On the damping function of shear relaxation modulus for entangled polymers |
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Authors: | Professor K Osaki |
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Institution: | (1) Institute for Chemical Research, Kyoto University, 611 Uji, Kyoto, Japan |
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Abstract: | Published data of the damping function of the shear relaxation modulus, h( ), are reviewed. This is the ratio of the relaxation modulus measured at a finite magnitude of shear, , to that at the limit of = 0. Majority of the data are in accord with the universal function of the Doi-Edwards tube model theory, in which the damping or the decrease of h( ) is attributed to the contraction along the tube of extended polymer chains. The weaker damping seems to be attributed to 1) comb-branching such as in LDPE; 2) lack of entanglement in too short chains; 3) bimodal molecular weight distribution. However, a star-branching does not cause a deviation from the tube model theory and a broadness of molecular weight distribution is not a major origin of a weaker damping. A star-branched polystyrene with 15 arms exhibits no strain dependence: h( ) = 1. For highly entangled systems with more than 50 entanglement points per molecule, the strain dependence is stronger than that of the Doi-Edwards theory. This could be due to a slip or an instability of deformation in the material. |
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Keywords: | Polymer entanglement nonlinear viscoelasticity damping function relaxation modulus tube model |
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