Chain and Ion Dynamics in Precise Polyethylene Ionomers

Abstract

We analyze the dynamics from microsecond-long, atomistic molecular dynamics (MD) simulations of a series of precise poly­(ethylene-co-acrylic acid) ionomers neutralized with lithium, with three different spacer lengths between acid groups on the ionomers and at two temperatures. At short times, the intermediate structure factor calculated from the MD simulations is in reasonable agreement with quasi-elastic neutron scattering data for partially neutralized ionomers. For ionomers that are 100% neutralized with lithium, the simulations reveal three dynamic processes in the chain dynamics. The fast process corresponds to hydration librations, the medium-time process corresponds to local conformational motions of the portions of the chains between ionic aggregates, and the long-time process corresponds to relaxation of the ionic aggregates. At 600 K, the dynamics are sufficiently fast to observe the early stages of lithium-ion motion and ionic aggregate rearrangements. In the partially neutralized ionomers with isolated ionic aggregates, the Li-ion-containing aggregates rearrange by a process of merging and breaking up, similar to what has been observed in coarse-grained (CG) simulations. In the 100% neutralized ionomers that contain percolated ionic aggregates, the chains remain pinned by the percolated aggregate at long times, but the lithium ions are able to move along the percolated aggregate. Here, the ion dynamics are also qualitatively similar to those seen in previous CG simulations.

Segmental Dynamics Measured by Quasi-Elastic Neutron Scattering and Ion Transport in Chemically Distinct Polymer Electrolytes

“…Previous work has shown that the addition of salt slows down the polymer dynamics on these timeand length-scales. 8,[12][13][14] The QENS data are used to determine the effect of added salt on the monomeric friction coefficient in both PCEA-and PEObased electrolytes. We explore the relationships between this parameter and ionic conductivity in the two systems.…”

Theory of Ionic Conductivity with Morphological Control in Polymers

“…The subject of ionic conductivity in polymeric materials with heterogeneous structures is of intense current interest, primarily due to the societal need for better battery alternatives. − Significant progress has recently been made toward formulating polymeric materials with enhanced ionic conductivities and at the same time not compromising on their mechanical stability. The procedures that have been implemented in this endeavor are primarily experiments and simulations. − Generally speaking, the investigated systems include solid polymer electrolytes such as salt-doped poly­(ethylene oxide), polymeric single-ion conductors, polymerized ionic liquids, and polyelectrolyte solutions in nanocapillaries. − …”

Polymer Dynamics in Block Copolymer Electrolytes Detected by Neutron Spin Echo

“…3,[7][8][9][10] In these cases, the translation of ions can be accommodated by segmental relaxation of the polymer chains. 11,12,21,[13][14][15][16][17][18][19][20] Thus, ionic conductivity of a well-studied polymer electrolyte, a mixture of poly(ethylene oxide) (PEO) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt measured by ac impedance spectroscopy, can be explained entirely by the segmental relaxation quantified by quasi-elastic neutron scattering (QENS). 14 To our knowledge, no attempt has been made to study the relaxation processes that govern polymer electrolytes under large applied potentials.…”

“…A popular approach for characterizing ion transport in electrolytes is ac impedance spectroscopy, which reflects the oscillation of ions in response to a small ac potential. ,, Another popular approach is pulse-field gradient NMR, wherein the Brownian motion of ions is quantified in the absence of an applied potential. ,− In these cases, the translation of ions can be accommodated by segmental relaxation of the polymer chains. − Thus, ionic conductivity of a well-studied polymer electrolyte, a mixture of poly­(ethylene oxide) (PEO) and lithium bis­(trifluoromethanesulfonyl)­imide (LiTFSI) salt measured by ac impedance spectroscopy, can be explained entirely by the segmental relaxation quantified by quasi-elastic neutron scattering (QENS) . To our knowledge, no attempt has been made to study the relaxation processes that govern polymer electrolytes under large applied potentials.…”

“…The normalized longest molecular relaxation time, τ d , n , was calculated from the NSE results, and was found to increase with increasing salt concentration. All of the rich literature on polymer electrolytes is narrowly focused on the transport of ions under small applied fields or in the absence of applied fields. ,− , We posit that our measurements of dynamics at long times (50–100 ns) are relevant to the operation of polymer-electrolyte-containing batteries at high currents, wherein the diffusion of salt ions in one direction must induce diffusion of polymer chains in the opposite direction.…”