ROTATION OF LIQUID-CRYSTALLINE MACROMOLECULES IN SHEAR-FLOW AND SHEAR-INDUCED PERIODIC ORIENTATION PATTERNS

We have measured the shear-induced rotation of the nematic director in a liquid crystalline polymer using poly benzyl glutamate (PBG) as mod el system. PEG is a well characterized synthetic poly (or amino acid) with rigid chain architecture and well defined conformations. For the experiments it is important to start out with a sample in which the mo lecules are highly aligned with a uniform director. This so-called mon odomain morphology is obtained by use of strong magnetic fields and su rface modifications of the sample holders. When shearing the monodomai n at a constant rate, the macromolecules rotate initially homogeneousl y until a periodic director pattern develops. These spatially periodic structures emerge in a narrow range of shear strain and, as shearing continues, disintegrate into a chaotic texture. By varying the initial monodomain director with respect to the flow direction (but within th e shear planes) we could show that the periodic patterns do not depend on the shear direction; they are governed by the director of the init ial monodomain. We observe conoscopically that at high shear rates the texture becomes uniformly aligned. The molecules are aligned preferen tially with an angle of about 4 degrees to the shear direction (agains t vorticity direction). Interestingly, this agrees very well with pred ictions made by Larson (1990).