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The relaxation of shear and normal stresses of nematic liquid crystalline polymers in squeezing and shear flows

Langelaan, Hubertus Cornelis, 1964-, Gotsis Alexandros

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The growth and relaxation of the shear and normal stresses in squeezing and simple shear flow are studied in this article for the nematic thermotropic liquid crystalline polymer Vectra A900. The shear stresses are found to relax very quickly to low values as soon as the flow stops, showing a long tail which is slow to die out. The normal stress never relaxes completely after a shear flow or a squeezing flow. There is good scaling agreement with strain in the case of the shear stress, whereas the agreement is less for the normal stress. The steady state values of the viscosity at 310 °C form the three‐region curve characteristic of liquid crystalline polymer fluids. Apparently the relaxation behavior of the normal stress is connected to the development of 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.

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