1990 Volume 18 Issue 1 Pages 33-38
Effect of flow history on the rheological properties of a well-known thermotropic liquid crystalline copolyester which was synthesized from poly (ethylene terephthalate) and 60 mol % p-hydroxybenzoic acid was investigated. The development behavior and steady values of shear stress, σ, and first normal stress difference, N1, and frequency dependence of dynamic viscoelastic functions were measured by means of a cone-plate type rheometer. Flow histories were controlled by steady and interrupted flows. The development of σ and N1 on a virgin sample shows a two-step overshoot. For sheared samples, the maxima of the stresses at the first overshoot drastically decreased, as compared with those for virgin samples. The initial behavior can not be recovered by resting the sheared samples for as long as 20 min. The values of dynamic viscoelastic functions measured after steady-state in shearing flow are significantly lower than those for virgin state. On the other hand, the steady state values of a and N1 are independent of the flow histories. It is concluded that the structure of the thermotropic liquid crystalline copolyester at rest changes to other one irreversibly after being subjected to steady shear flow, but the structure under steady-shear flow is unique and not affected by the shear histories.
