How the Microscopic Dynamics of Different Polymer Architectures Drive Nonlinear Extensional Flows

Many industrial processes elongate polymer liquids at rates much faster than the molecular chain's characteristic relaxation times. These nonlinear extension flows can strongly deform microscopic polymer conformations and drive dynamic transitions that produce large changes in polymer viscosity. Understanding how flow depends upon and drives such changes in polymer microstructure is essential for improving established and emerging fabrication methods like fiber spinning and 3D printing. However, most microscopic understanding of these nonlinear flows has been drawn from indirect techniques that infer molecular dynamics from macroscopic rheology. This has begun to change with the recent development of new experimental and numerical simulation techniques that allow researchers to control, sustain, and microscopically probe polymer dynamics during strong extension. Here, I’ll present molecular simulations for linear, star, and ring polymer melts and blends deformed in uniaxial extensional flow. In all three cases, coarse-grained molecular dynamics simulations reproduce the nonlinear rheology observed in extensional flow experiments, and also reveal the microscopic dynamics driving observed nonlinear trends. For some architectures, simple theoretical arguments can directly relate the elongated conformations of molecules to the nonlinear viscosity. In other cases, simulations show how extensional flows can drive polymers to topologically self-assemble or micro-phase separate in ways not seen in equilibrium. These new, far from equilibrium behaviors could provide new routes for controlling polymer microstructure during processing.