(420b) Photorheology and Gelation during Polymerization of Coordinated Ionic Liquids

Ionic
liquids present a novel and tunable medium for bulk polymerization of vinyl
monomers. 1-vinylimidazole (Vim) displays intriguing rheological behavior when
mixed with varying amounts of lithium bistriflimide (LiTf₂N) to form
coordinated ionic liquids (IL) and then photopolymerized via a free-radical
mechanism. Previous work has shown that increasing [LiTf₂N] results
in faster chemical conversion of monomer during photopolymerization. The
purpose of our study is to determine how differences in kinetics are manifested
in the development of mechanical properties. We used dynamic oscillatory
rheology to non-invasively monitor the in-situ photopolymerization of
coordinated ILs containing varying molar ratios of Vim:LiTf₂N.

The
coordinated ILs were all Newtonian solutions, and the viscosities exhibited a
power-law dependence on [LiTf₂N]. Polymerization caused large
increases in complex modulus (G*) due to the formation of entangled polymer
chains below the glass transition temperature. Three distinct regimes were
observed with respect to [LiTf₂N]: (1) at low [LiTf₂N],
samples increased in G* faster with [LiTf₂N] and behaved as
solutions; (2) at intermediate [LiTf₂N], G* growth was maximized and
samples underwent sol-to-gel transitions during polymerization; (3) at high
[LiTf₂N], G* growth slowed with [LiTf₂N] and samples
exhibited viscoelastic behavior. We attribute gelation to Li+ complexing with
imidazole pendant groups to form physical crosslinks. Our samples also
exhibited “dark curing”, the extent of which increased with [LiTf₂N].
EPR spectroscopy performed on a similar photopolymerizing system demonstrates
that Li+ cations can stabilize propagating radicals, leading to enhanced curing
following UV exposure. Taken together, these results demonstrate that the
rheological behavior of poly(ILs) can be tuned by varying the composition and
UV dosage. We then extended the study to observe the effects of other metal
salts on the rheology during and after poly(Vim) synthesis from coordinated
ILs.

Figure
1. A)
Complex modulus development during photopolymerization of coordinated ILs
containing varying molar ratios of Vim:LiTf₂N. B) Dependency of
final complex modulus after photopolymerization on [LiTf₂N].

Figure
2.
Post-polymerization frequency sweep spectra and postulated chemical structures
for three salt concentrations: A) neat Vim, B) 4:1 Vim:LiTf₂N, and
C) 2:1 Vim:LiTf₂N.