262848 Dynamics in Model Ionomer Melts As a Function of Polymer Architecture
Ionomers, polymers with a small fraction of covalently bound ionic groups, are of potential interest as solid, single ion conducting electrolytes in future batteries. A major challenge to their use in batteries is that their strong electrostatic attractions can make counterion diffusion unacceptably slow. Polymer connectivity is one design variable that can be potentially changed to create improved materials. To understand the effect of polymer architecture on the dynamics, we perform molecular dynamics simulations of coarse-grained polymers with explicit counterions. Our model captures the fundamental physics and is inexpensive enough to simulate the long time and length scales relevant to ionomer melts.
The polymers have a linear backbone with charged beads either in the backbone or pendant to the backbone that are spaced either periodically or randomly. The ratio of charged beads to backbone beads is varied from 1:3 to 1:9. The simulated structure factors reproduce the trends seen in experimental scattering of recently synthesized ionomers with controlled precise or pseudorandom spacing of charged groups along the chain. We have also calculated counterion diffusion constants, ionic cluster autocorrelation times, and other measures of dynamic behavior. These can change drastically depending on the polymer connectivity. Surprisingly, randomly spaced materials can have slower or faster counterion dynamics than periodically spaced materials, depending on whether the sequence is completely random or pseudorandom. This and other trends with possible relevance to conducting polymer design will be discussed.
Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.