A FINITELY EXTENSIBLE NETWORK STRAND MODEL WITH NONLINEAR BACKBONE FORCES AND ENTANGLEMENT KINETICS

In an earlier paper, a nonaffine network model of polymer melts was pr esented in which the rotation caused by shearing as well as the extens ion of the test strand are hindered by interactions with the network i tself. In that work, it was shown that such a strand motion leads to q ualitatively correct steady shear and elongational material properties , even though the strand disentanglement rate was constant and the str and force law was linear. These simplifications were accepted in order to emphasize the effects of the strand motion on material properties. In this paper, however, we show that these idealizations cause the mo del to fail in the start-up of shearing flow because no overshoot is s een in the shear stress growth function. To address this failure, the finitely extensible nonlinear elastic (FENE) network model is introduc ed in which the FENE force law replaces the Hookean force law used in the earlier finitely extensible network strand (FENS) model. Also, a n onlinear expression for the kinetics of strand disentanglement replace s the assumption of a constant rate of disentanglement. Material prope rties for the FENE network model are generated by stochastic simulatio n. The simulation results show that these modifications produce oversh oot in the shear stress growth function and result in a more consisten t description of finite network strand extensibility. (C) 1997 America n Institute of Physics.