Independent Observation of the Two Relaxations in Chain Anisotropy and in Entanglement Network of Condensed Polymers under Large Deformations

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Viscoelastic properties of well-entangled polymers can be expressed by two parameters, e.g., the plateau modulus and the longest relaxation time even outside the range of linear viscoelasticity. The former, a measure of the entanglement network, can be given by measurements of differential dynamic modulus by means of small oscillatory deformations superposed on a large deformation. There has been found the correspondence between nonlinear viscoelasticity and change in entanglement network. The network shows no change in linear viscoelasticity. In a large deformation, however, it really changes with the strain given to the material.Many investigators have been believed that viscoelastic properties of polymers are due to chain anisotropy and can be understood with strain-dependent memory function obtained by stress decay. The theory using reptation concept and tube-model is based on the way of thinking, too. It may be true for polymer dilute system. In considering polymer condensed syste … More m where intermolecular entanglement exists, however, we should take not only chain anisotropy but also change and recovery of entanglement network into consideration. The former and the latter are reflected in the changes in refractive index tensor and the height of rubbery plateau, respectively. Hence we can discuss the nonlinear viscoelasticity from the independent and concurrent measurements of birefringence and differential dynamic modulus. In this work, we have constructed a novel biaxial rheometer equipped with an optical train for birefringence and progressed the measurements. From the independent measurements of the two relaxations in the chain anisotropy and in the entanglement network structure, it was found that the entanglement network ruptured by large deformation takes a very long time to recover to the equlibrium one. The entanglement network does not recover even after the chain configuration relaxes completely. The change in entanglement network is found to play an essential role in nonlinear viscoelastic response. Less