Network Theory for Nonlinear Viscoelasticity under Elongational Flow

Abstract

A molecular network theory developed by one of the authors (Y.T.) is applied to explain much of the typical nonlinear viscoelastic behavior of concentrated polymer systems under uniaxial elongational flow. Three types of deformation, i.e., constant velocity, constant strain rate and large step strain are treated. The phenomenon of overshoot is observed in the curves of transient viscosity under steady flow. The steady elongational viscosity is a monotonically decreasing function of the strain rate. Trouton’s formula is satisfied in the limit of zero values for shear and elongational rates. The result of strain recovery after removal of stress following the steady flow is expressed by an integral equation which is numerically solved. The relaxation Young’s modulus strongly depends on strain. The results are in good agreement with the experiments.