THE RELAXATION OF SHEAR AND NORMAL STRESSES OF NEMATIC LIQUID-CRYSTALLINE POLYMERS IN SQUEEZING AND SHEAR FLOWS

The growth and relaxation of the shear and normal stresses in squeezin g and simple shear flow are studied in this article for the nematic th ermotropic liquid crystalline polymer Vectra A900. The shear stresses are found to relax very quickly to low values as soon as the flow stop s, showing a long tail which is slow to die out. The normal stress nev er relaxes completely after a shear flow or a squeezing flow. There is good scaling agreement with strain in the case of the shear stress, w hereas the agreement is less for the normal stress. The steady state v alues of the viscosity at 310 degrees C form the three-region curve ch aracteristic of liquid crystalline polymer fluids. Apparently the rela xation behavior of the normal stress is connected to the development o f certain structures in the melt. The structures depend on the type of flow and change slowly during the relaxation. This change can also be observed by the gradual increase of the dynamic moduli, G' and G '', measured as a function of time after the cessation of the shear flow. (C) 1996 Society of Rheology.