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It is well known that the flow behaviour of polymeric liquids can be, and often is, much more complicated than that of ordinary fluids. The ultimate reason for the complication is the molecular anisotropy, obviously very large in a polymer chain. Indeed, although most materials are isotropic at rest (because the molecular segments are randomly orientated), the anisotropy reveals itself when the polymeric liquid is set in motion, as the molecules become oriented. The flow-induced molecular orientation, and the consequent material anisotropy, even persists for a while after the flow has been switched off, i.e. during relaxation.

All the complexity of the rheological behaviour of polymeric materials is ultimately rooted in their molecular structure or, in the case of suspensions, blends, block copolymers, or other complex materials, is due to the “structure” of the material at the scale of, say, 1 µm. Thus a valuable alternative to the classical approach to rheology, originally founded in continuum mechanics, is one which starts from a consideration of the micro-structure of the system, and attempts to model those structural features which are most likely to be important in determining the rheological behaviour which is observed macroscopically. This is, generally speaking, the approach known as micro-rheological modelling, though in the present context only the case of solutions and melts of ordinary polymers will be considered, for which the names of “molecular modelling” or “molecular theories” are more often employed.

The theory of rodlike polymers in the nematic phase is now sufficiently well developed to allow predictions of rheological behaviour which compare favourably with many experimental observations. These include peculiar effects such as negative normal stresses, damped oscillations in start-up flows and oscillation frequencies inversely proportional to shear rate. The basic points of the theory are presented, together with a more direct interpretation of the above-mentioned phenomena.