flow after cessation of shear and must be coupled to the orientation periodicity resulting from director rotation. As a consequence, we propose that the back-flow effect is striking in the formation of band textures and should be taken into consideration in order to give a plausible and explicit mechanism of band formation.