Establishing a New Mechanical Nonlinear Coefficient Q from FT-Rheology: First Investigation of Entangled Linear and Comb Polymer Model Systems

Abstract

The nonlinear response of monodisperse linear and comb polymer melts has been investigated under oscillatory shear with Fourier-transform rheology (FT-rheology). The relative intensity of the third harmonics (I3/1), which quantify nonlinearity, was found to depend on the strain amplitude (γ0) at small and medium strain amplitudes quadratically regardless of the excitation frequency, temperature, and polymer topology. From these results, for the first time, we proposed new nonlinear coefficient Q, which is defined as Q ≡ I3/1/γ02, and we also defined zero-strain nonlinearity, Q0, as a constant value at relatively small strain amplitude (limγ0→0 Q ≡ Q0; e.g., the zero shear viscosity). In the case of the linear polymer melt, the Q value displays a constant value at small and medium strain amplitude. At large strain amplitude, we detect that the value of Q is finally reduced (Q(γ0) is decreasing). The investigated comb polymer shows very different behavior; the value of Q displays an overshoot (Q(γ0) is incr...