Prof Colby Seminar

Designing Ionomers for Facile Ion Transport

We synthesize single-ion conducting ionomers with low glass transition temperatures to

prepare ion conducting membranes for actuators and lithium battery separators. We use

dielectric spectroscopy to determine the number density of conducting ions and their mobility

from electrode polarization (using the 1953 Macdonald model) and the number density of ion

pairs from measured dielectric constant (using the 1936 Onsager model). This experimental

work concludes that the number density of conducting ions is tiny, and we discuss ways to

boost that using more polar polymers with weak-binding anions attached to the chain.

We use ab initio quantum chemical calculations at 0 K in vacuum to characterize ion

interactions and ion solvation by various functional groups on ion-containing polymers.

Simple ideas for estimating the ion interactions and solvation at practical temperatures and

dielectric constants are presented that indicate the rank ordering observed at 0 K in vacuum

should be preserved. Hence, such ab initio calculations are useful for screening the plethora

of combinations of polymer-ion, counterion and polar functional groups, to decide which are

worthy of synthesis for new ionomers. The results provide estimates of parameters for a

simple four-state model for counterions in ion-containing polymers: free ions, isolated ion

pairs, triple ions and quadrupoles. We show some examples of how ab initio calculations can

be used to understand experimental observations of dielectric constant, glass transition

temperature and conductivity of polymerized ionic liquids with either lithium or ionic liquid

counterions. In particular, recent calculations provide some important insight as to why

poly(ethylene oxide) is able to raise the dielectric constant to boost ion transport.