Linear-Nonlinear Dichotomy of Rheological Responses in Particle-Filled Polymer Melts

In the present study, systematic rheometric measurements are carried out to explore quasi-sinusoidal responses in the nonlinear regime of carbon black-filled monodispersed polyisoprene melts with molecular weights ranging from 3 to 390 kg/mol. Experimental results show that these quasi-sinusoidal responses emerge from a sharp transition as the molecular weight of the matrix, M_n, approaches and passes through a characteristic molecular weight M_c*. Below M_c*, the system typically shows the classic nonlinearity, where storage modulus G’ decreases as strain amplitude increases and the resulting stress waveforms are distorted from sinusoidal waves. Above M_c*, the system displays anomalous nonlinearity, where the stress responses at any given strain remain surprisingly sinusoidal regardless the drop of modulus G’. In this manner, the system actually exhibits characteristic linear-nonlinear dichotomy in the rheological responses in the nonlinear regime. The critical point M_c* is found to be a few times of the entanglement molecular weight M_e.