Normal stresses in flow of polyvinyl chloride plastisols

In the steady state flow of many liquids, such as polymer solutions and melts, the first normal stress difference, N1 = σ11 − σ22, is positive. However, with liquid crystal systems and some colloidal suspensions, negative values of N1 were reported in literature. In our past work with a commercial polyvinyl chloride plastisol, negative values were observed. During the steady state flow, the plastisol undergoes stress-induced phase separation into an immobilized layer and a mobile phase. The concentration difference between the two phases gives a rise to an osmotic pressure difference, Δπ, which is countered by a normal stress, N, generated by the flow. Because N is balanced with Δπ, N cannot be observed directly. In this work, N is identified as an isotropic and N1, directional. The disturbance among rotating particles in the mobile phase produces two effects; one is an increase of pressure, which is N; the other, N1 is associated with a small volume increase, which is directed towards the opening of the rheometer. The directional expansion is caused by the shear-stress gradient in the liquid between the rotating particles. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2769–2775, 2007