Rheology of liquid crystalline polymers

Concepts which may be used in interpreting the complex rheology of liquid crystalline polymers are reviewed. Some of them are taken from the nematodynamics of low molecular weight liquid crystals as described by the Leslie—Ericksen theory, others are intrinsic to the viscoelasticity of ordinary polymers. Tentatively, these concepts are used to construct an explanation of the band texture which is frequently observed when liquid crystalline polymers are sheared, and to interpret the complex behavior of normal stresses which exhibit both positive and negative values. INTRODUCTION The rheology of liquid crystalline polymers appears to be exceptionally complex. The phenomena observed during and/or after flow are extremely diversified and generally different from what is found in either ordinary polymeric liquids or low molecular weight (LMW) liquid crystals. Some understanding of these phenomena seems necessary, however, especially for what concerns molecular orientations. In fact, polymeric articles are manufactured via flow processes and the materials properties in the solid state are strongly influenced by flow induced molecular orientations. Limiting our attention to nematics only (possibly, to cholesterics), we may expect that in general a flow has the following effects: 1 — The distribution of molecular orientations about the nematic axis (director) is altered. 2 — The director itself is affected by the flow. The first effect is due to the polymeric nature and the consequent (relatively) large values of relaxation times. In LMW liquid crystals, the flow is never strong enough to compete with thermal motions and the equilibrium orientational distribution is preserved. In polymers, conversely, whether liquid crystalline or not, relaxation rates may be smaller than shear rates and the orientational distribution is therefore altered. In ordinary polymeric liquids, this effect (in the sense of deviation from isotropy, of course) is the only one which is present and yet the rheology is already complex. In particular, elastic phenomena are generated such as normal stresses in a simple shear flow. These, however, are always positive in ordinary polymers. The second effect is the only one that is present in LMW liquid crystals and, by itself, also gives rise to a complex rheology. The interplay between director and velocity gradient generates a viscous stress which is characterized (at the phenonenological level) by five independent viscosities, known as Leslie coefficients. Furthermore, the director is generally non—uniform in space. The spacial distortion of the director generates elastic stresses, named after Frank or Ericksen, which of course bear no relationship with polymer elasticity. Whenever the director varies considerably throughout the sample, with the possible occurrence of discontinuities (defects or disclinations), we speak of a polydomain structure, a domain being a region where the director is approximately uniform. Polydomains appear to be the rule rather than the exception in polymeric liquid crystals, much more so than in the LMW case. Flow processes are found to alter the structure of polydomains in more than one possible way. There is evidence that the flow may increase the defects or discontinuities (ref. 1) as well as decrease them to form a monodomain (ref. 2). There is also an accumulating evidence about the formation, during or soon after a shear flow, of a peculiar texture made up of bands perpendicular to the shear direction (ref s 3—5). The band texture