Morphology of Polymerized Ionic Liquids from X-Ray and Neutron Scattering: An Atomistic Simulation Perspective

Wednesday, 4 October 2017: 16:00
Chesapeake G (Gaylord National Resort and Convention Center)
H. Liu (University of Tennessee, Knoxville) and S. Paddison (University of Tennessee)
The design of solid-state electrolytes for electrochemical applications that utilize polymerized ionic liquids (polyILs) would greatly benefit from a molecular-level understanding of structure-property relationships. We herein use atomistic molecular dynamics simulations to investigate the structural properties of a homologous series of poly(n-alkyl-vinylimidzolium bistrifluoromethylsulfonylimide) poly(CnVim Tf2N) and present the first direct comparison of the structure factors obtained from scattering and simulations. Excellent agreement is found in terms of peak position and shape. Specifically, three characteristic peaks can be identified: low-q backbone-to-backbone peak or polarity peak, immediate ionic peak or charge peak and high-q pendant-to-pendant peak or adjacency peak. As the alkyl chain length increases, the backbone-to-backbone peak becomes stronger, moving to larger distance, the anion-to-anion separation slightly increases with weaker intensity, and the pendant-to-pendant or close contact peak remains essentially intact. The longer alkyl chains lead to the longer backbone-to-backbone separation and the larger nonpolar nanodomains. The quantitative cluster analysis along with color-coded snapshots vividly demonstrates that discrete nonpolar islands first form within the continuous polar network, then nonpolar domains grow beyond the percolation threshold, finally interconnect with the polar network into a bicontinuous ‘sponge-like’ nanostructure. Moreover, we exploit the selective labeling technique of neutron scattering using hydrogen/deuterium substitution to afford further insight into the morphology of poly(CnVim Tf2N). The neutron scattering profiles markedly depend on the isotopic substitution pattern. The total neutron structure factors of the backbone deuterated samples reveal the most noticeable low-q peak and are much more intense than those observed in single isotope neutron scattering. Our results strongly suggest that the X-ray scattering and distinct isotopic labeled neutron scattering data do not necessarily give rise to the same positioned scattering peaks. We hope these insights will lead to a fundamental understanding in structure and morphology of polyILs and pave a path forward towards the rational design of future polyILs for electrochemical devices.