Stress optical measurement of the third normal stress difference in polymer melts under oscillatory shear |
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Authors: | J. A. Kornfield G. G. Fuller D. S. Pearson |
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Affiliation: | (1) Chemical Engineering Department, Stanford University, 94305-5025 Stanford, California, USA;(2) Department of Chemical and Nuclear Engineering, and Materials Department, University of California, Santa Barbara, California, USA;(3) Present address: Department of Chemical Engineering, Caltech, Pasadena, California, USA |
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Abstract: | Results are reported for the dynamic moduli,G andG, measured mechanically, and the dynamic third normal stress difference, measured optically, of a series bidisperse linear polymer melts under oscillatory shear. Nearly monodisperse hydrogenated polyisoprenes of molecular weights 53000 and 370000 were used to prepare blends with a volume fraction of long polymer,L, of 0.10, 0.20, 0.30, 0.50, and 0.75. The results demonstrate the applicability of birefringence measurements to solve the longstanding problem of measuring the third normal stress difference in oscillatory flow. The relationship between the third normal stress difference and the shear stress observed for these entangled polymer melts is in agreement with a widely predicted constitutive relationship: the relationship between the first normal stress difference and the shear stress is that of a simple fluid, and the second normal stress difference is proportional to the first. These results demonstrate the potential use of 1,3-birefringence to measure the third normal stress difference in oscillatory flow. Further, the general constitutive equation supported by the present results may be used to determine the dynamic moduli from the measured third normal stress difference in small amplitude oscillatory shear. Directions for future research, including the use of birefringence measurements to determineN2/N1 in oscillatory shear, are described. |
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Keywords: | Polyisoprenes third normalstress difference oscillatory flow shearstress dynamicmoduli |
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