Development of orientation and texture during shearing of liquid-crystalline polymers

Abstract

Using polarizing microscopy, we study the development of orientation and texture during slow shearing of liquid-crystalline solutions of poly(γ-benzyl-L-glutamate). Shearing of high molecular weight samples that are initially coarsely textured, or are uniformly oriented over regions of 1 mm², produces a steady state striped texture, for each of three different initial average orientations. The director field in the striped texture is on average aligned parallel to the flow direction, but with misalignment that varies periodically as we move in a direction orthogonal to flow, thus giving the sample a striped appearance under crossed polarizers, with stripes parallel to flow. After the stripes form, they become finer with increased shearing, or with increased shear rate. Along with the birefringent stripes, there are defect lines, parallel to the flow direction with a spacing similar to that of the stripes, i.e. 10 μm or less at steady state. The existence of separate time scales for the development of steady state orientation, and for the shrinkage of the texture length scale to a steady state, is consistent with a phenomenological scaling theory that had been postulated to explain the rheological behaviour of these fluids. Although the steady state striped texture is independent of initial orientation, transient textures during start-up of shearing are highly dependent on initial orientation; for some initial orientations, bands perpendicular to flow occur temporarily during shearing.