posted on 2020-06-09, 14:37authored byU Hyeok Choi, Terry L. Price, Daniel V. Schoonover, Harry W. Gibson, Ralph H. Colby
Ion
transport and segmental dynamics were studied for one- and
two-armed norbornene ionic liquid monomers (ILMs) including imidazolium
(Im+)-bis(trifluoromethanesulfonyl)imide (Tf2N–) pairs, with different imidazolium pendant structures
containing either alkylene [(CH2)2] or oxyethylene
[(OCH2CH2)x=(OE)x, x = 1, 2, or 3] units as
the linkers between the norbornene and the imidazolium cation, using
dielectric relaxation spectroscopy and oscillatory shear. All ILMs
exhibit three dipolar relaxations, assigned to the fastest frequency
glassy β relaxation, attributed to local chain motions of pendant
groups, the intermediate frequency segmental α relaxation, associated
with the glass transition (Tg), and the
slowest frequency ionic α2 relaxation, attributed
to ions rearranging; the former has an Arrhenius character, while
the latter two have Vogel temperature dependences. The incorporation
of the OE linkers lowers the glass transition temperature (Tg), accelerating the α and α2 relaxations, and increases the static and Coulombic dielectric
constants (εs after the α2 relaxation
and εC between the α and α2 relaxations), compared to the monomer without the OE linker. From
the analysis of εs using the Onsager theory, the
one-armed norbornene ILM εs is well predicted, but
the two-armed ILMs exhibit lower experimental εs values
compared to the Onsager prediction. This suggests that there is more
ionic aggregation in the two-armed ILMs due to strongly overlapping
polarizability volumes, consistent with the Kirkwood g correlation factor less than unity and higher polarizability volume
overlap parameter, indicating that Im+Tf2N– ion pairs are strongly interacting and preferring
an antiparallel alignment, thereby lowering εs. Furthermore,
εC increases with the OEx linker length, consistent with Landau and Lifshitz’s mixing
rule, revealing that with higher OE content, the environment is more
polar for the ionic dissociation of the Im+Tf2N– ion pairs. This is directly reflected in ionic
conductivities (σDC), which are strongly correlated
with both Tg and εC.