INFLUENCE OF THE FLOW HISTORY ON STRESS GROWTH AND STRUCTURE CHANGES IN THE THERMOTROPIC LIQUID-CRYSTALLINE POLYMER VECTRA B950

The structure changes in the start-up flow of the thermotropic liquid crystalline polymer Vectra B950 are probed by performing transient exp eriments after various flow histories. The shear and normal stress gro wth curves of a squeezed sample and of a randomly oriented sample show a pronounced overshoot at low strains, whereas the stress growth curv e of a sample pre-sheared until steady state shows a gradual increase. This first peak is associated with the re-orientation of the director into the shearing plane. All stress transients show a second broad ma ximum at large strains that results from the generation of a steady de fect network. The effect of varying the relaxation period after pre-sh earing is reflected in the appearance of two peaks in the subsequent s tress growth curves. One of these peaks shifts linearly with relaxatio n period and the other is more or less fixed in position. The orientat ion of the molecules during steady shear flow is on average in the flo w direction. Intermediate orientation levels may exist in the transien t depending on the amount of strain. The material is able to maintain the flow-induced orientation distribution for a long time after cessat ion of flow. This is reflected in a similar fashion in the initial mag nitudes of the stresses and the dynamic moduli after various preshear strains. Moreover, the dynamic moduli decrease with time after cessati on of steady shear flow, indicating that the orientation increases dur ing relaxation.