Relating Molecular Architecture to the Extensional Rheology of Flowing Polymers

Thomas O’Connor
Sandia National Laboratories

Many industrial processes elongate polymer liquids at rates much faster than the molecule’s characteristic relaxation times. These nonlinear elongation 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 elongational flows. Here, I’ll present molecular simulations for linear, branched, and ring polymer liquids deformed in uniaxial elongational flow. Molecular simulations reproduce rate dependent trends in viscosity that are measured in elongation experiments and reveal the microscopic dynamics driving them. I’ll show where simple theoretical arguments can directly relate the conformations of elongated molecules to their extensional viscosity and discuss extending these ideas to more complex polymers.