Potential Universal Extensional Rheology in Concentrated Polymeric Liquids

Polymer dynamics are special, in that they are always insensitive to the chemical details of the monomers. However, recent experiments show that the nonlinear extensional rheology of concentrated polymeric liquids has a nonuniversal feature. In this work, the variation of segmental frictional coefficient under flows, which is thought to be the critical factor in explaining the observed nonuniversality, is investigated in the coarse-grained (CG) molecular dynamics (MD) simulations of polymer melts. The frictional coefficients in the simulations are quantified from the expressions we proposed very recently [Jiang, N.; van Ruymbeke, E., Macromolecules2023, 56 (8), 2911-2929], which are based on the analytical relationships between the frictional coefficients and the observable rheological and structural properties. After the validation of the simulations with experimental data and our expressions, it is shown that those frictional coefficients can be universally related to the projection areas of the polymer coils in the plane normal to the direction of elongation. Moreover, this projection-friction relationship indicates a Kuhn-scale criterion of extensional viscosity, which suggests that a similar extensional rheology will be observed when the materials have the same number of Kuhn segments per chain N-k and the same reduced Kuhn density n(k) defined as the ratio of the Kuhn length to the packing length. This criterion is tested on a series of new simulation systems designed to show similar steady-state extensional viscosities as a function of the reduced extensional rate, as well as the existing experimental data in the literature. The results of this work provide a helpful guideline in searching for the potential universal extensional rheology in the experimental samples.