Studies on nonlinear viscoelastic properties of well‐entangled polymers and change in entanglement structure

Abstract

Studies on nonlinear viscoelastic properties of well‐entangled polymers and change in entanglement structure by the author and his co‐workers are summarized. In concentrated solutions and melts of polymers with high molecular weights, the polymers are entangled with each other, forming a quasinetwork structure. Each molecule is repeating the process of leaving old constraint and entering a new one. In the range of linear viscoelasticity, the dissolution and reformation of entanglements are balanced. Hence the density of entanglement remains constant. Outside the range of linear viscoelasticity, however, the entanglement structure may be changed by external stimuli such as shear strain and shear rate. Such an idea was supported by the observations of transient viscoelastic phenomena in various types of shear rate jump experiments. The relationship between nonlinear viscoelasticity and change in entanglement structure was manifested from simultaneous measurements of stress growth and/or stress relaxation, and differential dynamic modulus by small oscillatory deformations superposed on a large deformation. The entanglement structure does not change from the equilibrium structure in the linear region, whereas it deviates from the equilibrium one in the nonlinear region. Such a change in entanglement structure arises from a decrease in entanglement density and an anisotropic configuration of the chain molecule.